Province of British Columbia MINERAL RESOURCES DIVISION Ministry of Energy, Mines and Geological Survey Branch Petrolelurn Resources Hon. Anne Edwards, Minister
GEOLOGY AND METALLOGENY OF THE STEWART MINING CAMP, NORTHWESTERN BRITISH COLUMBIA By Dani J. Alldrick
BULLETIN 85 MINERAL RESOURCES DIVISION Geological Survey Branch
Canadian Cataloguing in Publication Data Alldrick, Dani J. Geology and metallogeny of the Stewart mining camp. northwestern British Columbia VICTORIA BRITISH COLUMBIA (Bulletin, ISSN 0226-7497 ; 85) CANADA Issued by Geological Survey Branch March 1993 Includes bibliographical nferences: ISBN 0-7718-9358-2
,, Field research for this publication was carried 1. Geology - British Columbia - Stewart Region. 2. Metallogeny - British Columbia - Stewm Region. 3. out during the period 1982 through 1984.
Geology, Economic - British Columbia ~ Stewarl Region. I. British Columbia. Ministry of Energy, Mines and Petroleum Resources. 11. British Columbia. Geological Survey Branch. Ill. Series: Bulletin (British Columbia. Ministry of Energy, Mines and Petroleum Resources) ; 85.
QE187.A44557.11993 1’85 C93-092182-8 - SUMMARY The Stewart mining camp in northwestern British Columbia is underlain by an Upper Triassic to Lower Jurassic island-arc complex. The subaerial volcanic pile is constructed of calcalkaline basalts, andesites and dacites with interbedded sedimentary rocks. Lateral variations in volcanic rock textures indicate that the district was a regional paleotopographic high with a volcanic vent centredat Mount Dilworth. Early Jurassic calcalkaline hornblende granodiorite plutons of the Texas Creek Plutonic Suite represent coeval, epizonal subsidiary magma chambers at depths of 2 to 5 kilometres below the stratovolcano. From these plutons, late-stage two-feldspar porphyritic dikes cut up through the volcanic sequence to feed surface flows. Following subsidence this succession was capped by Middle Jurassic marine-basin turbidites. Mid-Cretaceous tectonism was characterized by greenschist facies regional metamorphism, east-northeast contraction, anddeformation. It producedupright north-northwest-trending en echelon folds and later east- verging, ductile reverse faults and related foliation. Mid-Tertiary calcalkaline biotite granodiorite of the Coast Plutonic Complex intruded the deformed arc rocks. The batholith, stocks and differentiated dikes of the Hyder Plutonic Suite were emplaced overa 30-million- year period from Early Eocene to Late Oligocene. The 200 mineral occurrences in the district formed during two mineralizing eventsthat were characterized by different base and precious metalsuites. One ore-forming episode occurred in EarlyJurassic time and the other in the Eocene. Both metallogenic epochs were brief, regional-scale phenomena. Some deposits from the younger mineralizing.episode were emplaced adjacent to older deposits. All Early Jurassic deposits were emplaced in andesitic to dacitic hostrocks at the'close of volcanic activity, about 185 million years ago. They have regional zoning patterns that are spatially related to plutons of the Texas Creek suite and to their stratigraphic position within the volcanic-sedimentary sequence. The Early Jurassic hydrothermal system acquired its characteristic suite of silver, gold, zinc, lead and copper from magmatic fluids. Early Jurassic deposits include gold-pyrrhotite veins, veins carrying silver, gold and base metals, and stratabound pyritic dacites. Gold-pyrrhotite veins formed adjacent to the subvolcanic plutons during late magma movement. Epithermal base and precious metal veins and breccia veins were formed along shallowerfaults and shears, and in hydrothermalbreccia zones along the contacts ofsubvolcanic dikes. Stratabound pyritic dacites are barren fumarole and hotspring-related deposits that formed on the pafeosurface from shallow groundwater circulation within hot dacitic pyroclastic sheets. Tertiay silver-rich galena-sphalerite veins are related to intrusion of Middle Eocene biotite granodiorite stocks of the Coast Plutonic Complex. All deposits are localized in brittle faults or fractures; most have southeast trends and subvertical dips. Likely sources for the silver in these Tertiary deposits are turbidite sequences within the Unuk River Formation and thicker, infolded turbidites of the Salmon River Formation. Tertiary skams and porphyry molybdenum deposits are also genetically related to the Middle Eocene plutons. All these deposit types can be selectively explored for by using diagnostic features such as stratigraphic and plutonic associations, alteration assemblages, gangue mineralogy and textures, sulphide mineralogy and textures, precious metal ratios and lead isotope signatures.
iii TABLE OF CONTENTS Page... 111 TexasCreek Batholith ...... 28 SummitLake Stock ...... 29 I TexasCreek Dikes ...... 29 Age ...... 31 1 Summary ...... 31 1 HyderPlutonic Suite ...... 31 1 HyderBatholith ...... 31 2 BoundaryStock ...... 32 3 HyderDikes ...... 32 3 Age ...... 34 Summary ...... 34 Basaltic Dikes of the Stikine VolcanicBelt 34 ...... 5 Petrochemistry ...... 34 5 VolcanicRocks ...... 34 5 Intrusive Rocks ...... 39 5 5 5 CHAPTER 4"STRUCTURE. METAMORPHISM. 5 ALTERATIONAND GEOCHRONOLOGY ...... 41
8 Structure ...... 41 9 Folds ...... 41
9 Faults ...... 41
IO Foliation ...... 42 H i s to r y 4210 Structural History ...... IO Metamorphism ...... 43 II Alteration ...... 43 Geochronometry ...... 47 13 Discussion ...... 47
13 Interpretation ...... ~ ...... SO 13 Conclusions ...... 51
13 SummaryGeologic History ...... 52 13 13 13 CHAPTER 5 .MINERAL DEPOSITS ...... 53 13 Lead Isotope Studies ...... 54 17 Deposit Classification Table ...... 56 18 Early Jurassic Deposits ...... 56 20 Gold-Pyrrhotite Veins: Scottie Gold Mine ...... 56 20 Geologic Setting ...... 56 21 Orebodies ...... 59 21 Mineralogy ...... 62 22 Alteration ...... 62 22 Genesis ...... 62 22 Silver-Gold-Base Metal Veins: The Big 23 Missouri Area ...... 64 23 Geologic Setting ...... 64 23 Orebodies ...... 66 25 Mineralogy ...... 69 25 Alteration ...... 70 26 Genesis ...... 70 26 Silver-Gold-Base Metal Veins: Silbak 26 Premier Mine ...... 71 26 Geologic Setting ...... 71 26 Orebodies ...... 73 27 Mineralogy ...... 79 28 Alteration ...... 80 28 Genesis ...... 81 28 Stratabound Pyritic Dacite: Mount Dilworth 28 and Iron Cap ...... 84
V TABLE OF CONTENTS .Continued page Page Eocene Deposits ...... 8519 . Tectonic environment discriminant plot ...... 37 Silver-Lead-Zinc Veins: Prosperityh'orter Idaho 20 . Modal compositions of plutonic rocks ...... 38 mine ...... :...... 8521 . Plutonic rock discriminant diagrams ...... 38 Geologic Setting ...... 85 22 . Metamorphic grade in the Stewart mining camp 43 Orebodies ...... :...... 88 23 . General relationships of temperature, deforma- Mineralogy ...... YO tion and argon loss ...... 49 Alteration ...... 90 24 . Igneous, metamorphic.. and thermal history of the Genesis ...... 91 Stewart dlstrict ...... 50 Skarns: Molly B, Red Reef and Oral M 25 . Growth curve for lead isotope evolution ...... 55 Prospects ...... 9 1 26 . Stewart camp galena lead isotope data ...... 55 Geological Setting ...... 91 27 . Geologic setting of the Scottie Gold mine ...... 58 Mineral Deposits ...... 91 28 . Fracturing mechanisms forScottie Gold mine ..... 59 Genesis ...... 91 29 . Genetic model for the Scottie Gold mine ...... 30 . Geologic map of the Big Missouri mine area ..... 63 CHAPTER 6 .METALLOGENY AND 31 . Cross-section through the Big Missouri area ..... 64 EXPLORATION ...... 93 32. Net fault offset in the Big Missouri mine area ..... 64
Regional Genetic Models ...... 93 33. DeDosit model for the Big Missouri~~ denosits ~ ~ ...... 65.. . r ~ ~ ~~ Early Jurassic :...... 93 34 Genetic model for Big Missouri deposits70 ...... the ...... Middle Eocene ...... 93 ~~ 35 . Seauence of development of edthermal settings 71 Metallogeny ...... 93 36 . Geologic cross-seciion through the Silbak Ke- Early Jurassic Metallogeny ...... 93 mier mine area ...... 72 Middle Eocene Metallogeny ...... 95 37. ~i~~rib~~i~~of ore at the silbak premier - ... " ...... Y* kxploratlon ...... mine 74. I~ 75 Gold-Pyrrhotite Deposits ...... :...... 98 38. ~~~l~~i~cross-section though ~~i~ zone, sil- ...... Silver-Gold-Base Metal Deposits 98 bak Premier mine ...... :...... 16 Stratabound Pyritic Dacites ...... 98 39. ~i~trib~~i~~of in the silbak Silver-Lead-Zinc Deposits ...... 98 Premier mine ...... 78 Skam Deposits ...... 99 40. Deposit model for Silbak Premier mine ...... 82 Porphyry Molybdenum Deposits ...... 99 41. ceneticmodel for the Silbakpremier ,,,in e ...... 83 42. Genetic model for stratabound pyritic dacite ...... 84 REFERENCES ...... 101 43 . Distribution of veins on Mount Rainey ...... 84
COLOUR PHOTOPAGE ...... 105 44. Geologic map of the Mount Rainey summit ...... 86 45 . Vein geometry at Prosperityh'orter Idaho mine 87 LIST OF FIGURES ... 46 . Cross-section through the ProsperitylPorter 1. Location of study area ...... vi11 Idaho mine ...... 88 2 . Physiographic and tectonic belts, terranes and 47 . Distribution of Early Jurassic and Eocene ore composite terranes of the Canadian Cordillera ..... 4 deposit types ...... 94 3 . Tectonic elements of northern British Columbia 6 48 . Genetic model for the Early Jurassic ...... 95 4 . Stratigraphy of Stikinia ...... 7 49 . Mineral potential in an andesitic stratovolcano 95 5 . Distribution of the HazeltonGroup and the 50. Deposit model for the Middle Eocene ...... 96 Stewart Complex ...... 8 51. Genetic model for the Middle Eocene ...... 97 6 . Evolution of stratigraphic nomenclature for the 52 . Metallogenic models for the Stewart camp ...... 97 Hazelton Group ...... 9 LIST OF TABLES 7 . Intrusive events in Stikinia and the Canadian Cordillera ...... 10 1. Textural distinctions between Premier Porphyry 8. Subduction zone geometry of the Canadian Cor- flow and Premier Porphyry tuff ...... dillera ...... 11 2 . Evidence for subaerial versus subaqueous depo- 9 . Geological history of Stikinia ...... 11 sition of the Unuk River Formation ...... 20 IO. Geologic setting of the study area ...... 14 3. Hyder Plutonic Suite - Sequence of intrusion 34 11. Geology of the Stewart mining camp ...... in pocket 4 . Major element analyses from stratified rocks ..... 36 12. Schematic stratigraphy of the Stewart camp ...... 15 5 . Major element analyses from intrusive rocks ...... 38 13. Evolution of stratigraphic nomenclature for the 6 . Structural history ...... 42 Stewart mining camp ...... 15 7 . Metamorphic minerals and phase transitions ...... 43 14. Phenocryst and fragment size variations in Stew- 8. Classification of alteration types by mineralogy, art stratigraphy ...... 16 intensity and extent ...... 44 15. Stratigraphy of the Mount Dilworth Formation 23 9 . Classification of alteration types by age and 16. Alteration screening plots ...... 35 genetic process ...... 44 17. Silica histograms ...... 37 IO. Dates from the Stewart mining camp in chrono- 18. Total alkali versus silica discriminant diagrams 37 logical order ...... 47
vi Page
30 32 45
45
46
60 60
61
61 66
66 66
61
68
68
69
71 77
19
80
88
89
89
vii Figure 1. Location of study area.
... VU1 - CHAPTER 1 - INTRODUCTION The Stewart mining camp has a long history of under- completed by the summer of 1905. Captain Gaillard later ground gold-silver production and, is entering a new era of achieved fame as one of the senior engineers on the Panama open-pit and underground precious metal mining. The dis- Canal construction project, and the Gaillard Cut is named in trict is abundantly mineralized, with over 200 widely distri- his honour. hute:d, varied mineral occurrences. This report documents Mining history in the region also began with the Gaillard the geologic setting of the mining camp and geologic fea- expedition when Lieutenant Mosier of theUnited States tures of the major mineral deposit types. New ideas about Navy discovered and staked the first copper showings in the the tectonic setting and metallogenic history of the district Outsider area on Maple Bay (Magee Bay), along the are also presented. southeast sidePortlandCanal. of the TheAlaska Brown Company of Prince of Wales Island, Alaska, began major LOlCATION AND ACCESS construction at this site in the summer of 1905 and 15 000 tomes of copper-gold ore were shipped to the smelter at The area lies near the eastern margin of the Coast Moun- Hadley, Alaska between 1906 and 1908. The existence of tains at the head of the Portland Canal, a fiord 115 kilo- this smelter was an earlyincentive to pr&pectors metreslong which marks the southeastern boundary and promoters alike, between the Alaska Panhandle and northwestern British During the fall and winter 1897 a Seattle promoter Columbia. Thefield area covers 750 square kilometres of centred near the Silhak Premier mine; it includes the town named Burgess (Bruges) talked enthusiastically ahout the of Stewart, British Columbia and the village ofHyder, untapped placer gold that he had seen in the headwaters of the Nass River. By the early spring of 1898 a party of 64 Ala& (Figure 1). Stewart canbe reached by road from‘adventurers,, few with any mining or prospecting experi- Van8couver and hasair service to Terrace and Smithers. The main access route within the area is the Granduc mine road, ence, set sail aboard the chartered steamer Discovery, with six months’ supplies aboard. They reached the head of the a gravel haul-road running north from Hyder to the Tide Portland Canal on May 4, 1898 and packed overland Lakte airstrip. through the Bear River Pass in search of the rich placer groundthat Burgess had reported in thearea around TOPOGRAPHY Meziadin Lake. No gold was found and themoved party The study area is near the end of the northward towards Bowser Lake, hut bynow the expedition Ranges of the Coast Mountains. Elevations range from sea had endured much physical hardship and still found no gold. to 2544 on Mitre Mountain, Topography is Amid growing discontent Burgess stole silently away,and rugged; the area is characterized by precipitous glaciated most ofthe rest of theparty made their way back to vall,ey walls and rounded ridge The dominant civilization at intervals. Whether Burgesshad ever visited topographic pattern is north.&ending ridges and intervening the region Or knew Of placer gold in the glaciated valleys (Figure 1). headwaters of the Nass remains a mystery, hut his objective was very wecific. Ironicallv. if the uartv had turned south instead bf north from Mezcadin Laki, they could not have HI!STORY failed to find the placer gold beds in Nelson, Porter (Del There isno history or evidence of a permanent settlement Norte) and Willoughhy creeks. at the head of the Portland Canal prior to 1898. However, Relying on the abandoned supplies, three men wintered the :area was one of the main hunting grounds for the Nass over, D.J. Rainey, James W. Stewart, and Ward Brightwell. (Na:a) indians who traditionally encamped twice yearly to They built a cabin at the foot of Mount Dolly,beside Rainey hunt hear and waterfowl and for herry-picking. Their name Creek. In the following seasons, their prospecting work for this site, Skam-a-kounst, means safe-house or strong- concentrated on locating placer gold, hut they noticed sev- house,and the remote headof this long fiord probably eral mineralized outcrops while hunting for game on the served as aplace of retxeat when parties of Haida raided the mountains. At first these were not considered valuable; in mainland or when intertribal wars broke out. Although they 1899, only the Grizzley Bear-claims (now the Roosevelt were never openly antagonistic, the indians did not wel- Group) were staked in Bitter Creek by Rainey, but gradually come the arrival of prospectors into their game preserve. all of these showings were relocated and staked. The Ameri- can Boy claims were staked on American Creek in 1900, In 1896, a preliminq survey of the canal was under- taken by the United States Army Corps of Engineers, led by and the first claims on Glacier Creek were staked in 1903. Captain D.D. Gaillard. They built four stone-and-concrete The general search for gold by prospectors straggling south storehouses along the canal; the final one was constructed at from the overcrowded Klondike, and the recording of min- Eaglle Point near what is now the border crossing to Hyder, eral claims at the head of the Portland Canal, slowly drew Alaska. This storehouse was constructed between Septem- more prospectors to this remote location. ber 4 and 21, 1896, and survives as an historic monument. Two other Stewart brothers, Robert and John, came north This site was also the starting point for the international from Victoria in 1902 and, although they spent a few sea- boundary survey that began in the summer of 1903 and was sons prospecting, their great vision wasof a prosperous
1 mining town with docks and railways serving mines in the located the outcrops of the veins that would become the hinterland. They established the post office in 1905,and Silverado mine workings. founded the Stewart Land Company and incorporated the InStewart, plans for therailway extension were townsite in 1907, soon after theInternational Boundary was postponed, then the Red Cliff mine closed in1912, and formally defined in 1906. It was standard policy that, in the Stewartk population dropped to less than two dozen people absence of any other name, the post office wasnamed during the FirstWorld War. Onlythree people wintered over Stewart after its founderand first postmaster, and the stead- in the town from 1917-1918. The opening of the Premier ily growing settlement also became known as Stewart. mine in 1918 began a new era of prosperity that continued Discoveries continued, fueling increasing stock specula- through to the Second WorldWar. A telegraph line con- tion in Victoria and Vancouver. In 1904, prospects known as nected Stewart to the outside world in 1919. By the late the Big Missouri, later renamed Golden Crown, were staked 1920s, the town had a population of 1500, with another 500 by Harrison and Raerick, two experienced Alaskan prospec- residents in Hyder. By 1929, before the stock market crash, tors. The first ore production, at the modest milling rate of there wereover 40 mining companies and syndicates active 80 tons per day, was achieved by the Portland Canal Mining in the district. The Premier mine provided stable employ- Co. Ltd. -at its GlacierCreek property. The Red Cliff copper- ment until the outbreak of war in 1939. With the mine shut gold deposit was the first major mine of this era, and the down during the war, Stewart’s population again declined richest gold deposit in the district. steadily, reaching 300 by 1950. By 1910, the population had grown to nearly 2000. As in Hyder was settled by American prospectors returning anyprospecting centre and burgeoning mining camp, from the Klondike gold rush. Originally Podand City, it rumours and speculations were part of daily life. Expecta- was soon renamed in honour of geologist F.B. Hyder. tions were heightened by the construction of a 12-mile Through the years of the First WorldWar, Hyder was railway to the Red Cliff mine,and by much publicised plans abandoned, but soon after the armistice of 1918, it returned to extend it through the Bear River Pass to the anthracite to life as Stewart hit world headlines (again) as the site of coalfields of the Groundhog camp. In early June, two rival the rich Premier mine. With the crash of 1929, and through prospecting teams came down from Glacier Creek with rock to the end of the Second WorldWar, Hyder was again samples containing coarse gold and stories of a “mammoth deserted. Most of the town was destroyed by a fire in 1948. gold reefof free-milling gold. Unknown in extent, but traced for at least20 miles, it ismore than a mountain of ore The village of Hyder, British Columbia blossomed during - an entire range of it in fact.” (The Portland Canal Press the 1920s, partly because of the Premier minetramway Ltd., 1910). The townbecame deserted overnight as the terminal and loading docks, but mainly because of its prox- Glacier Creek area was blanketed with new claims. A young imity to itsAmerican namesake during the years of prohibi- reporter who was in Stewart on an assignment to cover tion. Within a few years Hyder, Alaska’was connected to ‘frontier life’ for an English newspaper succumbed to the Stewart by road and Hyder, B.C. was soon reduced to pilings. atmosphere and excitement.His enthustiastic “eye- witness” report of the discovery of this mountain of gold Readers particularly interested in the early history of the was reprinted round the world, and Stewart changed forever. districtare referred to the displaysand archives of the By the end of the year the population reached 10000, with Stewart Museum, and to the archives of the Legislative another 2000 settled in the neighbouring village of Hyder. Library and Royal British Columbia Museum in Victoria. In the prospecting and staking frenzy that ensued, Charles Bunting and William Dilwoah found a small gossanous PREVIOUS GEOLOGICAL WORK knob in the woods overlooking the canyon of the Salmon River and staked the showing that would become the Silbak Geologic work began in the Stewart area in 1906. Studies Premier mine. The discovery outcrop and original claim up to 1965 were reviewed by Grove (1971, p. 19). Detailed group were centred on the site of the small glory hole. tables in Brown (1987) list all geological reports and maps Tunnelling on I-level and 2-level by the Salmon-Bear River for the Silbak Premier mine (p. 7) and for the whole of Mining Company Limited, began in 1912 and 1913 respec- north-central British Columbia (p. 9). Thesis studies in the tively, but no ore-grade rock was encountered until 1914 Stewartarea have beencompleted by White (1939), and the bonanza ore forwhich the Premier is justlyfamous, Seraphim(1947), Grove (1973), Galley (1981), Brown was not located until 1916. Limited production of direct- (1987),McDonald (1990b) and Alldrick (1991). Com- shipping ore began in 1918 and the mill was commissioned prehensive reference lists for every mineral property are in 1921. included in recently revised MINFILE listings. In the late spring of 1911, several hundred metres of the Prospectingand properties in theStewart (Portland toe of the Silverado Glacier tore loose and a roaring ice- Canal) area have featured prominently in the BritiBColum- avalanchecascaded down intothe bay, causingmajor bia Depment of Mines Annual Reports beginning with waves. The steamer Camosun, tied up at dock, was badly Carmicbael’s review in 1907. Mapping projects and deposit shaken fora few minutes, producing broken dishes and studies have been completed by Grove (1971, 1986) and tumbled cargo. Waiting only a few days for the veneer of Alldrick (this report). Several mapping programs by the crushed ice to melt, two brave or foolhardy prospectors Geological Survey of Canada include those of McConnell climbed the avalanche route. Spurred on by the train of (1913), Schofield and Hanson (1922). Hanson (1935) and boulders and blocks of well-mineralized float, they soon Anderson (1989).
2 Ongoing geological work includes regional-scale map- I am grateful to the management of Westmin Resources ping by R.G. Anderson of the Geological Survey of Canada Limited, Scottie Gold Mines Limited, Pacific Cassiar Lim- and studies in the Hyder area by the Geological Survey ited and Esso Minerals Canada Limited for arranging tours Division of the State of Alaska (Solie ef aL, 1991) and the and sampling trips. The writer has benefitted from discus- United States Bureau of Mines. sions with many geologists during this study; my thanks to R.G. Anderson, D.A. Brown, G.L. Dawson, S.M. Dykes, F1:ELDWORK P. Folk, J. Greig, P.J. McGuigan, J.M. Kenyon, H.D. Meade, W. Melnyk, D. Novak, A. Randall, N.L. Tribe, D. Williams Fieldwork for this study was carried out during the snm- andP.J. Wodjak for sharing their information andideas. mers of 1982, 1983 and 1984. Mapping traverses were rewrded on 1:lO 000-scale air photographs and compiled Geologists J. Mawdsley, I.C.L.Webster and J.P. Dupas on 1:25 000-scale field maps. Four major deposits, repre- provided valuable field assistance. senting significantly different deposit types, were selected Chemicalanalyses were completed by P. Ralph, for detailed examination. V. Vilkos, B. Bhagwanani and M. Chaudhry of the British The route to the present report was neither obvious nor ColumbiaMinistry of Energy,Mines and Petroleum direct. What started out as an exercise in mineral deposit Resources laboratory. R.L. Player prepared all thin sections, classification ultimately required modification of accepted polished sections and polished thin sections. The original : ages of major plutons, inversion of stratigraphic columns, manuscript was typed by D. Bulinckx. The task of drafting andl rotation of the dips of wholemountains of strata figureswas shared by J. Armitage, P. Chicorelliand through as much as 90" from orientations established for M. Taylor. decades. Thesis advisors A.J. Sinclair, R.L. Armstrong andC.I. Godwin provided advice about general procedures, guid- ACKNOWLEDGMENTS ance about specific research techniques, and improved the This study was proposed by A. Panteleyev in 1981 and reportwith thorough editing. J.M. Newelland B. Grant fonned the basis of the writer's Ph.D. thesis at The Univer- reviewed the final manuscript and offered many helpful sity of British Columbia. suggestions.
3 Figure 2. Physiographic and tectonic belts, terranes and composite terranes of the Canadian Cordillera (modified from Sutherland Brown, 1976, Armstrong, 1988 and Monger, 1984).
4 CHAPTER 2 - REGIONAL GEOLOGIC SETTING
The study area lies in the Coast Mountains along the minorintercalated tuffaceous shale, tuff and volcanic western margin of the Intermontane tectonic belt, adjacent breccia. to Ihe Coast Plutonic Complex (Figures 2a and 2b). Accord- ing to terrane concepts it lies entirely within Stikinia (Fig- STEWART AREA (WEST) ure 2c). The Intermontane Belt roughly coincides with the The large exposure of Hazelton Group rocks on the west- Int 5 Figure 3. Tectonic elements of northern British Columbia (modified from Wheeler and McFeely, 1987) 6 " I AGE 2 SCHEMATIC ROCK LITHOSTRATIGRAPHY (Ma JJ STRATIGRAPHY TYPES .- 0- STlKlNEVOLCANIC BELT E W V c 0 NDAKO GROUP/OOTSA LAKE GROU -- 50 - d ?i I I """""""" """""""" """"""""jslst cgl tuff """""""" SUSTUTGROUP KASALKAGROUP "100 SKEENAGROUP -- 150 BOWSERLAKE GROUP -TERRANE COLLISION - HAZELTONGROUP "200 STUHINI/TAKLAGROUP -- 250 i- -- 300 - ASITKA/STIKINE ASSEMBLAGE -- 350 -- 400 1-I Figure 4. Stratigraphy of Stikinia and younger overlap assemblages. 7 HAZELTON GP. BOWSER L. GP. Figure 5. Distribution of the Hazelton Group, the Stewart Complex and Bowser Lake Group rocks in Central British Columbia Smithersformations of the Hazelton Group defined by “FELSIC VOLCANIC- CHERT UNIT Tipper and Richards (1976). Consequently, Anderson (per- (WHITESAIL FORMATION (?): MjC) sonalcommunication, 1990) suggested redefining this Siliceous light grey, greenishgrey, and dark grey sequence as the Spatsizi Formation. volcanic rocks conformably overlie, and are, in part, interbedded with red and green volcanic units of the WHITESAIL AREA (SOUTH) Telkwa Formation. These rocks aretypically thin bed- The Whitesail Formation was defined by Woodsworth ded, and in places are finely laminated. Thepredomi- (1980), described in the Tahtsa Lake area by MacIntyre nant rock @pes are welded lapillituft; mottled cherty (1985), and dated at 18424 Ma (van der Heyden, 1989, tufi and banded or massive dacite and rhyodacite. p.58-59). The unithas a gradational upper contact with thin- Eutaxitic textures are common in the more siliceous beddedargillites, siltstones and cherts similar to the fragmental rocks. These felsic volcanic rocks grade Smithers and Salmon River formations. There are striking upward into alternating beds of mottled and banded similarities between the Whitesail Formation (MacIntyre, grey chert, siliceous argillite,and siltstone which may 1985) and the Mount Dilworth Formation of the Stewart be part of the Smithers Formation. Strataboundlenses area, which is described in this report. MacIntyre’s descrip- of pyrite and pyrrhotite, up to several centimetres tion of the Whitesail Formation follows: thick, are common in this part of the section.” 8 L Figure 6. Evolution of stratigraphic nomenclature for the Hazelton Group (see also Figure 13). L.A.M.=Lower Andesite Member; L.S.M.=Lower SiltstoneMember; M.A.M.=Middle AndesiteMember; U.S.M.=Upper Siltstone Member; U.A.M.=Upper Andesite Member; P.P.M.=Premier Porphyry Member. POST-HAZELTONSTRATA Eocene volcanic rocks of the Ootsa Lake Group were firstdefined byDuffell (1959).Volcanic rocks of the The Bowser Lake Group comprises a thick sequence of Endako Group were defined by Armstrong (1949). The Middle Jurassic to Upper Jurassic sedimentaryand rare OotsaLake Group is a succession of continentalcalc- volcanic rocks. The type area lies northeast of the study alkaline basalt to rhyodacite volcanic rocks. The Endako area. Chert-pebble conglomerates are diagnostic of the base Groupis preserved as scattered plagioclase-porphyritic of the group and these are overlain by shale, siltstone and basalt flows. int~raformationalconglomerates. More than 100 volcanic centres have been recognized in The LowerCretaceous (Hautervian to Albian) Skeena northwestern Stikinia. Souther (1977) labelled this region of Group consists of a sedimentary sequence of basal con- Miocene to Holocene cones and flows the Stikine volcanic glomeratesoverlain by a thick successionof distal tur- belt. Lavas are mainly olivine basalt and range from 17Ma bidlites. Minor amygdaloidal basalt flows are interbedded to Recent, with the youngest age 130 years B.P. (Grove, near the base of the sequence (MacIntyre, 1985). 1986). IMacIntyre (1985) definedthe Middle to UpperCre- taceous (IO5 to 80 Ma) Kasalka Group. The predomi- INTRUSIVE EVENTS nantlyvolcanic sequence consists of basalts to rhyolites The magmatic evolution of the Canadian Cordillera was overlying a distinctive,maroon basal conglomerate. episodic, with distinct lulls between magmatic pulses. Epi- Tuffaceous members interbedded with Sustut Group sedi- sodes varyin location,intensity, duration, extent and me:ntary rocks are probably products of contemporaneous Character, andappear to be linked to changes in plate “Kasalka” explosive volcanism. motion. Armstrong (1988) and Woodsworthet al. (in press) ‘The Sustut Group consists of a thick sequence of middle define eight magmatic periods in northern British Columbia Albian to Maastrichtian continental sedimentary rocks with (Figure 7). Magmaticevents which affected the Stewart minor conglomerates and air-fall ash tuffs. area are reviewed here. 9 Several plutons of economic significance were emplaced Armstrong (1988) characterizes the Early Tertiary (55 in the Canadian Cordillera during the Early Jurassic (210 to 45 Ma) magmatic episode as “a spectacular, shodived to 185 Ma): the Guichon Creek batholith, Copper Mountain event with rapid onset andequally rapid decline. Theactual intrusions, Iron Mask batholith and Island Plutonic Suite in duration could not have been much more than 5 million southern British Columbia, and the Texas Creek Plutonic years”. Suite in the Stewart area. All have been interpreted as the WithinStikinia, small high-level Eocene plutons and coevalsubvolcanic magma chambersto Early Jurassic dikes mark a period of widespread intrusive activity. Wood- island-arc volcanic complexes. sworth et al. have defined three suites: the granitic Nanika WithinStikinia, Early Jurassic plutons are relatively suite, granodioritic Babine suiteand gabbroic Goosly Lake small. All are interpreted as the sources of Hazelton vol- suite (Figure 7).Several stocks are related to major molyb- canic rocks. The calcalkaline Texas Creek suite, within this denum and copper-molybdenum deposits: Berg, Kitsault, study area, is represented by batholiths and stocks of dis- Ajax, Granisle and Bell. Intrusions are coeval with inter- tinctive coarse-grained hornblende granodiorite that contain mediate to felsic pyroclastic rocks of the Ootsa Lake and potassium feldspar megacrysts. Buck Creek groups and mafic volcanic rocks of the Endako Group. Middle Cretaceous (110 to 90 Mal.- zranitic rocks are concentrated in two broad geographic belts that coincide with two zones of metamorphic rocks in the Canadian TECTONICHISTORY Cordillera: the Omineca crystalline belt and the Coast Plu- tonic Complex (Figure 2B). Mid-Cretaceous intrusiverocks sUBDUCTIoNzoNEs are oarticularlv abundant in the western uart of the Coast Pluknic Com&x. Within this small timespan these I-type Prevailing tectonic models suggest there were two active plutons are interpreted as pre-, syn- and post-tectonic with subduction zones in the Triassic and Early Jurassic, provid- respectto the regional deformation and metamorphism ing magmas to the separate Insular and Intermontane ter- (Woodsworth et al., in press). ranes (Figure 8; Griffiths, 1977; Griffiths and Godwin, 1983; Armstrong, 1988). Polarity of Triassic-Jurassic sub- duction zones is uncertain (Marsden and Thorkelson, in press) but from Cretaceous time onward there is strong evidencefor eastward-dipping subduction (Armstrong, 1988). Polarity of the Eocene volcanic arc is well documented (Ewing, 1981; Armstrong, 1988): subalkaline quartz diorite bodies are present in the Coast plutonic belt; Intermontane rocks grade eastward from calcalkaline to alkaline; east- ernmost exposures are invariably alkaline, which is the normal pattern in wide subduction-related magmatic arcs (Gill, 1981). TERRANEACCRETION Relationships between terranesare summarized from Armstrong (1988) and illustrated in Figures 2, 3, 8 and 9. Earliest terrane linkage was between the Cache Creek and Quesnel terranes where Cache Creek rocks were caught up in an active accretion wedge (Monger, 1984; Paterson and Harakal,1974). The timing of theconnection between Stikinia and the Cache Creek is uncertain, but Upper Tri- assic rocks of both Stikinia and Quesnel terranes (Stuhini 160 THREE SISTERS SUITE: and Takla groups)are very similar and Early Jurassic Three Sdsrr pluton HO em ryenite (212-204 Ma) plutonic rocks are common to both terranes. Ho?ailuh syenite Therefore these three terranes were probably linked when TOPLEY SUiTE Early Jurassic arc-magmatism began in northern Stikinia BLACK LAKE SUITE TEXAS CREEK SUITE (Monger, 1984). Until Toarcian time, contrasting environments existed on STlKlNE SUITE: Slikins Hoioiiuh Hickman the partially assembled Stikinia, Cache Creek and Quesnel ALPINE ULTRAMAFICS terranes, and on the oceanic Slide Mountain Terrane to the 240 east. But all four terranes, which were independent of each other in the Permian (Monger, 1977), hadbeen at least Figure 7. Intrusive events in Stikinia and the loosely assembled by the end of the Early Jurassic to form Canadian Cordillera. Superterrane I (Figures 2c and 2d). IO EAST I WEST LATETRIASSIC - EARLYJURASSIC TAKLA- I SLIDE 1 PROTEROZOIC- I STUHINI-HAZELTON I CACHE CREEK I HAZELTON I MOUNTAIN PALEOZOIC MAGMATICARC OCEAN SEA I MIOGEOCLINE I I (ORBACK-ARC SEA) I I I I I Figure 8. Subduction zone geometry of the Canadian Cordillera. A hypothetical cross-section through Stikinia As Superterrane I was thrust onto North America in Early to Middle Jurassic time, these four loosely assembled ter- ranes collided sequentially with each other. All deformation within Superterrane I terminated before emplacement of late Middle to early Late Jurassic plutons. This large part of the Canadian Cordillera was therefore in place against North America by the mid-Jurassic, but perhaps not at its present latitude. The overlap assemblage of the Gravina - Nutzotin belt SKEENA CROUP (Berg et a[., 1972) links the Alexander and Wrangell ter- ranes in Alaska by Late Jurassic time, to form Superrerrane I1 (Coney et al., 1980). Ongoing work suggests links BOWSER UKE between these terranes in the Paleozoic. The Jurassic Bridge River - Hozameen - Tyaughton - Taku ocean separating Superterranes I and I1 closedbetween Early to Middle Jurassic. Thus, assembly of the Canadian Cordillera was largely complete by the Late Jurassic, and by midCretaceous an Andean-type magmatic arc wasestablished on theCor- -,a6 dilleran margin of North America. "300- GEOLOGIC HISTORY MIDDLE STIKINE ASSEUBUGE A synthesis of the preceeding stratigraphic, plutonic and " tectonicrelationships is summarized in this section and LOWER SllKlNE illustrated schematically in Figure 9. ASSEYBUGE 380 to 310 Ma: Thelower and middle Stikine assemblages may represent remnants of two mid-Paleozoic Figure 9. Geologic history of Stikinia. volcanic arcs. 11 280 to 250 Ma: The unconformable,upper Stikine tin, 1989). Renewed plutonism began about 130 Ma within assemblage may record a third magmatic arc complex. the Coast Plutonic Complex. 250 to 230 Ma: The Tahltanian orogeny deformed the 125 to 110 Ma: The magmatic lull continuedin the Stikine assemblage prior to deposition of Upper Triassic Intermontane Belt, but magmatic activity in the Coast Plu- volcanic rocks. tonic Complex increased. Deposition of Skeena Groupsedi- mentary rocks and KasalkaGroup volcanic rocks pro- 230 to 210 Ma: The volcanic arc assemblages of the gressed throughout this period. Stuhini/Takla Group wereproduced by twosubduction zones. The Cache Creek and Quesnel terranes were linked 110 to YO Ma: An Andean-type, continental-margin mag- in the Late Triassic, but Stikinia was not yet linked to other matic arc was fully established by the mid-Cretaceous. terranes. Renewed magmatism in the western part of the Coast Plu- tonic Complex and in the Omineca Belt peaked during this 210 to 190 Ma: In the earliest Jurassic, plutons intruded period. Accompanying tectonism, deformation and regional andpinned the collision zoneof the Cache Creek and metamorphism within the western Intermontane Belt pea- Quesnel terranes. The partially assembledSuperterrane I ked from l IO to 100 Ma. (Stikinia - Cache Creek - Quesnel) linked with the Slide Mountain Terrane in Toarcian time (190 Ma). YO to 70 Ma: Transpression related to oblique subduction produced right-lateral transcurrent faulting, carrying dis- On Stikinia, Upper Triassic Stuhinimakla mafic volcanic placed terranes northward(Monger, 1984). Right-lateral strata were gradational into Early Jurassic intermediate to movement along the Tintina fault on the west side of the felsk HazeltonGroup units. Early Jurassic batholiths OminecaBelt displaced the BowserBasin hundreds of throughout the Intermontane Belt were coeval subvolcanic kilometres northward (Eisbacher, 1981). magmachambers within the “Hazelton” island-arc com- plex, analogous to modem arcs of the southwest Pacific. Plutonism,with extrusive phases, was abundant in Stikinia and in the western and central parts of the Coast 190 to 155 Ma: One east-dipping subduction zone oper- Plutonic Complex. Sedimentary rocks and intercalated tuffs ated through the Late Jurassic. In the period 190 to 180 Ma of the Sustut Group were deposited during uplift of the the linked terranes of Superterrane I collided with North Omineca Belt to the east and the Coast Plutonic Complexto America. Starting in Toarcian time, Slide Mountain rocks the west (Eisbacher, 1981). were thrust eastward onto the North American continental margin. Between Toarcian and Bajocian time, Stikinia was 70 to 60 Ma: No evidence of volcanism or sedimentation ovemdden by the Cache Creek Terrane. Mid-Jurassic plu- was preserved on Stikinia during this short magmatic lull. tonspinned these consolidated terranes andthen a pro- 60 to 40 Ma: This period spans the final major magmatic longedmagmatic lull beganthroughout the Canadian episode in the Canadian Cordillera. Within Stikinia, plutons Cordillera. were small, high-level stocksand dikes with contempo- raneous volcanic sequences. The Smithers, Whitesail, Mount Dilworth, Salmon River andSpatsizi formations of theupper Hazelton Group 40 to 20 Ma: This period was characterized by localized accumulatedbetween 190to 180Ma. The BowserLake plutonism in the Coast Plutonic Complex. High-level gran- Group was deposited in the Bowser Basin between 180 and ite and granite porphyry intrusions include the stock hosting 150 Ma (Cookenboo and Bustin, 1989). The Bowser Basin the Quartz Hill molybdenum deposit. The Oligocene was was a slowly sinking depression that accumulated clastic marked by widespread, but volumetrically small emplace- sedimentfrom surrounding elevated areas. It hasbeen ment of lamprophyre dikes. devoid of magmatism since the Middle Jurassic, earning it Cooling, uplift and unroofing of the Coast Plutonic Com- the label “cold spot” (Armstrong, 1988). plex accompanied east-west extensionrepresented by north- 155 to 125 Ma: From Early Cretaceous onward a single striking faults. North to northeast-striking faults and joints eastward-dipping subduction zone developed on the Cor- controlled distribution of Oligocene to Recent intrusive and dilleran margin. The majormagmatic lull continued volcanic rocks. throughoutmuch of the Early Cretaceous.Sedimentary 20 to present: East-dipping subductionhas continued rocks of the Bowser Lake Groupaccumulated until 150 Ma. along our westcoast until the present. Throughout the From 150 to 125 Ma the absenceof both plutonic and Miocene a flood of alkali-olivine basalt issued from a multi- volcanicrocks coincides with regional gapsin the strat- tude of vents andfissures to form plateau lavas. Intermittent igraphic record, indicating a time of emergence, erosion and eruptionhas continued throughout Pliocene, Pleistocene external drainage (Armstrong, 1988; Cookenhoo and Bus- and into Recent time. 12 - CHAPTER 3 - LITHOLOGIC UNITS simplified geologicalmap of the study area is pre- attitudes, stratigraphic tops and fault offsets throughoutthe sentedon Figure IO; more detailed geology is presented at map area. 1:50 000 scale on Figure 11 (in pocket). Regional strati- Overall, the Unuk River Formation is a thick, monoto- graphypresented in this report (Figure 12) is the latest step nous sequenceof medium-green andesitic rocks that are inthe evolution of stratigraphic subdivisions started by preserved as weakly to moderately foliated greenschists. In McConnell(1913) and revised by Schofieldand Hanson more detail, the volcanic rocks range from dust to ash tuff, (1922), Hanson (1929, 1935) and Grove (1971, 1973, 1986) crystal tuff, lapilli tuff, monolithic pyroclastic breccia and (Figure 13). Grove (1973,1986) established the current lava flows. Plagioclase and, less commonly, hornblende or stratigraphic nomenclature. Only one modification is made, augite phenocrysts or crystal fragments characterize the the felsic volcanic sequence, Monitorrhyolite, which Grove volcanic rocks. Phenocryst distribution is illustrated in Fig- inciluded within the SalmonRiver Formation, has been ure 14. Individual tuff beds show little evidence of sorting raked to formational status in this studyand has been or preferred orientation of crystals or lithic fragments, renamed the Mount Dilworth Formationor Mount Dilworth except in zones of ductile deformation. dacite. Discontinuousexposures and abrupt lateral textural Fossil samples from the map area have beencollected by changes in the tuffs hinder tracing and correlation of indi- Schofield and Hanson (1922), Hanson (1935), Grove (1971, vidual beds along strike, but a general zoning pattern of 1986),Anderson (personal communication, 1986),and clast size and crystal abundanceswithin the sequencehas Brown (1987). Allpreviously reported and newly located been recognized (Figure 14). Lapilli tuffs are uniformly fossil exposures are shown on Figure 11. distributed butmedium to coarseash tuffs are mostabun- dant in the lower part of the sequence and crystal tuffs and HAZELTON GROUP crystal-lithic tuffsare somewhatmore abundant in the upper oart. Tuffbreccias are mesentnear the ton of both the The Hazelton Group is a sequenceof mixed volcanic and middle and upper members of the sequence. Very coarse sedimentaryrocks that spans the Lower Jurassic epoch.tuff breccias are knownonly at the topof the succession Volcanic units progressivelychange in composition from near the Mount Dilworth vent area. basalt through andesite to dacite over a total thickness of 5 kilometres. This compositionalchange is paralleled by a LOWERANDESITE MEMBER cha.nge in colour from dark olive-greento light grey, and by The Lower Andesite Memberis at least 500 metres thick, progressivechanges in associated phenocrysts (Figure 14). but thelower has not been defined, It is composed of massive to well-bedded ash tuffs. No lapilli were seen in UNUKRIVER FORMATION outcrop and no phenocrysts were noted in hand samples, but a thin section of ash tuffcollected near Flower Pot Rock has The UnukRiver Formation is a thick sequenceofmassive fine-grained (-o,5 mm) pyroxene grains and green to greenishgrey andesitic tuffs andlava flows with crystals, minor interbedded sedimentarv rocks. This succession was defined during studies north ofthe presentmap area (Grove, S,LTsTONE MEMaER 1973, 1986). This unit hosts all of the major gold deposits in the Stewart mining camp. The Lower Siltstone Member is a thin-bedded turbidite formation is exposed along a north.trending belt successionof dark grey to black siltstones thatranges from thmugh the centre of the map area.The base of this SO to greater than 200 metres in thickness. sequence lies to the west, enguifed in the Coast batholith, ANDESITEMEMBER but the formation is at least 4500 metres thick within the MIDDLE map area. The uppercontact is typically a sharp, ragged toThe MiddleAndesite Member is at least 1500metres sm'oothly undulating erosional boundary with significant thick and comprises dusttuff, ash tuff, lapilli tuff and minor paleorelief. Porphyritic andesite flows or tuffs are Overlain tuff breccia with interbedded medium to coarse volcaniclas- by grits to conglomeratesof the BettyCreek Formation. tic sedimentary rocks. Massivepyroxene-porphyritic flows crop out near the top of the member around the Granduc STRATIGRAPHY millsitealong and thenetwork road andveinRoanan to the other deposits southof the Alaskanborder. An isolated Significant revisions have been made to Grove's informal exposure of massive augite-porphyritic basalt in the Long stratigraphic divisions. Thesefollow from recognition of Lake area (Dupas, 1985) may be thestratigraphic equivalent regionally distributed feldspar-porphyritic andesite flows of the pyroxene porphyries of the Middle Andesite Member. and tuffs at the topof the formation, and two siltstone memberswithin the succession. The siltstone units mark UppER MEMBER neriods of.~ volcanic~~~~~~~~ ouiescence and orovide imnortant strat- =""" ~.~~ ~ igraphicand structural markerswithin the predominantly The Upper Siltstone Member varies in thickness from as andesitic sequence.They permit determination of bedding little as 50 metresnear the Indianmine to morethan a 13 9% INDEX MAP *e ~~~~ ~~~~~~ MINERAL DEPOSITS EASTGOLDMINE A SCOmEGOLDMINE B MARTHA ELLEN DEpbslT C DAG0 HILL DEPOSIT D BIG MISSOURI MINE@-I ZONE) E CONSOLIDATED SILVER BUTTE DEPOSIT F TERMINUS DEPOSIT G INDIAN MINE H SEBAKWEMINE I B.C.SILVER MINE J SILBAK PREMIERMINE K RIVERSIDE MINE L JARYIS VEIN M BAYVIEW DEPOSIT N PROSPERITY AND PORTER IDAHO MINES 0 LEGEND hqm. be, mhg Eocene bioliie granodiorilestocks 141.sls Emty Jurassic hornblende granodiorite stwks 4 Argillite, siltstone, sandstone 3 Dacite pyrwla~ticformation 2 Epiclaslic mcks, hematiic le flows and tuffs Andeshe id Argiiiile, siltstone 1c AndesRe tuns lb siilstone ArgillRe. la Andestetuns Figure 10. ,Geologic setting of the study area. 14 GROUP FORMATION MEMBER SCHEMATIC PLUTONIC SUITE STRATIGRAPHY upper SiltBtOne SALMON RIVER Upper Wacke dOUNTDILWORTH HYDERSUITE BETTY CREEK Premier Porphyry If Upper Andesite le (EARLY JURASSIC) Upper 5i1181one ld Premier Porphyry UNUK RIVER Lower Siltstone 1 b Lower Andesite la Figure 12. Schematic stratigraphy of the Stewart mining camp. Hanson Grove Read Galley Brown THIS 1923 1971 I %:I 1979 1981 I 1987 I STUDY I NW Formation Bower Bower Loke Loke ' Owside 1 I mop 1 Group Group Bear Rive Formation Biner Creek Farmatin Figure 13. Evolution of stratigraphic nomenclature for the Stewart mining camp. (See also Figure 6.) 15 BOWEN'S DISTRIBUTION SCHEMATIC DISTRIBUTION OF LITHOLOGIES OFPHENOCRYSTS STRATIGRAPHY REACllON UM RMR BETTY CREEK SEMES FWTION FWTW (relates to left side of- schematic column) Figure 14. Phenocryst mineralogy and fragment size variations in Stewart stratigraphy. kilometre in the northern part of the map area. The upper blocks that are up to 1.5 metres across, including an intact contact can be a sharp change from dark greyor black thin- hexagonalfragment of columnar-jointedandesite, 1.2 bedded siltstone to massive green andesitic tuff, as along the metres across. Overall variations in clast size and in pheno- western slope of Troy Ridge; but typically the thin-bedded cryst compositionand abundance are illustrated by Fig- strata are overlain with a sharp upper contact by the basal ure 14. bl,ack-tuff facies of the Upper Andesite Member. Offsets of Generally, massive fine-grained aphanitic ash tuffs have this unit provide evidence for major movement along the local fine plagioclase and hornblende phenocrysts set in an Millsite, Morris Summit and Slate Mountain faults. altered ash matrix. These rocks are pervasively chloritized On the west side of the Tide Lake Flats, the Upper andcontain up to 5 percent disseminated fine-grained Siltstone Member hosts precious metal veins at the East pyrite. Chlorite defines the penetrative foliation of the rocks Gold mine. Pyritic areas in the siltstones produce brightly and gives a phyllitic sheen to some broken surfaces. coloured, gossanous exposures that crop out from north of Fragmental rocks are open-framework volcanic breccias th" mine southward to the Millsite fault (Figure 11). Rocks and may represent air-fall lapilli tuff, ungraded pyroclastic of the Upper Siltstone Member, preserved as purple-brown flow breccias, or lava flow breccias. Exposures are typically hornfelsed siltstones, host calcsilicate veins and minor sul- massive with no preferred orientation, but local zones of phide mineralization in the Molly B, Red Reef and Oral M ductile deformation result in plate-like flattened fragments. adits on Mount Rainey (Figure 11). Correlation of these fragmental units is difficult, even over the short distances between outcrops and drill holes. The U:PPER ANDESITE MEMBER apparent lack of continuity may be due to abrupt textural The UpperAndesite Member is a thick sequenceof variations, slumping, postdepositional erosionor small- massive tuffs with minor flows and local lenses of sedimen- scale, postlithification faulting. ta:ryrock. It is about 2000 metres thick andcapped by Deep green to greyish green fragmental rocks are typ- regionally extensive crystal tuffs of the Premier Porphyry ically monolithologic; fragments are generally matrix sup- Member. ported and textures are best displayed on recently glaciated or deeply weathered surfaces. Fragments are typically angu- Bllack Tnff Facies lar, hut some exposures have subrounded and well-rounded The base of the upper andesite is marked by a massive, clasts. Fragmental rocks contain lithic, pumice and crystal black, fine-grained to locally fragmental unit, 0 to 250 fragments. Lithic fragments consist of varieties of ash tuff, metres thick. Thin sections reveal that it includes not only variable plagioclase and hornblende porphyritic rocks, and layers of carbon-impregnated ash tuff, feldspar crystal tuff, lithified fragmental rocks. The matrix is composed of fine and lapilli tuff, but also massive siltstone, feldspathic wacke lithic chipsand glass shardsin a groundmass of quartz, and granule to pebble conglomerates of roughly rounded feldspar and sericite, chlorite and carbonate-altered ash and volcanic debris. Therefore the unit consists of intimately dust. Some weathered exposures show resistant fragments mixedmassive sedimentary rocks and tuffs, transitional in a recessive matrix, others a resistant matrix with reces- be:tween underlying thin-bedded blacksiltstones and overly- sive fragments. Possibly the former represent air-fall lapilli ing massive green andesite tuff. Whole-rock analyses plot tuffs and the latter either hot pyroclastic flows or flow-top as andesite. breccias. The uppercontact of this unit gradesover 10to 50 Pyrite is disseminated in both fragments and groundmass centimetresinto green chloritic tuffs. Thisgradational as angular subhedral grains to euhedral cubes. Typically it colour boundary undulates across large, texturally uniform makesup about 2per cent of these rocks, butwithin outcrops. These relationships indicate that the black rocks 2 kilometres of the Silbak Premier mine, local exposures of are andesitic air-fall tuffs that were deposited on top of the intense chlorite-pyrite alteration containup to 5 per cent thin-bedded silts in a very shallow anoxic basin. The tuff disseminated medium-grained pyrite. apparently accumulated until it built up to and above sea Locally within the Upper Andesite Member, lenses of level, where it was preserved without carbon. maroon to purple to grey siltstone, sandstone and conglom- erate are preserved. Epiclastic rocks record periods of ero- Main Sequence sion and volcanic quiescence during the overall develop- A monotonous succession of greenstones and minorsedi- ment of the andesitic volcano. These hematitic sedimentary mentary lenses comprises the main sequence of the Upper units are distinctive and may be useful markers on a prop- Andesite Member. These rocks have previously been stud- erty scale. ied in detail byGrove (1971, 1973, 1986), Read(19791, A thin lens ((30 m) of black to dark grey sedimentary Galley(1981) and Brown (1987). It is difficult to dis- rock crops out high on the northwest slope of Troy Ridge tinguishindividual units because of the uniformgreen (Figure 11, in pocket). The unit is exposed around the shore colourand pervasive foliation of the rocks. Rocktypes of a long, narrow pond andconsists of thin-bedded siltstone range from dust and ash tuffs to lapilli tuffs and tuff brec- and gritty limestone. It wedgesout to thesouth but its cias; crystal tuffs display varying abundances of plagioclase northern extent is unknown. These rocks mark local suba- and hornblende. The top of the Upper Andesite Member is queous conditions on what is interpreted as a subaerial an.desitic lapilli tuff to coarse tuff breccia. Regionally, the volcanic edifice; conditions of formation might be analo- largest angular fragments range up to 40 centimetres long. gous to sediment accumulation in Spirit Lake at Mount St. In one small area on Mount Dilworth a single bed contains Helens, Washington. 17 PREMIER PORPHYRYMEMBER layering is variable, some exposures are almost massive. This unit does not normally carry potassium feldspar crys- The distinctive Premier Porphyry Member contains plag- tals, but a few small orbroken orthoclase rhombs were ioclase, potassium feldspar and hornblende phenocrysts and noted in widespread outcrops (arrow in Plate 2). marks the top of the Unuk River Formation throughout the map area. Regionally it can be divided into three units; on Premier Porphyry Flow morelocal scales a variety of additionalfacies are ThePremier Porphyry flow unit is adark green to preserved. medium greyish green or grey to black, massive, indurated, The rocks are texturally similar to dikes of Premier Por- crystal-rich rock with megacrystic potassium feldspar and phyry which cut both the underlying strata (Plate 1) and the smaller plagioclase and hornblende crystals. An interlock- Texas Creek batholith. Brown (1987, pp. 113.118) renamed ing, felted groundmass of microlites indicates the rock is a these units to avoid confusion between stratahound and flow. Phenocrysts include small (3 to 5 mm) white, sub- intrusivephases. Here, the writerprefers to stress the hedral to euhedral plagioclase and larger (1 to 5 cm) buff- genetic link between the intrusive and extrusive phases of coloured, euhedral orthoclase. Locally hornblende crystals these economically and stratigraphically important rocks. 5 to 10 millimetres long are visible, but they are generally All gold deposits in and around the Silbak Premier and Big obscured by strong chlorite alteration. The matrix is fine Missouri mines lie stratigraphically below this member. grained and usually chloritized. In thin section the rock has embayed, partially resorbed Massive to Layered Plagioclase Porphyry quartz grains. The large feldspar crystals are euhedral with The basal unit of the Premier Porphyry Member is mas- little rounding or fracturing. Plagioclase may be present as sive to crudelylaminated plagioclase-hornblende-phyric glomeroporphyritic clusters and all feldspars have a thin, rock. Plagioclase crystals (<6 mm) stand out clearly (Plate clear, inclusion-free rim. Both Read (1979) andGalley 2) but the hornblende crystals are smaller and only evident (1981) report minor amounts of biotite in thin section exan- in hand sample with careful study. Layering reflecting vari- ination of samples collected near the Big Missouri deposit. able grain-size sorting is typically 1 to 2 centimetres thick No biotite was noted in samples examined in this study. and the rock appears more indurated and less foliated than andesitic tuffs lower in the sequence. The definition of the Premier Porphyry nf'f The Premier Porphyry tuff is characterized by its maroon colour and by potassium feldspar megacrysts and plagio- Plate1.Chilled margin along irregular contact of Pre-Plate 2. Beddedplagioclase-hornblende tuff, uphillfrom mier Porphyry dike cutting bedded tuffsof Lower Andesite Premier glory hole. Note rare, coarse K-feldspar crystals Member, Unuk RiverFormation. Exposed in bank of (arrow).Massive to layeredplagioclase porphyry, Premier SalmonRiver nearPorphyryRock. FlowerMember, Pot Formation.RiverUnuk 18 Plate 3. Mappable hematitic regolith layer between massive beds of K-feldspar-porphyritic flow, north end of 49 Ridge. Premier Porphyry flow, Premier Porphyry Member, Unuk River Formation. TABLE 1 TEXTURAL DISTINCTIONS BETWEEN PREMIER PORPHYRY FLOW AND PREMIER PORPHYRY TUFF - FlOW nfr 0 rimmed feldspars no feldspar rims 0 6%~or no lithic chips 0 lithic clasts common 0 euhedral phenocrysts 0 broken andlor rounded phenocrysts 0 interlocking crystalline matrix 0 tine ash matrix 0 glomemporphyritic feldspars - clase and hornblende crystals. It is distinguished from the underlying green Premier Porphyry flow by its hematite content, which produces a purple to greyish purple colour, and by its noncrystalline aphanitic matrix. Based on micro- scopic textural features summarized in Table 1 this rock is interpreted as a massive, subaerially deposited, air-fall crys- tal tuff. Distinction between tuff and flow textures is best made by thin section study, althoughRead (1979) felt Plate 4. Milled fall-back breccia of dark green sla'bbed samples were adequate. K-feldspar-megacrysticboulders, east cliff of 49 Ridge. Vent facies,Premier Porphyry Member, Unuk River Other Facies Formation. I:n the Mount Dilworth area, the 100-metresection of Pre:mier Porphyry flows is divided in the middle by a thin angular to well-rounded (milled) clasts of Premier Porphyry hematitic sandstone layer (Plate 3) parallel to the borders of that is interpreted as a fall-backbreccia in a vent area (Plate 41, and very coarse tuff-breccia at the top ofthe the flow. The sandstone regolith was traced along the cliff Upper Andesite Member. The fissure itself is exposed in the face on the west side of 49 Ridge for 150 metres. This vertical .cliff at the south end of 49 Ridge where blocks of sedimentarylayer records a brief periodof subaerial black Premier Porphyry up to 3 metresin diameter are weathering, followed by continued extrusion ofsimilar suspended in a matrix of massive to foliated ash and crystals flONS. of similar material (Plate 5). This vent area was probably a A vent area hasbeen mapped at the south end of parasiticcone on the flanks of a largecomposite 49 Ridge. The facies indicators are: a wedge-shaped bed of stratovolcano. 19 Plate 5. Large angular block of black K-feldspar-megacrystic flow-rock in similar groundmass. Megabreccia exposure in the south cliff of 49 Ridge. Vent facies, Premier Porphyry Member, Unuk River Formation. PROVENANCE AND DEPOSITIONAL ENVIRONMENT TABLE 2 EVIDENCE FOR SUBAERIAL VERSUS SUBAQUEOUS The Unuk River Formation represents an andesitic vol- DEPOSITION OF THE UNUK RIVER FORMATION canic pile and intraformational epiclastic rocks. Volcanic rocks are subalkaline, calcalkaline, potassium and iron-rich 0 Conspicuous absence of pillowed flows in seven decades of mapping andesites with minor basalts to basaltic andesites. The for- 0 Meteoricwater comDonent in fluid inclusions(McDonald. 1990a) mation is regarded as a well-preserved composite strato- Key to Symbols in Following Lists volcano, with paleotopographic peaks at or near Mount P=present; A=conspicuously absent: Dilworth (Upper Andesite Member) and at the north end of ?=insufficient data or not applicable Long Lake (Middle Andesite Member). The stratovolcano Characteristics of Subaerial Fallout Deposits was a predominantly subaerial smcture with two brief (simplified from Fisher and Schmlncke, 1984) periods of marine transgression recorded by the thin-bedded ? -Minor lenticularity close to source siltstone members (Table 2). ? - Ash layers wedge out against steep surface irregularities Rocks of the Unuk River Formation have not yielded ? - Grading may be normal and reverse diagnostic fossils within the map area, neither has the base P - Fabric in beds is commonly isotropic P - Elongate fragments are uncommon of the formation been defined. Rocks low in the sequence P - Bedding planes are generally gradational; bedding planes are dis- have not been dated. Rocks of the uppermost Premier Por- tinct only where deposition ison weathered or erosional surfaces or phyry Member are interpreted to be 195 million years old different rock types based on correlation withU-Pb zircon ages from nearby A - Sorting is moderate to good ? - Size and sorting vary with distance within single layers Premier Porphyry dikes. The entire formation is regarded as P - Silicic and intermediate compositions more common than mafic an Early Jurassic, Sinemurian(?) to mid-Pliensbachian, vol- P - Intermediatecomposition commonly associated with large com- canic pile composed of onlapping to overlapping volcanic pasite volcanoes and derived sedimentary members from a series of nearby, P -Proximal facies include:lava flows, pyroclastic flows, domes, actively erupting volcanic vents. pyroclastic tuff breccias, avalanche deposits and debris flows P - intermediate facies include:coarser grined tephra, somelava flows, pyroclastic flows, ash falls and reworked fluvial deposits BETTYCREEK FORMATION ? -Distalfacies include: tine fallout tephra with no coeval coarse- grained pyroclastic rocks or lava flows The Betty Creek Formation is a complex succession of P -Coarser grained pyroclastic deposits gradually decrease, reworked distinctively coloured red and green epiclastic sedimentary pyroclastic deposits gradually increase away from source rocks interbedded with andesitic to dacitic tuffs and flows. Characteristics of Submarine Fallout Tephra It was first defined by Grove (1971) as a member of the (simplified from Fisher and Schmincke, 1984) “Bowser assemblage”. He redefined it (1973, 1986) as a A -Plane parallel beds extend for hundreds of lun2 formation within the Hazelton Group with its type area ? -Normal grading; crystal and lithic-rich bases to shard-rich tops along the canyon of Betty Creek in the northem part of the ? - Inverse grading only if pumice is present A - Basal contacts sharp, upper contacts diffuse map area. ? - Sorting: good to poor depending on bioturbation The formation varies in thickness from 4 to 1200 metres. P - Sizesand sorting varies irregularly, but there is an overallsize It is exposed continuously throughout the map area except decrease with increasing distance from source A - Ancient layers in terrestrial geologic settingsare altered to clays and for the 2-kilometre interval between Union Lake and Fetter zeolites; known as bentonites Lake where it may be absent or overprinted and obscured by A -Tephra commonly interbedded with pelagic calcareous or siliceous intense alteration. oozes, or withmuds and silts, dcpending on proximityto land. Commonlyinterbedded with nonvalcanic or tuffaceousshale or Sections through the Betty Creek Formation from Daisy siltstone Lake to Slate Mountain have no interbedded volcanic rocks. A -Terrigenous materials accumulate as interbedded turbidites In contrast, on Mount Rainey the andesites of the Unuk 20 River Formation are capped by a thick sequence of dacitic strong reddish shades lies within the intergranular clay frac- volcanic rocks and there are few interhedded sedimentary tion (Plate 6). rocks. The varying abundance of sedimentary and volcanic Striking exposures of multicoloured, heterolithic boulder rocksis probably controlled by paleotopographyand conglomerate are widespread. Rounded cobbles and boul- regional distribution of volcanic vents. The volcanic flows ders of volcanic rock withinthese beds range in colour from and tuffs may have been extruded from other nearby vol- red to purple, green and grey. Most of the clasts are andesi- caric centres to form onlapping units that interfinger with a tic volcanicrocks ‘similar in texture andcomposition to thick sedimentarywedge shed from the MountDilworth rocks in the underlying Unuk River Formation. Conglome- paleovolcano. rates are predominantlymatrix supported (Plate 7), hut The basal contact is typically marked by a sharp colour clast-supported“boulder beds”, with boulders up to change from greenish chloritic andesitic tuffs of the Unuk 40 centimetresin diameter, cropout along Bear River Rilrer Formation to maroon clastic sedimentary rocks. The Ridge. Individual conglomeratic beds are massive to cru- bar:al sedimentaryrocks arecommonly conglomerates dely sorted. Crudely sorted layers may have either reversely developedon lithologically similar underlying andesites. graded or symmetrically graded beds, whichare characteris- The contact varies from irregular to smoothly undulating. tic features of lahars (Fisher and Schmincke, 1984). The uppercontact is a sharp, smoothboundary between Monolithiccobble and boulder-conglomerate beds are purple, hematitic grits or wackes and overlying aphanitic common near the base of the sequence, particularly where massive dust tuffs of the Mount Dilworth Formation. the formation thins on Mount Dilworth and at the northeast endof Long Lake. The textures andmineralogy of the STIRATIGRAPHY angular to subrounded clasts are identical to those of the immediately underlying andesitic rocks. SEDIMENTARY UNITS The generaldepositional environment is subaerial Brightlycoloured sedimentary rocks range frommud- although some sedimentruy units exhibit waterlain textures. stones, siltstones, sandstones, wackes and grits up to coarse Scattered exposures of grit to mudstone beds have normal boulder conglomerates. The bedding, textures and colouring grading (Plate 6), crossbedding (Plate 7), scour marks of these beds are distinctive. The matrix is typically hemati- (Plate 8) andrhythmic bedding (Plate 6); they represent tic, brick-red to maroon to purple, hut local greenish and stream-channel deposits where debris-flow fans have been mottled purple and green units are present near the base of reworked on the lower flanks of a stratavolcano. On both the sequence. The iron oxide that gives the sandstones their limbs of the Dilworth syncline these sedimentary structures Plate 6. Rhythmic graded beds of coarse wacke (pale grey) to hematitic mudstone (darkgrey), east shoreof Daisy Lake. Strike north, tops to east towards Troy Ridge, Betty Creek Formation (See colour photo in back of report). 21 series ofvolcanic units appears to changeupward from andesitic to dacitic composition. The dacitic tuffs range from pale waxy yellowor yellow- green crystal tuffs and welded tuffs to pale green, coarse ash tuffs and dust tuffs. Crystal tuffs host medium-grained (0.5 to 1.0 cm), white, subhedral to euhedralfeldspar pheno- crysts. No hornblendephenocrysts have been noted, but there are rare fine-grained (15 mm) quartz crystals in some samples. Rare dacitic welded tuffs exhibit eutaxitic textures with flattened fiammi up to 12 centimetres long. PROVENANCE AND DEPOSITIONAL ENVIRONMENT The clastic sedimentary rocks are probably derived by weathering and erosion of Unuk River Formation tuffs and flows. Whole-rock analyses from interbeddedtuffs plot near the dacite-andesite boundary. The Betty Creek Formation is interpreted as a subaerial clastic apronof poorly sorted Plate 7. Medium-beddedhematitic wackes andpebble lahars and reworked debris flows interbedded with andesitic conglomerates,west side ofTroy Ridge. Grading and to dacitic volcanicrocks on the flanks ofan emergent crossbeds in layer below boulders and hammer show that andesitic stratovolcano constructed of Unuk River Forma- beds are upright. Betty Creek Formation. tion rocks. Areas where the Betty Creek Formation thins or wedges out represent paleotopographic highs. No fossils or isotopic dates have been obtained from the Betty Creek Formation, but constraints include the Early Jurassic, Toarcian fossil assemblage at the base of the over- lying SalmonRiver Formation, and Early Jurassic U-Pb datesfrom underlying andesitic dikes (194.8k2.0 Ma). Thus, the most likely age range for accumulation of the sedimentary rocks, tuffs and flows of the Betty Creek For- mationis mid-Pliensbachian to mid-Toarcian (195 to 190 Ma). MOUNTDILWORTH FORMATION The Mount Dilworth felsic volcanicsequence is com- posed of dense, resistant, variably welded dacite tuffs. Indi- vidual members display distinct lateral textural variations and textural changes that can he related to volcanic centres, paleotopography and depositional environment. In this study the unit has been raised to formational status and named the Mount Dilworth Formation or Mount Dil- Plate 8. Bedded coarse wacke (prey)with finer sandstone worth dacite. This felsic volcanic sequence is thin but con- (white) and siltstone (black), east and uphill from Premier tinuous throughout the region. It is a resistant cliff-former glory hole. Scour channel indicatesstrata are upright. Betty Creek Formation. andan important regional stratigraphic markerand thus deservesformational status. MountDilworth is the pre- ferred type area because it offers several sections that are continuously exposed in outcrop and that are free from the have consistent stratigraphic tops toward the synclinal axis complications of minor faults and folds. Additional attrac- on Mount Dilworth (Figure 11). tions of the Mount Dilworth area arerelative ease of access and a wide variety of discrete, mappable rock types and members. VOLCANICUNITS The formation is exposed in a continuous, northerly elon- Volcanic rocks interbedded with the sedimentary rocks gated oval outcrop pattern within the map area. This dis- include dust tuff, ash tuff, lapilli tuff, feldspar-crystal tuff tributionreflects the regional, doubly plunging “syn- and porphyritic lava flows. Units on the slopes of Mitre clinorium” in the centre of the area. Thickness ranges from Mountainand along the northern Bear River Ridgeare 20 to 120 metres, hut appears evengreater near Union Lake predominantly dacitic. Those exposed on the southern Bear and Monitor Lake where outcrop patterns are complicated River Ridge, uphill from the Silbak Premier mine, are a by faulting. deeper green colour and seem more andesitic in character. The basal contact is a sharp, gently undulating surface. As a generalization, within a single stratigraphic section, a Massive, aphanitic, grey to green to maroondust tuffs 22 THICKNESS UNITS (metres) overlie purple to maroon clastic sedimentary rocks of the Betty Creek Formation. In some exposures the contact is BLACK TUFF with minor marked by a 1 to 3-metre interval of interbedded hematitic mudstonelenses sedimentaryrocks and dust tuffs. The uppercontact is typically a sharp but ragged surface between coarsely frag- NORTH SOUTH mental felsic lapilli tuffs and overlying calcareous grits of Salmon River ...... the Salmon River Formation. Wherethe uppermost member FOrmOliO" ...... of the Mount Dilworth Formation is the Black Tuff Mem- ,,.,.,fi ber, the upper contact of the formation is smooth with a ...... sharp break between massive black tuffaceous mudstones UPPER LAPILLI and the overlying calcareous grits. TUFF -* a ov Q STRATIGRAPHY Stratigraphic relationships within the MountDilworth MIDDLE WELDED Formation are shown schematically on Figure 15 and on Plate 9. Despite local heterogeneities and lateral textural 0 11-25 r- up to 3 cooling variations, the three major members of the formation - the - , units lower dust tuff, middle welded tuff, and upper lapilli tuff - LOWER DUST TUFF can be distinguished in sections throughout the map area. massive to faintly layered Belly Creek formalion LOWER DUSTTUFF MEMBER The lowest member of the Mount Dilworth Formation is ]?@re 15. Stratigraphy within the Mount Dilworth Formation. a massive aphanitic dust tuff (fine ash tuff) composed of volcanic dust andfine lithic particles. This unit is regionally distributed throughout the map area and beyond.It blankets underlyingBetty Creek sedimentary rocks and ranges in thickness from 3 to 15 metres. The unit has sharp conform- able contacts with adjacent units. This rock is typically olive grey to grey, but bright turquoise-colouredzones are exposednear Summit and Divide lakes and local purple and bright maroon hematitic alteration zones give the rock a swirled or marbled pattern (Plate 10). At the southeast comer of Summit Lake, along the upper Granduc road, the dust tuff contains fine silica- filledvesicles and largeeuhedral pyrite crystals up to 1 centimetre across (Plate 11). At microscopic scale the rock is predominantly (>70%) fine ash, with fine ( MIDDLE WELDEDTUFF MEMBER The middle welded ash flow tuff is the most variable of the three regionally extensive members of the Mount Dil- worth Formation. It forms a series of laterally varying dacitic facies sandwiched between the other two members that are texturally more consistent. The lower contact is sharp and planar, the upper contact is sharp but irregular. In the Mount Dilworth to Monitor Lake area, sections through this member show that mixed fiammB and Plate 9. Complete stratigraphy of Mount Dilworih For- angular mation exposed in cliff along east si& of DaisyLake. felsic lapilli at the base are overlain by pumice-lapilli tuff. Overlying Salmon River Formation includes well-bedded Well-exposed sections haveprogressively more intense shales, siltstones and wackes and basal fossiliferous lime- weldingand compaction down-section (Plate 12); equi- stone.Looking north from Windy Point to Scottie Gold dimensionalpumice clasts grade downward into black minesite in background. glassyfiammb. Outsidethe Mount Dilworth area snb- 23 Plate 10. Stratigraphyin Mount Dilworth Formation, north end of49 Ridge. Note colour-mottling of Lower Dust Tuff. Height from base of photo to top of pyritic hob is 30 metres (see colour photo in back of report). Plate 11. Lower Dust Tuff Member of Mount Dilworth Plate 12. Middle Welded Tuff Member of Mount Dil- Formation, Windy Point roadcut. Turquoise-grey colour is worth Formation, north endof 49 Ridge. FiammC in welded typical; fine lithic chips and large euhedral pyrite crystal are tuff. Largest clast is 10 centimetres long. unusual variations. rounded pumice lapilli are mixed with varied siliceous lithic north of the exposure. Along the northwest slope of Mount clasts, but the rocks are not obviously welded. Dilworth a similar bed of white to cream-weathering mas- sive fine siliceons ash tuff forms a thin (30 cm) resistant Midway between the south edge of the Mount Dilworth layer at the base of the WeldedTuff Member. At this snowfield and Union Lake, one .outcrop has bedding and location this bed is overlain by three ash-flow cooling units, small dune forms (IO cm amplitude) in fine creamy ash tuff eachcontaining progressively less compressed pumice at the base of this member. These duneforms are interpreted clasts up-section, all included within the Middle Member. as base-surge features; no internal truncation of layers was Elsewhere along the Mount Dilworth ridge, only a single noted but asymmetry indicates that the blast was from the cooling unit has been noted within this member. 24 Plate 13. Upper Lapilli Tuff Member of Mount Dilworth Formation, south end of Mount Dilworth Note swirled bombs of flow-handed dacite. In thin section, rocks of this member contain fiamm6like PYRITIC TUFFMEMBER wisps onall scales. Veinletsand fragments of micro- A prominently gossanous, pyritic, fragment-rich unit is crystalline quartz (chalcedony) are common, as are broken exposed along the west side of Mount Dilworth (Plate IO) crystals of both plagioclase and potassium feldspar. In some and the east side of Summit Lake (Plate 9). This member samples, partially collapsed fiamme are wholly altered to ends abruptly ina gossanous cliff at the south end of Mount aggregates of chlorite, quartz, calcite and epidote. Dilworth, but progressively thins northward, finally wedg- ing out high on the western slope of Troy Ridge. Another UF~PERLAPILLI TUFF MEMBER area of exposurehas been documented by Dupas (1985, p. 313) at the northeast end of Long Lake where itwas ‘The regionally extensive upper member of the Mount appropriatelytermed the “IronCap” by early workers Dilworth Formation is a siliceous lapilli tuff to tuff breccia. (Plumb, 1956). The upper and lower contacts of this unit are The lower contact ofthis unit is generally sharp, irregular to typically sharp but. irregular. Locally the lower contact is undulating, but may be locally indistinct. The upper contact gradational over less than a metre; the upper contact may be is ,gradational to sharp against the pyritic tuff, gradational an interbedded sequence of sedimentary rocks and dacitic into the black tuff, and irregular but sharp against the basal tuffs. fos,siliferous wackesand calcareous grits of the Salmon Along the southwest edge of the Mount Dilworth Snow- River Formation. field the unit carries clasts of flow-banded siliceous vol- canic rock, clasts of massive felsic tuffs or flows, and large Fragments are predominantly swirled flow-banded dacite rounded boulders (up to 0.5 m diameter) of coarse crys- bombs or spatter (Plate 13), but clasts of other siliceous talline calcite aggregates. As in the underlyingmember, vo:lcanic rocks are also common. Regionally, the fragment fragmentsize decreases progressively northward from sizeranges up to a maximum of IO to 15 centimetres, and rounded boulders to cobbles, then to lapilli. the: overall size range is that of a lapilli tuff. However, along In thin section, the unit contains lithic fragments, broken the. southwest edge of the Mount Dilworth snowfield, large feldspar crystals and swirled flow-banded bombs up to 50 centimetres long are chips of microcrystalline quartz (chal- cedony). Lithic fragments mayhave microscopic scale preserved. This local increase in fragment size suggests that flow-handing. Disseminated pyrite comprises up to 50 per the southem part of Mount Dilworth wasa vent area for this cent of the matrix, but is more typically 10 to 15 per cent. member. The unitmay be partially weldedbut contains Pyrite is present in some lithic clasts hut not in others. In neither pumice fragments nor fiamm6. Along much of its samples from exposures north of Goat Creek, charcoal to stn ke length the groundmass is medium to dark grey. black varieties of this unit are impregnated with upto 30 per IIn thin section the groundmass has a massive to subtly cent tine pyrite, not carbon. Some samples are cut by fine layeredglassy or welded texture. Clasts are lithic chips, veinlets of chalcedonic quartz and some of these veinlets cryptocrystalline quartz and scattered feldspar crystals. Dis- have internal growth-layering in thin section. seminated pyrite is often present in the matrix and in some The pyritic unit isnot continuous along strike but is lithic chips. Minor (1%) biotite is presentin exposures exposed as a series of discontinuous lenses of strong pyritic along the southeast side of Summit Lake. North of Goat impregnation. Despite, or perhaps because of, careful map- Creek many of the finer lithic chips are well rounded. ping along its entire strike lergii, this unit seems complex 25 andmay represent penecontemporaneous but postdeposi- rhyolites during field examination by all workers. However, tional impregnation of pyrite into a variety of rock types. on the basis of all the analytical data this rock unit is dacite. Although the predominant rock type is lapilli tuff identical The formation represents fallout and pyroclastic flow to the underlying Upper Lapilli TuffMember, in some deposits from'a series of explosive subaerial felsic volcanic exposuresthe pyrite-rich unit is a carbonate-mudstone- eruptions that proceeded in quick succession, apparently cemented debris flow with heterolitbic volcanic and carbo- becoming progressively more violent. Based on clast size, nate clasts. For these reasons the Pyritic Tuff Member is one vent area was near the south end of Mount Dilworth. interpretedto represent pyrite impregnation around The formation was deposited subaerially on the flanks of an fumaroliccentres, possibly with related calcareous mud- exposed volcanic cone. The general absence of sedimentary filled brine pools. The host lithology is therefore the upper deposits between units confirms there was little or no time layer of the Upper Lapilli Tuff Member. between eruptions. The uppermost Black Tuff Member was deposited under subaqueous conditions, perhaps in a fault- BLACKTUFF MEMBER bounded caldera or crater lake. Interbedded sedimentary rocks within the Pyritic Tuff Member indicate that subsi- The Black Tuff Member is a thick unit of carbonaceous dence followed the final major eruption. crystal and lithic-lapilli tuffs with local mudstone and silt- stone lenses. Lapilli consist of crowded feldspar porphyritic There are no fossils associated with the felsic volcanic lava or crystal tuff, limestone, pumice and rare massive rocks of the Mount Dilworth Formation but the unit is pyrite. The rock is well displayed in the waste dumps at the immediately overlain by Toarcian limestones and is under- south end of the penstock tunnel near the Long Lake dam lain by the mid-Pliensbachian to mid-Toarcian Betty Creek and in abandoned drill core near the Lakeshore workings Formation. Thereforeit records a voluminous but short- south of Monitor Lake. lived volcanic event during middle to late Toarcian time (about 190-185 Ma). This member has been traced in outcrop from the south end of Mount Dilworth southward to the crest of Slate SALMONRIVER FORMATION Mountain and is also exposed in a few discontinuous out- crops south and southeast of Monitor Lake. It overlies the The Salmon River Formation is a thick assemblage of Upper Lapilli Tuff Member and either overlies or is the complexly folded, thin to medium-bedded siltstones and stratigraphic equivalent of the Pyritic Tuff Member to the wackes with minor interbedded intraformational conglom- north. The contact between the black tuff and the underlying erates, limestones and siliceous tuffaceous siltstones. Grove upper lapilli tuff is gradational. Bothunits host dissemi- (1973, 1986) selected the summit, northem slopes and east- nated sulpbides and pyritic lithic clasts, but the black tuff em cliffs of Mount Dilworth and Troy Ridge as the type alsocarries angular clasts of massive,medium-grained area for the formation. pyrite aggregates. East of Silver Lakes, bluish grey chal- Salmon River strata are preserved as a large trough-like cedony veins cut this unit in exposures in the Start adit and erosional remnant in the northeast part of the map area. The in outcrops to the north of the adit. formation is at least 1000 metres thick, although its top bas In thin section the crystal-rich tuffs are seen to contain up not been identified. to SO per cent feldspar crystalsand up to 5 per cent ragged This thick sedimentary sequence has a rough to undulat- chloriteflakes after biotite, together with a few well- ing, sharp contact with underlying felsic fragmental tuffs. rounded quartz grains. Compositions for plagioclase in sam- Thewriter regards this as a paraconformable contact, ples throughout the Mount Dilworth Formation range from formed during foundering and rapid subsidence of a sub- An,, to An,, with most samples falling into the andesine aerial volcanic edifice immediately after its last violent and range, An,, to An,,. The rock matrix is composed of voluminouseruptive event. Evidence that supports this volcanic ash, dust and fine carbon. Some samples have interpretation includes minor interbedded wackes with rip- crude bedding caused by a slight change in overall matrix ple marks in the upper metre of the Pyritic Tuff Member at grainsize. Fine chalcedony veinlets cut through some the northeast end of Long Lake, and interbedded tuffaceous samples. and pyritic limestones, siltstones and cherty units in the Two alternative interpretations might account for the lim- lower part of the Salmon River strata. ited strike extent of the thick, distinctive Black Tuff Mem- STRATIGRAPHY ber; it may represent an erosional remnant of an originally extensive unit,or deposition was restricted to the area itnow BASALFOSSILIFEROUS LIMESTONE MEMBER occupies. The latter interpretation requires that the black tuff was deposited in a topographic low such as a volcanic Thin, pyritic, fossiliferous limestone crops out at the base crater or caldera. If this depression was an anoxic water- of the formation throughout the area. The unit comprises filled basin it would account for local sediment lenses and dark grey to black carbonate-cemented grit with interbedded the intense carbon impregnation throughout the unit. lenses, pods and nodules of fossiliferous, gritty limestone. Both the calcareous grits and fossiliferous limestones con- tain sparsely disseminated pyrite, which is more abundant PROVENANCE AND DEPOSITIONAL ENVIRONMENT where the unit is underlain by the pyritic tuff. All rocks of the Mount Dilworth Formation look highly The calcareous grits locally contain scattered granules siliceous in handsample and have been identified as and pebbles of volcanic rock types. Thin conglomeratic 26 Plate 14. Fossil and mudchip-rich gritty limestone, Basal Fossiliferous Limestone Member of the Salmon River Formation. Southwest edge of Mount Dilworlh snowfield. la:yers at Summit Lakeand on Slate Mountaincontain The slates and siltstones locally contain minoramounts of roundedpebbles of pumice.These local pumice- disseminated pyrite, and pyrite seams outline some bedding conglomerate beds may represent rafted pumice, suggesting planesproducing characteristic handed, iron-stained minor felsic volcanism continued during marine transgres- weathered surfaces. The buff-weathering limestone lenses, sion. The grits also have up to 10 per cent fine-grained, pods and concretions are regionally distributed but thin, and angular, altered feldspar grains, indicating rapid erosion and are present near the base of the siltstone sequence. Grove deposition with little weathering and reworking of the (1986) and Anderson (personal communication, 1986) have sands. recovered Middle Jurassic fossilsuites from these strata. The limestone is buff to grey, sandy to gritty with some Siliceous beds, first reported by Grove (1973, 1986), were black siltstone chips or rip-up clasts. The thin member is studied in detail by Brown (1987) who describesthe beds as fossiliferous throughout the map area and locally contains planar to undulating layers of siliceous, radiolaria-bearing up to SO per cent fossil debris (Plate 14). The most precise shale less than S centimetres thick. fossil age comes from recent sampling of the Basal Lime- In thin section siltstones arecomposed of silt-sized stone Member on Troy Ridge by Anderson (personal com- quartz, feldspar and lithic chips in a carbonaceous, cal- munication, 1986). Fossils include abundant belemnites and careous clay matrix.Tuffaceous beds containglass-shard lesser pelecypods, including Weyla; many fossils are pre- outlines, bubble-wall fragments, plagioclase microlites and served as fragments. The key fossils forman overlap quartz fragments, indicating that minor or distant pyroclas- assemblage characteristic of Toarcian time. The upper contact of this basal member is typically marked bya bedding-plane fault that separates this unit fmm overlying thin-bedded siltstones. The fault is marked by a massive quartz vein in some areas. Where the fault is absent, the upper contact is conformable. L~OWERSILTSTONE MEMBER The lower SO to 100 metres of the mainsedimentary succession consists of black to grey, thin to medium-bedded calcareous siltstones and shales withminor intercalated limestones and siliceous tuffaceous beds. The rhythmically interbeddedsiltstones to fine-grained sandstones are exposed as beds S to 10 centimetres thick (Plate IS). Shale partings between these layers are a few millimetres thick. The siltstone is well bedded and finely laminated; shale is massive, intensely cleaved or weakly phyllitic. The siltstone sequence is characterized by abundant scour-and-fill struc- Plate 15. Rhythmically interbedded black limy siltstones tures, graded bedding and crossbedding; tops are toward the and white siliceous tuffaceous siltstones, Lower Siltstone ayis of the Mount Dilworth syncline. Member of the Salmon River Formation. (Brown photo.) 27 tic volcanism was contemporaneous with marine deposition distinctive, basal chert-pebbleconglomerate units of the (Brown, 1987). Bowser Lake Group suggests that the youngest rocks in the This member is interpreted as a sequence of rhythmically succession predate Bowser time and are therefore Middle bedded marine clastic sedimentary rocks derived from the Jurassic, but older than upper Bajocian. erosionof a predominantlyvolcanic terrain. The lower contact is usually marked by a zone of intense deformation INTRUSIVE ROCKS andquartz veining, 5 to 30 metres thick, adjacent to a bedding-plane fault. Plutonic rocks underlie about 100 square kilometres or 15 per cent ofthe map area. In the southwestern corner they UPPER WACKE MEMBER form a continuous mass of superposedplutons; to the north- east, discrete stocks and a variety of dikes are scattered Conformably overlying the Lower Andesite Member are throughout the stratified rocks. Based on field relationships, medium to light greywackes and intraformational con- modal and chemical compositions, textures and extensive glomerates (Plate 16). Quartz greywackes, greywackes and dating, intrusive rocks of the region have been groupedinto arkosic wackes are included in this sequence. Most of these two plutonic suites in agreement with guidelinesestablished rocksform fairly massivebeds a fewmetres to several by theNorth American Commission on Stratigraphic metres thick with minor interbeds of thin-bedded siltstone. Nomenclature (1983). Bothsuites are represented by The conglomerates consist of subparallel, black siltstone batholiths, stocks and a variety of dikes; the older suite also slabs and cobbles in a grey sandstone matrix. has sills and volcanic extrusive phases. Terminology con- Still higher in the sedimentary succession the strata are forms to the classification scheme of Streckeisen (1975). rhythmicallybedded dark grey siltstones. Theywere not The Early Jurassic Texas Creek granodioritesuite is examined in this study. characterized byan overall coarse grain size, abundant coarsehornblende and locally byvery coarse potassium PROVENANCE AND DEPOSITIONAL ENVIRONMENT feldsparphenocrysts or megacrysts. The EoceneHyder granodiorite suite is characterized by overall medium grain The clastic rocks of this formation were derived mainly size, biotite, equigranular plagioclase and orthoclase, and fromweathering and erosion of a volcanicsource area. Some beds may have formed by mixing of varying propor- trace amounts of fine-grained, golden sphene. The marked difference inhornblende versus biotite contentsand the tions of water-transported volcanic detritus and air-fall ash presence of alteration and foliation in the older Texas Creek and crystals from distant felsic pyroclastic activity. suite are the most reliable field distinctions between the Sediment was deposited on the flanks of an extinct vol- two. can0 in a shallow to moderately deep marine basin, rela- tively near subaerial source rocks. The thin rhythmic bed- dingsuggests transport waspredominantly by mass-flow TEXASCREEK PLUTONIC SUITE processes such as turbidity flows. The general absence of The Texas Creek Plutonic Suite or Texas Creek grano- fossils and bioturbation through most of the sequence indi- diorite suite includes the Texas Creek batholith, the Summit cates that the beds accumulated rapidly. Lake stock, Premier Porphyry dikes and a variety of related The basal fossiliferous limestone is Toarcian. Although dikes and sills within the map area. The Premier Porphyry an upper age cannot be demonstrated, the absence of the Member of the Unuk River Formationis an extrusive equiv- alent of these plutonic rocks. Major element analyses of five plutonic and dike-rock samples from the Texas Creek suite were completed in this study. These results are presentedin detail later in this chapter and showthat rocks of the Texas Creek granodiorite suite are suhalkaline, calcalkaline intrusions with elevated potassiumcontents. Analyses from subvolcanic Premier Porphyry dikes fall within the andesite field on a total alkali versus silica plot. TEXAS CREEK BATHOLITH Buddington (1929,p. 22) definedthe Texas Creek batholith. The pluton crops out in the southwestern part of the maparea and extends farto the west. It is roughly 15 kilometres in diameter with a total area of about 205 square kilometres. The Texas Creek batholith is composed of hornblende Plate 16. Intraformational conglomerate. Zone of rounded black slabs and blocks of siltstone within a thicker granodiorite to monzodiorite to quartz diorite. Two phases bed of coarsewacke. Upper Wacke Member of Salmon havebeen recognized: in the central andwestern areas, River Formation, Northwest edge of the Mount Dilworth within Alaska, the batholith is a very coarse grained equi- Snowfield. granular rock; alongthe eastern margin it is porphyritic with 28 a medium to coarse-grained groundmass. The porphyritic pink to buff enhedral orthoclase crystals similar to those in ph;ase has hornblende phenocrysts up to 2 centimetres long Premier Porphyry dikes and extrusive rocks. Phenocrysts and large potassium feldspar phenocrysts from 3 to 5 cen- enclose small enhedral crystals of hornblende and plagio- timetres long. Based on isotopic dates and field relation- clase. Potassium feldspar in the groundmass is generally ships, the equigranular phase was emplaced first, followed interstitial to hornblende andplagioclase. Plagioclase is by the porphyritic phase. Similar phase relationships have present as euhedral to subhedral lath-shaped crystals of been recognized in the Early Jurassic Lehto batholith on the oligoclase-andesine. 1sk:nt River (Britton et al., 1990). Quartz has bimodal grain size. It is both coarse grained .Along the eastern contact, the coarsely porphyritic mar- and contemporaneous with other coarse mineral phases, and girl of the Texas Creek batholith usually displays a narrow fine grained and interstitial to the earlier formed minerals. zone, up to a few tens of metres wide, of medium to dark Abundant hornblende is present as euhedral, columnar, greenish grey chloritic alteration that is sometimes accom- black prisms up to 2 centimetres long. Accessory minerals, paniedby a crude foliation. Shearing and broken grains including sphene, zircon, apatite and magnetite, form about indicate that thiszone resnlts’from crushing along the 1 per cent of the rock volume. Pyrite is rare. granodiorite contact. There is a general absence of contact The porphyry phase has varying degrees of alteration. me:tamorphism in the adjacent country rocks that suggests Sharply euhedral hornblende and interlamellar biotite are thasthere was not a major thermal or chemical contrast partly to completely altered to chlorite, but quartz shows bel.ween them and the intrusions, and that pressures from only slightly strained extinction. Plagioclase is extensively rising magma and late-stage fluid accumulations were altered to fine sericite aggregates, but some crystals have relieved by faulting, dike injections or by regular venting of thin, clear, unaltered rims. Potassium feldspar is fresh and vo:latiles to surface. unaltered. ‘The main phase in the core of the batholith was exam- ined along the West Fork of Texas Creek, west of the map SUMMIT LAKE STOCK are:a. Corerocks are massive, equigranular, medium to coarse-grained hornblende granodiorite, with up to 15 per The SummitLake stock is a remarkably fresh, medium to cent coarseeuhedral hornblende. This hornblende-rich, coarse-grained hornblende granodiorite. The rock is gener- ally equigranular with rare potassium feldspar phenocrysts. coarse-grained texture is a characteristic feature of the Thin sections showthere is finer grained interstitial Texas Creek batholith that can be recognized through all a alteration and deformation. groundmass among the coarse-grained hornblende, biotite, plagioclase, potassium feldspar and quartz. :In thin section, feldspar crystals have fine crystal inch- sions of apatite and lesser zircon. Near the margins of the Potassium feldspar crystals are characteristicallynnal- batholith, coarse inclusion-rich euhedral plagioclase grains tered. Some large crystals display a ring of fine beads or show a thin inclusion-free overgrowth rim. Similar textures blebs of quartz near the rim.Plagioclase (oligoclase) is 25 to are present in Premier Porphyry dikes and in extrusive rocks 40 per cent sericitized. Plagioclase crystals display subtle, of the Premier Porphyry Member. In many samples of the thin, clear rims hut the texture is not as obvious as examples equigranular phase, anhedral quartz displays interfingering, from the Texas Creek batholith to the south. Most quartz is sutured boundaries. Hornblende crystals are typically large, in the fine-grained interstitial groundmass. The few large euhedral to subhedral, and are commonly twinned. Some quartz grains are sutured. homhlende hosts small zirconcrystals. Many hornblende Total mafic mineral content is I8 to 20 per cent. Coarse, crystals have minor interlamellar biotite but no free biotite sharply euhedral, green hornblende is commonly twinned. was noted. Chlorite alteration of the amphiboles varies in Inclusion-rich red-brown biotite composes 2to 5 per cent of intensity, hut is a characteristicfeature. The granodiorite the rock. It is more readily chloritized than the typically also contains accessory apatite, sphene and zircon. fresh hornblende. The rock also contains rare tiny zircons Toward the eastern margin of the batholith much (60 to and interstitial rosettes of chlorite. 1011%) of the hornblende is altered to chlorite, the rock has Massive, nnsheared textures at the intrusive contact indi- subparallel microfractures filled by chlorite, and coarse cate passive emplacement, hut the granodiorite is somewhat inclusion-rich plagioclase grains are strongly altered to altered. The hornblende is wholly altered to very pale green sericite.Even where plagioclase is intenselysericitized, chlorite, quartz is only slightly strained, and plagioclase has pot.assium feldspar is fresh and unaltered. Quartz grains in minor (15 per cent) sericitization. these margin areas have moderately strained extinction. The porphyry phase of the batholith lies along its east- TEXAS CREEK DIKES em.margin. The rock is generallya feldspar-porphyritic, The Texas Creek Plutonic Suite includes a dike phase. coarse-grained hornblende granodiorite. This porphyritic Schofield and Hanson(1922) referred to these dikes as margin is up to several hundred metres wide, but is locally “Premier Sills”. Brown(1987) proposed the descriptive abs:ent, as near the wrecked bridge on the Texas Creek road term “potassium feldspar porphyry”. The writer has used at !:he south end of Mineral Hill. The rock is dull greenish the term “two-feldspar porphyry” in past articles, but “Pre- grey to light grey and the large phenocrysts give outcrops a mier Porphyry” is preferred because of its current wide- mottled appearance. spread use and the close relationship between these distinc- Phenocrysts comprise up to several per cent of the rock tive dikes and all the major ore zones at the Silbak Premier volume, and are 1 to 4 centimetres long, white to faintly mine. The rock is characterized by potassium feldspar 29 megacrysts and plagioclase phenocrysts in a fine-grained to Three varieties of Premier Porphyry dike are recognized aphanitic groundmass. based on detailed mapping and drill-core logging at the Premier Porphyry dikes cut all the five lower members of Silbak Premier mine’ (Brown, 1987): the Unuk River Formation and the eastem margin of the Potassiumfeldspar megacrystic, plagioclase Texas Creek batholith. They do not cut the Premier Por- hornblende porphyry is the “typical” Premier Por- phyry Member of the Unuk River Formation or any of the phyry dike. The extrusive equivalents of this dike-rock overlying strata. They are interpreted as subvolcanic feeder are Premier Porphyry tuff and Premier Porphyry flow dikes coeval with the extrusive Premier Porphyry Member of the Premier Porphyry Member. that marks the top of the Unuk River Formation (Figure 12). Plagioclase hornblende porphyry is texturally simi- Inoutcrop, Premier Porphyry dikes weather green to lar to the potassium feldspar megacrystic porphyry, but greyish green, have visible phenocrysts and are massive to containsfew orno quartz andpotassium feldspar weakly foliated. Outcropsare characteristically blocky phenocrysts. The extrusive equivalent of this dike-rock weathering and less foliated than the country rock andesitic is the massive to layered Plagioclase porphyry of the tuffs. In some exposures potassium feldspar megacrysts Premier Porphyry Member. weather out, leaving large rectangular pits. Intrusive con- Plagioclase porphyry is hornblende poor, least abun- tacts are rarely found inoutcrop, but good examples of dant, and may be a gradational phase of plagioclase chilled dike margins have been noted during Underground hornblende porphyry. mapping, drill-core examination, and mapping along the Salmon River. On both property and regional scales the megacrystic variety is the most abundant and will be described here as the type rock. Typical Premier Porphyry dikes (Plates 1 and 17) are medium to dark green rocks with large (up to 5.0 cm) potassium feldspar megacrysts, plagioclase phenocrysts (up to 8 mm) and hornblende phenocrysts (up to 1.0 cm) in an altered fine-grained to aphaniticgroundmass. Thefine- grained groundmass, lower hornblende content, and pres- ence of pervasive chloritic alteration distinguish hand sam- ples of Premier Porphyry dikefrom the coarse-grained, hornblende-rich, generally less altered rocks of the Texas Creek batholith and Summit Lake stock. Plate 17. Threeaspects of PremierPorphyry dike: (I) Type rock with moderate chloritic alteration: (11) Strongly sericite-carbonate-pyrite-altereddike-rock Plate 18. Photomicrograph of pressure shadow around from south rim of Premier glory hole; (111) Strongly chlo- pyrite crystalshows sequential(but penecontemporaneous?) ritized mylonite zone in Premier Porphyry dike adjacent to growth of sericite, quartz and chlorite, indicative of lower Lindeborg vein in underground workings of the Riverside greenschist facies metamorphism.Silbak Premier mine. mine. Crossed nicols. I Geologists at the minesite emphasize the difticulries of distinguishing between these rocks by respectively naming these same three dike types: Premier Porphyry proper. 0 Premier Porphyry probable. Premier Porphyry problematic. 30 In thin section, typical Premier Porphyry consists of core (main phase) of the batholith, the youngest from dike phenocrysts of orthoclase, plagioclase, hornblende and phases. rounded quartz in a groundmass of quartz and potassium feldspar crystals. Potassium feldspar usually constitutes up SUMMARY to 4 per cent ofthe rock; crystals average about a centimetre Rocks of theTexas Creek granodiorite suite were long. Rocks with up to 50 per cent phenocrysts are called emplaced in a shallow subvolcanicsetting below and within “crowded porphyry”. Rare phenocrysts up to 5 centimetres an Early Jurassic andesitic stratovolcano. Some dikes were long have been noted. Cores and rings of fine hornblende volcanic feeders that produced extrusive units on the paleo- and plagioclase inclusions are common. Some megacrysts surface. Intrusion andcrystallization proceededover a dislplay simple twinning. Two x-ray diffractometer analyses 25 million year period from the deepest, earliest batholithic of potassium feldspar megacrysts indicated compositions of rocks to the shallower, youngest dike phases. 88 and 96 per cent potassiumfeldspar which were classified Following emplacement, rocks of the Texas Creek suite as“low sanidine” based ondegree of AIM disorder weresubjected to hydrothermal alteration and associated (Brown, 1987). Grove (1971) concluded that the potassium sulphide mineralization, episodes of both brittle and ductile feldspar phenocrysts were metasomatic. However, Vernon faulting, and lower greenschistfacies metamorphism. These (1986) demonstrated primary igneous origins for potassium rocks were deformed in the same regional deformation that feldspar megacrysts and the megacrysts in rocks throughout affected stratified country rocks and werelater cut by intru- the map area are regarded as primary igneous phenocrysts. sions of the Tertiary Hyder Plutonic Suite. Plagioclase phenocrysts, mainly oligoclase, constitute 25 to 35 per cent of the rock and range from2 to 8 millimetres long. Like equivalent extrusive and plutonic rocks, samples HYDERPLUTONIC SUITE of l?remier Porphyry dikes often contain plagioclase pheno- In the Stewart region thecontinental-scale Coast Plutonic crysts that have thin clear rims around sericitized cores. Complex trends northwest. Part of the main body lies along Scattered, small (<4 mm) quartz eyes are presentin the southwest edge of the study area (Figure IO). Its eastern accessory amounts (up to 4%) in some dike exposures. They boundary isdefined as theeastern limit of continuous are typically nodular tolobate and strongly embayed, Eoceneplutonic rocks (Anderson, 1989). Buddington indicatingsubstantial resorption. Analyses from sub- (1929) included the main Coast Range batholith and all the volcanicPremier Porphyry dikes fall withinthe andesite satellitic plutonic rocks to the east in the “Hyder Quartz field on a total alkali versus silica plot. This is consistent Monzonite suite” or Hyder Plutonic Suite. with the petrographic evidence of strongly resorbed quartz The Hyder Plutonic Suite includes a batholith, a large crystals; free quartz was metastable in this magma. stock, several minorplugs and widespread dikes.The Euhedral hornblende phenocrysts make up2 to 8 per cent batholith lies within the eastern margin of the Coast Plu- of the rockand average 3 millimetres long, with local tonic Complex. Smaller stocks in the map area are outlying examples up to a centimetrelong. Hornblende is per- satellites of the Coast Complex that are mineralogically and vasively altered to chlorite with or without pyrite and may texturally similar to the main batholith. The varied dikes, only be preserved as chloritized remnants. Apatite, sphene, collectively termed Hyder dikes, are exposed as prominent zircon, pyrite andmagnetite are accessory minerals. The swarmsof regional extent and as randomly distributed, rock typically contains about 2 per cent very fine grained isolated dikes in the intervening country rock. Although the euhedral pyrite. Pyrite grains havepronounced quartz- plutons have associated dike phases, they lack preserved chlorite-sericitepressure shadows, indicating a post- extrusive equivalents. sulphidedeformation event at elevated temperatures Major plutons range in compositionfrom granite to (Plate 18). tonalite to quartz monzonite. These plot as calcalkaline The groundmass is fine grained to aphanitic and is gener- granites to granodiorites on a total alkali versus silica dia- ally strongly altered. Rare fresh samples have fine-grained, gram. Theserocks are characterized by overall medium interlocking to micrographic quartz and potassium feldspar. grainsize, biotite, equigranular white plagioclase and Alteration generally consists of abundant very fine grained orthoclase,and trace amounts of fine-grainedgolden senkite, lesser tine-grained carbonate, and minor chlorite sphene. In comparison with Early Jurassic plutons, these and pyrite. In some dike samples carbonate flooding is the Tertiary plutons are biotite rich, more siliceous and less dominant alteration; in others chlorite alteration is domi- altered.Associated dike phases range from aplite to nant. Plagioclase is altered to sericite aggregates with some lamprophyre. Emplacement of the Hyder suite hasproduced car’bonate, and the hornblende is primarily altered to chlo- variable contact metamorphism, including mechanical,ther- rite. Alteration is not pervasive or ubiquitous even within a mal and metasomatic effects. This regionally extensive suite single outcrop;some samples are fairly fresh with little hosts major molybdenum deposits and many minor silver, akration of the groundmass and with unaltered hornblende lead, zinc, gold and tungsten showings throughout the crystals. district. AGE HYDER BATHOLITH Isotopic dates from a variety ofplutonic rocks of the The Hyder batholith extends along the eastern side of the Tex.as.CreekPlutonic Suite rangefrom 211 to186 Ma, Coast Plutonic Complex from the Unuk River area south- spanning the Early Jurassic. The oldest dates come from the eastward along the Alaska border through the head of the 31 Portland Canal at Stewart to Observatory Inlet and Alice massive, lighter coloured, with a pinkish hue, and contains Arm (Grove, 1986, p. 69). This overall length is 175 kilo- conspicuous biotite. The Boundary granodiorite also resem- metres and the width averages 16 kilometres. The batholith bles the Hyder batholith. However, the Boundary stock is is bordered on the west by the Central Gneiss Complex and generally finer grained; its hornblendeneedles are more on the east by stratified country rocks. euhedral, more abundant and more distinct; and all its min- The batholith is well exposed along tidal zones of the erals are slightly inequigranular. PortlandCanal andin roadcutsand quarries at Stewart, Hyder and northward along the Granduc mine road to the HYDER DIKES Fish Creek bridge. It ranges in composition from biotite The StewartComplex hosts an extensive may of Tertiary granodiorite to quartz monzodiorite. The rock is fresh, light dikes and dike swarms, collectively termed the Hyder dikes. grey to pinkish grey, massive, medium grained, biotite rich Textures andcompositions are highly variable,ranging with minor hornblende and is locally slightly porphyritic. fromaplite to lamprophyre,but plagioclase-porphyritic Fine-grained, golden sphenecrystals are characteristic. granodiorite with biotite-rich, fine-grained toaphanitic, Large outcrops have blocky joint patterns. Intrusive con- Some workershave tacts are marked by biotite hornfels of argillaceous country light grey groundmass is common. restricted the name “Hyder dikes” to refer only to the rocks, epidote disseminations and veins in tuffaceous and granodiorite porphyry dikes, but LateTertiary aplite, micro- plutonic country rocks and local skarn development in cal- diorite and lamprophyre dikes are considered as late phases careous sedimentary rocks. of the Hyder plutonic episode in this study. Based on biotite K-Ar dates,the Hyder batholith is Eocene, 48.8 Ma(Smith, 1977). As noneof the major Hyder dikes have been grouped into four dike phases Tertiary dike swarms in the region cuts the batholith, this based on similar compositions, textures and relative ages. date places an upper limit on the age of the dike swarms. The oldest are massive, equigranular to porphyritic, fine to medium-grained, light grey biotite or biotite hornblende Locally, the batholith is spatially related to silver-rich granodiorites that are up to60 metreswide, and rarely, galena-sphalerite-freibergiteveins at the Porter Idaho, Sil- muchwider. These are cut bybuff to pinkish to white, veradoand Bayview mines near the town of Stewart. massive to flow-banded, saccharoidal aplite or “rhyolite” Regionally, discovely of the huge Quartz Hill molybdenum dikes up to 5 metreswide. Aphanitic to fine-grained, deposit in Alaska andthe Molly May - Molly Mack molyb- greyish green microdiorite or “andesite” dikes range up to denite prospects south of Anyox spurredinterest in the main 10 metres in width. These, in turn, are cut by thin, dark batholith of the Coast Plutonic Complex as a setting for brownish grey, variably porphyritic lamprophyre dikes that molybdenum deposits. are generally less than 50 centimetres wide. BOUNDARY STOCK Pale granodiorite porphyry dikes vary significantly in both composition and texture. Modal compositions range The Boundarygranodiorite stock, which straddles the from granite to diorite, but massive plagioclase-porphyritic international border southwest of Salmon Glacier, was not biotite granodiorite is most common. In hand specimen the examined in this study. The stock was described indetail by rock is characterized by pink to buff to white plagioclase Buddington (1929) and Smith (1977). phenocrysts, up to 5 millimetres long, that form up to 45per The Boundary granodiorite is an inequigranular, hypidio- centof the rock. Equigranular dikes are fine to medium morphic, medium-grained, biotite hornblende granodiorite. grained; plagioclase, biotite and less commonly hornblende, Hand samples are fresh, dull white, with scattered grains of orthoclase or quartz can be recognized. The groundmass of pale pink orthoclase andgrey, glassy quartz; they are speck- the porphyritic dikes is aphanitic to fine grained. A single led with mafic crystals and tiny crystals of honey-coloured U-Pb zircon date of54.85 1.3 Ma agrees with field relation- sphene are common. In contrast to the older Texas Creek ships that show thesedikes cutting all rocksexcept the batholith, whichit intrudes, the Boundarystock is more younger plutons and dikes of the Hyder plutonic suite. Plate 19. The Portland Canal dike swarm and the Granduc mine road. Looking east from Mount Bayard across the Salmon Glacier to Mount Dilworth. (Hemhling photo.) 32 .4plite. dikes aresparsely distributed throughout the Both major swarms are composed of four main dike rock region. They seem to bemore numerous within the two types, the third is composed only ofthe two youngest, more major southeast-striking dike swarms and within the Terti- mafic rock types. In the two major swarms intrusive rock ary plutons, and are probably genetically related to them. comprises more than 40 per cent of the bedrock; locally, Dikesgenerally range up to 5 metreswide and tend to only narrow lenses and slices of country rock separate the weave or meander through the country rock, rather than anastamosing dikes. The cumulative thickness of intrusive have a rectilinear form. However,within the major dike rock in these two swarms represents a northeasterly crustal sw.ums their trend is generally parallel to the resistant, extension of at least 1.5 kilometres. bounding plagioclase porphyry dikes. The Portland Canal dike swarm (Plate 19) is the long- Aplite forms fine-grained to saccharoidal, white to cream est of the three. It has been traced continuously southeast- to buff to pale pink dikes with distinctive flow banding and ward from Mount Bayard to Mount Dickie and finally to aphanitic chilled margins. Flow banding ranges from a few Mount Trevor, in the centre of the Cambria Icefield. Major millimetres to a few centimetres thick, but is not present in felsic dikes are exposed along the north wall of the Salmon all dikes. Some handsamples have distinct quartz eyes. Glacier northwest of Mount Bayard andfarther northwest in Aplite dikes are extensively sericitized and generally lack the cliffs of Scottie Dog Mountain near the Grauduc mine, mafic minerals. for a total lengthof 67 kilometres. The swarmhas an Aplite dikes cut the Hyder batholith and the granodiorite indicated width of 2 kilometres on the accompanying map porphyry dikes, but they are cut by younger lamprophyre (Figure11), but its boundaries are somewhat arbitrarily dikes. Rubidium-strontium dates from aplite indicate a Mid- drawn where dikes make up 40 per cent or more of the dle Eocene age of 44?4 Ma (Brown, 1987). bedrock. Dikes crop out for up to 500 metres outside these boundaries. Microdiorite dikes are greenish grey or medium to dark greyand sparsely porphyritic. They are distributed Dikesin the swarmgenerally trend east-southeast to throughout the region and are the dominant lithology of the southeastand dip steeply southwest,but dips flatten to Berendon dike swarm. These dikes crosscut Tertiruy Hyder about 45" where dikes cross thin-bedded argillites of the plutons, butthey are cut by younger lamprophyre dikes. Salmon River Formation south of Mount Dilworth.Individ- Microdiorite dikes range up to 10 metres wide and typ- ual dikes are up to 150 metres thick and can be traced to ically display chilled margins and blocky fractures. Hand depths of hundreds of metres and lengths of thousands of samples have an aphanitic to fine-grained texture and may metres. Myriad smaller dikes in the swarm, from a few to be )porphyritic.Dikes are composed of small phenocrysts of 60 metreswide, form a complex,anastamosing network. plagioclasewith or without hornblende, in a felted Dikes locally merge into small elongate stocks more than gro,undmass of interlockingplagioclase laths, lesser 400 metreswide. Toward these minor Tertiary plutons, hornblende laths, and minor interlocking interstitial quartz dikes are progressively closer together, thicker and more and potassium feldspar (Buddington, 1929). numerous, ultimately coalescing into the stocks. Lamprophyre dikes are sparsely distributed throughout Variations in texture and composition have been noted the region. Most are less than 50 centimetres wide. Wide within individual dikes as well as between dikes. The most lam.prophyre dikes rarely change attitude or thickness, hut abundant felsic dikesare equigranular ortine to coarse thinnerdikes (C20 cm)change attitude or pinch out porphyries, and may be flow brecciated. Modal composi- abruptly. Dike contacts are sharp, parallel surfaces. Chilled tions range between granite, quartz monzonite, granodiorite margins are common but quite thin. Closely spaced joints and quartz diorite (Grove, 1986). Other dikes in the swarm make the lamprophyres easily eroded, so that many dikes include aplites, microdiorites and lamprophyres. are marked by deeply incised clefts. No absolute age determinations have been made on any Lamprophyre dikes vary in mineralogy,texture and hand- of the dikes in this swarm, but Brown (1987) reported that specimen appearance. In outcrop, they are dark green, dark the Portland Canal swarm was more differentiated than the gre:y or dark brownish grey; fresh samples are charcoal to jet Boundary dike swarm (54.8 Ma, R.G.Anderson, personal black. All these dikes areshoshonitic (Rock, 19771, but both communication. 1985) andconcluded that the Portland biotite-rich minettesand hornblende-rich spessartites are Canal swarm was slightly younger. present. Minettes are characterized by prominent biotite Severalmineral occurences and a fewdeposits are phenocrysts, up to 3 millimetres long, hosted in a massive, located within this dike swarm. At some deposits, like the dark grey, aphanitic groundmass (Brown, 1987). Spessartite Martha Ellen orebody, post-ore dikescut across the mineral dikes are composed of 40 per cent, deep green, euhedral zones but are not themselves affected by the mineralization. hornblende, 56 per cent subhedral plagioclase, minor mag- At other showings, the Portland Canal dikes were the locus netite and trace hypersthene. for sulphide mineralization. Many dikes have beenfractured Elrown (1987)reported an Oligocene K-Ar date and faulted, the spaces in the dikes and in adjacent country (25.2 fl Ma) for a biotite lamprophyre. Carter (1981, p.88) rocks are filled with coarse-grained quartz and silver-rich also reports OligoceneK-Ar dates (36.5t1.2; 34.421.5) galenaand sphalerite, for example:Outland Silver Bar, from two lamprophyre samples in the Alice Arm area. Silver Basin, Lion, Unicorn, Silver Hill, Silver Tip, Spider, R.ocks in the Salmon River valley are cut by two major Lois, M.J. and Silver Crown prospects. None of these occur- sout.heast-strikingdike swarms of felsic to mafic dikes and rences have been major producers, althoughSilver Tip does by a third, less dense, belt of intermediate to mafic dikes. have significant reserves (Plumb, 1957). 33 TheBoundary dike swarm isthe second major intrusion is summarized in Table 3, based on field relation- southeast-striking swarm and lies along the international ships and isotopic dates. border between Cantu Mountain and Mount Welker, passing As the Boundary stock is slightly older than the Hyder through the Silbak Premier mine. It extends beyond the map and Bitter Creek plutons, their spatially associated precursor area in both directions. The swarm has a length of at least dike swarms (Boundary and Portland Canal respectively) 22 kilometres, from Mount Jefferson Coolidge to the Bear are probably also slightly different in age. It is impressive River, and a width of 3 kilometres. The Boundary swarm that these small age differences were correctly deduced for has the same general southeast strike and steep southwest the plutons (Buddington, 1929, p. 33) and for the dike dip (13S’/7OoSW) as the Portland Canal swarm and includes swarms (Brown, 1987, p. 52) on the basis of petrographic a similar variety of dikes. Granodiorite porphyry dikes are studies alone. especially abundant within this belt. The most spectacular, continuous exposures are at the summit of Mount Welker. SUMMARY The Berendon dike swarm is the third, narrower, less The main batholith of the Coast Plutonic Complex and crowded belt of dikes. It trends south along the west side of outlyingsatellitic stocks and dikes are interpreted as Tide Lake Flats, and across the upper portal (3600-level) subduction-relatedplutonic rocks emplaced above the areaat Scottie Gold mine to August Mountain.From eastward-subducting Pacificplate in early Tertiary time. August Mountain it swings southeast and continues overthe Emplacement of major intrusions ended when relative plate crest of Mount Dilworth. Southeast of Mount Dilworth, motions changed from colinear (high-angle) to transcurrent toward Mount Bunting, it merges with the wider Portland (oblique) geometry about 42 Ma (Armstrong, 1988). The Canal swarm. plutonic episode was followed by faulting, uplift and rapid This belt of dikes is significant because it is dominantly erosion. composed of microdiorite (andesite) dikes with many sub- parallel, thin lamprophyre dikes. Older, more felsic dikes are absent. These younger dikes clearly trend north to north- BASALTIC DIKES northwest. Therefore the northwesterly trend of younger, OF THE STIKINEVOLCANIC BELT more mafic dikes in the two major swarms is due todeflec- In Miocene time, alkali-olivineflood basalts formed tion or ‘capture’ of these dikes by the resistant, thicker, plateau lavas. The lava issued from regional networks of felsic dikes of the two main swarms. vents and fissures (Souther, 1972; Smith, 1973). This vol- canicactivity culminated in thelate Miocene to early AGE Pliocene andcontinued intermittently through to the present Granodiorite porphyly dikes cut all stratified rocks and day. all Early Jurassic plutonic rocks, but do not cut Tertiary Basaltic dikes have been reported by Buddington (1929) plutons and dikes. Based on these field relationships and on and, under the classification of lamprophyres, by Smith age determinations, the emplacement of these dikes slightly (1973, 1977; see Rock, 1977, p.140). The dark porphyritic preceded intrusion of the main Tertiary plutons. Thus the rock has augite, plagioclase and rare olivine phenoclysts in plutons obliterated the precursor dikes. Dikes of aplite, asubophitic groundmass of plagioclaseand pyroxene. microdiorite and lamprophyre cut all other rocks and proha- Thesewidespread dikes may have been thefeeders to bly originated as late phases of the batholith. Tertiary plu- Miocene volcanic flows that were eroded in this area, but tons and dikes of the Hyder plutonicsuite crosscut all are preserved elsewhere. regional folds, but are offset by most major and minor faults. In a few locations dikes are deflected along pre- existing fault zones. PETROCHEMISTRY Isotopic datesare available formost of the intrusive rocks Chemical analyses have helped classify rock types, deter- of the Hyder suite andrange from 55 to 25 Ma, a 30 million mine their type and degree of alteration, and establish their year period similar to the 25 million year span of dates from tectonic affinities. the Texas Creek suice. The general sequence of Tertiary VOLCANIC ROCKS TABLE 3 In this report, lithochemistry is compared and contrasted HYDER PLUTONIC SUITE - SEQUENCE OF INTRUSION at formational scale. The patterns show a systematicchemi- cal evolution that is parallelled by the colour index and Epwh Age (Ma) LithodemPlPhases phenocryst assemblage of the rocks (Figure 14). late Oligocene 35-25 Lamprophyredikes. The stratigraphic distribution of phenocrysts shows an early Oligocene 45-35? Microdioritedikes. evolution that closely resembles Bowen’s reaction series mid-Eocene 44 Aplitedikes. mid-Eocene 4849 HyderbatholithiBitter Creek stock. (Figure 14). The significance of different phenoclysts in mid-Eocene 52? PrecursorPortland Canal dike swann. andesites isreviewed by Gill (1981, Chapter 6). Plagioclase mid-Eocene 51-52? Boundarystock/Davis Riverstock. phenocrysts may range in composition from An,, to An,?; early Eocene 55? PrecursorBoundary dike swarm. plagioclase phenocrysts in the Unuk River Formation fall In early 55 Granodioriteporphyry dikes. Eocene the range An2, to An,,. Characteristic features of plagio- (Ages derived from isotopic dates andlor field relationships) clase phenocrysts in orogenic andesites are present in Stew- 34 IA Mg/Si ALTERATION SCREEN lB Mg/Ca ALTERATION SCREEN 6- 6- 0 GALLEY (1981) GALLEY (198' 1) A BROWN (1987) A BROWN (198; ') 0 THIS STUDY 0 THIS STUDY 5- \ 5- 4- i AA 0 \ 40 50 60 70 SI02 COO C Na ALTERATION SCREEN D Na:K CORRELATION 6 GALLEY (1981: A BROWN (1987) GALLEY 0 THISSTUDY * (1981) A BROWN (1987) A A 0 THIS STUDY 'rpilillzed' 5 5- 0 0 4 4- 0 N z0 rubalkaline 3 3- AA 2 2- 1 AA A .A A' A A 0 0 " I 0 1 2 3 4 5 6 7 SI02 Figure 16. Major element data plotted on the alteration screening diagrams from de Rosen Spence (1976). 35 art rocks, for example, inclusion-rich zones, regions of the total alkali versus silica plot (Sabine et aL, 1985). oscillatory zoning, and clear, normally zoned mantles. Gill Altered samples must be eliminated from the data setbefore concludes that hornblendephenocrysts, which are con- chemicalclassification schemes can beapplied to the spicuously abundant in Stewart, are relatively uncommon in remaining "unaltered" samples. Petrography is an essential andesites and are largely restricted to mediumand high- step for classificationof every volcanic rock and can readily potassiumandesites, to topographically and strati- identify all significantly altered rocks. Not only can altered graphically high levels of stratovolcanoes, and to formation rocks bescreened and eliminated from the chemical data set in magma chambers at high crustal levels. Finally, he notes by microscopic examination, but it is usually possible to that quartzphenocrysts in andesites are almost always reliably identify the original rock type despite its alteration embayed. This presence of metastable quartz may reflect overprint. To complement microscopic studies, de Rosen assimilation of sialicrock, mixing of maficand acid Spence (1976) developed three discriminant plots for sepa- magmas, or high-pressure crystallization. rating chemically altered volcanic rocks from datasets. Alteration is the principal cause of lack of concordance These plots identify alteration effects involving the most between classification by petrographic description and by mobile elements: sodium, magnesium, calcium and silica. TABLE 4 MAJOR ELEMENT ANALYSES FROM STRATIFIED ROCKS IN THE STEWART MINING CAMP MOUNT DILWORTH FORMATION LbNo Ti02Si02 AI203 Fez03 FeO MnO MgO CnO Na20 K20 P2Os LO1 TOTAL A426 65.87 1.11 12.27 17.49 0.060 1.033.18 0.52 0.920 0.00 5.00 107.45 A433 1.2666.12 13.51 0.027.68 0.003 8.11 0.03 4.450 0.00 6.00 107.18 G-113.4 0.71 67.4 1.14 2.24 0.05 6.22 0.00 2.96 0.09 4.00 98.21 G-4 0.47 62.2 17.8 1.20 1.64 0.17 4.95 3.37 3.27 0.17 3.08 98.32 66.6 0.39 15.9 G-I5 0.39 66.6 3.94 0.10 1.53 0.79 2.04 4.27 0.11 3.62 99.29 UNUK RIVER FORMATION fpremler Porphyry diker See also Table 5) LbNo Ti02Si02 AI203 Fez03 FeO MnO MQO- CsO NqO K20 PzOs LO1 TOTAL 59.38 0.49 14.98 4.60A405 14.98 0.49 59.38 0.00 0.360 1.47 3.74 0.00 8.100 0.20 0.00 93.32 A424 0.52 57.54 14.470.270 2.08 4.86 1.65 7.34 0.24 4.740 0.10 3.10 96.91 59.6 0.50 14.8E70 0.50 59.6 3.6 0.7 0.28 1.55 4.46 0.39 4.96 0.19 7.86 98.89 62.5 0.51 15.0 0.9 15.0 E15 0.51 62.5 0.15 3.4 1.20 4.96 2.47 2.83 0.22 6.40 100.54 61.6 0.49 15.1B366 0.49 61.6 0.80.30 3.4 1.49 3.83 0.17 7.89 0.22 5.72 101.01 E524 0.55 62.8 16.0 0.6 0.14 3.9 1.82 3.49 3.31 3.99 0.26 2.80 99.66 E5460.6 60.114.5 0.52 0.26 4.0 1.60 7.20 0.16 3.89 0.19 8.93 101.95 UNUK RIVER FORMATION(Upper Andesite Member - Main Sequence) LbNo si02 Ti02 AI203 Fez03 FeO MnO Me0 CaO Na20 K20 PzOs LO1 TOTAL B -40 57.4 B-40 0.83 22.6 1.6 5.1 0.09 1.02 0.30 0.26 5.93 0.09 4.4 99.62 E105 63.314.3 0.48 1.8 2.5 0.11 0.99 4.86 2.39 2.64 0.23 6.0 99.60 E96 56.2 0.81 19.4 7.4 1.4 0.14 0.98 1.20 0.61 5.45 0.12 4.5 98.21 B -294 57.1 B-294 0.61 16.7 0.4 4.3 0.11 1.62 5.36 1.89 3.23 0.25 7.5 99.10 E3406.3 56.415.4 0.73 0.48 2.08 5.65 0.95 3.47 0.30 5.75 97.52 E357 56.721.1 0.96 8.1 1.5 0.08 1.02 0.20 0.45 5.85 0.12 3.62 99.70 E3810.9 64.214.9 0.59 3.7 0.13 1.84 4.86 2.23 2.17 0.23 6.03 101.78 E382 62.1 0.644.0 17.4 2.0 0.16 1.87 4.29 5.25 1.46 0.24 2.5 101.91 E455 58.6 0.56 15.1 5.6 0.17 1.08 6.01 2.37 3.33 0.28 7.58 100.68 E543 57.7 0.68 14.2 3.9 2.6 0.26 1.09 7.02 0.71 3.5s 0.30 9.51 101.52 G -7 60.9 G-7 15.9 0.64 5.98 0.20 1.42 3.17 5.85 0.63 0.22 3.31 98.22 G -18 61.8 G-18 0.75 14.8 7.30 0.24 1.50 2.54 0.00 3.90 0.25 4.62 97.70 G -20 54.8 G-20 15.6 0.76 6.76 0.25 1.74 6.21 0.00 4.73 0.31 5.69 96.85 G-2 1 56.7 0.83 17.6 7.43 0.17 1.33 1.95 0.00 5.85 0.34 5.83 98.03 G -22 53.0 G-22 0.77 16.0 7.55 0.34 2.59 4.21 1.51 3.80 0.31 7.00 97.08 G -24 54.3 G-24 0.77 15.9 8.43 0.35 2.44 4.83 1.55 3.34 0.31 5.69 97.91 G-26 61.114.1 0.56 4.84 0.44 1.80 3.54 0.00 4.65 0.25 6.15 97.43 0-27 60.515.8 0.80 6.19 0.15 1.77 3.64 0.00 3.89 0.30 4.46 97.50 UNUK RIVER FORMATION (Middle Andesite Member) LbNo Si02 Ti02 AI203 Fez03 FeO MRO MQO~ CaO LO1 TOTAL A -344 45.55 0.96 16.24 13.47 0.210 4.06 7.38 3.02 1.240 0.34 92.47 0.34 1.240 3.02 7.38 4.06 0.210 13.47 16.24 0.96 45.55 A-344 Datasource: A=Alidrick(1991);B=Brown(1987):G=Galiey(1981) 36 The plots separate unaltered subalkaline rocks fromstrongly 0 Application of discriminant plots as alteration screens altered samplesand also separate unaltered subalkaline will help identify moderately altered samples that may rocks from unaltered alkaline rocks. have been overlooked, and will give important insight Screening plots were applied to all the available data sets into the bulk chemistry of the alteration process. (Fi;:ure 16); the results were alarming. Only 22 per cent of 0 All iron-based discriminant plots must be interpreted the writer's data, 19 per cent of Galley's data and 33 per withcaution because mostrock samples from the cent of Brown's data were retained as "unaltered" samples. region contain some pyrite and many have hydrother- A large proportionof the rejected samples fall in the mal chlorite alteration. "sodium-depleted" field on the N%O versus SiO, plot. Petrographic work indicates that these samples have been affected by replacementof sodium by potassiumin pla- gioclase. Sericitization in the Stewart area is common in most rocks and particularly intense in plagioclase phe- nocrysts. A plot of Na,O versus K,O (Figure 16d) shows a strong 1:l linear correlation between the two components, alsoimplying selective replacement of sodium by potassium. Sinceselective Na:K replacementdoes not affect the discriminant plots ofFigures 17, 18and 19, samples showing only sodium depletionor enrichment with no significant calcium or magnesium alteration were also retained in the dataset. From a total of 68 wholerock ana- lyses, 31 'least-altered' samples were retained for plotting (Table 4). Three recommendations are proposed for dealing with variably altered volcanic rocks in Hazelton Group strata of the Stewart complex: 0 Thorough petrographic description is an essential step that will identify all strongly altered samples and indi- cate alteration mineral assemblages. Figure 18. Major element data plotted on total alkali versus silica discriminant diagram of Le Bas et al. (1986). FLOWSAND DIKES PREMIERPORPHYRY MEMBER Ti02 UNUK RIVERFORMATION A 322 PREMIER PORPHYRY > U "_"""_ Ti 3 0 UPPERANDESITE MEMBER OCEAN ISLAND OCEAN ISLAND MIDDLEANDESITE MEMBER ALKALINE BASALT c!a UNUK RIVERFORMATION ------x x #Pi. p!= %*X ISLAND ARC CALCALKALINE PERCENT SILICA Mn0'10 ~205.10 Figure17. Silica histograms. Apparent bimodalandesite Figure 19. Major element data plotted on the tectonic population is an artifact of histogram division points. environment discriminant diagram of Mullen (1983). 31 TABLE 5 MAJOR ELEMENT ANALYSES FROM INTRUSIVE ROCKS Lab# SO, TiO, AI,O, F+O, FeO MnO MgO CaO Na,O K20 P20, LO1 TEXAS CREEK SUITE (EARLY JURASSIC) AlldrickA32715.7958.526.570.54 0.140 2.32 4.82 2.363.5500.23Texas Creek batholith (porphyry) Alldrick15.83A3256.2460.590.53 0.120 1.83 4.26 2.80 2.970 Summit0.18 stockL&e AlldrickA62414.4757.544.860.52 2.08 0.270 1.65 7.34 0.244.740 0.10 3.10 Premier Porphyry dike AlldrickA40514.9859.380.49 4.60 0.360 1.47 3.74 8.100Premier0.20 Porphyry dike BrawnB36661.600.49 15.10 0.80 3.400.300 1.49 3.83 0.177.890 0.22 5.72 Premier Porphyry dike Bm wn B70 59.60BmwnB70 0.50 0.7014.80 3.600.280 1.55 4.46 0.394.960 0.19 4.86 Premier Porphyry dikc Brown B15 62.50BrownB15 0.51 15.00 0.90 3.400.150 1.20 4.96 2.472.830 0.22 6.30 Premier Porphyry dike Brawn B524 62.80 0.55 16.00 0.6016.00Brawn0.5562.80B524 3.900.140 1.82 3.49 3.313.990 0.26 2.80 Premier Porphyry dike Brawn B546 60.10 0.52 14.50 0.6014.50Brawn0.5260.10B546 4.000.260 1.60 7.20 0.163.890 0.19 8.93Premier Porphyry dike Brown B360 78.10 0.31 10.00 3.0010.000.31Brown78.10 B360 0.050 0.15 0.51 2.704.150 0.01 1.43 Premier Porphyly dike Alldrick . A32658.20 0.5114.66 7.63 0.120 1.86 3.58 Salmon3.2400.192.96 River dike HYDER SUITE (MIDDLE EOCENE) Carter NC38 71.38 0.33 12.01 0.9912.01CarterNC380.3371.38 ~~.~ 1.07 6.720 0.19 1.50 B.C. Moly stock (Alice Arm) Carter14.640.69NC3066.980.54 2.02 0.040 1.00 3.08 3.76 4.650 0.20 1.18 Bell Moly stack (Alice Arm) CarterNC3365.78 0.5213.56 . 1.29 2.22 0.030 1.32 3.53 3.97 4.250 0.38 1.27 Ajax stock (Alice Arm) Carter NC35 71.16 0.31 14.230.36CarterNC3571.160.31 1.86 0.040 0.29 1.40 3.22 4.580 0.15 1.39 Roundy Creek stock (Alice Arm) CarterNC26 68.92 15.201.020.57 0.87 0.010 0.64 2.23 3.32 4.430 0.38 0.23 Valley stack (Alice Arm) * samples A405 and 8365 were collected independently from rhe same trench exposure PUARTZ HYDERSUITE (MIDDLE EOCENE) * HYDfR BATHOLITH A A BOUNDARY STOCK TEXAS CREEK SUITE /\(EARLY JURASSIC) / \ 0 1UAS CREEK BAlHOUTH -1.5'"I 40 50 60 70 80 % Si02 .. TEXAS CREEK GRANODIORITE $ PREMIER PORPHYRY DIKES A HYDER PLUTONICSUITE Figure 20. Modal compositions of plutonic rocks from studies by Buddington (1929), Smith(1977) and Grove Figure 21. Major element data from plutonic rocks plot- (1986), replotted on the discriminant diagram of Streckeisen ted on discriminant diagrams of Cox et ai. (1979) and (1975). Brown (1982). 38 “Silica contents. . , are the best single discriminants for andare chemically similar to.modern andesites of the distinguishingandesites from basaltsor dacites, they Chilean Andes. Premier Porphyry rocks are high-potassium, increase regularly relative to various differentiation indices high-silica andesites. Felsic volcanic rocks of the Mount and they have moreimmediacy to most people than do Dilworth Formation are low-silica dacites. combinational parameters.” - James B. Gill, 1981 The subaerial volcanoes of the Stewart camp formed ]Histograms of total weight per cent silica(Figure 17) are along the central axis of a volcanic arc and were established simple but effective diagrams that clearly separate the dif- on crustapproximately 30 kilometres thick. The ferent lithological groups and show the progressive, more topographic setting envisioned is an arc-island chain of fekic evolution up-section. major islands and isolated subaerial volcanoes (similar to The new I.U.G.S. classificationscheme (Figure 18) Indonesian examples or the eastern Aleutians), resting on shows that the “basalt” and “andesite” field terms are thickened or “continental” crust. appropriate. Stewart andesites are dominantly high-silica andesites. Both the total alkali - silica plot (Figure 18) and INTRUSIVEROCKS the. silica histogram (Figure 17) showthat Premier Porphyry There are scores of modal composition determinations for snbvolcanicintrusive rocks are high-silica andesites. mediumto coarse-grained plutonic rocks of the Stewart Finally, samples from the Mount Dilworth Formation plot camp, but major element chemical analyses are limited to as low-silica dacite, rather than rhyolite as concluded by 13 samples (Table 5). Grove (1971), Galley (1981) and Brown (1987). No modal composition determinations were undertaken Composite Harker silica variation diagrams have been in this study. Earlier data from Buddington (1929), Smith plottedfor four main stratigraphicdivisions (Alldrick, (1977) and Grove (1986) are replotted here (Figure 20) 1991) and correlate well with predicted patterns for cal- usingStreckeisen’s (1975) nomenclature. Texas Creek calkaline orogenic andesites (Gill, 1981). TheTiO, analyses rocks range from granodiorite to monzodiorite and quartz are almost all less than 1.2 per cent, indicating a convergent diorite whereas the Hyder suite ranges from granite (syeno- margin, island-arc environment (Gill, 1981). granite) to tonalite and is slightly richer in quartz and alkali Application of the tectonic environment discriminant feldspar. Due to the large area of overlap in modal composi- plot of Mullen (1983) in Figure 19 classifies most of the tions, a Streckeisen diagram alone would not be a reliable Stewart samples as calcalkaline rocks from a convergent discriminant plot for plutonic rocks in this area. margin, island-arc environment. Major element analyses from the Texas Creek Plutonic Race element studies indicate that the rocks are calc- Suite and from the Hyder Plutonic Suite in the Alice Arm alkaline,high-potassium, high-silica, orogenic andesites area are plotted on discriminant diagrams from Cox et al. (Brown, 1987). Chondrite normalization diagrams show (1979) and Brown (1982) in Figure 21. All plots clearly patterns consistent with those of the convergent plate mar- show that the Texas Creek batholith, Summit Lake stock gin suites (Erdman, 1985; Brown, 1987). and Premier Porphyry dikes are chemically similar and In conclusion, Hazelton Group volcanic rocks are a sub- likely comagmatic; the wider scatter of the dike analyses is alkaline, potassium-rich, iron-rich(?), calcalkaline suite that attributed to alteration. The more siliceous rocks of the ranges in composition from basalt through andesite to dacite Hyder plutonic suite are chemically distinct. over a stratigraphic thickness of 5 kilometres. This com- Rocks of the TexasCreek suiteare subalkaline, calc- positional change is paralleled by a progressive change in alkaline, high-potash monzonites to diorites that are chemi- colourfrom dark olive-green tolight greyand by pro- cally identical and probably comagmatic with Hazelton gressiveevolution of associatedphenocrysts. Augite- Group volcanic rocks. Rocks of the Hyder suite are sub- porphyritic basalts are subalkaline. Intermediate volcanic alkaline, calcalkaline granites to granodiorites and have no rocks are medium to high-potassium, high-silica andesites preserved extrusive equivalents. 39 CHAPTER 4 - STRUCTURE, METAMORPHISM, ALTERATION AND GEOCHRONOLOGY ~~ ~ ~ STRUCTURE FAULTS :Rocks in the study area display a variety of fabric ele- Faults are abundant on both local and regional scales, but ments and structures on all scales. The map area lies in a there are unresolved questions about absolute and relative single structural domain or sub-area. All rocks have been ages of displacement. Small-scale structures are ubiquitous suhjected to the same series of stress regimes,although and create difficulties in locating and correlating offset different rock types have deformed differently. Structural mineralized zones and in estimating dilution factors for ore elements include: reserve calculations. These brittle fractures are preserved as narrow fault breccias or pinching and swelling bands of I* primary bedding (So) measured in sedimentary rocks, gouge up to 30 centimetres thick. felsic volcanic rocks and rare sedimentary intervals in Major faults are separated into five groups: massive andesitic sequences, 0 regional-scale north-striking, subvertical shears I* northwest-trending folds (F,) that vary from open in northerly striking, west-dipping shears volcanic rocks, to tight to isoclinal in turbidites, southeast to northeast-striking “cross structures” that ’* minor axial-planar cleavage (SI)related to small, tight cut the northerly structural grain folds formed during regional-scale folding, dtcollement surfaces or bedding-plane slips that are ’* west-dipping foliation (FJ of brittle to ductile origin present near the base of the Salmon River Formation ’* west-plunginglineations (L3) andgeometrically mylonite zones. relatedextensional quartz veins and joints (S,) Regional-scale faults form major topographiclinea- (Brown, 1987), ments .that control drainage.These northerly to north- northeasterly striking, subvertical to steeply west-dipping, ,* southeastward-striking, subvertical ductile shear zones ductile to brittle faults include the Long Lake, Fish Creek, (FA SkookumCreek, Salmon River and Cascade Creek brittle faults of many scales, orientations and ages. structures. Examples of map-scale, northerly striking moderately west-dippingnormal and reverse faults are the Hams FOLDS Creek, Union Creek and Mineral Gulchfaults, and the series Folding is the dominant structural feature of area. The of parallel shears aroundand south of the Silver Butte main fold structure is a northerly trending regional-scale property. These structures probably originated as ductile, fold system of en echelon synclines. Individual synclines contractional reverse faults, and were reactivated as brittle trend north-northwest with steeply dipping to vertical axial fractures during later extensional episodes. planes. Each syncline is an open to tightly folded, doubly Easterly “crossstructures” arebrittle, subvertical plunging canoe-shaped structure. Volcanic and sedimentary faults that have strong, butnarrow, foliation envelopes. rocks of the Hazelton Group are deformed into open cylin- Theytrend from northeast to southeastand have lateral drical folds. Sedimentaryrocks of the overlyingSalmon offsets up to a kilometre. Examples are: theEast Gold, RiverFormation occupy the synclinal coresand display Millsite, Moms Summit, Dumas Creek, Windy Point and disharmonictight to isoclinal folds on many scales, Hercules faults. Bedding-plane faults or dkollement surfaces formed where Dkollement surfaces or bedding-plane slips are cam- ductility contrasts betweenthe siltstones andunderlying mon around the perimeter of the Salmon River Formation felsic volcanic rocks caused failure. outcrop area. These slips indicate detachment during fold- Intermediate-scale minor folds, tens ofmetres wide, ing near the contact with underlying dacitic strata of lower havebeen noted in several areas and are related to the ductility. folmation of the major fold system. Outcrop-scale minor Intrusion of the northwesterly striking dike swarms follds haveonly beennoted in siltstoneunits, and are was accompanied by at least 1.5 kilometres of northeast- grouped into two classes: ward extension. Many dikes have sharp, planar, brittle wall- rock contacts. 0 Strongly contorted minor folds are varieties of drag folds, and can be found whereversiltstone beds are cut Buddington(1929, p.16) first reported east-striking by faults or intruded by major dikes. “gneissic structures” or mylnnites in granodiorites and noted that they are parallel to strong foliation zones in the 0 Some outcrop-scaleminor folds within the Salmon greenstonecountry rock. Discretemylonite bands, a few River Formation are disharmonic space accommoda- metres wide at most, have been identified from the south- tions of plastically deformed siltstones that maintain ernmost exposures of the Texas Creek batholith to as far axial planar continuity with the regional fold pattern. north as CooperCreek. At the Silbak Premier mine, 41 mylonite zones are exposed in the disrupted, clast-rich, In outcrops of andesite tuff near the junction of the Silbak handed sulphide zone at the 2-level portal and in bedrock Premier and Big Missouri mine roads, intense foliation is trenches near the glory bole. There areat least three orienta- coplanar with a broad, multiple fault zone mapped in the tions to mylonite zones in the map area: 6-level adit at the Silbak Premier mine (P. Wodjak, personal 0 North-striking subvertical faults like the Long Lake communication, 1985), which is probably the Slate Moun- fault (Brown, 1987) and the Fish Creek fault (Smith, tain fault. 1977). West-dipping foliation is emphasized by flattened angular North-striking,west-dipping fault zones commonly volcanic fragments in the andesitic tuffs and similar flat- contain ductily deformed, flattened clasts. tened cobbles in the conglomerates. These flattened clasts Southeast-striking, steeply northeast-dipping mylon- indicate that many of the faults were ductile in nature and record east-west compression at the relatively elevated tem- ites deform and offset Jurassic ore at the Silbak Pre- peratures needed to producesemiductile to ductiledeforma- mier mine and provide the locus for the Tertiary ore veins at the Riverside mine. tion. Contraction was probably accommodated by a series of ductile reverse faults. Subsequent Tertiary doming due to Moderately west-dipping, flattened angular clasts and batholith emplacement produced an extensional regime that rounded cobbles are common in outcrops and underground reactivated many of these faults, producing west-dipping, exposures throughout the southern halfof the map area; extensional, normal faults with relict ductile fabrics in their these exposures are all examples of ductile deformation. In wallrocks. the past, the orientation of flattened clastshas been recorded as bedding, but where continuous exposures are available, it is possible to walk out of the deformed zone into similar STRUCTURALHISTORY rock with equant angular or rounded clasts. Thestructural history of the area is summarized in The extensive zones of cataclasite illustrated by Grove Table 6. The undulating fold axes of the regional-scale folds (1971, 1986) and the single zoneindicated by Smith (1977) could not be located in the field or recognized in dozens of TABLE 6 thin sections from these areas. Grove's photomicrographs STRUCTURAL HISTORY OF THE STEWART DISTRICT are typical of foliated, sericitically altered, fine lapilli tuffs within the Unuk River Formation. The writer concludes that all the "cataclasites" of the Salmon River valley are actu- allymoderately to strongly foliated, moderately altered 0 ...... andesitic lapilli tuffs and crystal-lithic tuffs. FOLIATION Secondary foliation is present as rare axial-planar slaty cleavage, as schistosity, as flattened clasts in conglomerates or fragmental tuffs, and as gneissic or mylonitic fluxion structures described in the preceding section. In andesitic rocks of the Unuk River Formation the dominant feature is a north-strikingpenetrative foliation dipping moderately westward (30" to 70"W). It remains one of the most readily observed yet difficult to explain phenomena of the Stewart mining camp. In this report it is interpreted as coalesced, overlapping, or superposed foliation envelopes surrounding most fault zones in the district. Theseenvelopes extend into wallrocks adjacent to both brittle and ductile faults. Envel- opes associated with minor brittle faults arenarrower, a few tens of metres at most; those associated with major ductile faults may be well over 100 metres widewith foliation intensity becoming progressively stronger toward the core of the shear. Faults are so widespread that most outcropsin the region have some degree of foliation; but this mecha- nism for its development also explains the seemingly ran- dom, local areas of unfoliated, unstrained, massive tuffs that 185 have been encountered during mapping - there is no fault 190 zone nearby.In any. single outcrop, the most prominent foliation represents the largest or youngest faulting in the 210 area as foliation envelopes from younger faults overprint and obscure pre-existing envelopes. The foliation envelope around a fault may be ohvious 230 even though the recessively weathered fault itself ishidden. 42 TEMPERATURE (C) canhe attributed toinhomogeneous contraction along 100 nearly horizontal, east-northeast stressktrain axes. The sim- 200 300 400 500 600 700 ilar geometry of west-dipping ductile shear zones and folia- tion implies a common origin with the folds during east- west contraction. However, upright folds probably formed early, followed by sequential development of the foliation. Eocene dike emplacement was accompanied by sigoifi- cant northeast-southwest crustal extension. The north- stxiking pattern of microdiorite and lamprophyre dikes indi- cates minor east-west extension in Late Tertiary time. METAMORPHISM Regional metamorphic grade throughout the area is lower greenschist facies. Two independent lines of evidence indi- cate that temperatures reached 290°C at pressures of 450 m,-gapascals (4.5 kilobars) about 110 million years ago. The key mineral assemblages preserved in the rocks, and other diagnostic minerals that are conspicuous by their absence, are listed in Table7. Relevant mineral phase transi- tions are also listed. The pressure-temperature boundaries defined by these equations are plotted on Figure 22 and indicate that the rocks inthe region have been meta- morphosed tolower greenschist facies. Pressure was 4502150 megapascals (4500 bars 2 1500 bars); tempera- ture was 300”225”C. Figure 22. Metamorphicgrade in the Stewartmining camp(adapted from Heitanen, 1967, Winkler,1979, and From field relationships, the timing of metamorphism Cho and Liou, 1987). Numbers refer to bounding univariant and deformation must lie between deposition of the young- equilibria in Table7C. Z=Zeolite; BH=Biotite hornfels; est sedimentary rocks (Bajocian, 175 Ma) and the onset of HH=Hornblende hornfels; PH=Pyroxene hornfels; intrusionof the unmetamorphosedHyder Plutonic Suite P=Prehnite;GS=Greenschist; EA=Epidote amphibolite; (E:arly Eocene, 55 Ma). Microscopic textures indicate syn- A=Amphibolite;CG=Cordierite granulite; G=Granulite; de:formational mineral growth, indicating that the physical BS=Blueschist: E=Eclogite. de:formation and thermalmetamorphism that affected the regionwere contemporaneous. Pressure shadows around euhedral pyrite are filled by chlorite or by quartz with minor monly curved, indicatingrotation of the stress field, or sericite (Plate 18). Chlorite and quartz crystals are com- rotationof the rockwithin a stressfield, during mineral growth. One unexpected result of the geochronometry work in TABLE 7 thisstudy was determination of the age and maximum METAMORPHIC MINERALS AND PHASE TRANSITIONS temperature of the regional metamorphic event (see under “Geochronometry”, this chapter). Resetting of potassium- TABLE 7A: Metamorphic miner& identiBed. argon dates from biotite, sericite and feldspar indicates a Albite Leucoxene (?) thermal peak of 28Oo22O0C was reached 110210 million CarbonatdCalcite Prehnite (Brown, 1987) Chlorite Pyrite years ago. Epidote Sericite ALTERATION TABLE 7B Metamorphic minerals conspicuous by their absence in petrographic studies. Alteration is a conspicuous feature in hand samples of all Biotite (minor primary biotite altered to chlorite) andesitic rocks and someintrusive rocks inthe area. At Hornblende (abundant primary hornblende altered to chlorite) microscopic scale all rocks have alteration effects except Ca-plagioclase Glaucaphane some samples ofthe Hyder Plutonic Suite. This section Chlaritoid Magnetite Cordierite Pumpellyite summarizes the types of alteration assemblages so that they Garnet (locally present in small skam zones) can be readily compared, and indicates their extent, inter- preted age and process of formation. TABLE 7C: Bounding univariant equilibria for equations in Figure 22. “Aiteration” is usedin its broadest sense to include 1. chlorile+white mica+quanz=cordierite+phlogopite+fluid metamorphic and surface-weathering products, as some of 2. chloritetwhite mica+quanz=cordierit+biotile+AI,SiO~+fluid these features were mapped as hydrothermal alteration in 3. Fe-chlorite+quanztmagnetite=almandine garnet 4. chloritetalbite+uremolite=glaucaphane+clz+fluid the past. This arrangement also allows comparisonand 5. chlorite+zoisile+quanz+fluid=lawsonite+p~~p~llyite contrast of chloritic assemblages formed by regional meta- - morphism and geothermal processes. 43 TABLE 8 TABLE 9 CLASSIFICATION OF ALTERATION TYPES BY CLASSIFICATION OF ALTERATION TYPES MINERALOGY, INTENSITY AND EXTENT BY AGE AND GENETIC PROCESS KEV W=Weak: M=Moderate; S=Strong: I=lntense EARLY JURASSIC DIAGNOSTIC MINING 1. BROAD GEOTHERMAL PROCESSES MINERALS REGIONAL CAMP PROPERTY LOCAL At depth; strong chlorite + pyrite (>I5 km) (5-15 km) (0.1-5 km) ( 44 int:rudes the unit just downstream from the toe of Berendon Strong chloritic alteration is locallized along much of Glacier. the eastern contact of the Texas Creek batholith where the margin is crushedand crudely foliated. The alterationis ‘The pyritic rock has been extensively sampled for assay interpreted as synvolcanic geothermal propylitic alteration and is generally barren, but hosts the high-grade veins of the concentrated in a strongly permeable zone along the mar- Eat Gold mine near its northern end, justbefore the altera- gins of the pluton. The crushing and foliation are due to tion intensity begins to diminish.The alteration process and late-stage intrusion and to volume changes in the interior of chemistry are probablysimilar to structurallycontrolled the large pluton after the outer margin had crystallized. sericite-carbonate-pyrite zones to the south, but this zone Strong epidote alteration is in andesiticto de!;erves special attention because of its stratigraphic con- pervasive dacitic country rocks around the perimeter of Tertiary plu- trol and itsdemonstration of a directspatial association tons. Significant, but less extensive zones of epidote altera- between the Early Jurassic stock, sericitic alteration and a tion are also common around Tertiary dikes, and are distrib- high-grade gold deposit. uted along and around permeable faults or shear zones that have been cut by Tertiary plutons or dikes. Alteration along shears may extend more than1 kilometre from the intrusion. Alteration is typically fine to medium-grained disseminated epidote that is preferentially weathered from outcrop sur- faces, and is only evident in fresh rock and drill core. Close to plutons, lithic fragmentsmay be wholly replaced by epidote (Plate 20). Inboth intrusive and country rocks, crosscutting narrow veinlets of epidote and minor specular hematite are common within a few metres of the contact. Strong carbonate flooding is locally preserved within the broad propylitic alteration zones in the massive andesite tuffs. Hand samples are very pale green to off-white and have a bleached or ‘silicified’ appearance. The rock is quite soft however, and reacts vigorously with hydrochloric acid. Thin sections show groundmass, phenocrysts and fragments are flooded byup to 35 per cent fine-grained carbonate. This alteration is interpreted as remnants of early (shallow) hydrothermal carbonate flooding that have escaped subse- quent overprinting by later (deeper) chloride-rich brines as the volcanic edifice grew. Plate 20. Andesite lapilli tuff. Strongly altered with clasts Intense chlorite alteration is common in wallrocks of wholly replaced by yellowish green epidote. Exposure in many ore shoots. It is most evident in the deeper levels of SilveradoCreek, 400 metresbelow Silverado millsite, the Silbak Premier mine (Plate 21). At shallower levels this Mount Rainey. alteration is either absent or overprinted and obscured by Plate 21. Photomicrograph showing strong chlorite-carbonate alteration in groundmassof Premier Porphyry dike rock. Silbak Premier mine. Plane polarized light (see colour photo in back of report). 45 subsequent sericite-carbonate-pyrite alteration. This classic Hornfels has a characteristic red-brown to purplish colour propylitic alteration is accompaniedby up to 5 percent and isso fine grained that original bedding is well preserved medium to coarse-grained disseminated euhedral pyrite. It in most exposures.Biotite hornfels rarely extends more than is attributed to intense propylitzation along long-lived fluid 100to 200 metresfrom the intrusive contact. It is well channelways. The transition to the broader propylitic envel- exposed in the Molly B and Red Reef adits near Stewart, ope of regional greenschist metamorphism is gradational, and in the Skookum (Mountainview) adit southeast of the but the two differing intensities are readily distinguished in Riverside mine. hand sample. Very coarse grained disseminated pyrite is distributed Strong sericite-carbonate-pyrite alteration is a charac- over a narrowzone in the wallrocksof ore shoots and teristic feature of the wallrock at all the deposits in the Big subeconomic sulphide zones at the Silbak Premier mine, at Missouri area, and adjacent to ore shoots in the shallower many deposits along the Big Missouri Ridge, and at many levels of the Silbak Premier mine. The rocks are strongly other prospects and showings. Euhedralcrystals range up to bleachedto an off-whitecolor and have a silicified 1 centimetre across andhave strongly striated surfaces. appearance, although they are not silicified. Altered wall- Quartzinclusions indicate porphyroblastic growth. This rocks in the Big Missouri area show progressive alteration pyrite may be an integral part of the associated propyllitic of andesitic tuffs (Plate 22). Originalrock textures are and sericitic alteration in the wallrock,but has a more commonly obscured. The carbonate component is generally restricted distribution and may be a useful guide to ore more abundant than the white mica, but broken rock tends within a broader propylitic or sericitic zone. The pyrite is tofail along the micaceous foliation, creating the interpreted to 'have formed by hydrothermal circulation appearance of a strongly sericitized rock, sometimes termed around structural conduits in the stratovolcano. sericite schist. Alteration may extend up to 50 metres out- Small skarn zones have formed where limy beds are cut ward from ore zones and grades abruptlyinto broad propyli- by Tertiary plutons and dikes. They have limited extent, a tic alteration over distances of only a few metres (Plate 22). few tens of metres long and a few metres thick at most, This alteration preceeded, accompanied and followed sul- reflecting the thickness of the original limestone or limy phide deposition along long-lived or reactivated channel- siltstone that has beenmetasomatized. Zoning includes wayswithin the stratovolcano. Itis regarded as typical symmetricallylayered coarse-grainedgarnet-quartz- hydrothermal sericitic alteration introduced by hydrother- diopside-tremolite. Good exposures are in the Molly B adit, mal brines at moderate to shallow depths. This assemblage in the Red Reef adit and elsewhere in scattered outcrops on is a common feature in the wallrocksof epithermal ore the lower northwest slope of Mount Rainey. All skam zones deposits (Silberman and Berger, 1985). are enveloped by larger alteration zones of biotite hornfels. Dense mats of fine-grained biotite hornfels are common In summary, complex, scattered and locally overprinted in siltstone units cut by plutons ofthe Hyder Plutonic Suite. alteration records four processes that affected the rocks at Plate 22. Progressive sencite-carbonate-pyrite alteration(bleaching) of chloriticandesitic lapilli tuff in Dag0 Hill drill hole at depths of 34,36 and 37.5 metres respectively. Note variation in lapilli textures. Alteration has virtually obscured lapilli in the most altered samplewhich is cut by a blue-grey chalcedonyand sulphide veinlet. Note abrupt transitionfrom chloritic to sencite-carbonate bleaching in central core piece. Dag0 Hill deposit, Big Missouri area. 46 differenttimes, at differentintensities and indifferent .in Table 11. Uranium-lead dates are presented with analyti- volumes: cal data in Table 12. 41 Oxidation due to subaerial weathering DISCUSSION 41 Hydrothermal alteration and related mineralization due to a suhvolcanic, synvolcanic geothermal convection It washoped that the newpotassium-argon .analyses system in an andesitic stratovolcano would reveal the age of mineralization at the Silbak Premier mine, the Dag0 Hill deposit nearthe Big Missouri mine, and 41 Lower greenschist facies regional metamorphism the Indian mine. However,significantly different dates were a Contact metamorphism and metasomatism peripheral obtained from twosamples of altered wallrock froma single to plutons. diamond-drill hole at the Dag0 Hill deposit. These results indicate that a simple interpretation of the dates, as direct GECOCHRONOMETRY measurements of the age ofalteration and mineralization, is not valid. In addition, contrasting dates obtained by Smith Thisreport presents a revised interpretation of the (1977) for hornblende and biotite separates from samples of geologichistory of the Stewart areabased on new Texas Creek granodiorite prove that there has been argon potassium-argon dates and all previouslypublished geo- loss from at least some mineral phases in these rocks. chronological data from the district. Isotopic dates from the Minerals will lose argon gas from their crystal lattices Stewart mining camp are erratically distributed over a 186 whenheated to moderatetemperatures over geologically million year range, from Triassic to Oligocene (Table 10). short periodsof time (Armstrong, 1966; Dodson, 1973; Dates from samples ofthe same rockunit vary significantly. York, 1984). The lowesttemperature at whichminerals However, when this broad array of dates is combined with rapidly lose argon has been termed the “threshold tempera- fiehi observations, closure temperatures for argon loss in ture’’(Armstrong, 1966). “closure temperature” (Dodson. minerals, and the concept of the “metamorphic veil” (Arm- 1973), and“blocking temperature” (York, 1984) and is strong, 1966), a simple explanation emerges. largely dependent on mineral type and grain size. Pamsh Isotopic dating near Stewart has been reported by Smith and Roddick (1984) have compiled closure temperaturesfor (1977), Alldrick et ai., (1986), Alldrick et ai., (1987a) and mineral phases and mineral groups (Table 13). Brown (1987). Dates from Smith (1977) and Brown (1987) Armstrong (1966) argued that temperature increases dur- are listed in Table 10. New potassium-argon dates are listed ing regional metamorphismwould drive off argonfrom TABLE 10 DATES FROM THE STEWART MINING CAMP IN CHRONOLOGICAL ORDER Analytical Mineral Method *21i t-6 K-Ar * 202 ? 6 K-Ar 195.0 ? 2.0 U-Pb 194.8 ? 2.0 U-Pb 19:!.8 ? 2.0 U-Pb 189.2 f 2.2 U-Pb 186 2 6 K-Ar * 130 ? 3 K-AT * 108 ? 3 K-Ar 101 t- 3 K-Ar 89.0 t- 3.0 K-AT 87.2 t 3.0 K-Ar 8’1.9 2 2.8 K-Ar 78.5 t- 2.8 K-Ar c1.9.~~~ ? 2.3~~ K-Ar 548 t- 1.3 U-Pb * 511.8 ? 2.0 K-Ar 5:!.2 f 4.0 K-Ar * 51.6 2 2.0 K-Ar = 50.5 ? 2.0 K-Ar * 50.4 2 2.0 K-Ar * 45b.9 2 2.0 K-Ar 48.4 t- 1.7 K-Ar * 47.3 t- 1.0 K-Ar 45.2 t- 1.6 K-Ar * 44.8 2 1.5 K-Ar 4;!.7 f 1.5 K-Ar 2%2 t 1.0 K-Ar Dates from Smith (1977). recalculated with IUGS decay constants. 47 TABLE 11 POTASSIUM-ARGON DATES FROM THE STEWART MINING CAMP Lwation Samole No. (Minfile No.) ~~ Premier G.H.' Southeast G.H.' Premier sericiteWhole-rock, Silhak "m of 22.6246.38k0.09 5.301 93.1 89.0k3.0 Lat. 56-03.1' Premier glory hole flooded n=3 Long. 130"00.7'104B-54)(MI PM84-29-232" Whole-rock,Premierfrom samplesericite Core 87.2?3.05.189 4.92k0.0295.2 17.079 Lac. 56"03.1' glory hole area flooded n=3 Long. 130W.7' (MI104B-54) PM84-25-369" Core sample fromPremier K-feldspar, 81.922.8from 4.870 94.3quartz 37.26811.44k0.03 glory hole area vein Lat.area 56O03.1' hole glory n=3 Long. 130"00.7'104B-54)(MI 81-58-25" HillDag0 zone, Whole-rock, sericite 4.13k0.07 16.681 89.8 6.038 101 23 Lat. 56O06.4' Big Missou" camp flooded n=3 Long.130'00.7' (MI IMB-45) 81-58-68" Whole-rock,zone, Hill Dagosericite 4.5220.03 . 14.087 86.0 4.659 78.5k2.8' Lat. 56'06.4' Big Missouri camp flooded n=3 Long. 130"00.7'104B-45)(MI IM-10' Galena Cuts zone, Wholorock,sericite 87.4 9.5425.6810.05 2.511 42.7k1.5 Lat. 56'04.6' Indianmine (MI 104B-31) flooded n=5 Long. 130'01.9' L-2' Blueberry vein Blueberry L-2' 90.9 Hornblende, 6.433 from 0.84710.017 11.359 186 k6 Lal. 5674.8' (MI 304B-133) lamprophyre dike n=5 Long. 130"04.3' A84-1-8' Roadside quarry On CreekBitterBiotite, 6.75k0.06 12.878 79.7 2.852 48.421.7 Lat. 56O02.0' StewanHwy. granodiorite n=5 Long. 129"55.0' DB-84-25' Trench 2190 at PremierWhole-rock K-feldspar 5.92_t0.05 14.721 82.2 3.717 62.9e2.3 Lat. 56-03.3' glory hole flooded n=2 Long. 130"00.5' B-383*Roadcut 2.0 km south of Biotite,Texas Creek 4.1720.02 7.416 84.4 2.659 45.221.6 Lat. 55-59.3, Riversidemine granodiorite n=2 Long. 130"03.8' AT84-27-52 Roadcut on eastBiotite,sideof lamprophyre dike 7.22k0.01 Lat. Lake56-04.4' Indian 68.0 n=2 7.133 1.477 25.221.0 Lom.~~~~ 130'02~4' ~~~ ~~. I K analyses by P.F. Ralph and B. Bhagwanani. Ministry of Energy, Mines and Petroleum R~SOUIC~S:D. Alidrick samples. K analyses by K. Scot, The Univmily of British Columbia: D. Brown (1987) samples. All Ar analyses by J.E. Harakal. The University of British Columbia. TABLE 12 URANIUM-LEAD ZIRCON DATES FROM THE STEWART MINING CAMP A84-I(13O005'55V 56"IQKrV) nm, ISO-215pm 16.3 459 13.6 8841 0.03035i16 0.2W2ill 0.M998i W 192.821.0 1929t0.9 194.2i 3.3 192.822.0 IW.432250E623232ON] m, <45pm 1.3 1908 55.6 8473 0.02952216 0.2041211 0.050141 09 187.5i1.0 IRX.620.9 201.42 4.0 m, <45pm 1.5 887 26.2 M99 0.02962i16 0.2048i11 0.050152 13 188.211.0 189.2109 ZM.02 5.8 AM-2(130"03'13'W 56'05'28'W) nm. >ISOpm 14.2 349 10.2 8110 0.M979217 0.2049i11 0.049902 06 189.2i1.1 189.320.9 190.3i 2.8 189.2i2.2 lW43MWE 62iM75Nl m. <75pm 0.3 898 27.8 1278 0.03056i17 0.2101273 0,049872162 194.ltl.l 193.726.1 189.1276.0 A84.3 (130'03'13"W56"OS'28"N)A84.3 om, >ISO&m 10.5 596 18.6 1839 0.03062tl7 0.2i13215 O.OSW6t 25 194.4i1.0 194.711.3 197.1211.7 195.0i2.0 [W.43444wE621M75NI nm, >ISOpm, abd 4.3 510 618915.6 0.03072i16 0.2117tll 0.049991 W 195.0CI.O 195.0i0.9 194.42 4.1 m, <45p 1.3 127238.36639 0.03034i16 0.2078ill 0.050161 W 190.8i1.0 191.7i0.9 202.61 4.1 ~84.5 (l3O0CQ'5VW56V3'Om) nm. >l5Opm 5.3 343 10.841081 0.03020i16 0.2WOi12 0.050181I7 191.8il.O 192.7i1.0 203.527.6194.8i2.0 1094367fdE6212240N] nm. >liOpm, abd 186411.7 378 2.9 0.03067216 0.2120t13 0.05013i 19 194.821.0 195.2i1.1 201.2i 8.9 AT-34-31 (13o"ol'22"W56V3'OYNj nm. >l5Opm 5.0 362.2 4.14 629 0.010M206 0.0102205 0.04789t 02 68.220.4 68.920.5 931t10.1 54.821.3 1094363WE 621213ONl m. <751rm 1.0 431.2 4.34 226 0.W853112 0.0554t14 O.M7!3+93 54820.8 54.8tl.355.6f4.0 I All analyspr by I.K. Monensen, Geological Survey of Canada, Ottawa. Dam provided by R.G. Andenon, Geological Survey of Canada, Vancouver. 3 The emr~apply to the last digits of the atomic ratios. 4 All ermn shown are lo ems,excepl for 20 emon in final column. lsoropic comporition of blank 61417.75, 7/4:15.5, 81437.3. isotopic composition of common lead is based on the Stacey and Kramer (1975) model: 6/4=11.152. 7/4=12.998. 8/4=31.23 at 3.7 Ga with 238UPPb=9.74, 2>2WMPb=37.19. Decay con slant^: 6/4=0.155125. 7/4=0.98485, 8/4=137.88. 48 r 4. PENETRAllVEDEFORMATION; -DUCTILESTRAIN BRlmE DEFORMATION: LARG E-SCALE CHEVRON FOLDS, CHEVRON LARGE-SCALE KINKS,- FAULTS TEMPERATURETHRESHOLD OF DIFFUSIVEARGON LOSS FROMMINERALS > K-Ar tDATE GEOLOGIC TIME .. 3. Zircon closureZircon GREENSCHISTLOWERFACIES METAMORPHISM AND 5 800 *C LARGE-SCALE FOLDING 600 - : RANGE OF HORNBLENDE 5- CLOSURETEMPERATURE -~ - - 530% MEDIAN 500 - - 20 a400-- ‘ W II: RANGE OF BIOTITE 3 CLOSURETEMPERATURE 5 300- W L 200 - 100 - oh-+; 4 * 4 K-Ar date K-Ar date K-Ar date K-Ar date > from TIMEGEOLOGIC from from from TIMEGEOLOGIC Hornblende K-feldspar sericite finecome biotite Figure 23. Relationships between temperature, deformation and argon loss versus geologic time, for potassium. argon samples. A. Relationships between temperature, deformation and argon loss from minerals in the metamorphic interior of an orogenic belt (from Armstrong, 1966). B. Relationships between temperature, deformation and argon loss for minerals dated in the Stewart mining camp. 49 TABLE 13 greenschist facies temperatures on potassium-argon dates CLOSURE TEMPERATURES FOR ARGON LOSS IN MINERALS for a variety of minerals. Mineral Closure Temperature INTERPRETATION Hornblende ...... 530” 5 40°C Muscovite ...... -350°C Biotite ...... 280” * 40°C Radiometric dates are displayed on Figure 24. This dia- K-feldspar ...... 130” + 15°C gram includes uranium-lead dates from zircons which are Microcline ...... -110°C estimatedto have closuretemperatures of greater than (From Parrish and Roddick 1984) 500°C and thus are not susceptible to thermal resetting at greenschist facies temperatures. The “date” of a mineral represents the last time the mineral cooled through its clo- sure temperature, whether that temperature drop resulted many minerals whose potassium-argon dates would then be from original cooling of an igneous magma or from cooling reset to record only the time of the final cooling of the after a metamorphic event. orogen. If thetemperatures were high enough, all potassium-argondates relating to the pre and syn- The broad band on Figure 24 represents the interpreted metamorphic history of the rocks would be lost or degraded thermal history of the Stewart mining camp. The fineblack to younger values. Thisconcept of a “metamorphic veil” is lines represent the interpreted thermal history of discrete igneous bodies such dikes and stocks. The cooling curve illustrated schematically fora high-grade region in Fig- as after each thermal peak is drawn through the data points, but ure 23a. Regional metamorphic grade in the Stewart area thethermal peak itselfrepresents the probable age of was lower greenschist facies indicating a thermal peak of emplacement. The temperature rise to each thqrmal peak, 300°C. From the closure temperatures listed in Table 13, and details of any prior, cooler interval, cannot be exactly only certain mineral groups should have argon loss during reconstructed. For igneous bodies the temperature rise can lower greenschist facies metamorphism. Figure 23b shows a be considered virtually instantaneous - a veltical line, hut schematicdiagram which predictsthe effects of lower for the regional metamorphic event the temperature rise is ~~ ~~~~ LATE TRIASSIC Island ArcVolcanism Texas Creak Granodiorite EARLY JURASSIC Texas Creek Granodiorite, 11 Late Maamatic Phase: Texas Creek Pomhvw Phase 220 200 180 160 1 40 120 100 80 60 40 20 11 GEOLOGIC TIME (Ma) Figure 24. Igneous, metamorphic and thermal history of the Stewart district is revealed in this plot of isotopic dates versus closure temperatures for the minerals dated. All data are listed in Tables 10, 11 and 12. 50 hidden behind the metamorphic veil and must be hypotheti- developed pressure shadows in thin section (Plate 18), also cal .- a dashed grey band. indicating that the mineralization and alteration predate Biotite separates from the Texas Creek granodiorite are metamorphism. The age of ore deposition at the Big Mis- clearly reset because their potassium-argon dates do not souri, Silbak Premier, Silver Butte, Scottie Gold and similar makh those for hornblende separates from the same sam- deposits is estimated at 185 Ma (Chapter 4). ples(Table IO). Also, the potassium-argon datesfor the reset biotites differ by 22 million years even though the CONCLUSIONS dates for the two hornblende separates lie within analytical error of each other. This is interpreted as the result of only The new data and interpretations presented in this report partial argon loss from the older biotite (Sample 3s-008 of allow significant revisions to the geologic history of the Smith, 1977). Partial argon loss occurs either when the district (Table 14). When isotopic dates are plotted against thermal peak is shortlived, such as country rock intruded by mineral closure temperatures, a simple thermal history can a narrow dike, or when it barely reaches a temperature equal be deduced: to the closure temperature for the mineral. Coarse-grained igneous biotites are estimated to have closure temperatures 1. Late Triassic to Early Jurassic volcanism and coeval of :!40"C to 320°Cand, as the thermal peak of regional subvolcanic intrusion (211 to 190 Ma) was followed metamorphism was probablynot a short-lived event, the by dike emplacement (190 to 185 Ma) and then by temperatures during regional metamorphism are inferred to turbidite deposition (Toarcian to Callovian, 185 to 160 have reached, hut not exceeded 300°C. It is also possible Ma). that the two biotites had slightly different closure tempera- 2. Moderate deformation associated with lower green- tures, orthat the maximum temperature during regional schist facies metamorphism reached its thermal peak metamorphism varied between the sample locations, which ahout 110+10 Ma. were 3 kilometres apart (Smith, 1977). A third biotite sepa- rate from the Texas Creek granodiorite, sample B-383 in 3. Stocksand dikes of the CoastPlutonic Complex Table 11, hasbeen dated at45.2 Ma. This sample was intruded the deformed rocks in early to middle Eocene coll,:cted near the northern contact of the Hyder stock and time, 55 to45 Ma, followed by a 20 million year the potassium-argon ratio of the biotite has been thermally period of aplite, microdiorite and biotite lamprophyre reset by the Tertiary intrusion. dike emplacement. Two datesfor sericite-rich altered dike-rock from the Silbak Premier mine, 87.2 and 89.0 Ma, fall within analyti- TABLE 14 cal tcrror of each other and are considered the mostrepresen- GEOLOGIC HISTORY OF THE STEWART MINING CAMP tativevalues for thermally reset sericite dates. The potassium feldspardate from the SilbakPremier mine, Age (Ma) Event 82 Ma, gives a reasonable additional control for the cooling 35-25Emplacement of biotite lamprophyre dikes. curve after regional metamorphism. 45-35Emplacement of microdiorite or 'andesite'dikes along NNW The dates for two sericite-rich altered andesite samples trend, locally deflected by biotite granodiorite dikes. from a single drill-hole at Dag0 Hill are different, 78.5 Ma 48-43Formation of argentiferous galena-sphalerite-freibergite vein deposits and spatially associated MoS, and WO, deposits. and101 Ma. Thin section study of these two samples 55-45Intrusion of Hyder, Boundary, Davis River, Bitter Creek and reveals that the deeper core sample, which yields the youn- MineralHill stocks of theCoast Range batholith. Biotite ger date,is composed of sericite and carbonate-altered granodiorite to biotite quartz monzonite.Continuing dike andesitic ash tuff. The shallower sample is similarly altered intrusion. andesitic crystal tuff containing large laths of hornblende 55 Crustalextension and intrusion of major WNW-trending bio- lite granodiorite dike swarms marked onset of emplacement that have been extensively altered to coarse sericite (All- of stocks at depth. drick et aL, 1987a). The closure temperature for hornblende 110-90Lower meenschist facies regional metamorphism reaches a is well above the thermal peakreached during regional thermaipeak. Moderate deiormation along north-trending metamorphism (Figure23) so the remnant hornblende fold axes. Major folds and slaty cleavage formed. 190-160Marine transgression, flysch sedimentation with minor inua- would probably retain some of the radiogenic argon gener- formational conglomerates (Unit 4). Relative quiescence. atedl since its original magmatic crystallization. The older 190-185Waning magmatic activity marked by emplacement of date: is thus attributed to a mixture ofargon from older hornblende lamprophyre dikes at depth. (Jurassic) hornblende andyounger (reset to Cretaceous) -190 Subaerial felsic volcanism (Unit 3). Emplacement of dikes at depth.Formation of gold-silver vein and breccia deposits. sericite. Depositionof barren- pyrite around fumarolic centres at Theformation of theDag0 Hill and Silbak Premier surface. deposits must predate the 110+10 Ma regionalmeta- 195-190Deposition of subaerialepiclastic sediments and interbedded andesitic to dacitic tuffs and flows (Unit2). (Ernplacement of morphic event. Black stylolites of insoluble residue are minor dikes at depth?) common within the coarsely crystalline carbonate gangue at 195Intrusion of porphyryphase of Texas Creekgranodiorite, -vera1 mineral deposits in the Big Missouri camp. These Premier Porphyry dikes, and extrusion of Premier Porphyry stylolites are pressure-solution features that indicate that the flows and tuff breccias. deposits predated regional deformation. Fine to medium- 215-195Andesitic volcanic activity (Unit I), predominantlysubaerial with two periods of marine transgression; coeval intrusion of grained euhedral pyrite crystals associated with the altera- main phase of Texas Creek granodiorite tion envelopes at the Big Missouri deposits exhibit well- 51 SUMMARY GEOLOGIC HISTORY end of volcanism was followed by subsidence of the snb- aerialvolcanic arc. Sedimentation was initially gritty, This section highlights the tectonic processes that have shallow-water fossiliferous limestones at the base of the affected the Stewart camp, by combining elements of the Salmon River Formation, followed by a thick accumulation geologic history of the study area, established in this report, of carbonaceous turbidites and wackes. with the tectonic evolution of Stikinia (Armstrong, 1988). As sedimentary rocks accumulated in the Bowser Basin Basaltic volcanic and sedimentary rocks of the Stuhini through the Late Jurassic, final amalgamation of the two Group were laid down on a metamorphosed platform of superterranes was completed to the west of Stikinia. From Stikine assemblage rocks. This was the first phase in the 150 to 125 Ma there is a lullin plutonism and volcanism and evolution of a single LateTriassic to Early Jurassic arc that region-wide gaps in the stratigraphic record that indicate a evolvedfrom basaltic flows (Stuhini Group) through period of tectonic and magmatic quiescence and widespread andesiticvolcanism (Unuk RiverFormation) to dacitic emergence and erosion. pyroclastics (Betty Creek and Mount Dilworth formations). Each volcanic phase was accompanied by emplacement of In the Early Cretaceous an. eastward-descending subduc- subvolcanic intrusions that are preserved as suites of pro- tion zone was established well west of Stikinia. The arc was gressively shallower, more siliceous plutons. Compositional fully developed by the mid-Cretaceous (I 10 to 90Ma) when evolution over this 40 million year period is attributed to magmatism peaked. Accompanying tectonism was charac- progressive thickening of the cmst. terized by greenschist faces regional metamorphism, east- Early Jurassic volcanism was predominantly subaerial. northeastcontraction, and deformation that produced a Intervolcanicperiods were marked by subsidence,sub- north-northwest-trendingundulating fold system. Folds were overprinted by east-verging ductile reverse faults that mergence and deposition of turbidites. Re-emergence of eachnew volcanic edifice was due to swelling over began in the Stewart area and propagated eastward across recharged magma chambers and to aggradation of the vol- the Bowser Basin. cano above sea level. Topographically, the volcano was part Transpression related to oblique subduction during the of asemicontinuous chain of largeand small volcanic Late Cretaceous (90 to 70 Ma) may have initiated large, islands, similar to the central and eastern Aleutians and to north-striking fault structures such as the Long Lake fault. many examples in the southwest Pacific such as the New These faultsremained intermittently active into lateTertiary Ireland - Solomon Islands chain. time. Bulk chemistry and prominent hornblende phenocrysts of No significant magmatic activity or sedimentation was the Unuk River Formation andesites suggest these lavas recorded in Stikinia over the period 70 to 55 Ma, but the erupted along the main axis of an arc. A general northwest following 10 million years, 55 to 45 Ma, spans the final trend to Hazelton volcanic centres is suspected (Alldrick, major magmatic episode in the Canadian Cordillera. Plutons 1989). The arc was established over a Mesozoic subduction emplaced in the Stewart area includethe batholith, satellitic zonethat is poorlyconstrained, but probably dipped stocks and extensive dikeswarms of the Hyder suite. Intru- northeasterly. sion was preceded and accompanied by northeast extension. Volcanism terminated abruptly in Toarcian time (187 to Waning magmatism between 45 and 25 Ma was marked by 185 Ma) with the deposition of the thin, widespread, dacite a succession of aplite, microdiorite and lamprophyre dikes, pyroclastic blanket of the Mount Dilworth Formation. The and by reactivation of old faults. 52 CHAPTER 5 - MINERAL DEPOSITS rrhe Stewm mining campis abundantlymineralized, into groups of similar deposit types. The first impression with widely distributed, texturally and mineralogically var- when lookingat a mineral depositsdistribution map is of the ied, precious and base metal deposits. Past production data unusually large number of mineral occurrences in the 750 and ore reserves are listed in Table 15. square kilometre study area (Figure 11) and the wide dis- In the first section of this chapter, empiricalgeologic tribution of these showings. The geologic map shows no criteria are used to sort these numerous mineral occurrences clear stratigraphic or plutonic controls to distribution. If TABLE 15 MINE PRODUCTION AND ORE RESERVES IN THE STEWART MINING CAMP IT" Jaman 1. 19921 PNpPrtY Minfile No. Date Past RSW"ps AU Prcd"d10" (cateporyi dt - (t0""W) EAS T GOLD 104B GOLD -EAST 033 441939-1954 1207.00 313.00 4.8031.300.07 104B 034 1981-1985 197 522 16.50 522 SCOTTIE197 1981-1985 GOLD 034 104B 16.00 1985(U) 132 1985(U) OOO (g) 19.20 17.00 - 1990(U) 28 992(m) 18.51 SPIDER 104A 010 22.2 1925, 14.20 X 238.003.503.90 - .1911-1016...... 1987 449 1 576 (g) 27.43 2.26 -MARTHA ELLEN 104B 092 1915, 1950. 26 11.80 2 610.00 14.00 19.00 14.00SILVER 610.00 TIP 2 11.80 104B26 043 1950. 1915, _,",1051 1057 1956(U) 816 (g) 6.20 4.20970.30 4.80 -KRTHSTAR 146078 104B 47 1987 (8)20.57 4.29 104B 084 1987 s-1 084 104B I709 209 (g) 2.71 7.20 1990 304 1990 000 2.40 10.00 - 1991 800 OOO (SI 2.20 10.00 104B 086 1987 7 529 7 -CREEK 1987 086 104B (g) 116.23 2.40 -BlCi MISSOURI046104B 1938-1942943768 2.37 2.13 104B 045 1934, 1950 14 48.00 3 952.00 0.46 952.00 3 48.00 14 1950DAG0 1934,HILL 045 104B 0.12 1987 557 1987 141 (g)38.06 2.54 1988-1989 3x4 000 1.20 10.00 - 1991 150 OOO (g) 1.20 10.00 PROVINCE 1987 147 104B 286 734 (g)12.69 2.43 1990 33 300 33 1990 2.46 21.88 1 .50 - 1991 100 OOO (g) 20.00 SILVER BUTTESILVER 209 96 1991(U) 1048 150 (m) 9.91 0.323.8565.900.67 1991(U) 279 387 (g) 17.31 36.69 - 1991 102539 . 8.89 55.50 INDIAN 104B 03 4.40 104B -INDIAN119.70 3.40 X70 12 I 1952 1925. 5.50 SILBAK PREMIERSILBAK 0.20 0.66274.00 13.00 1048 714 OS4 276 1919-1953.4 (includes SEBAKWE104B IS3 1959-1968, and B.C. SILVER) 155104B 1989(0) 500 6 W(m) 2.16 80.23 199OfO) 3 388 900(mi67.73 2.51 1990iuj 851 mimj 7.50 34.84 1991(0) 945 OOO(m) 2.80 41.30 1988-19912.27 1 060 593 67.00 - 1992(U+O) 000900 4 (g) 20.002.00 1925, 1927, 26 437 2.89 102.10 3.90 0.13 0.12 0.13 3.90073 102.10 l04BRIVERSIDE 2.89 437 26 1927, 1925, - 1941-1950 -DUNWELL 1926-1941 052103P 45 710 6.72 0.032.43224.401.83 UNITED EMPIRE 163103P 050 1925 2.10 ' 1 136.707.40 - 1934,1936 103P 085 1940, 1941 2.36 12.01 0.72 12.01 2.36 1941 1940, MOLLY- B085 103P 290 SILVERADO 13 103P 088 1927 3 662.40 PRO SPER ITY/ 103P 089PROSPERITY/ 103P 1922 27 268 1.00 0.10692.970.035.10 2 PORTER IDAHO 1924-1932, 1938,1939, 1947,1950, 1981 1989fU) X26 400 (e) 2.50 2.50 668.50 U= underground g=geological 0 =open pit rn = mineable 53 stratigraphy, plutonic associations or structural settings are bined with other geological data. The following interpreta- used to separate out 'groups' of deposits, deposits of dif- tions are derived in Figures 25 and 26 ferent ages are present within most of these divisions (Table 0 The position of the data clusters in Figures 25 and 26 16, in pocket). Deposit classifications based solely on ore indicates that all deposits sampled in this study are mineralogy, stratigraphic position, plutonic association or Phanerozoic, and probably post-Paleozoic. structural setting do not work in the Stewart mining camp 0 Comparing relative positions of the two data clusters and are probably not reliable anywhere in the region. with the progressive evolution of lead isotope ratios Examples from each deposit-type group aredescribed in shows that the galena from deposits in Cluster 2 are the second section of this chapter. These aregood examples significantlyyounger than galena from Cluster 1 of thedifferent deposit types, but they should not be deposits.This relative age relationship isconsistent regarded as "type deposits". Comparison of the features of with interpretations based on geological evidence. the Big Missouri and Silbak Premier deposits shows that 0 The two clusters of data shown in Figure 26 clearly there can be variations between deposits within one deposit- define two separate metallogenic events in the Stewart type group. area. 0 The tightness of the two data clusters indicates that LEAD ISOTOPE STUDIES each ore-forming event was a short-lived episode in geologic time, drawing homogeneous lead from a uni- In 1986, asuite of galenasamples, representing ten form or well-mixed or well-sampled source rock(s). deposits on eight properties, was submitted for lead isotope 0 Both data clusters represent deposits that are distr- analysis. The results of this work, reported in Alldrick et al., buted over a 30-kilometre strike length, thus the two (1987b), were so unexpected and definitive that additional metallogenic events that formed these deposits were samples were immediately processed. This section presents both regional-scale phenomena. galena lead isotope datafrom 21 mineral occurrences on 18 properties in the Stewart area. 0 Although the Indian and Silbak Premier mines are less than 2 kilometres apart, the data indicatethese deposits Occurrence locations ae shown on Figure 11. Table 17 formed at significantly different times, with the Indian presents a summary of the galena lead isotope data. The mine being the younger of the two. complete data set of 88 values is tabulated and plotted in The lead datasuppori theinterpretation of Alldrick (1991). isotope Jurassic and Tertiary metallogenic epochs based on field Small lead isotope data setsare useful for indicating studies and geochronology reported in Chapters 3,4 and 5. relativeage relationships among deposits, and between The two ore-forming episodes were not closely related deposits and hostrocks. Such 'fingerprint' interpretation of genetic processes. Specifically, Cluster 1 deposits formed lead isotope data is a simple but powerful tool when com- cogenetically with calcalkaline Hazelton Group volcanic TABLE 17 GALENA LEAD ISOTOPE DATA B.C. B.C. MINFILE LEADFILE Number Number BIG MISSOURI GROUP Creek GROUP104B-086 MISSOURI BIG 18.820 15.615 30415-006 1048-148 30415-002Calcite Cuts15.646 18.858 1048-002 Terminus 18.823 15.609 30415-007 104B-092 Manha Ellen 15.615 18.826 30415-AVG 104B-092 Hercules,30415-013 Dumas 15.612 18.734 304B-I47 Province 18.824 15.604 30415-AVG 104B-136 Province West 18.781 15.643 30415-005 104B-034 SCOTTIE GOLD 18.804 '15.608 30493-001 304B-054 SILBAK PREMIER GROUP Glory hole 18.836 15.617 30494-006 304B-053 Northern Lights 18.828 15.607 30494-AVG 304B-054 Z-Level pond 18.837 15.619 30494-AVG 104B-095 CONSOLJDATEDSILVER BUTTE Silver Bum 15.612 - 18.820 - 30495-AVG 18.82 15.62 CLUSTER AVERAGECLUSTER 15.62 18.82 103P-089 PROSPERITYPORTER IDAHO 19.121 15.622 30492-AVG 104A-010 SPIDER 19.085 15.609 30616-001 104B-095 CONSOLIDATED SILVER BUTTE Packerfraction 19.177 15.629 30720-001 103P-051 BAYVIEW 19.152 15.620 30765-AVG 103P-088 SILVERADO 15.640 19.159 30766-AVG 104B-369 START 19.134 15.638 30923-AVG 104B-031 INDIAN 15.623 19,155 30939-AVG 1030-001 JARVIS (HOWARD) 15.625 19.168 50055-AVG 104B-073 RIVERSIDE 19.176 15.639 50058-AVG ~ - 19.15 15.63 AVERAGECLUSTER 54 Clurler 2 Average (from Tabla 17) I, Modified from Godwinet a1. (1988). “i, , , , , , , , , I 10 9.1 IO 11 12 13 I4 15 16 ,, 18 18 206Pb/204Pb Figure 25. Growth curve for lead isotope evolution suggests relatively recent ages for the data in Table 17 and Figure 26. Lead evolution curves are for the models of Holmes (1946, 1947) and Houtermans (1946) (HHl and HH2), Stacey and Kramers (1975) (SK), and Godwin and Sinclair (1982) (SH). Ages are X109 years. Note that curveSK evolves from 3.7XIO9 years on curve HHI, and that curve SH departs from curve SK at lo91.887X years. 15.8 e 15.7 boN 2 I- % 15.6 15.5 Figure 26. Lead isotope plot of averaged galena lead isotope analyses for each mineral occurrence listedin Table 17. Inset shows plot of all galena lead isotope analyses. Lead evolution “growth curve” is from the model developed by Stacey and Kramers (1975). Ages arein millions of years. Analytical erroris 2u. Square symbols represent group means. A-Creek: B-Calcite Cuts; C-Terminus; D-Martha Ellen; E-Hercules; F-Province; G-Province West; H-Scottie Gold; I-Glow Hole; J-Northern Lights; K-2Level Portal; L-Silver Butte; 1-ProsperitylPorter Idaho; 2-Spider; 3-Packer Fraction; 4-Bayview; 5-Silverado; 6-Start; 7-Indian; 8-Jarvis; 9-Riverside. 55 rocks of the Stikinia Terrane about 190 million years ago. mined, all 186 remaining showings in the area could be Cluster 2 deposits are epigenetic veins related to Eocene categorized as Early Jurassic or Eocene deposits without intrusion of dominantly granodioritic plutons. The deposits resorting to additional lead isotope work. Analysis of these of Cluster 1 include both gold-silver-pyrrhotite veins, such features allowed separation of the 207 mineral occurrences asthe Scottie Gold deposit, and silver-gold-lead-zinc- into five groups with similar characteristics. The resulting copper deposits such as Silbak Premier orebodies. In con- sorted chart is presented as Table 16 (in pocket) and sum- trast, deposits of Cluster 2 are silver-lead-zinc veins charac- marized in Table 18. Features and genesis of deposits from terized by high silver grades and by spatially associated each of the five deposit-type groups are described in the molybdenum(Mountainview) or tungsten(Riverside) next section. occurrences. Empirical classification criteria do not indicate genetic Both data clusters are enriched in 206Pb relative to aver- processes but they do divide the wide variety of mineral age crustal growth curves such as the Stacey and Kramers deposits in the Stewart mining camp into only afew groups, (1975) model curve (Figure26), consequently the positions each with readily recognizable diagnostic features. Once of the ‘Jurassic’ and ‘Eocene’ clusters of data from this deposits had been sorted into groups with common charac- study are substantially at odds with ‘absolute’ ages indi- teristics, other more complex, poorly documented features cated on the Stacey andKramers (1975) model growth also proved to be diagnostic. For example, the style, inten- curve. This is an example of the local inadequacy of the sityand mineralogy of associatedalteration envelopes various world average lead isotopic growth curves for abso- seemed too variable; too erratic and too poorly documented lute dating of mineral deposits. The Stewart leads have to be useful diagnostic criteria during this study, but there evolved in a uranium-rich environment fora significant are characteristicalteration minerals or mineral assemblages portion of their recent history. associated with each of these deposit-type groups. For prospect classification, lead isotope analysis from galena in a small exposure or in a weakly mineralized vein EARLY JURASSIC DEPOSITS would indicate whether the mineral occurrence was related to the earlier gold-silver (?base metal) event or to the later Early Jurassic deposits have lead isotope ratios consistent s~~v~r~~e~d.z~nc(~mo~ybdenum~tungsten)mineralizing epi. with a Jurassic age; other geologic features of these deposits sode. the area, routine lead isotopeanalysis indicate a more specificlate Early Jurassic age. Some of would be a practical aid for exploration programs focused these deposits havebeen identified as Triassic, Jurassic, on specific metals. Themethod is an effective technique for Eocene and even Oligocene age deposits prior to this study, evaluating the commodity potential of a mineral showing or were not categorized. for setting exploration phorities on large claim groups that cover several varied mineral occurrences. GOLD-PYRRHOTITE VEINS: Lead isotope data from the Stewart mining camp do not SCOTTIE GOLD MINE provide absolute age dates for the formation of mineral The Scottie Gold mine lies on the east side of Moms deposits, but are consistent .with dates determined by other Summit Mountain at 1100 metres elevation (Figure 11). The methods. The formation of 21 varied mineral deposits can showings were first staked in 1928. Trenching, diamond be attributed to just mineralizing events: one in Early two drilling and several phases of underground development Jurassic time and one in Eocene time. Both metallogenic were canied out between 1931 and 1948. There was no epochswere brief, regional-scale phenomena. Deposits further physical work until the Northair Group optioned the from the younger mineralizing episode may be emplaced ground in 1978. Production started on October 1, 1981 and adjacent to or superposed on older deposits. continued until February 18, 1985. DEPOSIT CLASSIFICATION TABLE GEOLOGIC SETTING A variety of geological parameters were applied as classi- Ore zones are hosted in andesitic volcanic rocks near the ficationcriteria for the sorting of mineraloccurrences eastern edge of a large hornblende granodiorite stock. Host- (Table 16). The criteria were necessarily empirical - it was rocks are matrix-supported andesitic tuff breccias and lapilli essentialto first separate out groups of similar deposits tuffs with intercalated ash tuffs, volcanic sandstones and before an attempt was made to summarize the distinctive volcanic conglomerates of the Middle Andesite Member of characteristics of each group and to deduce processes of the Unuk River Formation. Massive tuffs vary from coarse formation for each deposit type. ash tuffs to fine-grained crystal-richtuffs, composed of plag- Twenty-one occurrences were firstsorted into two groups ioclase and plagioclase-pyroxene-hornblende phenocrysts. using lead isotope data as a “primary” diagnostic charac- Two kilometres northeast of the mine, thin-bedded silt- teristic, but the lead isotope signature is justone of several stones trend north, dip vertically and have tops to the east; distinctive geological features of the two deposit groups two kilometres west of the mine, several outcrops of thin- (Table 18). For these same 21 deposits, “secondary” diag- bedded wacke strike southeast, and dip steeply northeast nostic or characteristic featuresof the Jurassic and Tertiary with tops to the northeast. No bedding has been recognized depositgroups were identified (Tables 16 and 18). within the mine workings. Ultimately, 45 secondary diagnostic features were recogn- The mine sequence is intruded on the northwest by the ized. Once these other characteristic features were deter- Summit Lake stock, a coarse-grained equigranular to subtly 56 TABLE 18 DIAGNOSTIC FEATURESOF STEWART MINERAL DEPOSITS(From Table 16) DIAGNOSTIC DEPOSIT TYPES FEATURE :ATEGORIES EOCENE DEPOSITS JURASSIC DEPOSITS Sbarns Ag-Pb-Zn Au-Pyrrhotite Au-Ag-Base Metal Stratabound Veins Veins veins Pyritic Dacite PbPb Ratio 'Eocene' cluster 'Jurassic' cluster 'Jurassic' cluster h:Ag Ratio >1:2w, comb quam book SINCIUE callofom "WY growth zones (cockade) metamorphicoverprint metamorphic overprint massive veins rare vugs coarse quartz-calcite intergrowths chalcedony clasts ore clasts vein clasts vuggy abundant primary fluid inclusions chalcedonv veinlets metamorphicoverprint metamorphk overprint Alteration skam hornfels hornfels epidote epidote rare minor pyrite pyrite envelope envelopepyrite chlorite envelope broad chlorite envelope silica silica silica sericite K-feidspar carbonate suatabaund stratabound massive, tabular veins enstockworks, echelonveinveins (southeast strike, subvettical) breccia veins dike margin (granodiorite) dike margin (Premier Porphyry) disseminated disseminated metamorphicoverprint metamorphic overprint netamorphic overprint Salmon River Fm. Mount Dilwonh Fm Mount Dilwonh Fm. Betty Creek Fm. Premie; Porphyry Mbr. Middle And. Mbr. Middle And. Mbr. ?Lower Slst. Mbr. Lower Slst. Mbr. Hyder batholith Hyder batholith ? Boundary stock ?Boundary stock Ponland Canal dike swarm Boundan dike swarm granodiorite dikes Premier Porphyry dike Premier Porphyry dike Bluebeny dike Bluekny dike Summit Lake stock 51 A SECTION A’ SOUTHWEST NORTHEAST MINING LEVELS (FEET) 3600 - Figure 27. Geologic mapand cross-section through the Scottie Gold mine. Map shows the distribution of veins on surface. Country rock is massive andesite tuffs. 3000-level underground workings for reference. (Simplified from company plans.) 58 potassium feldspar porphyritic hornblende granodiorite. It OREBODIES crops out 500 metres to the west of the mine workings. The pluton has not beenintersected in drill holes or underground Ore zones on the Scottie Gold property are vein networks workings so theclosest approach to the ore zones is locallized within four complex, subparallel shear or fracture unknown, butis probably less than 500 metres.Contact zones (Figure 27). The vein networks are major structures relationships indicate relatively passive emplacement, but striking about 130" and dipping 75" to 80" northeast. The L, the pluton has produced adistinctive metasomatic alteration N andM Zones have a horizontal separation of about assemblage. Near the contact with the stock, andesites are 50 metres: the 0 Zone is roughly 1 IO metres farther to the bleached and impregnated with fine to very coarse grained northeast. The greatest oreproduction has come from accessoryhornblende (upto 3 cm long) andminor fine M Zone which is described here. Other zones had similar pyrite. Bleaching is due to carbonate3-sericite flooding. sulphide and gangue mineralogy and ore grades, but were Country rock and ore zones are cut by green microdiorite generally narrower with less complex structure. dikes and dark brown lamprophyre dikes of the Berendon M Zone is two parallel vein structures: the WestVein dike swarm. Lamprophyre dikes are spessartite with fresh, (Main Vein West, West Mine Vein, McLeod West Zone), fin.e hornblende phenocrysts and calcite-filled amygdules. and theEast Vein (Main Vein East, East MineVein, McLeod The Moms Summit fault is a regional-scale structure East Zone). These two major veins are connected by 'rungs' (F:igure 1 I) that trends north-northwest through the mine ofmany narrower veins, called the Sixties Veins (Fig- area between the 3000-level and 3600-level portals, dipping ure 28). Local flexures or rolls are common; the attitude of so:uthwest at 30" to 45". The fault lies east of the ore zones. the West Vein averages 130775"NE overall, but orientation Thlere isno consensusabout the absolutedisplacement in the 36-95 stope is 145"/63"NE. Sixties Veins trend and ac:ross this major fault. Specific geological features support dip 060"/8O0NW. The M zone extendsvertically at least 450 arguments for normal, reverse and dextral strike-slip move- metres from surface at 1280 metres elevation to the deepest ment. In plan, the relative sense of offset is dextral. Resolu- drill-hole intersections at 830 metres elevation. It is at least tion of this problem is an important keyto exploration along 150 metres long, but along its northwestern 100 metres, the its eastern side. Several small crosscutting faults mapped in Main Vein West is IO centimetres to 1 metre wide with only the mine workings do not significantly displace mineral- erratic gold values. The productive section of the M Zone is ization. roughly 40 metres long on every mine level. POSSiBLE FRACTURING MECHANISMS PROBABLEFRACTURING MECHANISM - - - - -L - - - 0 SHEAR COUPLE L ,Y .." . I ...... 1. Developmentof I//// ENVELOPE an extensionfracture .. (tensionfracture, J en echelon vein) """" 1"- wilhina shear zone lniilol growih of "Sixller' veins 01 mineralized extension fractures SHEAR COUPLE 2. Development of Riedel fractures SHEAR (conjugateshear lVELOPE fir*il-ord.r shear 10". fractures)within "" a shear zone 7 3. Development of ...... pinnateveins in antitheticconjugate shearfractures at WEST MAIN VEIN yT 60' tofault plane a Growih of East and West Main veins Figure 28. Possible and probable fracturing mechanisms controlling vein formation at Scottie Gold mine (modified from Hancock, 1972 and Hodgson, 1989). 59 West Vein is a simple, massive vein deposit with a moder- ward until it rolls away to a 110"-trend at the southeast end ate alteration envelope, whereas the East Vein ismore of the mine where gold values abruptly decrease and the complex, showing small-scale structural control and a more vein breaks up into horsetails and disappears. The northwest intense alteration envelope (Figure 28 and Plate 23). The limit of the West Vein has not been determined, hut the vein minor but mineralized Sixties Veins, connecting the two is at least 150 metres long. major veins,commonly constitute ore. There are enough The core of the vein is massive, fine-grained, distinctly differences among the West Vein, East Vein and the inter- pinkish to bronze-coloured, nonmagnetic pyrrhotite with connecting Sixties Veins that they are described separately. little or no associated pyrite. The only other component is The West Vein is the most massive, regular vein in the minor scattered knots or blebs of quartz, calcite and chlorite. mine (Plate 24), averaging 5 metres in width and locally This core zone ranges from 4 to 6 metres wide dong the reaching 7 metres. On every mine level, it trends southeast- productive part of the vein. The immediate margins of this Plate 23. Sheared and altered wallrock, Scottie Gold mine. A. Low reflectance view of strongly shearedwallrock shows rich pink tocoral manganese carbonate alteration (rhodochrosite). B. High reflectance view shows distribution of sulphide trains, mainly pyrite, within this sheared, altered wallrock (see colour photo in back of report). Plate 24. The M Vein (Main Vein West) in the back (roo0 of a drift along 3600-level, Scottie Gold mine. Note perfect symmetry of the massive pyrrhotite core and quartz-sulphide marginal zones. Pods, blebs and lenses of pyrrhotite are also present in foliated wallrock. Strapping is 15 centimetres wide. 60 pyrrhotite core consist of overprinted swarmsof quartz- tialdisseminations, blebs and seams offine-grained pyr- carbonate veins and veinlets hosting accessory to minor rhotite. All country rock between the Sixties Veins is varia- pyrrhotite,pyrite, sphalerite and galena with intervening bly altered as a result of overlapping alteration envelopes wallrock fragments or inclusions that are intensely altered to from adjacent veins. sericite,carbonate and chlorite. These marginal vein The abrupt mineralogical change between the pyrrhotite swarms are typically 0.5 to l metre wide on both sides of core and the symmetrical quartz-carbonate margins suggests the massive pyrrhotite core. Gold values within this mar- that these ore zones originated as composite veins. Subse- gind rock are erratic and only locally make ore. quent shearing and recrystallization has obscured any evi- The East Vein is narrower and more erratic than the West dence of a median suture and original fibrous growth. Ve:in and can be seen to pinch and swell, andbifurcate, Prior to this study, the form and distribution of the gold along its overall 130" trend,' parallel to the West Vein. In had not been determined; rare free gold required a hand lens detail this vein is two adjacent, parallel vein structures with for detection and was notedonly within the marginal quartz- screens of strongly altered wallrock sandwiched between carbonate vein zones. Gold valuesare distributed erratically them. Both veins makeore. Each consists ofa massive across and along all veins, even though veins are continuous sulphide core that is narrower but similar to the West Vein. and the pyrrhotitecores appear homogeneous. Whole stopes Thm: sulphide core iscomposed of roughly equal amounts of on the WestVein grade up to 50 grams goldper tonne, fin'e-grained pyrrhotite and pyrite, in contrast to the West locally exceeding 70 grams per tonne, but sections of the Vein which is dominantly pyrrhotite. Quartz-carbonate mar- East Vein and Sixties Veins may be either very high grade or gins of the East Vein are similar to the West Vein,hut wider, virtually barren in exposures that appear identical. Distrihu- and have veins and veinletspenetrating into thecountry rock. These quartz-carhonate vein swarms contain galena, sphalerite, pyrrhotite and pyrite with fragments of altered country rock. They are erratically gold bearing,locally constituting ore. The East Vein trends southeastward to outcrop. The intervening Sixties Veins connect the West and East veins. The Sixties Veinstrend 060" and dip 80" to the northwest. They are individually much thinner than either of the main veins, locally pinching down to 2 to 3 centimetres. However, they commonly make ore dueto consistently high grades of better than 70 grams per tonne gold. Sixties Veins are either miniature copies of the larger veins (massive pyrrhotite cores with quartz-carbonate-sulphide margins) or they consist only of quartz-carbonate gangue with substan- Plate 26. Photomicrograph of two gold grains within a chalcopyrite 'veinlet' filling a fracture through a pyrite grain, Scottie Gold mine. r, .. Plate 25. Photomicrographshowing typicalform and !size fordisseminated gold within massivepyrrhotite, Scottie Goldmine. Crossed polars show that gold blebs are locallized along pyrrhotite grain boundaries. Sample of M Plate 27. Photomicrographof gold grainwithin pyr- Zone ore from a working face that averaged 69 grams per rhotite 'veinlet' cutting large, round, fractured pyrite grain, tonne gold. Scottie Gold mine. 61 tion of silver values also appears to be random, hut silver ALTERATION was not routinely assayed. Andesitic volcanic rocks on the property have strong propylitic alteration with pervasive chlorite, minor epidote MINERALOGY and trace disseminated pyrite. Alteration intensity increases In order of abundance, opaque minerals include pyr- progressively within 10 metres of the ore zones. Pyrrhotite, rhotite, pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, pyrite and chalcopyrite are present as fine disseminations native gold, tennantite and rare chalcocite. and hairline-fracture coatings adjacent to the main mineral deposits and seem to be associated with the most abundant Pyrite commonly shows evidence of cracking, crushing chloritization. and shearing. Pyrrhotite, however, has totally annealed with roughly 120” triplejunctions (Plate 25). Cracks within Within the underground workings, wallrock alteration pyrite are filled by trace copper minerals (Plate 26) or by immediately adjacent to the quartz-carbonate vein margin is pyrrhotite (Plate 27). The chalcopyrite along these cracks abundantchlorite alternating with two lightercoloured represents crystalline growth along the fracture, the pyr- alteration types that have been described in hand samples as rhotite represents recrystallization along the walls of the silicification and as hematization producing a pink chert. crack that has turned a thin selvage of the host pyrite grain Thin sections show that the light, colourless alteration is into pyrrhotite. Pyrite and pyrrhotite also are present as dominantlysericite andcalcite with grains of crushed blebs and patches and as sheared streaksor strings of quartz.The pinkish alteration type (Plate 23) is entirely crushed grains in the quartz-carbonate marginal zones and in adjacent, intensely altered wallrock. Chalcopyrite is present in trace to minor amounts in both thepyrrhotite corezone andmarginal quartz-carbonate zone, but is more abundant in the core zone where it is distributed along fractures in pyrite (Plate 26), commonly with associated gold, and along grain boundaries in mas- sive, recrystallized and annealed pyrrhotite (Plate 25). Ten- nantite and tetrahedrite, distinguished by their respective greenish and bluish tints, have been noted in the core zone as late fracture fillings in pyrite, withass0ciate.d chalco- pyrite and gold. Sphalerite and galena arecommon in the quartz- carbonate envelope, but are virtually absent in the pyrrhotite core. Arsenopyrite is present locally within the pyrrhotite core as fine to coarse-grained euhedral, commonly cracked Figure 29. Genetic model for the Scottie Gold mine. crystals. Broken crystals areenveloped by pyrrhotite. Zones of arsenopyrite are not associated withthe highest gold composed of fine to coarse-grained carbonate aggregate that grades, but generally assay in the 15 to 20 grams gold per is free of hematite even under high magnification, and is tonne range. therefore probably rhodochrosite. Native gold is ubiquitous as fine blebs and grains in the Wallrock alterationranges from massive, unfoliated, pyrrhotitecore zone. Gold grains are typically 10 to alternating patches of chlorite, sericite-carbonate, and pink 40 micrometres in diameter, the largest grain observed mea- carbonate with scattered knots or blebs of sulphide aggre- sured 120 micrometres across. Gold is most commonly gate, to strongly foliated,banded segregations of alternating associated with chalcopyrite (Plate 26). Free gold is also green, white and pink alteration types with scattered streaks scattered throughout massive pyrrhotite (Plate 25). St is of sulphides. much less abundantand more erratically distributed in quartz-carbonate envelopes where it may he associated with GENESIS pyrrhotite-pyrite blebs or quartz-carbonate gangue. Gangue minerals include quartz, carbonate, sericite, chlo- Veins at the mine are locallized along complex, sub- rite, minor epidote and trace clinozoisite. The four most parallel shear or fracture zones. They have been called shear abundant gangue minerals display both massive and sheared veins, cymoid veins and loops, sigmoidal veins, extension forms, suggesting they were all deposited prior to shearing. veins, tension gashes and ladder veins. The zones have Major shear strnctures in the quartz-carbonate margin zone undergone post-ore ductileand brittle shearing that compli- and in the immediately adjacent wallrock are dominantly cates structural studies, nevertheless en echelon distribution replaced by sericite and carbonate with lesser amounts of of the Scottie Gold ore zones is evident in both plan and chloriteand quartz. Some large quartzchips floating in section (Figure 27). massive pyrrhotite are aggregates of long fibrous crystals Outcrop patterns and underground geometry of the Six- indicating growth prior to brecciation and shearing. Thin ties Veins suggests they are a series of en echelon tension sections show elongate strings of quartz granules resem- gash veins. However, the formation of the massive, planar bling mylonitic ribbon grains. “Main Veins” requires a more complex process. The ore- 62 Figure 30. Geology and mineral deposits in the Big Missouri mine area. (See Figure 11 or 12 for legend.) 63 -6A"LT ++ AXIS: ANTICLINE. SYNCLlNE MINERAL DEPOSIT. QUARTZ-CARBONATE BRECCIA VEIN WITH DISSEMINATED TO SEMI-MI\SSIYE SuLFlDEI -GEOLOGICAL CONTACT: FORMAT.Tl0NS , ...... ALTERATION HALO. SERICITE. SILICA. OUaRTL VE~N~NO OEOLOO~CALCONTACT: FACES. MEMBERS Figure 31. Cross-section through the Big Missouri mine area. (See Figures 11 or 30 for section location and Figure 11 or 12 for legend.) hosting structures at Scottie Gold mine began as a series of in nearby outcrops above and below the road range from 21' en echelon tension veins, the Sixties Veins, within a shear to 72" eastward and average about 55" east. envelope thatbecame a locus for subsequent shearing (Fig- The Upper Andesite Member is overlain on the east by ure 28). The tensional structures developed in the country the maroon and green Premier Porphyry Member which has rock around the Summit Lake pluton during late magma been traced along the eastern side of Dag0 Hill and seems to movement (Figure 29). Many variables affect the equi- mark the eastern limit and stratigraphic cap to all the min- librium stability fields of pyrite and pyrrhotite, but in gen- eral zones on the hill (Figure 31). era1 these minerals indicate crystallization under low oxy- gen fugacity with temperatures in the range 250°C to 350°C Near the northeast end of Silver Lakes, the Betty Creek (Nguyen et aL, 1989). Formation shows bedding dipping from 82" west to vertical with tops to the east. The formation thins down to only a SILVER-GOLD-BASEMETAL VEINS: THEBIG MISSOURIAREA TheBig Missouri mine and several other nearby orebodies and prospects lie on or adjacent to Big Missouri Ridge at an average elevation of 900 metres (Figure 30). GEOLOGIC SETTING All gold-silver occurrences in the Big Missouri area are hosted by the Upper Andesite Member of the Unuk River Formation. It is similar to descriptions in Chapter 2 and includes the basal black tuff facies that is well exposed on the hillside along and above the Granduc mine road. The main sequence includes typical medium-green andesitic ash tuffsand lapilli tuffs, abundantplagioclase-hornblende- phyric crystal tuffs, and minor tuff breccias. Figure 32. Net fault offset in the Big The Upper Andesite Member is underlain on the west by Missouri mine area. Combinedoffsets along the Cascade Creek and Union the Upper Siltstone Member. The contact is commonly Creek faults may be normal or reverse. A. If normal, ore sheared, but locally is exposed as an unsheared, undis- zones on the west and east sides of the faults are unrelated rnpted, "welded" contact of thin-bedded siltstones overlain andfaulted-off extensions of known zones may exist at by massive tuffs. Graded, rhythmically bedded turbidites depth. B. If reverse, ore zones on the west and east sides of have tops to the east. Dips along theGranduc mine road and the faults may have originally been continuous ore shoots. 64 few metres near Tunnel Lake. Tothe north, between Tunnel Intrusive rocksalong BigMissouri Ridge range from Lake andUnion Lake, itis either absent,recessively batholiths to dikes. The floor of the portal into the Silver weathered, or obscured by intense hydrothermal alteration. Butteworkings shows the eastern contact of potassium .All members of the Mount Dilworth Formation have been feldspar porphyritic Texas Creek batholith intruding andesi- identified in the Dag0 Hill area exceptthe pyritic tuffwhich tic tuff;this is the highest stratigraphic level of intrusion ends abruptly at the south end of Mount Dilworth. Orienta- known for these rocks. Road construction north ofthe Silver tion of distinctive fiammk in the Middle Welded Tuff Mem- Buttedeposit has exposed an irregular dike of Premier ber and subtle bedding in the Lower Dust Tuff Member Porphyry which cuts thin-bedded turbidites of the Upper range from shallowto moderately eastward dips. Fiammk at Siltstone Member. The southern end of the Martha Ellen the north end of Dag0 Hill strike south-southeastward and orebody is cut bya Tertiary granodiorite porphyry dike. dip 42" east-northeast. The fiamm6 are at the baseof a Faulting is abundant on a rangeof scales. The Silver single 10-metre-thick cooling unit of welded tuff that dis- Butte deposit lies within a fault-bounded, stratigraphically plays progressively more compressed pumice clasts down- down-dropped block. Bounding faults have steep westerly section. Base-surge dune formsexposed in this member dips. Two other major north-striking faults on the east side north of Union Lake also have tops to the east. of the ridge, the Union Creek and Cascade Creekfaults, dip Based on all bedding attitudes noted in adjacent units, moderately to steeply westward (Figures 30 and 32). Dis- massive andesitic tuffs of the Upper Andesite Member are tribution of outcrop patterns across the faults suggests rela- interpreted to dip eastward on BigMissouri Ridge. No tive normal offset of up to several tens of metres. Deter- bedding has been reported within this unit, despite several mination of direction and amount of offset on these two programs of detailed, property-scale mapping. Athin lens of faults is a key to resolving the number of ore zones and the wdl-bedded hematitic wackes, exposed along Big Missouri exploration potential on the property (Figure 32). road south of Silver Lakes, lies within the Upper Andesite In summary, the general structure on Big Missouri Ridge Member and dips steeply eastward (Figure 11). is an upright, steeply to moderately eastward-dipping strat- WEST Dag0 EAST MIssourl Hill Granduc Rldge SALMON Mlne RIVER Rmd Province/Province West (15') FORMATION MAIN SEQUENCE MAIN SEQUENCE I TEXASCREEK GRANODIORITE 500 I METRES NO VERTICAL EXAGGERATION Figure 33. Ore deposit modelfor Big Missouri mine. This interpretive cross-section shows the dip of each ore zone in parenthesis. Note that progressive steepening of ore zone attitudes coincideswith the curvature of the folded hostrocks, so that each ore zone must have formed roughly perpendicular to the paleosurface. Stippled areas are alteration envelopesthe around breccia veins. Broad outcrop exposures of alteration (e.g. Galley, 1981, and Grove, 1971, Figure 25) represent areas where the topographic slope roughly parallels the dip of an ore zone. Normal fault offset implies that additional ore zones could existat depth. 65 . igraphicsequence lying on the westernlimb of a major north-trending syncline (Figures 30 and 31). There are no features that indicate the existence of minorfolds within the Upper Andesite Member; the mineral deposits are tabular bodies that are well delineated by drilling and underground workingsand clearly show no evidence of small-scale folding. OREBODIES There are 25 deposits andprospects within a 2.5- kilometre radius of the Big Missouri mine (Figure 30). Five of these havebeen classified as Tertialy silver-lead-zinc deposits in this study, the remainder are classified as Early Jurassic gold-silver-base metaldeposits (Table 16). This section reviews the structural and textural features of the Early Jurassic deposits. All but one of these 20 deposits have a northerly strike and a gentle westward dip (-ZOO), and thus are oriented roughly perpendicular to stratigraphy (Figures 30 and 33). Based on their crosscutting attitude and the many distinctive textures and structural features describedin this section, these Early Jurassic mineral deposits are all interpreted as epigenetic veins. Depositdimensions rangefrom impressively largeto surprisingly small. The S-I vein system and its down-dip Plate 29. Cross-cut through the main orebody of the Big continuation (the original BigMissouri orebody) have a dip Missourimine shows ringed wallrock clasts representing length of 700 metres and a strike length of 120 metres. The classic cockade texture in the quartz-calcite gangue. 2816 Creek deposit, composed of semimassive to massive sul- cross-cut. Big Missouri area. Plate 28. Crudelysymmetric vein growth ofcoarse- Plate 30. Slabbed ore samplefrom the Province Zone grained quartz and calcite with late infilling of grey chal- shows clasts of early, sulphide-poor banded vein rock pre- cedony.DDH 81-58 at 34 metresdepth. Dago Hill deposit, served in moresulphide-rich quartz-carbonate vein material. Big Missouriarea. Big Big core.BQ drill Missouri area. 66 phbie, is at the opposite end of the size scale. This zone is the Province deposit. These clasts typically containonly 2 metres thick with a strike length of only 35 metres and an minor to trace amounts of sulphides and no carbon, and may up-diplength of only 15 metres(thought to terminate represent early (presulphide) brecciation episodes. Internal against a fault). textures are medium to coarse-grained crystal aggregates although some clasts have coarse comb quartz and others fiverage thicknesses and drill intersections on all show- ing,sare in the 1 to 3-metre range. Maximum thicknesses are show segments of cockade texture as alternating bands of mo.re difficult to determine, hot sections of the original Big quartz and carbonate (Plate 30). Missouri underground orebody were at least 6 metres thick Blue-grey microcrystalline quartz clasts are interpreted and. may have been wider. as fragments of chalcedony vein material. Blue-grey chal- In general, the strike length of ore shoots is significantly cedonic veins and veinlets are common in outcrop, trenches less;than dip length, so that their longestdimension is and drill core adjacent to deposits of the Big Missouri area consistently east-west, or in terms of the original (prefold- (Plates 22 and 28). Clasts are small (<3 cm), sulphide free ing) depositional setting, vertical. Certainly this is the case andsuhrounded. Sulphide clasts are rare, smallangular for S-1 zone and Dag0 Hill. Drill information indicates a chips of fine-grained pyritic aggregate. small but real southward rake to these zones, so that the Vein material consists of variably textured quartzand trend of their long dimension is close to west-southwest chalcedonicquartz, calcite, sulphide minerals, carbon, (Figure 10b in Dykes et aL, 1988). sericite and chlorite. Gangue minerals are commonly dis- Different veins have slightly different dips (Figure 33). tinctly laminated (Plates 28 and 30), with bands of fine to The Dago Hill deposits dip 25” west, Calcite Cuts dips 21” medium to coarse-grained crystalline quartz, crystalline cal- west, S-1 dips 18” west and Province and Province Westdip cite and, less commonly, blue-grey microcrystalline quartz, 10” to 15” west. These differences are real and measurable, carbonand sulphides. Undulating to arcuately banded or crustified gangue alongside vein walls(“dish structures” of hut are perhaps too small to be significant. Nevertheless it seems more than a coincidence that the variation is system- Galley, 1981, pp.68-71) drape around original irregularities atic. This radial or fan pattern is exactly what would be of the vein walls andclasts incorporated in earlier deposited expected from a series of originally vertical and parallel layers. Quartz textures range from cryptocrystalline chal- veilns after the host stratigraphy had been deformed in the cedonic quartz, through fine to medium-grained and locally ma~nnerthat this region has been folded (Figures 11,31 and coarse-grained crystalline comb quartz rich in primary fluid 33). inclusions. Quartz of differing textures may grow in adja- cent layers. Cockade texture, consisting of distinctive rings Vein contacts are sharp, butcomplicated by extensive of alternating, variably textured quartz-calcite-sulphide fracturing and peripheral quartz-carhonateand chalcedony layers may completely envelope wallrock clasts (Plate 29). veiln networks that criss-cross shattered, altered wallrocks (Plates 28 and 29). Someore zones have distinctive changesDocumented features such as symmetricgrowth-layers in:lithology between structural footwalland hangingwall (Plate 2% COllOfOrm growth (Plate 31), cockade texture rocks. However,some vein intersections, such as the inter- (plate 291, rebrecciatedveins (Plate 30). Coarse, elongate valfrom 10.7 to 12.5 metresin drill-hole 82-42 on the S-1 comb (Plate 28), fluid-inclusion trains and ~SY zone, have exactly the same lithology in the hangingwall and. footwall. Mineral deposits range from quartz-carbonate veins con- tairing. trace to minor amounts of sulphides, to zones of semimassivesulphide veins with accessory quartz- carlaonategangue. The dominant vein type is a complex qurutz-carbonate+sulphide-cemented brecciavein. Clasts are typically present (Plates 29 and 30), locally absent, and vary from sharply angular to smoothly rounded, but are not spherical. Clasts are wallrock andesite, quartz-carbonate vein material (indicating a rebrecciated vein), blue-grey microcrystalline to cryptocrystalline quartz (chalcedony) and sulphide chips. Wallrock clasts may be strongly bleached and sericite, car’bonate andsilica-altered. Some fragments are medium to dark green and moderately chloritized. Clasts are massive ash tuff, lapilli tuff and crystal tuff and are interpreted to have formed by plucking or ‘spallation’ of fractured wall- rock. Lack of significant rounding and abrasion in most clasts suggests they have not moved great distances within the vein. Quartz, carbonate and quartz-carbonate clasts vary Plate 31. Photomicrograph of colloformpyrite growth from irregular raggedchips in the Dag0 Hill veins, to layers with gangue-filledspaces. S-1 deposit,Big Missouri smoothlyrounded “pebbles” in at least some locations inarea. 67 cavities are characteristic of vein growth and open-space Carbon locally comprisesup to 15 percent of the vein filling in the subvolcanic environment (Dowling and Mor- volume at Dag0 Hill where recent mining has exposedlarge rison, 1989). blebs and pods upto 15 centimetres across. It also occurs in fine veinlets and small patches and commonly is associated Carbon, as amorphous carbon, is present in the veins as with pyrite. Open cavities containing euhedral, drusy quartz thin seams between adjacent quartz and quartz-carbonate and pyrite may be coated with a film of carbon (Galley, layers. Carbon is fine grained, powdery and charcoal to jet 1981, p.76). The most probable source for this material is blackin colour. Granulesrange up to 2 millimetresin the strongly carbonaceous turbidites of the Upper Siltstone diameter with lustrous, conchoidallyfractured surfaces. Member which stratigraphically underlie this area. Plate 32. Photomicrograph of fracture through pyrite grain hosting chalcopyrite, galena and two grains of electrum. Dago Hill deposit, Big Missouri area. Plate 33. Photomicrograph of characteristic featuresof delafossite: botryoidal texture, moderate pleochroism and strong anisotropy. S-l deposit, Big Missouri area (see colour photo in back of report). 68 Fineto medium-grained semimassive sulphides are significantly different paleodepths and pressure-temperature characteristic of the Silver Butte, Province West, Creek and conditionswithin the major fractures that cut the Upper Ma:rtha Ellen zones. Sulphides display crude book-structure Andesite Member. (interlayered sheets of gangue) at centimetre-scale. These zones generally have higher base metal values but lower MINERALOGY precious metal values relative to sulphide-poor sections of In order of abundance, opaque minerals include pyrite, veins, Pyrite is the most abundant sulphide; other sulphides sphalerite, minor to accessory chalcopyrite andgalena, trace noted in drill core andhand samples include sphalerite, tennantite and pyrrhotite, and rare electrum (Plate 32), galena and chalcopyrite. covellite and delafossite (Plate 33). Holbek (1983) identi- Alteredwallrocks are cnt bymultiple generations and fied trace amounts of electrum, acanthite, native silver, 0rie:ntationsof variably textured veins and veinlets of chal- freibergite and chalcocite. Galley (1981) also reported rare cedony, crystalline quartz, quartz-calcite and calcite. Mine pyrargyrite and polybasite. Mineral paragenesis deduced by geologistshave documented up tofive generations of Galley and Holbek indicates an overall simple sequence of cro!;scutting veinlets at Dago Hill (S. Dykes, personal com- sulphide precipitation that is consistent with progressively munication, 1984). All these wallrock veins contain trace to lower temperatures with time. minor sulphides, but late calcite veins carry the least sul- Sulphidemineral textures range fromdisseminated phides. No obvious asymmetry was notedin the distribution euhedral crystals to irregular crystal aggregates of mixed of these complex overprinted vein networks inthe structural sulphides. Grainsrange from micron size up to coarse hangingwall and footwall rocks. euhedral pyrite crystals and large sphalerite blehs or aggre- EIroad patterns of mineralogical zoning have been docu- gates up to 2 centimetres long. Many samples have cracked mented by Galley(1981) and Dykes et nl. (1986, 1988). and shattered grains of pyrite with remobilization of other These includeabundant galena and carbon at DagoHill sulphidesamong these fragments, suggesting post-ore ver:;us more abundant sphalerite, calcite and iron carbonate, deformationby local faultingor regionaltectonism. In and a general absence of carbon, at the Province deposit. undeformedsamples most sulphide minerals are closely Figure 31 shows that these deposits may have formed at associated with each other, indicating contemporaneous deposition, although there is clearly an early pyrite phase. Galenaand chalcopyrite are mostlyincluded within sphalerite but also are present as fine blebs. Chalcopyrite is also distributed as sinuous grains along fractures (Plate 32). Pyrite is present asdisseminations, veinlets and thin bands interlayered withgangue. It is the most abundant mineral in semimassive sulphide zones and also occurs as veinlets and disseminations in surrounding wallrocks. Dis- seminated pyrite in wallrocks has strongly striated surfaces andmay be of a different generationthan vein-hosted pyrite. Mostvein-hosted pyrite is earlier thanother sul- phides which enclose or partially replace it. Holbek (1983) reports minor post-ore pyrite, so that as many as three generations of pyrite may be present in these deposits. Vein-hosted pyrite is present as sharply euhedral crystals, sharplyangular fragments and "crushed"ragged chips. Some grains have internal crystalline skeletons, with later overgrowths in crystallographic continuity. Local, thin arcu- ate bands of pyrite grains and aggregates conformto thicker crustiform layers of enclosing gangue minerals (Plate 34). Some large pyrite grains have internal rings or diffuse bands ofencapsulated gangue and sulphide grains indicating oscillating pulsesof inclusion-rich growth (Fleet et aL, 1989; Barton et al., 1977; Barton and Bethke, 1987). Tiny internal blebsof pyrrhotite, tennantite and chalcopyrite were noted. Pyrite grains are commonly cracked with the fractures filled by chalcopyrite, sphalerite and galena. Sphaleriteis presentin both low-sulphide and semi- massive sulphide zones. It is locally more abundant than pyrite. Colour ranges from red to red-brown to purple to black. Spica1 textures are medium to coarse-grained sub- hedral to anhedral aggregates. Individual grains vary from Plate 34. Photomicrograph of pyrite encrustations around anhedral to amoeboidshapes that suggest remobilization. coarser gangue crystal (quartz). S-l deposit, Big Missouri Galley (1981) reports sphalerite at the Northstar-Lindeborg area. prospects that forms rims up to 1 centimetre thick around 69 andesite fragments; these in tum are enveloped bythin Gangue is primarily quartz with lesser carbonate, sericite, crusts of galena, forming cockade texture of alternating oxides and carbon. Holbek (1983) has documented minor sulphides. Thewriter has identified atoll sphalerite at micro- barite. scopic scale in samples from the underground workings that may represent skeletal growth or an early stage of develop- ALTERATTON ment of cockade texture. Andesiticvolcanic rocksbecome progressively more Galena is most common as bands of grains within semi- bleached toward each major vein (Plate 22). No apparent massive sulphides but also occurs in minor amounts in low- asymmetry, either in thickness or mineralogy, was noted in sulphide vein sections. Intergrowths with chalcopyrite or the alteration zones examined in core. These envelopes of sphaleriteindicate coprecipitation. Blebs of tetrahedrite- bleached, foliated pyritic rocks were mapped as rhyolite tennantite are scattered through some galena crystals. during early exploration work. Light coloured rocks are still Chalcopyrite has been noted in all deposits of the area in commonly referred to as“siliceous” or “silicified”, but minor to accessory amounts. The mineral is present as they aredominantly affected by sericiteflooding with intergrowths with other sulphides, as microscopic exsolu- lesser, patchy carbonate alteration. The bleached rock is tion blebs in sphalerite and in late fineveinlets, presumably generally quite soft; powdered material has a moderate to dueto remobilization. Secondary copper minerals, chal- strong reaction to dilute hydrochloric acid. cocite, covellite and delafossite, are intimately associated Several metres from major veins, core samples show with, or entirely replace, chalcopyrite. weak to moderate chlorite alteration (Plate 22). This pro- Preciousmetal values are contained in electrum, pylitic alteration may be overprinted by irregular zones of acanthite, freibergite and native silver and extremely rare fine sericitic alteration. Closer to the main veins the zones pyrargyrite and polybasite. In low-sulphide vein sections of sericitic alteration coalesce and the rock is bleached and silver sulphides and native silver are present as free dis- moderately to strongly sericitized with ragged patches of seminations in gangue. In semimassive sulphide vein sec- remnant chlorite alteration. Similar patches of later carbon- tions, they generally form aggregates with galena and chal- ate alteration overprintsericite. Closer still to the veins copyrite or inclusions in pyrite. Freibergiteltetrahedritel (<2 m), fine calcite is themost abundant alteration mineral tennantite minuteblebs within sphalerite, and areas of pervasive calcite flooding are common. Adja- ispresent as cent to the veins (<0.5 m). alteration may be intense serici- galena and pyrite. Gold occurs only as electrum and has been reported only from the S-1 and Dag0 Hill zones. tization, calcite flooding or silicification, or mottled patchy combinations of allthree alteration minerals. Fine to Consequently, it may be useful as a depth-dependent min- eral phase (Figure 33). Electrum is present as tiny rounded medium-grained disseminated pyrite accompanies all altera- grains entirely contained within chalcopyrite, sphalerite or tion types, but is most abundant (up to 15%) in strong pyrite (Plate 32). as coarser elliptical hlebs along sulphide sericitic alteration. grain boundaries orfractures, and asfree grains within GENESIS gangue. Grain sizevaries from 2 to 70 microns. The average gold:silver ratio for electrum is 1:1 (Holbek, 1983). The subtle, but real, progressive steepening of dip in the Big Missouri ore zones (Figure 33) is an important clue to the prefolding orientation of these deposits. Prior to mid- EAST Cretaceous deformation, ore zones in the Big Missouri area formed as parallel epithermal breccia veins emplaced in a 9”” -7-series of parallel, vertical, brittle faults (Figure 34). There was relatively minor offset across these faults, on the order of a few tens of metres at most; offset between blocks was more a matter of differential settlingrather than major tectonic disturbance (Figure 35c). Faults formed as part of a series of synvolcanic radial fractures around an active vol- canic centre. ‘Radial’ fracture sets may appear to be truly radial on a regional scale, but in detailare commonly composed of several parallel sets,for example Spanish Peaksand Dike Mountain, Colorado (Knopf, 1936; Johnson, 1961). Hydrothermal fluids were concentrated abovea local dome in the underlying Texas Creek batholith, exposed near the mill portal of the Big Missouri mine and inunderground workings at the Silver Butte deposit. Dikes of Premier 34. Genetic model for the ore deposits of the Big Figure Porphyry that crop out along the Granduc mine road may Missouri ridge area shows their prefolding configuration as a series of subparallel epithermal breccia veins emplaced have penetrated up into the mineralized region. Fluids pre- along minor faultsin a crater or caldera-like setting(see also cipitated breccia veins into several parallel fault zones. A Figure 35). This diagram is an actual cross-section through series of fluid pulses deposited ore and gangue repeatedly the Mount Skukum epithermal OE deposits (MacDonald, along each fault. Possibly only one fault structure at a time 1987). served as a fluid conduit; other faults becoming conduits as 70 the 'active' fault sealed due to precipitating ore and gangue of high-sulphide ore shoots at depth versus stratigraphically minerals. shallower deposition of low sulphide-ores -fit present Big Missouri data. Metal and mineral distribution is less well documented at Big Missouri than at Silbak Premier, and post-ore faulting has disturbed the continuity of individual ore shoots (Fig- SILVER-GOLD-BASEMETAL VEINS: ure 33). However, two Silbak Premier patterns - elevated SILBAK PREMIER MINE precious metal grades in low-sulphide ores, and deposition The Silbak Premier mine is located 17 kilometres by road from Hyder and 20 kilometres by road from Stewart, below treeline on the western slope of the Bear River Ridge. Brown (1987, pp.8-13) provides a detailed history of ore production. GEOLOGIC SETTING Like the Big Missouri area to the north, Silbak Premier mine and several nearby showings are allin the Upper Andesite Member of the Unuk River Formation (Figures 11 and 47a). The member is similar to the detailed descriptions in Chapter 3 except that the basal black tuff facies has not beenidentified in this area. The main sequence includes medium todark green, moderately to strongly foliated andesitic ash tuffs, lapilli tuffs and crystal tuffs with abun- dant plagioclasethornblende crystals. Andesite is notice- ablydarker green and morestrongly chloritized than elsewhere in the region. Local beds ofhematitic and chlori- tic conglomerate with well-rounded volcanic cobbles crop out uortb and northwest of the mine workings between the glory hole and the Cooper(Lesley) Creek bridge. Con- glomerates are purple, green, and mottled purple and green in colour. No sedimentary bedding has been noted in the immediate mine area (200 m radius). Ductile deformation has produced moderately westward-dipping (40") penetrative foliation andductile shear zones characterized by spectacularly stretched clasts of angular lapilli and rounded cobbles that have allpreviously been recorded as evidence ofwest- dipping bedding in these strata (Langille,1945). Many bedding measurements shown in the immediate mine area (Brown, 1987) were taken from elongate cobbles that may have been deformed (D.A.Brown, personal communication, 1987). Figure 35. Sequence of development of Early Jurassicsub- volcanic settings in the Stewarf mining camp. The Big Missouri and Silbak Premier epithermal ore deposits likely formed penecon- B temporaneously, hut stratigraphic and facies evidence suggests that their geologicalsettings formed sequentially. CaseA-1: the thicker section of Betty Creek Formation strata that overlies the Silbak Premier deposits suggests the Silbak Premier setting represents a later parasitic vent on the flank of a major stratavolcano that was centred at the Big Missouri area. Case A-2 the setting for the Silbak Premier ore deposits formed beneath an early central vent that was later buried by pyroclastic and epiclastic debris from a later, but nearby, central vent in the Big Missouri area. CaseA-2 seems to fit the detailedlithostratigraphic. evidence best. Epilhermal veins form along sub-parallel fauns. B. Subsequent caldera formation in late 'Mount Dilworth' time Lake acts as a thermal dam, PREMIER MINE creates block faults and a basin for deposition of the Black Tuff forming 'YelesMpefl veins. facies. C. Main period of epithermal mineralization coincides with along the hangin wall formation of Pyritic Tuff facies and accumulation of Black Tuff C contact of ring dig- facies. 71 To the west, the Upper Siltstone Member underlies the indicators are consistently to the east, so that westward- Upper Andesite Member. The sedimentary unit is offset in dipping exposures represent overturned strata. this area by major faults and thus provides a useful iudica- tion of relative direction and amount of offset that can be Overlying the Upper Andesite Member to the east, the Premier Porphyry Member has its used to reconstruct fault-truncated ore zones and guide three facies exposed on the wooded slopes eastward and uphill from the Silbak exploration. From north to south: Premier glory hole. Layering in the lowest massive to lami- 0 The Upper SiltstoneMember crops out west and nated tuff unit dips gently eastward (20"). downslope from the Indian mine workings where it dips moderately eastward (57") with tops to the east. Farther east and uphill, bedded wackes and siltstones of the Betty Creek Formation dip moderately eastward (30" to 0 Exploration work in 1988 located an exposure on the hilltop south of Indian Lake and west of the Woodbine 50"). Scour channels indicate tops to the east. Along the prospect. Strata here are vertical: no tops indicators crest of the Bear River Ridge, abundant bedding exposures were noted (A. W. Randall, personal communication, indicate a broad synclinal trough with local minor folds 1988). developed in interbedded sedimentary rocks and tuffs (Fig- ure 11). 0 Southeastward, the next area of outcrop is at the con- fluence of Cascade Creek and Salmon River where a In summary, the general structure is a vertical to moder- large well-washed outcroparea of thin-bedded silt- ately eastward-dipping sequence lying on the western limb stones is disrupted by the Boundary dike swarm. Dips of a north-trending syncline (Figure 36). This structural range from 60" west to vertical, no tops indicators were setting fits observed sedimentary bed orientations and tops noted. indicators, and is critically important for resolving the intru- Nearby, Brown (1987) mapped siltstones in Logan sive history of the dikes in the mine and the genesis of the Creek just upstream from theGranduc mine road. Beds mineral deposits. here dip steeply westward, averaging 80°, but show Volcanic rocks at the mine are intruded by a variety of tops to the east, indicating that strata are Overturned. dikes and other irregular intrusive bodies. Dikes of Premier 0 Farther south, geologists from Esso Minerals Canada Porphyry, a potassium feldspar megacrystic plagioclase- measured dips averaging 80" east, with tops to the east, amphibole porphyry, are the most abundant intrusive rock on outcrops in Cabin, Boundary and Daly creeks. and they are spatially associated with most, and possibly all, Therefore the Upper Siltstone Member in the Silbak ore zones. Premier Porphyry dikes are described in Chap- Premier area has dips ranging from steeply westward to ter 3 from their type exposures at the Silbak Premier mine. moderatelyeastward but dominantlysubvertical; tops The dikes have a variety of forms including: WEST E-W CROSS-SECTIONTHROUGH THE SILBAK-PREMIER MINE UWNTSTEVENSON STEWART AREA 1 JURASSIC TERTIARY Hazelton I2l Biotite quartz monzonite Epiclastic sequence JURASSIC Andesilic pyroclaslics flows and Texas Creek hornblende granodiorite Lower and upper siltstone members a n Premier Porphyry -dikes andflows Figure 36. Geologic cross-section through the Silbak Premier mine area. (See Figure 11 for cross-section location.) 72 0 branching or anastomosing dikes which incorporate manysmall, previously subeconomic ore shoots are screens of country rock or brecciated mineral zones, recovered in current open-pit mining. Past production and fan.like radiating of a few to several dikes, published reserves for the Silbak Premier mine are listed in and Table 15. 0 laree-scale curved dikes. Production from two distinct brecciavein and stockwork ,~ ~ ~~~ ~ 1~ ~~~ ~~ ~ ~. explanation of how folded and unfolded examples trends. The intersection area contained the widest ore shoots within a few hundred metres of each other. (up to 20 m) and those with the highest gold and silver grades. Other intrusive rocks in the mine area are dikes and a small stock of the Hyder plutonic suite. One major grano- The new open pit is located in the richest part of the old diorite porphyry dike, the 100-metre-wide "Main Dike", Premier underground workings where the Main and West cuts through the centre of the Sehakwe ore zone. Tertiary zones intersect (Figure 37a). This area also corresponds to a microdiorite and lamprophyre dikes also cut through the concentration of converging Premier Porphyry dikes (Fig- mine workings. ures 38 and 37a). The many small, subparallel ore shoots Small-scale faulting has been a major problem in under- have .an aggregate thickness of 20 to 50 metres. ground exploration in the mine area. Many ore shoots are The Main zone trend of ore deposits strikes 050" and offset a few tens of metres or less, but locating the continu- extends 2000 metres northeast from the glory hole area. The ing ore shoot can be a major delay and expense. Dikes, trendincludes the original Premier, B.C. Silver and altered wallrock and mineralized zones are so abundant that Sebakwe ore deposits and the Bush showings. The Main it is difficult tobe certain that the same ore shoot has been zone dips about 60" northwest at surface and progressively found. curves and flattens to a dip of about 30" near 6-level (Fig- Regional-scalefaults have important implications for ures 38 and 37d). exploration. The location of the Upper Siltstone Member Convergence of the Premier Porphyry dikes from depth we$t of the Woodbine prospect indicates that relative nor- towardsurface meansthat continuous ore shootsare mal offset occured along the Slate Mountain fault and Cas- localized between PremierPorphyry dike and andesite cadle Creek lineament (Figure 36). Deposits such as Wood- country rock at depth, but lie between separate but similar bine, Fork, Power and Hope, in the hangingwall block, may Premier Porphyry dikes near surface (Figure 38). Ore shoots have been emplaced at a similar stratigraphic level as the at the southwest end of the Main zone trend were composed West zone in the footwall block and the mineralized dike of two extensive subparallel sheets Langille (1945), but in contact in each fault block may be the same structure the Sebakwe workings at the northeast end of the trend, ore (Figure 37). zones consisted of many small, en echelon shoots. The most enigmatic structure in the mine area is a ductile At its southern end the Main zone is abruptly truncated at to brittle shear that trends suhparallel to the West zone and the West zone which trends 290". The West zone is vertical deforms ore atmany sites, including a spectacular exposure to steeply dipping throughout its extent. Mineralization and at the 2-level portal. This structure is well exposed in alteration are localized along the southern contact of a large tre~ncheson the south edge of the glory hole as bleached, subvertical Premier Porphyry dike (Figure 37a). The zone is ductily deformed Premier Porphyry. It is interpreted as a 350 metres long and is exposed as a single breccia zone, 5 to post-ore ductile shear, with later superimposed brittle fract- 10 metres thick, open to depth. The northwestward exten- uring, that developed subparallel to the long linear dike of sion of this trend includes the Hope,Power, Fork and Premier Porphyry because of the high resistance of the Woodbine prospectsover a total strike length of 1500 massive dike and relative ductility or instability of the metres. sulphide and breccia zones. Northern Lights is a blind orebody in the structural A hematitic, ductile shear zone crops out north of the hangingwall of the Main and West zones. It displays the mine workings and has been intersected in drilling in the same two distinct zone orientations (Figure 37a). glory hole area (McDonald, 1990a). The zone shows pro- Ores of the Silbak Premier mine are hosted in massive nounced stretching of both angular and rounded clasts that breccia veins andin peripheral vein networks or stock- havebeen mapped as bedding by early workers. Conse- works. Ore was deposited during one pan of an extensive quently it has been regarded as a stratigraphic marker for history of hydrothermal brecciation and veining that has dezades, for example Langille's (1945) "purple tuff", and it been divided into three stages (McDonald, 1988): pre-ore was a key piece of evidence for west-dipping strata in the breccias and veins, ore-stage breccias and veins, and post- mine area. This shear zone is certainly an important planar ore veins. marker, but it is not a stratigraphic unit. Pre-ore breccia is a hydrothermal breccia confined to andesitic volcanic rocks immediately adjacent to massive OREBODIES unbrecciated Premier Porphyry dikes. It consists of rounded The historic Silbak Premier underground operation pro- to angular fragments of country rock andesite ranging from duced orefrom eighteen separate shoots (Grove, 1971). 1 to 15 centimetres across. Fragment abundance ranges 73 NORTHWEST SOUTHEAST WEST ZONE paleovertical directions Q isolines in MAIN ZONE spread reflects curving ore zones loooFWT ELEVATION ll \ 500FWT ELEVATION C Figure 37. Three-dimensional distribution of ore zones at the Silbak PremierA. mine. Simplified geology mapof the Silbak Premiermine area. (Modified from Alldrick, 1987 and Brown, 1987.) B. Stereonet shows good correlation between "paleovertical" directions obtained from bedding measurements,and from probable "paleovertical" precious metal zoning in the ore shoots. C. Longitudinal section through the West Zone orebodies shows zoning of goldsilver ratios. (Modified from Grove, 1986.) SOUTHWEST NORTHEAST- D Figure 37. Three-dimensional distribution of ore zones at the Silbak Premier mine. D. Longitudinal section through the Main Zone orebodies shows zoning of goldsilver ratios. (Modified from Grove, 1986.) WEST EAST MINERALIZED BRECCIA VEINS -1 Level STOPES I Level 1W 1 Figure 38. Geologic cross-section through Main zone, Silbak Premier mine, Mine Section 2245’4. (Modified from P. Wojdak, written communication. See Figure 11 or 12 for legend.) from 25 to 90 per cent (McDonald, 1988). Breccia matrix is tion. The ore types are best considered as four stylesof min- medium to dark green andconsists of pyrite,chlorite, eralization that can be shown as an inter-related matrix sericite, quartz and carbonate.Pre-ore breccia may have (Table 19). been overprinted by later veining and brecciation; large Brown(1987) puts the dividing line between low- clasts of pre-ore breccia are foundwithin the ore brecciasof sulphide and high-sulphide categories at 15 per cent sul- the 602 ore zone. Quartz-carbonate and quartz-chlorite pre- phide content; McDonald (1988) defines low-sulphide veins ore veins cut pre-ore breccia, but are cut,in turn, by all later and breccias as those with less than 5 per cent sulphides, vein and breccia types. high-sulphide veins and breccias as those with 20 to 45 per Ore-stage veins and breccias contain most of the base cent sulphides. Vein stockworks are areas of overprinted metal sulphides and host all the precious metal ore of the vein patterns that, together with zoned sulphide grains and mine area. They seem to record an extended period of layered sulphides, provide evidence for multiple mineraliz- multiple, superimposed ore-bearing vein and breccia forma- ing episodes. Breccias are areas of even greater dilation than tion.Veins and minerals display internal structures and the veins, so that these descriptive categories for veins, vein textures indicating several pulses of mineral deposition and stockworks and breccias are gradational. The distinction growth, and these are transected by veins of later genera- between low-sulphide and high-sulphide ore types is more 76 sharply defined than the textural divisions. Even so, large textured quartz, chalcedonic quartz, crustiform banding and low-sulphide breccia bodies may have local pods of high- open vugs lined with quartz, pyrite and sphalerite. Fluid sulphide breccia within them. inclusion studies indicate ore depositionoccurred at an A vertical zoning pattern is indicatedby the general average depth of 600 metres (McDonald, 1990a,b). These distribution of these ore types (McDonald, 1988); high- important veins and breccia bodies are described and com- sulphide veins and breccias are moreprominent in the lower pared below in the same numerical order used in Table 19. levels of the deposit and in the West zone, low-sulphide (1) Vein Stockworks and Veins. Veinstockworks are veins and breccias are more prominent near surface and in irregular networks of banded to massive veinlets that vary the Main zone. All styles of mineralization constitute pre- from 0.5 to 4 centimetres in width (Plate 35). Stockworks cious metal ores. are generally peripheral to breccia zones and appear to be Brecciasand veins are characterized by distinctive lateral,gradational extensions outwardfrom the breccia ganguetextures indicating deposition in a subvolcanic bodies. environment,for example, cockade structure, comb- (2) Breccias. Breccia bodiesare extremely irregular, with subsidiary arms or splays off the main body. Breccia zones range up to 20 metres wide and have been located throughout the entire extent of Main and West zones, from surface to the deepest mine workings. Most breccias pass gradually into vein stockwork zones, but some are bounded by faults. Breccia matrix comprises up to70 per cent of the rock and hosts most of the precious metal minerals. Clasts include wallrock and ore fragments. Wallrock clasts are composed of either andesite or Premier Porphyry, hut the original lithology of strongly altered clasts can be difficult to determine (Plate 36). (3) Low-sulphide Veins and Breccias. There is a pro- gression from weakly altered hostrock, to strongly altered hostrock cut byminor quartz veins, to intenselyaltered hostrock criss-crossed by low-sulphide vein stockworks, to low-sulphide breccias (siliceous breccias) (Figure 39). (4) High-sulphide Veins and Breccias. High-sulphide Plate 35. Shattered wallrock with vein network or ‘stock- veins, vein stockworks, and the matrix to brecciamay work,’ 602 orebody, 6-leve1, Silbak Premier mine. contain up to 45 per cent pyrite, galena, sphalerite, chal- copyrite, pyrrhotite, tetrahedrite, native silver and electrum. Quartz, chalcedony, calcite, barite and albite gangue min- erals are either randomly intergrown with the sulphides or locally form crude hands (McDonald, 1988). (5) Low-sulphide Veins. Low-sulphide veins and stock- works are preferentially localized in Premier Porphyry and along porphyry-andesite contacts. They are more abundant TABLE 19 STYLES OF MINERALIZATION AT THE SILBAK PREMIER MINE 1 2 VEIN SMCKWORKS BRECCIAS AND VEINS 3 Siliceous breccias 4 7 S HIGH SULPHIDE Base metal veins Base metal breccias (>20% sulphide) Sulphide veins Sulphide breccias Plate 36. Sulphide-cemented breccia of bleached, rounded wallrock clasts exposed in the floor of the Silhak (Numbers refer to sections in texO Premier glory hole, south wall. 71 2 VEIN COUNTRY ROCK pc MASSIVE VEIN OR YEINr/ I 7 QUARTZ BRECCIA VEIN STOCKWORK Figure 39. Distribution of ore structures and alteration zones in the Silbak Premier mine. Note variation in horizontal scale closer to surface and in the Main zone and generally con- stitute ore with the highest si1ver:gold ratios(Randall, 1988). Low-sulphide vein stockworks contain dissemina- tions and patches of pyrite, sphalerite, chalcopyrite, pyr- rhotite, tetrahedrite, native silver and electrumin quartz-rich gangue with associated calcite. (6) Low-sulphide Breccia. Low-sulphide breccia is the most extensive of the four "end-member" ore types and also the highest grade ore. Alternate names are: precious metal breccia, siliceous breccia, bonanza ore and breccia veins. Total sulphide content is less than 5 per cent, com- posed mainly of pyrite with minor sphalerite and galena. Precious metal bearing minerals are polybasite, pyrargyrite, acanthite, tetrahedrite, freibergite, native silver, gold and electrum (Randall, 1988). Gold and silver values are not distributeduniformly within thesezones. Higher grade "bonanza" sections consistof local concentrations of silver sulphosalts with electrum and native silver in later fractures (Brown, 1987). Clasts are predominantly bleached, altered porphyv and andesite with rare chalcedonic clasts (Plate 37). Near the margins of the breccia, clasts are sharply angular and more distinct,closer to the centre they are rounded and less distinctdue to intense silicification. Breccia matrix is mainly chalcedonic quartz with patchy sulphide aggregates. Local high-sulphide zones have been discovered within these low-sulphide breccias. (7) High-sulphide Veins. High-sulphide veins and vein- stockworks are restricted to the immediate margins of high- sulphide breccias, extending only 2 to 3 metres from the breccia contact. They are most common in the lower parts of the deposit and in the West zone. (8) High-sulphide Breccias. High-sulphide breccias are the most visually striking ore type (Plate 36) although precious metal contentis consistently lower than low- 78 sulphideores. Sulphides are dominantly pyrite with pyrite in gangue(Plate 18); porphyroblasticgrowth of ' accessory sphalerite and galena, and minor chalcopyrite and coarse-grained, striated pyrite; and parallel foliation in sul- pyrrhmatite. Base metal sulphidesare massive textured, phides and wallrock along the West zone. medium to coarse grained and commonly vuggy. Breccia boundaries may he sharp fault contacts, a gradation into a MINERALOGY highdphide vein stockwork, or gradational contacts into There are significantvariations in mineral volcanic wallrock with scattered large patches of sulphide textures, and sulphidemineralogy, aggregate grading outward into a disseminated pyritic halo. gangue gangue and sulphide abundances, precious metal contents and precious metal This ore type is characteristic of West zone and deeper ratios both within and among ore zones. Sulphide minerals levels of Main zone. includepyrite, sphalerite and galena with accessoryto A different variety of high-sulphide breccia ore is minor chalcopyrite and pyrrhotite and rare arsenopyrite. expolied at the 2-level portal, the Ladder trench and the Silverand gold-hearing minerals include tetrahedrite/ Hope: prospect. Striking surface exposures of semimassive freibergite, polybasite, pyrargyrite, acanthite, native silver to massive polymetallic sulphides are distinctly layered. and elecuum (Brown, 1987; McDonald, 1988). McDonald Layering is regarded as post-ore, tectonically induced folia- distinguishes between tetrahedite (0 to 20% contained sil- tion within and adjacent to the major ductile shear that ver) and freibergite (greater than20% contained silver). parallels the West zone. Gold is present mainly as electrum whereas silver occurs Post-ore veins are quartz-carbonate and quartz-chlorite primarily within tetrahedrite, polybasite and freibergite. en echelon sets. They resemble pre-ore veins, so crosscut- Base metal sulphides, tetrahedrite, electrum and native ting relationships must be noted hefore classification is silver are present in both low-sulphide and high-sulphide possible. ores. Polybasite, pyrargyrite, freibergite and acanthite are Evidence for post-ore deformation includes: fractured specific to low-sulphide breccias and veins (McDonald, anddislocated pyrite enveloped by moreductile chal- 1988) where total metallic mineral content is less than 5 per copyrite, galena and sphalerite; pressure shadows around cent. High-sulphide breccias and veins contain 20 to 45 per cent pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, arse- nopyrite, tetrahedrite, native silver and electrum. Pyrite is the mostabundant sulphide mineral; grains range from microscopic dust up to 1 centimetre in'diameter. Larger grains may host complex rings of inclusions of other sulphides (Plate 38) indicating asequence of pulses of inclusion-rich crystal growth (Fleet et al., 1989; Barton et a[., 1977; Barton and Bethke, 1987). Fractured grains are enveloped by more ductile sulphide minerals. Pyrite aggre- gate typically forms monomineralic layers in book structure or handed veins. Sphalerite is the next most abundant sulphide mineral with irregular grains ranging up to 8 millimetres across. Microscopic study shows most sphalerite is inclusion-rich withchalcopyrite and rare galena and sulphosalts. Sphalerite is typically brown, but McDonald (1988) also reportsamber, deep red and black varieties. Galena is mostly a minor component but locally comprises up to 10 per cent of the vein material. Chalcopyrite is present in trace to minor amounts, generally as minute inclusions within sphalerite. Some high-sulphide ore contains as much as 4 per cent chalcopyrite McDonald (1988). Pyrrhotite is present in minor amounts in most high- sulphide ores, associated withthe more abundant pyrite. High-sulphide breccias exposed inthe 6-level workings contain up to 8 per cent pyrrhotite as large intergranular blebs. It may be significant that these zones are among the deepest exposures in the mine as pyrrhotite crystallizes at higher temperatures than pyrite. Plate 38. Photomicrograph of concentrically-zonedMcDonald (19881 has identified small(0.1 to 0.3 mm) pyrite and pyriteaggregate~record episodes of inclusion-rich rectangular to subhedral crystals of arsenopyrite~~ ~~ in ~ high- ~~~ I mineral growth interspersed with inclusion-free layers. This sulphideores. Arsenopyrite has also been reported oscillatoryzoning orgrowth banding is attributedto peripheral to the low-sulphide ore zones (Phillips, 1989). periodic changes in the physical and chemical conditionsof the er al.. 1989). deuositine. ~ ~~ svstem", (Fleet 2-level.Silbak Tetrahedrite and argentiferous tetrahedrite are present in Premier mine. low-sulphide and high-sulphide ores. The mineral forms 79 irregular grains or occurs as inclusions within galena and cious metal mineralization is everywhere bounded byat sphalerite. Silver contentof tetrahedrite increases with ele- least a thin envelope of siliceous alteration. vation and the mineral becomes freibergite above 2-level Sericitic alteration (potassic).Intense sericiticalteration (McDonald, 1988). with accessoly pyrite. silica and minor potassium feldspar Polybasite is present only in low-sulphide ores where it is characterizes the next alteration envelope outward from the the most abundant silver mineral. It forms both irregular ore. This alteration may develop within the outer part of a aggregates and minute inclusions in tetrahedrite. Pyrargyrite vein-stockwork ore zone (Figure 39). In these areas, mar- is a minor component of low-sulphide ores, forming irregu- gins of individual veins and veinlets may have a silicified lar grains that are intergrown with polybasite (McDonald, wallrock envelope up to one centimetre thick. 1988). Acanthite is present in trace amounts, always in Carbonate alteration. Carbonatealteration bleaches contact with galena. still-recognizable country rocks. Altered rock has an overall Native silver is present as minute inclusions in pyrite, pale grey or bone colour but original textures, fragments galena,tetrahedrite and polybasite or as free grains in and phenocrysts can still be seen. Pyrite is present only in quartz. It alsoforms replacement rims around, and fine trace amounts or is absent in this zone, suggesting selective veinswithin, grains of polybasite,pyrargyrite and dissolution and replacement by calcite. This alteration type tetrahedite. has been mapped as potassic or even as siliceous alteration Electrumis present in bothlow-sulphide and high- in the past, but carbonate-flooded rocks are quite soft and sulphide ores as irregular blebs within pyrite and galena. powdered rock reacts strongly with dilute acid. Although The composition of electnnn varies within individual grains the inner boundary seems gradational or patchy, the outer and throughout the deposit; gold contents range between 35 boundary of this alteration zone is marked by a relatively to 56 weight per cent (McDonald, 1988). Grains are more abrupt colour changeover as little as a few centimetres or a silver-rich toward the rims. more gradual change over a metre or less. McDonald(1988) determined a primaryparagenetic Chloritealteration (propylitic). Propyliticalteration sequence for metallicminerals. Both low-sulphideand consists of pervasive chloritization of wallrock, producing a high-sulphide veins and breccias have a sequence from distinctive deep green colour (Plate 39). Associated with sulphides to sulphosalts to native metals. In general, low- this alteration is^ up to 5 per cent fine to coarse-grained sulphide ores areearlier than high-sulphideores; low- euhedral pyrite and minor to trace carbonate, sericite and sulphide veins and breccias are cut by high-sulphide veins epidote that areusually only noticeable in thin section. This and breccias. alteration is distinct from the much less intense regional metamorphic greenschist facies alteration. Differences in Gangue minerals include chalcedony and quartz, together alteration should be routinely documented during reconnais- with carbonate, potassium feldspar, albite, barite and rare anhydrite. ALTERATION A sequence of fourdistinctive alteration envelopes is developed around Silbak Premierore zones and isan impor- tant exploration guide at mine scale. Spatial distribution of these alteration zones is shown on Figure 39. Widths of the different alteration zones viry with depth; widths of the silica, sericite and carbonate alteration all increase toward surface. These zones are locally discontinuous in the lower mine levels. The mineralogy, evolving fluid chemistry, and spatial and chronological sequence of the alteration envel- opes is consistent with deposition in a subvolcanic, epither- mal environment (Heald et al., 1987; Siems, 1984). Siliceous alteration (silica). Wallrock immediately adja- cent to breccia orebodies and within vein stockwork ore zones is silicified (silica-flooded) and hosts accessory dis- seminated fine-grained potassium feldspar, and dissemina- tions and patchy aggregates of mixed sulphides. Feldspar and sulphides together do not exceed 10 to 15per cent of the rock volume. Sulphides locally have enough precious metal content to make ore. Alteration is sufficiently intense that original rock features are obscured in hand sample, but polished slabs or thin sections reveal the volcanic or he- Plate 39. Chlorite-altered wallrock clasts host early dis- seminated sulphide mineralization. Chlorite-altered wal- mier Porphyry dike textures. Silica flooding extends only a lrock was then cut by thick, coarse-grained quam-calcite few metres outward from major siliceous breccia bodies and veins, subsequently cut by bright white quartz veinlets, and may fade out within a vein-stockwork zone as the overall then rebrecciated and cemented by a base-metal-rich sul- vein density decreases outwards (Figure 39). However, pre- phide phase. Silbak Premier glory hole. 80 same mapping in the region. Propylitic alteration should be the Silhak Premier deposits. This has been considered by regarded as a distal indicator of ore potential, defining an Burton (1926), White (19391, Grove (1971, 1986) and Barr are:a worthyof follow-up prospecting or geochemical (1980). Burton felt that somesupergene enrichment of surveys. native silver and polybasite was probable, but White and all ]Pyrite alteration. McDonald (1988) concluded that the subsequent workers up to McDonald (1990a) concluded that pyritic envelope or halo around both the low-sulphide and the ores areprimary, based on their examinations of mineral high-sulphide ore types may he independent of spatially textures and determination of paragenesis. Silbak Premier asociated silica and potassic alteration envelopes. In con- ores were emplaced at a considerable depth; McDonald trast, the writer has noted slightly varying pyrite textures concluded 600 metres depth based on fluid inclusion stud- tha.t seem to be characteristic of each alteration zone (Fig- ies. The deposit was subsequently covered by many hun- ure: 39). In either case, the pyritic envelope could be mapped dreds of metres of volcanic and sedimentary rocks. There is as an independent alteration phenomenon and regarded as no lithostratigraphic evidence of a downcutting Jurassic ye11 another proximal indicator of ore. erosional episode that could have exposed the uppermost oresto weathering or supergene enrichment. Burial was .Silica alteration hosts very fine to fine-grained anhedral followed by regionalmetamorphism. Mid-Cretaceous to granular disseminated pyrite resembling dust orfine- deformationtextures postdate the entire paragenetic ground pepper. Potassic alteration is associated with gener- sequence of ore minerals, including the paragenetically late ally coarser, more abundant (up to 8to lo%), fine to polybasite (McDonald, 1988). Therefore there was no time medium-grained euhedral disseminated pyrite that is more in the history of the deposit that it was unroofed, until its dktinctive and produces a shotgun-pellet or coarse-ground very recent exposure by Quaternary glaciation, and there pe:pper sizepattern. Pyrite is virtuallyabsent from the was no period when supergene processes could have gener- carbonate-altered zone and the propylitic alteration zone ated the paragenetically youngest minerals in these ores. hosts fine to coarse-grained euhedral disseminated pyrite with smooth faces. Pyrite in the propylitic alteration zone is GENESIS diritinctly coarser and more abundant (5% versus 2%) than in the regionally metamorphosed greenschist facies country A simple intrusive sequence relates the different Premier rocka~ Porphyry dike rock types to stratigraphically stacked extru- Zoning. As elevation increases, the decline in the total sive equivalent rocks. This intrusive ‘model’ accounts for amount of sulphide minerals (Brown, 1987) coincides with all the types of intrusive rock and predicts that hydrothermal an increase in silver sulphosalt minerals (McDonald, 1988), breccia veins willbe concentrated in exactly the same an increase in the silver togold ratio (Hanson, 1935; Grove, position (relative to dikes and the paleosurface) as many of 1971, 1986), and a decrease in fluid inclusion homogeniza- the Silbak Premier ore zones. The model resolves the prob- tion temperatures (McDonald, 1990a,b). lems of: Low-sulphide veins and breccias are dominant in the 0 ‘folded’ and ‘unfolded‘ subvolcanic dikes and sills and upper levels of the deposit andin the northwest-striking extrusive flows M.ljn zone. High-sulphide veins and breccias are dominant 0 the phenomenon of “telescoped” epithermal deposits in the lower part of the deposit and in the west-northwest- (Barr, 1980; Grove, 1986) striking West zone. 0 apparentlateral mineralogical zoning (McDonald, Metallic minerals display strong vertical zonation within 1990a,b). the mine. Minerals change from sulphosalt assemblages in In addition it accounts for unusual features such as the the topographic top of the deposit to more base-metal-rich hematiticrock-flour dikes first documented by Grove asremblages at the bottom. Below 3-level, ore minerals are (1971). pr(2dominantly pyrite, galena, sphalerite and minor chal- Premier Porphyy dikes in the mine area record three copyrite.Silver-bearing minerals are present mainly subvolcanic intrusive events that each produced extrusive between3-level and surface (polybasite, pyrargyrite, equivalent rocks now preserved as the three mappable units acanthite and freibergite). Tetrahedrite, native silver and of the Premier Porphyry Member. The events must have e1,ectrum maintainrelatively constant abundances. occurred over a relatively short period of time as littleor no Tetrahedrite is foundthroughout the deposit but above sediment accumulated between them. The volcanic edifice 2-level the content of silver exceeds 20 per cent, indicating wasemergent and the mine area represented a local a changeto freibergite. In addition, the greater abundance of topographic high (Figure 35). The peculiar dike pattern at silver sulphosalts near the top of the deposit means silver Silbak Premier can be projected into a pre-erosional cross- gr,ades and si1ver:gold ratios increase with elevation (Fig- section (Figure 40) that emphasizes the difference in orien- UP2 37). tation between early subvertical plagioclase hornblende por- This mineralogical and metal zoning may provide away- phyry dikes and later, curving potassium feldspar megacrys- UFI indicator and correlates well with homogenization tem- tic dikes. perature isotherms of fluid inclusions that indicate that The intrusive sequence and genetically related hydrother- temperatures decreased upwards within the ore zones from mal brecciation and veining are illustrated schematically in 250°C to 205°C (McDonald, 1990a). Figure 41. The initial subvolcanic magma (Figure 41a) was Higher ore grades near surface suggest that secondary or composed of plagioclase hornblende porphyry, texturally supergene enrichment may have affected the upper levels of similar to the ‘type’ Premier Porphyry dikes but lacking the 81 characteristic potassium feldspar megacrysts. The accom- lized in the fracture as massive subvertical dikes, including panying extrusive unit was deposited as a thinly laminated the one that now forms the northern contact of the West to thin-bedded, rhythmically bedded, crystal-rich tuff (mas- zone (Figure 37). sive to laminated plagioclase porphyry, Plate 5). The origi- This second eruption produced the Premier Porphyry nal conduit may have been a single elongatefissure. A large flow and was the most voluminous of the three eruptions. It bodyof this plagioclase hornblende porphyry lies smc- was followed by a second period of quiesence and magma turally above the main ore zones at the Silbak Premier mine. subsidence. This created increasing stress in rocks overlying It isan apparently homogeneous mass that has little breccia- the magma chamber (Figure 41b). The hemispherical dis- tion, alteration or mineralization and has consequently been tribution of thisstress field is governed by decreasing a disappointing exploration target despite its similarities to lithostatic pressures as the surface is approached. As stress the thinner, curving potassium feldspar megacrystic dikes. increased, shear fractures propagated through the caprock Pressure release was followed by adiabatic cooling within along a parabolic trace due to the hemispherical shape of the the subvolcanic magma chamber during a period of quies- tensile stress field.This mechanism, originally proposed by cence (Figure 41a) that initiated growth of potassium feld- Anderson (1937), has been presented with modifications by spar megacrysts andquartz crystals. Seismic activity or Phillips(1986). Magma, now rich in newlyformed magma subsidence caused a new fracture to form near the potassium feldspar and quartz, intruded the opening shear existing volcanic neck, allowing injection and extrusion of fractures to form ring dikes, and extruded at surface (Fig- the potassium feldspar megacrystic magma. Magma crystal- ure 41c). These ring dikes were thinner but more numerous WEST EAST METRES low - em- MHI- 400 - MO - 0- Figure 40. Ore deposit model for Silbak Premier mine. Pre-erosional geologic reconstruction in the Silbak Premier mine area. Compare with Figures 38 and 41. le=Upper AndesiteMember: lf=Premier PorphyryMember (undivided); 2a=Betty Creek Formation sedimentary strata; 2b=Betty Creek Formation tuffs; 6c=Premier Porphyry dikes, 82 i. EXTRUSION 2. MAGMA INTRUDES SHEAR FRACTURES D.HYDROTHERMAL CONVECTION Figure 41. Genetic model for the Silbak Premier mine (modified from Anderson, 1937 and Phillips, 1986). than the earlier central feeders. The extrusive equivalent is fluid pressure increased and exceededlithostatic load, major maroon Premier Porphyry tuff, and perhaps the upper part and minor fractures popped open abruptly. These fractures of the PremierPorphyry flow, which represent the last extended the existing fluid-filled fracture networkboth phases of this eruptive event. upward along the dike contact and outward into the wall- rock. Each new fracture opening caused a small, abrupt drop .4n integral part of the final intrusive-extrusive event was in fluid pressure which resulted in mineral precipitation that the: formation of hydrothermal breccia zones on the hang- reflected the chemistryof the solutions at that moment. ingwall sides of the curved shear fractures (Phillips, 1986). Mineral layers in ore display textures reflecting periods of Hydrothermal solutions derivedfrom late magmaticvol- slow, static high-pressure mineral Precipitation during coal- ati:les rose upward from the area of the top of the magma ing, alternating with periods of mineral ‘dumping’ during chamber (Figure 41d). Pathways to surface were intermit- abrupt pressure drops. ten.tly sealed due to mineral deposition and lithostatic load Fracturesoccasionally propagated tothe surface and so that fluids accumulatedin lower pressure, dilational spacesabove the magmachamber. These spaces existed resulted in breaching, flash boiling and catastrophic pres- along the hangingwall side of the ring dikes due to minor sure drop throughout the system with considerable mineral contraction during crystallizaiion and cooling. In contrast, precipitation and accompanying chaotic wallrock spalling the: footwall of a ring dike wasa closed, high-pressure into the evacuated and unsupported fluid chambers. As the environmentbecause the dike itself provided a dome- chambers closed, rock fragments in the centre of the zone shaped impervious cap. werecrushed and ground in a natural autogenous mill. Consequently, older clasts in the core of the breccia veins Hydrothermal solutions gradually accumulated, produc- are crudely rounded in contrast to sharply angular, younger in$: a fluid sheet that progressively precipitated minerals. As clasts near the breccia-vein margins. The shallow fractures 83 that were open to surface sealed quickly; then the more pebble-dike. Such structures characteristically cut the bar- typical cycle of moderate to high-pressure fluid accumula- ren caprock that overlies the mineralized interval of an tion and propagation of minor fractures resumed. epithermal vein system (Sillitoe, 1985, Figure 14). A rock cut along the old track between the B.C. Silver and Sebakwe portals sits stratigraphically above (eastward STRATABOUNDPYRITIC DACITE: and uphill from) the ‘blind’ B.C. Silver ore shoots. Therock MOUNTDILWORTH AND IRONCAP consists of green ash tuff shattered into a sharply angular The stratabound pyritic dacite tuff exposed along the mosaic breccia with a matrix of red, hematitic rock-flour western edge of the Mount Dilworth Snowfield has been (Grove, 1971, Plate IXc). This feature was formed by a gas described in detail in Chapter 3. This unit forms a striking blast and is a monolithologic equivalent of a hydrothermal gossan that has attracted attention for decades. No other TOWARDS VENT- sulphide minerals havebeen noted in this area and no Pond or hoirprlng Fine significantprecious or base metal values have been (buried by next ash flow) disseminated , reported. As part of this study, samples of the unit were collected for assay at 300-metre intervals between its southern termi- nation and Summit Lake. It was hoped that results might indicate an anomalously high zone of base or precious metalsthat would warrant additional sampling and ultimately drill testing of the down-dip extension. Base metaland precious metal values from 14 samples were uniformly low; ‘peak’ values are: less than 3 ppm gold, less than 10 ppm silver, 0.006 per cent copper, 0.07 per cent lead and 0.016 per cent zinc. Iron ranges up to 14 per cent and cos- so*- dch~[ the rocks are relatively high in vanadium, thallium, ger- Deothermal Mds manium, chromium, barium, beryllium and fluorine when Figure 42. Genetic model for the stratabound pyritic compared to most other oresand altered rocks in the district dacite of the Mount Dilworth Formation. (Table 16). ...... I Figure 43. Distribution of veins at the Prosperity, Porter Idaho and Silverado mines near the summit of Mount Rainey. (From plans of Pacific Cassiar Limited. See Figure 11 for topographic and geologic setting and latitudenongitude.) 84 The Iron Cap prospect at the northeast end of Long Lake p.355). The Silverado deposit is exposed on the north face is :another exposure of the same lithology in the same of the mountain, 2.5 kilometres northwest of the Prosperity1 straiigraphic position. The showing has been described by Porter Idaho mine. Dupas (1985,p.313) and Plumb (1956, p.8). Because Plumb’sreport is unpublishedthe relevant section is GEOLOGIC SETTING reproduced here in full: Silver deposits on Mount Rainey are hosted within an “This name derives from the iron-stained erosion surface andesitic to dacitic volcanic sequencethat is the strat- of a single gently-dipping basaltic volcanic flow exposed igraphic extension of rock types hosting precious metal for 3,000 feet along the south side of Joan Creek. It forms deposits in the Salmon River valley to the north (Figures 10, the uppermost horizon of the Bear River volcanic rocks 11,44 and 47c). The Hyder batholith intrudes these volcanic and dipsat about 20” conformably under the lowest rocks on the west and north sides of Mount Rainey (Fig- sedimentary beds of the Salmon River Formation. The ure11). The pluton is medium to coarse-grained biotite upper four feet or so is heavily impregnated with fine- granodiorite; the core is unaltered but the outer 100 metres grained iron pyrite that appears to fill vesicles in the of the batholith is cut by a network of widely spaced (50 to dense, black, fine-grained rock. In addition to pyrite this 100 cm apart) epidote veinlets. Volcanic rocks at the contact rock contains a little chalcopyrite, galena and sphalerite are sheared and cut by epidote and chlorite veinlets. LISwell as minute blebs and veinlets of opaque, pale blue The intruded volcanic section comprises a thick sequence chalcedony. Whilethe gossan is widespread, no eco- of green andesitic coarse ash tuffs interpreted as the top of nomic concentrations of the base or noble metals were the Unuk River Formation (Figure 11). A section of massive found. purple epiclastic conglomerate, 100 metres thick, crops out “Two open cuts had been blasted by the owners near the as a prominent knob on the ridge top, 700 metres east of the topand bottom of the zone,and they report low but intrusive contact and is interpreted as the base of the Betty consistent values in silver, lead and zinc. One sample, Creek Formation. The overlying volcanic sequence farther No. 326, was channeled by the writer across eight feet in east (Figure 44) is a complex succession of andesitic crystal the upper cut (elevation 4,420 feet) and assayed 0.20 and lithic tuffs, including lapilli tuffs and medium to coarse ounces in silver with a trace of gold. No other areas tuff-breccias, and interbedded dacitic crystal tuffs, lapilli worthy of sampling were observed and nothing further tuffs, welded and thinly bedded tuffs, with local thin epi- was done with this zone.” clastic conglomerate beds, black siltstones and argillaceous limestones. A thick section of massive felsic tuffsand At both the Mount Dilworth and Iron Cap occurrences, interbedded coarse tuff-breccias is exposed on the ar&e at pyritic dacite caps a blanket of dacitic pyroclastic rocks. This represents penecontemporaneous or postdepositional the head of Barney Glacier to the northeast of the mine impregnation of pyrite into avariety of rock types, although workings. The artte continues eastward to the peakof the predominant rock type is dacitic lapilli tuff. In some Mount Magee. Beyond the peak, siltstones of the Salmon exposuresthe pyritized rock is a carbonate-mudstone- River Formation havebeen identified, so theunmapped cemented debris flow with heterolithic volcanic and carbo- peak area is the expected location of the stratigraphic exten- nate clasts. The pyritic tuff is interpreted as local pyrite sion of the Mount Dilworth Formation and the basal fos- impregnation around fumaroles, hotsprings and calcareous- siliferous limestone of the Salmon River Formation (Fig- mud-filled brine pools that were scattered about on the ure 11). cooling volcanic sheet (Figure 42). The overall mike of volcanic units in the mine area is north-southwith dips moderately to steeply westward to vertical, but large variations in the strike have been noted. EOCENE DEPOSITS No tops indicators .were found. In order to correlate the Eocene deposits have lead isotope ratios consistent with a rocks in the mine area with the stratigraphic sequence estab- Tertiary age; other geologic features of these deposits indi- lished to the north, the sequence at the mine would have to cate a more specific Eocene age. Some of these deposits be overturned to the east (Figure 11). were regarded as Tertiary prior to this study, others were Hostrocks to mineral zones at ProsperityPorter Idaho are regarded as Jurassic or Cretaceous, most were not cate- predominantly dacitic volcanic rocks, varying from lapilli gorized. The ProsperityPorter Idaho mine was examined in and crystal tuffs to welded tuffs with minor units of thinly detail. bedded tuff. In coneast, hostrocks at Silverado to the north- west are underlying andesitic lapilli tuffs and coarse tuff- SI:LVER-LEAD-ZINCVEINS: breccias. Various felsic and lamprophyre dikes crop ant in and around the Silverado deposits but are less evident in the PROSPERITY~~ORTERIDAHO MINE ProsperityPorter Idaho workings. )High-grade silver-lead-zinc veins of the Prosperity and Silver mineralization on Mount Raineyis present as veins Porter Idaho mines crop out on the upper slopes of Mount within a set of major subparallel brittle fault zones or Rainey, 4.5 kilometres southeast of Stewart (Figure 11). “shears” (Figures 44, 45, 46 and 47c). Six of these shear Mine workings are on the south face of the mountain at structures, spaced roughly 175 metres apart, havebeen 1550 metres elevation (Figure 43). Published reserves are explored at ProsperityRorterIdaho whereas four faultstruc- 826 400 tonnes at 668.5 grams silver per tonne with 5 per tures are known at Silverado. At ProsperityPorter Idaho the cent combined lead-zinc (Canadian Mines Handbook, 1989, main shear zones trend 165” and dip 60” westward. These 85 JURASSIC HAZELTON GROUP LOWER TO MIDDLE JURASSIC 3 FELSIC TUFFS;DUST TUFFS, COARSE ASH TUFFS 2 DACITE TUFFS,CRYSTAL TUFFS, WELDED TUFFS 1 ANDESITE TUFFS,COARSE ASH, TUFFS. CRYSTALTUFFS. LAPlLl -, ~ TUFFS, TUFF BRECCILS,-LOC~ INTERBEDDED EPICLASTICS VEIN/MINERALIZED SHEAR ZONE...... --- FAULT...... - STRIKE AND DIP...... '!?L GLACIER ORSNOWFIELD...... a BUILDINGS., ...... I AIR PHOTO BASE BC 02019 NO. 219 Figure 44. Geological map of the Mount Rainey summit around the Prosperity,Porter Idaho and Silverado mines. 86 I / I Figure 45. Plan of the ProsperitylPorter Idaho mine workings. Plan showsthe surface trace of the veins, the underground mine workings, the location of the veins within the main levels, and the location of the three channel sample sections for this study. 87 HORIZONTAL ANDVERTICAL SCALL 50 1W 150 % u. M€R€S .o 0 0 N N STOPED TO SURFACE - we 0 SURFAC€ TRACE ELEVAlION 1600 m - / 1 WAKE VEIN 1' D TUNNEL WEST 1430 m EAST Figure 46. Cross-section through the FTosperityiPorter Idaho mine workings, along map section 14,000 N. (See Figure 45.) The direction of offset along the shears has not been determined. Slickenside orientations range from horizontal to directly downdip. As there are no marked lithological changes across the shears, and in one area no lithological change at all, even on a microscopic scale, total offset is probably minor, 200 metres or less. OREBODIES The Porter Idaho shear zones are continuous structures, up to 13 metres wide, hosting discontinuous,mineralized lenses or shoots. In unmineralized or weakly mineralized areas material within shear structures consists of varying amounts and sizes of intensely sheared wallrock fragments and blocks set in a gouge or clay matrix. Some shear zone sections are silicified,others are carbonate-cemented (Plate 40), still others aresoft unlithified gouge. Mineralized zones pinchand swell within shear zones resulting in well- mineralized shoots up to 13 metres wide and 250 metres long. High-grade ore shoots extend from surface to a depth Plate 40. Polishedslab shows weakly mineralized, of 200 metres where old mine workings end, still in miner- lithified fault breccia. Pragments arecountryrockdacite tuff alization. There is no mineralization inthe country rock and bull quartz chips from pre-ore quartz veins. Prosperity between the parallel shears. vein, FTosperityiPorter Idaho mine. The distribution of sulphides is complex in the high-grade zones do not splay and are cutbut not offset by lamprophyre sections. Ore lenses consistof one or typically two veins of dikes in the underground workings. All shear zones con- massive sulphide, each about 60 centimetres wide. Veins are tinue northward until covered uphill by talus or ice. South- hosted in recemented fault gouge and in screens of sheared, ward, they terminate at, or are displacedby, a major nortl- altered and mineralized country rock. Massive sulphide dipping east-west fault zone called the Big Rig fault. At veins typically follow, or are near, the footwall and hanging- Silverado, the main mineralized shears trend 155" and dip wall of the main shear structure. They locally converge and 65" westward and the structures split along horse-tail-like swell to form a single vein, up to 2 metres wide, anywhere splays. within the shear. These larger veins are composed of argen- 88 Plate 41. Well-mineralized fault breccia.A. High reflectance view shows sphalerite (white). Minor galena and pyrite and trace ,chalcopyrite are also present.B. Low reflectance view shows strongly bleached and fractured wallrock clasts. Blind vein, Prosperity/ Porter Idaho mine. Plate 42. Photomicrograph of native silver within and adjacent to galena, enveloped by sphalerite with trains of chalcopyrite exsolution blebs. D vein, ProsperityPorter Idaho mine. tiferous galena, minor brown to black sphalerite and quartz. some outcrops manganese alteration predominates, produc- A'djacent to the massive sulphide veins, the shear zone is ingmassive to sheared sooty black gossan; in other mineralized with disseminations, blebsand veinlets of exposuresalteration is predominantlybuff-orange- quartz, buff-weathering carbonate, black manganese oxide weathering carbonate. Sulphide minerals are partially pre- and sulphides. served in some shear zone outcrops, but are leached from On surface, shear zones are recessive and exposures are others, leaving a limonitic boxwork. Shear zones exposed spme. A typical outcrop is a well-sheared zone containing by recent glacial retreat have less oxidation of sulphides; distinctive black and orange coarsely mottled rock. This galena and sphalerite are exposed at surface and traces of colour isdue to manganese and carbonate alteration. In yellowish, powdery greenockite have been noted. 89 TABLE 20 SEQUENCE OF ORE DEPOSITION AT THE PROSPERITYE’ORTER IDAHO MINE RELATIVE GROUND STAGE TIMESPAN SULPHIDES GANGUE PREPARATION ALTERATION AND [AGE] Long ?Minor Surface Weathering: Minor reactivation carbonate; carbonate of shears manganese oxides; hematitic mud. IV Minor Coarse banded calcite, Patchy reactivation coarse quartz, carbonate of shears; quanz+calcite, overprint Late cross-faults; quartz+hematite+chlorite, 11 Late dikes qtz/chlor/qtz+chlod I. qtz+chlor+calcite I11 Short Silver-bearing Sericite flooding, Intense base metal minor fine-grained sericitic sulphides quartz knots, overprint of ser>>qtz>>chlor+carb chlorite, local wallrock silicification Short Major brittle shear [-50 Mal (catadasis) Moderate ExtensionalI Moderate Disseminated Quam veins Chloritized [-55 Ma1 cracks euhedral pyrite wallrocks Snowfields and glaciers on Mount Rainey have retreated minor chlorite and carbonate; and a later phase of quartz- substantially in recent years, exposing extensionsof known chlorite-calcite veining that additional work might resolve veins and reinforcing the concept that the shear stmctnres into different pulses. Crosscutting all of these, the final vein are continuous through the mountain between Prosperity/ phase is an episode of ribbon veins of coarse-grained cal- Porter Idaho and Silverado (Figure 43). Discovery of new cite.Overprinted patches of fine Carbonate, manganese vein outcrops has increased the mineralized strike length to oxide and hematitic mud are attributed to recent surface a horizontal distance of 750 metres with a vertical extent of weathering and redeposition from groundwater. 335 metres. Icefield retreat also exposed other areas on the The sequenceof ground preparation, veining, mineraliza- mountain for prospecting. tion and alteration has been determined from underground mapping and microscopy. This is summarized on Table 20. MINERALOGY There was only one main pulse of sulphide mineralization; In hand sample, massive sulphide veins consist of an local areas of physical sulphide remobilization along nar- aggregate of coarse galena and black sphalerite (Plates 40 row, reactivated shear planes may account for descriptions and 41) with accessory pyrite and quartz; wire silver has of late crosscutting sulphide veins. been noted with the aid of a hand lens (Plate 42).Accessory amounts of tennantite (freibergite) are often suspected in ALTERATION hand sample examination, particularly in high-silver sam- The sequence of alteration is schematically represented ples, hut if the trace amounts of these minerals noted in all on Table 20. Shear zones have sharp borders against the microscopic studies aretypical, then the grains seen in hand country rock. Extensive zones of intense chloritization up to sample are probably sheared or anhedral galena. 10 metres wide and local zones of silicification, up to 0.5 Earlyworkers reported a mineralsuite of galena, metre wide can be seen in hand sample. Both alteration sphalerite, native silver, ruby silver, tetrahedrite, and minor typesare accompanied by minordisseminated fine to amounts of pyrite,chalcopyrite and acanthite. A medium-grained pyrite. Thin section study shows that all petrographic description of a grab sample of massive sul- chloritized rocks are dacite, although in places they are phide vein material adds polybasite, arsenopyrite and trace mapped as andesite. Some ‘silicified’ wallrock has upto electrum to the mineral suite (J. McLeod, personal com- 20‘per cent pervasive sericite; the rock is not so much munication, 1982). Petrography from this study identified silicified as bleached and sericitized. galena,sphalerite, pyrite, chalcopyrite, tennantite, Epidote weathers from surface exposures and hadnot tetrahedrite and native silver, adding only ubiquitous trace been recognized as an alteration mineral at ProsperityPorter tennautite to the establishedlist of minerals. Idaho, although it is conspicuous at Silverado (Plate 20). Gangue minerals include early quartz veins, now pre- Drill coreshows abundant fine to medium-grained dissemi- served as angular chips; synmineralization sericite flooding nated epidote in dacitic wallrocks for several metres dis- with associated knots of fine prismatic quartz crystals and tance away from the shears, but it is replaced or overprinted 90 by ,chlorite or sericite and silica alteration immediately batholith and scattered examples of all the main rock types adjacent to the shears. of Hyder dikes: plagioclase-porphyritic granodiorite, aplite, microdiorite and lamprophyre. GENESIS Intensealteration zones are developed where the The geologic setting of Eocene deposits is remarkably batholith cuts the limy turbidite sequences. Biotite hornfels conisistent eveN though hostrocks vary fromthe oldest rocks imparts a purplish colour to the altered turbidites, hut hed- of the stratigraphic column through to the youngest, and ding is preserved. Calcsilicate skarn zones consist of con- include Early Jurassic to Eocene plutonic rocks (Table 16). centric zones of medium to coarse-grained red garnet, green All occurrences are localized in brittle faults or fractures diopside and white tremolite that obscure the original rock and there is .a dominant trend for mostof these veins, type and fabric. southeastward with a subvertical dip. Economic concentra- tions of argentiferous sulphides appear to be restricted to the MINERAL DEPOSITS pm: of the mineralizedshear that transects more brittle, resistant rocks. For example, deposits are hosted in dacitic The Molly B prospect consists of two separate calcsili- vob:anic rocksof the Mount Dilworth Formation (Silver cate skarn zones. One hosts disseminated scheelite, molyb- Tip, Lion, Start, Unicorn), or the BettyCreek Formation denite, pyrite, chalcopyrite, pyrrhotite and sphalerite. Three (ProsperitylPorter Idaho), or in massive granodiorite of the hundredmetres to the south, asecond mineralized zone Texas Creek batholith (Riverside). There is a close spatial consists of pyrrhotite,chalcopyrite, pyrite and trace ass,ociation hetween silver-lead-zinc deposits and Early to sphalerite. Between 1940 and 1941, the southern skarn zone Mi(idle Eocene igneous rocks, both earlier dikes (55 Ma) produced 290 tonnes of ore grading 2.36 grams per tonne and later plutons (52-48 Ma). gold, 12.01 grams per tonne silver and 0.7 per cent copper. A less obvious correlation, hut one that is critical for The Red Reef prospect, staked in 1904 hy pioneer pros- explaining the difference between Jurassic and Eocene pector D.J. Rainey, consists of large patches, blebs, dis- metal suites, is a close spatial association between Tertiary seminations and stringers of pyrrhotite, pyrite, minor chal- ore deposits and Jurassic turbidite sequences. Reduced, car- copyrite and trace bornite in calcsilicate skarn. The highest honaceous turbidite sequences characteristically have ele- reported assays from an array of grab and chip samples are vated silver contents. Contentsof 20 to 30 ppm silver 3.0 per cent copper and trace gold and silver. represent a geochemical concentration factor of the order of The Oral M prospect was discovered in the early 1930s 100 to 500 (Laznicka, 1985). The minor turbidite units and named in honour of the wife of Harry Melville, man- within the Unuk River Formation near theIndian, Silverado, ager of the Premier mine. Oral Mis an auriferous, sulphide- SilverBasin and Riverside deposits, and the thicker, rich quartz vein hosted by a brittle shear zone that cuts infolded turbidite sequence of the Salmon River Formation hornfels and skarn. The vein strikes 130"and dips 75" ne;u the ProsperitylPorter Idaho, Unicorn, Lion, Silver Tip southwest.Disseminated, patchy to locally semimassive and Start deposits, are probably the main source bedsfor the sulphides in the vein are pyrrhotite, pyrite and chalcopyrite. metals of these Tertiary deposits. Exploration work from 1936 to 1941 comprised adits and open cuts on the surface together with several drill holes. SICARNS: Limited production was achieved in 1939 when 4.68 tonnes MOLLYB, RED REEF AND ORALM PROSPECTS of hand-cobbed ore was shipped, grading 28.5 grams per tonne gold, 68.6 grams per tonne silver and 3.5 per cent Three 'skarn' deposits and several small showings crop copper. In 1941, 7.26 tonnes of ore was shipped assaying out on the lower northwest slopeof Mount Rainey.All three 28.1 grams per tonne gold, 163.9 grams per tonnesilver and deposits on MountRainey are veins or veinstockworks 11.84 per cent copper. Recent chip samples across the adit cutting across country rock that is variably hornfelsed or facereturned values of 23.0grams per tonne gold, 27.8 skarned. The deposits were not examined as part of this grams per tonne silver and 2.04 per cent copper (Davis, study, and questions remain about the age and genesis of 1990). these mineral occurrences. The following summary is com- piled from references listed in MINFILE. Elsewhereon the claim groups, narrow veinlets of coarse-grained silver-rich galena-sphalerite, similar to the GEOLOGICAL SETTING Silverado deposits, cut andesite tuffs. Bedrock on the mountainside consists of volcanic and GENESIS se:dimentary rocks of the Unuk River Formation. Strata of th.e UpperAndesite Member crop out onmuch of the These veins have a similar strike and dip to the silver-rich mountain, hut the deposits are hosted by the turbidites of the Tertiary veins and a close spatial relationship to the Eocene Upper Siltstone Member. In this areathe turbidites are Hyder batholith. Sulphidesare hostedin massive hull distictly limy, with minor limestone interbeds, and lapilli quartz, and sulphides and gangue show no sign of post-ore tuffs within the overlying Upper Andesite Member contain shearing. Although they are not skarn depositssensu sfricru, clasts ofthin-bedded limestone.Intrusive rocks on the the veins are locallized within stronglyhornfelsed and mountainside all belong to the Hyder Plutonic Suite and strongly skarned rocks that were certainly more brittle than include medium-grained biotite granodiorite of the Hyder the unrecrystallized extensions of these units, exposed in the 91 area. Preferential mineral deposition in more brittle country veins. Thesefeatures suggest the Mount Rainey skarn rock types is also a characteristic of Eocene vein deposits. deposits might he Early Jurassic. However, these veins have the higher gold contents and A third possibility is that these deposits represent Early higher gold-silver ratios of the Early Jurassic deposits. In~~~~~~i~ gold.pyrrhotite veinsthat were remobilized during particular, the veins at Oral M have gold-silver ratios that emplacement of [he E~~~~~batholith, but no of an are characteristic of the gold-pyrrhotite veins like the Scot- ~~~l~ ~~~~~~i~ stock remains, tie Gold deposits. Moreover, these veins on Mount Rainey have aclose association between goldand copper thatwas Although the skarn-hosted veins on Mount Rainey pres- noted at a m~croscop~cscale in the scottieGold ores. ent problems to deposit classification, they also demonstrate ~i~~ll~,the pyrrhotite-dominated sulphide assemblage is the potential usefulness of a technique like lead isotope alsocharacteristic of the Early Jurassicgold-pyrrhotite 92 CHAPTER 6 - METALLOGENY AND EXPLORATION REGIONAL GENETIC MODELS of a much older (Mesozoic) arc -has been recognized as the metallogenicassociation with the greatest variety of Regionalgenetic models for eachmetallogenic epoch accumulatedmetals and mineralization styles. Laznicka compareand contrast the different geologic settings and (1985)provides exampleswhich include the Cretaceous geneltic processes that formed the distinctly different, but volcanic belt of PuertoRico, the “Green Tuff” belt of penecontemporaneous,deposit types. Determination of Japan, and the Hazelton Group of British Columbia. Mod- genetic relationships among Early Jurassic deposit types is em analogues exist in the New Ireland - Solomon Islands more: complex because the effects of mid-Cretaceous defor- chain and the eastern Aleutians. mationand metamorphism must be understood and ‘removed’ before developing these conceptual models. EARLYJURASSIC METALLOGENY EARLYJURASSIC Eastward-plungingsubduction generated arc volcanism For Jurassic deposits, Figure 47a emphasizes the strat- on crust 25-30 kilometres thick (Gill, 1981), consisting of igraphic position of the mineral deposits, and indicates that the metamorphosed Paleozoic rocks of the Stikine assem- all deposits except pyritic dacites crosscut stratigraphy, that blage (Figure 52a). Volcanism continued through 40 million is, they are epigenetic. The interpreted paleotopographic years, evolving from basalt to dacite. Hydrothermal convec- setting for the Early Jurassic deposittypes is shownin tion cells developed around cooling subvolcanic plutons and Figu.re 35. Figure 48 emphasizes the variety of structural dikes and deposited ores at depths ranging from as much as conduits and depositional sites that were available in and 4 kilometres up to the paleosurface. The hydrothermal sys- aroundthe Early Jurassic volcano. Subvolcanicshear- tems at different depths should be regarded as separate and hosted vein deposits (Figure48) are: deposits that are transi- independant convection cells with separate chemical, ther- tion,al in setting beween the settings for porphyry copper mal and pressure characteristics. There are broad horizontal andepithermal deposits; deposits that are transitional in bands with little or no record of mineral deposition or major depth of emplacement andin mineralogy and textures hydrothermal alteration that separate the three roughly hori- zontal zones of distinctly different mineral deposit types between mesothermal and eDitherma1 denosits (“leotother- . mal”ofLindgren, 1933; ’“telescoped” of Budiington. (Figure 47b). 1935); and deposits emplaced along the thermal transition Hydrothermal fluids may have derivedtheir characteristic betweenthe ductile and brittle shearingregimes (Nesbitt, Suites Of silver, gold, zinc, leadand copper from the domi- 1988;Colvine, 1989; Foster, 1989). nantly andesitic countryrocks (Gill, 1981;Laznicka, 1985), Many features of the geologic setting and ore deposits of but Gill argues that mostof the metals indeposits hosted by the Stewart dishct are depicted in Figure 49, which was andesitic volcanicrocks are ofprimary magmatic origin. ori@Jy developed for island-arc porphyryandepithermal There is a high Content in magmas in sysrems in the southwest Pacific (Branch, 1976). volcanic arcs; in orogenic andesites this vapour lies in the H-C-0-S-CI-Fsystem. Exsolution of vapourfrom the residual andesite liquid causes magma depletion in the ele- M1:DDLE EOCENE ments which partition strongly into the vapour. Although The genetic model for Eocene deposits emphasizes: the H20is the most abundantvolatile constituent in an orogenic black turbidite source-rocks, the hydrothermal heat-source andesite magma, the magma is more likely to become first provided by the Middle Eocene intrusive rocks, and struc- saturated with CO, or a sulphur species than with H,O. turally and stratigraphically controlled deposition in well- Orogenic andesite magmas typically contain 200 to 2000 shattered brittle rock units. Figures 47c, 50 and 51 show the ppm sulphur, andcould be sulphur-saturated, coexisting spatial relationships between deposits and igneous rocks, with a sulphide liquid, or solid, or anS-species vapour. the range of hostrock types, and the most favourable host- Subaerially erupted andesites lose much of this excess SO, rocks. Figure 51 illustrates geologic settings that have con- by exsolution during eruption. centrated economic quantities of metals. Once the vapour phase forms in chlorine-rich systems, the vaoour is an effective solvent for alkalis.I metals and MK’TA 1.l .J3CF.NY silica &ill, 1981). Chloride comolexes remove metals from ~ ~ ~ ~‘~~ ~~~ ~~~~~~~” ~..” remarkably similar. Differences in pluton composition, in will also transport gold and silver at the upper end of this de:Pfh Of Ore emplacement, and in textures, mineralogy and temperature range, but at the lower temperatures ofepither- metalcontent of deposits are attributed to differences in ore deposition, precious metals will be transported by crustal thickness,crustal composition and effects of bisulphide complexes (~~1-2(B~~~~~ and ~~~l~~,1989). regional metamorphism. Bergerand Henley predict that the totalprecious metal This specific tectonic setting of ‘stacked arcs’ - a young content of epithermal deposits is proportional the concentra- (Eocene) arc constructed on a foundation or crust consisting tion of H,S in the hydrothermal system. 93 HYDERDIKES a TEXASCREEK PWTONIC SUITE tc-TexasCreak Balhollth ti-Summil Lake Stock PramlarPorphyry Dikes 8TRATIFIEO ROCKS SRF-SALMON RIVER FORMATION UDF-MOUNT MLWORTH FORUATIOH &F-BOn CREEKFORMATION UNUK RIVERFORMATION pp-PremierPorphyry Member UA-Upper Andorlte Member US-Upper Sllbtono Member MA-MIddIe Andeslle Member LS-Lowar Sillatone Member LA-Lower Andesite Member EARLY JURASSIC MINERAL POTENTUL STRATA80UND PYRITICOACITE 0 SILVER-GOLD-BASEMETAL VEINS GOLD-PYRRHOTITE VEINS Figure 47. Schematic north-south longitudinal section showing the distribution of Early Jurassic and Eocene ore deposit types and prospective areas within the Stewart mining camp. A. Distribution of Early Jurassic mineral deposits. B. Prospective areas for Early Jurassic mineral deposit types. C. Distribution of Eocene mineral deposits. D. Prospective areas for Eocene mineral deposit types. Vertical exaggeration -221. 94 Subvolcanic shear-hosted gold-pyrrhotite veins Welded Ash Flows formed in en echelon tension gashes developed in the imme- with fumarolic and diate country rock around the plutons during late magma movement. These veins were locallized along the thermally controlled brittle-ductile transition envelope that surrounded each subvolcanic stock. Epithermal silver-gold-base metal veins and breccia veins were deposited at critical pressure-temperature transi- tions in shallower subvolcanic faults and shears andin hydrothermalbreccia zones along dike contacts. These deposits formed from many pulses of mineralizing fluid. Mixing of cool, meteoric groundwater withhot sulphur, chlorine, and metal-bearing magmatic fluids is the most likely mechanism for base and precious metal deposition, because there is no evidence of boiling (Brown,1989; Seward, 1989; McDonald, 1990b). Base-metal-rich deposits of the Stewart camp are similar to low-sulphidation epither- mal vein deposits (adularia-sencite type) that are common in the Phillipines (Sillitoe, 1989). Stratabound pyritic dacite tuffs formed when residual trapped volatiles and magma erupted in a final series of violent dacitic pyroclastic blasts. Venting fumarolic fluids and hotspring pools scattered on this cooling volcanic sheet impregnated local areas with abundant fine disseminated pyrite. The area subsided beneath sea level before subaerial erosion could dissect the composite volcano or remove any ...,.. of the deposits. I Figure 48. Genetic model for Early Jurassic mineral MIDDLE EOCENE METALLOGENY deposit types. Eastward-plunging subduction generated the plutons of the Coast Plutonic Complex that were emplaced within a NO VERTICALEXAGGERATION 0 5 10 15 km EPITHERMAL 5 km ,HOTSPRINGS \ VOLCANOGENIC MASSIVE SULPHIDES LAKE - , 0""""""""" """"""" TRANSITIONAL 5 km PORPHYRY CU-Au SUBVOLCANIC Figure 49. Mineral potential in an andesitic stratovolcano (modified from Branch, 1976). 95 ... 0 0 0 0 MINERALIZATION .. "... ALTERATION BlOTiTE HORNFELS .,. -,, I I /EJ EPiDOTE k SPECUIARHEMATITE I. ...... RIVE ...... LEGEND 2 - BElTY CREEK FORMATION 1 - UNUK RIVER FORMATION le- Upper Andesite Member Id- UpperSiltstone Member hqm - HYDER PLUTONICSUITE IC - MiddleAndesite Member tCg - TEXAS CREEK PLUTONICSUITE 1b - Lower Siltstone Member la- Lower Andesite Member Figure 50. Ore deposit model for the Middle Eocene. 96 """"""""" TYPES OF EOCENEMINERALIZATION """""""""_ VEINS IN BRITTLE FAULTS """""""""" . ------Ag-Pb-Zn """""_" ""_ """""""...... ------ ------PORPHYRY MOLY . ------Olsremlnotad Ag in homfeired sediments . ------""""" . ------ vvvv +++++++ vvv vvvv ++++++++ ++++++++ ++++ +++ + + + -EOCE+NE + + + +++ + + + + BIOTITE+ + + + + + 'GRANODIORITE, +++ ++++ ++++r 11, +++++++++++ ++I-++++++++++++++++ Figure 51. Genetic model for Middle Eocene mineral deposit types. A MAGMATIC BACK-ARC FORE-ARC B TRENCH FORE-ARC ARC OCEAN TRENCH tl4SlN BStewort camp 1. PORTER IDAHO 2. RIVERSIDE Kitsoult Camp 3. KITSAULT "\ ASTHENOSPHERE 4STHENOSPHERE 4. AJAX \ Figure 52. Early Jurassic and Middle Eocene metallogenic models for the Stewart camp. A. Early Jurassic tectonic selling and ore deposits. B. Eocene tectonic setting and ore deposits. Width of diagram=200 kilometres. Vertical exaggeration -41. 91 plate of continental crust 50 to 70 kilometres thick (Gill, SILVER-GOLD-BASEMETAL DEPOSITS 1981), consisting of Paleozoic and Mesozoic sedimentary and volcanic rocks (Figure 52b). Brittle dacitic units and Although the entire thickness of the Upper Andesite Jurassic plutons, shattered by mid-Cretaceous tectonism, Member of the Unuk River Formation is prospective for provided local sites of extensive fracturing favourable for base and precious metal deposits, the strata immediately mineral deposition in Eocene time. underlying the Premier Porphyry Member should be specifi- cally targeted. Large pyritic alteration halos usually provide The Eocene magmatic pulse began with dikeswarms extensive gossans that areproximal indicators of mineraliz- followed by intrusion of the main batholith and satellitic ation. Thorough prospecting will be the singlemost produc- hypabyssalplutons. Hydrothermal convection cells tive tool, although soil geochemistry can be expected to developed around these cooling intrusions; the volume of work well. Geophysical surveys have not been particularly country rock affected was proportional to the size of the helpful. Low-sulphide veins may yield consistently higher igneous body. Associated mineral deposits vary in size precious metal values than the visually more impressive according to the amount of circulating fluid andthe cooling high-sulphide veins. period for the pluton. Specific prospects that deserve reassessment are:High The thick turbidite sequence of the Salmon River Forma- Ore, several showings in the Stoner-Clegg-O’Rourke area, tion and minor turbidite members of the Unuk River For- showings on the Silver Coin and Dan claims, the belt of mation contributedsilver, lead and zinc-to theEocene gossans 2 kilometres long between Dag0 Hill and Union hydrothermal system. The metals precipitated out as coarse- Lake, the Oxedenta-49-Yellowstone trend and the nearby grained galena-sphalerite-pyriteveins adjacent to dikes or Dumas, Lila and Harry showings, and Rainbow. in fractures and shear zones some distance away from larger plutons. Skarn zones developed where plutons cut through STRATABOUNDF’YRITIC DACITES limyturbidites and silver-rich porphyrymolybdenum deposits formed around the apices of smaller stocks. Although the uvritic dacites in the Mount Dilworth area Biitton et al., 1990).Consequently, the Iron Capzone EXPLORATION deservesa thorough re-examination in view of reported minor base metals and anomalous silver values (Plumb, Areas favourable forexploration for the six main depopit 1956). typesare highlighted on Figures47b and 47d. Local Extensive pyritic intervals recur in this unit regionally: at geological settings within these broad zones will be more the toe of Frankmackie Glacier, at the head of D.C. Glacier, highly prospective. at the toe of Knipple Glacier, along the south wall of Knipple Glacier, along the north arm ofTreaty Glacier, along the upper canyon of Stone Creek, on the Unuk River GOLD-PYRRHOTITEDEPOSITS near the mouth of Coulter Creek and at the Eskay Creek property east of Tom MacKay Lake (Alldrick and Britton, The perimeter of the Early Jurassic the inner plutons and 1988; Alldrick et al., 1989). These regionally distributed be the focus for margin of the pluton should exploration pyritic zones are favourable sites for subaerial hotspring- deposits similar tothe Scottie Gold mine. A zoneextending related deposits or forsubmarine volcanogenic massive from 100 metres inside the pluton to 400 metres beyond the sulphide deposits, both along the upper dacite contact and in contact should be thoroughly prospected or covered by soil- the immediately overlying pyritic sedimentary rocks. sampling grids. The outerexploration limit can be extended if dikes or alterationdistributions suggest a hidden apophysis of the pluton. SILVER-LEAD-ZINCDEPOSITS Gossans are strong but small. Areas where dozens of Prospective settings for these deposits are the perimeters small veins and veinlets are clustered, such as the Shasta of Eocene plutons and the interiors and perimeters of the prospect or the banks of the Salmon River downhill from majorEocene dike swarms. The greatest potential lies the LowerDaly-Alaska workings, should be carefully alongside the Hyder batholith, especially where the pluton investigated andmapped. Aseries of samples of bigh- cuts brittle units (dacites, Texas Creek batholith, skams). sulphide rock may return widely varying gold values; this The exploration potential in the dike swarms is alsogreatest should not discourage a thorough evaluation. where dikes cut these same resistant units. 98 Deposits do notprovide strong or large gossans. The Skam and hornfels are not associated with the plutons of mineralized shears are recessive weathering, making them the Lower Jurassic Texas Creek suite in the Stewart camp, difficult prospecting targets, andthey do notgenerate but a molybdenite-bearing skam is well exposed in a rock geo'physical anomalies. Peripheralepidote alteration in quarry adjacent to the Bronson stock in the Iskut mining country rocks is a subtle but important proximal indicator. camp, and the nearby McLymont Creek gold prospect has been identified as anEarly Jurassic skam deposit(Ray Southeast-striking mineralized structures are persistant er al., 1991). The potential for similar skam mineralization andmay host up to fourwidely separated mineral zones in the Stewart camp hasnot yet been investigated; the over a distance of a few kilometres. The intersections of eastern tip of the rock spine that separates the North and thesesoutheast-striking structures with thefew north- South arms of the Berendon Glacier (Figure 11) deserves a striking structures that are known to hostmineralization careful look. should be specific prospecting targets. PORPHYRYMOLYBDENUM DEPOSITS SKARNDEPOSITS Silver-rich porphyry molybdenum deposits in the Alice Skarnmineralization may bepresent wherever the Arm - Kitsault mining camp are developedaround the Eocene batholith or stocks cut turbidite sequences, conse- contact, at the apices of small to moderate-sizeEocene qumtly the perimeters of Eocene plutons are also the most stockswhere they intrudeturbidite sequences (Carter, prospectivearea for skarn deposits. Limestoneand limy 1981). No showings of this type have been identified in the siltstone beds will be selectively converted to skarn; silt- Stewart camp but the optimum setting, around the perime- sto:nebeds will be recrystallized to biotite hornfels. As ters of the Eocene batholith and stocks, is also the most pyrite is relatively minor, and acid generationis buffered by prospective area for Eocene veins and skams. Prospecting abundant carbonate, these deposits do not produce striking work for any of these deposit types should proceed with an gor;sans. awareness of the potential for all Eocene deposit types. 99 REFERENCES Albirick, D.J. (1983): Salmon RiverProject, Stewart, British westernBritish Columbia; in CurrentResearch, Part E, Columbia (104B/1);in Geological Fieldwork 1982,B.C. Min- Geological Survey of Canada, Paper 89-1E. istry of Energy, Minesand Petroleum Resources, Paper Armstrong,J.E. (1944a): Preliminary Map, Smithers, British 1983-1, pages 182-195. Columbia; Geological Survey of Canada, Paper 44-23. Alldrick, D.J. (1984):Geologic Setting of thePrecious Metal Armstrong,J.E. (1944b): PreliminaryMap, Hazelton, British Deposits in the Stewart Area (104B/I); in Geological Field- Columbia; Geological Survey of Canada, Paper 44-24. work 1983, B.C. Ministry of Energ3 Mines and Petroleum Armstrong, J.E. (1949): Fort St.James Map Area; Geological Resources, Paper 1984-1, pages 149-164. Survey of Canada, Memoir 252. Albirick, D.J. (1985): Stratigraphy and Petrology of the Stewart Armstrong, R.L. (1966): WAr Dating of Plutonic and Volcanic Mining Camp (104B/1); in Geological Fieldwork 1984, B.C. Rocks in OrogenicBelts; in Potassium-ArgonDating; Minisfry of Energy, Mines and Petroleum Resources, Paper Schaeffer, O.A. and Zahringer, J., Editors, Springer-Verlag, 1985-1, pages 316-341. Berlin, pages 117-133. Albirick, D.J. (1986):Stratigraphy and Structure in the Anyox Armstrong, R.L. (1988): Mesozoic and Early Cenozoic Magmatic Area (103P/5); in Geological Fieldwork 1985, B.C. Ministry Evolution of the Canadian Cordillera; Geological Sociely of of Energy, Mines and Petroleum Resources, Paper 1986-1, America, Special Paper 218, pages 55-91. pages 211-216. Barr, D.A. (1980):Gold in theCanadian Cordillera; Canadian Alldrick, D.J. (1987):Geology and Mineral Deposits of the Instituie Mining and MetaNurgy, Bulletin, Volume 73, Salmon River Valley, Stewart Area (104A. B), 150 B.C. of 000; Number 818, pages 59-76. Ministry of Energy, Mines and Petroleum Resources, Open File 1987-22. Barton, P.B., Jr. and Bethke, P.M. (1987): Chalcopyrite Disease in Sphalerite:Pathology and Epidemiology; American Miner- Alldrick, D.J. (1988): Detailed Stratigraphy of the Stewart Mining alogist, Volume 72, pages 451-467. Camp; in Precious Metal Deposits of theStewart Mining Camp, GeologicalAssociation of Canada, FieldTrip Barton, P.B., Jr., Bethke, P.M. and Roedder, E. (1977): Environ- Guidebook C, 15 pages. ment of Ore Deposition in the Creede Mining District, San Alldrick, D.J. (1989): Volcanic Centres in the Stewart Complex, in Juan Mountains, Colorado: Part Ill. Progress Toward Inter- pretation of the Chemistry of the Ore-forming Fluid for the Geological Fieldwork 1988, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1989-1, pages 233-240. OH Vein; Economic Geology, Volume 72, pages 1 -24. Alldrick, D.J. (1991): Geology and Ore Deposits of the Stewart Berg H.C., Jones, D.L. and Richter, D.H. (1972): Gravina-Nutzotin Mining Camp, British Columbia; unpublished Ph.D. thesis, Belt; Tectonic Significance of an Upper Mesozoic Sedimen- The University of British Columbia, 344 pages. tary and Volcanic Sequence in Southern and Southeastern Alaska and Adjacent Areas, United States Geological Survey, Alldrick,D.J. and Britton, J.M. (1988):Geology and Mineral Professional Paper 800D, pages 1-24 DepositsoftheSulphureIsArea(104.4/5, 12; 104B/8,9);B.C. Ministry of Energy, Mines and Petroleum Resources, Open Berger B.R. and Henley, R.W. (1989): Advances in the Under- File 1988-4. standing of EpithermalGold-Silver Deposits, with Special Reference to the Western United States; in The Geology of Alldrick, D.J., Britton, J.M., Webster, I.C.L. and Russell, C.W.P. Gold Deposits: The Perspective in 1988, Economic Geology, (1989): Geologyand Mineral Deposits of the Unuk Area Monograph 6, pages 405-423. (104B/7E, 8W, 9W, IOE); B.C. Ministry of Energy, Mines and Petroleum Resources, Open File 1989-10. Boyle, R.W. (1979): The Geochemistry of Gold and its Deposits; Alldrick, D.J., Brown, D.A., Harakal, J.E., Mortensen, J.K. and Geological Survey of Canada, Bulletin 280, 584 pages. Armstrong, R.L. (1987a): Geochronologyof the Stewart Min- Branch, C.D. (1976): Development of Porphyry Copper and Strat- ing Camp (104B/1),in Geological Fieldwork 1986,B.C. Min- iform Volcanogenic OreBodies During the LifeCycle of istry of Energy, Mines and Petroleum Resources, Paper AndesiticStratovolcanoes, in Volcanism in Australasia, 1987-1, pages 81-92. Elsevieiel: Amsterdam, p.337-342. Alldrick, D.J., Gabites, J.E. and Godwin, C.I. (1987b): Lead Iso- Britton, J.M., Fletcher, B.F. and Alldrick, D.J. (1990): Snippaker topeData from the Stewart Mining Camp(104Bll). in Map Area (104B/6E, 7W, low, 11E); in Geological Field- Geological Fieldwork 1986, B.C. Minisrry of Energy, Mines work 1989, B.C. Ministry of Energy, Mines and Petroleum and Perroleum Resources, Paper 1987-1, pages 93.102. Resources, Paper 1990-1, pages 115-125. Alldrick, D.J. and Kenyon, J.M. (1984): The .Prosperity/f'orter Brookfield, A.J., Compiler, (in preparation):Terrane Map of Brit- IdahoSilver Deposits (103P113); in GeologicalFieldwork ish Columbia, Compiled from Tectonic Assemblage Map of 1983, B.C. Ministry of Energy, Minesand Petroleum the Canadian Cordillera, J.O. Wheeler. Resources, Paper 1984-1, pages 165-172. Brown, D.A. (1987):Geological Setting of the Volcanic-hosted Alldrick, D.J., Mortensen, J.K. and Armstrong,R.L. (1986): SilbakPremier Mine, Northwestern British Columbia; Uranium-LeadAge Determinations in the Stewart Area unpublished MSc. thesis, The Universily of British Columbia, (104B/1), in GeologicalFieldwork 1985, B.C. Ministry of 216 pages. Energy, Mines andPerroleum Resources, Paper 1986-1, pages Brown, G.C. (1982): Calcalkaline Intrusive Rocks: Their Diver- 217-218. sity, Evolution,and Relation to Volcanic Rocks; in Andesites, Anderson, E.M. (1937):The Dynamics of Formation of Cone Thorpe, R.S., Editor, John Wiley and Sons, pages 437-461. Sheets, Ring Dykes and Cauldron Subsidences;Royal Society Brown, K.L. (1989): Kinetics of Gold Precipitation from Experi- of Edinburgh Proceedings, Volume 56, pages 128-157. mental Hydrothermal Sulfide Solutions. Economic Geology Anderson, R.G. (1989): A Stratigraphic, Plutonic and Structural Monograph 6, The Geology of Gold Deposits: The Perspec- Framework forthe Iskut River Map Area (NTS 104B),North- tive in 1988, pages 320-327. 101 Buddington, A.F. (1929): Geology of Hyder and Vicinity, South- Northem Cordilleran Precious Metal Deposits; Society of eastem Alaska: UnitedStares Geological Survey, Bulletin Economic Geologists, Field Trip Guidebook, 24 pages. 807, 124 pages. Eisbacher, G.H. (1981): Late Mesozoic-Paleogene Bowser Basin Buddington, A.F. (1935): High-temperature Mineral Associations Molasse and Cordilleran Tectonics,Western Canada: in Sedi- at Shallow to Moderate Depths: Economic Geology, Volume mentation and Tectonics in Alluvial Basins, Miall, A.D., Edi- 30, P.205-222. tor, Geological Association of Canada, SpecialPaper 23, Burton, W.D. (1926):Ore Deposition at Premier Mine, British pages 125-151. Columbia; Economic Geology,Volume 21, Number 6, pages Erdman, L.R. (1985): Chemistry of Neogene Basalts of British 586- 604. Columbia and Adjacent Pacific Ocean Floor: A Test of Tec- Carter, N.C. (1972): Toodoggone River Area, B.C.; in Geology, tonicDiscrimination Diagrams; unpublished MSc. thesis, Exploration and Mining in British Columbia 1971, B.C. Min- The University of British Columbia, 294 pages. istry of Eneqy, Mines and Petroleum Resources, pages 63-70. Ewing, T.E. (1981): Petrologyand Geochemistry of the Kamloops Carter, N.C. (1981): Porphyry Copper and Molybdenum Deposits Group Volcanics, BritishColumbia; CanadianJournal of of West-central British Columbia; B.C. Ministry of Eneqy, Earth Sciences, Volume 18, pages 1478-1491. Mines and Petroleum Resources, Bulletin 64, 150 pages. Fisher, R.V. andSchminke, H.-U. (1984):Pyroclastic Rocks: Cho, M. and Liou, J.G. (1987): Prehnite-Pumpellyite to Greensch- Springer-Verlag, New York, 472 pages. ist Facies Transition in the Karmutsen Metabasites, Vancou- Fleet, M.E., MacLean, P.J. and Barbier, J. (1989):Oscillatory- ver Island, B.C.: Journalof Petrology, Volume 28, pages zonedAs-bearing Pyrite from Stratabound and Stratiform 417-443. Gold Deposits: An Indicator of Ore Fluid Formation: Eco- Colvine, A.C. (1989): An Empirical Model for the Formation of nomic Geology, Monograph 6, pages 356-362. Archean Gold Deposits: Products of Final Cratonization of Foster, R.P. (1989): Archean Gold Mineralization in Zimbabwe: theSuperior Province, Canada. EconomicGeology Implications for Metallogenesis and Exploration, Economic Monograph 6, The Geology of Gold Deposits: The Perspec- Geology Monograph 6, The Geology of Gold Deposits: The tive in 1988, pages 37-53. Perspective in 1988, pages 54-70. Coney, P.J., Jones, D.L. and Monger, J.W.H. (1980): Cordilleran Galley, A.G. (1981): Volcanic Stratigraphy and Gold-Silver Occur- Suspect Terranes: Nature, Volume 288, pages 329-333. rences on the Big Missouri Claim Group, Stewart, British Cookenboo,H.O. and Bustin, R.M. (1989):Jura-Cretaceous Columbia: unpublished MA. thesis, University of Western (Oxfordian to Cenomanian) Stratigraphyof the North-central Onfario, 181 pages. Bowser Basin, Northem British Columbia;Canadian Journal Gill, J.B. (1981):Orogenic Andesites and PlateTectonics; of Earth Sciences, Volume 26, pages 1001-1012. Springer-Verlag, New York, 390 pages. Cox, K.G., Bell, J.D. and Pankhurst, R.J. (1979): The Interpreta- Godwin, 12.1. andSinclair, A.J. (1982): Average Lead Isotope tion of Igneous Rocks; Allen and Unwin. London. Growth Curves for Shale-hosted Lead-Zinc Deposits, Cana- Davis, J.W. (1990): Wild Weasel Project: unpublished company dianCordillera: Economic Geology, Volume 77,pages report, Taiga Consultants Ltd., 16 pages. 675-690. Dawson, G.M. (1877): Report on Explorations in British Colum- Godwin, C.I., Gabites, J.E. and Andrew, A., (1988): Leadtable: A bia; Geological Survey of Canada, Report on Progress, Galena Lead Isotope Database for the Canadian Cordillera; 1875-1876, pages 233-265. B.C. Ministry of Energy, Mines and Petroleum Resources, de RosenSpence, A.F. (1976):Stratigraphy, Development and Paper 1988-4, 188 pages. Petrogenesis of the Central Noranda Volcanic Pile, Noranda, Griffiths, J.R. (1977): Mesozoic - Early Cenozoic Volcanism, Plu- Quebec; unpublished Ph.D. thesis, University of Toronto. tonism and Mineralization in Southern British Columbia: A Diakow, L.J. (1990): Volcanism and Evolution of the Early and Plate Tectonic Synthesis: Canadian Journal of Earth Sci- Middle Jurassic Toodoggone Formation, Toodoggone Mining ences, Volume 14, pages 1611-1624. District, British Columbia: unpublished Ph.D. thesis, Univer- Griffiths, J.R. and Godwin, C.I. (1983): Metallogeny andTectonics sity of Western Ontario, 178 pages. of Porphyry Copper-Molybdenum Deposits in British Colum- Dodson,M.H. (1973): Closure Temperature in CoolingGeo- bia: Canadian Journal of Earth Sciences, Volume 20, pages chronologicaland Petrological Systems; Mineralogy and 1000-1018. Petrology, Contributions, Volume 40, pages 259-274. Grove, E.W. (1971): Geology andMineral Deposits of the Stewart Dowling, K.and Momson, G. (1989): Application of Quartz Area, British Columbia; B.C. Ministry of Energy, Mines and Textures to the Classification of Gold Deposits Using North Petroleum Resources, Bulletin 58, 229 pages. Queensland Examples: in The Geologyof Gold Deposits:The Perspective in 1988, Economic Geology, Monograph 6, pages Grove, E.W. (1973): Detailed Geological Studies in the Stewart 342-355. Complex, Northwestern BritishColumbia: unpublished Ph.D. thesis, McGill University, 434 pages. Duffell, S. (1959): Whitesail Lake Map Area, British Columbia: Geological Survey of Canada, Memoir 299, 119 pages. Grove, E.W. (1986): Geology and Mineral Deposits of the Unuk River Salmon River - Anyox Area; B.C. Ministry Energ) Dupas, J.P. (1985): Geology of the Spider Claim Group on Long - of Mines and Petroleum Resources, Bulletin 63, 152 pages. Lake (104N4); in Geological Fieldwork 1984,B.C. Ministry of Enerav, Mines and Pefroleum Resources, Paper 1985-1, Hancock, P.L. (1972): The Analysis of En-echelonVeins, Geologi- pages 308-315. cal Magazine, Volume 109, Number 3, pages 269-276. Dykes, S.M., Meade, H.D. and Galley, A.G. (1986): Big Missouri Hanson, G. (1929): Bear River and Stewart Map-areas, Cassiar Precious - Base Metal Deposit, Northwest British Columbia, District,British Columbia; Geological Survey of Canada, Memoir 159, 84 pages. in~~ MineralDeoosits of the NorthemCordillera: Canadian I ~~ ~ Institute ofMining and Metallurgy, Special Volume 18, pages Hanson,G. (1935): Portland Canal Area, British Columbia: 202-215. Geological Survey of Canada, Memoir 175, 179 pages. Dykes, S.M., Payne, J. and Sisson, W. (1988): Big Missouri Pre- Heald, P., Foley, N.K.and Hayba, D.O. (1987):Comparative cious - Base Metal Deposit, Northwest British Columbia, in Anatomy ofVolcanic-hosted Epithermal Deposits: Acid- 102 Sulfate and Adularia-Sericite Types; Economic Geology, Vol- McDonald, D.W. (1988): Progress Report for Westmin Resources ume 82, Number I, pages 1-26. Limited;unpublished report, Westmin Resources Limited, Henley, R.D. and Ellis, A.J. (1983): Geothermal Systems, Ancient May, 1988,72 pages. and Modem; Earth Science Reviews, Volume 19, pages 1-50, McDonald, D.W. (1990a): Temperatureand Composition of Fluids Hielanen, A. (1967): On the Facies Series in Various Types of in the Base Metal Rich Silhak Premier Ag-Au Deposit, Stew- Metamorphism; Journal of Geology, Volume 75,pages art, B.C.; in GeologicalFieldwork 1989, B.C. Ministry of 187-214. Energy, Mines and PetroleumResources, Paper 1990.1, pages Hodgson, C.J. (1990): Uses (and Abuses) of Ore Deposit Models 323-335. inMineral Exploration, Geoscience Canada, Volume 17, McDonald, D.W. (1990b):The Silhak Premier Silver-Gold Number 2, pages 79-89. Deposit:A Structurally Controlled, Base Metal Rich Cor- Holbek, P.M. (1983):Ore Petrography of the Big Missouri dilleran Epithermal Deposit, Stewart,B.C.; unpublished Ph.D. Deposit,Northwestern British Columbia; unpublished thesis, University of Western Ontario, 41 1 pages. directed studies report, The University of British Columbia, Monger, J.W.H. (1977): Upper Paleozoic Rocks of the Western 35 pages. Cordillera and their Bearing on Cordilleran Evolution;Cana- Holmes, A. (1946): An Estimate of the Age of the Earth; Nature, dianJournal of Earth Sciences, Volume 14,pages 1832- Volume 157, pages 681-684. 1859. Holmes, A. (1947): A Revised Estimate of the Age of the Earth, Monger, J.W.H. (1984): Cordilleran Tectonics: A Canadian Per- Nature, Volume 159, page 127. spective; SocietL; Geologique de France, Bulletin, Volume Houtermans, F.G. (1946): The IsotopicAhundances in Natural XXVI, Number 2, pages 255-278. Lead and the Age of Uranium, Natunuissenschaften, Volume Mullen, E.D. (1983): MnO/TiO,P,O,: A Minor Element Discrim- 33, pages 185.186. inantfor Basalt Rocks of OceanicEnvironments and its Hudson, T., Smith, J.G., and Elliott, R.L. (1979): Petrology, Com- Implications for Petrogenesis; Earth and Planetary Science position and Age of IntrusiveRocks Associated with the Letters, Volume 62, pages 53-62. QuartzHill Molybdenite Deposit, Southeastern Alaska; Nesbitt, B.E. (1988): Gold Deposit Continuum: A Genetic Model Canadian Journal of Earth Sciences, Volume 16, pages for Lode Au Mineralization in the Continental Crust; Geol- 1805-1822. ogy, Volume 16, pages 1044-1048. Johnson, R.B. (1961): Patternsand Origin of Radial Dike Swarms Nguyen, P.T., Booth, S.A., Both,R.A. and James, P.R:(1989): The Associatedwith West SpanishPeak and Dike Mountain, WhiteDevil Gold Deposit, Tennant Creek,Northern Ter- South-central Colorado; Geological Society of America, Bul- ritiory,Australia; in The Geology of GoldDeposits: The letin, Volume 72, pages 579-590. Perspective in 1988, Economic Geology, Monograph 6, pages Kindle, E.D. (1954): Mineral Resources, Hazelton and Smithers 180-192. Areas; Geological Survey of Canada, Memoir 223 (revised). NorthAmerican Commission on StratigraphicNomenclature Knopf, A. (1936): Igneous Geology of the Spanish Peaks Region, (1983): North American Stratigraphic Code; American Asso- Colorado; Geological Society of America, Bulletin, Volume ciation of Petroleum Geologists, Bulletin, Volume 67, Num- 47, pages 1727-1784. ber 5, pages 841-875. Langille,E.G. (1945): SomeControls of OreDeposits at the ONeill, J.J. (1919): Salmon River District, PortlandCanal Mining Premier Mine; Western Mine6 Volume 18, Number 6, pages Division, British Columbia; Geological Survey of Canada, 44-50. Summary Repon, 1909, Part B, pages 7-11. Lai:nicka, P. (1985):Empirical Metallogeny, Developments in Parrish, R. and Roddick, J.C. (1984): Geochronology and Isotope Economic Geology; Volume 19, Elsevier: Amsterdam, 1758 Geologyfor the Geologist and Explorationist; Geological pages. Association of Canada, CordilleranSection, Short Course Leach, W.W. (1909): The Bulkley Valley and Vicinity; Geological Number 4, 71 pages. Survey of Canada, Summary Report, 1908. Paterson LA. and Harakal, J.E. (1974): Potassium-ArgonDating of Leach, W.W. (1910): TheSkeenaRiverDistrict; GeologicalSuruey Blueschists from PinchiLake, Central British Columbia; of Canada, Summary Report, 1909. Canadian Journal of Earth Sciences, Volume 11, pages LeBas, M.J., LeMaitre, R.W., Streckeisen, A.L. and Zanettin, B. 1007-1011. (1986): Chemical Classification of Volcanic Rocks based on Phillips, P. (1989): Silhak Premier; the Total Alkali-Silica Diagram; Journal of Petrology, Vol- The Northern MinerMagazine, ume 27, pages 745-750. May, 1989, pages 15-16. Lin.dgren, W. (1933): Mineral Deposits, 4th edition; McGraw-Hill, Phillips, W.J. (1986): Hydraulic Fracturing Effects in the Forma- New York, 930 pages. tion of Mineral Deposits; Institution of Mining and Metal- MacDonald, B.W.R. (1987): Geology and Genesis of the Mount lurgy, Transactions, Section B, Applied Earth Science, Vol- Skukum Tertiary EpithermalGold-Silver Vein Deposit, ume 95, pages B17-B24. Southwestern Yukon Territory(lOSD/SW); unpublished Plumb, W.N. (1956):Report on the M.J. Mineral Deposits, MSc. thesis, The University of British Columbia. unpublished company report, British Silbak Premier Mining MacIntyre, D.G. (1985):Geology and MineralDeposits of the Company Limited, 10 pages. TahtsaLake District, West-central BritishColumbia; B.C. Plumb, W.N. (1957): Preliminary Geological Report on the Silver Ministry of Energy, Mines and PetroleumResources, Bulletin Tip Mine; unpublished company report, British Silbak Pre- 75, 82 pages. mier Mining Conpany Limited, I3 pages. Ma.rsden, H. and Thorkelson, D.J. (in press): Geology of the Portland Canal Press Ltd., (1910): MammothReef of Free Milling Hazelton Volcanic Belt in British Columbia; Implications for Ore Found; The Portland Canal Mine6 Volume 1, Number the Early to Middle Jurassic Evolutionof Stikinia; Tectonics. 19, p. 1. McConnell, R.G. (1913): Portions of Portland Canal and Skeena Randall, A.W. (1988): Premier Gold Project: Geological Setting MiningDivisions, Skeena District, British Columbia; and Mineralization of the Silbak Premier and Big Missouri Geological Survey of Canada, Memoir 32, 101 pages. Deposits; in Precious Metal Deposits of the Stewart Mining 103 Camp, Geological Association of Canada, Geology and Met- Souther, J.G. (1972): Telegraph Creek Map-area, British Colum- allogeny of Northwestern British Columbia Workshop, Field bia; Geological Survey of Canada, Paper 71-44, 38 pages. Trip Guidebook C, 13 pages. Souther. J.G. (1977): Volcanism and Tectonic Environments in the Ray, G.E., Jaramillo, V.A. and Ettlinger, A.D. (1991): The McLy- Canadian Cordillera;Geological Association of Canada, Spe- mont Northwest Zone, Northwest British Columbia: A Gold- cia1 Paper 16, pages 3-24. rich RetrogradeSkam? (104B); in Geological Fieldwork Stacey, 'J.S. and Kramers,J.D. (1975): Approximation of Ter- 1990,B.C. Ministry of Energy Mines and Petroleum restrial Lead Isotope Evolution by a Two-stage Model; Earth Resources, Paper 1991-1, pages 255-262. and Planetary Science Letters, Volume 26, pages 207- 221. Read, P.B. (1979):Preliminary Geological Mapping of theBig Steiger, R.H. and Jager, E. (1977): Subcommission on Geochronol- Missouri Property near Stewart, Northern British Columbia; ogy - Convention on the Use of Decay Constants in Geo- and unpublishedreport, Geotex Consultants Limited, 17 pages. Cosmochronology; Earth and Planetary Science Letters, Vol- Rock, N.M.S. (1977): The Nature and Origin of Lamprophyres: ume 36, pages 359-362. SomeDefinitions, Distinctions, and Derivations; Earth- Streckeisen, A. (1975): To Each Plutonic Rock Its Proper Name; Science Reviews, Volume 13, pages 123-169. Earth-Science Reviews, Volume 12, pages 1-33. Sabine, P.A., Hamson, R.K. and Lawson, R.I. (1985): Classifica- SutherlandBrown, A. Editor(1976): Porphyry Deposits of the tion of Volcanic Rocks of the British Isleson the Total Alkali- Canadian Cordillera, The Canadian Institute of Mining and Oxide-SilicaDiagram, and the Significance of Alteration; Metallurgy, Special Volume 15, 510 pages. British Geological Survey Report, Volume 17, Number 4, 9 Thomson, R.C., Smith, P.L. and Tipper, H.W. (1986): Lower to pages. Middle Jurassic (Pliensbachian to Bajocian) Stratigraphy of Schofield, S.J. and Hanson, G. (1922): Geology and Ore Deposits the Northern Spatsizi Area, North-central British Columbia; of the Salmon River District, British Columbia; Geological Canadian Journal ofEarth Sciences, Volume 23, pages 1963- Survey of Canada, Memoir 132, 81 pages. 1973. Schroeter, T.G., Lund, C. and Carter, G. (1989): Gold Production Thorkelson, D.J. (1988): Jurassic and Triassic Volcanic and Sedi- and Reserves in British Columbia; B.C. Ministry of Energy, mentary Rocks in Spatsizi Map Area, North-central British Minesand Petroleum Resources, OpenFile 1989-22, 55 Columbia; in Current Research, Part E, Geological Survey of pages. Canada, Paper 88-1E, pages 43-48. Seraphim, R.H. (1947):The Scotty Group, Unpublished BSc. Tipper, H.W. and Richards, T.A. (1976): Jurassic Stratigraphy and History of North-central British Columbia;Geological Survey thesis, The University of British Columbia, Vancouver, B.C., of Canada, Bulletin 270, 73 pages. 31 pages. van der Heyden, P. (1989): U-Ph and K-Ar Geochronometq of the Seward, T.M. (1989): The Hydrothermal Chemistryof Gold and Its Coast Plutonic Complex, 53"N to 54"N. British Columbia, Implications for Ore Formation:Boiling and Conductive and Implicationsfor the Insular-IntermontaneSuperterrane Cooling as Examples, Economic Geology Monograph 6, The Boundary; unpublishedPh.D. thesis, The University of British Geology of Gold Deposits: The Perspective in 1988, pages Columbia, 392 p. 398-404. Vernon, R.H. (1986): K-feldspar Megacqsts in Granites - Pheno- Siems, P.L. (1984): Hydrothermal Alteration for Mineral Explora- crysts,not Porphyrohlasts; Earth-Science Reviews, Volume tionWorkshop Lecture Manual; University of Idaho, 580 23, pages 1-63. pages. Wemicke, B. and Klepack, D.W. (1988): Escape Hypothesis for Silberman, M.L. and Berger, B.R. (1985): Relationship of Trace- the Stikine Block; Geology, Volume 16, pages 461-464. element Patterns to Alteration and Morphology in Epithermal Wheeler, J.O. and McFeely, P. (1987): Tectonic Assemblage Map Precious-metal Deposits; Reviews in Economic Geology, Vol- of the Canadian Cordillera and Adjacent Parts of the United ume 2, pages 203-232. States of America; Geological Survey of Canada, Open File Sillitoe, R.H. (1985):Ore-related Breccias in Volcano-plutonic 1565. Arcs; Economic Geology, Volume 80, pages 1467-1514. White, W.H. (1939): Geology and Ore-deposition of Silbak Pre- Sillitoe,R.H. (1989): Gold Deposits in Westem PacificIsland mier Mine Limited; unpublished M.A.Sc. thesis, The Univer- Arcs: The Magmatic Connection; in The Geology of Gold sity of British Columbia, 78 pages. Deposits:The Perspective in 1988, Economic Geology, Winkler,H.G.F. (1979): Petrogenesis of Metamorphic Rocks; Monograph 6, pages 274-291 Springer-Verlag, New York, 345 pages. Smith, J.G. (1973):A Tertiary LamprophyreDike Province in Woodsworth, G.J. (1980): Geology of Whitesail Lake (93E) Map Southeastern Alaska; Canadian Journal of Earth Sciences, Area, British Columbia; Geological Survey of Canada, Open Volume IO, pages 408-420. File Map 708. Smith, J.G. (1977): Geology of the Ketchikan D-l and Bradfield Woodsworth, G.J., Anderson, R.G., StNik, L.C. and Armstrong, Canal A-I Quadrangles, Southeastern Alaska; United States R.L. (in press): Plutonic Regimes, Chapter 15, in Decade of Geological Survq Bulletin 1425, 49 pages. North American Geology; Geological Survey of Canada, Solie, D.N., Riefenstuhl, R.R.and Gilbert, W.G. (1991): Prelimin- Special Volume. ary Results of Geologic and Geochemical Investigations in York, D. (1984): Cooling Historiesfrom 40Ar/'9Ar Age Spectra; in the Hyder Area, Southeast Alaska, Department of Natural Annual Review, Earth and Planetary Sciences Letters, Vol- Resources, State of Alaska, Public Data File 91-8, 11 pages. ume 12, pages 383-409. 104 Plate 6. Rhythmic graded beds of coarse wacke Plate 10. Stratigraphy in Mount Dilworth Forma- (grey) to hematitic mudstone (maroon), east shoreof tion, north end of 49 Ridge. Note colour-mottling of Daisy Lake. Strike north, tops to east towards Troy Lower Dust Tuff and spectacular gossansof the Pyri- Ridge. tic Tuff. Height from base of photo to top of pyritic knob is 30 metres. Plate21. Photomicrograph showing strong chlorite-carbonate alteration in groundmass of Pre- ~nierPorphyry dike rock. Silbak Premiermine. Plane Plate 23. Sheared and altered wallrock, Scottie !polarized light. Feldspar crystal is 2.4 mm long. Gold mine. Left. Low reflectance view of strongly sheared wallrock shows rich pink to coral manganese carbonatealteration (rhodochrosite). Right. High reflectanceview shows distribution of sulphide trains, mainly pyrite,within this sheared, altered wallrock. Scale bar in centimetres. Plate 33. Photomicrograph of characteristic fea- Plate37. Low-sulphide breccia shows late, tures of delafossite. Left. Bouyoidaltexture, rose- coarsely intergown calcite-quartz gangue. Clasts are brown colour, moderate pleochroism. Right. Strong locally ringed by early sequentiallayen of pyrite and anisotropy in shades of blue or gfeenish blue. S-1 mixed base metal sulphides (dark grey). Note single deposit, Big Missouri area. clast of blue-grey chalcedony. 2-level trench, Silbak Premier mine. 105