Geology and Mineral Deposits, East Flank of the Elkhorn Mountains, Broadwater County, Montana

GEOLOGICAL SURVEY PROFESSIONAL PAPER 665 Geology and Mineral Deposits, East Flank of the Elkhorn Mountains, Broadwater County, Montana

By M. R. KLEPPER, E. T. RUPPEL, V. L. FREEMAN, and R. A. WEEKS

GEOLOGICAL SURVEY PROFESSIONA~·L PAPER 665

A descriptive report of the geology, mineral deposits, and mines in an area east of the Boulder batholith l i.

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UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1971 .A

UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY W. A. Radlinski, Acting Director

) ., " Library of Con~ess catalog card No. 70-611921

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For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 _ CONTENTS

Page Page Abstract ______1 Igneous rocks-C_ontinued Introduction ______2 Composite intrusive bodies______27 ; Location and access ______2 Thermal metamorphism______29 '· Fieldwork and acknowledgments ______2 Quartz latite porphyry dikes of Eocene(?) age______29 Previous work ______3 Basalt flows______29 Sedimentary rocks ______3 Rhyolite flow______30 Paleozoic rocks ______3 Structural geology______30 Park Shale ______3 Folds______31 r Pilgrim Dolomite ______3 Faults______31 Red Lion and Maywood Formations------~ 4 Faults associated with folding______31 Jefferson Dolomite ______~ __ 4 Northwest- and northeast-trending faults______32 Three Forks Shale ______4 Range-front faults------32 Madison Group ______4 Other faults ____ :______32 Big Snowy Group and Amsden Formation ____ _ 5 Summary of. Late Cretaceous and Tertiary tectonics_ 33 Quadrant Formation __ ------____ _ 6 Mineral deposits______34 Phosphoria Formation and related strata ______6 History and production______34 Mesozoic rocks ______7 Types of metallic mineral deposits______36 Swift Formation ______7 Oxidation and enrichment______39 Morrison Formation ______7 Description of deposits______39 Kootenai Formation ______7 Iron Age Gulch and Weasel Creek areas (Win- Colorado Formation_-_-- ______8 ston and Beaver Creeks)______39 Lower black shale unit ______8 Parkarea______42 Middle siliceous mudstone and sandstone Hassel area______43 unit ______9 Radersburg area (Cedar Plains) ______44 Upper black shale unit ______10 Quartzite Ridge area______46 Slim Sam Formation ______11 Other areas and deposits______48 Tertiary rocks ______------______12 Nonmetallic mineral deposits______48 Oligocene sedimentary tuff ______12 Possible future discovery and development of min- Sedimentary tuff of uncertain age ______13 eral deposits______48 Oligocene tufa ______----______13 Measured sections______49 Unconsolidated Tertiary and Quaternary deposits __ ~ 13 1. Morrison, Swift, Phosphoria (and related strata), Fan deposits and mantle ______13 Quadrant, and Amsden Formations near In- Gravel deposits of uncertain age ______14 dian Creek______49 Quaternary deposits ______------______14 2. Morrison and Swift Formations near Montana Glaciation and glacial deposits ______14 Silver Star mine______52 DepositEt resulting from mass wasting ______15 Alluvium ______3. Kootenai Formation and parts of the Colorado and 15 Morrison Formations______53 Igneous rocks ______15 Elkhorn Mountains Volcanics ______4. Colorado Formation near south end of mapped 16 area______54 Intrusive rocks related to the Elkhorn Mountains 5. Colorado Formation along and near the Indian 20 Volcanics------Creekroad______56 Younger intrusive rocks ______23 Mafic rocks ______24 6. Slim Sam Formation (part) and Colorado Forma- Intermediate and felsic rocks ______24 tion near Iron Mask mine______58 Granodiorite and related rocks ______24 7. Lower part of middle member of the Elkhorn Porphyritic quartz monzonite ______26 Mountains Volcanics______60 Quartz monzonite porphyry and related References cited______62 rocks ______27 Index______65 m

/ IV CONTENTS ILLUSTRATIONS

[Plates are in pocket]

PLATE 1. Geologic map and structure sections of the east flank of the Elkhorn Mountains.

2. Geologic maps of the Little Olga lower adit (Kleinschmidt mine), Edna No.2 adit, Silver Saddle adit, and January mine, Winston mining district. 3. Geologic map of parts of the Marietta, Gold Dust, Switzerland, and Sitting Bull mines, Park mining district. 4. Geologic maps of the Diamond Hill mine area, Blacksmith mine, and Little Giant mine, Hassel mining district, and Santa Anita and Ruby mines, Quartzite Ridge mining district. Page FIGURE 1. Index map showing area of this report and of recent reports on adjacent areas ______2 J 2. Diagrams showing nomenclature of coarser grained igneous rocks ______16 3. Triangular diagrams showing modes of intrusive rocks, east flank of Elkhorn Mountains------­ 22 4. Sketch map· of Clancy, Townsend, Devils Fence, and Radersburg quadrangles, showing principal mines and mining . areas------34. 5. Map showing distribution of metallic mineral deposits along the east flank of the Elkhorn Mountains __ ------37. 6. Geologic map of the Iron Cross magnetite mine ______--: __ ------______48

TABLES

Page TABLE 1. Chemical analyses, norms, and modes of igneous rocks------18 2. Summary of mine production, east flank of Elkhorn Mountains------35

1 GEOLOGY AND MINERAL DEPOSITS, EAST FLANK OF THE ELKHORN MOUNTAINS, BROADWATER;COUNTY, MONTANA

By M. R. KLEPPER, E. T. RUPPEL, V. L. FREEMAN, and R. A. WEEKs

ABSTRACT mafic rocks ranging from syenogabbro to calcic syenite ; many The Elkhorn Mountains are an oval-shaped mountain mass stocklike b:XUes, from a few acres to a few square miles in in west-central Montana that is located between the city of outcrop area, consist of felsic rocks ranging in composition Helena and the valley of the Jefferson River. This report de­ from syenodiorite to granite ; in the central part of the area scribes the geology and mineral deposits of the east flank of a few small intrusive bodies are composite and consist of this mountain mass, a strip about 6 miles wide and 26 miles one or more mafic rock types and one or more felsic types. long. Where age relations can be deter~ined the mafic rocks are older than the felsic rocks. For descriptive purposes the felsic The stratified rocks include a thick and, for the most part, rocks can be subdivided into three petrographic groups : ( 1) conformable sequence of marine and nonmarine sedimentary medium-ln'ained equigranular roc~s, mainly granodiorite but rocks, which are overlain by a volcanic pile of Late Cretaceous b • ranging from syenodiorite and monzonite to quartz monzomte, age and by consolidated and unconsolidated sediments of Cen­ (2) porphyritic quartz monzonite characterized by conspicu­ ozoic age. The prevolcanic sedimentary rocks range in age from ous K-feldspar megacrysts, and (3) porphyries ranging from Middle Cambrian to Late Cretaceous and include a sequence granodiorite ·to granite in composition. Age relations of rocks about 4,000 feet thick of marine limestone, dolomite, and sub­ within the felsic group could nowhere be determined. Medium­ ordinate clastic rocks of Paleozoic age and a sequence almost grained equigranular rocks are widespread and much more as thick of marine and nonmarine rocks of Mesozoic age that abundant than the other petrographic types which are re­ are dominantly shale, mudstone, siltstone, and fine-grained sand­ stricted to small bodies in the north half of the area. stone, but that also includes limestone and some coarser grained The structure of the mapped area i!s simple in gross aspeet cla·stic rocks. The youngest of these sedimentary rock units is the Slim Sam Formation, a sequence that is mainly marine but is complex in detail. In late Cretaceous and early Tertiary sandstone in the lower part and nonmarine tuff in the upper time the layered rocks were folded into a broad syncline, part.· which trends north through the central part of the ~rea, and is broken by many faults of simHar trend that formed con­ The Slim Sam is gradationally overlain by the Elkh?rn temporaneously with the folding ; the syncline is one of a series Mountains Volcanics of Late Cretaceous age, which in this of north-trending folds between the Boulder batholith to the area comprises mainly rhyodacitic and trachyandesitic pyro­ west and the Lombard thrust to the east. Later, the rocks clastic rocks of the lower member of the fO'rmation and rhyo­ were broken by northeast- and northwest-trending tear faults. litic welded tuff, trachyandesitic and rhyodactic tuff, and tuff The youngest faults ·are range-front faults that bound the breccia of the lower part of the middle member ; the upper momitain front in the north part of the area ; movement along part of the middle member and the upper member of the Elkhorn the range-f:ront faults began before deposition of Cenozoic Mountains Volcanics have been eroded. sediments and has been recurrent almost to 1the present time. The sedimentary and volcanic rocks are cut by many bodies The east flank of the Elkhorn Mountains has been a significant of intrusive igneous rocks, including fine-grained rocks, mainly tSource of metallic minerals and has yielded metals valued at porphyries, related to and of the same age as the Elkhorn about $17 million. Gold mined from lode deposits in the Raders­ Mountains Volcanics that were emplaced before or at the outset burg, Winston, Park, and Hassel districts and from placer of folding, and by younger generally coarser grained intrusive deposits at Radersburg, along Indian Creek, and along Weasel rocks that were emplaced subsequent to volcanism and folding. Creek accounts for about three-fourths of the total value. The intrusive equivalents of the Elkhorn Mountains Volcanics Silver, lead, zinc, and copper account for most of the remainder. comprise porphyritic rhyodacite, trachyandesite, and trachy­ andesite in sills, dikes, and small partly concordant bodies rang­ The metallic mineral deposits comprise auriferous pyrite veins ing to syenodiorite porphyry and granodiorite porphyry in larger that contain sparse quartz, quartz veins that contain auriferous pyrite, base- and precious-metal veins that typically have quartz partly concordant bodies; the largest of these, the Rattlesnake gangue, silver-lead-zinc replacement deposits in carbonate rocks, intrusive, c.rops out over an area of 20 square miles and is im­ skarn deposits at or near intrusive igneous contacts, a titanif­ perfectly laccolithic in form. erous magnetite deposit, and gold placers. Most of the deposits The younger intrusive rocks form small, commonly discord­ in the Radersburg and Hassel districts are auriferous pyrite ant stocks or bodies of unknown configuration, and they range veins; the deposits in the Winston area are quartz veins with in composition from gabbro to granite. These rocks span about auriferous pyrite and base- and precious-metal veins. Gold Placers the same compositional range and were emplaced during ap­ were mined at Radersburg, along Indian Creek from Hassel to proximately the same time interval as .the Boulder batholith the mouth, and along Weasel Creek. The replacement deposits to the west. .A: few small stocks and dikes consist wholly of of silver, lead, and zinc are mainly in the Quartzite Ridge area. 1 . 2 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

The principal skarn deposits are west of Radersburg and at 112° Diamond Hill; titaniferous magnetite has been mined west of Radersburg. Perhaps as much as half the ore mined from the several types of vein deposits has been oxidized or partly oxi­ dized; only a few veins have been profitably mined below the zone of oxidation. Despite extensive mining and prospecting, the area is con­ sidered favorable for discovery of additional mi.peral deposits. For example, auriferous py.rite veins or ancient gold ·placers might be concealed beneath the relatively thin Cenozoic deposits south of the Keating and Ohio Keating mines near Radersburg, and the untested outwash gravels along and near Cold Springs Gulch would seem to warrant testing for gold. l

INTRODUCTION LOCATION AND ACCESS The oval-shaped mountain mass west of the Missouri River valley between Helena, Mont., and the J e:fferson River valley is generally referred to as the Elkhorn Mountains. This report describes the geology of the east flank of that range -and the adacent part of tJhe Townsend Valley, an area of about 150 square miles (fig. 1) encompassing the west hal£ of the Townsend 15-minute quadrangle -and the northwest quarter of the Radersburg 15-minute quadrangle. U.S. Highway 287 and the N orthem Pacific Railway cross the northeast corner of the area. Townsend, the seat of Broadwater County, is on the highway and rail­ road about 6 miles east of the area and 34 miles south­ east of Helena. Two villages, Radersburg and Winston, are within the area. Both were formerly centers of min­ ing activity but are now inhabited mainly by ranohers. The area is uninhabited except for these communities, a few I'lanches near them, arrd, from time to time, small mining camps in the mountains. The former mining 0 5 10 15 20 25 MILES community of Hassel (also known as St. Louis) is I " I I I I I I I uninhabited. FIGURE 1.-Ind't:'.X map showing location of present Most parts of the area, except the headwaters of report area and of adjacent areas described in recent Beaver Creek, are accessible by mountain roads. A~ti­ reports. tudes range from 4,200 feet in the Townsend Valley to 8,600 feet ~along ~the divide between Beaver and Crow Creeks. graphic base map by use of a Mahan Plotter. Prin­ cipal ·responsibility for mapping different parts of the FIELDWORK AND ACKNOWLEDGMENTS area is shown on plate 1. E. J. Lyons, H. W. Smedes, The area described in this report is part of a large D. E~ Brambilla, D. M. Pinckney, H. B. Nickelson, A. region in and around the BoUlder batholith that has W. Berg, and R. F. Gosman assisted in the fieldwork been mapped by the authors and their associates. Most at various times. of the fieldwork on which the report is based was done The authors are indebted to many individuaJls and between 1949 ·and 1953. The geology of the area was mining companies for information and maps of min­ plotted directly on preliminary topographic maps pre­ ing properties. Particularly helpful were Albert Dance, pared at a scale of 1 : 24,000 by Fairchild Aerial Sur­ Harry Anders, Palmer Engh, and Fred Sclme:i!der of veys, Inc., except for a strip about 2 miles wi'de along Townsend, Arthur Berg of Radersburg, Arthur and the west edge of the Radersburg quadrangle, within Harold Hogan and Mrs. Charles Muffley of Winston, which the geology was plotted on aeriaJl photographs the late Carl Trauerman of Butte, Herb Carver of Hel­ in the field and subsequently transferred to the topo- ena, Harry Kleinschmidt of Lincoln, Al Nugent of 1SEDIMENTARY ROCKS 3 Wallace, Idaho, Edmund E. Pohl of Seattle, Wash., ments of Tertiary 'age, most of which are only wea:kly and officials of the Anaconda Co. The authol'ls also ru-e consolidated. indebted to W. A. Cobban and the late J. B. Reeside, Most of the sedimentary and volcanic units 'are nvther Jr., both of the U.S. Geological Survey, for material uniform throughout :the Elkhorn Mountains ~and near­ assistance in interpretation and correlation of the · lby. They !have ibeen fully described by Klepper, Weeks, Cretaceous sedimentary rocks. ~and Rup~l ( 1957), Freeman, Ruppel, and Klepper (1958), Roibinson ;(1963), Nelson (1963), and Smedes PREVIOUS WORK (1966), and >accordingly most of them are treated only Several1previous geo'logic studies and investigations summ,arily in this report. New information, such as of minel'lal deposi~ts included i)arts of the area. Stone measured sections and lists of fossils, is included, how­ L (1911) e~amined the minel'lal deposits ,and mapped the ever. geology of partt of the area in reconnaissance. Bard PALEOZOIC ROCKS (1910) and Winchell (1914) :briefly described !the geol­ P~aleozoic sedime:rutary rocks are exposed in two ogy ~and some of the ore deposits of the Radersburg dis­ north-trending ibelts, one about a mile wide along the trict, Condit, Finch, and P~ardee ( 1928,. p. 177-178) re­ po!1ted on occurrence of phospib.aJte rock in the Elkhorn · southwest margin of !the area, ~and the other !aibout half ~a mile wide along lthe east margin, norlth of Crow Creek. Mountains, ~and Corry ( 1933) described some of the gold placer deposits in and adjacent to the mountains. Rocks in the western outcrop belt ~are on the east flank Reed (1951) summarized available information, includ­ of a broa:d north-trending dome (Klepper ~and otJhers, ing :production data for mines in Broadwater CounJty. 1957). Those in rthe eastern outcrop belt ~are pant of !the Reports iby l(nopf (1913), Billingsley (1916), Billings­ east flank of the ibDoad syncline north of 'Crow 'Creek (Freeman and others, 1958). ley ~and Grimes (1918), and Pardee and Schmder (1933), provide va'luruble inform~ation on the regional PARK SHALE st:vatigraphy, structure, peJtrology, and ore deposits. The The oldest formation exposed is the Park Shale of following geologic maps ~and reports have ibeen pu1b­ Middle Cambrian age, which crops out along Johnny lished recently of the ~areas that surround the present Gulch at the west margin of the area. The formation mapped area: The Canyon Ferry .quadrangle Ito the consists mainly of olive-green, gray, and brown shale north mapped by Mertie, Fischer, and Hobbs (1951), but contains a few thin beds of argillaceous limestone, the Elkih.orn MountJains to the west mapped by Klepper, siltstone, and silty sandstone. The formation is 215 Weeks, and Ruppel (1957), Smedes (1966), and Kno.pf feet thick on the north slope of Johnny Gulch (Klep­ (1963), the Three Forks quad!'langle to the south per and others, 1957, p. 10). mapped lby Robinson (1963), ~and the Townsend VraHey to the east mapped by Freeman, Ruppel, ~and Klepper PILGRIM DOLOMITE (1958), Lorenz and McMurtrey (1956), and Nelson The Pilgrim Dolomite overlies the Park Shale with ( 1963) . The ore deposits of the nor.th pant of the Pa:rk apparent conformity. It is 482 feet thick along the mining district have recently Jbeen discussed by Schell north slope o.f Johnny Gulch in SE* sec. 17, T. 5 N., ( 1963) , and those of the Winston mining district, by R. 1 W. (Klepper and others, 1957, p. 11). The lower Earll (1964). GI'Iavity ~and aeromagnetic daJta have been part of the formation; about 50 feet thick, is oolitic and published by ICinoshita, Davis, Smedes, ~and Nelson pebbly bluish-gray limestone overlain by dolomite that weathers mottled light and dark gray. The middle part (1964) ~and Kinoshita, Davis, and Robinson (1965). of the formation, about 200 feet thick, is mainly gray SEDIMENTARY ROCKS limestone with mottles ·and ribbons of yellowish-gray silty dolomite; it closely resembles the mottled lime­ A sequence of consolidated sedimentary rocks rubout stone of the lower and upper parts of the Cambrian 13,000 feet thick and ranging in age from late Precam­ Meagher Limestone, not exposed in this area. The upper ibri,an Ito Cretaceous is exposed in ~and near the Elkhorn part, about 230 feet thick, is medium-bedded, partly Mountains. These rocks underlie a volcanic pile of La;te oolitic, mottled light- and dark-gray dolomite overlain Cretaceous age that in pl~aces was ~art 'least 10,000 feeJt by very :resistant, rough-weathering, thick-bedded light­ and perhaps as much as 15,000 feet thick. The Creta­ gray dolomite. ceous and older rocks are strongly folded and faulted The Pilgrim Dolomite is considered to be Late Cam­ ~and are cut by the Boulder ibath:olivh and related in­ brian. Fossils from 10 feet above the base, at the south trusive rocks of Late Cretaceous age. The m:ajor valleys end of the Limestone Hills, about a mile east of the that flank the mountains .are filled with ash-rich sedi- mapped area (250ft northwest of an abandoned quarry 4 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA in NE% sec. 4, T. 5 N., R. 1 E.), are· typical of the formation. Small lenses and discontinuous thin layers lower Oedaria subzone (A. R. Pahner, 'vritten commun., of dark dolomite breccia, some of which do not conform 1951) ; the collection contains: Oedarina cordillerae to bedding and probably were formed by collapse, are (Howell and Duncan) , B olaspidella wellvillensw present locally. The uppermost 25-40 feet of the forma­ (Lochman and Duncan), Nixonello cf. N. montanemis tion is distinctive :faintly mottled light-gray dolomite Lochman. in beds 1-3 feet thick. Fossils from the middle part of the formation about The formation appears to lie conformably on the 41h miles southwest of the area are characteristic of the Maywood Form,ation and to be overlain conformably middle Oedaria subzone (Klepper and others, 1957, by the Three Forks Shale. It is 478~727 feet thick in p. 11). measured sections nearby (Klepper and others, 1957, p. 14-15; Freeman and others, 1958, p. 548-549) and j RED LION AND MAYWOOD FORMATIONS appears to be 500-600 feet thick in this area. Varicolored argillaceous rocks and impure carbonate rocks above the Pilgrim Dolomite are 134 feet thick THREE FORKS SHALE on the north slope of Johnny Gulch in NE% sec. 17, The Three Forks Shale is poorly exposed in several T. 5 N., R. 1 W. (Klepper and others, 1957, p. 13). The narrow strike valleys along and near the west margin of lower part of the sequence is referred to the Red Lion the area southward from sec. 8, T. 5 N., R. 1 "'\V. It is Formation of Late Cambrian age, and the upper part, about 300 feet thick and consists largely of dark-gray, to the Maywood Formation of Middle and Late De­ greenish-gray, and brown shale. About 200 feet above vonian age (Klepper :and others, 1957). The Red Lion the base, fossiliferous argillaceous gray limestone, 10- appears to lie conformably on the Pilgrim Dolomite. 25 feet thick, in places crops out as a low ledge. The The Maywood is disconformable on the Red Lion; in upper part of ·the formation, about 50 feet thick, is dolo­ fact, near the south end of the Limestone Hills about mitic and calcareous siltstone and mudstone that a mile east of the area, the Red Lion was largely or weathers yellowish brown and yellowish gray. wholly removed by erosion before the Maywood was de­ The Three Forks Shale is l·argely of Late Devonian posited. Because the two formations are similar, thin, age. The upper part is probably a correlative of the and generally poorly exposed, they were mapped as a Sappington Sandstone Member as used by Berry (1943) single unit. The unit is wholly or partly equivalent to of nearby areas and therefore probably contains beds of the Dry Creek 'Shale of earlier literature (Deiss, 1936; J\1:ississippian age near the top. Recently, Robinson Emmons and Calkins, 1913; Lochman-Balk, 1950; (1963, p. 34-38) provided detailed new information and , Sloss and Laird, 1947; Hanson, 1952, Robinson, 1963). summarized the extensive literature on the age of the Along Johnny Gulch the Red Lion Formation is 93 Sappington Member of the Three Forks. feet thick and consists of varicolored siltstone, shale, MADISON GROUP and fine-grained sandstone; most of the rocks are cal­ careous ·and weather to shades of red and yellowish The Three Forks Shale is conformably overlain by a brown. The overlying Maywood Formation is 41 feet thick sequence of gray limestone assigned to the Madi­ thick and consists mainly of silty dolomite and dolo­ son Group of Mississippian age. The lower part of the mitic or calcareous siltstone and shale; the rocks sequence, assigned to the Lodgepole Limestone, is about 700 feet thick and consists largely of thin-bedded lime­ weather to shades of gray and yellowish brown. stone. The upper part of the sequence, about 1,000 feet JEFFERSON DOLOMITE of thick-bedded limestone, is assigned to the Mission The Jefferson Dolomite of Late Devonian age con­ Canyon Limestone. These formations crop out in a sists mainly of fetid dolomite that weruthers to a dark­ belt about a mile wide extending southward along the gray nodular rock commonly with a brownish cast, but west edge of the mapped area from sec. 32, T. 6 N., the formation also con~ains beds of gray limestone and R. 1 W., and in a narrower belt along the east edge of dolomite, which weather lighter shades of gray, and a the area north of Crow Creek. few thin beds of intraformational pebble conglomerate. The Lodgepole Limestone consists almost entirely Much of the rock is formed of clastic carbonate grains. of thin and distinctly bedded medium-gray limestone, Most beds are a few inches to a few feet thick, some 'are typically with a distinct bluish cast on weathered sur­ laminated, and some are cross-laminated; small chan­ fruces. Much of it is lithi:fied carbonate sand. Many beds nels are present locally. Sm·alllenses and discontinuous are fossiliferous and have a fetid odor; some consist thin layers of chert and partings of yellowish-gray largely of waterworn fossil debris. In the lower part of mudstone are present sparingly in the lower half of the the formation the beds range in thickness from 1 inch 'SEDIMENTARY ROCKS 5

to 1 foot and average 3-4 inches; thin interbeds and the Quadrant Formrution. The assemblage is probably partings of yellowish- or reddish-gray calcareous shale 250-320 fe~ thick in most places, but it may ibe as thin or mudstone rure characteristic. In the upper part of 1as 200 feet in ·a few a;>laces al,ong rthe western belt of the formation, beds 2-3 feet thick are interlayed with outcrop ~and perhaps is as much !as 350 fe~ thick in the much thinner beds, mudstone partings are rare, fos­ vicinity of 1ndi,an Creek; ·a;t Indi~an Creek, however, siliferous beds are less abundant, and thin siliceous the parttern of .rock distribution is peculiar and rthe ap­ seams and chert lenses are sparingly present. parent thickness may be grea;ter than Jthe true thickness The Lodgepole rests with abrupt but apparently con­ as a resulrt of rep~tition of beds iby unrecognized minor formable contact on calcareous or dolomitic siltstone folds and faults of small displacement or of deposition of the Three Forks. It grades upward into the Mission on an irregular surface of moderate local relief, perhaps ( '-.. Canyon through a thickness of 100-200 feet. The con­ 'a karst. tact was placed at the top of the uppermost unit of Though rthe conta~t with lthe underlying Mission thin-bedded limestone. Canyon Limestone was nowhere observed, indirect evi­ The Mission Canyon Limestone consists largely of dence indicates Jthat it is a disconformity (IClep:per ~and thick and indistinct beds of medium- to light-gray others, 1957, p. 19-20). Lenses and l~ayers of limestone fine- to coarse-grained limestone; a few beds of darker breccia 'are a;bundant in rthe upper part of Jthe Mission gray limestone are present near the top and bottom of Oanyon Limestone. Some of the breccia is cemented or the formation. The upper half contains sparse nodules, pa11tly cemented by red and yellow limy mud. In a few lenses, and thin layers of gray chert. Beds containing pl,aces tthe mud is laminated ~and inclined ·rut the same abundant small twiglike bodies of white crystalline cal­ angle as rthe enclosing Hmestone. The observ,rutions indi­ cite are common in the middle part. cate !that the ibrecda formed, probrubly by collapse of The formation weathers light gray to medium gray caverns, ~and was cemented 1prior ito regional folding. and is locally mottled light olive gray. Weathered sur­ The similarity of some of rthe cave fillings lbo the over­ faces differ in detail; some rure rough and cuspate and lying rocks suggests that near-surface oaves formed and others are smooth or only slightly rough. Thin, irre­ coUapsed just before or while the Big .Snowy ~and Ams­ gular, siliceous crusts coat some surfaoos and stand den rocks were being depos]ted. fn most places the sur­ in slight relief. face on whiPermian rocks of the area, and of the Elkhorn massive quartzite as much as 25 feet thick are present Mountains in general, would be referred to the Shed­ in the upper 100 feet. Some of the quartzite b~ds are horn Sandstone and to the Tosi Chert Member and conspicuously crossbedded. The formation is 327 feet the Retort Phosphatic Shale Member of the Phosphoria thick in NW% sec. 29, T. 6 N., R. 1 E., about a mile Formation (Mcl(elvey and others, 1959, pl. 3, South , north of Crow Creek (Klepper and others, 1957, p. 20- Boulder Creek Section and pl. 2, East Madison Sec­ 21) . Outcrops suggest that the thickness is about the tion). In the thickest section of phosphatic rocks of same throughout the area. Permian age in and near the area (Klepper and others, The Quadrant Formation is more resistant to weath­ 1957, p. 22-23), the lower quartzitic part, about 25 feet ering and erosion than the overlying and underlying thick, would correlate with the lower member of the rocks, and it typically forms ridges. The formation be­ Shedhorn Sandstone of their usage, the overlying 28 haved as a competent but brittle unit during deforma­ feet of chert and phosphate rocks represents the Retort tion, and therefore it formed broad folds and broke and Tosi Members of the Phosphoria Formation, and along numerous fractures. Many smal'l cross-faults dis­ the remainder of the formation is the upper member of place the Quadrant but die out in overlying and under­ the Shedhorn Sandstone. In thinner sections consisting lying less competent rock, as is well illustrated south of almost entirely of quartzite and chert and devoid of Johnny Gulcll. Some of the faults shown on the map phosphate rock, such as the one along Indian Creek, (pl. 1) an!d other parallel fractures along which dis- correlation with the formal and informal units pro­ placement was negligible channeled ore-bearing solu­ posed by McKelvey and others is uncertain. For this tions ; along or adjacent to many of these fractures lead reason and because these rocks constitute at most a and silver minerals and gold replaced favorable ddlo­ single cartographic unit at the scale of the present mite beds in the Quadrant Formation. study, the authors chose to ~retain the familiar term Phosphoria Formation and related strata. PHOSPHORIA FORMATION AND RELATED STRATA The Phosphori·a Formation is overlain disconform­ The Phosphoria Formation and related strata in the ably by the Swift Formation of Late Jurassic age. The Elkhorn Mountains consist mainly of chert and quartz­ disconformity between the Phosphoria and ~Swift For­ itic sandstone, but in places includes one or two thin mations represents all of Triassic time, most of Jurassic beds of phosphate rock, dark-colored· shale or mudstone, time, and perhaps ·part of Permian time. Regionally, the 1SEDIMENTARY ROCKS 7

Phosphoria Form·ation and its equivalents thin north­ place. In some places, the upper 40--100 feet of !the for­ ward and eastward from southwestern Montana. North­ mation includes dark-gray shale •and one or ·a few thin ward thinning of all units of Phosphoria age and the lenticular beds of impure lignite overlain by alternating absence of phosphate rock or phosphatic mudstone varicolored dominantly ·argillaceous rocks, and salt­ from the middle part of the sequence north of Indian and-pepper sandstone, speckled with dark chert grains, Creek indicate thB~t most of the ·thinning is due to non­ simHar lbo that in !the overlying Kootenai Formation. deposition. Locally, however, part of the formation may The interbedding 10f sandstone of Koo!tenai aspect with have been eroded before the Swift Formation was argillaceous rock of Morrison aspect suggests that in deposited. ·these places rbhe !two forma;tions intergrade. On the MESOZOIC ROCKS other hand, dark-gray shale 1and salt-·and-pepper sand­ SWIFT FORMATION stone were not recognized •and ·are [>ro!bably ,a;bsent in other places, which suggests thalt elsewhere there is a Th~ Swift Formation is ·an inconspicuous 15- to 30- disconformity at the top of the Morrison Formation foot-thick unit of brown and brownish-yellow friable and !that locally ·as much •as 50 feet of beds of the Morri­ calcareous marine sandstone speckled with grains of son was eroded before the Koo:tenai was deposited. The black chert and limonite. In places, one or more beds latter intterpretaJtion is in !harmony with tha;t of Cob­ near the base are crowded with broken and specifically ban ·and Reeside ( 1952, cha11t) , who indicated that .rocks unidentifiable shells of Ewmiaroti8, Ostrea, and Oamp­ of earliest Cretaceous age •are probrubly not present in tonectes (R. W. Imlay, oral commun.). A basal con­ Montana. glomerate as much as 3 feet thick, consisting of pebbles KOOTENAI FOMATION and cobbles of chert in a mB~trix of medium- to coarse­ grained sandstone, is present at many localities, and The Kootenai Formation of Early Cretaceous age, small pebbles :are present in some higher beds. Imlay a heterogeneous ·assemJblage of continental rocks, crops (oral commun.) examined these rocks in the field and out in two north-trending belts, one near the west edge referred them to the upper unit of the Swift Form-ation of the southern part of !the ·area and the other near the of the Ellis Group of Late Jurassic age (Imlay and east edge between Crow Creek and Whitehorse Gulch. others, 1948, chart). The rela;tively uniform, though The lower ,part of the formation consists chiefly of l slight, thickness of the formation in and near the area coarse- to medium-grained speckled quartz-chert (sallt­ indicates that it was deposited on a nearly flat surface, ·and-pepper) sandstone. ~he middle ~and !thickest part and the disconformity between it and the underlying consists mainly of red shale and mudsbone or claystone Phosphoria Formation represents a long time of non­ ·and green or greenish-gray siltstone. The upper part deposition but .probably only slight erosion. is gr·ay limestone interbedded w~th dark shale and silt­ The formation is too thin· to be mapped separately stone. The formation is 445 fe~ ,thick ~along the north and is included with the Morrison Formation on the slope of Johnny Gulch (Klepper and others, 1957, p. 25) geologic map. ~and ·529 feet thick h:alf a mile soulth of Indian Creek (p. 53-54). The thickness of the formation seems to be MORRISON FOMATION within this range throughout most of the ~area, but in a The Morrison Formation, a continental deposit of few places irt may be as great as 700 feeJt, judging from Late J ur.assic ·age, overlies ttJhe Swift Formation wiJth outcrop width and attitude of :bedding. apparent conformity. It consists of varicolored beds The /base of the formation is marked iby fairly re­ of shale, mudstone, siltstone, fine-grained sandstone, sistant crossbedded salt-and-pepper sandstone 15-50 and limestone. At most places the formation is 425....J5·50 feet thick tha;t contains abundant chert grains and /that fe~ !thick, but in a few ,pl,aces, perhaps ;as a result of resembles ibut is generally !thicker tJhan the lenticular deformation, the apparent thickness is as much ·as 7·50 sandstones in the upper part ·of !the Morrison Forma­ feet. tion. Similar sandstone is interlayered with shale, mud­ The :formation· consists predominantly of mudstone stone, and siltstone throughout the :formation, except and sHtstone ibult includes many thin beds of fine­ near the to:p, but !the ibeds are generally lenticula-r •and gr·ained sandstone, shale, ·and dense limestone, many of thinner, and most •are finer grained ·and contain less which weather yellowish brown or yellowish gray. Red che11t than those at the base of the formation. shale and mudstone ·are relatively :a!bundant in some The salt-and-pepper sandstones are gray, medium to places, as in the se~tion measured just soU/tJh of the area coarse grained, and locally conglomeratic and show (p. 52-53), and are almost absent elsewhere, as in scour markings and cross-lamination. The sand grains the vicinity of Indian Creek (p. 49-52). The pro­ in different beds and in some single beds are angular portions of other rock ty:pes 1also differ from place Ito to rounded and poorly sorted to well sorted. The dark- 8 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA colored grains are chert; most of the light-colored the lithologic sequence is distinctive and exposures are grains are quartz. Most sandstone beds contain inter­ good enough to permit placement of the contact within _.stitial clay minerals, and some also contain sparse 10-20 feet. chlorite and sericite that appear to replace clay min­ COLORADO FORMATION erals. Small and discontinuous conglomerate lenses con­ The Colorado Formation consisting of marine and tain pebbles and cobbles of quartzite and chert as much nonmarine sandstone, shale, and siliceous mudstone, as 3 inches across. A few lenses also contain pebbles and crops out along the east edge of the mapped area from cobbles of limestone. Chert grains form as much as two­ Crow Creek to Whitehorse Creek and thence discon­ thirds of some sandstone beds in the basal part of the tinuously northward along the range front to Beaver formation but are much less abundant in the middle Creek. Another discontinuous belt of outcrop extends part of the formation and are absent or sparse in the from near the southwest corner of the area to the junc­ upper part of the formation. Accessory minerals are tion of Crow Creek and Muddy Lake Creek. The con­ leucoxene, iron oxides, tourmaline, zircon, and anatase. tact with the underlying Kootenai Formation is prob­ The grains are cemented by secondary quartz, mostly as a:bly a minor disconformity, because in·some places the enlargements of quartz grains, and by calcite. basal sandstone of the Colorado Formation rests on The middle part of the formation, generally 350-400 limestone whereas in other places as much as 40-50 feet feet thick, consists of varicolored beds of siltstone, of dark argillaceous rocks lies between the limestone and argillaceous siltstone/ claystone/ shale, silty shale, and the sandstone. fine-grained sandstone. Olive gray to greenish gray, The formation has been subdivided into three units grayish red, and grayish purple are the most common (Klepper and others, 1957, p. 26-28), the lower and colors. Most of the beds are calcareous, and some contain upper units consisting chiefly of dark shale and the abundant irregular concretions of limestone. middle unit~ chiefly of sandstone and siliceous mudstone. The upper part of the formation, generally about 50 The formation is 1,542-1,639 feet thick near the south feet thick, is charar.terized by beds of dark-gray lime­ margin of :the area (p. 54-56), 1,140 feet thick ~long stone that form low ridge$ and weather light gray. Some Indian Creek (p. 56-58), 1,211 feet thick near the of the limestone beds contain abundant fresh-wwter fos­ Iron Mask mine (p. 58-60), rand 858 feet thick about sils, principally gastropods, and some thin beds are com­ 4 miles north of the area ( Srriedes, 1968, p. 13). The posed almost entirely of fossil detritus. Dark claystone greater pant of the nonthward decrease in thickness and siltstone are interbedded with the limestone, and in is due to thinning of the lenticular nonmarine sand­ places dark-gray or olive argillaceous siltstone overlies stones in the middle ·!lnit of the formation, but the upper , the uppermost limestone bed. black shale unit also appears to thin northward from T. C. Yen (written commun., 1952) identified the about 275 to 66 feet (Smedes, 1966, p. 12). Local incon­ following Prosobanchian gastropodE, species that are sistencies in appare:1t thickness, as for example the common in the Kootenai Formation elsewhere, in col­ great apparent thickness of the lower black shale unit in lections from limestone beds near the top of the forma­ the vicinity of Slim Sam Gulch, are probably due to tion about one-half of a mile south of Indian Creek: unrecognized deformation. Lioplacodes cf. L. convewwul!us Yen, Lioplacodes cf. L. cretaceus (Stanton), Oharydrobia cf. 0. cretaccea LOWER BLACK SHALE UNIT Yen, .~ Reesidella sp. undet. The lower black sha.le unit comprises: (1) quartzitic The lithologies of the Kootenai and Morrison Forma­ sandstone and siltstone (bottom), (2) dark-gray to tions are similar enough that it is difficult to distin­ grayish-black shale and a :few thin interbeds of siltstone, guish between them in areas of poor exposure. and (3) dark-gray earbonaceous argillaceous fine­ Placement of the contact between the two formations grained sandstone and siltstone. The unit is 328 feet is particularly difficult where lenticular salt-and-pepper thick at the south edge of the area and 240-255 feet thick sandstones are present and dark gray shale is absent in the vicinity of Indian Creek and the Iron Mask mine. or inconspicuous in the upper part of the Morrison Its true thickness is probably within or near these limits Form·ation. In such places the contact was arbitrarily throughout; local discrepancies are probably due to un­ mapped at the bottoom of the first thick salt-and-pepper recognized faults or minor folds. The basal quartzitic sandstone beneath predominantly red and green argil­ sandstone may rest on an erosion surface, but the relief laceous rocks and may deviate by 50-100 feet from a con­ of this surface must haV£1 been negligible, for the thick­ sistent stratigraphic position. In most places, however, ness of the sandstone is relatively constant. In all well-exposed sections the basal part of the for­ 1 Included in the term "mudstone" in the measured section (p. 53-54). mation is sandstone, a few feet to 20 feet thick, overlain 1SEDIMENTARY ROCKS 9 by siltstone and thin interbeds of sandstone and dark and chert that are interlocked by secondary outgrowths shale, aggregating 30-45 feet in thickness. Many bed- on quartz grains. ding surfaces are irregula.r, are coated by films of dark The basal sandstone and siltstone is probably equiva­ clay and micaceous minerals, and exhibit ropy markings lent to the Fall River Formation of the typical great resembling worm trails. Some beds are laminated and Plains section (Cobban and Reeside, 1952) and to the some are cross-laminated. The basal sandstone is fine to Flood Member of the Blackleaf Formation of the Sweet­ medium grained and is in beds as much as 4 feet thick. grass Arch section (Cobban and others, 1959). The over­ It typically weathers very pale orange or grayish yellow lying black shale yielded fossils determined by Cobban and crops out as discontinuous low ribs. The siltstone (written commun., 195~) as "bwoeramus oomanoheanus is generally greenish gray or gray, thinly bedded, and and/. bellvuensis, guide fossils to the Skull Creek shale l_ argillaceous and carbonaceous. It weathers to blocky or of the Black Hills and the Purgatoire formation of sla;bby fragments. The sandy and silty rocks grade up- Colorado." The lower part of the dark sandstone ~above ward into dark shale. the shale also yielded an I nooeramus, probably /. bell- In thin section the basal sandstone is a clean ortho- vuensis ,at one locality (p. 56, unit 11 of section in quartzite, consisting of very fine to medium-sized well- sec. 4, T. 4 N., R. 1 W., USGS Mesozoic location 23028) sorted tightly interlocked grains, of which 70-95 per- and is accordingly also a probable equivalent of the cent are quartz, 3-25 percent are chert, and a trace to a . Skull Creek. few percent are chlorite, muscovite, clay minerals, tour­ maline, zircon, and black iron minerals. Original pore MIDDLE SILICEOUS MUDSTONE AND SANDSTONE UNIT space has been filled by quartz, which occurs partly as The middle siliceous mudstone and sandstone unit of secondary enlargements of quartz grains and partly as the Colorado Formation comprises: ( 1) fine-grained cement unrelated to grains in the plane of the thin sec­ sandstone, siltstone, and mudstone (bottom), (2) mud­ tion. Original boundaries of a few rounded to sub­ stone and siliceous mudstone, and ( 3) sandstone. The rounded quartz grains can be detected by lines of in­ unit is largely nonmarine. It ranges in thickness from clusions. The chert has largely recrystallized to a mosaic 930 feet near the south end of the area to about 600 feet of very fine grained quartz. The boundaries of the re­ in the vicinity of Indian Creek and the Iron Mask Mine. ., crystallized chert grains 'are serrate. Four miles north of the area it is 578 feet thick ( Smedes, The basal sandstone and siltstone is overlain by a se­ 1966, p. 13). Part of this variation may be due to unde­ quence about 100 feet thick consisting mainly of fissile tected small faults, but most of it is probably due to dark-gray to grayish-black shale and silty shale but con­ original depositional differences and perhaps to intra­ taining many thin beds of olive and gray siltstone. formational erosion, as suggested by lenticular coarse­ Roughly spheroidal calcareous concretions as much as grained conglomeratic sandstone beds in the upper part. 18 inches in diameter are sporadically present; they The base of the unit has been mapped at the bottom of weather to a rusty color. The shale breaks down readily a distinctive flaggy greenish-gray to medium-light-gray to thin chips or flakes that resist decomposition and fine-grained sandstone that crops out throughout the form very little soil. area. The top has been mapped at the bottom of a se­ Overlying the shale is a sequence consisting mainly quence that consists mainly of dark-gray shale. of dark fine-grained argillaceous sandstone with inter­ Except for a relatively clean basal sandstone, the beds of siltstone and shale. This sequence is about 165 rocks in the lower part of the unit are chiefly thin­ feet thick at the south end of the mapped area and 100- bedded olive-gray and gray siltstone and fine-grained 110 feet thick farther north, between Indian Creek and sandstone that consist largely of quartz and chert, in Whitehorse Gulch. The dark color is due to carbonaceous proportions that range from 10: 1 to 1: 1, hut also con­ and argillaceous material that comprises 15-25 percent tain a trace to 20 percent of feldspar and 20-30 percent of most of the rocks. The fine-grained sandstone is made interstitial clay. Secondary chlorite and calcite and very up of angular· to subrounded grains of quartz and chert small amounts of mica, opaque iron minerals, and zircon in roughly equal amounts, together with lesser amounts also are present. The relative abundance of fresh feld­ of muscovite, biotite, clay, and black opaque minerals. spar, which was not observed in any of the underlying Locally, some of the sandstone beds are speckled with rocks, and the presence of many shardlike grains of limonite and weather rusty. At a few places north of quartz suggest admixture of material from a contempo­ Crow Creek several thin beds of clean sandstone are ex­ raneous but probably distant volcanic source. The thick­ posed. These are composed of medium-grained well­ ness of this sequence in which sandstone and siltstone sorted and probably originally rounded grains of quartz predominate ranges from about 400 feet near. the south. 10 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA end of the mapped area to about 200 feet in the vicinity basis of spairse fossils and. lithology, particularly the of Indian Creek and the Iron Mask Mine. peculiar silic~ous mudstones of probable volcanic origin. The overlying sequence, which consists of olive and Rocks 140-145 feet below the top of the measured sec­ dark- to light-gray silieeous mudstone interbedded with tion along ln. dian Creek (p. 56) conta·in fossils indic­ olive to gray mudstone, siltstone, and subordinate sand­ ative of GreBnhorn or early ·carlile age, and rocks 30- stone, ranges in thickness from 200 to 300 feet. The si­ 35 feet :below the top of the un~t (p. 56) contain fos­ liceous mudstone, described in detail by Klepper, Weeks, sils indicativ-e of middle Carlile age (W. A. Cobban, and Ruppel (1957, p. 27-28), is characterized by white, written corrunun., Sept. 15, 1958). The lower part of the light-gray, and light-greenish-gray colors on weathered overlying upper black shale unit in the same section surfaces, by blocky, hackly, or splintery fracture, and by contains fossils indicative of latest Carlile and earliest abnormal hardness. Subrounded :to shardlike silt-sized Niobrara age (W. A. Cobban, written commun., May 31, ) grains of quartz and feldspar are commonly present in a 1951, and S€~pt. 15, 19·58; J. B. Reeside, Jr., written cryptogranular matrix that may be either devitrified commun., Oct. 3, 1952) . glass (purr.. iceous dust) or a product of the decomposi· Lists and age interpretations of fossils collected from tion of siliceous ash by sea water with penecontempo­ measured S84)tions are incorporated in pages 56-58. raneous deposition of the released silica (Rubey, 1929, Information on fossils collected elsewhere in the area p. 168-169). The siltstone, sandstone, and nonsiliceous are summari:?:ed below : mudstone interbedded with the siliceous mudstone are A collection (USGS Mesozoic loc. 23833) from the thin-bedded olive ,and gray rocks that fracture to splint­ middle part of the siliceous mudstone and sandstone ery or haclcly fragments. These rocks are composed of unit in the CE•nter of sec. 27, T. 5 N., R. 1 W., contains quartz and chert grains and lesser amounts of feldspar, Tempskya gr'J,ndis Read and Brown, "a widespread. and mica, opaque minerals, and fine-grained rock fragments. typical Mowry species" ( J. B. Reeside, Jr., written Clay minerals and locaUy :abundant calcite are the commun., Se:;>t. 24, 1952). cementing materials. Most grains are angular or shard­ A collection from poorly exposed rocks that are fault­ like. A large part of the material is probably from a ed and cut by igneous intrusions in the SE:l)J,NW:l)J, sec. contemporaneous but distant volcanic source. 35, T. 6 N., H 1 W., (USGS Mesozoic loc. 23885) con­ The upper pa,rt of the unit, 165'-240 feet t!hick, con· tains: sists of sandstone and subordinrute siltstone, shale, and I noceranvus n. sp. aff. I. fragilis Hall and Meek conglomerate. The sandstone beds are of two types: I noceran~us sp. fragments of very large shells , light-gray, medium-grained quartz-chert (salt-and-pep­ Lucina aff L. juvenis Stanton per) sandstone and olive-gray, very fine grained to fine­ Oollignoniceras woollgari (Mantell) var. praecox grained thinly laminated sandstone. The salt-and-pep· Hass per sandstone commonly contains scattered pebbles of Another collection nearby, in the center of the NW chert and locally contains lenticmla.r beds of chert­ 1)J, sec. 35, (USGS Mesozoic loc. 23884) contains : pebble conglomerate. Both types of sandstone are com­ posed mainly of angular to subrounded quartz and Lucina j-Utvenis Stanton chert grains and a small amount of interstitial clay Pseudomelania ~ sp. minerals, secondary chlorite, and accessory plagioclase, H aminea ~ sp. muscovite, opaque i,ron minerals, zircon, and tourmaline 0 ollignoniceras sp., fragments and are locaUy cemented by calcite and iron oxide. The These collections indicate equivalence to the lower salt-and-pepper sandstone is similiar to the basal sand­ part of the Carlile Shale ( J. B. Reeside, Jr., written stone of the Kootenai Formation, except that it con­ commun., Sept. 24, 1952). Accordingly, the rocks are tains grains of fibrous chalcedony. The other sand· probably equivalent to rocks in unit 24 of the Indian stones are characterized by small amounts of feldspar Creek section (p. 57). and by a diflty appea,rance due to interstitial clay min­ erals, chlorite, and calcite cement. Some of the argil­ UPPER BLACK SHALE UNIT laceous sandstones are characterized by undulose beds, The upper black shale unit consists of d.ark-gray to ripple marks, 'and fossil-:worm trails. They are prob­ black fissile shale and, particularly in the lower part, ably lagoon deposits. thin interbed~; of very fine grained sandstone and silt­ The lower and middle parts of the middle siliceous stone. Sphero:ldal, brown-weathering limestone concre­ mudstone and sandstone unit are correlated with the tions 8-12 inehes in diamocer are locally abundant in Mowry Shale of the reference sequence for the Western the shale, and thin layers rich in feldspar crystals are Interior (Cobban and Reeside, 1952, chart lOb) on the interbedded in the uppermost part. In small isolated ex- •SEDIMENTARY ROCKS 11 . posures, it is difficult or impossible to distinguish be­ containing layers rich in feldspar crystals, and a few tween the black shales of the lower and upper units. beds of volcanic sandstone also are present. The vol­ Thin beds of very fine grained sandstone and silt­ canic sandstone contains suhrounded to angular and stone comprise most of the lower 50 feet of the unit shardlike grains of quartz, crystals of plagioclase, flakes and 5-10 percent of the rest. These beds consist of of fresh biotite and of an altered mica, and small frag­ tightly packed angular grains of chert and quartz in ments of microcrystalline volcanic rock ; in some beds a proportion of about 3 : 1 ; the matrix is comn1only calcite cement is moderately abundant. Most of the argillaceous. Accessory minerals include plagioclase, material in the volcanic sandstone is probably slightly muscovite, biotite, tourmaline, and secondary calcite. reworked debris of contemporaneous ash falls from a The unit is about 280 feet thick in the vicinity of nearby source. The subrounded quartz grains are prob­ Indian Creek and the Iron Mask Mine (pl. 1) and may ably nonvolcanic. In general, quartz decreases and pla­ be thicker at the south end of the mapped area, though gioclase increases in abundance from the bottom to the the ·apparent ·thickness of 381 feet there (p. 55) may top of this part of the formation. be greater than the true thickness due to repetition of At many places, ·and perhaps generally, one or a few beds by unrecognized strike faults similar to those ·beds of sandstone consisting la.rgely or entirely of de­ mapped in sees. 19, 30, and 31, T. 9 N., R. 1 E. Four trital titaniferous magnetite are present in a grada­ miles north of the area the unit is only 66 feet thick tional zone between rocks with sparse volcanic material (Smedes, 1966, p. 13). below and rocks with abundant volcanic material above. The upper black shale unit contains a fauna similar The magnetite-rich rocks probably accumulate as plac­ to that of the lower two faunal zones of the Niobrara ers along bea:C:hes of a receding sea. Formation of the reference sequence for the Western The upper part of the formation, 400-700 feet thick Interior (Cobban and Reeside, 1952). Lists and age in most places but perhaps as much as 1,000 feet thick assignments of fossils collected from a well-exposed locally, consists mainly of crystal tuff, crystail-lirthic tuff, section along the Indian Creek road and from a section and volcanic sand~tone but contains beds of la.pilli-tuff, near the south boundary of the area are included among sandstone, mudstone, and siliceous mudstone. The tuff the measured sections (p. 54-58). In the Colorado and lapilli-tuff consist of ·angrilar grains of plagioclase Shale of north-central Montana and in the Cody Shale and small fragments of fine-grained volcanic rock and of Wyoming, about four more Niobrara faunal zones less abundant grains of quartz, mica, hornblende, and are known above the zone of Scaphites ventricosis (W. opaque minerals in a m·atrix of indeterminate micro­ A. Cobban, written commun.), the highest zone recog­ crystalline material, possibly devitrified glass, that is nized in the east flank area. partly altered to chlorite and sercite. The sandstones are similar to the same rock types in the lower part of the SLIM SAM FORMATION formation. The mudstone is olive and dark gray; most The Colorado Formation grades upward into a se­ beds contain ·rubund'ant sand-sized grains, and many are quence that consists mainly of sandstone in the lower hard and siliceous and break to hacky fragments. In part and of tuff in the upper part. The name Slim Sam general, bedding is less distinct and ibeds are consider­ Formation was assigned to these rocks by Klepper, ably thicker in the upper part of tJhe formation th1an Weeks, and Ruppel (1957, p. 28-31), and the type lo­ in the lower part. Some beds are cross-la;minated. cality was designated along the east margin of the The few :£ossils collected from the lower p~rt of the Slim Sam Basin, in the area of the present report. The formation indicate that it is of Colorado age and mainly formation crops out along the eastern front of the Elk­ if not entirely of marine origin. Scaphites· cf. S. ventri­ horn Mountains from Staubach Creek southward to cosus Meek and Hayden, common in the Niobrara For­ Crow Creek (pl. 1) and along the west edge of the area mati•on, and probably of the faunal zone next above the from Eureka Creek south ward to the Slim Sam Basin. basal zone of the formation (W. A. Cobban, written Farther south it has been faulted and disrupted by large commun., 1952), was collected from ruternating hlruck intrusive masses· and is present only discontinuously. shale and flaggy sandstone beds in the basal 50 feet of The lower part of the formation, 250-450 feet thick, the formation on the east side of Eureka Greek, ·albout is chiefly gray, greenish-gray, and yellowish-gray thin­ one half of a mile west of the. mapped area ( SW% bedded to medium-bedded sandstone, largely or wholly SW14 sec. 20, T. 7 N., R. 1 W.). "Mactra" arer~.aria of nearshore marine origin. Most beds consist mainly Meek, "a pelecypod not lmown from rocks as young as of quartz and chert grains and contain some argil­ the Montana group" (W. A. Cobban, written ~ommun., laceous matrix ; a few beds contain a little feldspar 1952) was collected from beds near the top of the and mica. Partings and thin beds of dark shale, somo dominantly nonvolcanic lower part of tJhe formation, 12 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 300 and 375 feet above the base in NE·lh sec. 4, T. 4, N., Western Interior (Klepper and others, 1957, p. 26) is R. 1 W., and from ·about 200 feet above the base of the strengthened. formation south of Kimber Gulch. At some places in and around the Elkhorn Moun­ The upper part of the formation contains carbonized tains, including most of the east flank, the Slim Sam wood in many places and a silicified tree stump, ap­ Formation appears to grade upward into the Elkhorn parently in place, at one locality, but no identifiable Mountains Volcanics. But 1in other places nearby, the fossils were found. Consequently, its age has not been Elkhorn Mountains Volcanics rest with angular dis­ fixed as closely as the a.ge of the lower part. It could be cordance on Slim Sam and older rocks (Klepper and of late Nidbrara, Telegraph Creek, or early Eagle age others, 1957, pl. 1, south-central part; Freeman and of the reference sequence for the Western Interior others, 1958 pl. 42, northwest part T. 4 N., R. 1 W.; (Cobban and Reeside·, 1962, pl. lOb), but it is no Smedes, 1966:, p. 21). In these places Slim Sam and J younger, £or the overlying Elkhorn Mountains Vol­ older rocks were folded and eroded before the Elkhorn canics are older than rocks of early Claggett age ( equiv­ l\1ountains Volcanics were extruded. The evidence is alent to lower part of Pierre Shale of the reference compelling that folding affected some parts of the area sequence), as discussed elsewhere in this report. east of the Boulder batholith after the Slim Sam For­ The titaniferous magnetite zone in rocks transitional mation was deposited and before the Elkhorn Mountains from the lower to the upper parts of the formation is Volcanics were extruded, and a few miles north of the believed to have accumulated as placer deposits along east flank area weak folding concurrent with deposi­ beaches as the sea retreated eastward. A similar but tion of the Slirn Sam Formation is inferred ( Smedes, thicker and more persistent zone of titaniferous mag­ 1966, p. 21). In the area of the present report, how­ netite farther north in Montana is at the top of the Vir­ ever, the general apparent conformity within the Slim gelle l\iember of the Eagle Sandstone (W. A. Cobban, Sam Formation and between the Slim Sam Formation oral commun.) which Stebinger (1915, p. 62-63) con­ and the Elkhorn Mountains Volcanics indicates little sidered to be a sandy beach or nearshore deposit laid prevolcanic deformation. down in a retreating sea. Though the magnetite-bear­ ing rocks in these two areas are mineralogically similar TERTIARY ROCKS and accumulated under similar conditions, they are not Ter.tiary sedi12entary rocks include two units of necessarily ·contemporaneous. The gradational relation waterlaid !tuff and one of calcareous tuf·a. The !tuffs are between the lower and upper parts of the Slim Sam part of tJhe ibasin fill in the Townsend ¥alley; one is of Formation in the present area suggests uninterrupted Oligocene ·age ·and one, of uncertain age, and both are deposition of fossiliferous marine rocks, not more than small ~aDts of mo:re extensive uni.ts discussed iby P·ardee 50 feet below the magnetite zone, and the magnetite­ (1925), Merttie, Fischer, ·and Hobbs (1951), Lorenz and rich rocks. If this is true, the upper part of the Slim McMurtrey (1956), Freem·an, Ruppel, ~and Klepper Sam Formation is more likely to be of late Niobra,ra (1958), Nelson (1963), Kinoshita, Davis, 1Smedes, ·and Nelson (1964), Ro:binson (1967), and White (1954). and Telegraph Creek age than of early Eagle age. On the other hand, at a few places within the area the OLIGOCi~NE SEDIMENTARY TUFF lower part of the Slim Sam is abnormally thin, and Gently warped ;beds of sedimentary tuff are poorly son1e.of it may have been removed by erosion before the exposed south rof the Limestone Hills and along Johnny upper part was deposited. A few miles north of the :Gulch. They overlie the pre-Teritiary rocks with marked area the upper part of the Slim Sam is unconformable !angular unconformity ·and are overlain iby several units on rocks of the lower part and is overlain with angular of Qualternary and :perhaps late Tertiary age. Beneath unconformity by the Elkhorn l\1ountains Volcanics ~the mantle of younger deposiJts the unit is probably con­ (Smedes, 1966, p. 16, 21 112). The magnetite zone has tinuous wi1th the Oligocene sedimenltary tuff that crops not been recognized here. Whether its absence is due to out to rthe east and :3outh (Freem,an and others, 1958; nondeposition or erosion is not known. Though these Dunbar Creek Form·.%tion in Rohinson,-1963, 1967) and relations suggest a significant time lapse between the is also considered equivalent to the Tertiary unit 2 de­ lower and upper pa·rts of the Slim Sam Formation, they scribed by Mertie, Fischer, ~and Hobbs (1951) in the seem to indicate a n1ore significant lapse between the north end of the Townsend Valley. Slim Sam and the Elkhorn Mountains Volcanics. Ac­ The sedimentary tu:ff includes beds of shale, mudstone, cordingly, the earlier tentative correlation of ·the upper sandstone, and small lenses of poorly cemented con­ part of the Slim Sam Formation with the Telegraph glomerate and conglHmeratic sandstone ~that contain Creek Formation of the reference sequence for the angul,ar to subrounded fragments ·as much as 10 inches .SEDIMENTARY ROCKS 13 long of pre-Ter1tiary rocks. Bedding ranges from thin SEDIMENTARY TUFF OF UNCERTAIN AGE ~and distinct 1to very ;thick ~and indistinct. The rocks are In the northeast corner of the mapped area, poorly soft, porous, typically cream and buff colored, slightly exposed rocks resembling the Oligocene sedimentary consolidruted, poorly sorted, and contain granules of tuff have been mapped separately. The rocks include older rocks, angular grrains of feldspar and quantz, ·and pale-orange to yellowish-gray silty tuff, some of which glass shards in a light-yellowish-gray matrix of calcite is bentonized, gray and white calcareous volcanic silt­ ~and :benton~tic clay. The content of volcanic material in stone, and deeply weathered conglomerate. Bedding is different beds ranges from nearly 0 to 100 percent ·and obscure and poorly exposed, but in a few places the typically is more than half. Some beds 1are composed beds seem to be horizontal or to dip gently eastward. largely of bentonite; grayish-brown compaot fine­ The lffu.ickness of the unit i'S unknown. l grained rocks when ·fresh, they swell and become por­ Mertie, Fischer, and Hobbs (1951, p. 31) suggested ous and cream colored on weathering. The lenses of that these rocks are probably a part of the Oligocene conglomerate nnd conglomerrutic sandstone typically sequence. Freeman, Ruppel, and Klepper (1958, p. 512) con~ain little or no megascopically identifi.aJble volcanic favored the interpretation that they are younger and mruterial. rest unconformably upon the Oligocene rocks. The greatest exposed thickness of Oliogocene sedi­ mentary tuff in the mapped area is about 1150 feeJt, but OLIGOCENE TUFA the thickness probably ranges from 0 lbo ~as much as 300 Calcareous tura and handed travertine, probably an­ feet. The rocks !thicken southward ~possibly to about cient spring deposits, are exposed along Johnny Gulch 1,000 feet (Freeman •and others, 1958, p. ·508) 1and Ito near the west edge of the mapped area and in an un­ rthe east where Robinson (1967) found the Durubar named tributary south of Johnny Gulch. The larger Creek Formation to be as much as 1,600 feet thick. exposure, in Johnny Gulch, is about 2,500 feet long, 500 Vertebrate fossils of early Oligocene age from the feet wide, and 200 foot thick. Most of the rock is very sedimentary tuff at four localities in and adjacent to pale yellowish brown, gray-weathering, finely crystal­ the rna pped area were identified by the late M. Jean line, porous calcareous tu:f.a; the rest is hard compact Hough (written commun., Oct. 25, 1950, and Dec. 5, rtraver.tine. Fibrous, la;minated, ·and twigJike structures 1952) as follows ('See also F-reeman and others, 1958, are present locally. p.509,510): The tufa contains leaf imprints or molds from which NW1,4SE:LJ.t sec.18, T. 5 N., R.1 E. the late R. W. Brown (written oommun.) identified Sequoia affinis Lesquereaux and Alnus or Betula sp. M esohippus hypostylus Osborn indicative of Oligocene age. Relative ages of the Leptomeryw esulcafirus Cope Oligocene tufa and the Oligocene sedimentary tuff are Peltosaurus ~ sp. indeterminate. NW~SW1,4sec.19,T.5N.,R.1 E. Titanotheriomys sp. UNCONSOLIDATED TERTIARY AND QUATERNARY Palaeolagus sp. DEPOSITS Bathygenys sp. FAN DEPOSITS AND MANTLE Slh sec.13, T. 4 N., R.1 W. A poorly exposed group of surficial deposits of diverse Brontotherium sp. origin were mapped as fan deposits and mantle. Gravel Titanotheriomys cf. T. veterior (Matthew) that was deposited in alluvial fans along the east flank Palaeolagus cf. P. brachydon Matthew of the Elkhorn Mountains, especially north of Indian H yracodon sp. Creek, comprises most of the unit;· the rest consists of pediment gravel, eolian silt, poorly sorted hillwash, ·and N E~NW:LJ.t sec. 130, T. 4 N., R. 1 W. local deposits of alluvial gravel and finer debris. Pedi­ Oylilndrodon cf. 0. fontis Douglass ment gravel and eolian silt, the "mantle" of Freeman, Leptomeryw esulcatus Cope Ruppel, and Klepper (1958), form a veneer on the Oli­ Lithology and bedding suggest that some of the gocene sedimentary tuff and older rocks south of Crow Oligocene sedimentary tuff was formed partly of air­ Creek. Deposits of hillwash and alluvium are scattered borne pyroclastic debris that was deposited directly in throughout the 'area. a body of standing water and prurtly of mixed volcanic The unit includes. deposits that range in age from and nonvolcanic material washed from the surrounding Pliocene ( ~) to Holocene. The pediment gravel was hills. A small part of the unit seems to have been de­ deposited during the Pliocene Old Valley cycle of posited in a fluviatile environment, perhaps as a delta. Pardee (1950), and the oldest fan gravel may be as old. 14 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA The youngest parts o:f the :fans as well as the hillwash that destroye.d nearly all burt !the most resistant volcanic and alluvium are Holocene. rocks. The moraines at the mouth of Beaver Creek con­ The :fan deposits 'and mantle are composed o:f gravel sist of subrounded to subangular resistant :fragments with a sand and clay matrix. The gravel is entirely o:f of volcanic and intrusive rocks -as much as 18 inches local origin, and its composition reflects the bedrock ~across in a closely packed heterogeneous maJtrix. A few exposed at higher altitudes nearby. The source of the exceptionally large blocks are preserved; one on a hill eolian silt is unknown, but its distribution, centered on north o:f Be~~ver Creek is 30 :feet on ~a side. Some o:f rthe a broad divide between the Townsend and Three Forks larger iblocks have .polished faces and are fainrtly valleys, offers a clue. Robinson (1963, p. 85-86) sug­ grooved and. striated. The deposits in Crow Creek are gested that the silt was removed :from Oligocene sedi­ generally sbni1ar to those in Beaver Creek hut ·are ob­ mentary tuffs by northerly winds 'and was deposited scured lby Vt3getaJtion 1and soil. They are composed o:f IJ where the sweep o:f the wind was obstructed. cobbles 1and iboulders of Elkhorn Mountains Volcanics ~and ;related intrusive. rocks in a matrix of sand ·and GRAVEL DEPOSITS OF UNCERTAIN AGE pe;hbles and have no ~apparent sorting or stratification. Gravel deposits ,of uncertain but probruble late Plio­ In a ,few places, rthe !boulders are 1as much ~as 25 :feet long, cene or early Pleistocene ·age and origin were mapped in and many ~are distinctly hlocky or angular. On the hill a :few pl,aces. A veneer o:f gravel covers three small areas north o:f !the Vosburg mine (sec. 34, T. 8 N., R. 1 W., near the front o:f the range east o:f Sheps Gulch. The pl. 1), ·a sm::~:ll deposit of large boulders, some o:f which gravel is o:f local origin ~and .pro:hrubly is contempora­ ~are deeply ·disintegrated and one o:f whi0h is striated, neous wittJh the gr.avel rthat :forms the apron along the is believed !lX> have been deposited during this older pe­ mountain fronlt. An elong3ite body o:f very coa·rse debris riod o:f glacia;tion. east o:f !the mouth ·o:f Oold Springs 'Gulch is composed o:f The largo older outwash :fan :forming the prominent cobbles, boulders, and 'a few !blocks, ~as much as 15 feeJt hills east and west o:f Winston and extending eastward across, o:f Quadrant quartzite and Madison limestone de­ (Freeman and others, 1·958, p. 513-514) beyond the rived :from rthe Limestone Hills to the east. The debris mapped area is made up o:f coarse bouldery and cobbly appears to fiU a channel,and is older than gravel re­ debris o:f volcan'ic and dioritic intrusive rocks from the ferred tlo older outwash £an deposits. A ~train o:f coarse Beaver CrHek drainage. The debris is unusually coarse locally derived debris in an unnamed tributary of and contains relatively abundant subangular pieces, Johnny Gulch south ·of the KeaJting Gulch stock appears some severftl :feet across. Though most o:f the debris is rto grade into .and be of the same age ~as the older out­ of little-weathered resistant rocks, other pieces .are deep­ , wash :f.an deposit in Johnny Gulch. ly weathered. This deep weathering, the advanced dis­ QUATERNARY DEPOSITS section and ilie destruction of ~the surface :form o:f lthe fan and o:f related moraines, and the overlapping by :Surficial deposits o:f Pleistocene age are .widespread younger :f2m deposits strongly support an early Pleisto­ 1along lthe east flank of the Elkhorn Mountains. They cene age for these deposits. include glacial deposits and relaJted outwash f~ans, de­ Coarse gravel of the older outwash fan flanks the posits :formed iby mass wasting, ~and alluvial deposits. valley o:f Crow Creek from Radersburg almost to Power GLACIATION AND GLACIAL DEPOSITS Gulch and extends up Cold Spring Gulch to Indian Glaciers accumu1ruted on the high main mass of the Creek. Th~ gravel consists o£ slightly weathered cobbles Elkhorn Mountains, west .of the mapped ·area, 1at least and bould,ers, mostly subrounded rto rounded and less three times during the Pleistocene (Klepper and others, than a foot across, o£ resistant fine-grained volcanic rocks and dioritic intrusive rocks.· South o:f Crow Creek 1957, 1p. 43; Ruppel, 1962, ~p. 12), and ·at least twice !the glacial ice was so extensive rthaJt it spread into canyons the grave1 is more than 200 feet thick in many places, in the easlt flank area. '.Dhe earliest g1adation in the but north o:f Crow Creek it is probably much thinner, ·range, probaibly early Pleistocene, seems also to have or only a -veneer. The deposit is thought to be a remnant been the most extensive, for its poorly preserved mo­ o:f a larg£1 composite outwash :fan built by Crow Creek raine deposits are present near the mouth of Beaver and Indian Creek during the early period of glaciation Creek ·at altitudes of aboult '5,000 feeJt and in Crow and is probably o:f about the same age as the old :fan at Creek 1at ,altitudes of ~rubourt 5,700 feeJt. The most strik­ tJhe moutb. o:f Beaver Creek. The saddle 1at the head o:f ing deposit of tJhis early glaciation is rthe large older Cold Spring Gulch clearly was a spillway for melt water outwash fan of Beaver Creek near Winston. The older from Indian Creek. The Indian Creek outwash :fan was moraines rand the older outwash fans ·are similar in built southward during the early period o:f glaciation composirtion; both ~are characterized by deep weathering to coalesj~e with the outwash :fan being built in Crow

\ IGNEOUS ROCKS 15 Creek, and thus there is no old fan at the present mouth area. To the north of Beaver Creek, fans at the mouths of Indian Creek comparable to those at the mouths of of Staubach, ·Pole, and Antelope Creeks were deposited Beaver Creek and Crow Creek. The present canyon of at about the same time. Beaver Creek is now entrenched Indian Creek, below Hassel,and the Cold Spring ·Gulch into its fan. At the head of the fan the trench is 40-50 spillway, probably was cut in later Pleistocene time by feet deep, about 200 feet wide, and is mostly cut in a headward eroding stream that pirated the ancestral bedrock. Downstream the· trench is progressively Indian Creek. broader and shallower and is wholly in gravel. East Relatively thick, coarse older outwash gravel deposits, of U.S. Highway 287 the fan is not trenched and is composed mainly of sedimentary rocks from the middle probably still accumulating. and upper reaches of Johnny 'Gulch, are preserved along The younger Crow Creek fan differs from the the lower reaches of Johnny Gulch. Though the head of younger Beaver ·Oreek fan in that it has not been incised Johnny Gulch was not occupied by glaciers, the coarse by the creek but is still being constructed. It consists gravel probably was deposited during a period of largely of rounded or subrounded boulders and cdbbles greatly accelerated erosion that coincided with the early of volcanic rocks and dioritic intrusive rocks but also Pleistocene glaciation. Thus, the gravel on Johnny includes quartzite and other rocks from resistant Pale­ Gulch is probably a correlative of the old Beaver Creek zoic and Mesozoic sedimentary formations. and Crow Creek-Indian Creek fans. The fan-shaped Terrace deposits related to the glacial deposits of part of the deposit has been much dissected, and it late Pleistocene age are present at several places along grades imperceptibly into gravelly mantle that may Beaver and Crow Creeks as gravel veneers on rock include old pediment veneer as well as material that benches (Klepper, and otJhers, 1957, p. 62). The benches was redistributed as the fan was dissected. were probaJbly cut and veneered with gravel during the The deposits of the two late Pleistocene (Wisconsin~) maximum outwaSh phase of the last glacial period. The glacial episodes have not been mapped separately, al­ deposits consist of· coarse rounded gravel containing though both episodes must be represented in the exten­ boulders several feet long. The surface of the gravel de­ sive younger outwash .fan deposits and in the few posits slope at about the same angle as the modern morainic deposits. All the deposits ·are characterized by streams, which are entrenched from 20 feet to ·as much fresh rock fragments and by fairly well preserved as 75 feet below them, and they appear to be graded to surface form. the surface of the younger fan deposits. The best preserved moraine is near the junction of the South Fork, Badger Creek, and Beaver Creek, where a DEPOSITS RESULTING FROM MASS WASTING terminal moraine marks the maximum advance of the Surficial deposits resulting from mass wasting in­ last valley glaciers to occupy these valleys; this last clude delbris originally deposited by glaciers, by streams advance was probaibly in late Wisconsin time, for the as alluvial fans, and by weathering of the bedrock. moraine is not dissected and it contains little-weathered These deposits have been so modified by postglacial boulders as much as 15 feet across. Other boulde·r mass-wasting processes, probably mainly soil creep, deposits that may be partly destroyed moraines are solifluction, and frost action, that near-surface features preserved in Weasel Creek. attributaJble to the original depositional process have Younger outwash fans similar in rock content and been largely destroyed. Accumulations of rock waste in degree of preservation to the late Pleistocene moraines clearly recognizaJble landslide deposits were mapped in the Elkhorn Mountains (Klepper and others, 1957, separately from the other mass-waste deposits. p. 43; Ruppel, 1962, p. 12) are much more widespread ALLUVIUM in the mapped area than deposits of till. Before these fans were deposited, the older outwash fans were partly Deposits mapped ·as alluvium include gravel, sand, eroded. The younger fans occupy about the same sites and silt accumulating in the valleys of the present as the older fans, but they were graded to a lower level ; streams, and similar deposits in small fans now forming their tops are 200-300 feet below adjacent remnants of or recently formed at the mouths of many gulches. Many older fans. other such deposits are present but are too small to be The younger Beaver Creek fan consists of un­ shown. weathered cobbles and boulders of volcanic rocks and IGNEOUS ROCKS dioritic intrusive rocks, mostly subrounded or rounded. Extrusive and intrusive igneous rocks are abundant The south edge of the fan distinctly overlaps the upper along the east flank of the Elkhorn Mountains. Extru­ part of the mountain front gravel apron, but it grades sive rocks of the Ellmorn Mountains Volcanics of Late into the lower part of this apron east of the mapped Cretaceous age cover about one-half of the area, and 16 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA remnants of much younger, probably middle to late Ter­ hiatal and seriate texture if a substantial part of the tiary, basalt and rhyolite flows 'C'rop out in the central groundmass is too fine grained to be identified by con­ part. The many bodies of intrusive rocks fall into two ventional microscopic methods. Medium- and ooarse­ main groups: ( 1) intrusive equivalents of Elkhorn grained roc:k:s with conspicuous phenocrysts are re­ Mountains Volcanics, for convenience called volcanic ferred to as :porphyritic. The term "phenocryst" is used intrusives, and (2) intrusives younger than Elkhorn without genetic connotation for crystals that stand out Mountains Volcanics. The volcanic intrusives comprise from a finer grained goundmass. mainly seriate syenodiorite or granodiorite porphyry in The younl~er intrusive rocks form generally discord­ larger partly concordant bodies and hiatal rhyodacite ant stocks, plugs, or bodies of unknown subsurface con­ and trachyandesite porpihyry in smaller sills and dikes figuration that were emplaced after the enclosing rocks that were emplaced before folding or during the earliest had been fo1ded. Most of the bodies consist of medirnn­ J stages of folding. grained and subequigranular rocks, but a few are made The nomenclature used in this report for igneous up of porphyry. Compositionally, the rocks can be con­ rocks that are coarse grained enough for thin-section veniently asdgned to one of two groups: ( 1) mafic rocks modes to be determined is shown in figure 2. Finer and (2) intermediate to felsic rocks. Some bodies are grained volcanic rocks for which modes cannot be de­ simple and comprise only one rock type; some are com­ termined are classified on the basis of eihemical compo­ posite and consist of one or more mafic and felsic types. sition of representative samples, in accordance with the Lead-alpha ::tge determinations of zircon from two post­ system proposed by Rittman (1952). Though the term volcanic int:rusives are 72-76 million years (Jaffe and "andesite" was applied to many rocks in the field, none others, 1959:, p. 80-81), determinations indicating Late of tJhe analyzed rocks are andesite in the Rittman classi­ Cretaceous ~Lge according to the time scale of Holmes fication. The terminology applied to fragmental vol­ ( 1959) ; theBe age determinations are in the same age canic rocks is that of W entwortJh and Williams ( 1932, range as roc.ks of the Boulder batholith 10-20 miles to p. 43-53) . The term "porphyry" is used for rocks of both the west. Four thin dikes of quartz latite porphyry probably QUARTZ are equivalent to the Lowland Creek Volcanics of Eocene age (Smedes, 1962) nearby. ELKHORN MOUNTAINS VOLCANICS , The Elkhorn Mountains Volcanics of Late Cretaceous age (KleppE:r and others, 1957, p. 31-41), comprise three members in the type locality : a lower member consisting mainly of rhyodacitic and trachyandesitic pyroclastic rocks, a m:~ddle member· characterized by rhyolitic 5 35 65 95 welded ~tuffs alternating with andesitic and rhyodacitic K-FELDSPAR PLAGIOCLASE tuff and tuff breccia, and an upper member com­ ROCKS WITH PLAGIOCLASE OF posed mainly of airlaid tuff and of waterlaid tuff and COMPOSITION An o-so conglomerate that are largely debris eroded from the QUARTZ lower members. Only the lower and middle members are present along the east flank of the Elkhorn lfountains. Rocks of the lower member underlie most of the eastern slope of the Elkhorn Mountains in the northern part of the Jnapped area and much of the mountainous central part. Farther south, they form part of the roo£ of the Rattlesnake intrusive body. Cenozoic sediments in the southeastern part of the mapped area are prob­ ably underlain at shallow depth by rocks of the lower 5 35 65 95 member. The lower part of the middle member occupies K-FELDSPAR PLAGIOCLASE much of the northwestern part of the mapped area, but ROCKS WITH PLAGIOCLASE OF COMPOSITION An:;o-too south of Indian Creek it is present only in a small fault block in_~ec~ 2, T. 6 N., R. 1 W. Rocks of the upper part FIGURE 2.-Nomenclature of coarser grained intrusive igneous rocks. of the inidd1e member and the upper member are not IGNEOUS ROCKS 17

present probably due to erosion rather than to nondepo~ (partly or completely replaced by aggregates of chlo­ sition. rite, epiodote, magnetite, quartz, and leucoxene) , sparse North of Johnny Gulch the Slim Sam Formation biotite (partly altered to chlorite and sphene), and rare grades upward into the Elkhorn Mountains Volcanics. K-feldspar. Most rock fragments are trachytic, pilotaxi­ South of Johnny Gulch, however, the volcanics trun­ tic, or cryptofelsic trachyandesite and rhyodacite; some cate Slim Sam and older rocks that had been tilted or are amygdaloidal basalt and brown devitrified glass. gently folded at the beginning of the volcanic episode Fragments of prevolcanic sedimentary rocks are ex­ (Freeman and others, 1958, pl. 42). The volcanics are tremely rare. unconformably overlain by Oligocene and younger Because of greater permeability the pyroclastic rocks rocks and by unconsolidated sediments. are more altered than the associated flows, dikes, and ·~ The volcanic pile is cut by several large irregular in­ sills. Hornblende and pyroxene are com.pletely replaced, trusive masses that are broadly concordant with but and plagioclase is saussuritized in many of these rocks. that locally sharply crosscut the enclosing rocks and by Calcite and epidote are common in veinlets and irregu­ many dikes and sills; most, if not all, of these rocks are lar replacement masses. of about the same composition and age as theElkhorn The lava flows in the lower member, as much as 100 Mountains Volcanics and are products of the same feet thick, consist of trachyandesite and amygdaloidal magma or magmas that yielded the volcanic rocks. The basalt. The flows are mineralogically and texturally Rattlesnake mass, south of Crow Creek, is the largest · closely similar to the related dikes and sills but are gen­ of these. erally more altered. LOWER MEMBER Thin units of welded tuff are interlayered with the The lower member of the Elkhorn Mountains Vol­ pyroclastic rocks of the lower member. The welded tuff canics ranges in thickness from 2,500 to 5,000 feet and is gray to greenish gray and contains a!bundant flattened consists mainly of tuff, tuff breccia, and breccia inter­ and bent pieces of dark chloritic devitrified pumice as layered with a few lava flows and a few thin units of much as an inch long, angular rock fragments, and welded tuff. These rocks are mostly dark gray, greenish broken crystals of labradorite surrounded by micro­ gray, and grayish green, but shades of brown, red, and crystalline intergrowths of quartz, feldspars biotite, purple are common. Chemical analyses of three com­ chlorite, sericite, magnetite, and indeterminate mate­ posite samples indicate that the average composition of rial. Mafic minerals are completely replaced by chlorite, the lower member (table 1, samples 1-3) is in the tra­ calcite, epidote, and magnetite. Chemical analysis in­ chyandesite-rhyodacite range. dicates that a representative sample (table 1, sample Tuffs form massive unstratifi~d units that probably 4) is quartz latite. accumulated from air falls of ash and stratified units of MIDDLE MEMBER which at least some were deposited in water. Crystal tuffs and crystal-lithic tuffs are the dominant rock types; The middle member of the Elkhorn Mounta:ins Vol­ by an increase in abundance and size of lithic fragments canics consists of rhyolitic welded tuff interlayered with these rocks grade into tuff breccias and breccias, which crystal tuff and lapilli tuff and less abundant volcanic are very poorly sorted and commonly exhibit complete breccia. The fragmental rocks are similar to those in size gradation from the largest lithic fragments to cryp­ the lower member. A measured section of the lower tocrystalline matrix. J\iost of the finer grained tuff is 2,500 feet of the middle member is given on pages .... well bedded and delicately laminated and is locally 60-62. The total exposed ~thickness is ·about 3,000 crossbedded, channeled, and mud cracked; these fea­ feet. Farther west, the middle member is as much as tures indicate that the ash was deposited in shallow 7,500 feet thick (Smedes, 1966, p. 26). water. Thick, poorly bedded units of breccia and tuff Welded tuff units range in thickness from a few feet breccia are probably mud flows. The fact that most of to about 200 feet, but most are between 20 and 60 feet the crystal and lithic fragments are angular, even in the thick. They range in color from light to dark shades well-bedded rocks, indicates a n1inimum of transport of gray, greenish gray, red, and purple. Only a few and reworking. thin dark-greenish-gray units resemble the quartz latite Fragments of intermediate plagioclase and of vol­ welded tuff of the lower member. Some of the units canic rocks are by far the predominant clastic compo­ appear nearly homogeneous and apparently are welded nents. The plagioclase is complexly twinned, commonly from bottom to preserved top ; others consist of a shows progressive and oscillatory zoning, and ranges in strongly welded base that grades upward into a poorly composition from An4o Ito An65 • Other crystalJ~agments welded or unwe,lded zone. Most of the welded rocks include quartz, magnetite, pyroxene, and hdrnblende contain abundant flattened fragments of devitrified 18 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

~ABLE I.-Chemical analyses, norms, and modes of igneous

Sample Field J,aboratory No. Rock type 1 Geologic occurrence Location No. No.

Elkhorn Mountains Volcanics

L------52Wl42 ______53-81CW 8 ______Trachyandesite ______Middle part of lower meml•er (composite of NEX sec. 25, T. 7 N., R.1 w______chips). 2 ______52W143 ______53-82SCW. ______Labradorite trachyandesite .• Lower member (composite or chips from basal Iron Mask mine crosscut, sec. 30, T. 7 N., R. 1 1,000 ft). E. 3 ______52KL ______53-71CW __ ------Labradorite rhyodacite...•.• Lower member (composite of chips across 1,200 SEX sec.16, and NEU sec. 21, T. 8 N., R. 1 W. ft). 4 ______52R65 ______53-77CW 8 ______Quartz latite ______Welded tuff, lower member .. ·------NEU sec.19, T. 7 N., R. 1 E ______5------52K7 ______53-72CW •------Rhyolite ______Welded tuff, middlememba~ •• ______NEUSWU, sec.16, T. 8 N., R. 1 W ______

Intrusive rocks related to Elkhorn Mountains Volcanics

6------52R66 ______53-78SCW s______Syenodiorite porphyry ______Small stock in lower member of Elkhorn NWUsec.19, T. 7 N., R.1 E. ______Mountains Volcanics. 7 ______61K659 ______158775 &______Calcic granodiorite ______Thick sill east of Santa Anita mine ______NE>-:1' sec .• ~ T. 4 N., R. 1 W------8 ______52K127 ______53-736W a______Calcic syenodiorite.. ______Sill in middle member of Elkhorn Mountains NEUNWn sec. 9, T. 8 N., R.1 W. ______Volcanics. g ______Winchell None given 7______Andesite (Winchell's (1914) Rattlesnake mass. ______Rena mine, sec. 7, T. 5 N., R. 1 E------No. 1 usage). 10 ______49K13 ______51-939CW 9______Syenodiorite ______Dutchman intrusive, southeastern lobe of NEU sec. 32, T. 5 N., R.1 w______"' body. 11------51K15------53-69SCW a______Syenodiorite porphyry ______Small intrusive in lower member of Elkhorn NWUNW>-:1', sec. 30, T. 7 N., R. 1 E------. Mountains Volcanics. 12------49K12 ______51-938CW o______Syenodiorite______Dutchmanintrusive600ftno::thwestofsample NE>-:1' sec, 32, T. 5 N., R.1 W------14locality. . 13 ______52R6 ______53-76SCW a______Syenodiorite porphyry ______Stock near Iron Mask mine .... ------Sec. 19 T. 3 N., R. 1 W ------14 ______49K14 ______51-941CW 9______Granodiorite ______Dutchman intrusive, central part______Boundary between sees. 29 and 32, T. 5 N., R. 1 w.

Younger intrusive rocks

l5 ______49K9 ______51-931CW 9______Granodiorite ______Keating Gulch stock ______NE>-:1' sec.16, T. 5 N., R.1 W ______16 ______60S599 ______G-3167 9______Syenogabbro------Orphan Boy mafic dike ______NE>-:I'NE>-:1' sec. 4, T. 8 N., R.1 W ------17------49K10 ______51-936CW 9______Syenodiorite. ------Keating Gulch stock ______------SE>-:1' sec. 9, T. 5 N., R. 1 W------18 ______49Kll ______51-937CW 9______Granodiorite ______Slim Sam stock ______NWUSE>-:1' sec. 20, T. 6 N., R.1 w ______19 ______49K8 ______51-934CW o______dO------Keating Gulch stock------NE>-:1' sec.16, T. 5 N., R.1 W______20 ______52K184 ______53-64SCW a______.dO------Antelope Creek stock ______SW>-:I'SW>-:1' sec. 33, T. 9 N., R. 1 \V ------21. ______52K19L ______53-75SCW a______.• dO----- ______Antelope Creek stock (composite of samples ___ -- __ ----_-- __ ------throughout stock). 22 ______61K66QO ______1587768 ______Quartzmonzonite ______LoneMountainstock ______SW>-:I'NE>-:1' sec. 6, T. 4 N., R. 1 E ______23 ______49K7 ______51-933CW 9______Granodiorite ______Keating Gulch stock ______NE>-:1' sec. 9, T. 5 N., R.1 W ------24 ______51K1a. ______53-68SCW •------Porphyritic quartz mon- Vosburg stock ______Kleinschmidt mine, SW>4SEUsec.34, T.8N., zonite. R. 1 W.

Sample Field No. Laboratory No. Rock type 1 Geologic OCCUITI:nce Location No.

Elkhorn Mountains Volcanics

L------52Wl42 ______53-81CW a______Trachyandesite ______Middle part of lower memb~r (composite of NE>-:1' sec. 25, T. 7 N., R. 1 w ______chips). 2 ______52W143 ______53-82SCW a______Labradorite trachyandesite.. Lower member (composite of chips from basal Iron Mask mine crosscut, sec. 30, T. 7 N., 1,000 ft). R. 1 E. 3 ______52KL ______53-71CW. ------Labradoriterhyodacite ______Lower member (composite of chips across SEX, sec.16, and NEU sec. 21, T. 8 N., R.1 W. 1,200 ft). 4 ______52R65. ______53-77CW a______Quartz latite. ------Welded tuff, lower member __ , ______NE% sec. 19, T. 7 N., R. 1 E------5 ...... --- 52K7 ______53-72CW •------Rhyolite ______Welded tuff, middle member .. ______NEUSWX, sec. 16, T. 8 N., R. 1 W.

Intrusive rocks related to Elkhorn Mountains Volcanics

6 ______52R66 ______53-78SCW 3 ______Syenodiorite porphyry ______Small stock in lower memher of Elkhorn NWX sec.19, T. 7 N., R.1 E ______Mountains Volcanics. 7 ______61K659 ______158775 5 __ ------Calcic granodiorite .• ------Thick sill east of Santa Anita mine. __ ------NEX sec. 4, T. 4 N., R. 1 w______------8------52K127 ______53-736W 3_. ------Calcic syenodiorite ______Sill in middle member of Elkhorn Mountains NEXNWU sec. 9, T. 8 N., R. 1 w______Volcanics. 9 ______Winchell No. L None given 7 ______Andesite (Winchell's (1914) Rattlesnake mass ______------Rena mine, sec. 7, T. 5 N., R. 1 E------usage). 10 ______49Kl3 ______51-939CW 9______Syenodiorite ______Dutchman intrusive, south•lastern lobe of NEU sec. 32, T. 5 N., R. 1 w______body. n ______51Kl5 ______53-69SCW s______Syenodioriteporphyry ______Small intrusive in lower member of Elkhorn NWXN, sec. 30, T. 7N., R. 1 E______Mountains Volcanics. 12 ______49K12 ______51-938CW 9 ______Syenodiorite. ______Dutchman intrusive 600 feet northwest of NEU sec., 32, T. 5 N., R.1 w___ ------______sample 49K14locality. 13 ______52R6 ______53-76SCW a______Syenodiorite porphyry ______Stock near Iron Mask mine._------Sec. 19 T. 3 N., R. 1 W__ ------14 ______49Kl4 ______51-941CW 9______Granodiorite ______Dutchman intrusive, central )art ______Boundary between sec. 29 and 32, T. 5N., R. 1 w.

See footnotes at end of table. IGNEOUS ROCKS 19 rocks from the east flank of the Elkhorn M otmtains

------Chemical analyses (weight percent) Norms (percent) 8!02 TiOa Ah03 Fe20a FeO MnO MgO CaO Na20 K20 P20a Loss on Total q or ab an wo en fs mt 11 ap cc c hm ignition2

Elkhorn Mountains Volcanics-Continued

54.8 0.76 17.0 4.5 3.4 0.16 3. 6 6. 6 3.6 1.6 0.31 3.0 99.3 9.5 9.5 30.4 25.5 2.2 9.0 1.6 6.5 1.4 0.7------56.2 . 66 16.8 4. 0 3.1 .18 2. 6 5.4 2. 6 3.0 .38 5.1 100.0 15.2 18.9 23.1 25.6 .0 6.8 1.9 6.8 1.4 1.0-----0.4----- 59.6 . 72 17.4 3.3 3.2 .16 2. 6 5. 2 2. 6 3.4 .34 1.5 100.0 16.4 20.1 22.0 23.6 .0 6.5 2.3 4.8 1.4 .8----- .8----- 60.7 . 74 16.6 1. 7 3. 5 .12 1. 8 4. 2 2.1 5. 0 .32 2.3 99.1 16.8 29.5 17.8 18.7 .0 4.5 4.0 2.5 1.4 .8----- .9----- 69.4 . 56 15.4 1. 7 0. 86 . 08 . 37 1. 6 2. 6 6.3 . 09 .80 99.8 26.5 37.2 22.0 7.3 .0 .9 .o 1.4 1.1 .2 ..... 1.6 0.7

Intrusive rocks related to Elkhorn Mountains Volcanics-Continued

53.4 1. 0 17.2 4. 2 5. 0 0. 22 3. 6 6. 0 3. 0 2.8 0. 39 2.4 99.2 6.5 16.5 25.4· 25.2 0.8 9.0 4.5 6.1 1.9 0.9------

54. 9 . 63 17. 0 4. 6 3. 6 . 20 3. 9 8. 1 2. 8 2. 1 .42 ~2.0 100.0 9.8 12.4 23.7 27.6 4.0 9.7 2.1 6.7 1.2 1.0 0.1------55. 4 • 74 13. 4 3. 3 5. 5 . 68 6. 8 7. 8 2. 0 2. 1 .49 1.1 99.3 10.0 12.4 16.9 21.4 5.9 16.9 7.4 4.8 1.4 1.2 ------

56. 61 . 71 17. 91 4. 22 2. 7 .a am a~ aro an .46 (8) 99.84 11.9 16.0 26.2 27.0 1.5 5.5 1.3 6.1 1.3 1.1 .3------57.3 .66 17.1 2.8 3.6 .08 2.8 7.0 3.8 2. 6 .47 2.3 100.5 8.0 15.4 32.1 21.9 4.1 7.0 3.4 4.1 1.3 1.1 ------57.8 . 68 17.1 3. 8 3. 6 .16 2.6 6.0 3. 2 2.1 .35 1.9 99.3 14.5 12.4 27.1 26.1 .6 6.5 2.6 5.5 1.3 .8 ------·------58.1 .57 17.4 3.9 2.1 .15 2.4 6.9 3.4 2. 9 .34 1.8 101.0 11.6 17.1 28.8 23.7 3.5 6.0 .o 5.6 1.1 .8 ------··- 58.6 • 44 16. 8 3. 6 2. 6 .18 2. 5 6. 0 3.6 2. 2 . 26 2.8 99.6 13.8 13.0 30.4 23.2 2.0 6.2 1.4 5.2 .8 .6------60.2 . 52 17. 2 3. 6 2. 2 .17 2. 0 5. 6 3.5 2. 8 .32 1.6 99.7 15.4 16.5 29.6 23.0 1.1 5.0 .5 5.2 1.0 .8------

Younger intrusive rocks-Continued

55.2 1.1 16.4 4.1 5. 0 0.18 4.1 7.3 3. 0 2.3 3.1 0. 66 100.0 8. 2 13. 6 25. 4 24. 5 4. 0 10.2 4. 3 5. 9 2.1 0. 7 ------55.46 . 81 16.93 2.12 5. 63 .16 4. 01 7.22 3. 01 3.18 . 36 0. 71-.13 99.7 4. 2 18. 8 25. 5 23. 3 4. 1 10.0 7. 5 3.1 1. 5 .9 0.1 ------56.2 • 76 18.9 4.6 3.0 .13 2. 7 7. 0 3. 7 2.4 • 42 1. 0 101.0 8. 2 14. 2 31. 3 27. 9 1. 7 6. 7 0. 7 6. 7 1.4 1.0 ------­ 60.5 .62 17.2 3.4 2. 5 .14 2.4 5.8 3.4 2.6 .30 1. 2 100.0 15. 8 15. 4 28. 8 24. 0 1. 2 6.0 1. 0 4. 9 1.2 .7 ------61.3 .58 18.1 3. 0 2. 9 .14 2.1 5.8 3.6 1. 7 . 28 1.2 101.0 18. 1 10. 0 30. 4 26. 9 . 0 5. 2 2.1 4. 3 1.1 .7 ----- 0. 5 ----- 61.6 . 53 16.5 2. 5 3.2 .68 2.6 5. 2 3.4 3.0 ,22 .88 100.0 15.3 17.7 28.8 20.9 1.4 6.5 3.2 3. 6 1.0 .5 ------­ 63.5 .50 15.7 2.6 2.8 .14 2.4 4.6 3. 3 3.2 • 21 1. 4 101.0 18. 8 18. 9 27. 9 18. 6 1. 2 6.0 2.4 3.8 1.0 .5 ------63.5 .55 15.6 3. 9 1. 4 .14 2. 2 3. 9 4.2 3.6 .38 0. 84 100.0 15. 6 21. 3 35. 5 13. 1 1. 4 5.5 . 0 3. 4 1. 1 • 9 . 1 --.-- 1. 6 65.8 .50 16.2 3. 2 1.0 .07 1. 4 4.4 3. 3 3.4 .20 1. 0 101.0 22. 9 20. 1 27. 9 19. 4 . 5 3.5 . 0 2. 0 1. 0 . 5 ------1. 8 66.4 . 29 16.4 1. 6 1.3 .11 . 76 4.2 4. 2 3.6 .12 1. 0 100.0 19. 1 21. 3 35. 5 15. 3 2. 0 1. 9 .8 2.3 .6 .3 ------

Quantitative spectrographic analyses (parts per million) 10 u Mode (volume percent)

Cu Pb Co Ni Ga Cr Sc La Zr Be Ba Plagioclase K·feldspar Quartz Biotite Hornblende Augite Other 12

Elkhorn Mountains Volcanics-Continued

0. 012 0. 0042 0. 0020 0. 0021 0. 0042 0. 011 0. 0015 0. 0 0. 0094 0. 0001 0. 014

Intrusive rocks related to Elkhorn Mountains Volcanics-Continued

·------·------·---····------·------50 (An12) 24 3 10 chlorite 8 4

\.,.I •••. ------•.• --.. --••••...••••• --.••..••. --••.•••.. ------.--.. ------... --- 53 (An43-55) 14 4 ------23 2 4 0. 0029 0. 0037 0. 0022 0. 0009 0. 0054 0. 0013 0. 0032 0. 0 0. 012 0. 0001 0. 21 45 (An1o·52) 15 4 ------19 12 5 ------·------__ ----... ------______. ______----- _____ . ------·· 50 (An!o-oo) 24 2 14 2

. 0012 . 0037 . 0018 . 0 . 0046 . 0009 . 0010 • 0 . 0085 . 0 . 14 60 (An•o) 19 3 ------9 6 3 -·-----.• ---...•• -----_----· .. ----••.•.••...... ______.. _. __ .. ----.--..... ------52 (An45) 17 10 ------15 3 3 20 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

TABLE I.-Chemical analyses, norms, and modes of igneous rocks

Sample Field No. Laboratory No. Rock type 1 Geologic occurrence Location No.

Younger intrusive rocks

15 ...... •.. 49K9 ...... 51-931CW o______Granodiorite .•...... Keating Gulch stock ______NE~ sec. 16, T. 5 N., R. 1 W ______16 ....•.•.• 60S599 ...... G-3167 o______Syenggabbro ______Orphan Boy mafic dike ______NE~NE~ sec. 4, T. 8 N., R.1 w ______17 ....•.... 49K10 ...... 51-936CWo------Syenodiorite ______Keating Gulch stock .•...... SE~ sec. 9, T. 5 N., R. 1 W______18.------49Kll ______51-937CWo ______Granodiorite ______Slim Sam stock ______NW~SE~ sec. 20, T. 6 N., R. 1 w______19 ...... •.• 49K8 ...... • 51-934CW o______Granodiorite ... ------Keating Gulch stock ______NE7;1: sec. 16, T. 5 N., R. 1 w ______20 ...... •.. 52K184 ...... 53-64SCW a______Granodiorite ... ------Antelope Creek stock.•... ------SW~SW~ sec. 33, T. 9 N., R. 1 W...... 21...... 52K19L ...... 53-75SCW a______Granodiorite ______Antelope Creek stock (Composite of samples throughout stock). 22 •.....•.• 61K6600 ..•.... 158776 e______Quartz monzonite ______Lone Mountain stock ______------SW7;i:NE~ sec. 6, T. 4 N., R. 1 E.------23 ...... 48K7 ______51-933CW o______Granodiorite ______Keating Gulch stock ______NE~ sec. 9, T. 5 N., R. 1 w ______24 .... ----- 51K1a ______53-68SCW •- .• ____ Porphyritic Quartz Vosburg stock·------Kleinschmidt mine, SW~ SE7;1: sec. 34, T. 8 N ., monzonite. R. 1 W.

1 Volcanic rocks (samples 1-5) are named according to Rittman (1952); intrusitive rocks (samples 6-11) are named on basis of their modes (fig. 2). 2 Includes gain due to oxidation of FeO. a Chemical analysis by rapid method by J. M. Dowd, Katherine White, and F. S. Borris. • Analysis by rapid method by J. M. Dowd. ~Analysis by rapid method by P. L. D. Elmore, I. H. Barlow, S.D. Botts, and G. W. Chloe. e Reported as HzO;

pumice. Chemical analysis (table 1, sample 5) indicates intrusive rock~; most lava flows also closely resemble that a composite sample of typical welded tuff fron1 one or another of rbhe intrusive rocks, so much so rthat this me1nber is rhyolite. in -areas of poor exposures and struotural complexity The welded tuffs consist of plagioclase and biotite flows cannot .hf, distinguished from intrusives with con­ phenocrysts, rock fragments, and flattened chunks of fidence. Similarly, plagioclrase, augite, and hornblende, devitrified pumice in a devitrified groundmass. Plagio­ in that order, are the most abundant large mineral clase in anhedral angular fragments that are generally grains in flow~, intrusives, and fragmental rocks, and zoned, twinned, and unaltered makes up less than 10 their optical properties rare identical or closely similar. percent of most rock, and biotite phenocrysts comprise The inttrusivo bodies range in size from dikes ·and sills less than 5 percent; magnetite grains, which are less than ·a foot rthick and a few tens of feet long to the oxidized to hematite near the margins of the pheno­ large grossly concordant Rattlesnake mass that is ex­ crysts are abundant. Rock fragments compose 5-50 posed over about 20 square miles. Most of ·the sills are a percent of different units and are mainly of trachyande­ few tens of feet rthick, but some are several hundred site, basalt containing chlorite-filled vesicles, microcrys­ feet thick; several are more than 2 miles long. Sills are talline rhyodacite, and chunks of brown devitrified more ·abundant in shales of the l{ootenai Formation and spherulitic glass. Collapsed pumice,, ranging from 5 to Colorado Formation. Different bodies were emplaced ,at 30 percent in different units, is crumpled and cut by different times during rthe period of volcanic aotivi,ty as perlitic cracks. Groundmass and pumice are devitrified indicaJted by crosscutting and truncating relaJtions. All and in some rocks are recrystallized to mosaics of very the concordant and quasi -concordant masses were al­ fine grained quartz, feldspars, chlorite, biotite, sericite-, most certainly empl,aced before the major episode of magnetite, minor calcite, and abundant leucoxene dust. folding. The texture of 1the rocks and the shapes of most The reconstituted pumice typically contains more seri­ bodies indicate •3m placement at relatively shallow depth. cite than does the groundmass. The larger intrusive masses consist 1nainly of seriate syenodiorite porphyry that grades either into granodio­ INTRUSIVE ROCKS RELATED TO THE ELKHORN rite porphyry or toward diorite porphyry. Some of tlhe MOUNTAINS VOLCANICS rocks in the Dutchman intrusive are medium grained Several large irregular partly concordant bodies con­ subequingranular. Smaller dikes and sills, however, sisting mainly of syenodiorite porphyry and granodrio­ are mainly hiat~~l rhyodacite or trachyandesite porphyry rite porphyry (fig. 3) and many dikes and sills of with abundant indeterminate groundmass. The rocks trachyandesite poTII>hyry ·and rhyodacite porphyry cut are mainly gray, greenish gray, and purple and con­ Elkhorn Monntains Volcanics and olders rocks. These sist of plagioclase, augite, hornb'lende, magnetite, and intrusives ,are similar in composition [;o the associruted rarely hypersthene and olivine. On plate 1, map sym­ volcanics and are products of ithe same magm·a or bols distinguish those rocks in which hornblende is the magmas. Lithic fragments in .the tuffs ·and breccias, dominant mafic mineral as determined in the field and except for rare fragments of underlying sedimentary those in which augite or olivine is dominant; many in­ rocks, are indistinguishable from the principal types of trusive bodies, particularly the larger ones, contain IGNEOUS ROCKS 21

from the east flank of the Elkhorn Mountains-Continued

Quantitative spectrographic analyses (parts per million) 1o u Mode (volume percent)

Cu Pb Co Ni Ga Cr Sc La Zr Be Ba Plagioclase K.feldspar Quartz Biotite Hornblende Augite Other 12

Younger intrusive rocks-Continued

------.-.--- __ ----.. _- -- .. --.------51(An45-6o) 9 8 13 9 8 2 ------49(AnaMo) 20 3 8 7 11 2 ___ ------.------___ . __ . ---- ______.. _-. _____ .. ---- -·--- __ .. ______-- .. __ ------53(An4HO) 18 7 _------16 2 4 _----- ______. ______------____ . ______----- ______---- ___ ----- 55(An42) 18 14 3 7 ------3 ------______------55(An4o) 12 13 5 13 ------2 ... 0012 0.0029 0.0 0.006 0.0030 0.0013 0.0012------0.0075 0.0001 0.15 56(Anao-•5) 18 10 6 9 ------· 1 0. 0014 • 0037 . 0 . 0011 . 0028 . 003 . 0012 . 0078 . 0096 . 0002 .13 ------.. ------.------.----.- _. ______------.---- ______45(An2s-a2) 30 12 1 8 1 3 --_.- _.- --. ______. ____ . _... _. _. ____ . __ . ______. ___ . _------. 51(Ana5-:l6) 2 ______---- ______. ______.. ____ .. ______. 60(An2o-aa) 22 16 ------9 ------28 15 ------6 ------1

7 Analysis by Chase Palmer (Winchell, 1914, p. 176). s Other constituents reported include 0. 29 percent H20-, 1.16 percent H20+, 0.13 percent C02, 0. 03 percentS, and 0.14 percent BaO. g Analysis by rapid method by S.M. Merthold and E. A. Nygaard. 1o Quantitative spectrographic analyses by E. L. Hufschmidt. n Elements not detected: B, Ag, Ge, Mo, W, Sn, As, Sb, Bi, Zn, Tl, In, Y, and Nb. ' 2 Includes apatite, epidote, opaque minerals, sphene, chlorite, calcite, and sericite.

both hornblende and augite. For example, in some parts tion angle of about 18°. Twinned and nontwinned of the Ratt~esnake mass, hornblende needles are con­ grains are about equally abundant. In rocks having spicuous, in other par~ts hornblende and augite appear augite as ibhe dominant mafic phenocryst, the horn­ to be roughly equal, and in some places neither is con­ blende typica:lly is altered to oxyhornblerrde with ox­ spicuous. Groundmass textures in narrow dikes and idized margins, small extinction angle, and the follow­ sills generally are pilotaxitic or trachytic; in larger bod­ ingpleochroism:.X=medium brown; Y=mediumolive ies such ~as the Rattlesnake and Dutchman intrusives the brown; Z=dark olive brown. Euhedral to subhedral groundmass is typically a fine-grained mosaic of plagi­ equant crystals of magnetite as muelh as 0.5 mm across oclase, !{-feldspar, and magnetite, with lesser amounts are scattere1d in a groundmass or as inclusions in horn­ of augite, quartz, chlorite, and an interstitial carbonate blende and augite. mineral. Accessory minerals include sphene, leucoxene, Many of these rocks are deuterically ·al·tered, and the hematite after magnetite, apatite, and zircon. Small original groundmass minerals and textures are replaced :vugs in smne rocks are filled by one or more of the fol­ by fine-grained intergrowths of albite, epidote, calcite, ' lowing minerals : Quartz, epidote, actinolite, chlorite, sericite, chlorite, magnetite, and quartz. Sericite re­ and K-feldspar. places the calcic cores of plagioclase phenocrysts, augite Percentages of phenocrysts in samples of syenodio­ phenocrysts are altered to chlorite, hypersthene and rite porphyry examined under a microscope range as olivine phenocrysts are changed to serpentine, and horn­ follows (table 1, fig. 3) : plagioclase, 19-41; augite, blende phenocryBts are partly replaced by chlorite, mag­ 3-11; !hornblende, 0-12; hypersthene, 0-6; and mag­ netite, epidote, and sphene. Thin veinlets of calcite and netite, 2-5 percent. Plagioclase phenocrysts are equant chlorite which cut phenocrysts and groundmass are com­ to lathlike euhedra1 crystals 5 mm (millimeters) to mon. Where cut by younger intrusives, the rocks have

less than 0.5 mm long and are strongly zoned from An70 been thermally metamorphosed ·and the groundmass re­ cores to Anzo rims. Delicate oscillatory zoning is com­ crystallized to a mosaic of albite, orthoclase, quartz, mon, and 1nany grains have zones of "glass" inclusions. epidote, and magnetite, which superficially resembles Albite ;and cmnbined albite-Carlsbad twinning are the original microgranitic texture, but is mineralogi­ most coinmon; pericline twinning, less so. Augite phe­ cally different and is restricted to rocks in which the nocrysts occur either as single euhedral prismatic grains phenocrysts also are recrystallized. <0.5-3.5 mm long or in c~umps of smaller subhedral A few distinctive dikes and sills in the vicinity of crystals. They are faintly pleochroic from pale green Kimber Gulch and Whitehorse Gulch are olivine-rich to colorless and have a 2V=51-58+2°. Most of the syenodiorite porphyry that contains phenocrysts of grains are zoned, and multiple twinning is common. olivine, 'augite, and a few of biotite in a groundmass I-Iornblende fonns euhedral LatJhs as much as 5 em consisting chiefly of cryptoperthite and plagioclase. (centimeter) long and is fresh or partly replaced along These olivine-bearing rocks cut the Slim Sam Forma­ the margins by finely granular clinopyroxene and mag­ tion and are cut by hornblende- and augite-bearing netite. It has distinct pleochroism (X= colorless to pale syenodiorite porphyry. They are clearly older than most brown; Y =olive green; Z =deep green) and an extinc- of ~he syenodiorite porphyry and probably are intru- 22 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

EXPLANATION QUARTZ A Orphan Boy intrusive

~ Jenkins Ridge stock* c Aldrich GuiCh intrusive*

QUARTZ

Diorite or Diorite gabbro K-FELDSPAR K-FELDSPAR PLAGIOCLASE • MAFIC INTRUSIVE ROCKS

EXPLANATION AA DD Quarry Gulch intrusive* Indian Creek intrusive* EXPLANATION BB EE A C Power Gulch intrusive* Mammoth intrusive* Vosburg stock Pole Creek stock c B D .Limestone Hills intrusive Edna stock Eagle Basin stock

QUARiZ •

Diorite iorite

K-FELDSPAR PLAGIOCLASE K-FELDSPAR PLAGIOCLASE ROCKS IN COMPOSITE INTRUSIVE BODIES PORPHYRITIC QUARTZ MONZONITE, GRANODIORITE PORPHYRY, AND RELATED ROCKS

EXPLANATION A F Antelope Creek stock Monte Cristo stock B G South Fork Stock Bluebird stock

c~ H Keating Gulch stock* Silver Wave stock D I Diamond Hill stock Vulture stock Diorite E J Slim Sam stock Lone Mountain stock K-FELDSPAR PLAGIOCLASE GRANODIORITE AND RELATED ROCKS

FIGURE 3.-Modes of intrmdve rocks, east flank of Elkhorn Mountains, Mont. Mafic minerals subtracted and mode recalculated to 100 percent. *,underscore indicates a calcic (An>Go) rock. IGNEOUS ROCKS 23 sive equivalents of some of· the lowest Elkhorn Moun­ geneous and that it consists ·of rocks ranging from tains Volcanics. quwrtz-poor granodiorite to syenodiorite; most of the The Rattlesnake intrusive body, which is crudely lac­ rock has the texture of a porphyry, but some is medium­ colithic or sill-like, e:rtends 10 miles southward from grained subequigranular. The Dutchman intrusive Crow Creek to about a mile beyond the southern limit closely resembles the Rattlesnake mass in ~hemical and of· ·the area; it is 1-31h miles wide. Most o~ the rocks mineralogical composition but is generally coarser are syenodiorite or granodiorite porphyry consisting grained and contains little Uiiidentifiable groundmass, largely of hornblende, augite, and plagioclase in differ­ lacks large inclusions, is more regularly shaped, and has ing proportions. Quartz 'and K-feldspar are sparse or not st:vongly disrupted the enclosing rocks. These sim­ unrecognizable in thin section but are significant com­ ilarities and differences probrubly result from emplace­ ~· ponents of norms of similar rocks. The base of the mass, ment and crystallization 'of hatches of similar magma at along its west side, though discordant in detail, is significantly different depths beneath the surface, the grossly concordant with rocks of the Colorado Forma­ Rattlesnake mass beneath a cover no more than a few tion, the Slim Sam Formation, and the lower part of thousand feet thick a.nd the Dutchman mass a:bout ·a mile the Elkhorn Mountains Volcanics. The contact with the deeper. Elkhorn Mountains Volcanics that forms the roof is YOUNGER INTRUSIVE ROCKS irregular but appears to be broadly concordant. Rem­ The Elkhorn Mountains Volcanics and related shal- nants of older rocks within the mass are abundant, and 1ow intrusives are cut and tJlrermally metamorphosed by some are large; most of the larger remnants are severely younger Upper Cretaceous and Paleocene generally faulted and 'are locally folded. The mass is thought to coarser grained intrusive rocks, in stocks, dikes, plugs, have been emplaced as follows: sheets of magma, prob­ and bodies of unknown subsurface configuration. The ably fed through a north-trending zone of steep frac­ rocks range from gabbro to granite (table 1, fig. 3) ; most tures, were emplaced along bedding surfaces in the are medium- to coarse-grained equigranular rocks; a Colorado Formation and younger rocks. As the sheets few of the smaller bodies are fine-grained porphyry. grew they swelled and coalesced, disrupting septa of the The rocks -can readily be assigned to either a mafic group host rocks and strongly deforming and embaying the or an intermediate and felsic group. The mafic rocks roof rocks, which were at most a few thousand feet range from gabbro to calcic syenite. The more felsic thick. Locally, the magma probably burst through to rocks range from syenodiorite to granite; rocks in the the surface as suggested by the abundance nearby of t larger bodies are mainly equigranular and range from coarse debris of probable mudflow origin consisting of syenodiorite to quartz monzonite. Three stocks of por­ the same petrographic types that constitute the Rattle­ phyritic quartz monzonite and several small bodies of snake mass. For the most part the roof was shouldered granodiorite porphyry, quartz monzonite porphyry, and up and aside but not engulfed; and ·accordingly, the leucocratic granite porphyry cut the Elkhorn Moun­ blocks of older rocks are not products of stopping but tains Volcanics and related intrusives in the north \half rather are remnants of septa that once separated sills. of tJhe area. In the central part of the area a few com­ Large inclusions of older rocks are abundant, but small posite bodies similar to the composite in-trusives of the inclusions are rare; no features suggest 'assimilation. In southern Elkhorn Mountains (Klepper and others, 1957, several places zones of healed intrusion breccia separrute · p. 51) consist of one or more mafic rock types and one or rocks of slightly different lithology or texture. more felsic rock types. In each of the few examples The Dutchman intrusive ·body crops out along the where ·age relations can be determined, felsic intrusive west margin of the Radersburg quadrangle (pl. 1 and rocks are younger than associated mafic intrusives. section/-/') in sees. 29 and 32, T. 5 N., R.1 W. Its west­ Relative age of the several compositional and textural ward continuation has 'been mapped 'by Klepper, Weeks, varieties of felsic and intermediate intrusive rocks, how-. and Ruppel (1957, p. 45). The body is strongly discord­ ever, could not be determined with certainty. In only ant along its north and east margins in the mapped area one place, near Hassel, are rocks of two different masses but is roughly concordant with the enclosing Devonian of intermediate and felsic intrusive rocks in contact with rocks along most of its west m·argin. Several sill-like one another. There, the Silver Wave and Diamond Hill offshoots extend southward from the body, and the out­ stocks are in contact, but relatively intense hydrother­ crop pattern of the entire intrusive body suggests that mal alteration and poor exposures obscure the contact it is crudely sill-like. Field study, ex·ruminrution of thin relations. The Silver Wave stock is finer grained and sections ( ta:ble 1, fig .. 3), and chemical analyses of three rather strongly hydrothermally altered. The Diamond representative samples (table 1, samples 10, 12, 14) in­ Hill stock is porphyritic in places, is cut by aplite and dicate that the Dutchman intrusive is relatively homo- quartz veins, and is less affected by hydrothermal altera- 24 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA tion in the vicinity of the contact with the Silver Wave crystals are slightly altered. K-feldspar in subhedral to stock. As shown on section 0-0, plate 1, the Silver Wave anhedral interstitial grains commonly forms about 10 stock is interpreted as the older of the two. percent of most of the rocks examined under the micro­ Differences in types of mineral deposits seem to be scopic but reaches as much as 30 percent. The K-feldspar associated with different rock types of the intermediate includes cryptoperthite, anorthoclase or sanidine, and and felsic group. South of the Diamond Hill mine, for cloudy microperth.ite thrut poikilirtically encloses plagio­ example, the intrusives are dominantly intermediate in clase and in places is in micropegmatitic intergrowth composition and are equigranular and fine to medium with quartz. The K-feldspar is intergrown with the grained, and the veins are notably poor in quartz and marginal part of plagioclase crystals in some rocks. A have been valuable primarily for gold associated with small amount of quartz, commonly less than 5 percent, abundant pyrite. North of the Diamond Hill mine, the is present in small anhedral interstitial grains. Acces­ f intrusives are generally more felsic and richer in quartz, sory minerals include apatite, sphene, and zircon. and some are porphyritic; in this area the veins are The mafic rocks are younger than the Elkhorn Moun­ dominantly quartz and havev been valuable for their tains Volcanics and related intrusive rocks and, wher­ gold, silver, lead, and zinc content. ever relations can be determined, they are older than •ll' felsic intrusive rocks. They are discordant and intrude MAFIC ROCKS rocks of the Colorado and Slim Sam Formations and The Aldrich Gulch intrusive, (pl. 1, section D-D) the Elkhorn Mountains Volcanics. One of the dikes ap­ Jenkins Ridge stock, and two dikes, all west of Glendale pears to be truncated by the South Fork stock. Rocks Butte in the south-central part of the mapped area, the of the Orphan Boy intrusive (pl. 1, section A-A'), Orphan Boy intrusive (pl. 1, section A-A) in the which strongly metamorphosed Elkhorn Mountains northern part, and the mafic parts of several small com­ Volcanics, are in turn recrystallized by granodiorite of posite intrusive bodies in the central part consist mainly the Antelope Creek stock. The rocks are similar to the of calcic syenodiorite and calcic monzonite but include mafic facies of the composite intrusives of the southern rocks ranging from mafic syenite to gabbro. Most of the Elkhorn Mountains (Klepper and others, 1957, p. 51- mafic rocks form part or all of small discordant stocks 52), and to.ga:bbroic to monzonitic rocks that are among or plugs; a few form dikes. An intrusive just west of the oldest rocks of the Boulder batholith (Knopf, 1957, the mapped area similar to the Orphan Boy intrusive p. 90-91). has been described by Smedes ( 1960; 1966, p. 64, 66) as a segment of a ring dike marginal to the Antelope Creek INTERMEDIATE AND FELSIC ROCKS stock. Rocks ranging in composition from syenodiorite to ' Most of tihese rocks ~are dark gray, medium ,to coarse granite (table 1, fig. 3) comprise many stocklike intru­ grained, equigranular to porphyritic, and are character­ sive masses along the east flank of the Elkhorn Moun­ ized especially by their content of mafic minerals, which tains. All these masses, which range in area from a few in most rocks ranges from 30 to 55 percent, and by their acres to a few square miles, are sharply discordant, and relatively high potassium content in the form of K­ different masses cut folded rocks ranging in age from feldspar or biotite, or bdth. The rocks are hypidiomor­ Devonian to Late Cretaceous. Most consist of medium­ phic granular to porphyritic aggregates of feldspar and to coarse-grained subequigranular rocks, but a few small mafic minerals, and they contain phenocrysts and phe­ bodies consist of fine-grained porphyry and a few are nocrystic clusters of mafic minerals. The mafic minerals coarse grained ~and porphyritic. Three groups are de­ •• are olivine, augite, and hypersthene, hornblende, and fined on the basis of composition and texture: ( 1) gran­ biotite in different combinations and proportions. Oli­ odiorite and related rocks, (2) porphyritic quartz mon­ vine is in part replaced by serpentine and in part rimmed zonite, and ( 3) granodiorite porphyry and related rocks. by pyroxene and biotite; pyroxene is replaced and rimmed by hornblende and biotite, and hornblende and GRANODIORITE AND RELATED ROCKS biotite are replaced in part by chlorite and sparse cal­ Rocks clustering near the boundary between grano­ cite. Magnetite and perh31ps other black opaque minerals diorite, syenodiorite, quartz monzonite, and monzo­ are associated with the ·mafic minerals and prob­ nite (table 1 ; fig. 3) are the most abundant and wide­ ably formed as a result of release of iron during re­ spread of the younger intrusive sui1te and make up most placement reactions. Plagioclase, forming 25-50 percent of the larger masses of th31t suite. The rocks ~are mainly of different rocks, ranges from labradorite ~to andesine, medium gray to greenish gray, fine Ito ·medium grained, and typically is in slightly zoned subhedrallaths smaller 1and subequigranular; seriate or porphyritic textures than the associated mafic minerals. The cores of some are rare. Mineral composition falls generally within the IGNEOUS ROCKS 25

following lim~ts: plagioclase, 40-60 percent; K-feld­ feet wide. The stock consists of medium-grained sub­ spar, 15-35 percent; quavtz, 1-20 ,percent; and ma;fic equigranular rocks of uniform appearance that cluster minerals, 15-35 percent. Plagioclase ty:pically is in around the intersection of the monzonite, syenodiorite, weakly 2ioned subhedral .to ·anhedral crystals !that range quartz n1onzonite, and granodiorite fields (fig. 3 ; table

in composition from '3ibouit An55 at the core .to An35 'at 1) ; locally, near the margins subequigranular rocks the border; rthe highest and lowest An contents ·observed grade into rocks with porphyritic or seriate textures. A

were An75 in a core an An25 in a rim. The centers of lead-alpha age of 76 million years was determined on zoned crystals are replaced to some extent by clino­ zircon from rocks of the Lone Mountain stock (Jaffe zoisite or epidote, calcite, albite, sericite; or by mixtures and others, 1959, p. 81). of .these minerals. Most of the K-feldspar is orthoclase, Keating Gulch stock.-The Keating Gulch stock cuts but l·ocally, 'anorthoclase ·and (or) orthoclase crypto­ sharply and irregularly across upper Paleozoic and perthi,te( ~) are ·present. K-feldspar forms anhedval in­ Mesozoic rocks. Two small bodies west of the stock and terstitial g~airis, .which ·are generally smaller rbhan the one to the south consist of rocks thait resemble those accompanying grains of plagioclase, and larger irregu­ in the stock ~and are probably connected with it at depth. larly shaped crystals that poikilitically enclose and re­ The stock ~and its outliers consist of gray or greenish­ · .• place plagioclase and mafic minerals. Par:t of the !{­ gray medium-grained, looailly porphyritic rocks; the feldspar is in micro:pegmatiitic intergrowrth with quartz. main body of the stock consists of granodiorite and The orthoclase typically is cloudy and speckled with syenodiorite, and !two smaller outliers to the west are brown, semi opaque mineral dust; anorthoc1ase and (or) monzonite (fig. 3 ; table 1) . The marginal part of !the orthodase cryptoper:thite ( ~) are clear. Quartz forms stock includes quartz-rich rocks of .peculirar !texture that small interstitial anhedral grains ·and appears Ito have are probably metasoma;tically replaced sedimentary been the last mineral to crystallize. rocks. The m,afic minerals ·are hornblende, pyroxene, and Slim Sam stock.-The intrusive body cutting upper biotiite. They range in size from grains a few hundredths Paleozoic, Jurassic, and Lower Cretaceous sedimentary of ,a millimeter long in the groundmass to phenocrysts rocks in and west of sees. 20, 21, 28, and 29, T. 6 N., as mubh as 20 mm long. The m·afic grains in most roclns R. 1 W., (pl. 1, section D-D') was named the Slim Sam are about rthe same size ·as the felsic ones, but hornblende stock by Klepper, Weeks, and Ruppel (1957, p. 50}, and pyroxene crystals are generally larger !than biotite who briefly described the rock and reported one chemical crystals .in the same rocks. Hornblende is .the dominant analysis. Though exposures are poor, the map pattern mafic mineral in ·most rocks, ~and it occurs as suhhedral suggests that the stock is discordant in most places but ' to ragged anhedral crystals that 'are partly ·altered to may be concordant along its northeast margin. The bioti:te, chlorite, epidote, and calcite. Pyroxene is wide­ stock consists of relatively homogeneous medium­ ly present burt is rarely dominant; it is in ragged sub­ grained granodiorite and quartz monzonite (fig. 3). hedral crystals rthat typically ~are rimmed and partly South Fork stock.-The South Fork stock, along the replaced by uralitic hornblende rand brown biotite. South Fork of Crow Creek, is a discordant mass of Most pyroxene is colorless or faintly pleochroic from medium- to fine-grained subequigranular hornblende­ colorless to pale green, has an extinction angle (Z/\c) biotite quartz monzonite near granodiorite (fig. 3) that of 45-50°, a 2V of 40-60°, and is 1probably pigeonitic. cuts folded sedimentary rocks of the Colorado and Slim Biotite typically is in small subhedral to anhedval flakes Sam Formations. The rocks resemble those of the Slim replacing hornblende ~and pyroxene or, less commonly, Sam stock about a mile to the south. is disseminated through the rock; much of it is partly Silver Wave stock.-The Silver Wave stock on In­ al.tered rto chlorite and epidote. dian Creek cuts discordantly across the Elkhorn Moun­ Accessory minerals includes apatite, sphene, magne­ tains Volcanics and is inferred to be cut by the Diamond tite, and small amounts of hematite and ilmenite com­ Hill stock (pl. 1, sections B-B', C-C'), though contact monly 'associated with par:tly altered mafic minerals ·and relations are obscured by hydrothermal alteration and very sparse small crystals of zircon. poor exposure. The rocks of three large outcrop areas Lone Mountain stock.-The Lone Mountain stock, are believed to be connected at shallow depth. The named and described by Freeman, Ruppel, and Klepper northern part of the stock is separated from the central (1958, p. 520-521), underlies an ·area of several square part by Holocene alluvium; the central and southern miles in the south of the mapped area (pl. 1 and section parts are separated by a plate of intensely recrystallized F -F'-F"). The stock cuts sharply across gently dip­ Elkhorn Mountains Volcanics. A small elongate body of ping Elkhorn Mountains Volcanics and has thermally intrusive rock along Cold Springs Gulch is similar to metamorphosed them in a zone at least several hundred the stock and probably part of it. The stock consists 26 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA mainly of syenodiorite but includes granodiodte, mon- A'), extends northward and westward beyond the zonite, and diorite (fig. 3). Most o£ the rocks are fine- to mapped area. The part to the north was briefly de­ medium-grained subequigranular; some are seri3tte and scribed by Mertie, Fischer, and Hobbs (1951, p. 44-45), a few are porphyritic. Grain size o£ rocks near the bor­ and that to the west, by Smedes (1966, p. 66-67). The der of the stock averages 1 mm or less, ·and grain size of stock is composed of medium- to fine-grained subequi­ rocks from the central part of the stock averages 1-2 granular granodiorite o£ uni£orm appearance and com­ mm. The rocks near the border of the stock contain a position (fig. 3 ; table 1) . It cuts and strongly meta­ few small inclusions of country rock and are a little morphoses rocks o£ the middle member of the Elkhorn darker than the coarser grained rocks in the central Mountains Volcanics. The Orphan Boy mafic intrusive part. In the vicinity of the Diamond Hill mine the rocks is separated from the stock by a screen of basalt of the are hydrothermally altered. Elkhorn Mountains Volcanics; both are thermally Diamond Hill stock.-The Diamond Hill stock, at the metamorphosed. The stock is cut by thin dikes of aplite, junction of Indian Creek and its West Fork (pl. 1, by many stringers and veinlets of quartz., and by a few section C-C') , consists of biotite quartz monzonite of quartz latite dikes. uniform composition (fig. 3). The dominant rock type Other stooks.-The Bluebird stock, near the Blue­ is medium gray, fine to medium grained, sparsely por­ bird mine in· sees. 1 and 18, T. 6 N., R. 1 E., and sees. phyritic, and contains biotite and plagioclase crystals as 12 and 13, T. 6 N., R. 1 W., is composed o£ homogeneous much as 5 mm long in a xenomorphic intergrowth of medium- to fine-grained subequigranular to porphy­ smaller grains of K-feldspar, quartz, and plagioclase. In ritic hornblende granodiorite (fig. 3). It is a small, a few places the rock is conspicuously porphyritic, and strongly discordant body that cuts the Elkhorn Moun­ it contains euhedral pink phenocrysts of K-feldspar as tains Vol

In the east flank area, the contact between the Slim Sam of the major folds and east-trending faults normal to Formation and the Elkhorn Mountains Vokanics 'ap­ the trends of the folds. pears to be gradational and there is no evidence of pre­ The north-trending faults are the oldest faults rec­ volcanic deformation. ognized in the mapped area, and some are contempo­ The folded rocks are cut by many north- and east­ raneous with the early part of the folding. Many of them trending steep faults. Many of these faults displace are very steep reverse faults, and in a general way they Elkhorn Mountains Volcanics 'and related intrusive flank the synclinal trough, a location suggesting that rocks and were formed following volcanism, during the they formed during later stages of folding as the trough principal episode of folding. Others, however, are oc­ area was squeezed between the flanking anticlines. .Some, cupied by dikes related to the Elkhorn Mountains Vol­ however, must have formed during the early stages of canics and were formed during volcanism, probably folding for they are cut by younger faults or are cut during the ea;rly stages of folding. Northwest-trending or occupied by dikes related to the Elkhorn Mountains cross faults appear to have formed too, as brittle rocks Volcanics tha't were intruded during early stages of failed to accommod·ate the folding. folding (Freeman ·and others, 1958, p. 524). The compression that produced the north-trending A noteworthy example of these old north-trending folQ.s and associated faults culminated in eastward faults is the Whitehead Ranch fault, which trends thrust faulting on the Lombard fault zone east of the north-northwest from .the vicinity of the East Pacific mapped area (Freeman and others, 1958, p. 525), and mine to the Antelope Creek stock. North of Beaver the rocks in the rna pped area were broken by north­ Creek it is mostly concealed by gravel flanking the west- and northeast-trending tear faults at about the range front. South of Beaver Creek it displaces a sill same time. Still younger are range-front faults, along of older intrusive rock; to the north, however, it is trun­ which there has been recurrent movement from pre­ cated by a sheet of older intrusive rock. The displace­ Oligocene time to the present (Freeman and others, ment is greatest just north of Beaver Creek where rocks 1958, p. 526-529). near the base of the middle 1nember of the Elkhorn FOLDS Mountains Volcanics are faulted against rocks of the The pre-Tertiary layered rocks between the Boulder Slim Sam Formation, a stratigraphic displacement of batholith and the Missouri River (fig. 4) are folded about 3,000 feet. A mile south of Beaver Creek the into a series of gently plunging anticlines and synclines displacement is not more than a few hundred feet. that trend north to N. 20° E. The mapped area .(pl. 1) Though the fault bounds the range north of Beaver covers part of the flanks of the two well-defined anti- . Creek, the topographic relief and the displacement are clines and the intervening broad syncline. The anti­ in opposite directions, and the front of the range is cline to the west is described by Klepper, Weeks, and a fault-line scarp. Ruppel (1957, p. 56) and that to the east, by Freeman, The east-trending faults which are nearly vertical Ruppel, and l(lepper (1958, p. 524). faults along the mountain front south of Winston and In the central part of the area, in Cold Springs between Eureka Creek and Slim Sam Gulch, trend Gulch, the syncline is well defined and simple. The east west-northwest to about east-west and displace Cre­ limb dips more steeply than the west limb, and both taceous sedimentary and volcanic rocks as much as 500 limbs are complicated by minor folds. North of Indian feet. These faults cut and are cut by north-trending Creek the S.ilver Wave and Diamond Hill stocks occupy faults, and thus both sets formed during the same time the approximate position of the trough (pl. 1, section interval. Both are cut by all the other fault systems. 0-0'). Between these stocks and the Whitehorse fault, The faults most probably are tensional breaks produced the rocks are massive, ·almost structureless volcanic duringtfolding. breccias, and. the position of the trough line cannot be Faults of similar trend displace Paleozoic and • recognized. South of Crow Creek the rocks have been Mesozoic rocks as much as 100 feet along the north­ invaded and disrupted by the Rattlesnake intrusive, trending ridge in the southwest corner of the area. and the synclinal trough has been largely obliterated. These faults are best developed in the Quadrant Forma­ tion and are part of a system that .radiates from the FAULTS large dome west of the area (Klepper, and others, 1957, FAULTS ASSOCIATED WITH FOLDING p. 57) and that formed during folding as brittle beds Faults that formed during folding are the oldest failed to accommodate. Two of these faults, just south faults recognized in the mapped area. They include of Johnny Gulch, displace a sill of diorite porphyry north-trending faults that parallel the axial traces similar to the Rattlesnake intrusive mass. 32 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA Sulfide minerals, in places in minable concentrations, mapped area (South Fork fault of Smedes, 1966, p. have been deposited along and adjacent to many east­ 97-98) the relative displacement is about 3,000 :feet, trending faults. north side down, and the fault terminates westward against the Beaver Creek fault, which is subparallel NORTHWEST- AND NORTHEAST-TRENDING FAULTS with the Weasel Creek :fault. Northwest- and northeast-trending faults and line­ The northeast and northwest-trending faults are com­ aments are among the most prominent structural fea­ plementary sets that occupy theoretical shear positions tures of the east flank of the Elkhorn Mountains, for for east-west compression. However, movement on they control parts of the valleys of some of the main many of the northeast-trending faults has been domi~ streams. The Indian Creek fault clearly displaces Creta­ nantly vertical rather than horizontal as would be re­ ceous and older rocks southeast from the Diamond Hill quired in the proposed model. Thus it seems likely that stock but is recognized only as a prominent lineament many of the faults were subjected to different stresses to the west. The fault is vertical or nearly so, and the that resulted in vertical displacements after they had movement along it appears to have been principally left­ formed by shearing during east-west compression. Some lateral, for ~ast of Hassel it displaces a steep north­ of thmn may also have acted a;s tears during the thrust­ trending fault about 2,000 feet. In the Limestone I-Iills, ing that culminated the compressive episode (Freeman east of the mapped area, the fault has been described as and others, 1958, p. 525-526). A few small northeast­ a tear fault (Freeman, and others, 1958, p. 525). and northwest-trending faults are cut by rocks related Crow Creek parallels Indian Creek and the Indian to the Elkhorn Mountains Volcanics, and thus they were Creek fault but is not so clearly controlled by a fault. formed early in the period of compression. Most or all The valley of Crow Creek and of Wilson Creek, one of the displacement along the larger faults, such as the In­ its principal tributaries west of the mapped area, is al­ dian Creek and 1Veasel Creek faults, was later, however, most straight for 12 miles and forms a striking line­ for these faults displace even the youngest rocks of the ament. But if this exceptional valley is controlled by a Elkhorn Mountains Volcanics and related intrusions. fault, the displacement cannot exceed a few hundred feet. Most probably it is controlled by a sheared or RANGE-FRONT FAULTS closely jointed zone along which there has been little if A system of linked and branching, roughly north­ any displacement. trending steep normal faults cuts rocks along the front The Weasel Creek fault extends south-southwest from of the range between Indian Creek and the Beaver the northeast corner of the Elkhorn Mountains along Creek fan. The principal fault (or faults) of this range­ upper Weasel Creek to the Kleinschmidt mine and n1ay front system is covered by fan gravel ; the block to the be continuous with a fault in Eureka Creek. The fault west has been uplifted at least 2,500 feet, for in sec. 8, T. splits north of the Freiburg stock and may be hinged, 7 N., R. 1 E., the middle part of the Kootenai Forma­ for north of the Freiburg stock the block east of the tion is faulted against rocks of the lower member of the fault has been dropped, perhaps more than 1,000 feet Elkhorn Mountains Volcanics. Recurrent displacement near the front of the range, whereas the block .east of the along some of these faults was an important factor in fault in Eureka Creek has been uplifted at least a few outlining the basin (now the valley of the Missouri hundred feet and perhaps as much as 1,000 feet. River) in which Oligocene and younger sediments were The northeast-trending Whitehorse fault occupies deposited, and the range was lifted again along some of upper Whitehorse Gulch and cuts across the spur north there faults after the youngest rocks and unconsolidated of Whitehorse Gulch at the front of the range. The sediments in the valley had been deposited (Freeman block north of the fault has dropped 500-1,000 feet. The and others, 1958, p. 527-529). The youthful appearance fault may merge with the Weasel Creek fault to the of the range front suggests that much of the displace­ west; to the east it is cut off by the range-front fault ment occurred in late Pliocene or early Pleistocene time. • system. In a few places there are faint suggestions of north­ South of the Vosburg stock a steep fault that trends trending scarps in fan gravel of probable Pleistocene west-northwest and that parallels the Indian Creek age east of the range front, but there is no indication fault separates rocks of the lower and middle members that younger Pleistocene or Holocene fans have been of the Elkhorn Mountains Volcanics. The block north displaced. of the fault has been dropped at least several hundred OTHER FAULTS feet and perhaps much more. The fault terminates east­ On the south side of Indian Creek west of Diamond ward against the Weasel Creek fault. West of the Hill, faulted welded tuff and other rocks of the middle STRUCTURAL GEOLOGY 33 member of the Elkhorn Mountains Volcanics form a canism. In the Elkhorn Mountains and nearby the ma­ graben within rocks of the lower member. The displace­ jor folds trend about north and presumably resulted ment along the faults that bound the graben need be from east-west compressive forces. A single broad no more than 500 feet, for the surrounding rocks are troughlike fold covers most of the mapped 'area. It is probably within 500 feet of the top of the lower member cut by many faults that developed at several different of the formation and the rocks in the graben are the stages, some during and some subsequent to folding. basal part of the middle member. The proximity of the Intrusive masses are abundant in the trough of the graben to the Silver Wave stock and its outliers suggests fold; the earliest of these are hypabyssal bodies related that the gra:ben may have formed by collapse of part of to the Elkhorn Mountains Volcanics; many of them are the roof of a magma reservoir. concordant or partly concordant and were probably A few other faults have no apparent relation to re­ injected during the initial stages of folding. gional structures, and proba;bly are related to nearby Many faults formed during folding as rocks in the intrusive bodies. The sedimentary and volcanic rocks trough were squeezed between adjacent anticlinal or within and next to ·the Rattlesnake intrusive body, for do mal folds. Steep north -trending normal ~and reverse example, are cut by many short faults of diverse trend. faults are occupied or cut by older intrusives, and these These faults almost certainly were formed during the relations indicate that they formed during an early emplacement of the body, for they are restricted to the stage of folding. East-trending faults of about the same rocks near the body and are not part of the regional pat­ age appear to represent failure of brittle rocks in ten­ tern. Most of these faults are truncated by the nearby sion. Some north ,and east-trending faults displace in­ intrusive, but a few of them displace the intrusive. Most trusive rocks related to the volcanics; others appear complexly faulted are isolated remnants of sedimentary to control the margins of such bodies; many contain rocks surrounded by intrusive rocks, within which dis­ concentrations of sulfide minerals. The evidence indi­ placements of several hundred feet occur. Most of the cates that the faul:ts are of several different ages, or blocks within the intrusive probably were septa between that if all were fonneJd at about the same time, different sill-like bodies during the early stage of emplacement ones were later reactivated. and were severely disrupted during a later more force­ The conjugate shear directions for eastward-directed ful stage of emplacement; some may have been carried compression is represented by ex,tensive northwest- and up by magma that rose along a north-trending feeder northeast-trending faults. The northwest-trending dike or dikes, and some may have foundered from the fau1ts ·are well developed; horizontal motion is hrdi­ roof. cated along a few, and vertical movement is dominant Small faults near the mafic intrusive along the ridge along most. One of the northwest set of fau}ts becomes south of Jenkins Gulch may be associated with the em­ a tear fault associated with eastward displacement on placement of the mafic intrusive or with the emplace­ the Lombard fault, a major thrust east of the present ment of the South Fork stock. map area. Dominant movement along the northeast­ SUMMARY OF LATE CRETACEOUS AND TERTIARY trending fault appears to have been vertical. ':Dhe pat­ TECTONICS tern of faulting established during folding and the The Elkhorn Mountains, of which the present study emp:lacement of the older intrusives establirshed struc­ area is a part, were the site of marine sediinentation ~tural blocks which have responded individually to sub­ during much of Paleozoic time, but beginning in Meso­ sequent regional and local stresses. zoic time they began to emerge and be covered by non­ The younger intrusives were emplaced after the fold­ marine sediments. The margin of the seas advanced and ing and related faulting. Some of them appear to oc­ withdrew across the area several times during the Cre­ cupy zones of weakness developed during the deforma­ taceous, as indicated by several alternations of marine tion. These show little evidence of forceful injection; and nonmarine rocks. directed textures are present only locally along the Folding and volcanism within or near the area are margins of a few of the larger bodies. first recorded in rocks of the Slim Sam Formation which 'I'he east front of the Elkhorn l\{ountains is partly contains shardlike quartz crystals and feldspar crystals bounded by a system of north- to norlhwest-tren!ding and which locally was weakly folded before the Elk­ linked faults, along which the mountains appear to have horn Mountains Volcanics were erupted, mainly or en­ been elevated repeate

111"30' EXPLANATION D Quaternary and Tertiary rocks

Younger Tertiary intrusive rocks

Older Tertiary.. and Upper Cretaceous ·intrusive rocks

Upper Cretaceous volcanic rocks

Mesozoic sedimentary rocks

15'

Paleozoic sedimentary rocks

Precambrian sedimentary rocks

Contact •

Mine with recorded production in excess of $300,000

. I.

0 5 10 MILES

FIGURE 4.-P.vincipal mines and mining areas in Clancy, Townsend, Devils Fence, and Rader~burg quadrangles. MINERAL DEPOSITS 35

TABLE 2.-Summary of mine production, east flank of Elkhorn Mountains [Data from Reed (1951), Stone (1911), Winchell (1914), Pardee and Schrader (1933lr_. Corry (1933), Trauerman and Waldron (1940), Reyner and Trauerman (1950), U.S. Geol. Survey (1883-1923), and u.S. Bur. Mines (1924-31, 1932-53)] .

District Period Gold Silver Lead Zinc Copper Total value (dollars) (ounces) (tons) (tons) (tons)

Beaver Creek t ______1864-1904 ______$1, 750, 000 730, 000 2, 050 0 ·o $2, 500, 000 1905-28 ______250, 000 175, 000 950 0 25 500,000 1929-56 ______750,000 300,000 2,400 500 75 1,500,000 TotaL ______2,750,000 1,205,000 5,400 500 100 4,500, 000

Cedar Plains a______1864-1904_~------2, 200,000 No data 70 0 140 2, 310,000 1905-28 ______3, 000, 000 180,000 2,530 0 1, 170 3, 820,000 1929-56 ______2, 700,000 80, 000 650 250 50 3, 000,000 ' TotaL ______7, 900, 000 260,000 3,250 250 1, 360 9,130,000 Park and Hassel a______1864-1904 ______1905-28 ______850, 000 50, 000 0 0 0 900,000 50, 000 25, 000 220 100 40 100,000 1929-56 ______1,250,000 175,000 2,000 1,250 10 2,200,000 TotaL ______2,150,000 250,000 2,220 1, 350 50 3, 200,000 Grand totaL ______12,800,000 1, 715, 000 10, 870 2, 100 1, 510 16,830,000

1 Mainly mines in Iron Age Gulch and Weasel Creek areas. 2 Mainly mines in Radersburg area and Quartzite Ridge area. a Mainly mines in Park and Hassel areas.

primarily from the !(eating mine. Mines in the Winston of this type is the East Pacific on Weasel Creek, from (Beaver Creek) district, principally those in Iron Age which ore valued at more than $2 million has been mined Gulch and Weasel Creek, contributed about 30 percent (Reed, 1951). Other mines that have produced note­ of the total value in gold, silver, lead, and subordinate worthy quantities of a similar type of ore are the J anu­ amounts of zinc and copper. Most of the remaining 30 ary, Kleinschmidt (Little Olga), Stray Horse, and percent is accounted for by gold and, to a lesser extent, mines of the Custer-Iron Age-Hyantha group. by silver and lead from mines in the Park and Hassel Some mines have produced ore valuable mainly for districts, by silver and lead from mines in the Quartzite silver and lead. Important among these are the Jo Ridge area, and by byproduct copper from the Keating Dandy, North Home, and Santa Anita, in the Quartzite gold mine at Radersburg. Ridge area. The Ruby n1ine in the same area has yielded The initial mineral discoveries in the area were placer oxidized zinc-lead ore. gold at Radersburg, Weasel Creek, and Indian Creek in Different lode mines have been productive at different 1866 and 1867. Intensive placer mining in these three times since about 1870. The greater part of the produc­ areas during the next 10-15 years, intermittent small­ tion from the Beaver Creek district was before 1904, scale operations thereafter, and mechanized operations from the Custer-Iron Age-Hyantha group, the East Pa­ along Indian Creek during the 1940's yielded gold worth cific mine, and the l{leinschmidt group; in recent years at least $1 million and perhaps as much as $2 million. the January mine and the East Pacific mine dumps have Some of the large lode deposits-the !(eating, Diamond yielded most of the ore reported from the district. Most lEU, and East Pacific-were discovered shortly after of the production from the Park district before 1930 the placers were found, and most of the more produc­ was gold from the Diamond Hill, Blacksmith, and Giant tive lode deposits were known before 1900. group mines; since 1930 the Marietta mine has been ':Dhe !(eating mine west of Radersburg was long fa­ the largest and the most consistent producer. The Ra­ mous for its gold ores. Other principal gold mines along dersburg (Cedar Plains) district is noteworthy for the east flank of the Elkhorn Mountains include the gold from mines at Radersburg, particularly from the Ohio !(eating and Black Friday near Radersburg, the Keating mine which has accounted for more than half Diamond Hill, Giant group, and Blacksmith in the the output of gold from the district and for most of I-Iassel distrid, the Custer-Iron Age-Hyantha group in the copper. Mines in the Quartzite Ridge area, the Jo Iron Age Gulch, and the Marietta in the Park district. Dandy, North Home, Santa Anita, and Ruby, have Some mines have yielded complex ore from which yielded most of the lead and silver from the district. In gold, silver, and lead, and small quantities of zinc and recent years a deposit of titaniferous magnetite near copper have been recovered. Perhaps the principal m·ine Radersburg has been 1nincd on a small scale. 36 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA TYPES OF METALLIC MINERAL DEPOSITS riferous pyrite veins, whereas the nearby more felsic '.Dhe metallic mineral deposits of the area are assigned Diamond Hill stock is cut by quartz veins with aurif­ to the groups listed below: erous pyrite and by quartz veins with base and precious 1netals, hut it lacks quartz-poor auriferous pyrite veins. Auriferous pyrite veirrs (quartz-poor). Quartz veins with auriferous pyrite. AURIFEROUS PYRITE VEINS (QUARTZ POOR) Quartz veins with base and precious metals. Steep veins that contain auriferous pyrite, sparse Silver-lead-zinc replacements in carbonate rock. marcasite, very sparse chalcopyrite, and traces of Skarn deposits. pyrrhotite, galena, sphalerite, and chalcocite have been Titaniferous iron ore. the principal source of go~d in the Radersburg and Has­ Gold placers. sel areas. The veins are a few inches to several feet wide Most of the deposits (fig. 5) in the Radersburg and and a few hundred to at least 2,500 feet long. The Keat­ Hassel areas are auriferous pyrite veins with only minor ing vein has been exp~ored to a depth of about 1,200 quartz; the deposits in the Beaver Creek area are quartz foot. Simple veins consist of discontinuous bands of veins with auriferous pyrite and quartz veins with base pyrite sheathed by altered rock along a single fracture. and precious metals. The silver-lead-zinc replacement In more complex veins, stringers ·and pods of pyrite deposits are m·ainly in the Quartzite Ridge area. Skarn occur in a zone of sheared and strongly altered rock as deposits are small and scattered. Deposits of titanif­ mucih as 20 feet wide. Quartz a.nrd calcite, or other car­ erous iron deposits of economic interest are known only bonate minerals, are sparse anrd erratically distributed near Radersburg. Gold placers have been worked near as gangue. Rock within and adjacent to •the veins is Radersburg and along Indian and Weasel Creeks. silicified, sericitized, kaloinized, and pyritized. Replace­ Most of the important veins trend near north or near ment has been more important than fracture filling. east and are steep, but some veins in the Park and Iron The veins are associated with younger intrusive bodies Age Gulch areas dip gently. Recurrent movement took of' nonporpihyritic granodiorite and related rocks that place along most veins during and after mineralization; contain few or no aplite dikes or quartz veins. some veins have been brecciated and then healed by two In the zone of oxidation, which in places is at least or more generations of quartz and sulfide minerals, and a few hundred feet deep, the pyrite is largely altered in some veins brecciated ore is incorporated in gouge. to i·ron oxides, and the veins are aggregates of varicol­ Most of the vein deposits :are clustered in the vicin­ ored iron oxi1des, locrully with sparse manganese oxide ity of bodies of younger intrusive rock, and some veins and flecks or films of secondary copper minerals ; the cut these intrusives. Accordingly, a genetic Telationship pyritized wallrocks are bleached almost white. In the between the intrusives and the veins is inferred. A Radersburg district, where veins are abundant and oxi­ further correlation between type of vein and type of in­ dation intense, most of the rock in an area a mile long trusive rock is suggested. South of the Silver Wave by one-half a mile wide is altered and bleached (pl. 1). and Diamond HiH stocks the younger intrusives are Many veins were profitably mined in the oxidized intermediate in composition, meldium-grained equi­ or partly oxidized zone, but only a few veins, such as granular, virtually lacking in phenocrysts, and are the Keating and Ohio Keating, were profitably worked cut by almost no aplite and pegmatite dikes or quartz beneath this zone. This strongly suggests that oxida­ veinlets; in this area most of the veins consist of altered tion significantly improved ~the grade of ore, but avail­ rock with auriferous pyrite and very lit·tle quartz and able assay data are too few to support any quantitative 3ire enclosed by strongly altered rocks. North of the estimate. In the Radersburg area, at least, much or most Silver Wave stock the intrusives ten!d to be more felsic of the oxidation was pre-Oligocene, for Oligocene sedi­ and porphyritic, contain large K-feldspar.phenocrysts, mentary rocks overlie the oxidized part of the !{eating and are cut by many quartz veins :and vein~ets and by vein ('\Vinchell, 1914, p. 182). This fact is of possible small 31plite arrd pegmatite dikes ; the mineralized veins economic importance ; for if similar veins exist beneath in this area contain quartz as the principal gangue, and the relatively thin blanket of Tertiary sediments south many contain g·alena, sphalerite, cili.alcopyrite; arseno~ of the Radersburg district, .the chances are good that pyrite, ·and tetraheJdrite in addition to pyrite. Altera­ they are oxidized. tion halos adjacent to veins generally are narrower than they are to the south. Genetic relationship of vein type QUARTZ VEINS WITH AURIFEROUS PYRITE to intrusive rock type is most strongly suggested in the Gold veins north of the Hassel area are distinctly Hassel area where the Silver Wave stock and surround­ different from those farther south. They consist mainly ing volcanic rocks are cut by abundant quartz-poor au- of quartz but contain auriferlous pyrite, locally in MINERAL DEPOSITS 37

111 °37'30" EXPLANATION

Quaternary and Tertiary rocks No. Mine 1 Orphan Boy 2 Gold Bug 3 Custer 4 Edna No.2 Younger Tertiary intrusive 5 Hyantha 6 Iron Age rocks 7 Maine 8 Sunshine 9 Monte Cristo 10 Freiburg 11 Silver Saddle 12 East Pacific Older Tertiary.. and Upper 13 January Cretaceous intrusive rocks' 14 Sunrise 15 Stray Horse 16 Little Bonanza 17 Vosburg 18 Kleinschmidt (Little Olga) 19 Champion 20 Upper Cretaceous volcanic 21 ~~1t~~~ef rocks 22 Marietta 23 Bullion King 24 Little Annie 25 Jawbone 26 Park-New Era-Venezuela 27 Parker · Mesozoic sedimentary rocks 28 Shep 29 Buckeye 30 Phoenix 31 Iron Mask 32 JohnL 33 Lookout Paleozoic sedimentary rocks 34 Mammoth 35 St. Louis 36 Queen Bee 37 Silver Wave 38 Central Contact 39 Spring Hill 40 Diamond Hill u 41 Little 'Giant D 42 Blacksmith 43 Bluebird . Fault 44 Monarch Dashed where approximately 45 Quartzite located. U, upthrown side·; 46 Horn Silver 47 Rothfus D, downthrown side 48 Iron Cross 49 Jebro 50 Rena ----+---- 51 North Butte Syncline, approximately located 52 Black Bear 53 Beauty 54 Surprise 55 Delome Quartz vein •with base and 56 Hidden Treasure 57 Greenhorn precious metals 58 Ohio-Keating 59 Keating 60 Etta ... 61 Hard Cash Quartz vein with auriferous 62 Kahoka pyrite 63 North Home 64 Congress 65 Iron Age (Allen) • 66 Black Friday Auriferous pyrite vein 67 Bluebird (quartz poor) 68 Gopher .. 69 Cyclone 70 Jo Jo 71 Jo Dandy Silver-lead-zinc• replacement 72 Gold Cross 73 Santa Anita deposit 74 Ruby (Ruby Silver) 75 Monterey • ••• 76 Ralls Skarn deposit Quartzite Ridge Titaniferou!J iron deposit district

Gold placers

0 2 3 4 5 MILES Size of symbol indicates relative size of deposit

FIGURE 5.-Distct:bution of metallic mineral deposits along the east :flank of the Elkhorn Mountains. Geology generalized from plate 1. 419-246--71----4 38 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA abundance, and in places ankerite or another carbonate ore body of this type was about 1,500 feet long and was mineral. Some veins contain a little molybdenite a:r:td mined to a depth of 700 feet (Reed, 1951, pl. 13). Some chalcopyrite, and a few contain sparse arsenopyrite, of the veins have been enriched by oxidation, but many galena, and sphalerite. Veins of this type are mainly have been profitably mined well beneath the zone of and perhaps entirely fissure fillings, and most of them oxidation. are frozen to their walls. Many veins are less than a SILVER-LEAD-ZINC REPLACEMENTS IN CARBONATE foot thick, but a few veins locally are as much as 4 feet ROCKS thick. They occupy fractures of diverse trend, most of Replacements in limestone and dolomite of the Quad­ which dip less than 30°; none were observed to dip more rant Formation and Mission Canyon Limestone have steeply than 45 o. In several places veins of this type been explored south of Johnny Gulch. The deposits are are cut by steeply dipping base- and precious-metal lenticular or pipelike aggregates of gailena, sphalerite, veins; the reverse relationship has not been observed. and pyrite, or their oxidation productions, in a gangue Veins of this type have been important sources of gold of cryptocrystalline quartz (chalcedony and jasper) in the Iron Age Gulch area and have yielded some gold and calcite or dolomite. A few contain a little barite. in the Weasel Creek and Park mining areas; much of Most of the deposits have been mined only in the zone the gold has been from oxidized or partly oxidized of oxidation or partial oxidation where iron oxides, parts of veins. Most of the veins of this type are as­ cerussite, smithsonite, and subordinate amounts of sec­ sociated with felsic intrusive rocks that are porphyritic ondary minerals of lead and zinc (hemimorphite, angle­ ·and that are cut by numerous quartz veins and · stringers. site, pyromorphite, wulfenite, and probably others) strongly predominate over sulfide minerals. . QUARTZ VEINS WITH BASE AND 'PRECIOUS METALS The deposits are localized along steep east-trending Many veins in volcanic and intrusive rocks north of fractures. In the Quadrant Formation most of the de­ Indian Creek and a few south of Indian Creek contain posits are steeply plunging pipelike bodies in dolomite gold and silver and most or all of the following metallic beds. The few ore bodies in the Mission Conyon Lin1e­ minerals : pyrite, galena, sphalerite, arsenopyrite, chal­ stone are irregularly shaped lenses in limestone beds or copyrite, and tetrahedrite. Such veins characteristically in premineral collapse breccias adjacent to or between are associated with felsic intrusive rocks that are cut steep fractures. Pipes or lenses are from a few square by aplite dikes and quartz veins and stringers. The feet to a few hundred square feet in cross section and are amount and proportions of the minerals differ from vein as much as a few hundred feet in length. to vein: for example, veins of this type at the Marietta A few small deposits (Na:da, Buckhorn) in the mine are valuable mainly for gold; at the Iron Mask, for Quadrant Formation are similar in habit and control zinc and lead; at the Kleinschmidt, for silver, lead, and to these replacement deposits, but the ore is porous iron zinc; and at the January and East Pacific, for lead, oxide that contains free gold and little or no silver, lead, silver, gold, and zinc. Quartz is the common gangue or zinc. mineral-and is abundant in some veins. Many veins con­ SKARN DEPOSITS tain small quantities of a carbonate mineral as well, In a few places, particularly at the margin of the most commonly probably ankerite, but in some veins it Keating Gulch stock, limestone and associated beds have is siderite, in some veins, calcite, and in some veins, been converted to masses of coarse-grained silicate min­ dolomite. The veins are mainly replacement deposits erals, mainly garnet, epidote, vesuvianite, quartz, and • and partly banded fracture fillings in silicified, seri­ calcite. Some of these skarns contain magnetite, hema­ citized, argillized, and pyritized rock. Typically, they tite, pyrite, pyrrhotite, chalcopyrite, and secondary iron consist of broken altered rock laced with lenses and and copper minerals. The largest known body of this stringers of sulfides, gangue minerals, and gouge, but type, at the Rothfus mine, yielded a few thousand tons in places they are clean -cut fracture fillings accom­ of partly oxidized copper ore. panied by little or no gouge. Recurrent fracturing dur­ Volcanic rocks in the lower part of the Elkhorn ing and after mineralization has brecciated many of Mountains Volcanics adjacent to the Silver Wave stock the veins. have been hornfelsed and locally replaced by coarser Veins of this type are as much as several feet thick textured aggregates of epidote, chlorite, quartz, garnet, and a few thousand feet long ; a few have been mined and pyrite that resemble skarn and locally contain gold through a vertical distance of 600 or 700 feet. Most ore and scheelite. Most of the coarse-textured silicate rock bodies were only a few feet thick and a few tens or a is along fractures in a zone of intensely fractured rock few hundred feet long, but in the ·East Pacific mine an near the intrusive contact. The distribution of these MINERAL DEPOSITS 39 silicate rocks and the intimate intergrowth of pyrite veins yielded most of the ore 1nined before 1900 anJd and scheelite with the silicate minerals suggest that they part of the ore mined since. In the Radersburg district and the associated metallic minerals are genetically only the !(eating and Ohio Keating auriferous pyrite related to the Silver Wave stock. The silicate rock and veins sustained profitable operations beneath the zone the stock are cut by many ~pyritic gold veins, some of of oxidation. In the Park, Weasel Creek, and Iron Age which contain scattered crystals of scheelite, and by a Gulch areas the importance of oxidation on the quartz · few small base- and precious-metal veins. veins with auriferous pyrite and the base- and precious­ metal veins was not so great, but nevertheless it seems TITANIFEROUS IRON ORE certain th31t the most profitable mines (E·ast Pacific, Lenticular beds of titaniferous magnetite are locally Custer-Hyantha-Iron Age groups, Kleinschmidt) were present in the middle of the Slim Sam Formation. These \ mostly in oxiJdized ore. Though unoxidized ore bodies beds are th~ught to be lithified beach concentrations or parts of ore bodies were profitably mined from these deposited as the Late Cretaceous sea withdrew, probably and other mines, most of them were relatively small and in Niobrara time. In the vicinity of Radersburg, ore has the grade of ore was generally lower than that in the been mined from a slightly metamorphosed deposit of oxidized zone. this type near the margin of the Rattlesnake intrusive Most deposits are oxidized or partly oxidized to a body; tlwse magnetite-bearing rocks are distinctly depth of at least 100 feet, and some are largely ox~dized harder, tougher, and of higher grade than those at other to depths of several hundred feet. Winchell (1914, localities. Though the magnetite grains appear to be p.173) reported that the !(eating vein was oxidized to a· detrital, some recrystallization and perhaps even some depth of 175 feet and partly oxidized for an additional migration of iron probably took place during thermal 59 feet. The East Pacific vein is reported to be oxidized metamorphism. Samples of the ore contained 43.1 and to a depth of about 300 feet (Pardee and Schrader, 1933,

44.1 percent Fe and 9.6 and 9.3 percent Ti02 • Most of p. 219). The importance of oxidation was probably two­ the titanium is probably in solid solution in the mag­ fold : gold was liberated from the pyrite with which it netite ( titanomagnetite), for neither exsolved ilmenite was associated in primary ore, and thus recovery was nor any other titanium minerals were recognized in the facilitated ; and the grade of ore was increased by few polished and thin sections examined. removal of pyrite, sphalerite, and arsenopyrite. Fur­ thermore, liberation of gold during oxidation was a GOLD PLACERS necessary first step in the formation of placer deposits. ... Gold placers have been mined at several places. South Much of the ore in the silver-lead-zinc replacmnent and west of Radersburg, a pediment of probable late bodies in the Quartzite Ridge area also is oxidized. Pliocene age is veneered by gravel and is cut by a For example, the unusually rich ore from the J o Dandy network of shallow gravel-filled channels (pl. 1), many mine was mainly lead carbonate, a.nd virtually the entire of which have yielded placer gold. The age of the gold­ output of the Ruby mine has been secondary minerals of bearing gravel. probably ranges from late Pliocene to zinc and leaJd. Holocene. The source of the gold is the numerous oxi­ The history of oxidation in the ·area is complex and dized veins that crop out in the low hills that border imperfectly known. In the Radersburg area, and per­ the placer area to the west. haps ·elsewhere, the veins were oxidized in latest Cretace­ The placer gold ·along Indian Creek, in the mapped ous and early Tertiary time and subsequently oovered, area, is mainly in remnants of gravel terraces of prob­ at least in part, by Oligocene (and Miocene~) sediments. able late Pleistocene age that border the present chan- This cover has been erodeJd from the productive part of ·nel of the creek, and partly in the present channel the Radersburg area in relatively recent time (late gravels. East of the area, gold is in gravel-filled chan­ Pliocene~ to Holocene), but oxidized veins may lie nels on a pediment of probable late Pliocene age as well beneath the Tertiary cover farther south. North of In­ as in gravel terraces along the present stream. The dian Creek, some veins have probably been exposed to source of the gold is veins in the Park and Hassel areas. weathering and oxid31tion since l31test Cretaceous or Placer gol1d also was recovered from postglacial gravel early Tertiary time. along the present stream channels of Weasel and Beaver Creeks. DESCRIPTION OF DEPOSITS OXIDATION AND ENRICHMENT IRON AGE GULCH AND WEASEL CREEK AREAS At least one-third and perhaps more than one-half of (WINSTON AND BEAVER CREEKS) the total value of metal produced from the area has The Iron Age Gulch and Weasel Creek areas include been fron1 oxidized or partly oxidized ore; oxidized most of the deposits that are referred to the Winston

\ ----· 40 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA region or district (fig. 5) or the Beaver Creek district tudinal section of the Custer mine (Reed, 1951, fig. 12) of earlier reports (Reed, 1951; Pardee and Schrader, suggests that an ore shoot, or several closely spaced ore 1933; Stone, 1911; U.S. Geol. Survey, 1883-1927; U.S. shoots, in the Custer vein was of comparable size. Bur. Mines, 1925-32, 1932-56; Earll, 1964; Schell, 1963). Most of the mines are wholly or partly inaccessible, As the principal ore deposits in these two areas have and consequently little information could be obtained on been described by Pardee and Schrader (1933, p. 211- the characteristics of veins and ore shoots at depth ex­ 277), Earll (1964, p. 4-29), and Schell (1963), only a cept in the Kleinschmidt (Little Olga) mine. In this resume of pertinent features is given here. Earll (1964, mine a long south-trending crosscut adit was driven to p. 30-43) also described tJhe results of a geochemical explore several east-trending steep veins at depth (Par­ survey of the Winston district. dee and Schrader, 1933, pl. 31). The Little Olga vein, Most of the deposits in both areas are fissure veins which yielded partly ·oxidized silver-lead-gold ore with­ adjacent to or within stocks of quartz-rich felsic intru­ in 250 feet of the surface, was intersected at a depth of sive rocks. The mines in the Iron Age Gulch area cluster 500-750 feet below the outcrop. A level was driven 400 around the Edna stock and have yielded ore from steep feet along the vein from the intersection, and a raise at base- and precious-metal veins and from gently dipping least 180 feet long was driven on the vein. In these work­ gold -bearing quartz veins with auriferous pyrite. In ings the vein was 2-4 feet thick and consisted of altered the Weasel Creek area the greater part of the ore has rock ·and gouge cut by stringers of pyrite, sphalerite, been from steep east-trending base-and precious-metal and galena. Only a few small pockets of ore were rich veins, but some ore has also boon mined from quartz enough to be mined. Several other veins were inter­ veins with auriferous pyrite; these mines are near sev­ sected by the crosscut, but they were too narrow or of eral stocks, of which the Vosburg is the largest. too low grade to be mined. None of them could be cor­ Mineralogy of the veins is fairly uniform. The gently related with certainty with veins near the surface a:bove dipping veins cons'ist of limonite-stained quartz and a the crosscut (Irish Syndicate, Cynosure, Harmony, and little pyrite at the surface and, where less affected by two unnamed veins shown on pl. 31 of Pardee and weathering, of clear glassy quartz with pyrite. The veins Schrader, 1933). appear to heal fractures and contain little or no gouge. A map published by Reed (1951, fig.13) suggests that The steep veins contain pyrite, argentiferous galena, the East Pacific ore shoot pinched and became discon­ sphalerite, subordinate chalcopyrite and arsenopyrite, tinuous below the No. 4 adit (about 400 ft. below the and sparse tetrahedrite in a gangue of quartz and locally outcrop and 100 ft. below the bottom of the oxidized carbonate minerals. Gold is present, probably mostly in zone as reported by Pardee and Schrader, 1933) and pyrite. Much of the ore mined from these two areas was was less than 200 feet long on the bottom level of the oxidized. Pardee and Schrader (1933, p. 214-215) re­ mine, about 300 feet below theNo. 4 adit. ported that most of the oxidized ores were valuable 2 mainly for gold and that in the unoxidized ores "gold CUSTER MINE (3) appears to be carried mostly by pyrite, but its distribu­ The largest mine in the Iron Age Gulch area, the tion is irregular" and that "silver is associated mainly Custer, explored a steep base- and precious-metal vein with the galena." that trends N. 30° E. Ore was mined from a single shoot These veins have been explored over a vertical distance 2,400 feet in length (Mrs. Charles MufHy, oral commun.; of 3,400 feet, from an altitude of 4,250 feet at the bottom see also Reed, 1951, fig. 12). Gold was the principal of the Custer mine (Reed, 1951, fig. 12) to an altitude of metal produced. The vein cuts the Edna stock and the 7,650 feet along the outcrop of the Little Olga vein Elkhorn Mountains Volcanics, but the ore shoot was (ICleinschmidt mine) ; the deepest workings on a single confined to that part of the vein in the volcanic rocks. vein, about 750 feet, are at the East Pacific mine. l\Iost The mine was operated through two vertical shafts and ore shoots do not exceed a few hundred feet in length an adit. It was largely inaccessible in 1952, and only a and depth and a few feet in thickness; a few are much small part of the adit level· was examined. Where seen, longer and deeper, but, except for small local bulges, the vein comprises two splits striking about N. 30° E. ; none has exceeded 5 feet in thickness. An ore shoot in the east split is about vertical, and in a 12-inch width it the East Pacific mine was almost completely stoped for a contains chiefly pyrite and smaller amounts of galena, length of 1,200-1,500 feet through a vertical distance of arsenopyrite, chalcopyrite, and sphalerite in brecciated 400 feet, mainly in the oxidized zone, and was partly quartz and a carbonate mineral (ankerite~) . Gouge stoped for 300 feet below this in the sulfide zone (Reed, along the vein walls contains brecciated sulfides and 1951, fig. 13). Vein thickness in most places did not ex­ 2 The number in parentheses following each mine name is the number ceed 2 feet and in some places was a foot or less. A longi- of that mine on plate 1 and in figure 5. MINERAL DEPOSITS 41

fragments of altered andesite. The west split dips JANUARY MINE ( 1 3) 48°-65° mv. and consists of brecciated andesite, gouge, The January mine exploited a base- and precio1JS­ and, locally, of brecciated sulfides. metal vein for a length of 450 feet and over a vertical distance of about 400 feet. The vein trends N. 70°-90° EDNA NO. 2 MINE (4) W. and dips about 40°-70° N. (pl. 2). Ore has been Most of the shallow workings in the Iron Age Gulch mined from a discontinuous shoot that rakes to the area apparently explored gently dipping gold-bearing east and is as much as 160 feet l'Ong. West of the ore quartz veins that cut granite porphyry of the Edna shoot the January vein has not been identified with stock. One of tJhe workings on the Edna No. 2 clailn certainty, but a number of faults along the projected (pl. 2) illustrates many of the features seen in the strike of the vein contain brecciated sulfides and altered workings that were examined. In this adit, the con­ rock. The ore ranges in thickness from a few inches tinuity of any single gently dipping vein is difficult to to about 4 :feet and typically is 12-18 inches thick in a prove because of the many steep faults. Displacements vein that is 1-15 feet thick and consists of altered a.nd on the faults are small, but are enough to hinder sys­ broken :vock laced with strands of gouge and with lenses tematic mining. Similar gently dipping ·veins some­ and stringers of sulfide and gangue minerals. Post­ what less disrupted by faults were developed on the mineralization movement along the vein is indicated adjacent Edna claim. The veins typically are one-hal:£ by brecciated sulfides and thick gouge. Sulfide minerals to 8 inches thick, and. the gold content reportedly ranges are pyrite, galena, sphalerite, chalcopyrite, tetrahed­ :from a trace to a few ounces per ton. rite, and arsenopyrite. The gangue is quartz and r-hodo­ chrosite. Minerals in the oxidized ore include limo­ HYANTHA MINE ( 5) nite, cerussite, pyromophite, pyrolusite, and gold. Reed The Hyantha mine yielded lead, zinc, silver, and gold (1951, p. 39) reported the average metal content of from a base- and precious-metal vein that trends N. the ore to be: copper, 0.11 percent; lead, 5.0 percent; 50°-70° E. and dips 60° N. to vertical. The vein cuts zinc, 3.4 percent; silver, 5.3 oz (ounces) per ton; gold, several gently dipping quartz veins with auriferous py­ 0.108 oz per ton. rite. The vein is explored by a 340-:foot-deep vertical A. second vein, the Montana-Idaho, was explored for shaft and by two adits. At the shaft collar, the vein is a short distance on the No. 1level. This vein trends N. as much as 4 feet wide, occupies a fault in the Elkhorn 70°-90° E., dips 60° N. to vertical, and appears to be Mountains Volcanics, and consists of pyrite, galena, and offset by the January vein. It differs from the Januacy­ sphalerite in brecciated quartz and aJbundant gouge. vein in containing abundant arsenopyrite and pyrite A. short distance west of the shaft, the vein cuts into the and little galena or sphalerite. Both veins cross the con­ Edna stock and probably continues as one or more of tact between the Elkhorn Mountains Volcanics and the barren northeast-trending, steeply dipping faults the January stock, and the January vein contains ore in the Edna No.2 mine (pl. 2). on both sides of the contact.

EAST PACIFIC MINE ( 1 2) KLEINSCHMIDT (LITTLE OLGA) MINE (18) The East Pacific mine, the most productive mine in ,The l{leinschmidt (Little Olga) mine (pl. 2) the vVeasel Creek area, yielded ore having an estimated (Prurdee and Schrader, 1933, p. 31; Reed, 1951, p. 40) value of more than $2 million (Reed, 1951, p. 38, fig. explores two sets of veins (Little Olga and Quartette) a. 13). It was largely inaccessible in 1952. The mine ex­ in the porphyritic quartz monzanite of the Vosburg ploited a vein that trends N. 65°-80° E., dips 65°-85° stock. Most of the production has been from the Little N., and cuts the Elkhorn Mountains Volcanics near the Olga vein, a steep east-trending base- and precious­ Freiburg stock of porphyritic quartz monzonite. The metal vein as much as several feet thick that contains vein is a filled fracture that has banded structure and pyrite, galena, sphalerite, chalcopyrite, stephanite, and • is accompanied by minor replacement in the adjacent tetrahedrite in a gangue of quartz, siderite, altered wallrock. Vein width ranges from a few inches to sev­ rock, am.d gouge; some ore was mined from other eral :feet. The vein contained several shoots of base­ steeply dipping base- and precious-metal veins and and precious-metal ore, within which were narrow from thin north-trending gently dipping quartz veins chimneys of high-grade gold ore. The principal ore that contained pyrite and sparse ,arsenopyrite, molyb­ minerals were argenti:ferous galena, auriferous pyrite, denite, and gpld. The gently dipping veins are cut and sphalerite, and smaller amounts of chalcopyrite and slightly displaced by the steep veins. The steep veins tetrahedrite in a gangue of quartz and one or more car­ contain considerable broken rock and gouge, and the bonate minerals. country rock bordering them is sericitized, pyritized, 42 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA and argillized. The gently dipping veins are largely Granodiorite on the dump of a caved shaft at the fillings that heal the fractures ; gouge and broken rock Venezuela mine and thermally metamorphosed volcanic are uncommon along them and the adjacent altered zone rocks in the vicinity suggest :that a sizable body of is generally no more than a foot thick. younger intrusive rock lies at shallow depth. A crosscut was driven 3,300 feet southward from the MARIETTA MINE (22) head of vVhi~tehorse Gulch to explore at a depth of wbout 500 feet beneath the Little 0 lga lower adit (pl. 2) . The Marietta mine (pl. 3; Reed, 1951, p. 48; Schell, In this crosscut the Quartette vein was not identified 1963) explores several gently to moderately dipping and the Little Olga vein. consisted of altered broken north- ·and northeast-trending lenticular veins in the rock and gouge cut by a few narrow stringers of sulfides. Elkhorn Mountains Volcanics and associated intrusives. Only a few small pods of ore were mined above this Most of the ore has come from a north-trending zone crosscut and adjacent to a raise driven from it. that includes the Marietta and Blue veins (pl. 3). The veins contain pyrite, arsenopyrite, galena, sphalerite, OTHER MINES and sparse chalcopyrite in a gangue of quartz and a Several other mines near vV easel Creek yielded ore manganiferous carbonate mineral. Limonite and man­ from veins similar to those described above. The Stray ganese oxide stain .are prevalent in oxidized and partly Horse {15) mine expored two parallel, east-trending, oxidized ore. The veins are a few inches to rarely as steep veins in tthe Elkhorn Mountains Volcanics, both much as 6 feet thick, are in part banded, and are mainly typically a foot or less thick. The veins are displaced fracture fillings. Adjacent to the veins the volcanic as much 'as 20 feet by steep north-trending faults. rocks are argillized, pyritized, and locally sericitized. Mineralization is similar to that in the January vein. The dip of the veins is variable but generally low, and The Little Bonanza mine explored a small ore shoot on some of the thicker ore bodies are localized by rolls. yielded small amounts of high-grade silver-lead ores. Most of the ore is valuable mainly for gold, which ap­ The Little Bonanza mine e~plored a smrull ore shoot on parently is in ·auriferous pyrite and arsenopyrite in a vein trending N.75° E. and@ping 65° NW. in the unoxidized ore. Lead and silver are important locally, Elkhorn Mountains Volcani9s The Silver Saddle mine for example in a flat part of the Marietta vein in the explores a mineralized fracture zone th31t trends north­ Gold Dust intermediate level (pl. 3). Zinc and copper east and dips 30°-45° SE. (pl. 2) in the Elkhorn Moun­ are present only in minor quantities. tains Volcanics. A few small pods of ore containing The north-trending vein zone is cut by faults or fault argentiferous galena, pyrite, aiid sparse sphalerite in a zones of several trends {pl. 3). Displacements along the quartz gangue were mined. Between ore pods the vein faults are small and range from a few feet to rarely as consists of gouge seams in bleached, slightly silicified, much as 20 feet, even though in some places the rocks and pyritized volcanic rock. are intensely fractured in zones 10 feet or more across. Because of the fracturing and displacement of the veins PARK AREA and their lenticularity and varia:ble dip, correlation of The Park area at the head of Indian Creek centers veins in the north-trending zone is uncertain. The most around the ~1arietta mine in and near sec. 15, T. 7 N., persistent fauLt trends N. 30° W. along the course of the R. 1 W. It is underlain by rocks of the Elkhorn Moun­ intermediate level, dips steeply, and has an a;pparent tains Volcanics, mainly tuff, and a few small bodies of offset of 12-16 feet, southwest side down. On the south­ intrusive rocks related to the volcanics. Most of the west side of the fault the Blue vein trends nearly paral­ ore mined has been from gently dipping lenticular lel to the fault but dips very gently. This vein has been quartz veins with base and precious metals, valuable stoped for a length of 200 feet over a vertical distance mainly for gold, but in places containing important of at least 150 feet. Northeast of the fault, the north­ quantities of lead and silver. In some veins arsenopyrite trending Marietta vein or vein zone has been mined is relatively abundant. The veins are mostly 2-3 feet intermittently for about 700 feet through a vertical dis­ thick with local swells to as much as 5 feet. Some of tance of more than 250 feet. them have been mined for several hundred feet along strike and vertically 250-300 feet. Most veins dip 35° OTHER MINES or less and in places roll to almost horizontal. In some Gently to moderately dipping veins of diverse trend places rolls appear to localize thicker parts of veins. similar to those in the Marietta mine have yielded gold The veins commonly are frozen to the walls. They ore with different amounts of lead and silver at the contain a few thin gouge seams and small horses of Park {26), New Era {26), and Venezuela {26) mines, altered rock. a short distance southwest of Marietta, at the L~ttle MINERAL DEPOSITS 43 Annie (24) mine about 1,000 feet east of the Marietta, stock, none has been recognized cutting the Diamond and at the Vulture (21) mine at the head of Eureka Hill stock. Creek. Vuggy milky quartz characterizes the vein ma­ IRON MASK MINE ( 31) terial at the Vulture mine. l\1assive and partly vuggy The Iron Mask mine explored a north-trending steep­ veins of white quartz and small amounts of a carbonate ly east-dipping base- and precious-metal vein that cuts mineral, probably siderite, and pyrite yielded gold ore the lower member of the Elkhorn Mountains volcanics. at the Ohampion (19) mine (NE~SElJ.., sec. 4, T. 7 The mine yielded about 23,000 tons of ore of the :fol­ N., R. 1 W.) and at. the Bi~ Chief (20) mine near the lowing average grades: 4 percent lead, 7.4 percent zinc, head of 'Vhitehorse Creek. A northwest-trending steep 0.2 percent copper, 4.9 oz. per ton silver, and 0.005 oz. vain on the Jawbone ('25) claim (NWlJ..,NW~ sec. per ton gold (Reed, 1951, p. 46). The mine is developed ... 17, T. 7 N., R. 1 W.) also yielded gold ore from the by three vertical shafts and a 2,300-:foot-long crosscut oxidized part of a pyritic vein that contained quartz, adit at a depth of 570 feet beneath the outcrop (Reed, calcite, and ankerite or siderite. High-grade silver-lead 1951, p. 46). In and above this adit the vein was explored ore is reported to have been mined from a nearly ver­ and partly stoped for a length of about 600 :feet. tical pipe along a north-trending fault at the Bullion The only exposure o:f the vein at the time of this study King (23) mine (Albert Dance, oral oommun., 1952), was in a sublevel 100 :feet below the adit, where the about 1,000 feet northeast of the Marietta mine. At the thickness of the vein ranged :from 3 to 8 :feet, and aver­ Parker (27) mine in the val'ley of Eureka Creek, base­ aged about 4 feet. In the north heading of this sublevel and precious-metal ore containing pyrite, galena, sphal­ the vein consisted of a thin footwall gouge, 16 inches erite, and arsenopyrite was mined from a pipelike body of altered :fractured tuff with disseminated sphalerite, in a steep vein of N. 70o E. trend. galena, pyrite, and quartz, a 4-inch-thick band of sphal­ HASSEL AREA erite and pyrite and some galena, 36 inches of fractured and altered tuff laced with stringers of a manganiferous Veins in the Hassel area are around and in the Silver carbonate mineral and sulfides, and a 1-:foot-thick hang­ Wave and Diamond Hill stocks. North of Diamond ing-wall gouge containing sparse brecciated sulfides. Hill most of the veins are of the base- and precious­ Strands of gouge overlap and split within the vein metal type; the most productive, explored by the Iron zone. The ore shoot on this sublevel was about 90 feet Mask mine, yielded 23,000 tons of zinc-lead-silver ore long. The tuff adjacent to the vein is bleached and con­ (Reed, 1951, p. 46). South of Diamond Hill, the veins tains disseminated pyrite and sericite. are mainly of the quartz-poor auriferous pyrite type, Unoxidized ore contains pyrite, sphalerite, and ga­ similar to those in the Radersburg area. Oxidized gold lena, and sparse chalcopyrite and tetrahedrite in a ore from auriferous pyrite veins at the Diamond Hill, gangue of quartz, calcite, and a manganiferous carbo­ Blacksmith, and Little Giant mines accounted for most nate mineral. Samples of oxidized ore on dumps of work­ of the production from the Hassel area. The Little Giant ings along the outcrop of the vein contain limonite, jar­ and Blacksmith mines explore the oxidized parts of osite, psilomelane, pyrolusite, cerussite, smithsonite, and narrow, vertical or steeply dipping east-trending veins. sparse galena and sphalerite. The Diamond Hill mine explored a north-trending belt A parallel vein, the Red Wing vein, is exposed in the of thermally metamorphosed and hydrothermally .al­ crosscut adit and at the surface. This vein trends north, '& tered (pyritized) volcanic rock cut by lenticular pyritic dips 80° E., is 3-4 feet thick, and contains abundant veins and stringers of diverse trend ; ore was mined limonite and manganese oxide and sparse chalcopyrite from open pits and underground workings in the oxi­ in a gangue of quartz and two carbonate minerals. dized zone, and the mineralized rock beneath the zone of oxidation was explored by a long adit with appended DIAMOND HILL MINE (40) short drifts. The Diamond Hill mine is in a north-trending belt The Diamond Hill stock is cut by many aplite dikes of metamorphosed tuff of the Elkhorn Mountains Vol­ and by small quartz veins which contain pyrite and canics adjacent to the Silver Wave stock (pl. 4). The sparse chalcopyrite, and which are similar to the quartz mine explores one of the first lode deposits discovered veins with auriferous pyrite in the Beaver Creek and on Indian Creek, and its workings include eight adits~ Iron Age Gulch areas. The stock is also cut by a few six open pits, and numerous prospect pits and shafts. thin base- and precious-metal veins. Though quartz­ Reed (1951, p. 45) stated that these workings explore poor auriferous pyrite veins are the principal vein type and develop the deposit ·for a horizontal distance of in the volcanic rocks nearby and in the Silver Wave 2,700 :feet and a vertical distance of 250 feet. The mine 44 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA has yielded about $550,000 worth of ore valued chiefly andesite with very little quartz. Below the zone of oxi­ for its gold content, but it also contains minor scheelite dation auriferous pyrite is the only sulfide present, and and base-metal sulfides. apparently the ore could not be profitably mined. T-he Rocks in the north-trending metamorphosed zone country rocks, lapilli tuff, tuff breccia, and crystal tuff are silicified and pyritized and locally contain epidote, of the Elkhorn Mountains Volcanics, are hydrother­ chlorite, garnet, calcite, and sparse scheelite; in the zone mally altered and contain ·abundant epidote, kaolinite, of oxidation they are bleached. The zone is cut by er­ sericite, and finely disseminated pyrite. They are in­ ratically distributed but abundant seams and irregular tensely fractured and cut by many quartz stringers. stringers, pipelike shoots, and distinct veins of aurifer­ Weathered rocks are heavily seamed and stained by ous pyrite in a gangue of calcite, perhaps other carbo­ limonite. nate minerals, and quartz. Lenses of massive auriferous BLACKSMITH MINE ( 4 2) pyrite are present in places. The pyrite veins and lenses The Blacksmith mine (pl. 4), an adit with appended contain native gold and sparse chalcopyrite and schee­ drifts, two shafts, and prospect pits southeast of the lite. Scheelite also is sparsely disseminated in epidote­ Little Giant mine, explores a series of narrow anas­ chlorite-garnet rock with no particular concentration tomosing north-trending steep veins that consist mainly near pyrite veins and lenses and probrubly is older than of altered rock and limonite with a little quartz. The the pyritic mineralization. A few quartz-pyrite veins ore, mined for its gold content, was similar to that in that contain galena and sphalerite appear to be younger the Little Giant veins, and as in the Little Giant mine tihan the more widespread pyritic mineralization. it apparently was not profitable below the zone of oxi­ Rock alteration and metallization appears to have dation. The mine reportedly yielded ore valued at about occurred in several episodes. During emplacement of $250,000 (Reed, 1951, p. 44). The volcanic country rocks tJhe Silver Wave stock, the invaded andesitic tuffs were are bleached ·and limonite stained for several feet ad­ hornfelsed and, locally along fractures, epidote-chlorite­ jacent to the veins. gaTnet rock was formed; 'PYrite and prdbably most of the sparse scheelite was deposited at this time. Some­ OTHER MINES what later, probably during the final stages of consoli­ A number of mines and prospects in the Hassel area dation of the Silver Wave stock, rocks in the fractured explore base- and precious-metal veins that contain zone were hydrothermally altered, and stringers and pyrite, galena, sphalerite, and ·arsenopyrite in a gangue lenses of •pyritic gold ore were deposited, partly as frac­ of quartz and ankerite or another carbonate mineral. tur~ fillings and· partly ·as replacements of altered vol­ In this group are the Central (38) and.Queen Bee (36) canic and intrusive rocks. Following emplacement and mines, which yielded ore from steep N. 50° W.-trending consolidation of the Diamond Hill stock, a few quartz veins; the St. Louis (35), Spring Hill (39), and the veins containing pyrite, galena, and sphalerite were Mammoth ( 34) mines, which explore steep N. 80° W.­ formed. The final stage of mineralization in the area is marked by the filling of fractures by calcite. trending veins, ·and the Silver Wave mine (37), which developed an ore body on a N. 50° W.-trending vein at LITTLE GIANT MINE ( 4 1 ) its intersection with a N. 70° E.-trending vein.

At least 14 na.rrow, east-trending steep pyritic gold RADERSBURG AREA (CEDAR PLAINS) veins in hydrothermally altered voleanic rocks south Except for a deposit of t~taniferous iron ore and a of tJhe Diamond Hill mine have been explored iby the few narrow base- and precious-metal veins, the ore de­ Little Giant and nearby mines. The veins are explored posits of the Radersburg area are auriferous pyrite by several adits, three shafts, and many prospect pits veins (Winchell, 1914; Corry, 1933) in a gangue of and trenches, all in the oxidized zone. A:. typically east­ strongly altered wallrock and relatively sparse quartz, trending vein of this group is shown on plate 4. The esti­ calcite, and an iron-bearing carbona;te mineral. Most of mated value of the ore from these mines is about $300,- these veins trend nearly north and dip 65 o W. to 000 (Reed, 1951, p. 47), nearly all of it mined before vertical. They range in thickness from less .than a foot 1901. The veins occupy narrow but persistent fractures to as much as 5 feet and in length from a few hundred that range in thickness from 1 inch to rarely 5 feet, feet to about 3,000 feet. The Keating vein has been ex­ and in most places are less than 2 feet thick. The east­ plored to a depth of about 1,200 feet. trending fractures are displaced a few feet by at least Most of the veins are clustered in an area of llibout two steep faults that trend N. 25°-35° E.; locally these 1 square mile in which the country rocks are intensely faults are weakly mineralized. The ore in the oxidized altered and include two gradational zones (pl. 1) : an zone is native gold in ·a gangue of limonite and altered inner zone (heavy stipple on pl. 1), which is almost en- MINERAL DEPOSITS . 45 tirely in the Rattlesnake intrusive and contains most the main inclined shaft, and it formed a stringer zone of the veins, and an outer zone (light stipple) in both over a 1- to 2-foot width north of the shaft. According intrusive and extrusive rocks. Rocks :in the inner zone to Winchell ( 1941, p. 132), the vein was followed south­ are highly fractured, pyritized, silicified, sericitized, ward on the fourth level into "broken ground" consist­ and argillized. Most of the rock in this zone has been ing of unstratified boulders as much as 18 inches across oxidized, bleached white or light gray, and is seamed of altered and oxidized rock of local origin. Winchell and coated with limonite. The alteration is most intense correctly considered this material to be part of Tertiary adjacent to the major veins, but no distinct zonation deposit. To the north, the vein narrows and was not related to specific veins was recognized. Rocks in the followed to its termination. The vein is displaced as outer zone are green or greenish gray, and the effect of much as 25 feet by a few faults (Winchell, 1914, p. pervasive alteration seems to have been mainly the 179). formation of epidote, chlorite, and locally pyrite; ad­ The ore is auriferous pyrite in altered pyritic rock jacent to individual veins in this zone, however, an en­ with a little quartz and calcite, and sparse chalcopyrite, velope of wallrock is bleached and pyritized, and it marcasite, pyrrhotite, ·galena, sphalerite, and chalco­ resembles the strongly altered rock of the inner zone. cite (Stone, 1911, p. 93). Quartz and calcite are more The difference in intensity of alteration between the abundant in parts of the vein that do not constitute ore. inner and outer zones appears to he· related to abun­ Near the surface, the oxidized part of the vein contains dance of fractures; part of the inferred difference be­ abundant limonite, a little malachite, and gold. The ore tween zones, however, may be more apparent than real was cmnpletely oxidized to a depth of 175 feet and because of differences in original rock type and degree partly oxidized _for an additional 60 feet (Winchell, of oxidation, which tend to emphasize effects of altera­ 1914, p. 173). tion. In places the rocks are so intensely altered that diagnostic features have been obliterated and the con­ OHIO-KEATING MINE (58) tact between intrusive and extrusive rocks cannot be The Ohio-Keating mine, formerly the second most located with confidence. productive in the area, explored an auriferous pyrite vein in altered rock of the Rattlesnake intrusive paral­ KEATING MINE (59) lel to arid about half a mile west of the Keating vein. The Keating mine has been the most productive in The mine, inaccessible in 1952, was opened by a 550- the area and was for years one of the foremost gold foot-deep inclined shaft with five levels. The Ohio vein producers in Montana. During the period 1901-48 it yielded about 200,000 tons of ore with an average grade yielded 412,471 short tons of ore from which were re­ of 0.31 oz gold per ton (Reed, 19.51, p. 27). The vein covered 157,538 oz of gold, 43,557 oz of silver, and trends about N. 15° W. and dips about 80° W., it is 1,064 tons of copper (Reed, 1951). The deposit was about 1-5 feet wide and contains abundant auriferous discovered ~n 1866 and was worked at intervals until pyrite and very sparse chalcopyrite and bornite. Quartz, 1947, when underground operations were suspended. calcite, and lenses of altered country rocks are conspicu­ Since then, small shipments have been made from the ous in parts of the vein that contain little or no ore dumps. Total recorded production of gold is about $4 (Winchell, 1914, p. 179) . The vein is offset a few feet million (Trauermann and Waldron, 1940, p. 9), of by two cross-faults (Winchell, 1914, p.179). whi'Ch a small part may have come from nearby small mines. The n1ine was flooded and inaccessible in 1952. OTHER DEPOSITS The average recovered metal content of the ore was 0.38 Ore has been mined from a number of pyritic gold oz gold per ton, 0.25 percent copper (Reed, 1951), and veins in the vicinity of the Keating and Ohio-Keating a little silver. veins (fig. 2). Most of the veins trend nearly north The Keating mine is aJong one of the most persistent and dip steeply. Perhaps the most productive has been of a number of steep north-trending auriferous pyrite the Black Friday (66), which trends about N. 5° W. veins in a large are•a of intensely altered rock that is and dips 70° W. The Cyclone (69) vein, which trends partly tuff of the Elkhorn Mountains Volcanics and is N. 15° E. and dips 60°-75° W., is 1-4 feet .thick and partly porphyritic rocks of the Rattlesnake intrusive. may be a continuation of the Black Frid:ay vein. A few The !(eating vein has been explored by three shafts veins in this vicinity contain base metals in addition and 10 levels over a length of 2,200 feet and for a depth to auriferous pyrite. Ore containing galena, cerussite, of 1,100 feet (Reed, 1951, p. 25). The vein trends north wulfenite, pyrolusite, quariz, limonite, and probably to N. 5° E., dips aibout 70° W., and is about 1-5 feet vanadinite (Winchell, 1914, p. 180), abundant pyrite thick. Typically, it was about 20 inches thick south of and traces of calcite was mined from a vein trending 46 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

N. 68° vV. and dipping 65° NE. at the Congress (64) to have exrtended 1-3 feet ·along 1the fracture and 7-20 mine, and lead-silver-gold ore was mined from a vein feet along bedding and .to have had an average c~oss trending N. 60° vV. and dipping 80° NE. at the Gopher section of 2 by 12 :feet. A:t least three smaller pipes were mine (68). mined, two in !these same dolomite beds along a frac­ .IRON CROSS MINE ( 4 8 ) ture ahouJt 30 feet south of the main fracture, and the third in ,a higher dolomite bed along a fracture 35 feet At least two lenticular beds of titani:ferous magnetite north of ~the main fracture. The dolomite a;bove :the thick in sandstone and sandy crystal tuff in the Slim Sam qua·rtziite was explored f.or a length of 350 feet at 'aJbout Formation have been exploited at the Iron Cross mine. 'the level of the collar of the vertical shaft and for about The ore beds are :folded (fig. 6), and the principal ore 800 feet in ·a drif.t driven toward the Sanrta Anita mine bodies are along a fold crest .and a fold trough. The ( 73) , on the 500 level, but no other pipes were found. magnetite accumulated as placer sands, but the massive Most .of the ore was oxidized 'and consisted of cerus­ appearance of the ore and the localization of the richest silte, .pyromorphi1te, limonite, and calcite along with and thickest ore bodies along the crests and troughs of galena, manganese oxides, and wulfenite. The galena folds suggest that some of it was redistributed, perhaps content increased in depth, and ore from rthe bottom by hydrothermal solutions. :lt level of the shaft was a mixed sulfide-carbonate ore Ore is mined from several small open pits and sold (Carl Trauerman, oral commun.). :for use in a special type of cement made at Trident, Mont. SANTA ANITA MINE ( 7 3) QUARTZITE RIDGE AREA The Santa Anita. mine, a short distance sou:th of the Pipelike and lenticular bodies of silver-lead-zinc ore Jo Dandy, consists of a 4715-:foot adit wi1th connooting have replaced favorable rocks adjacent to east-trending stopes 'and inclined shafts (pl. 4) and explores a min­ steep faults of slight displacement in the Quartzite eralized fault thaJt cuts the Quadrant Formation ~and Ridge area, south of Johnny Gulch and the Rattlesnake 1the upper part of rthe Amsden Formation. The :fault intrusive. Most of the bodies are in thin dolomite beds trends N. 7·5° .to 85° W., dips from veriticallto 65° 8., between quartzite beds in the Quadrant Formation. At and offsets the beds about 60 :feet. The ore was similar least two ore bodies :are in the Mission Canyon Lime­ 1to ·the lead-silver ore of rthe Jo Dandy but was mainly stone, where one of then1 is partly localized in a col­ in lenticul,ar lba;bular hodies along the fault; a few small lapse breccia. ore :bodies were in irregular replacements projecting JO DANDY MINE ( 71) from :bhe fault in1to :f.avor3ible dolomite beds. The oxi­ dized ore was a mixrture of limonite, cerussite, smith­ The Jo Dandy mine, the most productive of the mines sonite, ·and manganese oxides with minor galena, sphal­ tha.t explore silver-lead-zinc replacement deposits in erite, and pyrite. The mine has yielded ~rubout 1,600 tons carbonate rocks, since 1920 has yielded about 9,300 tons of ore since 1910 with an average of 23 percent lea;d, of ore with an average grade of 34 percent lead and 5.7 oz. per ton silver, and 0.02 oz. per tton gold (Reed, 11.9 oz per ton silver. The deposit is opened by a 600 19,51, p. 28). The rock in the upper part of the Amsden foot-deep vertical shaft with six levels. The mine was Formrution and a;t the ;base of the Quadrant Formation inactive in 1952. at 1the west end of the ·adit is st~ongly breocia;ted and The ore is in pipelike .replacement bodies ·at inter­ ox:idized. This oxidized breccia zone is thought Ito have sections of east-trending nearly vertical fractures with :formed by leaching of carbonaJte rocks along the fault • some of rthe nor.th-·trending dolomite .beds in the Quad­ and subsequent collapse during an early Teritiary epi­ rant Formation; the main host beds •are immediately sode of weathering. a;bove and ibelow the thick quavtzite bed vhat forms the ridge crest in this vicinity. The I.argest ore !body was RUBY MINE (RUBY SILVER) (74) beneaJth lthe thick quar.tzite ·and was mined from the The Ruby mine (pl. 4), southwest of the Santa Anita, surface Ito rthe 600-foot level. The cross-sectional dimen­ explores a replacement deposit of zinc-silver-lead ore in sions of tlhis ore pipe near the surface were 2-3 feet the upper part of the Mission Canyon Limestone ad­ ·along the :f~adture •and 10-15 :feet along bedding; ron jacent to two N. 75° W. vertical small faults. The mine the 300-foort level they were about 16 feet along the includes two inclined shafts with interconnecting stopes. fracture •and 4 feet along bedding (A. R. Berg, oral The workings extend about 75 feet along the strike and commun., 1950). The second l·argest ore pipe was along 300 :feet down the dip of beds that trend about N. 15° tJhe same :fracJture at its intersedtion with the car.bonate E. and dip 40°-55° SE. About 4,000 tons of ore with a bed ·above the thick quartzite bed ; !this pipe is reported recoverable grade of 4.4 percent lead, 8.8 percent zinc, MINERAL DEPOSITS 47

0 1000 2000 FEET CONTOUR INTERVAL 20 FEET DATUM IS MEAN SEA LEVEL

EXPLANATION

(/) :::> Elkhorn Mountains Volcanics, Porphyritic pyroxene­ Syenodiorite 0 LJJ lower member bearing dike rock porphyry· u <( 1- w a:: u Slim Sam Formation

~ 28~ Magnetite ore Axis of .syncline, showing plunge Low-grade----- magnetite-bearing tuffaceous sandstone Strike and dip of beds Contact Dashed where approximately -:;_ located Strike and dip of over- · __...L 40 __ turned beds

Fauit, showing dip • Approximately located Spring

Axis of anticline, showing. plunge

FIGURE 6.-Iron Cross magnetite mine, Broadwlllter County, Mont. Geology by V. L. Freeman and D. E. Brambilla, 1952. 48 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS', MONTANA and 5 oz. per ton silver has been mined (Reed, 1951, but few of these have been produotive. In and around p. 27). the Antelope Oreek, Monte Cristo, Vulture, and Mam­ The limestone beds in which the ore occurs have a moth intrusives are narrow veins that contained gold; maximum thickness of 35 feet. Most of the ore near the some of them also contained silver and lead. Near the surface was in two distinct zones, each several feet thick Slim Sam and !{eating Gulch stocks are gold-bearing and 5-10 feet apart; in the lower workings only the skarn (Monarch mine ( 44) ) , and gold (Quartzite mine upper zone was followed, and the ore in it thinned. In (45)) and silver-lead (Horn Silver mine (46)) replace­ the upper zone, from which most of the ore was ob­ ment bodies in carbonate rocks tained, several feet of high-grade oxidized lead ore was underlain gradationally by several feet of high-grade NONMETALLIC MINERAL DEPOSITS oxidized zinc ore (A. R. Berg, oral commun., 1949). Potentially useful deposits of limestone, dolomite, The ore bodies consisted of cerussite, smithsonite, hemi­ and building stone are found in the different sediment­ morphite, limonite, drusy quartz, calcite, dolomite, and ary formations, and abundant sand and gravel are pres­ jasper and were sheathed by partly silicified limestone. ent in the surficial deposits Galena, sphalerite, and pyrite were sparsely present Low-grade bentonite, potentially useful as a sealing near the bottom of the mine. A little fluorite was seen agent for dams and canals, is present along Beaver in a few specimens of ore. Creek a few miles northeast of the mapped area (Mertie, Fischer, and Hobbs, 1951, p. 89-90). The bentonitic NORTH HOME MINE ( 6 3) strata probably extend southward into the mapped area The North Home mine, north of Johnny Gulch, also but are concealed beneath yotmger deposits. Bentonitic explored replacement silver-lead ore bodies in the upper strata are also present in the Oligocene sedimentary part of the Mission Canyon Limestone. Between 1901 rocks near Johnny Gulch, but the deposits are probably and 1950 it yielded about 4,000 tons of ore with an too small and too impure to be exploited. Similarly, average grade of 2.3 percent lead, 17.4 oz. per ton silver, beds of phosphate rock in the Phosphoria Formation and 0.06 oz. per ton gold (Reed, 1951, p. 26). are too thin and too low grade to be of commercial The ore bodies were developed by steep shafts, both interest. of which were inaccessible in 1952. The main ore body, Lignitic shale near the top of the Morrison Formation in beds immediately beneath a thick collapse breccia, has been prospected in the SW:lh sec. 32, T. 7 N., R. 1 E., was followed to a depth of about 400 feet. The shapes a few hundred yards west of the Indian Creek road, but and trends of open stopes suggest that the main ore body the deposit is to thin and too impure to be of economic was a lens that raked 60°-70° N. in beds that trend N. interest. 15° E., and dip 65° SE. It was 25 feet long and 4 feet POSSIBLE FUTURE DISCOVERY AND DEVELOPMENT thick at the surface and was about 70 feet long and as OF MINERAL DEPOSITS much as 5 feet thick at a depth of 40 feet. Below 40 feet . ']}he area has long been readily accessible and has the stope is filled. The other ore body in the basal part been explored and exploited for more than 100 years, of the collapse breccia was pipelike and was followed so that veins and ore bodies that crop out almost cer­ to a depth of about 150 feet. Near the surface the cross tainly have been found. The possibilities for discovery section of the pipe was 10 by 6 f·eet. and future development of ore in the area are of three The ore bodies are partly sheathed by silicified lime­ kinds: (1) concealed deposits o~ the same kinds that stone and limestone breccia and appear to have formed have been mined in the past, ( 2) extensions of known partly by cavity filling and partly by replacement. Most deposits, and (3) development of low-grade deposits of the ore that was mined was oxidized. It consisted of under more :favorable economic conditions or through limonite, cerussite, smithsonite, cerargyrite, pyrmor­ development of cheaper mining and recovery methods. phite, and traces of galena in a gangue of dolomite, ba­ Concealed deposits or extensions of known deposits may • rite, calcite, aragonite, manganese oxides, brown jasper, be discovered as a result of detailed geological studies gray quartz, clear drusy quartz, and traces of fluorite. and geochemieal prospecting (:for example, see Earll, Ore deposition was preceded by silicification and per­ 1964, p. 30-3). As technology improves, perhaps t'he haps by the formation of caverns in massive limestone low-grade gold deposits, such as those at the Diamond and in earlier formed collapse breccia. Hill and Keating mines, could be profitably mined. Parts of the area are judged worthy of more detailed OTHER AREAS AND DEPOSITS geologic, geochemical, and perhaps geophysical study Elsewhere in the mapped area small mineral deposits in the search :for concealed deposits or extensions of occur in and near most of the younger intrusive bodies, known deposits. MEASURED SECTIONS 49

AURIFEROUS PYRITE VEINS tively small replacement bodies of silver-lead-zinc ore. Auriferous pyrite veins in the Radersburg and Hassel Geochemical prospectJing in carbonate rocks known to areas have accounted for most of the value of metal be favorable host rocks would probably be a logical first produced from the mapped area. The oxidized and en­ step. riched parts of the known veins have been largely or GOLD PLACERS wholly mined. Large tonna.ges of unoxidized vein mate­ The gravel along Cold Springs Gulch (p. 14) was rial remain but apparently have not been profitable to deposited by an ancestral Indian Creek that headed mine. For example, the Keating mine was reopened dur­ in the Park and Hassel areas, which are noted for their ing 1947 and about 18,000 tons of ore was mined (U.S. gold veins; these areas contributed the placer gold Bur. Mines, 1947, p. 1449); if the total output of lode mined from younger gravels along the present channel gold from the Cedar Plains district during that year of Indian Creek. The gravel along Cold Springs 'Gulch (2,472 oz, U.S. Bur. Mines, 1947, p. 1446) was from this accordingly may contain placer concentrations of gold ore, the indicated recovery was 0.138 oz per ton '($4.83 derived from the Park and Hassel areas. per ton at $35) . A 2,800-foot-long adit in unoxidized Placer deposits of gold may also be buried beneath ·.i. material at the Diamond Hill mine is reported to be in the Oligocene sedimentary tuff in the vicinity of Raders­ "ore assaying from 80 cents to $2.50 a ton" (Stone, 1911, burg. The vein deposits in the Radersburg area were p. 82) based on $20.67 per oz price for gold; the equiv­ exposed to erosion before deposition of the tuff alent value is $1.35 to $4.25 per ton at $35 per oz price (p. 39), and gold eroded from the veins could have of gold. The tonnage of gold -bearing material that may been deposited with gravels in now-buried stream chan­ be of about this grade is larger, probably at }east several nels, or concentrated in the lower part of the Oligocene hundred thousand tons, of which a part might be amen­ able to open-pit mining. Some of this rock also contains sedimentary unit itself. small quantities of scheelite, but most of it almost cer­ Neither the gravel in Cold Springs Gulch nor the tainly does not contain as much as 0.1 percent wo3. base of Oligocene sedimentary tuff near Radersburg Undiscovered oxidized ore bodies in auriferous pyrite appears to have been prospected for placer gold, but veins may exist beneath Cenozoic deposits south of the both areas are considered promising. !(eating and Ohio-l(eating mines in the Radersburg area. The veins in this area were exposed to oxidation MEASURED SECTIONS and enrichment in Tertiary time (p. 39), and the belt SECTION 1.-Morris on, Swift, Phosphoria (and related st1·ata), of mineralization west of Radersburg can reasonably be Quadrant, and Amsden Formations near Indian Oreek extended southward beneath the Cenozoic cover, which [Measured by M. R. Klepper, 1948, along and near road on northwest side of Indian Creek (pl. 1) NW:!;.I, sec. 5, T. 6 N., R. 1 E., to approxi­ is not more than a few hundred feet thick. Oxidized mate center sec. 32, T. 7 N., R. 1 E.] pyritic gold veins also might be sought beneath the Kootenai Formation: gravel-covered area in Cold Springs Gulch. Sandstone, "salt-and-pepper," gray; speckled with darker chert grains ; light gray to medium light QUARTZ VEINS WITH BASE AND PRECIOUS-METALS gray and medium coarse grained in lower 9 ft; AND QUARTZ VEINS WITH AURIFEROUS PYRITE greenish gray, finer grained, and less conspicuously Small and intermittent production can be expected speckled in upper 14.6 ft. Approximately 24 ft thick. from quartz veins with base and precious metals and Disconformity. from quartz with auriferous pyrite veins. Almost with­ out exception veins of both types are narrow, and few of Morrison Formation: Feet 129. Concealed ------11 the known ore shoots extend more than several hundred 128. Sandstone, gray, speckled, "glassy," relatively feet in length or depth. Furthermore, values beneath the fine grained------1.5 oxidized zone tend to be erratic. From these considera­ 127. Concealed ------2 tions, it seems unlikely that new deposits or extensions· 126. Limestone, olive-brown, very finely crystalline, sandy or silty; in beds 1 ft. thick; weathers of known deposits will be found that will yield more brown ------10 than several thousand tons of direct shipping ore or that 125. Mudstone, olive and gray, fissile; interbeds of would yield enough ore of lower grade to sustain a mill silky dolomite; weathers yellowish brown ---- 24 of capacity larger than about 50 tons per day. 124. Siltstone, light-olive-gray to yellowish-orange, dolomitic, blocky______10 REPLACEMENT BODIES IN CARBONATE ROCK 123. Dolomite, gray, dense, siltY------3 122. Concealed ------6 More intensive exploration in the Quartzite Ridge 121. Sandstone, gray, silty; speckled with dark chert area might lead to the discovery of high-grade but rela- fragments ------6 50 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 1.-Morris on, Swift, Phosphoria (and related strata), SECTION 1.-Morrison, Swift, Phosphoria (and related strata), Quadrant, and Amsden Formations near Indian Oreek-Con. Quadrant, and Amsden Formations near Indian Oreek-Con.

Morrison Formation-Continued Feet Morrison Formation-Continued Feet 120. Sandstone, salt-and-pepper, medium-gray with 96. Siltstone and limestone; hackly to fissile silt­ faint greenish tint ; in beds about 0.5 ft. stone interlayered with thicker beds of silty thick ; lower 2 ft. silty and :tine grained, 'upper limestone ; weathers dark yellowish orange__ _ 15 6 ft. medium coarse grained______B 95. Limestone, brownish-olive, silty; weathers 119. Concealed; mostly if not wholly underlain by brownish ------0.5 dark carbonaceous mudstone and one or more Q4. Siltstone and limestone; sim'ilar to unit 96, but lignite. layers______20. 5 siltstone is calcareous ______11.5 118. Concea;led ------10 93. Concealed; underlain at least in part by brown­ ish-gray, olive, and grayish-orange calcareous 117. Mudstone, yellowish-orange, punky, blocky; two ~ lignite beds, one 0.5 ft. thick and one 0.7 ft. siltstone that weathers nearly dark yellowish thi<;!k, in upper 2 ft of unit______6 orange 17 116. Concealed ; soil in part reddish______45. 5 92. Siltstone, 1b~ownish-gray, olive, and grayish­ 115. Shale, olive-gray and dark-greenish-gray, hackly orange, calcareous, fissile to thin-bedded ; to pencilly ______..,;,______16 weathers nearly dark yello'wish orange ______16 ).. 114. Concealed ------9. 5 91. Limestone, olive-gray, silty, indistinctly ·bedded; 113. Mudstone, olive-gray and brownish-olive; in weathers gray to brown ______4 part calcareous______8 Total thickness of the Morrison Forma- 112. Concealed------8. 5 111. Mudstone, olive-brown; in beds 0.1 ft thick_____ 2. 5 tion ------523 110. Concealed ------4 Swirt Formation: 109. Mudstone, olive and dark-gray------13 90. Concealed ------7 108. Limestone, siltstone, and sandstone; olive and 89. S-andstone, brownish-gray, speckled, fine- olive-gray silty limestone, limy siltstone, and grained ; consists of whitish to glassy. surb­ fine-grained sandstone in beds 0.2l>--0.5 ft 'angul,ar to subrounded quartz grains in fine­ thick; in part fissile ; abundant darl{-gray grained br,own-stained sand matrix; unit cherty fragments in the upper 6 ft o·f unit; weathers !brownish gray------11 weathers orange brown to drab brownish 88. San:dstone, moderate-dusky-yellow, speckled, gray------27.5 fine-grained, with subrounded grains, very cal­ 107. Limestone, olive and gray, silty; weathers dark careous, frialble, locally pebbly------1 yellowish orange to moderate yellowish brown or lighter______15.5 Total thickness of S.wift Formation______19 106. Concealed ------9 105. Sandstone, light-brownish-olive, speckled, fine- Disconformi ty. grained, calcareous; weathers to medium light Phosphoria Formation ·and rel•ated strata: gray ------5 87. Chert, dark-gray, mottled dull red, ·brown, and 104. Quartzite, light-olive, dense, calcareous______1 white ------1 103. Limestone, dolomite, and siltstone ; brownish­ 86. Quartzite, dark-gray, vitreous, very fine grained; olive-gray, pale-yellowish-brown, and yellow­ contains opaque white-weathering ovoids ish-brown, silty and dense limestone and dolo­ (proba,bly ,phosphatic material) as much ·as 14 mite and limy or dolomitic siltstones with car­ in. long, :and near ~top a few pebbles of bonate rocks predominant; siltstones com­ quartzite ------16 monly in beds less than 0.02 ft thick, carbon- 85. Qua·rtzite, light-grayish-brown, very fine grained, ate rocks in beds as much as 0.7 ft thick; unit vitreous; one bed______3 weathers dark yellowish orange, grayish or­ 84. Sandstone, dolomitic, lig.bt.,brownish gray, fine­ ange, and light olive______31 grained; in beds as much :as 1 rt :thick______10 102. Concealed ------86 83. .Sandstone, silty, orange.;brown, strongly limo- 101. Siltstone, weathers orange brown______2 nitic, .punkY------1 100. Siltstone, punky, calcareous, or dolomitic; in 82. Igneous rock; 3lh ft .thick. part very fine grained sandstone; unit weath- 81. ,Concealed ------1 ers yellowish brown______3. 5 80. Chert and very fine-grained quartzite, light-gray 99. Sandstone, pale-yellowish-brown, light-olive, and and light-bro,wnish-gray; weathers to yellow- light-gray, speckled, fine-grained, som·ewhat ish OI,ange, grayish orange, :and yellowish gray crossbedded; in beds 0.2-1 ft thick; weathers rough surface ; some beds are chert, some nre pale yellowish brown______19. 5 quartzite, and some contain both chert and 98. Siltstone, orange-brown, calcareous, fissile ; up- quartzite ------15 per half· of unit mostly concealed______22. 5 97. Siltstone, calcareous; weathers dull yellowish Total 'thickness of Phosphori!a Formation brown ______..,;, ______11.5 and rel·ated strata______47 MEASURED SECTIONS 51

SECTION 1.-Morris on, Swift, Phosphoria (ana related strata), SECTION 1.-Morris on, Swift, Phosphoria (ana related strata), Quadrant, ana Amsden Formations near Indian Greek-Con. Quadrant, ana Amsden Formations near Indian Greek-Con.

Quadrant Formation: Feet Quadrant Formation-Continued Feet 79. Quartzite, pinkish-gl'ay to light-grayish-brown; 44. Dolomite, dense gray; weathers white ______1 in !beds 1-2 ft thick______7 43. Concealed ------2 78. Concealed ------5 42. Dolomite, sandy, gray------5 77. Quartzite, simila.r to unit 79______11 41. Concealed ------2 76. Concealed ------33 40. Sandstone, dolomitic, yellowish-gray and very 75. Dolomite, sandy ; weathers light gray------3 light gray, fine-grained; upper part quartzitic_ 4 74. Quartzite, pale-gray and brown, fine-grained 39. Dolomite, sandy; weathers very light gray____ 3 vitreous ; weathered surface is brown ·and 38. Dolomite, honey-colored, sandy------1 rough ------3 37. Quartzite, very light gray and yellowish-gray, 73. Dolomite, light-gray, thin-'bedded, sandy to fine fine-grained------1.5 crystalline ------.------2. 5 36. Covered------72. S•andstone, dolomitic_. honey-colored, very fine 35. Sandstone, dolomitic, pale-yellowish-brown, thin- grained; weathered surface very rough______2 71. Dolomite, sandy, light-gray ; contains irregular bedded ------2 chert masses; 2 in. 1band of gray chert at top__ 2 Total thickness of the Quadrant Formation_ 250 70. Dolomite, dense; weathers very light gray; in ·beds as much .as 2 ft thick; contains thin sandy layer nea·r tOP------10 Contact between Quadrant Formation and Amsden 69. Quartzite, white very fine grained, thin-bedded__ 4 Formation is gradational and is placed at bed above 68. Dolomitie, cherty; weathers pale bluish gray; uppermost red or pink rocks. contains fossil fragments______1 Amsden Formation : 67. Concealed ------3 34. Quartzite, dolomitic, pale~red; weathers pale 66. Quartzite, cherty matrix with quartz grains, brownish gray______1 yellowish-gray ------1 33. Siltstone and fine-grained sandstone, in part 65. Concealed ------15 dolomitic; weathers light yellowish gray, pink- 64. Quartzite, white, very fine grained______4 ish gray, and white ; thin bedded ( :JA;-:14 in.), 63. Concealed ------6 in part crossbedded and very fine grained 62. Dolomite, dense; weathers light gray, contains sandstone ------10 3 beds of quartzite each 1 in. thick near top__ 2 32. Siltstone, red; in part slightly dolomitic______3 61. Quartzite, dolomitic, pale gray, dense; middle 31 .. Conglomerate; weathers pale red; fragments part weathers to pitted surface______4 are from underlying bed------1 60. Quartzite; similar to unit 61, but weathered sur- 30. Dolomite, silty, very pale gray, thin-bedded____ 0. 5 face is ribbed______4 29. Sandstone, pale-red, coarse and gritty, cross- 59. Concealed ------11 bedded ------1 58. Dolomite, cherty, dense; weathers light gray; 28. Sandstone, slightly dolomitic, very light gray, contains thin beds of quartzite in upper halL_ 15 very fine grained______0.5 57. Quartzite, honey-c.olored; bedding thick and in- 27. Sandstone, grayish-red, very fine grained, distinct ------18 oolitic? ------1 56. Concealed ------11 26. Concealed; float is grayish-red mudstone and 55. Sandstone, dolomitic, fria:ble______1 very fine grained sandstone______10 54. Dolomitic, sandy; l-in. layer of gray chert near 25. Dolomite, silty, pale-red______1 top------4.0 24. Siltstone, dolomitic, pale-reddish-gray------0. 5 53. Quartzite, dolomitic, honey-colored______1 23. Quartzite, dolomitic, pale-gray to pinkish-gray- 2 52. Sandstone, calcareous, or limestone, sandy, al- 22. Shale, papery, grayish-red______3 most white, punky______5. 5 21. Dolomite, pinkish-gray and blue-gray, dense___ 0. 5 51. Dolomite, dense, light gray ; contains chert lam- inae in lower half______11. 5 20. Quartzite, light-gray, slightly dolomitic; • weathers yellowish gray and brown ; in beds 50. Concealed ------8 about 3 in. thick______1.5 49. Sandstone, calcareous, or limestone, sandy, light- gray, friable ______10.5 19. Concealed; float is red siltstone______7 48. Quartzite, light-gray to light-yellowish-gray; 18. Dolomite, pale-bluish-gray, dense______4 weathers pale brown ; upper 4 ft. vitreous ; 17. Ooncealed; float is red siltstone______5 lower 1 ft calcareous and friable______5 16. Dolomite, reddish-gray to pinkish-gray, dense; 47. Sandstone, yellowish-gray, calcareous, fine- in beds 6-8 in. thick______3 grained ------2 15. Concealed ------~------7 46. Dolomite, dense; weathers white______1 14. Dolomite, gray, dense______1 45. Dolomite, very finely crystalline, light-gray ; 13. Dolomite, dense, pale-olive-gray; weathers · weathers darker gray------1 grayish orange______6 52 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 1.-Morrison, Swift, Phosphoria (and related strata), SECTION 2.-Morrison and Swift Formations near Montana Quadrant, and Amsden Formations near Indian Greek--Con. Silver Star mine-Continued

Amsden Formation-Continued Feet Morrison Formation-Continued Feet 12. Limestone, fine-grained, dark-g.ray ; weathers 41. Mudstone, medium-gray, blocky weathering, con- bluish gray to yellowish gray ; most beds in tains a few thin beds of yellowish-gray silt- lower 10 ft are 4-10 in. thick and in upper 18 stone and fine-grained sandstone______4 ft are 12-18 in. thick; chert present as nodules 40. Sandstone, yellowish-gray, fine-grained, friable__ 1 and thin layers; 3 ft above base is bed with 39. Shale, dark-gray, fissile to papery------5 8Jbundant small horn corals overlain hy 4-in. 38. Concealed; mainly sandstone float______43 bed of black calcareous paper shale; dark­ 37. Sandstone, light-gray to light-brownish-gray, in gray pencilly shale in beds as much as 1 in. part color laminated; speckled with dark chert thick present throughout; beds of fossiliferous, grains ; some beds very argillaceous ; some con­ light-g.ray crystalline limestone 5.5-7.5 ft tain abundant chips of black shale; thin to below tOP------34 medium bedded, in part crossbedded______19 11. Limestone, dolomitic, or dolomite, calcareous, 36. Concealed; lower half of interval covered by dark dense, yellowish-gray; weathers light gray; in shale and mudstone float______52 beds 8 in. to 2 ft. thick______15 35. Mudstone, dull-red and olive, slightly calcereous; 10. Concealed ------38 weathers to small chips; alternates with dark­ 9. Dolomite, calcareous in lower half of unit, pale olive, very calcareous siltstone and fine-grained gray locally stained pink, dense______11 sandstone; unit weathers to plates and chunks 8. Concealed ------12 averaging 1 in. on a side------19 7. Dolomite, dense, light-gray ; weathers with brown- 34. Concealed; abundant hematite-stained light-gray ish hue; in beds 6-8 inches thick; interbedded with dusky-red siltstone______3 sandstone float near middle of intervaL______30 6. Siltstone, grayish-red to dusky-red on fresh and 33. Siltstone and very fine grained sandstones, olive­ weather surfaces______3.5 gray, calcareous; interval contains a few beds of dull-grayish-red silty limestone-'------15 5. Concealed ------5 4. Limestone, light-gray to almost white, fine- 32. Shale, alternating dull-red and green; interval grained; contains chips of red siltstone______6 partly covered------21 3. Concealed ------7 31. Concealed ------27 2. Dolomite, light"'gray partly stained pink; in beds 30. Limestone, greenish-gray, dense, sublithographic_ 4 6 in. thick ; contains a few thin interbeds of 29. Concealed, red soiL______21 grayish-red siltstone------15 28. Limestone; similar to unit 30------5 1. Concealed; float is red siltstone, quartzite and 27. Concealed ------2 dolomite; interval main1y or wholy underlain 26. Limestone, greenish-gray, dense______1 by red siltstone______96 25. Concealed; lower part probably underlain by olive and brownish siltstone and fine-grained sand­ •rotal thickness of Amsden Formation ______315 stone and upper half 'by red shale------~----- 20 24. Sandstone, calcareous, friable; alternating with Mission Canyon Limestone : sandy and silty limestone ; most beds are olive Limestone, massive, cliff-forming; weathers light gray. or brownish yellow on fresh and weathered not measured. surfaces ------11 23. Concealed ------11 SECTION 2.-Morrison and Swift Formations near Montana 22. Limestone, impure ; alternating with calcareous Silver Star mine shale ------5 [Measured, by 1\f. R. Klepper, 1948, in SW~ sec. 16, T. 4 N., R. 1 W., 21. Limestone, dolomitic, crystalline, pale-yellowish- about 2,000 ft east of Montana Silver Star mine (2 miles south of brown ------1. 5 southwest corner of mapped area)] 20. Shale, red and grayish-yellow, flaggy, calcareous_ 4 Kootenai Formation (basal part only) : Feet 19. Mudstone and shale, mostly grayish-red, chippy 46. Sandstone, salt-and-pepper, light-gray speckled to hackly; contains conspicuous pale-greenish­ with darker chert grains, coarse-grained in gray to grayish-red limy concretions______18 beds as much as 1 ft thick; partly cross­ 18. Shale, grayish-red; few thin interbeds of dense • bedded; basal part gritty and locally pebbly___ 50 brownish-gray limestone______4. 5 Disconformity 17. Siltstone, pale-yellow, calcareous; grades upward Morrison Formation : into thin-bedded dolomitic limestone______1 45. Concealed; some brown-weathering mudstone in 16. Shale, grayish-red, greenish-gray, and olive, partly mottled, very slightly calcareous, fissile float ------23 44. Sandstone, speckled, medium-grained, cross- to hackly ; contains concretions and at least one bedded ------1 0.5-ft bed of dense greenish-gray limestone____ 14 43. Concealed ------15 15. Mudstone, greenish-gray, calcareous------1 42. Siltstone and rsandstone, fine-grained, yellowish­ 14. Limestone, pale-greenish-gray, dense, argillaceous 0. 5 gray, thin-bedded, friable______4 13. Siltstone, greenish-gray______1 MEASURED SECTIONS 53

SECTION 2.~Morrison and Swift Formations near Montana SECTION 3.-Kootenai Formation and parts of the Colorado and Silver Star mine-Continued Morris on Formation-Continued

Morrison Formation-Continued Feet Kootenai Formation-Continued Feet 12. Limestone, weathers light gray; blocky fractur- 58. Limestone, medium-dark-gray, irregularly bedded ; ing; contains a few small irregular cherty con­ weathers medium g-ray to medium light gray-- 2. 5 cretions------0.5 57. Siltstone, micaceous; contains 0.5-ft-thick platy 11. Sandstone, greenish-gray, very fine grained_____ 0. 5 gray limestone bed in middle .of unit______3 10. Limestone, pinkish-gray, dense, silty; weathers 56. Limestone, gray, platY------1 greenish-gray ------0. 5 55. Sill of Late Cretaceous and Paleocene •age, decom­ 9. Concealed ------1 posed; contains abundant biotite phenocrysts; 8. Dolomite, grayish-orange, calcareous, fine- 2ft .thick .. grained ------4 54. Limestone. dark-gray, very fine grained; contains 7. Concealed; pale-yellow clayey soiL______9 ·scattered pebbles ·and chips of limestone 6. Siltstone, dusky-yellow; weathers into blocks similar to that in un~t 53; weathers light gray about 1 in. on a side______2 to light olive gray------5 5. Concealed; float is mixed fine-grained sandstone, 53. Limestone, medium-dark-gray, granul·ar, fetid; shale, and dense limestone______16 fossil gastropods abundant; weathers medium gray rto da·rk yellowish brown______2. 5 Total thickness Morrison Formation ______438 52. Limestone, medium-gray to medium-dark-gray, Swift Formation: very fine to medium grained; contains a few 4. Sandstone, grayish-orange, pale-yellowish-brown, g·astropod-rich layers; weathers medium light and light-olive-gray, fine- to medium-grained, gray------5.5 thin-bedded, locally crossbedded, friable; in 51. Limes·tone, similar to unit 53, but only slightly part argillaceous and in part weakly cal- fetid------1.5 careous ------18 50. Concealed------10.5 3. Sandstone, gray, calcareous, . friable; contains 49. 1Shale, grayish-red to grayish-purple and greenish- abundant fragments of pelecypod shells______8. 5 gray to light-olive-gray; grayish red predomi- 2. Concealed --~------3.5 nant in lower 48ft, greenish gray predominant in upper 23.6 ft; fissile to thinly platy; con- Total thickness Swift Formation______30 ·tains irregular thin, lenticul·ar, and ovoidal Disconformity. dense limestone concretions from 4.3 to 14 ft Phosphoria Formation : a1bove base, and 0.2-rt-thick brown-weathering 1. Chert ------Not measured dense concretionary limestone tbed 48 :ft above base------71.5 SECTION 3.-Kootenai Formation and pa1·ts of the Colorado and 48. Shale, pale-red to grayish-red, and subordinate Morris on Formations light-olive-gray to greenish-gray interbeds; fissile, chippy, rand hackly; zone of irregular [Measured by E. T. Ruppel and 1\L R. Klepper, 1948, in SW:IA, sec. 5, T. 6 N., R. 1 E.] · ovoidal to spherical 'brown-weathering limestone concretions fr:om 12.1 -to 13.3 ft above base____ 17 Colorado Formation (part of lower unit only) : Feet 67. Shale, medium-dark-gray, fissile, papery to thin:ly 47. Mudstone, light-olive-gray to olive-gray, and platy; in part contains very small flakes of subordinate grayish-red inter:beds, platy to fissile ------3 mica ------~------41 66. Siltstone, dark-gray, hard, blocky; alternating 46. Limestone; pale-purple to pale-reddish-purple, beds of dark-gray and very dark gray, fissile, locally with pale-green tint, very fine gvained, papery shale in. lower 20.5 ft of unit; two mod­ subconchoidal fracture; weathers dark yel­ lowish brown______0. 5 erately thick beds of similar shale in the upper 28.6 ft; weathers yellowish gray______49 45. Mudstone, light-olive-gr:ay to olive-gray, and 65. Concealed ; probably underlain by olive and gray grayish-red interbeds in upper 6ft, platy to sub­ compact blocky siltstone______27. 5 fissile ; thin zone of limestone concretions 64. Siltstone, dark-gray, compact, blocky______6 20.6 ft ahove base is similar to those of unit 63. Sandstone, light-olive-gray to yellowish-brown, 49 ------~ 27.5 _tl ch~an, quartzitic; weathers pale yellowish 44. Concealed ---.------20 brO\Vn ------2. 5 43. Mudstone, light-olive-gray, platy----,------4 42. Limestone, dark-gray, dense; contains scattered Total measured thickness of Colorado small calcite crystals; very fossiliferous____ 0. 5 Formation ------126 41. Shale, yellowish-gray to light-olive-gray, fissile Kootenai Formation : to chirppy______3 62. Siltstone, dark-gray, coarse-grained, compact, 40. Shale, dusky-'blue to grayish-purple, fissile to blocky ; very calcareous in lower 2 ft______5. 5 chippy ------3 61. Concealed ; shale intervaL______4 39. Concealed ------3 60. Limestone, dark-gray, dense to fine-grained, 38. Limestone. pale-purple to grayish-red-purple, lumpy-weathering ------2 very fine gr:ained; weathers dusky yellowish 59. Concea·led ------12 brown ------1 54 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 3.-Kootenai Formation ana parts ot the Colorado ana SECTION 3.-Kootenai Formation ana parts ot the Colorado ant! Morris on Formatio11r-Continued Morris on Formatio11r-Continued

Kootenai Formation-Continued Feet Kootenai Formation-Continued Feet 37. !Shale, gr·ayish-red, and grayish-olive interbeds 15. ·Sandstone, pale-reddish..ibrO'Wn, very fine grained, in lower 6 ft, fissile ; a:bundant limestone con­ !blocky; contains a few thin ·beds of grayish­ cretions similar to those in unit 49 in upper yellow, fissile to lumpy mudstone______4 1 ft______16 14. Conglomerate, •subrounded gray limestone and 36. Shale, greyish-olive, fissile, micaceous ; contains orange siltstone fragments up to 0.25 in. in .thin, lenticular, and flattened grayish-olive diameter in a matrix of sand and mud______2 dense limestone concretions in upper 4 ft_____ 15 13. Mudstone, intel'lbedded yellowi!sh-gray and pale- 35. Mudstone, light-olive-gray to greenish-gray, fis­ red, fissile to platy and ll];J]lpy ------15 sile, pencilly, and chippy; contains .sparsely 12. Concealed ------~------2 disseminated light-gray dense limestone concre­ 11. Sandstone, pale-yellowish-~brown, slightly cal- tions up to 2 in. in diameter ; upper 1 ft is prin­ careous, very fine grained; contains sparsely ' cipally medium-gray concretionary limestone disseminated ·black chert pebbles______1 that weathers to ir.regular rounded pieces up to 10. Concealed ------7 t8 in. in diameter------16 9. IJi.mes·tone, pale-yellowish-brown, dense, sulb- 34. Mudstone, light-greenish-gray, very thin and i·r­ litJhographic, and yellowish-gray mud!stone; regularly bedded ; weathers very light gray____ 6 edgewise pelblble conglomerate zone in upper 33. Mudstone, greenish-gray ·and yellowish-gray, fis- 0.5 ~t------7 sile to 'blocky; pencilly in lower 5 ft; weathers 8. Mudstone, oUve-gray, somewhat hackly; contains yel!lowish orange______10 .fmJ:aU limestone chips or flattened concretions_ 9 32. Shale, grayish-purple tto very dusky puJ:l)le, very 7. Concealed ------9 fi·ssile; conspicuous rounded su'blithographic 6. Mudstone, similar to unit 8, •but contains small limestone concretions up to 5 in. in diameter concretions of light-olive-gray dense limestone in zones 2 ft •thick near the top and ibottom in upper pa1'1t------17 of 16 unit______5. Mudstone, dark-gray, platy; weathers light gray_ 1 31. Concealed; red shale float______19 4. Concealed ------·------8 30. Siltstone, yellowish gray to grayish yellow in 3. Sand!stone, pale-yellowis'h.J,brown to light-gray, lower half and pale red to grayish red in upper salt-and-pepper medium-grained, homogeneous, half of unit; sparse limestone concretions quartzitic, very indistinctly 1bedded to massive; throughout ------9 weather.s to large irregular blocks up to 6 ft 29. Sandstone, white, clean, in beds 0.4 ft thick; in diameter______36 a few interbeds of grayish-yellow mudstone, in part irregularly ripple marked ______6 Total ·thickness of ~the Kootenai Formation __ 529 28. Sandstone, pale-yellowisho~brown, limonitic; con­ tains subordinate rounded black chert grains; Morrison Formation : medium-grained; in ·beds 0.4-1 ft thick______17 2. Concealed ------35 27. Sandstone, yellowi~h-gray, fine-grained, compact; 1. Sandstone, light-olive-gray, medium-fine- to me­ in ·beds 0.5-0.7 ft thick; contains interbeds dium-gra-ined, limonitic, in 'beds less than 1 ft of yellowish-orange, grayish, ·and grayish-red, thick, which are crossbedded on a small scale__ i$1 rp1aty, fissile, irregularly lumpy sandstone___ _ 9 26. Concealed; float is red mudstone ______8.5 25. Mudstone, pale-red to grayish-red, fissile to platy, SECTION 4.-Coloraao Formation near south end of mapped and lumpy ______area 2 24. Sandstone, medium-grained, limonitic______0.5 [Measured by M. R. Klepper, 19:49, along shallow gully that approxi­ mately coincides with boundary between NE* and SE:J4 sec. 4, T. 4 23. Mudstone, s'imilar to unit 25------13.5 N., R.l W.] 22. Mudstone, yellowiS'h...orange, p1aty to chippy___ _ 1.5 Slim Sam Formation: Feet 21. Concealed ------4 20. Conglomera·te, fine-grained sandstone matrix and Measured section published in Klepper, Weeks, and pebbles of yellowish siltstone and light-gray Ruppel ( 1957, p 31). Basal unit is sandstone, light-olive-gray, thin-bedded, :li:mestone ------2 non-resistant; contains a few thin beds of silt­ 19. Concealed ------6 18. 'Sandstone, pale-reddish-brown, very fine grained_ 2 stone in lower half. "Mactra" arenaria Meek, a 17. Sandstone medium-gray, "salt-and-pepper" rue­ pelecypod not known from rocks as young as the dium grruined, compact, quartzitic; in 1beds 0.25-- Montana Group was collected from beds about 300 0.75 ft thick______10 ft (USGS Mesozoic loc. 23039) and 375 ft (USGS Mesozoic loc. 23040) a·bove the bas·e of the forma­ 16. Mudstone, intel'lbedded pale-red and grayish-yel- tion ( W. A. Cobban, written commun. May 31, . low, chippy to lumpy; 0.3-ft thicik ibed of red­ 1951) ------·----- 55 dish-brown fine-grained 'blocky sand!stone at base of unit------~------13 Gradational contact. l\1EASURED SECTIONS 55

SECTION 4.-0olorado Formation near south end of mapped SECTION 4.--,0olorado Formation near south end of mapped area-Continued area-Continued

Colorado Formation : Feet Colorado Formation-Continued Feet Upper black shale unit: Middle siliceous mudstone and sandstone unit-Con. 66. Shale (80-90 percent), dark-gray to very ·52. Mudstone, similar to unit 54------24 dark-gray, chippy to fissile, and dark- 51. Mudstone, yellow, olive, and gray, siliceous; gray very fine grained micaceous sandstone in layers 4 in. to 1 :flt thick______19 (10-20 percent) ; shale contains sporadic 50. Concealed ------28 moderate-yellowish-brown-weathering lime- 49. Mudstone, 'Siliceous ; similar to unit 51 ; con- stone concretions up to 6 in. in diameter. tains 4-ft-thick bed of quartzite near top__ 22 Collection (USGS Meso~oic loc. 23032) 48. Concealed ------21 ,,. near top of unit contains Inoceramus cf. I. 47. Mudstone, very light to dark gray and olive- umbonatus Meek and Hayden, Scaphites gray, siliceous, blocky ; contains a few thin ventricosus Meek and Hayden, ·and Scap- layers of hard sandstone______18 hites cf. S. tetonensis Cobban. Another col­ 46. Concealed ------20 lection (USGS Mesozoic loc. 23031) near 45. Mudstone, medium-gray, siliceous______2 middle of unit contains Inoceramus um­ 44. 'Sandstone, light-olive-gray, medium-grained, bonatus Meek and Hayden, Ostrea congesta -speckled ------3 1Conrad, and Baculites asper Morton. A col­ 43. Concealed; float is 'blocky siliceous mud- lection ( USG'S Mesozoic loc. 23030) near stone ------22 the 'base of the unit contains Inoceramus cf. 42. Mudstone, pale-greenish-yellow and medium- I. umbonatus Meek and Hayden, and Ostrea dark-gray, siliceous, blocky to subconchoi- congesta Conrad. These collections are in­ . dal !fracture______10 dicative of the next faunal zooe rubove the 41. Quartzite, very light gray with thin brown zone of Inoceramus deformis of earliest layevs ------10 Niobrara age (W. A. Cobban, written 40. Concealed ------17 commun., May 31, 1951)------254 89. Mudstone, greenish-gray, si'liceous, •blocky __ 4 65. Concealed; probable duplication by faulting__ 97 38. Concealed ------12 64. Shale, very dark gray, and olive-gray; mud- 87. Sandstone, very li'ght gray, thi.on-'bedded, stone ; contains conspicuous brown- croSJsbedded, speckled ______,______8 weathering limestone concretions______30 36. Mudstone; 'alternation of units from 1 to Total thickness of upper black shale 6 f.t thick colored grayish olive, lighrt gray, unit______maximum 3 ____ 381 ·and medium gray; contains 1 thin layer minimum_____ 284 of siliceous mudstone and sever·al of 'light- gray standstone------50 Gradational contact. 85. Sandrstone, :fine-grad.ned______8 Middle ·siliceous mudstone and sandstone unit: 84. Mudstone, grayish-olive, b:ackly to chippy; 63. Concealed; sandstone float______22 contain·s a few thin layers of fine-grained 62. Sandstone, coarse-grained, crossbedded ; gray sandstone------14 speckled with chert and feldspar(?) 83. ;Sandstone, speckled very light gray .and grains ------4 medium-gray; one coar.se-grained 'bed art 61. Concealed; floa;t irs sandstone ·similar to base ------8 unit 62------24 32. Mudstone, :similar to uniJt 8!4------20 60. Sandstone, quartzitic, medium-light-gray, 81. !Sandstone, olive- ·and :medium-gray, fine­ very fine grained, hard ·and 'blocky; con- grained; contains a few thin beds of dark- tains scattered woody fragments______4 gray shale------~------63 59. Concealed ------45 30. Concealed ------10 58. Sandstone, quartzitic, medium-d.ark-gray____ 5 29. Mudstone or shale, olive-gray, medium-gray, 57. Sandstone, lig:ht-gray and olive, thin- to me- and medrium-dark-gray, chippy to :firssile__ 85 dium-bedded; in pavt speckled with chel'lt 28. Sandstone, siltstone, shale, and a few lime- and feldspar grains; contains a few inter- stone concretions, drab-colored______39 'beds of 'blocky mudstone______88 27. Mudstone and .shale, very dark .gray; con- 56. Concealed ------56 tain·s 'brown-weathering limestone concre- 55. 'Sandstone, light-gray, ·speckled, crossbedded ; tions near base------6 contains a1bundant woody fr.agments near 26. Sha'le and sandstone, very dark gray_____ 4 ·barse and one bed of blocky quartzitic 25. ·Sandstone, similar to ·unit 23------7 sandstone near tOP------37 24. Mudstone, medium-dark-gray and olive-gray, 54. Mudstone, med-ium-dark-gray, siliceous, blocky ------12 blocky ------7 23. Sandstone, medium-gray ; •speckled with chert 53. Concealed ______:______87 and feldspar(?) grains______7 22. Concealed ------6 a About 100 ft. thicker than this unit at other localltles: some beds 21. S.andstone, similar to unit 19______8 are probably duplicated by a concealed fault or faults in interval 65 ; true thickness is greater than 284 ft. and probably less than· 381 ft. 20. Concealed; sandstone float______24 56 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 4.-..0olorado Formation near south end of mapped SECTION 4.-Golorado Formation near south end of mapped area-Continued area--Continued Colorado Formation-Continued Gradational contact-Continued Middle siliceous mudstone and sandstone unit-Con. Feet Lower black shale unit-Continued Feet 19. Sandstone, medium- to Ught-gray, very fine 5. S'iltstone, dark-gray and olive, thin-bedded grained; contains a zone of roundish cal­ and almost fissile ; cont.a.ins irregular careous sandstone concretions ne.ar middle limonitic concretions in upper 4 ft______13 of unigular and wavy. Collection Upper black shale unit : (USGS Mesozoic loc. 23028) from middle 29. Shale, very dark gray to black. Fossil col­ part of unit contains Inoceramus cf. I. lection (USGS Mesozoic loc. 23037) bellvuensis Reeside; 1this fossil indicates about 20 ft below top of unit contains : a probable correlation with the Skull Creek Ostera congesta Conrad; Baculites asper Morton; and Scaphites ventricosus Meek Sh.ale of the Black HiHs (W. A. Cobban, and Hayden. Fossil collection (USGS written commun., May 31, 1951) ------37 Mesozoic loc. 23034) near base of unit 10. Concealed; very dark gray shale float______34 contains : Pinna dolosoniensis McLean ; 9. Sandstone, similar to unit 7------2 Inoceramus umbonatus Meek and Hay­ Fine-grained diorite sill 40 ft thick den; Ostrea congesta Conrad; Volsella n. 8. Mostly covered; outcrops are sandstone sim- sp. ; Pholadomya aff. P. papyracea Meek ilar to un'it 7------47 and Hayden ; Lucina sp. ; Legwmen cf. 7. Sandstone, medlium-dark-gray with 'li.mon!tic L. ellipticum Conrad; Gyrodes conradi specks, medium-grained ; conrt;;lins finely Meek; Baculites asper Morton; Baculites dispersed organic matter 'and grains of cf. B. codyensis Reeside; Scaphites teto­ black shale; ~bedding typically wavy and nesis Cobban. This collection is indica­ irregular ------19 tive of the next faunal zone above the 6. Concealed; very dark gray and dark-gray zone of Inoceram~us deformis of earliest shale float is predominant ; sandstone float Niobrara age (W. A. Cobban, written is subordinate______106 commun., May 31, 1951) ------56 MEASURED SECTIONS 57

SECTION 5.-0olO?·ado Formation along and near the Indian SECTION 5.-0olorado Formation along and near the Indian Greek road-Continued Greek road-Continued Colorado Formation-Continued Colorado Formation-Continued Upper black shale unit-Continued Feet Upper black shale unit-Continued Feet 28. Shale, very dark gray to black ; upp~r 36 underlying unit are proba!ble equivalents of ft mostly covered and may contain di- the middle part of the Carlile and those in orite porphyry sill about 5 ft thick____ _ 72.5 the overlying shale are equivalents of -the 27. Shale, very dark to black; mostly outcrop ; lower part of the Niobrara (next fa·un•al zone some interbeds of very fine grained black above zone of Inoceramus dejormis of ear­ sandstone. Fossil collection (USGS mes­ liest Niobrara age)------50 ozoic loc. 23033) near middle of unit contains Inoceramus umbonatus Meek Total thickness of upper black shale uniL 273. 5 and Hayden; Inoceram~ts n. sp. Ostrea congesta Conrad; Pholadomya cf. P. Middle siliceous mudstone and sandstone unit: papyracea Meek and Hayden ; Veniella 25. Sandstone, tan, s.alt-and-pepper. Collection goniophora Meek; Legumen cf. L. ellip­ 58K2 (USGS Me'Sozoic loc. D1749) fr.om ticum Conrad; Baculites asper Mor­ top of unit contains: Inoceramus cf. I. ton Idiohamites? sp. Scaphites sp. This perplemus Whitfield, M actra, 1sp., and Tel- collection is indicative of the next faunal Una sp. The. impressions of Inoceramus zone above the zone of Inoce1·amus de­ suggest the species perplemus of middle jormis of earlest Niobrara age (W. A. Carlile age (W. A. Co'b'ban, written com­ Cobban, written comm., May 31, 1951) _ 95 mun., Sept. 15, 1958) ------37 26. Sandstone and subordinate, dark gray ; 24. Sandstone, sHtstone, and thin inter:beds of foul-bottom type lithology and structure, grayi'sh-'bl.aok shale; ·sandstone is argil­ with undulose bedding, ripple marks, and laceous foul-bottom lithology having worm trails. Fossil collection (USGS scattered fossil fragments, undulose 1bed­ Me~ozoic loc. 23035) from top of unit ding, ripple marks and worm trails ; contains Inoceramus cf. I. dejormis poorly exposed. Collection 58Kl (USGS Meek, indicative of earliest Niobrara age Mesozoic loc. D1748) from 1base of unit (W. A. Cobban, written commun., May contains Inoceram~ts f sp., Ostrea 'sp., and 31, 1951). Another collection ( 58K4), preserved fragments of gastropods. The possibly from the same bed but prob­ fr.agmen ts of oysters suggest a small ably from another bed a few feet lower smooth species that is common in rocks in the section, contains Inoceramus cf. of Greenhorn and early Oarlile age (W. I. incert1.ts Jimbo, Ostrea sp., Baculites A. Oo'bban, written commun., Sept. 15, sp., and Scaphites sp. Inoceramus cf. I. 1958) ------109 incertus marks the very highest Carlile 23. Sandstone, tan, salt-and-pepper, pebb'ly, bed in the Western Interior region (W. thick-bedded ------37 A. Cobban, written comm., Sept.15, 1958) Sill, diorite porphyry, not measured; section A collection (USGS Mesozoic loc. 24000) from offset. 10-25 ft above base of the unit (2~0 ft 22. Concealed; d-iorite porphyry float obscures below collections listed above) contains Ino­ bedrock, but proba1bly underlain by sand­ ceramus dejormis Meek, Inoceramus, un­ stone, siltstone, and mudstone ; may in- named species resembling I. labiatus ( Schlo­ clude some thin sills______92. 5 theim), PholJadomya papyracea Meek and 21. Mudstone, siliceous, ·and some mudstone Hayden, and Scaphites sp. cf. S. ventricosus pebble conglomera,te ; poorly exposed, less group and is indicative of the basal Niobrara than o~e-quarter of interval is oUJtcrop__ 21 zone (J. B. Reeside, Jr., written commun., 20. Concealed ; probably underlain by siliceous Oct. 3, 1952) . .Another collection ( 58K3) mudstone, mudstone, and siltstone______118 from about the same bed contains Inocera­ Sill, dior:ite pol"phyry, 32ft thick. mus sp. and Cardium cf. 0. pauperculum 19. S•andstone, ·greenish-gray 'to pale-olive, me­ Meek. (W. A. Cobban, written commun., dium-coarse-grained, feldspathic; inter- Sept. 15, 1958) . bedded with dark-gray shale______70. 5 The presence of rock's that are pr·o'bably o·f 18. Sandstone, greenish-gray to pale-olive, me- highest Oarlile age between rocks of earliest dium-coarse-grained, feldspathlc______12 Niobrara age suggests that 10-20 ft of beds 17. Concealed ; but probalbly underlain by has 'been duplicated by an unrecognized fault sandstone simHar .to 18------14 wi,thin unit 26. In any event the contact !be­ Sill, d·iorite porphyry, 14.5 f.t thick. tween rocks of Carlile 'and Niobrara age is 16. Concealed; probably underla·in by sand- within thi:s unit, for rocks a;t the top of the stone similar to 18------43 58 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 5.-0olorado Formation along and near the Indian SECTION 6.-SZim Sam FormatiQn (part) and Colorado Forma­ Greek road-Continued tion near Iron Mask mine

Colorado Formation-Continued [Measured by M. R. Klepper and V. L. Freeman, 1~52, near Iron Mask Mine in SE% sec. 30, T. 7 N., R. 1 W., and adJacent parts of sees. Middle siliceous mudstone and sandstone unit- 29, 31, 32] Continued F'eet Feet 15. ·Sandstone, greenish-gray, medium-fine- Slim Sam Formation (lower part only): grained, feldspathic, distinctly 'bedded 71. Sandstone, medium-gray to light-olive-gray, me­ dium-grained, feldspathic and argillaceous; and thin~bedded______4.5 beds *-4 in. thick with a few *-1 in.-thick 14. tConcea'led; .float similar to 1'5------19 medium-dark-gray shaly interbeds; one 8-in. 13. Sandstone, greenish-gray, medium-fine­ crystal tuff bed 44ft above base------'---- 71 •grained, feldspathic, slightly to moder- 70. Sandstone, medium-gray, fine- to medium­ 8:tely calcareous______4. 5 grained, feldspathic, thin-beddfd ( * -2 in.)- 2 12. Concealed ; proba!bly underlain by sand­ 69. Concealed ; except for 1 ft of sandstone similar stone ,similar to rthat in unit 13. Sand­ to that in unit 70------15 stone fi<>:at nearby from this unit or 68. Sandstone, medium-gray, slightly mottled, within 50 ft a1bove it (USGS Mesozoic 3 loc. 23413) contains Lingula cf. L. sub­ argillaceous ------6.5 ,, spatulata Hall and Meek, Ostrea ano­ 67. Concealed ------•. mioides 'Meek, Trigonia sp., and Mactra 66. Siltstone and very fine grained sandstone, me- sp.; fossils th'at indicate a correlation dium-gray and olive; some- interstitial clay and thin lenses of crystal tuff; thin-bedded __ with 'the Mowry ·and Aspen formations 2 ( W. A. Cobban, written commun., May 31, 65. Concealed ------J.3.5 64. Siltstone and very fine grained sandstone, like 1951) ------29 Total thickness of middle siliceous runit 66------28.5 mudstone and sandstone unit ___ .___ 611 63. Siltstone, interbedded olive and medium- to Lower 1Mac'k shale unit: dark-gray, hackly to rubbly fracturing______3 11. Concealed ------4 === Measured thickness of Slim Sam Forma- 10. 1Sandstone, fine-grained, argillaceous, .and dal"k-gray poorly eX!posed 'Siltstone_____ 83 tion ------144.5 9. Sandstone, very fine-grained, and siltstone, Colorado Formation : inter;bedded with dark-gray papery Upper black shale unit : ,shale : all •are rusty weathering, in P.arrt 62. Shale, grayish-black, fissile ; local thin in­ mottled and irregularly laminated in terbeds of siltstone and very fine grained shades of medium and dark gray ; three sandstone form about 5 percent of inter­ lbeds, each 6 in. thick, of light-gray val ; weathers with some limonite 57 coarse-gr.ained salt~and-pepper sand- staining ------61. Shale, dark-gray to grayish-black, fissile, stone in upper 13 fL------72. 5 8. Shale, very dark gray to grayish.Jblack, and thin blocky siltstone beds ; diorite dike 5-6 ft thick cuts middle part of fis~le ------10.5 unit; 1- to 3-in.-thick bed·s of dark­ Sill, diorite porphyry 15.5 ~t thick. gray, very fine grained sandstone com­ 7. Shale; similar to that in unit 8------62 mon 28-40 ft above base; large brown­ ·Sill, diorite porphyry 11 ft thic~. weathering limestone concretions com- 6. Shale; similar to that in unit 8------23. 5 mon in upper 11 ft of unit______45.5 5. Siltstone, olive-gray, blocky, and brown­ 60. Concealed ------8.5 weathering, very fine grained sandstone ; 59, Sandstone, very fine grained; interbedded poorly exposed; top 1-2 ft of interval is with siltstone; dark-gray and olive ____ _ 3 fine- to medium-grained light-gray Sill, diorite porphyry with conspicuous horn­ quartzite, similar to unit 4------35 blende phenocrysts, approximately 40 ft 4. Quartzite, light-gray to olive-gray, salt­ thick. ·and-pepper fine- to medium-grained, mas- · 58. Shale, dark-gray, fissile; upper part con­ sive bedded______15 tains about equal amounts of shale and olive fine-grained laminated sandstone in Total thickness of lower black shale %- to 3-in.-thick bedS------19 unit ------255. 5 57. Shale, grayish-black, fissile, ·and numerous Total thickness of Colorado Forma- very thin beds of dark-gray siltstone; tion 1,140 limonite staining common ______32.5 56. Concealed ------30.5 Kootenai Formation: 55, Shale, dark-gray to grayish-black, fissile to 3. Concealed ------19 finely chippy, few 1-in. thick beds of silt- 2. Mudstone, siltstone------11 stone ; in part limonite stained along 1. Limstone, light-gray-weathering; contains fos- fractures ------11.5 sil gastropods------Not measured 54, Concealed ------20 MEASURED SECTIONS 59

SECTION 6.-SUm Sam Formation (part) and OoZorado Forma- SECTION 6.-SUm Sam Formation (part) and OoZorado Forma- tion near Iron Mask mine-Continued ,tion near Iron Mask mine-Continued

Colorado Formation-Continued Colorado Formation-Continued Upper black shale unit-Continued Jt'cct Middle siliceous mudstone and sandstone unit­ 53. Sandstone, very fine grained, and siltstone, Continued Peet medium-dark-gray to dark-gray, micace­ 38. Mudstone, gray ·and olive, splill'tery and ous, distinct bedding in 1- to 6-in.-thick chippy ------19 beds ; weathers blocky near base to hack­ 37. ·Sandstone, light-gray, fine-grained, thin- ly or crumbly in upper two-thirds of unit- 24 lbedded, calcareous------2.5 52. Sandstone, dark-gray, fine grained; with 36. Siltstone, medi•um-gray, light-gray, and one 3-ft-thick zone of olive siltstone; olive, splintery- and rubbly-weathering; chippy weathering______13 middle part calcareous ______29 51. Sandstone, medium- to dark-gray, fine­ 35. Mudstone, very light~gray, siliceous, grained, argillaceous, laminated, in part blocky ; weathers white, a few· dark- light gray streaked; subordinate very gray siltstone interbedd•s______..; ______13 dark gray ~shale interbedS------16.5 Sill, diorite porphyry, 7 :Dt tllick. 34. Siltstone, olive and dark-gray; interbed­ Total thickness of upper black shale ded siliceous and nonsiliceous beds and prominent 'beds of light-gray-weathering unit ------281 speckled coarse-grained sand·stone 7-9 ft Middle siliceous mudstone and sandstone unit: •albove base ______31 50. Sandstone, medium-gray, coarse-grained Concealed ; prolba•bly underlain lby diorite sill salt-and-pepper; indistinctly bedded with or dike, 8ft thick. irregular streaky laminae ______7 33. Siltstone, light-olive and greenish-gr.ay, Sill, diorite porphyry about 6 ft thick. 1spl·intery; medium- to dark-gray !blocky 49. Sandstone, similar to that in unit 50 ______3 siliceous mudstone ______29 48. Sandstone, pale-olive, medium-grained, ·Sill, diori'te porp'hyry 1.•5 rt thick. thin- to medium-bedded; in part cross­ 32. ;SUtstone, olive, light-gray, and dark-gray, laminated, slightly calcareous in part_ __ 24.5 blocky fracturing, chippy to rub'bly 47. Shale, olive and dark-gray, fissile; silt­ wea:thering ; some 'beds are siliceous ; stone ; a few thin beds of fine-grained diorite porphyry sill 2 ft thick cuts uuiL 22 olive sandstone ______15 31. ·sandstone, light-·gray, medium-grained; 46. Sandstone, medium-gray to light-olive-gray ·and olive siltstone______6.5 and light-yellow-brown, limonite-stained, 30. Mudstone, dark-gray, siliceous ______5 fine-grained; in beds 1-12 in. thick; sub­ 29. Concealed ------20 ordinate dark-gray to very dark gray 28. •Siltstone, greeni1sh-gray ~o pale-olive, splin- fissile shale in beds %,-6 in. thick ______36 tery; subordinate olive to d·ar:k-gray sili­ 45. Sandstone, mottled medium-gray and dark­ ceous mudstone and light-gray sand- gray, fine-grained, argillaceous; inter­ stone ------22 bedded with dark-gray subfissile silt- Sill, diorite porphyry, 7ft thick. stone ------12 27. Mudstone, dark-gray, siliceous, fi·ssile to 20 44. Concealed ------hackly ------6 43. Sandstone, very fine grained, and siltstone, 26. Siltstone, olive, :hackly fracture ______7 medium-dark-gr.ay, blocky; some beds 25. Concealed ------12 thinly laminated and cross-laminated; 24. Mudstone, olive and gray, siliceous, blocky sUJbordiuate dark-gray !Shale ______9 fracture ; intertbedded wit1Jh. irregular­ Sill, diorite porphyry, ·about 13ft thick. fracturing olive siltstone and very fine 42. Sandstone, medium-light-gray to medium­ grained .sandstone ______22 gray, ·salt-and-pepper; coarse.-grained 23. Mudstone, :siliceous, •blocky fracture; inter­ -wii1Jh peblble conglomerate near 1base a-nd bedded with gray ·and olive hackly- to scattered pebbies above; medium ·bedded splintery-fracturing siltstone to very and distinctly •bedded. Lower .halif is finer fine grained sandstone______10.5 gra'ined, less friable ______19.5 22. Sandstone, speckled, medium- to coarse- 41. ·Sandstone, light-olive-gray, fine- to me- grained ------1 dium-fine-grained, medium-bedded ____ _ 18.5 21. Siltstone, pale-olive, olive-gray, and green- ·Sill, diorite porphyry, 15 ft thick. ish-gray; similar to unit 17; two conspic­ 40. Mudstone, greenish-gr.ay to pale-olive, sub­ uous medium-grained sandstone beds, fissile to rU!b'bly or hackly -weathering; one 3 f.t albove base, other 22 ft ·above a few thin ·blocky weathering 1siliceous lbase ; diorite porphyry sill. 2.5 ft tbick mudstone beds------22 14.5 ft above base (thickness not in- 39. Sandstone, medium-tight-gray, fine- cluded) ------27 ·grained, thin-bedded, calcareous ______8.5 Sill, diorite POll>hyry. 4.5 ft thick. 60 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA

SECTION 6.-Slim Sam Formation (part) and Colorado Forma­ SECTION 6.-Slim Sam Formation (part) and Colorado Forma­ ,tion near Iron Mask mine-Continued tion near Iron Mask mine-Continued

Colorado Formation-Continued Feet Colorado Formation-Continued Feet Middle siliceous mudstone and sandstone unit­ Lower black shale unit-Continued Continued 3. Quartzite, yellowish-gray to light olive- 20. ·Sandstone, medium-gr.ay, fine-grained, cal­ gray, fine-grained______16 careous, thin-bedded and irregularly 'bedded; weathers greenish-gray______5 Total,thickness lower black shale unit_ 240 19. Siltstone, similar to unit 17______22.5 Total thickness Colorado Formation_ 1, 211. 5 Sill, diori1te portphyry, 3-4 ft thick. Se0tion off­ .set on top of sill. Kootenai Fol'IIIl.ation: 18. 'Siltstone, olive-gray, sandy, subhackly 2. Claystone and siltstone______36 fracture ------12 1. Limestone; contains abundant fossil 17. Siltstone, pale-olive, olive-gray, and green- ,gastropods ------Not measured i,sh-gray; a few dusky-yellow 'blocky rto chippy fr.acturing beds, and common SECTION 1.-Lower part of middle member· of the Elkhorn Moun­ dark-gray chippy to hackly or pencd.lly tains Volcanics fracturing 1beds ; a few 1beds 6-12 in. thick [Measured by M. R. Klepper, H. W. Smedes, and R. A. Weeks, starting of medium.. grained ·argillaceous sand­ at road at line between F.ecs. 15 and 16, T. 8 N., R. 1 W., extending stone ; most of the coarser grained ;bed·s along spur through the NW%,NE* sec. 21, and ending along the line between sees. 17 and 20] are calcareous------53.5 Top of measured section: Feet 16. Concealed ------5 15. Sandstone, olive-gray, medium- and fine- 63. Welded ash-flow tuff, dark-gray; weathers to me- grained, and su!bordinarte siltstone ; cut dium gray or medium light gray; contains abun- by a 3- to 4-ft-thick diorite sill (thick­ dant flatJtened pumice ribbons, many of which ness not included)------25 are more than 18 in. long, and sparse rock 14. Sandstone, medium-gray ~to medium-dark­ fragments ------> 100 gray and some olive-gray beds, fine­ 62. Covered------~------17 grained, t~in- to very thin-bedded ( lk- 61. Crsst.al tuff and crystal lithic 1tuff, poorly ·bedded ; 1 in. bed·s) ; weathers olive gray ; me- coutains sparse to a'bundant fragments up to dium-gray to dark-gray platy siltstone__ _ 29 8 in. in diameter; ma,trix is dominantly silt- 13. Concealed ------29 size crystal tuff ; lithic fragments resemble 12. 'Sandstone, medium- to greenish-gray, me­ fragments in unit 58------33 dium..fine-grained, well-'bedded; in beds 60. Crystal tuff ·and crystal lit,hic ·tuff, indistinctly 1 in. thick with one 3.:f.t..,thick 1bed near to distinctly bedded ; most layers are fine middle o'f unit ; ·argillaceous and ·slightly grained, a few are coarse ·grained, and a few calcareous ------30 are lapilli tuff______12 59. Covered ------10 Sill, pyroxene andesite, a1bout 10 ft thick. Sec­ 58. Tuff breccia; lithic fragments, which average less tion offset. than 4 in. in diameter but are as much as 10 in., Total thickness of middle siliceous mud- stone and sandstone unit______from half to two-thirds of the unit; most frag- 690.5 ments are of same apparent composition as the Lower black shale unit : crystal tuff matrix; some fragments are sharply 11. Concealed ; probably underlain by gray bounded, others are vaguely outlined. Vague sandstone and dark-gray siltstone______11.5 bedding at 60 ft above base and sparsely 10. Sandstone, dark... gray, and dark-gray ·snt- throughout upper 50ft indicates that transport- ·stone ------3 ing agent was able to accomplish some sorting ; Sill, diorite porphyry, 141ft thick. probably of hot mudflow origin______160 9. Sandstone, dark-gray, and dark-gray silt- Probable erosional surface of slight relief. 57. Crystal and crystal lithic tuff, poorly bedded; stone ------12.5 8. Sandstone, l·ighrt olive-gray, ~speckled, me- contains sparse lapilli throughout; number of lapilli increases upward ______..;___ 35 dium-grained ------2.5 7. Mostly covered; underlain by gr.ay mottled 56. Covered ; probably underlain by rocks similar to argillaceous sandstone and dark-gray those in unit above______16 pencilly fracturing siltstone______85 55. Tuff breccia, similar to unit 58; largest fragment 6. Mostly concealed ; underlain by black to observed was 3 in. across______2 very dark gray papery shale ______54.5 54. Crystal· tuff, dark-gray to dark-greenish-gray, 5. Ooncealed ------26 massive to well-bedded, dominantly fine­ 4. Argillite, dra'b and olive, and some blocky grained; sparse 14- to lh-inch lapilli in beds :siLtstone and rfirre-grained sandstone ; ir­ near middle of unit------65 regular lumpy 'bedding, 'some ripple 53. Ash flow tuff, poorly welded______3 marks and worm ·trails on 'bedding 52. Crystal tuff, well-bedded; contains a few mud- planes ------29 stone laminae and one thin mudstone dike_____ 2 MEASURED SECTIONS 61

SECTION. 1.-Lower part of middle member of the Elkhorn Moun­ SECTION 1.-Lower part of middle member of the Elkhat·n Moun­ tains Volcanics-Continued tains Volcanics-Continued Feet Feet 51. Tuff breccia, medium-gray with a greenish cast; 39. ·Ash-flow tuff, welded, dark-gray, very dense; weathers light olive gr:ay, grayish orange, and contains abundant collapsed pumice ribbons up greenish gray; forms blocky to rubbly cliffs; to 6 in. long and abundant lithic fragments up s~ightly rounded fragments are ~-9 in. across to 3 in. in diameter______24 and average about 3 in. ; all are of the same 38. Crystal tuff, grayish-'black to dark-gray; weathers lithology and strongly resemble the crystal tuff pale yellowish green ; poorly bed

SECTION 7.-Lower part ot middle member of the Elkhorn Moun­ SECTION 1.-Lowe1· part ot middle member ot the Elkhorn Moun­ tains Volcanics-Continued tains Volcanics-Continued Peet Feet 25. Crystal tuff, medium-grained ______43 2. Ash flow tuff, welded, grayish-black; weathers 24. Crysbal tu~, dark- to medium-gray; mostly mud­ medium light gray; contains abundant flattened stone with coarser grained plagioclase crystals ; pumice fragments, many of which are altered, some layers are contorted and brecciated; re­ some with a bleached halo and some entirely sembles welded tuff, but is probably a contempo- bleached ------49 raneously deformed mudflow deposit______6 1. Concealed; float is welded tuff______29 23. Crystal tuff, particles of sand and silt size in distinct beds______23 Total thickness of measured section ______2, 483 22. Crystal tuff, d~ark-gray; weathers olive gray; some layers contorted and locally brecciated; REFERENCES CITED similar to unit 24 ______.:______5 Bard, D. C., 1910, The Radersburg mining district of Montana 21. Crystal tuff, distinctly bedded, coarse-grained ; and some interesting featur

Hanson, A. 1\f., 1952, Cambrian stratigraphy in southwestern Rittmann, Alfred, 1952, Nomenclature of volcanic rocks pro­ · Montana: Montana Bur. Mines .and Geology Mem. 33, posed for use in the catalo.gue of volcanoes, and key-tables 46 p. for the determination of volcanic rocks: Bull. Volcano!., Holmes, Arthur, 1959, A revised geological time-scale: Edin­ ser. 2, v. 12, p. 75--102. burgh Geol. ·Soc. Trans., v.17, pt. 3, p.183-216. Robinson, G. D., 1963, Geology of the 'l"hree Forks quadrangle, Imlay, R. ,V., Gardner, L. ·S., Rogers, C. P., Jr., and Hadley, Montana, with sections on Petrography of igneous rocks, by H. D., 1948, Marine Jurassic fol"mations of Montana: U.S. H. F. Barnett: U.S. Geol..Survey Prof. Paper 370, 143 p. Geol. Survey Oil and Gas Inv. (Prelim.) Chart 32. ---1967, Geologic map of the Toston quadrangle, south­ Jaffe, H. ,V., Gottfried,. David, Waring, C. L., and ·worthing, western Montana: U.S. Geol. Survey Misc. Geol. Inv. Map H. W., 1959, Lead-alpha ·age determinations of accessory I-486. minerals of igneous rock (1953-1957).: U.S. Geol. Survey Rubey, W. W., 1929, Origin of the siliceous Mowry shale of the Bull. 1097-B, p. 65--148. Black Hills region: U.S. Geol. Survey Prof. Paper 154-D, Kinoshita, ,V. T., Davis, ,V. E., and Robinson, G. D., 1965, Aero· p. 153-170 [1930]. magnetic, Bouguer gravity, and generalized geologic map Ruppel, E. T., 1962, A Pleistocene ice sheet in the northern of Toston and Radersburg quadrangles and part of the Boulder Mountains, Jefferson, Powell, and I-'ewis and Clark Devils :H'ence quadrangle, Gallatin, Broadwater, and Jef­ Counties, Montana: U.S. Geol. Survey Bull. 1141-G, p. ferson Counties, Mont.: U.S. Geol. Survey Geophys. Inv. G1-G22. .... Map GP-496. Schell, E. M., 1963, Ore deposits of the northern part of the Park Kino.sbita, W. T., Davis, W. E., Smedes, H. W., -and Nelson, W. (Indi'an Creek) district, Broadwater County, M'ontana: H., 1964, Bouguer gravity, aeromagnetic, and generalized Montana Bur. Mines and Geology BulL 3'5, 47 p. geologic map of Townsend and Duck Creek Pass quad­ Sloss, L. L., and Laird, W. M., 1947, Devonian system in central rangles, Broadwater County, Mont. : U.S. Geol. Survey and northwestern Montana : Am. Assoc. Petroleum Geolo­ Geophys. Inv. Map GP-439. gists Bull., v. 31, no. 8, p.1404-t:430. Klepper, l\1. R., 'Veel{S, R. A., and Ruppel, E. T., 1957, Geology Smedes, H. W., 1960, Monzonite ring dikes along the margin of of the southern Elkhorn Mountains, Montana, Jefferson and the.Antel'ope Creek stock, Montana [abs.]: Geol. Soc. Amer­ Broadwater Oounties, Mont.: U.S. Geol. Survey Prof. Paper ica Bull., v. 71, no. 12, pt. 2, p. 1977. 292, 82 p. [1958]. ---1962, Lowland Creek volcanics, an upper Oligocene forma­ Knopf, Adolph, 1913, Ore deposits of the Helena mining region, tion near Butte, Montana: Jour. Geology, v. 70, no. 3, p. · Mont.: U.S. Geol. Survey Bull. 527, 143 p. 255--266. ---1957, 'J:he Boulder bathylith of Montana: Am. Jour. Sci., ---1966, Geology and igneous petrology of the northern Elk­ v. 255, no. 2, p. 81-10:3. horn Mountains, Jefferson and Broadwater Counties, Mon­ ---1963, Geology of the northern part of the Boulder bathy­ tana: U.S. Geol. Survey Prof. Paper 510, 116 p. Uth and adjacent area, Montana: U.S. Geol. Survey Misc. Smedes, H. W., and Thomas, H. W., 1965, Reassignment of the Geol. Inv. Map I-881. Lowland Creek Volcanics to Eocene age: Jour. Geology, v. tochman-Balk, Christina, 1950, Status of Dry Creek shale of 73,no.3,p.508-510. central Montana: .Am. Assoc. Petroleum Geologists Bull., v. Stone, R. W., 1911, Geologic relation of ore deposits in the Elk­ 34,no.11,p. 2200-2222. horn Mountains, Montana: U.S. Geol. Survey Bull. 470-B, Lorenz, H. ,V., and McMurtrey, n. G., 1956, Geology and occur­ p. 75--9-8. rence of grotmd water in the Townsend Valley, Montana, Stebinger, Eugene, 1915, The Montana group of northwestern wUh a sect,ion on Chemical quality of the ground water, by Montana: U.S. Geol. Survey Prof. Paper 90-G, p. 61-68. H. A. Swenson: U.S. Geol. Survey "rater-Supply Paper 1360-C, p. 171-290. Trauerman, C. J., and Waldron, C. R., 1940, Directory of Mon­ McKelvey, V. E., and others, 1959, The Phosphoria, Park City, tana mining properties: Montana Bur. Mines and Geology and Shedhorn formations in the western phosphate field: Mem. 20, 13'5 p. U.S. Geol. Survey Prof. Paper 313-A, p.1-47. U.S. Bureau of Mines, 1925--31, Mineral resources of the United Mertie, J. B., .Jr., Fischer, R. P., and Hobbs, S. ,Y., 1951, Geology States [annual volumes for the years 1924-30]: Washington, of the Canyon Ferry quadrangle, Montana: U.S. Geol. D.C., U.1S, Govt. Printing Office. Survey Bull. 972, 97 p. [1952]. ---1932-56, Minerals yearbook [annual volumes for the Nelson, ,V, H., 1963, Geology of the Duck Creek Pass quadrangle, years 1931-52]: Washington, D.C., U.S. Govt. Printing Montana: U.S. Geol. Survey Bull. 1121-J, p. J1-J56. Office. Pardee, J. '1'., 1925, Geology and ground-water resources of . U.S. Geological Survey, 1883-1927, Mineral resources of the Townsend Valley, Montana: U.S. Geol. Survey Water-Sup­ United Stwtes [annual volumes for the years 1882-1923] : ply Paper 539, 61 p. Washington, D.C., U.S. Govt. Printing Office. ---1950, Late Cenozoic block faulting in western Montana: Wentworth, C. K., and Williams, Bowel, 1932, The classification Geol. ·Soc. America Bull., v. 61, no. 4, p. 359-406. 1and terminology of the py;roclastic Tocks, in R,eport of Com­ Pardee, J. T., and Schrader, 1!"'. C., 1933, Metalliferous deposits of mission on Sedimentation, 1930-32: Natl. Research O'ouncil the greater Helena mining r~gion, Montana: U.S. Geol. Sur­ vey Bull. 842, 318 p. Bull. 89, p. 19-53. Reed, G. C., 1951, Mines and mineral deposits (except fuels), White, T. E., 1954, Preliminary analysis of the fossil vel'ltebrates Broadwwter County, Montana: U.S. Bur. Mines Inf. Circ. of the Oanyon Ferry Reservoir area [Mont.]: U.S. Natl. Mus. 7592, 58 p. Proc., v.103, no. 3326, p. 395-438. Reyner, M. L., and Trnuerman, C.. J., 1950, Directory of Montana 'Vinchell, A. N., 1914, Mining districts of the Dillon quadrangle, mining properties, 1949: Montnna Bur. Mines and Geology Mont., and adjacent areas: U.S. Geol. Survey Bull. 574, Mem. 31, 125 p. 191 p.

INDEX

.[Jtallc page numbers indicate major references]

A Pan Page H Page Harmony mine•• ______.______40 Acknowledgments •. ______----______2 East Paclflc vein •••·------39 Aldrich Gulch intrusive______24 Edna No. 2mine.------" 41 Hassel area. ___ ------______---_----_ 35, 36, 48 Alluvium______15 Edna stock ______------______27,40 . History and production ___ ------34 Amsden Formation ______·______5 Elkhorn Mountains Volcanics. _____ 12, 14, 16, 24,60 Horn Silver mine______48 measured section______49 Elmore, P. L. D., analyst______20 H~!Jgh, M. Jean, fossil identification______13 Andesite, defined______16 Hufschmldt, E. L., analyst______21 Anteiope Creek stock ... ------~ 24,26,29,31 F Hyantha mine ______-_-_-- __ --- 41 Auriferous pyrite veins ______96,46 Fall River Formation______9 Faults. __ ------6, 91 I B Felsic rocks~. ___ ------__ ------_____ 23 Igneous rocks______15 Barlow, I. H., analyst______20 Fieldwork •. ------2 Indian Creek, composite bodies______28 Folds.- ___ ---- ______-·-______30, 91 gold. ______---- ___ 34, 35, 39 Basalt flows ______------~- 29 Beaver Creek area ______15,99 Fossils. fan. ______------_____ 14 Bentonite. ______---_----______48 Alnus sp. _------_____ ·__ 13 fault ______------___ 32 Big Snowy Group______5 Bathygenys sp __ ------_ 13 stock. ___ ------______------_ 31 Black Friday mine. ______--______35 Betula sp. _-- __ -- _. ----- _- _------. ______13 Intrusive bodies, composite______27 Black Friday vein .. ----~------45 Bolaspidella wellvillensis. ------4 Intrusive rocks, related to tho Elkhorn Moun- Blacksmith mine .• _------______35, 43,44 Brontotherium SP------13 tains Volcanics______20 Camptonectes ______-- ______Irish Syndicate vein______40 Blue vein •... ------42 Bluebird stock______26 Caninfa. ______------______.: 5 Iron Age Gulch area _____ ------·------38,89 Cedaria ______.___ ----______4 Iron Cross mine. ______-- __ ---_-_--_____ 46 Borris, F. S., analyst.------20 Botts, S.D., analyst______20 Charydrobia cretaccea __ __ - -- _------____ _ Iron Mask mine •• ------8, 10, 11, 38,49 Boulder batholith ______------16, 24,31 Collignoniceras woolgarL _____ • ______10 J Brown, R. W., fossil identification______13 sp ______------_____ --- 10 Buckhorn mine______38 Cylfndrodon fontis .• _------___ ----- __ ___ ·13 January mine ____ ------35, 38,41 EumicrotiB. __ ------_____ 7 January stock ______------26,41 Jawbone claim. ______-_------_ 43 Haminea SP------10 c Jefferson Dolomite______4 Campanian time______33 Hyracodon SP------13 Inoceramus bellvuensis. ______Jo Dandy mine •• ---:·------35,46 Cedar Plains. See Radersburg. comanchean us bellvuensis ______Central mine.------44 K Champion mine______43 {ragUiB. _------10 Keating Gulch stock______25 sp_____ ------"______10 Chloe, G. W., analyst______20 Leptomeryx esulcatus _- __ ---______13 metamorphism______29 Claggett Formation______12 Keating mine ••• ------35, 45,48 Cobban, W. A., cited ______9, 10,11 Lioplacodes convexfculuB •• _ ------_ 8 cretaceus •• ___ ------_____ -______8 Keating vein ______------~- 36,39 Coburg Basin stock______27 Lucina juvenis ___ ------_- _-- ______10 Kibbey Formation. ______---- __ ------5 King mine ______- ___ ------43 Cody Shale._------11 Mesohfppus hypostylus______13 Kleinschmidt mine ______3S,40,4/ Cold Springs Gulch, graveL------49 Mactra arenaria __ ------_--- ______11 Collapse breccias ___ ------______38 Kootenai Formation... ------~---- 7, 8, 10,20,32,53 Colorado Formation ______8, 20,23,24,28 Nixonello montanensis. __ ------4 Ostrea •• ______•• __ ------______7 measured section ______53, M, 56,58 Palaeolagus brachydon______13 L Colorado Shale.______11 sp. ___ --- __ ------_- _- _- ______13 Congress mine._------46 Late Cretaceous and Tertiary tectonics______33 Copper•• ______----- ______35, 43, 42,45 Peltosaurus sp. ___ • ------_--- ___ -- ______13 Lead ______35, 38, 42, 43, 46,48 Cretaceous rocks. ______---______3 Pseudomelania SP------10 Llgnitic shale••• ------48 Reesidella sp ______------______-" ______8 Limestone Hills intrusive______28 Crow Creek fan.------14,15 Scaphites ventricosis. ------_---- _--- ______11 Custer-Iron Age-Hyantha group mine______35 Little Annie mine. ______------___ 42 Custer mine•• ______-----______40 ventricosus. __ ------______11 Little Bonanza mine______42 Cyclone vein. ______.______45 Sequoia affinis. _------_-·------13 Little Giant mine.-- __ -- __ ------_-- ___ 43,44 Spirifer brazeriana ______Little Olga mine______35, 40,41 Cynosure vein. ____ ~------40 brazerianus ______--- ______Little Olga vein------40 Tempskya orandis______10 Location ______------____ ------__ ----- 2 D Titanotheriomys veterior _ ------13 Lode mine.•••• ------35 Diamond Hill mine_------24, 26,35,49,48 sp __ ----- __ -_------______~ 13 Lodgepole Liln.estone______4 Diamond Hill stock ______23, 26, 31, 32,36,49 Freiburg stock______26 Dikes._------·16, 20, 21, 23, 24, 26, 29,43 Lombard facies •• __ ------_------~-- 5 Dowd, J.M., analyst.------20,30 Lombard fault •• ------33 G Lone Mountain stock •. ------~------25 Dry Creek Shale.------4 Giant group mine______35 metamorphism. ___ - ______------29 Dunbar Creek Formation ••• ------~------12 Glacial deposits. ______---- __ ----_____ 14 Lower black shale unit______8 Dutchman intrusive. __ ------__ ------_____ 20,23 Glaciation______14 Lower member, Elkhorn Mountains Vol- Gold ______34, 36, 38, 40, 41, 42, 43, 44, 45, 46, 48,49 canics._---- ______------_-- 17 E Gold Dust leveL------42 Lowland Creek Volcanics ______16,29 Eagle Basin stock.------27 Gold placers._------___ 36, 89,49 Eagle Formation.______12 Gopher mine.----- __ ------••• ____ ~______46 M Early Pleistocene. ______------14 Granodiorite. _____ ------__ ---- ____ ------_____ 24 Madison GrOUP------4 East Pacific mine.------35,38,40,41 Gravel deposits ______------14,49 Madison limestone _____ - __ ----- __ ------__ --_ 14 65 66 INDEX

Page Page Page Mafic rocks.------23,24 Phosphoria Formation.------6, 7 Silver-lead-zinc replacements ______36,38 Mammoth composite body______28 measured section. ___ ------______--_____ 49 Silver Saddle mine______42 Mammoth mine______44 Pierre Shale ___ ------______.---___ 12 Silver Wave mine______44 Marietta mine ______35, 38,42 Pilgrim Dolomite______3 Silver Wave stock ______23, 25, 27, 29, 31, 36, 38,43 Mass wasting______15 Placer gold. ______35, 39,49 Skarn deposits ______------______36,38 Maywood Formation______4 Pliocene Old Valley cycle._------13 Skull Creek shale. ______----_ 9 Plugs. ______. 16, 23,24 Meagher Limestone.• ------3 Slim Sam Formation ____ 11,12, 17, 21, 23, 24, 29, 31,39 Merthold, S.M., analyst______21 Pole Creek stock.------27 measured section______58 Mesozoic Rocks______7 Porphyry, defined. __ . ______--._._.-- 16 Slim Sam stock.------25 Metamorphism, thermaL______29 Power Gulch composite body______28 Solution breccia. ______--~- ______------___ 5 Middle member, Elkhorn Mountains Vol- Precambrian rocks.------3 South Fork stock ______24,25,33 canics.------______------17 Previous work.------3 Spring Hill mine.------44 9 Purgatoire formation. _____ -- .. __ --_ ... -- .. --- Middle siliceous mudstone and sandstone unit. Stray Horse mine ..• ------35,42 Mineral deposits______94 Structural geology______30 future discovery and development______48 Q Sulfide minerals______32 Mines ______._----___ 39 t Quadrant Formation __ ------______6, 31,38 Swift Formation______6, 7 See also individual mine name. measured section ______....• c .• ____ --- 49,52 Mission Canyon Limestone ______4,38 measured section._------49 Monarch mine______48 Quadrant quartzite .. -- ____ ---- __ -_------_- 14 Montana-Idaho vein______41 Quarry Gulch intrusive._------_____ 28 T Monte Cristo stock______26 Quartz latite porphyry_------29 Quartz monzonite. __ --______26 Moraines.------______----.:_ 14, 15 Tectonics, Late Cretaceous and Tertiary---__ 33 Quartz monzonite porphyry______27 Telegraph Creek Formation. ______.---__ 12 Morrison Formation______7, 8 Quartz veins, auriferous pyrite ______36,49 Terrace deposits .. ___ ------__ ---- __ ---- __ ---_- 15 measured section.------49, 52,53 base and precious metals ______36, 38, 49 Tertiary rocks ______------______-- ____ ---__ 12 Mowry Shale.------10 Quartzite mine______48 Three Forks Shale._------_ 4 Quartzite Ridge area •. ------35, 3!.1, 46 Titaniferous iron ore. ______11, 12, 35, 36, 39, 44,46 N Quaternary deposits .. ___ ------__ ------13, 14 •rosi Chert Member______6 Nada mine._------______------. 38 Queen Bee mine ______------44 New Era mine.______42 · u v Niobrara Formation ______------11,12 R North Home mine.------3.'i, 48 Radersburg area ______------___ 34, 35, 36, 3!.1, 44. 4!! Nygaard, E. A., analyst______21 Upper black shale unit------···----·----- 10 Range-front faults ___ -- ______-- ______---_- 32 Veins ______--·--- ______----_ 23,36 Rattlesnake intrusive. ______20, 21, 23, 30, 31, 33, 3\J Venezuela mine______42 0 Red Lion Formation______4 Vosburg mine .. ------14 Ohio Keating mine ______35,45 Red Wing vein______43 Vosburg stock •. ------__ ------·-- 26, 2\!, 32 Ohio Keating vein __ ------36,39 Reeside, J. B., Jr .• cited______10 Vulture mine ______--- ___ ----- 43 Oligocene sedimentary tuff______12 Replacement bodies, carbonate rock _____ ----- 49 Vulture stock._---·------_------26 Oligocene tufa •. _____ . ______--- ___ - 13 Retort Phosphatic Shale Member______6 Ore pipe ______46,48 Rhyolite flow. ___ ------30 Orphan Boy intrusive ______24, 26,29 Roads ______------______------2 w Outwash fan. ___ . ___ ------______-----·-- 14 Rothfus mine.------____ -- __ ------38 Weasel Creek area .• ------·---- 34, 35, 38,89 Oxidation and enrichment ______- ______39, 49 Ruby mine ______35,46 Weasel Creek fault._------32 Oxidation zone. ______36, 40, 43, 44, 45, 46, 48 White, Katherine, analyst______20 Ruby Silver mine .•. ------~------46 Whitehead Ranch fault______31 p s Whitehorse fault______--·- ______------31,32 St. Louis mine______44 Winston Creek.------~------39 Paleozoic rocks.------3 Winston district. ______-__ 35 Palmer, A. R.. fossil identification. ___ ------4 Santa Anita mine ______------35,46 15 Palmer, Chase, analyst______21 Sappington Sandstone Member______4 Wisconsin glaciation ______----- Park area. ______- 35, 38, 39,42 Scheelite. ___ --- __ ------.------_ 38, 39, 44, 4!J Park mine._------42 Sections, measured. __ .-_-_---.-_-----.------49 y z Park Shale______3 Sedimentary rocks ____ . ______-.-- __ -_-_------3 Parker mine ______. ____ ------43 Shedhorn Sandstone ..•. _____ -. ______-- ____ --- 6 Younger intrusive rocks______2li Phenocryst, defined. ______- _____ ------16 Sills .• ______·------______20,21 Yen, T. C., fossil identification______8 Phosphate rock. ______----- 48 Silver. ______------· 35, 38, 4~. 43, 45, 46,48 Zinc ______•• ______• _____ • ______35, 38, 42, 43, 46