Arkose-Hosted, Aquifer-Controlled, Epithermal Au-Ag Mineralization

EconomicGeolog•j Vol. 84, 1989, pp. 1891-1902

Arkose-Hosted,Aquifer-Controlled, Epithermal Au-Ag Mineralization,Wenatchee, Washington

JACOBMARGOLIS Corona Gold Inc., P.O. Box 11305, Reno, Nevada 89510

Abstract

The Wenatchee district, site of the Cannon mine, is located on the eastflank of the Cascade Rangein centralWashington. Epithermal gold mineralization of Eoceneage (44 Ma) is hosted by the EoceneChumstick Formation, a sequenceof interbeddedarkosic sandstone, conglomerate, and mudstone.This study focuseson the structureand hydrothermalalteration and mineralization south of the Cannon mine. Hydrothermalalteration and mineralizationis hostedby Chumstickarkose, minor Eocene dacite,and andesitedikes, and locally,Eocene felsic volcanic rocks (flows, ash flows, eruption breccia)and extends for at least4 km southeastof the Cannonmine. The district-scalestructure is dominatedby a postmineralization,northeast-verging fold andthrust belt whichconsists of threemajor southwest-dipping reverse faults and associated folds produced by the propagation of the faults. Arkose-hostedalteration consists of zonesof silicificationwhich grade symmetrically outward to argillizationand a widelydispersed, district-scale propylitic alteration. Alteration is strata boundwithin units of arkosicsandstone and conglomerate bound or cappedby mudstone-rich sections.Features of the propyliticzone includethe growthof pyrite within detrital biotite andthe replacementofplagioclase by carbonateand epidote. The argillicassemblage includes hydrothermalkaolinitc and sericite. Silicification consists of silicichydrothermal breccias and pervasivequartz flooding closest to the argilliczone; hydrothermal K feldsparis common. Typically,a transitionalsubzone containing abundant quartz veins (quartz stockwork) occurs betweenthe argillic and siliciczones. In decreasingorder of abundance,metallic phases are pyrite, arsenopyrite, marcasite, stibnite (latestin paragenesis),chalcopyrite, hessitc, electrum, native silver, sphalerite, galena, pyr- argyrite,and boulangerite. Gold contents are highestin silicicalteration and the quartz-stockwork subzone.Where arseniccontents are high, gold is concentratedin the argillic zone. In the latter case,strong correlations between gold and arsenic suggest that goldmay have been transportedas a goldthioarsenide complex; precipitation of the goldand arsenicmay have beenpromoted by the progressivesulfidation of biotite. Two laterallyextensive strata-bound alteration zones (•_2 km alongstrike) have been delineated,one largelybound by mudstonesin the hangingwall of a postmineralizationreverse fault, and one in the footwallof the fault and bound aboveby a mudstoneand ash-flowtuff. The absenceof alterationstratigraphically above the footwallzone indicatesthat it is the stratigraphicallyhighest mineralized aquifer in the district. Hydrothermalfluids migrated laterallyalong at leasttwo laterallyextensive aquifers which were stratigraphicallystacked. The shallowerfootwall zone is enrichedin As, Sb, andAu relativeto the deeperhanging wall zone, which is richer in Ag, Cu, Te, and Se. This variationin depth is similarto patterns observedin active geothermalsystems and epithermalprecious metal depositsand implies somedegree of exchangeof hydrothermalfluids between the aquifers,possibly via leaksin aquitards.

Introduction 0.214 oz/tonAu and 0.5 oz/tonAg when miningbe- THEWenatchee district (Fig. 1) is locatedon the east gan (Bartholemew,1986). flankof the CascadeRange in centralWashington and Similarepithermal mineralization, consisting of al- is the site of arkose-hostedgold mineralizationof tered and mineralized arkose,extends southeast from Eocene age. Productionfrom the Lovitt mine, also the Cannonmine, to the dormantLovitt mine, and to known asD reef (Fig. 2), from 1949 to 1967 totaled the subsurfaceof WenatcheeHeights (Fig. 2). This 1,036,572 tonsof ore averaging0.396 oz/tonAu and report focuseson the structureand alterationsouth- 0.607 oz/tonAg (Pattonand Cheney, 1971). In 1985, east of the Cannon mine in the area of Wenatchee AsameraMinerals (U.S.) Inc. commencedproduction Heights and describesthe district-scalestructure, from the Cannon mine or B reef to the northwest, form of mineralization, ore controls, and the nature which contained reserves of 5,256,000 short tons at of the hydrothermalsystem.

0361-0128/89/1000/1891-12$3.00 1891 1892 JACOBMARGOLIS

fault (Fig. 1) wasactive during Chumstick deposition (Evans,1987). Near Wenatehee,pre- and postmineralization Eocene intrusions are restricted to a narrow, northwest-trendingzone on strikewith the Eagle Creek fault (Fig. 1). It is inferred that this basement fault provided ingressfor magma during basin formation.Premineralization dikes and stocks range from basalticandesitc, possibly of alkalieaffinity, to hornblende andesitcand daeite and are variably altered (Gresens,1983; Margolis,1987). Mineralization at the Cannon mine has been dated at 44 Ma (K-Ar on adularia;Ott et al., 1986). Por- phyritierhyolite of the Wenateheedome, adjaeent to _ .... the Cannonmine, is interpreted as a postmineralization intrusion based on its lack of alteration and K- Ar age of 43 Ma (Gresens,1983; Ottet al., 1986). The ChumstickFormation was regionallyfolded prior to the depositionof the WenateheeFormation •• • • •Q/ of Oligoeeneage (34 Ma; fissiontrack; zircon in tuff- ',, aceousbed; Gresenset al., 1981). The Wenatehee Formation unconformablyoverlies the Chumstick Formation and older rocks and consists of at least 300 modi•ed •romCrese•s (]9• •a t•bor e• •]. (]9•, ]9•. Pre- m of quartzosesandstone, shale, and minor tuffaeeous beds(Hauptman, 1983). Dikesand sills of hornblende metamorphic•d p]uto•ic rocks;S•, Sw•e •iofi•e C•e•. •, andesire,dated at 29 Ma (K-Ar on hornblende;Gre- •uater•r• Terfi•r• u•ff•: Tb, Columbi•Bive• •s•]t; Tc, Chum- sens,1983), intrude both the Wenatehee and Chum~ stick Formations.

Eocene;st¾pIe•, O]i•oce•e. W = ci• o• Wen•hee. Geologyof WenatcheeHeights Norcovolcanic complex GeologicSetting Much of the area south of the Cannon mine is cov- Wenatcheelies within the Chiwaukumgraben, a ered by basalticdiamictite of Plioceneage and un- strike-slipbasin defined by the Entiat fault zone to consolidateddeposits of Quaternary age (Fig. 2). A the east and the Leavenworth fault zone to the west sectionof alteredfelsic volcanic rocks of Eoceneage, (Fig. 1; Gresens,1983; Johnson,1985; Evans,1988). informallytermed the Norco volcaniccomplex, un~ AlthoughQuaternary deposits obscure the position derliesdiamictite in the vicinity of the Norco well, a of the Entiat fault southof the city of Wenatehee,the wildcatwell drilled for naturalgas in the 1930s (Fig. fault may curve to the southwestbeneath Wenatehee 2). All informationon the Norcovolcanic complex is Heights where it is informallycalled the Wenatehee from examination of 12,000 ft of drill core from six Heightsfault zone (Margolis,1987; Figs. 1 and 2). drill holes.The Norco volcaniccomplex consists of a The Entiat fault systemwas apparently active before, basalsection of porphyriticrhyodacite flows and flow during, and after depositionof Chumsticksediments breccias. The flows are overlain by more silicic, within the graben (Laravie, 1976; Vance, 198,5; stronglywelded rhyolite ash-flowtuffs and interbed- Evans, 1988). There is no evidenceof intrusiveor ded, discontinuous horizons of bedded breccias, as hydrothermal activity southeastof the trace of the much as 60 m thick, which may be hydrothermalex- Wenatehee Heights fault zone. plosiondeposits (Margolis, 1987). A K-Ar ageof 47.3 The area between the Entiat and Leavenworth _ 1.8 was obtainedfrom biotite in a relatively unal- faults consists of the Chumstick Formation of Middle- tered rhyodacite flow at the base of the section.The to-Late Eocene age, a basin-fill sequenceof conti- volcanicrocks vary in thicknessfrom 100 to 200 m; nental arkosiesandstone with interbedded mudstone, their completeareal extent hasnot been delineated. siltstone,and conglomerate.Interbedded tuffs yield Similarlithologies have not been found elsewherein agesbetween 49 and 41 Ma, althoughthe top of the the Chiwaukumgraben, although thin air-falland ash- formation may be close to the Eoeene-Oligoeene flow tuffs are present in the ChumstickFormation boundary (Tabor et al., 1982; Evans, 1988). The (Gresenset al., 1981; McClincey,1986). Norcovol- ChumstickFormation hosts the Wenateheeepither- caniccomplex volcanism predates mineralization be- mal mineralization(Margolis, 1987). The EagleCreek causeboth the Norco volcaniccomplex and at least Tir

30 ,•,50 Tc

Canyon

Norco well •Q :"NVC:'

WENATCHEE

HEIGHTS

QIs

Tc QIs •0 , MILES 0.5 1 ,KILOMETERS 0.5 1

QIs

QUATERNARY •--• QuaternaryDeposits(undiff.) • Landslides

TERTIARY Pliocene• BasalticDiamictite TERTIARYINTRUSIVE F• Andesite Oligoceneli•• Wenatchee Formation [• Rhyolite ß T[•]Argillic Alterationand ofSilk=k= Tc •- T•']Andesite o RhyoliticAsh-Flow Tuff o T[• Gabbro "' [-• ChumstickFormation

.•.-'" contact;dashed where approximate;dotted where concealed •r•irsefault; teeth onhangingwall; dashedwhere ... approximate;dotted where concealed fault; dashed where approximate;dotted where concealed

/25 strikeand dip of bedding (•)horizontal -/.vertical • synclineaxis :• anticlineaxis NVC = Norco Volcanic Complex

FIG. 2. Geologicmap of WenatcheeHeights modifiedfrom Gresens(1980, 1983) showingthe locationsof the Cannon mine, Lovitt mine, Compton's Knob, and known extent of the Norco volcanic complex.The city of Wenatcheeis locatedimmediately north of the Cannonmine. 1893 1894 JACOBMARGOLIS

100 m of overlyingarkose are weakly altered and the time of deformationto between29 and 34 Ma, mineralized. the age of the Wenatchee Formation. The footwall Comptontuff fissurereverse fault in the Lovitt mineand the Dry Gulch reverse fault were recognizedpreviously A stronglywelded, felsicash-flow tuff, about30 m (Lovitt and Skerl, 1958; Gresens,1983), but the re- thick, has been delineatedby drilling and mapping lationshipof the faultsto mineralizationand folds was from the areaof the Norcowell northwestto the rhy- not resolved.The fold andthrust pattern was super- olite exposedat RoosterComb, several hundred me- imposedupon a regionalanticlinorium in the Chum- ters northeastof the Lovitt mine (Fig. 2). This unit, stickFormation which developed prior to deposition informallynamed the Comptontuff by Margolis of the Wenatchee Formation. The axis of this anticli- (1987), containsquartz, biotite, and oligoclasephe- norium coincides with the easternmost reverse fault nocrysts,is partially perlitic, and hasbeen dated at (Fig. 3), and the southwest-dipping,generally bed- 46.2 +_1.8 Ma (K-Ar, biotite). The similarityin phe- ding-parallelreverse faults failed to propagatein the nocrystassemblage between this tuff andthe Norco northeast-dippinglimb of the anticlinorium. volcaniccomplex tuffs, the proximityof the tuff to In the area of exposedand near-surfacemineral- the Norco volcaniccomplex, and its age suggestthat ization south of the Cannon mine, the structure con- it may be an outflowsheet from the Norco volcanic sistsof a fault-propagationfold (Lovitt anticline) cored complex. by the footwallfissure reverse fault (Figs.3 and 4). The term "footwall fissure" was first used because Structural Geology the fault marks the footwall to the D reef mineraliza- The district-scalestructure is dominatedby a post- tion at the Lovitt mine (Lovittand Skerl, 1958). Both mineralization,post-Wenatchee-age, northeast-verg- the work of Patton (1967) within the mine and data ing fold andthrust belt. The belt is at least6 km wide frommore recent drilling show that the fault is listric by 15 km along strike and consistsof three major anddips southwest, and that dragfolding exposed in southwest-dippingreverse faults and associatedfolds the footwall northeast of the fault indicates reverse producedby the propagationof the faults (Fig. 3). movement(Fig. 4). Right-lateral,nearly north-striking Gresens(1983) recognizedthat andesitcwhich in- faultsoffset the D reef and the footwallfissure by as truded the plane of the Dry Gulch fault is part of the muchas 100 m (Lovitt andSkerl, 1958; Patton,1967) Oligoceneintrusive complex (29 Ma) and constrains and offsetalteration patterns (Margolis, 1987). The

CROSS SECTION ALONG SQUILCHUCK CANYON

Dry Gulch fault Halvorson Lovitt Mine SW faultHalvorson syncline •..- ,• PitcherHW7-•%-•i,•

• •o o ' •o• •ooo•MILE •'•']Quaternary FWZ - - - ;FEET [] Diamictite NO EXAGGERATION l• WenatcheeFormation ß Mineralization [] ChurnstickFormation

DRY GULCH AREA

FIG. 3. Geologic crosssection A-A' along SquilchuckCanyon; see Figure 2 for location of section line. The sectionillustrates the postmineralizationfold and thrust pattern, the fault-propagationfold (Lovitt anticline) at the Lovitt mine cored by the footwall fissure,and the unconformablerelationship between Tc and Tw. Also note the pre-Tw anticlinoriumin Tc, and the axisof this fold at the Lovitt mine. HWZ = hanging-wallzone, FWZ = footwall zone, PA = propylitic alteration.Cross section at lower left is alongDry Gulch, one mile northwestof sectionA-A '. EPITHERMALAu-Ag MINERALIZATION, WENATCHEE, WASHINGTON 1895

COMPTON'S KNO•

200 FEET

,'i':[]PROPY LITIC ':[]ARGILLIC ]SlLICIC "•R ILLHOLE (with bedding angle)

HIGHEST GRADE Au,Ag .•LTERATION CONTACT

D-REEF B SW /•/•; NE

• P '%'•S200 feet

[] ß ß ^A.G,L.,c S SlLIClC

FIG. 4. A. Crosssection oriented southwest-northeast through Compton's Knob southeastof Squil- chuckCreek showingthe generallystrata-bound nature of alteration,the positionof the hanging-wall and footwallzones, and the absenceof alterationstratigraphically above the Comptontuff. Note that the footwallzone is overturnedby dragfolding along the footwallfissure fault. B. Crosssection through the Lovitt mine on the northwestside of SquilchuckCreek. (FWF = footwall fissure).D reef is the hanging-wall zone, the footwall zone is the silicic and argillic alteration beneath the Compton tuff. Note the drag folding (steepenedto overturnedbedding) in the footwallto the footwallfissure and the absence of aheration above the tuff. anglebetween the footwallfissure and the right-lat- fabric; thinner beds of unaltered arkosic sandstone eral faultsis about30 ø , suggestingthat the right-lat- within the mudstoneare disruptedand boudinaged. eral faultsare tear faultswhich were activeduring The footwall fissureis a narrow zone of discontinuous, the northeastcompression that causedthe reverse en echelon faults accommodatedby discontinuous faulting. mudstones.At the northwestmargin of the D reef, a The footwallfissure at the D reef is markedby a similarsection of variablydeformed mudstone marks mudstone-richsection which, due to its incompetence the hangingwall of the mineralizedarkose; the min- with respectto adjacentaltered arkose, preferentially eralizationis, therefore,generally strata bound (Fig. accommodatedbedding-parallel deformation. The 4). Ott et al. (1986) showedthat mineralizationin the mudstone-rich section is as much as 15 m thick and Cannonmine is similarlyconfined between mudstone- characterizedby a slickensided,anastamosing scaley rich sections. 1896 JACOBMARGOLIS

Form of mineralized zones to zones of argillizationand widely dispersedpro- Fromthe areaof RoosterComb southeast through pylitization. Figure 5 illustratesthe principal min- the area of Compton'sKnob (Fig. 2), two tabular eralogicchanges. Alteration contacts within arkose zonesof mineralization,generally parallel to bedding, are gradational,but commonly,carbonaceous mud- havebeen delineated(Figs. 3 and 4). One zone,the stonesand siltstones sharply separate alteration types. hanging-wallzone, occursin the hangingwall to the The descriptionof alterationgiven below pertains to footwall fissure;the secondzone, the footwall zone, arkose,the dominantlithology. occursin the footwallof the fault (Fig. 4). The hang- UnalteredChumstick arkose contains quartz (40%), ing-wallzone includesthe D reef of the Lovitt mine plagioclase(40%), orthoclase-microcline(10%), bio- and alterationat and subjacentto Compton'sKnob tite (5%), fragmentsof volcanic,plutonic, and meta- (Fig. 4). morphicrocks, and minor accessory minerals. Detrital The footwall zone is concealedby landslidede- biotiteis blackin handspecimens, unaltered, and lo- positssoutheast of SquilchuckCanyon and only poorly cally constitutesas much as 20 percentof the arkose. exposednorthwest of the canyonat the Lovitt mine. Propylitic alteration The zonelies east of Compton'sKnob and the D reef andconsists of silicifiedsandstone and conglomerate The mostconspicuous feature of propyliticalter- stratigraphicallybelow a laterallycontinuous mud- ationis the occurrenceof pyrite within grainsof de- stone which underlies the Compton tuff. On the trital biotite (Fig. 6A) whichare distinctlybronze in northeast limb of the Lovitt anticline, the footwall hand specimens,unlike the black unaltered biotite. zone dipsnortheast about 35 ø to 45 ø, but becauseof Nearlyall sulfidein the propyliticzone occurs as py- postmineralizationdrag folding by the footwallfissure rite in altered biotite grains;the pyrite is typically reverse fault, the sectionis steep to overturnedad- parallel to biotite (001) cleavage.The district-wide, jacent to the fault (Figs. 3 and 4). Throughoutthe propylitizedChumstick arkose had been previously district,alteration occurs only stratigraphically below interpreted as a different formation by Gresens the mudstoneand Comptontuff. If this zone is lat- (1983). erallyextensive, it shouldoccur on the southwestlimb Argillic alteration of the Lovitt anticline beneath the Wenatchee For- mation (Fig. 3). The alteration is termed argillic becauseof the presenceof kaolinite. Sulfidesare mostabundant in Hydrothermal Alteration the argillic zone, occurringas stringersor dissemi- nations,the latter chiefly within former biotite sites. Hydrothermalalteration consists of tabular zones Adjacent to the silicic zone, sulfideveinlets are cut of silicificationwhich grade symmetricallyoutward by multiplegenerations of quartzveins and the arkose

UNALTERED PROPYLITIC ARGILLIC SlLIClC

minor quartz quartz -• quartz

plagioclala • montmorillonite kaolinire •

carbonate

pink alblta

epldote

biotite pyrite + Mg-rich biotite • pyrlte + K-mica ,,

chiorlte + Fe-Ti oxidel • Fe-Ti oxidel )

K-feldlpar ß- K-feldlpar overgrowthl

FIG. 5. Diagramillustrating the principalmineralogic changes resulting from alterationof arkose. Curved arrowsindicate addition of quartz and K feldspar.Light arrowsindicate stability, heavy arrows indicate reactions. EPITHERMALAu-Ag MINERALIZA TION, WENATCHEE, WASHINGTON 1897 is more completelysilicified. This is the quartzstock- In the argillicallyaltered rocks of the footwallzone, work subzone(Fig. 6F). The quartz veinsare rarely there is a strongpositive correlation between As and greater than 3 cm wide, with notableexceptions in Au (Fig. 8). Althoughthe Au:Asratio variesamong the D reef, where veinsare aswide asi m. Fracturing different drill-core intervals, within each interval the of the arkose and concomitantvein emplacement ratio is uniform.There is no systematicpattern of As could occuronly after initial periodsof argillic and andAu concentrationas a functionof depthor location silicic alteration, which served to create a more brittle alongstrike. hostthrough the depositionof hydrothermalquartz in the matrix. Discussion Silicic alteration Two typesof silicificationare present at Wenatchee Hydrothermalaquifers Heights:silicic breccias and quartz flooding adjacent to the argilliczone. In general,silicic breccias grade When mineralizationtook place at 44 Ma, the symmetricallyinto a zoneof quartzflooding which in ChumstickFormation was still being depositedand turn gradesinto the argilliczone with an intervening the sectionmust have lain nearlyfiat. Premineraliza- quartz stockwork. tion foldsin the Cannonmine (Ott et al., 1986) may Silicicbreccias are characterizedby angular,silic- reflect local deformation related to the inferred sub- ified clastsin a siliceousrock-flour matrix which typ- jacent EagleCreek fault and/orto intrusiveactivity. ically containscarbonate. In areasof quartzflooding Earlyworkers such as Lovitt andSkerl (1958) stressed the clastictexture of the arkoseis preserved,and most the importanceof largestructures such as the footwall sulfideis confinedto the alteredarkose with onlymi- fissurein focusingmineralizing fluids. However, these nor sulfideoccurring in quartzveins. The quartzveins structuresclearly postdatemineralization. Most re- are most commonin the quartz-stockworksubzone cently, Ott et al. (1986) recognizedthe importance at the transitionfrom argillicto silicicalteration. of permeabilityand the generallystrata-bound form In the siliciczone detrital K feldsparis largelypre- of the alteration in the Cannon mine. served,and manyof the grainshave overgrowths of Throughout the district it is apparent from the secondaryK feldspar(Fig. 6B). Apparently,preex- strata-bounddistribution of the alterationthat hydro- isting K feldspargrains served as a nucleusfor the thermal fluids migrated laterally along aquifers of precipitationof hydrothermalK feldspar. sandstoneand conglomerate at leastpartially confined betweenor cappedby mudstones(Fig. 9). Someof Geochemistryand sulfidemineralogy the resultingmineralized zones (aquifers) are at least Figure 7 illustratesthe distributionof sulfidesin 2 km long (alongstrike) but otherssuch as the E reef, alterationzones. Except for pyrite (•2 mm)and stib- several hundred meters southwestof the D reef, are nite (•1 cm),the sulfidegrains rarely exceed0.5 mm relatively discontinuous.The unsilicifiedmudstones in diameter.The Au and Ag contentsincrease from boundingthe silicifiedsandstone aquifers preferen- the propyliticto the siliciczone. Where present,the tiallyaccommodated bedding-parallel slip during later quartz stockworksubzone is richest in Au and Ag, Oligocenedeformation. Although this studyis con- whichoccur as electrum and sulfides within the quartz cerned with the area south of the Cannon mine, the veins. Outsideof the quartz stockworksubzone, Au steeplydipping and deformedmudstones bounding contentsare higher in argillicalteration in the footwall the isolatedorebodies in the Cannon mine (Morrow zone andin silicicalteration in the hanging-wallzone. andFoilis, 1988) mayrepresent the northwestexten- Reportson the mineralogyof the D reef (Moody, sion of the footwall fissure fault zone. 1958; J. M. Guilbert, 1963, unpublishedreport for The absenceof alterationstratigraphically above Day Mines, Inc.) indicate that it is similar to miner- the Compton tuff indicatesthat the alteration zone alization at Compton'sKnob in that both contain immediatelybeneath it (footwallzone) represents the abundanthessitc, electrum, chalcopyrite, and appar- stratigraphicallyhighest mineralized aquifer in the ently no arsenopyrite.Guilbert (1963) reportednau- districtand that it isstratigraphically above the hang- mannitcfrom the D reef althoughin this studyno Se ing-wall zone. Thus the mineralizedaquifers were phasewas detected. Gullbert's finding does, however, stacked. indicatethat Se contentsare at least locally high in There are geochemicaldifferences that resulted the hanging-wallzone. from this stratigraphicstacking. The shallowerfoot- Significantmineralogic differences between the wall zone is enrichedin As, Sb, and Au, compared footwall and hanging-wallzones are summarizedin with the deeperhanging-wall zone which is richerin Figure 7. Geochemically,the hanging-wallzone is Ag, Cu, Te, and Se. This variationin geochemistryis richer in Ag, Te, Cu, and Se than the footwall zone, similarto patternsobserved in activegeothermal sys- which is richer in As, Sb, and possiblyAu. tems and many epithermalprecious metal deposits ;-f [ j -'

i" .-.

...

_: .

. ..

ß . ,- .,.•; * ß.'.': _**;2 7.. •';-'" ... "* .• ":" .:• '?'7"•'/•'s• : '•-:- ;-....x • --- .--: 4.•' - ß-,,• ' ....;.-.;:' ' .•C-, *;' • •', ...... 4":,, "?;' ....-..-.-.. . .;:.. ;- .•..•

,....

• .

....

1898 EPITHERMALAu-Ag MINERALIZATION, WENATCHEE, WASHINGTON 1899

PROPYLITIC ARGILLIC SILICIC ZONEIN V•-IICH COMMENTS MOST ABUNDANT ••--'"• • In propylltlc zone, PYRITE restricted to biotite

Overgrowths on •' ARSENOPYRITE• eartier-formed pyrlte Commonly intergrown • MARCA$1TE '• with pyrite •IBNITE/NATIVE$1• Latest-stage veinlets Overgrowths on •HALCOPYRIT•• X pyrlte and haslite • Rimmedby pyrlte •PHALERITE O In the ergllllc zone O u. X Late overgrowthe HESSITE on pyrlte

X Rarelyidentified + In veins ELECTRUM indicated by geochemistry GALENA] .,TIRE,g I X

•"-sPYRARGYRITE/DULANGERITEI X electrumClosely andassociated chelcopyrlte with

FIG. 7. Distributionof sulfides,native metals,and tellurideswithin the silicic,argillic, and propylitic zones south of the Cannon mine. Also shown is the strata-bound zone in which each mineral is most abundant.Identification of theseminerals was by electronmicroprobe.

(Ewers and Keays, 1977; Buchanan,1981; White, it is unlikelythat pervasivefractures were presentto 1981). Assumingthat mineralizationin the aquifers channelsolutions in the poorly consolidatedarkose. was coeval, sucha distributionimplies somedegree One problemthat remainsunsolved is the direction of exchangeand interactionof hydrothermalfluids of fluid flow, whichmay be addressedby district-scale between the major aquifers,possibly via leaks in studiesof mineralogic,isotopic, and fluid inclusion- aquitardsas shownin Figure 9. temperaturezoning. The lateralextent (perpendicular The depth from the palcosurfaceto the Compton to strike)of mineralizedaquifers is alsounknown. A tuff at the time of mineralization is not known. Based third unresolvedproblem is the positionof the Can- on the structureand distributionof propyliticalter- non mine ore zones with respect to the two major ation (Fig. 3), the stratigraphicdistance between the mineralizedaquifers to the south. two majoraquifers south of the Cannonmine is about 300 m. The lateral flow within aquifersand the dam- Alteration paragenesis ming by the mudstoneand Comptontuff probably Textures of alteration mineralsand sulfides(Fig. prohibitedthe dischargeof solutionsat the surface, 6D, E, and F) indicate that the alteration zoneshave at least in the area that hasbeen studied. Furthermore, encroachedupon one anothertoward unalteredar-

FIG. 6. A. Photomicrographof pyrite grains(opaque) within Mg-richbiotite (top) in the propylitic zone; sampleC9-602; scalebar is 250 t•m. B. Photomicrographof a detrital K feldspargrain with a rim of hydrothermalK feldspar;silicic hanging-wall zone, sampleC9-192. Scalebar is 250 t•m. C. Three coalescedspherical growths of chalcedony(center) in a quartzvein; silicic footwall zone, sample C 12-592. Chalcedonyfibers display negative elongation; scale bar is 250 t•m.D. Backscatteredelectron imageof elongatepyrite (gray)within alteredbiotite rimmedby arsenopyrite(white); argillic footwall zone,sample C5-1278. Photoillustrates the overprintingof argillicalteration on earlier formed propylitic alterationrepresented by the elongatepyrite in biotite. E. Backscatteredelectron image of euhedral pyrite (gray)rimmed and partially replaced by hessite(white); silicic hanging-wall zone, sampleCiC- 750. Photoillustrates overprinting of Ag-Te-richsilicic alteration on earlier formedargillic pyrite-rich zone. F. Two core samplesof the quartz stockworksubzone representing the transitionfrom argillic to silicic alteration in arkose. Photo illustrates the abundantquartz veins (gray) cutting previously formed stringersof sulfide (dark gray) which formed during argillic alteration. Samplesjrd3-1343, C1C-1124. 1900 J.4COBMARGOLIS

5000

4000 4000

A A AAA 3000 3000

2000 2000 ß ß ß

ß 1000 ß ß ß ß o foloo 201O0 301O0 lOOO 2o1oo 3o1oo ARSENIC (ppm)

FIG. 8. Plotsillustrating the strongcorrelation between arsenic and goldin two drill holeseast of SquilchuckCreek. Circles -- alteredarkose, triangles -- alteredfelsite. Left figureis from footwall zone near Compton'sKnob (holeC-16); right figureis from argillizedarkose overlying the Norcovolcanic complex(hole jrd-3). Alteredfelsite is notthe Comptontuff, but possiblya thinnerash-flow tuff (?)or a rhyolitedike. In all caseswhere dataare available,argillized felsite of the Norcovolcanic complex hasa higherAu:As ratio than associated arkose. Analyses on five-footintervals by instrumentalneutron activationanalysis (INAA) by Bondar-Cleggand Co., Ltd., Vancouver,B.C. kose--silicic alteration has overprinted argillically and minor native antimony,commonly within calcite, alteredarkose, and argillic alteration has overprinted representthe finalstage of sulfidemineralization. The propyliticallyaltered arkose. The best evidencefor late precipitationof stibniteand native antimony may this is that elongatepyrite of the propylitic zone, havebeen promotedby decreasingactivity of sulfur whichformed within biotite, isprogressively replaced with time (Bartonand Skinner, 1979). andrimmed by arsenopyritein the argilliczone (Fig. 6D). In addition,sulfide veinlets of the argilliczone Precipitationmechanisms arecut by a quartzstockwork at the edgeof the silicic Conclusionsconcerning precipitation mechanisms zone (Fig. 6F), and veinletsof kaolinitccommonly are morepreliminary and speculative than conclusions cutpropylitic alteration. Veinlets of euhedralstibnite reachedabove concerningthe nature of the hydro-

N • o S

UNALTERED ARKOSE

Coma)tonluff • '*'.":.' ...... ,.i•.•,-,-.-..•...----.-•*•...... ß ß-•*.:', .... Volcanic. FOOTWALLZONE "'... •2 '• •'.* Ctm•t•.','

?if,oNE .:......

•OPYLITICALTER ATIO• • (BELOWCOMPTON TUFF) • III

•T TO •ALE

FIG. 9. Schematicrepresentation of the district-scalehydrothermal system discussed in the text. Notethe dominantly lateral fluid flow (arrows), leaks in aquitards,the stratigraphically stacked position ofthe precious metal-rich footwall zone and the base metal-rich hanging-wall zone, and the distribution of alteration.A northerlydirection of fluidflow is shownfor the purposeof illustration,but the actual directionof flow is not known.Black = mudstone,open stipple = argillic alteration,dense stipple = silicicalteration, lines = veins.The relationshipbetween alteration at the Norcovolcanic complex andthe aquifersto the northis notknown; nor is the relationshipbetween the Cannonmine ore zones and the two major aquifers. EPITHERMALAu-Ag MINERALIZATION, WENATCHEE, WASHINGTON 1901 thermal systemand the alterationparagenesis. It is Minerals(U.S.), Inc. andthe WashingtonMining and hopedthat theseideas guide future researchefforts. Mineral Resources Research Institute. The author ac- Boiling:Comb-structured quartz veinsin zonesof knowledgesthe supportiveand criticalcomments of silicic alteration commonlycontain centers of finer Eric Cheney of the University of Washington.The grainedanhedral quartz or are cut by veinsof similar manuscriptwas improvedby the commentsof two material.Chalcedonic spheres (Fig. 6C) are alsocom- EconomicGeology reviewers. monin the latestveins of anhedralquartz. These fea- September23, 1988; June9, 1989 tures may have resultedfrom silicasupersaturation REFERENCES during rapid decompressionalboiling, which would Bartholemew, A., 1986, Cannon gold mine: Internat. Mining, causethe precipitationof chalcedonyand amorphous September,p. 34-35. silica gel, now anhedralquartz and sphericalchal- Barton,P. B., and Skinner,B. J., 1979, Sulfidemineral stabilities, cedony,respectively (Fournier, 1985). Other possible in Barnes,H. L., ed., Geochemistryof hydrothermalore de- features derived from boiling such as calcite-ce- posits:New York, Wiley Intersci., p. 404-460. Buchanan,L. J., 1981, Precious-metaldeposits associated with mentedbreccias are outlinedby Margolis(1987); all volcanic environments in the southwest: Arizona Geol. Soc. Di- are late in the paragenesiswithin the silicic zone. It gest, v. 14, p. 237-262. is likely that initial silicificationformed impermeable Evans, J. E., 1988, Depositional environments,basin evolution areaswhich led to increasedhydrothermal fluid pres- and tectonicsignificance of the EoceneChumstick Formation, sure. Sudden rupturing would then result in de- CascadeRange, Washington: Unpub. Ph.D. thesis,Seattle, Univ. Washington,325 p. compressionalboiling (Fournier, 1985; Hedenquist Ewers, G. R., and Keays,R. R., 1977, Volatile and preciousmetal and Henley, 1985) and the emplacementof boiling zoningin the Broadlandsgeothermal field, New Zealand:ECON. fluidsinto the resultingfractures and breccias. Fluid- GEOL., v. 72, p. 1337-1354. inclusion studies at the Cannon mine indicate that Fournier, R. O., 1985, The behavior of silica in hydrothermal solutions:Rev. Econ. Geology, v. 2, p. 45-61. hydrothermalfluids boiled (Klisch,1989). Gresens, R. L., 1980, Deformation of the Wenatchee Formation Gold thioarsenidecomplexing: The correlations andits bearing on the tectonichistory of the Chiwaukumgraben, betweenAu and As (Fig. 8) in the argillicalteration Washington,during Cenozoictime: Geol. Soc.America Bull., of the footwall zone suggestthat gold thioarsenide v. 91, pt. 2, p. 115-155. complexingoccurred (cf. Seward,1984) andthat the -- 1983, Geologyof the Wenatcheeand Monitor quadrangles, Chelan and Douglas Counties, Washington:Washington Div. arsenopyritemay be auriferous.Preliminary micro- Nat. ResourcesBull. 75, 75 p. probe studies, however, failed to detect Au in the Gresens,R. L., Naeser, C. W., and Whetten, J. W., 1981, Stra- arsenopyrite.In the hanging-wallzone at Compton's tigraphyand ageof the Chumstickand Wenatchee Formations: Knob,As contents are lower,arsenopyrite is rare, and Tertiary fluvial and lacustrine rocks, Chiwankum graben, Washington:Geol. Soc. America Bull., v. 92, pt. 2, p. 841- gold precipitatedas electrumin the siliciczone. 876. Sulfidationof biotite:Petrographically, it is clear Hauptman, J. L., 1983, The sedimentologyof the Wenatchee that iron for the abundantpyrite was providedby Formation: Late Paleogene fluvial and lacustrine strata of the detrital biotite in the arkose.Influx of sulfur-bearing east-centralCascade Range, Washington State: Unpub. Master's solutions reacted with iron in black detrital biotite to thesis,Seattle, Univ. Washington,164 p. Hedenquist,J. W., and Henley, R. W., 1985, Hydrothermal erup- producepyrite andsecondary Mg-rich bronze biotite. tionsin the Waiotapugeothermal system, New Zealand:Their This sulfidationof biotite beganin the zone of pro- origin, associatedbreccias, and relation to preciousmetal min- pylitic alteration(Fig. 6A) and continuedin the zone eralization: ECON. GEOL., v. 80, p. 1640-1688. of argillic alterationwhere biotite is completelyal- Johnson,S. J., 1985, Eocene strike-slipfaulting and nonmarine tered to sericite and pyrite. This processmay have basinformation in Washington:Soc. Econ. PaleontologistsMin- eralogistsSpec. Pub. 37, p. 283-302. destabilizedAu and As sulfidecomplexes and pro- Klisch,M.P., 1989, Fluid inclusionsand geochemicalmechanisms moted the simultaneousprecipitation of Au and As for gold deposition,Cannon mine, Wenatchee, Washington within argillic alteration of the footwall zone. Al- [abs.]:Geol. Soc. America Abstractswith Programs, v. 21, p. thoughgold wasnot precipitatedduring initial sulfi- 102. Laravie,J. A., 1976, Geologicfield studiesalong the easternborder dationin the propyliticzone, precipitation of Au and of the Chiwankumgraben, central Washington: Unpub. Master's As would occur after a certain degree of sulfur fixa- thesis,Seattle, Univ. Washington,56 p. tion; and this wasapparently attained during argillic Lhotka,P. G., and Nesbitt,B. E., 1989, Geologyof unmineralized alteration.The conceptof iron-richphases as favor- and gold-bearingiron formation,Contwoyto Lake--Point Lake able reactivesites for sulfideand goldprecipitation region, Northwest Territories, Canada:Canadian Jour. Earth Sci., v. 26, p. 46-64. in environmentsof sulfidecomplexing has received Lovitt, E. H., and Skerl, C. C., 1958, Geology of the Lovitt gold increasingattention (Phillips et al., 1984; Neall and mine, Wenatchee, Washington:Mining Eng., v. 10, p. 963- Phillips, 1987; Mountain and Wood, 1988; Lhotka 966. and Nesbitt, 1989). Margolis,J., 1987, Structure and hydrothermalalteration associated with epithermal Au-Ag mineralization, Wenatchee Acknowledgments Heights, Washington: Unpub. Master's thesis, Seattle, Univ. Washington,90 p. This research was conducted as a master's thesis at McClincey, M., 1986, Tephrostratigraphyof the ChumstickFor- the Universityof Washingtonand funded by Asamera mation: Unpub. Master's thesis,Portland State Univ., 122 p. 1902 JACOBMARGOLIS

Moody,D. W., 1958, An examinationof the ore mineralsof the Patton,T. C., andCheney, E. S., 1971, L-D goldmine, Wenatchee, Lovitt mine, Wenatchee,Washington: Unpub. Master'sthesis, Washington:New structuralinterpretation and its utilization Seattle, Univ. Washington,22 p. in future exploration:Am. Inst. Mining Metall. PetroleumEn- Morrow, E., and Foilis, E., 1988, Geologyand mineralizationof gineersTrans., v. 250, p. 6-11. the Cannonmine, Wenatchee,Washington, in Schafer,R. W., Phillips, G. N., Groves,D. I., and Martyn, J. E., 1984, An epi- Cooper, J. J., and Vikre, P. G., eds., Bulk, mineable precious- geneticorigin for Archeanbanded iron-formation-hosted gold metal depositsof the westernUnited States.labs. I: Reno, Geol. deposits:ECON. GEOL., v. 79, p. 162-171. Soc. Nevada, p. 733. Seward,T. M., 1984, The transportand depositionof gold in Mountain, B. W., and Wood, S. A., 1988, Chemical controlson hydrothermalsystems, in Foster,R. P., ed.,Gold '82: Rotterdam, the solubility,transport, and depositionof platinumand pal- A. A., BalkemaPub., p. 165-180. ladiumin hydrothermalsolutions: A thermodynamicapproach: Tabor, R. W., Waitt, R. B., Frizzell, V. A., Jr., Swanson,D. A., ECON. GEOL., v. 83, p. 492-510. Byerly,G. R., andBentley, R. D., 1982, Geologicmap of the Neall, F. B., andPhillips, G. N., 1987, Fluid-wall rock interaction Wenatchee1:100,000 quadrangle,central Washington: U.S. in an Archeanhydrothermal gold deposit:A thermodynamic Geol. Survey Misc. Inv. Map 1-1311, 25 p. model for the Hunt mine, Kambalda: ECON. GEOL., v. 82, p. Tabor, R. W., Frizzell, V. A., Jr., Whettern, J. T., Swanson, 1679-1694. D. A., Byerly,G. R., Booth,D. B., Hetherington,M. J., Waitt, Ott, L. E., Groody, D., Foilis, E. L., and Siems,R. L., 1986, Stra- R. B.,Jr., and Zartman, R. E., 1987,Geologic map of theChelan tigraphy,structural geology, ore mineralogy,and hydrothermal 30-minuteby 60-minutequadrangle, Washington: U.S. Geol. alterationat the Cannonmine, Chelan County, Washington,in Survey Map 1-1661, 1:100,000, 30 p. Macdonald,A. J., ed., Gold '86: Willowdale,Ontario, Konsult Vance,J. A., 1985, EarlyTertiary faulting in the NorthCascades Internat. Inc., p. 425-435. [abs.]:Geol. Soc.America Abstracts with Programs,v. 17, p. Patton, T. C., 1967, Economicgeology of the L-D mine, Wen- 415. atchee, Washington:Unpub. Master's thesis, Seattle, Univ. White,D. E., 1981, Activegeothermal systems and hydrothermal Washington,29 p. ore deposits:ECON. GEOL. 75TH ANNIV.VOL., p. 392-423.