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, conglom- erate, 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-stock- work 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 de- lineated,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 postmin- eralization Eocene intrusions are restricted to a nar- row, northwest-trendingzone on strikewith the Eagle Creek fault (Fig. 1). It is inferred that this basement fault provided ingressfor magma during basin for- mation.Premineralization dikes and stocks range from basalticandesitc, possibly of alkalieaffinity, to horn- blende 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 postminerali- zation 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 mc 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