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Mineralogy, Textures, and Relative Age Relationships of Massive Sulfide Ore in the West Shasta District, California

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Mineralogy, Textures, and Relative Age Relationships of Massive Sulfide Ore in the West Shasta District, California Econom/cGeo/og• Vol. 80, 1985, pp. 2114-2127 Mineralogy, Textures, and Relative Age Relationshipsof MassiveSulfide Ore in the West ShastaDistrict, California STEPHEN S. HOWE U.S. GeologicalSurvey, 345 MiddlefieldRoad, Mail Stop901, Menlo Park, California 94025 Abstract The Devonian massive sulfide orebodies of the West Shasta district in northern California are composedprimarily of pyrite, with lesseramounts of other sulfideand gangueminerals. Examinationof polishedthin sectionsof more than 100 samplesfrom the Mammoth,Shasta King, Early Bird, Balaklala,Keystone, and Iron Mountainmines suggests that mineralization may be dividedinto six parageneticstages, the last five each separatedby an episodeof deformation:(1) precipitationof fine-grained,locally colloformand framboidalpyrite and sphalerite;(2) depositionof fine-grainedarsenopyrite and coarse-grainedpyrite, the latter enclosingtiny inclusionsof pyrrhotite;(3) penetrationand local replacement of sulfideminerals of stages1 and 2 alonggrowth zones and fracturesby chalcopyrite,sphalerite, galena, ten- nantite, pyrrhotite, bornite, and idaite; (4) recrystallizationand remobilizationof existing minerals,locally increasingtheir size and euhedralismand promotingtheir aggregation;(5) depositionof quartz, white mica, chlorite, and calcite;and (6) formationof bornite, digenite, chalcocite,and covellite during supergene enrichment of severalorebodies at the Iron Mountain mine.Despite regional greenschist facies metamorphism and local heating by intrusivebodies, enoughof the originaldepositional features of the ore remainto suggestthat the depositsin the districtformed by processessimilar to thosethat formedKuroko- and Besshi-type massive sulfidedeposits. Mineralogic and textural evidence do not supporta secondmajor episode of massivesulfide mineralization during the Permian. Introduction mineralogyand texturesof the ore in mostof the ma- IN recent years, detailed studiesby Barton (1978), jor massivesulfide deposits in the districtand incor- Eldridge (1981), and Eldridge et al. (1983) of the porates these observationsinto a parageneticse- mineralogyand texturesof ore in the relativelypris- quencesummarizing the relativeage relationships of tine Kuroko-typemassive sulfide deposits have re- the sulfide and nonsulfide minerals. These features of suitedin a dramaticrevision in the understandingof the West Shastaore are comparedwith thoseof Ku- the paragenesisof thesedeposits. This modification roko- and Besshi-typemassive sulfide deposits, and hasplayed an importantrole in the developmentof the bearingof thesecharacteristics on depositional the most recent model for the formation of Kuroko- processesis examined.Finally, recentK-Ar datingof type deposits(cf. Ohmoto and Skinner, 1983). The the West Shastaore by Kistler et al. (1985) is exam- success of these studies has stimulated a reexamination inedin light of the relativeage relationships outlined of ore mineralsand texturesin metamorphosedmas- here. sive sulfidedeposits, including Besshi-type deposits General Features of the West Shasta (Yui, 1983) and the more intenselydeformed Appa- Orebodies and Ore lachiandeposits (Craig, 1983), in an attempt to un- The geologicsetting of the massivesulfide deposits ravel their depositionaland metamorphichistories. in the West Shastadistrict is well describedby Kinkel As Craig (1983) points out, this challengemay be et al. (1956), Alberset al. (1981), Caseyand Taylor overwhelming in highly metamorphoseddeposits (1982), Reed(1984), andAlbers and Bain (1985) and containingsulfide minerals that reequilibratereadily. is onlybriefly summarizedhere. The orebodiesoccur On the other hand, microscopicexamination of de- within the upperpart of the middleunit of the Early positsdominated by refractorysulfide minerals, such DevonianBalaklala Rhyolite, on or near the axesof as pyrite and sphalerite,which have not been sub- broad synclinal,and lesscommonly anticlinal, folds jected to metamorphismabove greenschistfacies, alonga northeast-trendingzone 13 km longby 3 km shouldreveal a wealth of information. For this reason, wide (Fig. 1). The largestdeposits consist of massive, a microscopicinvestigation of ore in the massivesul- generallylenticular and fiat-lying, pyritic bodies with fide depositsof the West Shastacopper-zinc district lengthsand widths commonly an order of magnitude in northernCalifornia was initiated as part of a broader greaterthan their thicknesses;contacts between these studyof the district (Albers,1985). bodiesand barren or weaklypyritized wall rocksare The present paper describesand illustratesthe sharp.Stockworklike sulfide veins and disseminations 0361-0128/85/478/2114-1452.50 2114 MASSIVE SULFIDE MINERALOGY, TEXTURES & AGE RELATIONS 2115 122030 ' MASSIVE sulfideminerals, in decreasingabundance, are pyrite, SULFIDE MINES SAMPLED chalcopyrite, sphalerite, galena, tetrahedrite-ten- GOLINSKY nantite (hereafter referred to as tennantite), arseno- pyrite, pyrrhotite, bornite, idaite, and two unidenti- BIRD fied phases.The nonsulfideminerals, in decreasing HASTA KING abundance,are quartz, white mica, calcite, chlorite, ,KLALA and iron oxides. Digenite, bornite, covellite, and 4Oø45' KEYSTONE chalcocitecomprise the secondarysulfide minerals. MOUNTAIN The growthhabits, textures, and associations of the mineralscomprising the ore are alsobroadly similar BIOTITE QUARTZ DIORITE throughoutthe district,allowing the depositionaland metamorphichistories of the depositsto be divided ALBITE GRANITE into six parageneticstages (Fig. 2): (1) "primitive" pyrite + sphalerite;(2) pyrite ___arsenopyrite; (3) 40o30, • PALEOZOIC chalcopyrite + sphalerite + galena + tennantite SEDIMENTARY ROCKS + quartz + white mica; (4) recrystallizationand re- mobilization;(5) quartz + white mica + calcite;and BALAKLALA RHYOLITE (6) supergeneenrichment. These stages are described below. COPLEY GREENSTONE ß Stage1: "Primitive" pyrite + sphalerite 0 5 MILES MINES Mineralization in the West Shastadistrict appears FIC. 1. Locationmap of principalmines and the generalized to have begun with the precipitationof framboidal, geologicsetting of the West Shastadistrict. The Mule Mountain colloform,and very fine grainedpyrite, collectively stockis shownhere asalbite graniteand the ShastaBally batholith termed "primitive" pyrite after Barton (1978). asbiotite quartz diorite. Framboids,a largeproportion preserved as iron oxide pseudomorphs,have been observedin ore from the occurlocally beneath the massivelenses. The Balak- Mammothmine where they reach a maximumdiam- lalaRhyolite is intrudedby the EarlyDevonian Mule eter of 25 ttm. Colloformpyrite occursin ore from Mountain stock,which is itself intrudedby the Late the Mammoth and Keystonemines as aggregatesof JurassicShasta Bally batholith.The districthas un- curvilinearbands up to 2 mm in length (Fig. 3A) or dergoneregional greenschist facies metamorphism. asindividual spheroids 15 to 90 ttmin diameter.Some The ore is composedmainly of massivepyrite with spheroidshave been selectivelyreplaced by chalco- relativelyminor amounts of chalcopyrite,sphalerite, pyrite,sphalerite, and galena along concentric growth galena,and quartz. Compositional layering is present zones (Fig. 3B) whereasother spheroidshave been locally.The orehas been deformed and recrystallized completely replaced, leaving circular voidsentirely to varying degrees,compromising the preservation filled by later sulfides(termed here "bullet hole tex- of originaldepositional textures and resultingin a ture") (Fig. 3C). Porousand pitted aggregatesup to somewhatchaotic mixture ofbreccia fragments, crys- 4 mm in diameterof very fine grained(•10 ttm)sub- tal aggregates,and minor crosscutting veins. hedralto euhedralpyrite crystalsconstitute the most The ore graderanges from 2.0 to 6.0 percentCu abundantand widespreadprimitive pyrite. andfrom 1.3 to 8.9 percentZn. Accordingto Kinkel Similaraggregates of fine-grainedsphalerite, up to et al. (1956), the lack of zinc assaysat many of the 4 mm in diameterand locallyporous and pitted, also mines preventsthe determinationof Cu/Zn ratios. appearto be earlyformed or primitive.The temporal Minor amountsof goldand silver have been recovered relationshipbetween primitive pyrite and sphalerite from the massivesulfide ore and from overlying is not clear, but both phasespredate stage2. gossan. Mineralogy,Texture, and Paragenesis T•_BLE1. Locationand Number of SamplesExamined Singlypolished thick and thin sections,and doubly polishedthin sections,of 137 samplesfrom six mines Mine Number in the districtwere examined(see Table 1) usingre- Mammoth 10 flectedand transmitted light microscopes. A scanning ShastaKing 2 electronmicroscope and an electron microprobe were Early Bird 2 not availableduring this study. Balaklala 9 Keystone 1 The mineralogyof the ore is relativelysimple and Iron Mountain 113 consistentin all of the samplesexamined. The primary 2116 STEPHEN S. HOWE STAGE i 2 3 4 5 6 selectivereplacement of coresand growth zonesby MINERAL chalcopyrite(Fig. 3H), whereasother fragmentsare PYRITE ARSENOPYRITE extensivelyembayed (Fig. 4A). Chalcopyrite also PYRRHOTITE formsgashlike veins and fills V-shapedvoids in ag- BORNITE, IDAITE CHALCOPYRITE gregatesof primitive pyrite and sphaleritefrom the SPHALERITE GALENA Mammothmine during this episode of stage3. At the TENNANTITE Mammoth and ShastaKing mines,chalcopyrite re- UNKNOWN B QUARTZ places arsenopyrite euhedra formed in stage 2 WHITE MICA (Fig. 4B). CHLORITE CALCITE Minute, rounded,oval inclusions and thin stringers DIGENITE of pyrrhotite, bornRe,
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