The Occurrence of Gold at the Getchell Mine, Nevada
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THE OCCURRENCE OF GOLD AT THE GETCHELL MINE, NEVADA. x PETER JORALEMON. ABSTRACT. The Getchellveins are lenticularreplacement bodies lying alongarcuate branchesof a complexrange-front fault system. The faultzone cuts all rocksof the districtand is tentativelydated as late Tertiary. The moreintensely mineralized portions of the depositform a shallow blanketwith rootsthat projectdownward into areasof sparsemineraliza- tion. The goldshoots are restricted to areasOf intense mineralization. Native gold and native silver are the only economicminerals. The greatbulk of thegolc• occurs in minutebut microscopically visible particles. Somegold may also occur in submicroscopicparticles and some may be in solid solutionin pyrite and carbon. The ore minerals,dissolved in alkali sulfidesolutions, are believedto havebeen deposited •vhen the sulfide/onconcentration in the hydrothermal liquiddecreased, making unstable the doublesulfides of gold;iron, and arsenic. The Getcheildeposit is similarin manyways to the Nevadaquicksilver depositsand present-dayhot-spring deposits. The Getcheilore occurrence may representa gradationfrom the commonepithermal gold depositto the cinnabar deposit. It is therefore placedclose to the feeblestend of the epithermalgroup. INTRODUCTION. In the past, a number of important gold mines have producedmillions of dollarsin gold from ore in which no gold couldbe seen. The Getchellgold depositis the.most recently discovered of thesemines of "invisible"gold. Be- causegold had not beenseen in the ore,either at Getcheilor at similardeposits, the problemof the state, or occurrence,of the gold has been subjectto con- siderablecontroversy. A prime purposeof the presentstudy has beento as- certain whether the modernmicroscopic and polished-sectiontechniques are capableof sheddingsome light on the problemsof gold occurrence. Location and ttistory.--The Getcheil mine is located in the northeastern cornerof the OsgoodRange in east-centralHumboldt County,Nevada (Fig. 1). It lies 26 milesnorth of the town of Golcondaand some15 milesnorth- west of Red House, the nearestrailroad station. The gold depositis in the old Potosi mining district but was itself not discovereduntil 1934. Between1934 and 1945, the depositwas extensively exploredby both surfaceand undergroundworkings. A 1,000-tonmill was erected,and in 1944 and 1945, the Getchellmine had becomethe largestgold producerin Nevada. In 1945,mining had nearlyexhausted the oxidizedore and it becameapparent that milling practicesin use were not suitablefor xUndertaken in part as a SamuelFranklin Emmons Memorial Fellowship award. 267 268 PETER ]ORALEMON. treating the sulfideore. Mining operationswere halted and an intensive metallurgicalresearch program was begun. In 1947, a pilot mill was started treating100 tons a day. By'thesummer of 1948a satisfactoryflow sheethad beendeveloped and constructionbegan on a 1,500-tonmill. Production,at presentlimited to 1,000 tonsa day, will be increasedupon the completionof the mill. Acknowledgments.--Thewriter is indebtedto the staff of the Getcheil mine,and particularlyto Mr. R. A. Hardy, for financialassistance and whole- heartedcooperation. G. S. Koch and R. S. Creelyably assistedin the two summers of field work. 114ø H I OHWAY •..,..o, •oo ......i --- RAILROAD FIG. 1. Index map of Nevada showinglocation of Getchellmine. The laboratoryportion of this studyhas beencarried on with the aid of a SamuelFranklin EmmonsMemorial Fellowship. The facultyof theDepartment of Geologyat HarvardUniversity has made valuablecriticism and given much'assistance in the laboratorywork. Pro- fessorL. C. Graton in particularhas donatedgenerously of his time and experience.All polishedsections were preparedby Mr. CharlesFletcher at theI-Iarvard Laboratory of MiningGeology. OCCURRENCE OF GOLD .4T GETCHELL MINE, NEV.4D.4. 269 GENERAL GEOLOGY. The Osgood;Range is apparentlya typicalGreat BasinRange. Its steep flanksare toppedby rollinguplands in thecenter of the rangeand it is bounded on at leastone sideby normalfaults that are responsiblefor the uplift of the range. The mountainsare composedof sedimentsof Paleozoicage that havebeen cut by a seriesof faultsinto isolatedstructural blocks (16). 2 The attitude of the rocksvaries widely from blockto block. The sedimentsare intruded by a large stock of Mesozoicgranodiorite and a seriesof related dacite porphyrydikes, and by smallTertiary andesiteporphyry dikes. Basaltflows anda smallbody of rhyolitetuff are productsof Tertiaryvolcanism. Irregular patchesof the tuff nearthe minehave been silicified and closelyresemble the opalitefound in certainNevada quicksilver deposits (3). •". •. •, "" '•'""C;?••-:½i':* .., •':'•.•.:-- .;.... •' ..: Fro. 2. Getchell Mine, Nevada. The scars on •e hill behind •e camp mark the footwalls of ½e open DescriptiveGeology oI the GetchellMine.•The beddedrocks of the mine area are interbeddedlim•ston• and carbonaceousargillit• (Fig. 5). Th• most stri•ng feature of the sedimentsis the extreme l•nticularity of the lim•ston• beds, as shown in Figure 3. Limestone formations s•v•ral hundred feet thick may drastically thin or pinch out completelyin a strike length of less than 1,•0 feet. The sedimentsare intrudedby the larg• granodioritestock, the noaheastern lobe of which is cut off by the fault zon• along which the gold ore bodies occur. Later andesiteporphyry dikes, trending more or less northerly, are apparentlylocalized in and n•ar this fault zone. A roughly conicalpipe of alteredrhyolite porphyry tuff li•s to the eastof this fault. • Numbers in parenthesesrefer to •ibliograpby at end of paper. 270 PETER JORALEMON. Structure.--The principalstructure of the sedimentsis a fold whoseaxis plungesat about 45ø northeast. Along the southernlimb of the fold, the sedimentsstrike northerly and dip moderatelyto the east, and to the north they are isoclinallyfolded and dip to the northeast. This structurehas been importantin localizingthe Getchellore bodies. The Getchell fault zone is the dominant structural feature of the mine area. It trendsin a northerlydirection and is composedof a persistent,footwall strand dipping moderatelyto the east, with steeper,arcuate hanging-wall branches(Figs. 3, 4). The footwall strand is shown by well-developed mullion and drag-foldstructures to be a rift fault, alongwhich the hanging- wall block movedan unknowndistance with respectto the footwall block (Fig. 6). Later normalmovement, indicated by steeplypitching slickensides engravedon the mullion faces, took place along this footwall strand. The steeper,hanging-wall branches were formed at that time and occur above flexuresin the footwallfault wherethe dip flattens,as shownin Figure 4. ORE DEPOSITS. GeneralFeatures.--The gold ore bodiesare sheet-likemasses that lie along the variousstrands of the Getchellfault zone. They extendat least7,000 feet horizontallyand 800 feet down the dip, and vary in width from a few feet to more than 200, averagingabout 40 feet wide. The vein pattern,extremely complexat the surface,becomes simpler with depthand, deeperthan 800 feet belowthe surface,all the branchveins apparently join to form a dingleper- sistentstructure, illustrated in Figure 4. Gan#ueMineraIs.--The veins consistof shearedand mineralizedargillite and limestonecut by quartz and calciteveins and containinga soft, plastic gumbothat has replacedthe sediments. The gumbo is the principal gold- bearer. Barite, gypsum,fluorite, and chabaziteare minorgangue minerals and occurin smallisolated pockets within the veins. The shapeof the moreheavily mineralizedportions of the veins is that of an irregular, relatively shallow blanketwith three root-like projectionsthat extenddownward into the region of sparsemineralization, as illustratedin the longitudinalprojection, Figure 3. The three "roots" of the ore body appearto mark the original major channel- ways along which the hydrothermalfluid passedin greatestabundance. There are striking differencesin the mineralizationwithin as contrasted with outsidethese major channelways. Within theseroots, now markedby high-gradegold ore, depositionhas beenlargely by non-selectivereplacement. In theseareas, all rocktypes and early gangueminerals have been replaced with equalease by the gangueminerals and the sulfidesof the later phasesof the ore-formingperiod. The larger part of the material depositedhere is ap- parently magmaticallyderived, and the processof depositionhas been one of wholesalereplacement. In the sparselymineralized stretches between the ore shoots,replacement has been limited and extremely selective,and all materialexcept small amounts of auriferouspyrite appearsto havebeen locally derived. In these outlying ar•easof the veins, mineralizationinvolved the local reworkingof rock mineralswith only minor introductionof material OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 271 272 PETER J'OR.,'ILEMON. 5300 + -t +Cd SECTION SECTION B'B' •9oo 5800 57oo 5600 5500 5400 5•tO0 SECTION G G' •'1'• Rhyolitutuff • Limestone• Unminerolizudfou$l :i:...:-•Andesite porphyry -•--'] Argillitu • Centoct f'G[•-•ß Gronodioritu • Goldvein --"• Grodotiono$coatoct 4• • 8• ' I• Feet FIG. 4. Geologiccross-section through Getcheil ore body. OCCURRENCE OF GOLD AT GETCHELL MINE, NEVADA. 273 foreign to the rocks. Thus. calcite veins are restricted to beds of limestone and quartz veinsand lensesoccur only in argillite. The gumbo,the importantgold-bearer, is unusualin that while it appears to be a fault gouge, it actually consistsof minute subhedralquartz crystals embeddedin a nearly submicroscopicintergrowth of quartz