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Geology and Sulfide Mineralogy of the Number One Orebody, Ruby Creek Copper Deposit, Alaska

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Geology and Sulfide Mineralogy of the Number One Orebody, Ruby Creek Copper Deposit, Alaska EconomicGeology Vol. 81, 1986, pp. 1675-1689 Geology and Sulfide Mineralogy of the Number One Orebody, Ruby Creek Copper Deposit, Alaska LAWRENCE R. BERNSTEIN* AND DENNIS P. COX U.S. GeologicalSurvey, 345 MiddlefieldRoad, Mail Stop984, MenloPark, California94025 Abstract The Number One orebodyis the largestand mostsulfide-rich ore zone (at least 200,000 tonscontaining 8.4 wt % Cu) at the Ruby Creek copperdeposit, in the southwesternBrooks Rangeof Alaska.Pyrite and copper-bearing sulfide minerals are concentratedwithin the matrix of a dolostonebreccia body, which is enclosedby phyllite andcalcic marble of Middle to Late Devonianage. The Number One orebodyhas three mineralogicalzones that gradeinto each other: (1) an outer zone,widest toward the hangingwall, containingmostly pyrite with minor amountsof chalcopyriteand traces of carrollitcand sphalerite;(2) an intermediatezone con- tainingmajor chalcopyriteand pyrite, minor tennantite-tetrahedrite,bornitc, carrollitc, and sphalerite,and tracesof galena;and (3) a core zone containingmajor bornitc, chalcopyrite, pyrite, and chalcocite,minor carrollitc, digenite, and sphalerite,and tracesof galena,covellite, and the germanium-bearingsulfides renierite and germanitc.Small clots of anthracitelike organicmaterial (anthraxolite) are foundthroughout the ore. Muchof the pyrite is fine grained andwas deposited before the othersulfides, being increasingly replaced by Cu-bearingsulfides from the outer zoneto the core. Someof thispyrite recrystallizedinto coarsergrains having cobaltiferousrims, and these grainswere generallynot replaced.The sulfideminerals are commonlypseudomorphous after lath-shaped crystals within the dolomiteclasts; no unreplaced exampleswere found,though the crystalsclosely resemble those of marcasite.The abundant fine-grained,porous pyrite alsomay represent replacement of marcasite.The originand timing of brecciationand ore depositionremain unknown, though dolomitization and ore deposition appearto have occurredin preexistingcarbonate breccia. Close mineralogicaland geologic similaritiesare noted with the Kipushi Cu-Zn-Pb depositin Zaire and with severalother carbonate-hostedcopper-rich organic-bearing deposits. The commonconcentration of the rare metalsCo, Ge, and Ga in thesestructurally and mineralogicallycomplex ores should increase their economic attractiveness. Introduction repeatedhere. Hitzman (1983, 1986) alsoprovided THE Ruby Creek copperdeposit, in the southwestern a reinterpretationof RunnelIs'sulfur isotopedata, BrooksRange of Alaska, consistsof sulfide-richdo- detailed analysesof anthraxoliteand other organic lostone breccia zones within Middle Devonian back- matter,an hypothesisconcerning the genesisof the reef faciescarbonate rocks. The ore consistspredom- deposit,and analogiesto other deposits. inantlyof copper-richsulfide minerals partly replac- Thisreport describes the geometry,internal struc- ing earlierfine-grained pyrite. Sulfides containing Zn, ture, and sulfidemineralogy of the largestand most Co, Pb, As, Sb, or Ge are presentin minor to trace sulfide-richorebody at RubyCreek, the NumberOne quantities.Anthracitelike organic material (anthrax- orebody.The surfaceexposure at the BergCamp area, olite)is foundsporadically distributed throughout the in the westernpart of the RubyCreek deposit, is also ore. Brecciation,apparently of more than one gen- described.The sequenceof crystallizationand im- erationand at scalesranging from millimetersto me- plicationsto the genesisof the ore arethen discussed. ters, greatly complicateszonal relationsof the min- Locationand Descriptionof Workings erals. The Ruby Creek depositis locatedabout 24 km The depositwas investigatedby Runnells(1963, 1969), who reportedthe resultsof geologic,miner- north of Kobuk village in the CosmosHills, Alaska alogical,and sulfurisotope studies. More recently, (67ø05'00"N, 156ø56'40"W), about350 km north- west of Fairbanks.Copper-bearing sulfide mineral- Hitzmanet al. (1982) andHitzman (1983, 1986) de- scribedthe geologyof the depositand surrounding izationcrops out at the originaldiscovery site at Berg areas in considerable detail; this material will not be Campand is also encountered in numerousdrill holes within an area of about1.5 kme (Fig. 1). In these sites, gradesof 1 to 3 wt percent Cu are common, * Present address:Center for Materials Research, Stanford with chalcopyritegenerally being the predominant University, Stanford, California 94305-4045. coppermineral. In a few drill holes,mainly within 0361-0128/86/609/1675-155•.50 1675 1676 L. R. BERNSTEINAND D. P. COX 4,780,000N Airstri ./•'-----•---- L --••_•e_• /Approximate limitof ( It 2 • 22 --'•x--'---'•/. diamonddrilling // F,gure2 /// 4,775,000N Coordinates in feet from Alaska StatePlane System, zone 6 0 250 500METERS 0 500 1000 1500 FEET FIG. ]. Locationmap of the Ruby Creek area. Shadedareas show the approximatesubsurface limits of bornite-chalcocitemineralization as indicated by diamonddrilling. the shadedareas shown in Figure 1, bornite-chalco- wt percentCu (C. T. Penney,unpub. Kennecott Corp. cite ore occursthat hashigher Cu grades. report, 1968). In 1965 the KennecottCorporation sank a shaftto Methods investigatea large high-gradeore zone encountered in the drill holes,which became known asthe Number Surfaceexposures at Ruby Creek, includingBerg One orebody.(In thisreport, we refer to thishigh- Camp, were examinedin 1979, 1980, and 1981. Un- gradezone as the "orebody"for simplicity.We do dergroundareas could not be examineddue to flood- notimply that this zone everywhere necessarily meets ing that occurredin 1967. Approximately12,000 ft any particulareconomic definition of ore.) A north- (3,700 m) of Ax diamonddrill core that penetrated trendingcrosscut at the 975-ft (300 m) level, aswell the Number One orebody and adjacent areas was asunderground drilling, servedto outlinethe dimen- loggedand sampled.From these data, crosssections sionsof the orebody.The orebodycontains a mini- were constructedthat were subsequentlyused to mum of 200,000 shorttons of chalcopyrite,bornitc, producegeologic plans at levels980, 1000, and 1020. andchalcocite ore with anaverage grade of about8.4 From the drill cores, 177 polishedsections were NUMBERONE OREBODY, RUBY CREEK DEPOSIT 1677 prepared.These sections were observedmicroscop- weatheringdolostone and less abundant dark gray, icallyin reflectedand transmitted light; minerals and fenestral, fossiliferousdolostone. These resemble textures were identified and recorded and the pro- Hitzman's(1983, 1986)hydrothermal and diagenetic portionsof the sulfideminerals were estimated.Ap- dolostones(A andB), respectively.The lightgray do- proximately20 polishedsections were examinedon lostoneis cutby numeroussparry dolomite veins that a CambridgeStereoscan scanning electron micro- terminateat the edgesof clasts(Fig. 3). Most clasts scope(SEM) equippedwith an energydispersive X- rangefrom 3 to 10 cm across,with clastslarger than ray elementalanalyzer at the U.S. GeologicalSurvey 50 cm acrossconstituting 18 percentof the mapped in Menlo Park, California. The latter work assistedin area. Calculationsshow that in any linear measure mineral identificationand in determiningthe distri- throughthe breccia,such as a diamonddrill core, bution of elementswithin mineral grains.Approxi- over50 percentof the sample would likely be in clasts mately15 polishedsections were analyzedon anARL- 1 to 3.3 m across. SEMQ electronmicroprobe at the U.S. Geological The matrix of the brecciaat Berg Camp is partly Surveyin Menlo Park; the operatingconditions and weathered,making its detailedcomposition difficult standardsare givenin Tables2 and3, below.Powder to determine.In the northwestpart of the exposure, X-ray diffractionwas used to identifyseveral minerals. the matrixis composedof fine-grainedto sparrydo- Semiquantitative emission spectroscopy was per- lomite that shows little contrast with the dolostone formedby ChrisHeropoulos of the U.S. Geological clasts.Within this area, breccia clastsare locally sur- Surveyin MenloPark on severalhand-picked sulfide, roundedby areasof red-brownsoil that is probably carbonate,and anthraxolitesamples of about 50 mg derivedfrom the weatheringof pyritic,iron-rich do- each.Additional types of datawere collectedon ren- lomitic matrix material. A continuous band of this soil ierite fromthis deposit; these are presented by Bern- lies between the breccia and the adjacentphyllite. stein (1986b). To the southeastand south,pyrite is increasingly abundantin the matrix,giving the brecciaa darker Geology color(Fig. 4). Chalcopyriteand borniteare alsoin- Copperdeposits at RubyCreek lie within the Bor- creasinglyabundant in the matrixto the southeast. nite Carbonatesequence of Hitzman(1983), a 1,000- Somewhite sparrydolomite veins cut the matrixin m-thick unit of Middle Devonianage composedof places. marble,dolostone, and minor graphiticto calcareous A gamma-rayscintillometer was used to measure phyllites. These rocks were metamorphosedto radioactivityin the areaof Figure2, to searchfor the greenschistfacies during the Cretaceous,with the presenceof pitchblendeor other uraniumminerals marbleand phyllite being penetrativelydeformed. notedby RunnelIs(1969). Radiationlevels of three The dolostoneacted competently during deformation to ten timesthe backgroundlevel were foundto cor- and blocksof dolostoneup to severalhundreds of respondto the areasof red-brownsoil notedabove. metersin size are now enclosedin marble and phyl- No correlationwas foundbetween radioactivityand lite. The origin of brecciain
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