EconomicGeology Vol. 86, 1991, pp. 960-982 Zoning and Genesisof the Darwin Pb-Zn-Ag SkarnDeposit, California: A ReinterpretationBased on New Data RAINER J. NEWSERR¾, Departmentof Geology,University of Alaska,Fairbanks, Alaska 99775 MARCO T. EINAUDI, Department of Applied Earth Sciences,Stanford University,Stanford, California 94035 AND HARVEY S. EASTMAN Bond Gold Exploration,4600 S. Ulster Street,•400, Denver, Colorado80237 Abstract The > 1-million-metric-tonDarwin Pb-Zn-Ag-Wskarn deposit has been previously described as a group of sulfidereplacement bodies zoned away from the Darwin quartz monzonite plutonand formed from magmatic fluids at •325øC. Detailedsurface mapping and available radiometricdata, however,indicate that the Pb-Zn skarnsulfide bodies are appreciably(>20 Ma) youngerthan the Darwin pluton, and undergroundmapping and core loggingindicate there are severalskarn sulfide pipes with strongconcentric zoning. One of the pipesis zoned arounda deepgranite porphyry plug. The pipesexhibit outward zoning in wt percentPb/Zn and oz/ton Ag/wt percentPb (both ratios<0.5 core, >1.0 margin).The pipesshow minera- logicalzoning, with a core definedby higher sphalerite-galena,higher chalcopyrite,darker sphalerite,more abundantpyrite inclusionsin sphalerite,and evidencefor multiple sulfide depositionalevents. In contrast,both graphitein marble and pyrrhotite in sulfideores are zonedaround the Darwinpluton, which suggests that pyrrhotitestability is influencedby pre- Pb-Zn skarn(Darwin pluton related?) bleaching of marblebeds. Garnet zoning is highlycom- plex, with four generationsidentified by petrographicand compositionalrelations; younger garnettypes are moreabundant in upperand lateral parts of the pipe. Retrogradealteration of garnet is concentratedin the upper and laterally distalparts of the skarn,but garnet in apparentequilibrium with sulfideis present throughout the verticalextent of skarn.Systematic mineral compositionalpatterns include outward increase in hedenbergite + johannsenite componentsin clinopyroxene(<2->20 outward),increase in Sb-I- Bi contentsof galena,initial increasefollowed by decreasein Mn contentsof sphalerite(range from <0.2->1% Mn), and an initial increasefollowed by outwardvariable increase and decreasein FeS contentsof sphalerite(range of <3->20% FeS).Previously published sulfur isotope data are compatible with a decreasein sulfurisotope ratios outward around the pipe core.Published isotopic data combinedwith temperatureestimates from phase homogenization and arsenopyrite-sphalerite geothermometryshow a systematicdecrease in temperaturefrom the skarnsulfide pipe center (>425øC) to the margin(<300øC). Comparisonof stopemaps to isothermcross sections indicates that the bulkof minedsulfides were from areassurrounding the pipe core,in whichtemperatures declined from approximately 375 ø to 300øC (gradient of løC/m). Combined mineral compositionand assemblageand sulfurisotope systematics indicate that asthe ore fluidsflowed outward they underwentpro- gressivedecrease in oxidationstate ('•'1 log unit) and increasein pH (2-3 units);upward- movingfluids underwent initial decreasein oxidationstate and increase in pH followedby a reversalto higheroxidation state and lower pH. The processof ore depositionwas chemically complexand may have involvedremobilization of earlier depositedsulfides. Realistic ore depositionalmodels at Darwin require simultaneouschanges in (at least)temperature, pH, and oxidation state. Introduction which wasdirect-shipping, high-grade material, and 0.1 milliontons of W ore. Averagegrades of sulfide THE DARWINdistrict, in southeastCalifornia (Fig. 1), ore mined after 1945 were approximately6 percent containsPb-Zn-Ag as well as Cu- and W-rich veins Pb, 6 percentZn, 0.2 percentCu, and 6 oz/ton Ag and skarns(Hall and MacKevett, 1962; Newberry, (Hall and MacKevett, 1962). The Darwin mine per 1987). Production from the district includes more se consistsof a seriesof isolatedPb-Zn-Ag _ W ore- than 1 million metric tonsof Ag-Pb-Znore, muchof bodies(Defiance, Thompson, Essex, Independence; 036!-0128/91/1248/960-2353.00 960 DARWINPb-Zn-Ag SKARN DEPOSIT 961 KEY --]Tert-Quaternarysediments .:•Granite porphyry,aplite, pegmatite ?• 'Coso-type'granite •tzß rnonzonite- qtzsyenite !-.'•Diorite •/•, -qtz monzonite -•Penn-Permiancarbonate turbidites ii!..i•Devono-Mis sippian carbonates •t•fm•t• thrtmtfm•t • fold •xi• Mineralization ekam•,.I- vein• veinsc• .Pb-Zn-Ag . X Davis thrusl system Cu_+Au n ß w o ß c•,/r,ovearm• • • Darwin I = Defiance ae*le'N+ townsite 2 = Essex 117ø36 ' W O• 0..5i 1I kilometer8 FIG. 1. Generalizedgeologic map of the Darwin district,showing distribution of variousore types. The Darwin mineconsists of severalorebodies located along the westmargin of the Darwin stock;two of the majororebodies are the Essex(1) andDefiance (2). Modifiedfrom Newberry(1987) andStone et al. (1989). Fig. 2) whichwere linked by a commonhaulage level enization(Czamanske and Hall, 1975), and fluid in- in 1945. clusionsystematics (Rye et al., 1974). Basedon their Althougha relativelysmall deposit, Darwin is no- study,Rye et al. (1974) concludedthat the Pb-Zn-Ag table for a pioneeringS isotopestudy (Rye et al., sulfidestage of mineralizationoccurred at tempera- 1974) in whichthe authorsintegrated light stable iso- turesof 325 ø _ 55øC andinvolved magmatic hydro- tope analyseswith the knowngeology; and for com- thermal fluidsflowing out of the magmawhich pro- parativeattempts at determiningmineral formation ducedthe adjacentDarwin stock.These authors noted temperaturesby techniquessuch as minor element horizontalzoning of S isotopevalues away from the distribution(Hall et al., 1971), sulfidephase homog- Darwin stockand concluded that ore depositiontook 962 NEWBERRY,EINAUDI, AND EASTMAN EXPLANATION •[] GRANODIORITECOSO r'•GRANITEPEGMATITE, PORPHYRY APLITE '•GRANITE MATRIX BRECClAPORPHYRY- •_• • SKARN/ENDOSKARN--MATRIX BRECCIA • ,,=.•MONZONITEDARWIN QUARTZ & ,'• DARWINMONZODIORITEQUARTZ & QUARTZ MONZON L[• QUARTZDIORITE PORPHYRYSYENITE Z •. • UNDIFFERENTIATEDHORNFELS MAR.LE ,,Z," .• CHERTYLIMESTONEARO •1 &GRAPHITIC LIME i_. SULFIDE-QUARTZ- ,• CALCITE_.+SK• • FAULT • THRUSTFAULT-- SAWTEETH ON UPPER PLATE ._....-- CONTACT FIG. 2. Detailed geologicmap of the Darwin mine area, basedon unpublishedmapping by R. Newberry and T. Sisson(1980-1982). Note the Pb-Zn-Agskarn veins that cut and displacethe Darwin pluton. placeprimarily due to pH increaseat constantfoe and mineralized and virtually unaltered (Hall and temperature,caused by fluid reactionwith carbonate MacKevett, 1962; Newberry, 1987), which may be and calc-silicate rocks. incompatiblewith its beingthe sourceof hydrother- Althoughwidely quoted in the isotopicliterature, mal fluids;and (3) the associationof ore-gradeW with there are severalfeatures of the Darwinstudy as pre- ore-gradePb-Zn-Ag is uncommonin limestonere- sentedby Rye et al. (1974) whichinvite scrutiny: (1) placementand/or skarn deposits (e.g., Einaudiet al., the Darwin quartz monzodiorite,with probablesol- 1981), raisingthe possibilityof multiple, unrelated idustemperatures > 800øC (Piwinskii,1973; New- hydrothermalevents. There alsoare problemswith berry, 1987), is located<50 m from the sulfidede- the relative timing of plutonismand Pb-Zn-Ag ore posits,but if thisstock was the sourceof hydrothermal deposition;the 174-Ma (U-Pb; Chen, 1977) Darwin fluids, then temperaturesof sulfide ore formation stockis cut by the Davisthrust (Eastman, 1980; New- could be expectedto range up to severalhundred berry, 1987), with movementdated at 154 to 148 degreeshigher than 325 ø +_55øC and skarn-related Ma (Dunne et al., 1978), but Pb-Zn-Agores cut the ores shouldbe present; (2) the Darwin stockis un- thrust and hence postdate thrusting (Hall and DARWINPb-Zn-Ag SKARN DEPOSIT 963 MacKevett,1962; Eastman,1980) andmust be at least (Sylvester et al., 1978). Contact metamorphicre- 20 m.y. youngerthan the Darwin stock. crystallizationof impurecarbonate rocks formed ido- Newberry (1987) resolvedsome of theseapparent crase-wollastonitecalc-silicate hornfels, garnet-rich conflictsby showingthat there are severaldifferent skarnoid, and bleached marble around the Darwin plutonicsuites associated with differentmetasomatic stock (Fig. 2; Hall and MacKevett, 1962; Eastman, skarnsuites in the Darwin district. Newberry (1987) 1980; Newberry, 1987). Tungsten-bearingskarns are further showedthat the contact-typeW skarnsare locally presentalong contactsof the more differen- geneticallyrelated to the Darwin stockand that the tiated unitsof the Darwin stockwith the surrounding Pb-Zn-Ag veins and skarnsare related to younger carbonate-bearingrocks and are distributedsymmet- quartzporphyry bodies. Supporting geologic data in- rically aroundthe Darwin stock(e.g., Fig. 2; New- cludethe presenceofPb-Zn skarnsalong faults which berry, 1987). Thrustingalong the Davis fault system cut acrossand displacethe Darwin stock(Figs. i and (Fig. 1) took place at 154 to 148 Ma (Dunne et al., 2) and graniteporphyry and brecciabodies (Fig. 2) 1978) and resultedin 1- to 3-km eastwarddisplace- which intrude the Darwin stock and contain clasts of ment of the upper plate,juxtaposing the Cosobatho- W-bearingbut Pb-Zn-poorskarn. Newberry (1987) lith andits adjacentCu skarnswith the Darwin pluton suggestedthat the Pb-Zn-Ag-Wore associationmay and its adjacentW skarns(Newberry, 1987). Subse- have resultedfrom superimpositionof Pb-Zn skarns quent to thrusting,a seriesof graniteporphyry dikes on older W skarns. and brecciapipes (Fig. 2) intrudedthe metamorphic