Abstract

The >1-million-metric-ton Darwin Pb-Zn-Ag-W skarn deposit has been previously described as a group of sulfide replacement bodies zoned away from the Darwin quartz monzonite pluton and formed from magmatic fluids at approximately 325 degrees C. Detailed surface mapping and available radiometric data, however, indicate that the Pb-Zn skarn sulfide bodies are appreciably (>20 Ma) younger than the Darwin pluton, and underground mapping and core logging indicate there are several skarn sulfide pipes with strong concentric zoning. One of the pipes is zoned around a deep granite porphyry plug. The pipes exhibit outward zoning in wt percent Pb/Zn and oz/ton Ag/wt percent Pb (both ratios <0.5 core, >1.0 margin). The pipes show mineralogical zoning, with a core defined by higher sphalerite-galena, higher chalcopyrite, darker sphalerite, more abundant pyrite inclusions in sphalerite, and evidence for multiple sulfide depositional events. In contrast, both graphite in marble and pyrrhotite in sulfide ores are zoned around the Darwin pluton, which suggests that pyrrhotite stability is influenced by pre-Pb-Zn skarn (Darwin pluton related?) bleaching of marble beds. Garnet zoning is highly complex, with four generations identified by petrographic and compositional relations; younger garnet types are more abundant in upper and lateral parts of the pipe. Retrograde alteration of garnet is concentrated in the upper and laterally distal parts of the skarn, but garnet in apparent equilibrium with sulfide is present throughout the vertical extent of skarn. Systematic mineral compositional patterns include outward increase in hedenbergite + johannsenite components in clinopyroxene (<2->20 outward), increase in Sb + Bi contents of galena, initial increase followed by decrease in Mn contents of sphalerite (range from <0.2->1% Mn), and an initial increase followed by outward variable increase and decrease in FeS contents of sphalerite (range of <3->20% FeS). Previously published sulfur isotope data are compatible with a decrease in sulfur isotope ratios outward around the pipe core. Published isotopic data combined with temperature estimates from phase homogenization and arsenopyrite-sphalerite geothermometry show a systematic decrease in temperature from the skarn sulfide pipe center (>425 degrees C) to the margin (<300 degrees C).Comparison of stope maps to isotherm cross sections indicates that the bulk of mined sulfides were from areas surrounding the pipe core, in which temperatures declined from approximately 375 degrees to 300 degrees C (gradient of 1 degrees C/m). Combined mineral composition and assemblage and sulfur isotope systematics indicate that as the ore fluids flowed outward they underwent progressive decrease in oxidation state ( approximately 1 log unit) and increase in pH (2-3 units); upward-moving fluids underwent initial decrease in oxidation state and increase in pH followed by a reversal to higher oxidation state and lower pH. The process of ore deposition was chemically complex and may have involved remobilization of earlier deposited sulfides. Realistic ore depositional models at Darwin require simultaneous changes in (at least) temperature, pH, and oxidation state.

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