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Abstract

In order to interpret the origin of the ores in the Aspen district (central Colorado), a mul-tidisciplinary analysis of the deposits in one representative mine, the Smuggler, was completed. The Smuggler orebody is hosted by an unconformity-related sedimentary breccia (breccia I) at the top of the Red Cliff Member of the Mississippian Leadville Limestone. Lesser amounts of ore are present in the matrix of a younger solution collapse breccia (breccia 11) and in unbrecciated calcareous shale of the Belden Formation, which immediately overlies the Leadville. Breccia II contains clasts of porphyry derived from the Laramide-aged Aspen Mountain sill. A set of high-angle faults (the Delia-Smuggler fault system) which cut the Aspen Mountain sill are mineralized, and the Smuggler orebody is thickest at the intersections of these faults and breccia I.

The mineralization event can be subdivided using mineral paragenesis into three principal hypogene stages. The early stage is characterized by open-space growth of barite, pyrite, marcasite, sphalerite, galena, and chalcopyrite in breccia 1. The intermediate stage is characterized by replacement and veining of the early-stage sulfides by tennantite, sphalerite, galena, chalcopyrite, pearceite, acanthite, and native silver. Late-stage galena, sphalerite, and pyrite replace the matrix of breccia II.

The early-stage ores have fluid inclusions which homogenize at 252° to 314°C (barite) and 237° to 268°C (sphalerite). The sulfide-sulfate isotopic fractionation in the early-stage minerals (barite: δ34S = 12.3-14.2‰; py = -11.4 to -11.6‰; cpy = -11.9 to -12.1%,; gal = -13.6 to — 13.9‰) indicates a similar temperature of 230° to 270°C. This sulfur is interpreted to have been derived by partial reduction of evaporitic sulfate in the Belden Formation. Barite has a fi180 value of 10.2 to 13,4 per mil, indicating a δI8Oflutd value of 6 to 9 per mil. The δD values of bulk fluid inclusion waters in barite (—112 to — 125‰) and early-stage galena (—105 -to — 108‰) represent minimum values for δD of the ore fluid. Tennantite and pearceite of the intermediate-stage ores contain fluid inclusions which homogenize at 250° to 261 °C. The δ34S values of tennantite (0.5‰), pearceite (—3.9‰), and intermediate-stage galeua (—3,4 to —9.4‰) suggest sulfur derivation from both igneous and sedimentary sources. Lead isotope ratios in most samples of early- and intermediate-stage ore are identical to those of Laramide to mid-Tertiary igneons rocks in central Colorado, although one ore sample shows evidence for a more radiogenic lead source. Galena in the late-stage ores has a δ34S valne of— 22.3 to —27.8 per mil. This sulfur most likely originated by reduction of sedimentary sulfate at a temperature of about 140° to 180°C.

The Aspen Mountain sill is hydrothermally altered throughout the Aspen district, including around the Smuggler orebody. The principal alteration assemblage in the Smuggler mine consists of quartz-mica-kaolinite-pyrite. Most of these rocks have a whole-rock δ18O value of 12′.9 t6’ 14.2 per mil and a δD value of-79 to -99 per mil (δ18Ofluid = 6-7‰, δDnilId = -50 to —70‰), and disseminated pyrite has a δ34S value of 0.9-1.7 per mil. These isotopic compositions indicate the passage of magmatic water. The altered porphyry is highly anomalous in Ba, Pb, Zn, and Ag, the principal ore components.

These geologic and geochemical relationships imply that the deposits in the Aspen district formed in an evolving magmatic hydrothermal system. The initial upwelling hydrothermal fluid evolved from barium to base metal rich and ascended along the Delia-Smuggler fault system. This fluid mixed with locally derived meteoric or formational water containing sulfate derived from gypsum and anhydrite in the Belden Formation. Fluid mixing took place in openings in preexisting sedimentary breccias in the Leadville Limestone and resulted in open space-textured barite and then sulfide deposition. Chemical evolution of the upwelling fluid to a composition rich in Ag and containing S−2 resulted in deposition of silver sulfosalts and the contribution of igneous sulfur to the deposit. Late-stage hydrothermal fluids produced large openings above breccia I, which collapsed to form breccia II. Final mineralization in the matrix of breccia II took place as the temperature was decreasing, probably by a similar mixing mechanism as the early-stage ores. Mineralization took place after the Laramide (kbout 70 Ma), and based on fission-track data presented in Bryant et al. (1990), possibly before 55 Ma.

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