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INTRODUCTION marcasite (another form of iron sulfidey, sphalerite (zinc sulfide), galena (lead sulfide), and chalcopyrite (copper iron sulfide) in the earlier breccia. The intermediate stage involved the replacement and veining of early-stage sulfides by tennantite (a complex silver-copper-zinc-iron arsenosulfide), sphalerite, galena, chalcopyrite, pearceite (silver copper arsenosulfide), acanthite (silver sulfide), and native silver. The final stage of primary mineralization resulted in the replacement of the younger breccia matrix by galena, sphalerite and pyrite. Geochemical and radiological studies indicate that the sulfides are the result of the reduction of evaporitic sulfur (as sulfate) of the Belden Formation and the introduction of igneous sulfur of Laramide to mid-Tertiary age. The Aspen mountain sill is hydrothermally altered throughout the Aspen district to the extent that no sample of "fresh" rock has ever been found. The altered sill contains anomalously high amounts of the principal ore components, barium (Ba), lead (Pb), zinc (Zn), and silver (Ag). The Aspen district ore deposits apparently formed in an evolving magmatic hydrothermal system. Original rising hydrothermal fluid evolved from barium to base metal (Pb, Zn) rich, and ascended along the fractures and faults of the De11a-Smuggler fault system. The rising hydrothermal fluids mixed with surface- derived or formation waters containing sulfates derived from gypsum and anhydrite in the Belden Formation, particularly in the open spaces of the (early) breccias and other solution cavities in the carbonates of the sedimentary sequence, most abundant by far in the Leadville Limestone. Continued chemical changes in the upwelling fluid to enrichment in silver and sulfide resulted in the deposition of silver sulfides and sulfosalts and the contribution of igneous sulfur to the deposits. Late-stage upwelling magmatic fluids dissolved large openings above the sedimentary (weathered) breccia between the Leadville (Red Cliff Dolomite) and the Belden Formation, which collapsed to forma separate and distinct breccia, the matrix of which was mineralized during the final stage as temperatures were decreasing, probably in a similar manner as the early stage ores. Available evidence suggests that all significant mineralization took place after the beginning of the Laramide mountain-building period {about 75 million years ago), but, from geologic and geochemical studies, apparently before 55 million years ago. By far the richest silver deposits in the Aspen district in general and the Smuggler mine in particular were found in the "supergene enrichment zone," those areas above the stagnant water BIUC@ A. CO1130B - XXV - SMUGGLER BIBLIOGRAPHY