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Abstract

In Part I, we argue that some mahto orebodies in the Leadville district are mid-Tertiary modifications of preexisting mineralized paleokarst cave systems. Some of the ores in these mantos are geologically, mineralogically, and geochemically distinct from ores of the main Leadville district, are Late Mississippian in age, and are unrelated to mid-Tertiary magmatism. These older paleokarst cave-hosted ores are termed “Sherman-type” ores. They are best observed in the Sherman mine east of the main district, although they are regionally dispersed throughout the Leadville Dolomite and are not localized near mid-Tertiary intrusive centers. Sherman-type ores are contemporaneous with cave development, sediment filling, and brec-ciation during a Mississippian karst event. The morphology of orebodies is fully to partially integrated cave systems. Orebodies preserve relict authigenic cave breccia, ore minerals as detrital sediments, Molas Formation clays, and huntite all in cave fill. We continue, in Part II, with evidence from fluid inclusion, stable isotope, and geochemical studies, to evaluate the sources of fluids and metal components in Sherman-type ores.

Fluid inclusion temperature and salinity data, stable isotope data from carbonates, quartz, sulfides, and barite, and metals geochemistry all show that a regionally extensive moderate temperature (150°-200°C) but high salinity Ca-Mg-Na brine was present during ore formation in Late Mississippian caves within the Leadville Dolomite. Cl/Br and Na/K inclusion fluid chemistry suggest that this regional brine was evaporated seawater, possibly remaining in the Paleozoic section after isolation of the Mississippian seaway trough to the east of the ancestral Sawatch uplift. Locally this regional brine underwent deep circulation along major basement fractures, was heated to>300°C, diluted with entrained surface meteoric water that reduced its salinity, and returned to the regional brine reservoir. Sulfate sulfur from dissolution of evaporites in pre-Mississippian rocks (δ34SσS ≈ 14‰) was quantitatively reduced during deep circulation of brine in the Frecambrian rocks. The deep circulating brine scavenged metals from Precambrian rocks, including radiogenic J-type lead, silver, zinc, and copper, and returned them to the regional brine.

Near upwelling centers of hot deep circulating brine, the host Leadville Dolomite dolostone contains elevated background levels of Ag, Pb, Zn, and Cu. Proximal to upwelling centers, the Leadville Dolomite is more enriched in lead and zinc fixed as carbonates and sulfides. With lateral regional brine flow and loss of lead and zinc from the fluids, rocks exposed to the brine became relatively enriched in silver and copper sulfides. Fluid inclusion, stable isotope, and rock geochemistry data and unique mineralogy show that a sharply defined interface existed between the high density regional brine and the overlying cold dilute surface and ground waters. This interface shifted through time in response to changing piezometric base levels and karst cave advancement and with high-angle faulting in the Mississippian trough. Mixing pulses of either the regional brine (≈high silver ores) or the deep circulating brine (≈lead- and zinc-dominant ores) with cold water in phreatic cave passages produced Sherman-type ores. Mixing of brines with surface waters within caves was controlled by the local karst hydrology within individual cave systems.

Caves are within the upper 8 to 15 m of the Late Mississippian paleosurface, indicating that the brine interface and fluid mixing in caves occurred at very shallow depths. Waters in phreatic caves were at times ambient, very dilute cold surface waters, at other times mixed ore flnids that attained 100° to 120°C. Rocks and contained brine fluids below the brine interface reached at least 200°C, and fluids near centers of upwelling were ≈300°C. Necessary conditions for Sherman-type ore formation include: (I) mixing of a regional brine (having sufficient lateral extent to provide recharge to a deep penetrating fracture system) with an upwelling brine below highly permeable karst cave systems in reactive carbonate rocks, (2) a sharp fluid interface with significant chemical contrasts between surface waters in phreatic caves and underlying evolved brines, and (3) injection of evolved brines into surface waters in the caves during cave development and sedimentation. We view Sherman-type mineralization as a variant of Mississippi Valley-type mineralization in a tectonically active area and on a much reduced scale.

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