Abstract

The Uncompahgre caldera, and the Lake City caldera nested within it, each have fossil hydrothermal systems and associated mineral deposits that formed during multiple episodes of mineralization during Oligocene and Miocene time. New lead isotopic analyses for 51 ore samples, mainly galena, combined with previously obtained data for ore minerals and rocks, suggest likely lead source rocks and fluid-migration paths. Most values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in galena from veins match the respective lead isotopic ratios for their host rocks; for example, all have 206Pb/204Pb typically in the range 18.4-19.0. The source for most vein lead is therefore probably the host unit for the vein. Some mixing of lead from other sources also occurred. Ratios of 206Pb/204Pb > 19.0 probably indicate a component of more-radiogenic lead from a Middle Proterozoic source. Lower lead isotopic ratios, 206Pb/204Pb < 18.5, suggest mixing with less-radiogenic lead from either Miocene rhyolitic volcanic and intrusive rocks or early Oligocene intermediate-composition volcanic rocks. Hydrothermal flow in the Uncompahgre caldera was predominantly west to east down the topographic slope. Discharge was mainly in closed topographic lows marked by lacustrine deposits. Hydrothermal circulation in the Lake City caldera was controlled by local topography and post-caldera intrusions and was isolated from flow in the Uncompahgre caldera and Eureka graben. Richer ore deposits may be associated with ring fault-related conduits that extend through the volcanic cover to more-radiogenic Middle Proterozoic basement at depth. As in the rest of the San Juan Mountains, lead originally came from a predominantly ∼1450 Ma source. Enough variation in 207Pb/204Pb was produced by orogenic events at ca. 1450 Ma, ca. 1760 Ma, and earlier to explain most of the 207Pb/204Pb variation present today in Tertiary volcanic rocks and hydrothermal veins.

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