U-Pb isochron age and Pb isotope systematics of the Golden Fleece vein; implications for the relationship of mineralization to the Lake City caldera, western San Juan Mountains, Colorado

A U-Pb isochron age of 27.5 + or - 0.5 m.y. has been determined for the Golden Fleece vein, southwestern Colorado, which contains an assemblage of pitchblende and gold-silver tellurides unique within the Lake City area. Initial Pb isotope ratios of ore samples from the vein ( ²⁰⁶ Pb/ ²⁰⁴ Pb = 19.08-19.58; ²⁰⁷ Pb/ ²⁰⁴ Pb = 15.643-15.682; ²⁰⁸ Pb/ ²⁰⁴ Pb = 38.533-38.84) are among the most radiogenic values reported from the San Juan Mountains and suggest that a significant component of common Pb, and possibly other metals, was derived from upper crustal Precambrian rocks by deeply circulating hydrothermal fluids. The 27.5-m.y. age of the Golden Fleece vein is identical, within analytical uncertainty, to the age of the quartz latite lavas that it cuts. This clearly documents the relationship of the vein to the waning stages of activity associated with the Uncompahgre caldera (27-29 m.y.) rather than to early activity related to the 23.1-m.y.-old Lake City caldera. Other veins in the Lake City area have Pb isotope ratios and mineral assemblages that are distinctly different from the Golden Fleece, and most of these veins are thought to be genetically related to the younger Lake City caldera.