Exploring the ancient volcanic and lacustrine environments of the Oligocene Creede caldera and environs, San Juan Mountains, Colorado


The 26.9-Ma Creede caldera is a large resurgent ignimbrite-subsidence structure that hosts world-class epithermal Ag-Pb-Zn deposits set within the San Juan locus of the Southern Rocky Mountain volcanic field (SRMVF). This field guide reviews the understanding of the SRMVF and central San Juan magmatic locus and then provides a summary of the Oligocene Creede Formation, the caldera moat fill, and ancient Lake Creede, which occupied the caldera for ∼100 k.y. after the calderaforming eruption. The summary of the volcanic setting and Creede Formation provides a framework for discussing the hydrothermal system that deposited Ag-Pb-Zn ores in the Creede district ∼1.8 m.y. after the caldera formed. This guide also highlights controversial aspects of the Lake Creede environment, such as the duration of the intracaldera sedimentation, proposed pseudomorphs after ikaite (CaCO3·6H2O) in the lake sediments and tufa, and the paleolimnology of ancient Lake Creede.

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GSA Field Guides