Reconstruction of the fossil hydrothermal system at Lake City caldera, Colorado, U.S.A.: Constraints for caldera-hosted geothermal systems
Reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings. The 22.9 Ma Lake City caldera in Colorado, U.S.A., is well-exposed and contains an exhumed fossil shallow hydrothermal system. In this study, alteration mineralogy, vein textures and fluid inclusions are used to characterise the temperature and composition, spatiotemporal variability, and structural controls of the hydrothermal system. At paleo-depth equivalent between 1 and 2 km, the hydrothermal system was dominantly moderate temperature (up to ~290 °C), low salinity (<3% NaCl equivalent) and neutral to weakly acidic pH. There is evidence for boiling in veins throughout the exposed depth range; however, boiling textures are most common at the highest elevations of the system (~1150 m paleo-depth), and in structurally controlled fluid conduits. Quartz-illite alteration assemblages in the centre of the caldera indicate slightly more fluid-dominated conditions compared to those that formed propylitic alteration on the caldera margin. These alteration types reflect contrasting fluid pathways; a more pervasively fractured and faulted resurgent dome in the caldera centre, compared to fewer, larger conduits in the basement granite at the caldera margin. Based on the lack of high-temperature (<350 °C), hypersaline (>20 wt% NaCleq) fluid inclusions within the caldera centre, we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established. In contrast, in the eastern portion of the caldera, late distinct magma batches provided high temperature (up to ~540 °C) and hypersaline (up to ~65% NaCl eq.) magmatic fluid input above intrusions. Our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings. We also recognize the contrasting hydrothermal manifestations of a waning, degassed magma batch “left-over” from a caldera forming eruption, compared to fresh, volatile-rich magma.