LAUSR

Abstract 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 ( 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.