Polymetallic vein formation through fluid flashing at the Sunnyside intermediate-sulfidation epithermal deposit, Colorado, USA

Sunnyside is a well-preserved Miocene polymetallic vein deposit located in the Western San Juan Mountains of Colorado, USA. The steeply dipping veins extend vertically for ~ 600 m and can be traced laterally over a combined length of ~ 2100 m. Fracture-controlled fluid flow dominated during the pre-ore stage. Subsequent ore deposition along major extensional structures took place at far-from-equilibrium conditions resulting in the formation of ore mineral dendrites in a silica matrix that was originally noncrystalline. Recrystallization of the noncrystalline silica to quartz caused extensive microtextural modification of the veins during and after the ore-stage. Microtextural evidence suggests that essentially all quartz in the ore-stage veins originated from a noncrystalline silica precursor. The deposition of ore mineral dendrites and noncrystalline silica is interpreted to have occurred during repeated fluid flashing events over the lifetime of the hydrothermal system. A period of quasi steady-state fluid flow occurred during the post-ore stage resulting in the formation of gangue minerals in open spaces in the veins. Fluid inclusion evidence suggests that the veins at Sunnyside formed at the transition between the epithermal and porphyry environments at ~ 1300–1900 m below the paleowater table at temperatures ranging up to ~ 345 °C.