Abstract

Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase CO2-bearing fluid containing ∼2–12 wt% NaClequiv. Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38–50 wt% NaClequiv) and vapor-rich inclusions trapped together at temperatures of 560–350 °C and pressures of 550–140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36–46 wt% NaClequiv) and vapor-rich inclusions trapped at 380–330 °C and at 160–120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor ∼500 m below the base of the orebody at a paleodepth of ∼2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400 °C was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wall-rock alteration style.

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