Copper deposition during quartz dissolution by cooling magmatic-hydrothermal fluids: The Bingham porphyry

Textures visible in cathodoluminescence consistently show that the bulk of vein quartz (Q1), characterized by bright luminescence, crystallized early in the vein history. Cu-Fe-sulfides are precipitated later in these veins, in a microfracture network finally filled with a second generation of dull-luminescing Q2 quartz. Mapping of brine and vapor inclusion assemblages in these successive quartz generations in combination with LA-ICPMS microanalysis shows that the fluids trapped before and after Cu-Fe-sulfide precipitation are very similar with respect to their major and minor-element composition, except for copper. Copper concentrations in inclusions associated with ore formation drop by two orders of magnitude, in a tight pressure-temperature interval between 21 and 14 MPa and 425-350 °C, several hundred degrees below the temperature of fluid exsolution from the magma. Copper deposition occurs within a limited P-T region, in which sulfide solubility shows strong normal temperature dependence while quartz solubility is retrograde. This permits copper sulfide deposition while secondary vein permeability is generated by quartz dissolution. The brittle-to-ductile transition of the quartz-feldspar-rich host rocks occurs in the same temperature range, which further enhances vein reactivation and promotes cooling and expansion of fluids ascending across the transition from lithostatic to hydrostatic conditions.