Temporal and spatial distribution of alteration, mineralization and fluid inclusions in the transitional high-sulfidation epithermal–porphyry copper system at Red Mountain, Arizona

Abstract

Red Mountain, Arizona, is a Laramide porphyry Cu system (PCD) that has experienced only a modest level of erosion compared to most other similar deposits in the southwestern United States. As a result, the upper portion of the magmatic–hydrothermal system, which represents the transition from shallower high-sulfidation epithermal mineralization to deeper porphyry Cu mineralization, is well preserved.Within the Red Mountain system, alteration, mineralization and fluid inclusion assemblages show a systematic distribution in both time and space. Early-potassic alteration (characterized by the minerals biotite and magnetite) is paragenetically earlier than late-potassic alteration (K-feldspar–anhydrite) and both are followed by later phyllic (sericite–pyrite) alteration. Advanced argillic alteration (pyrophyllite–alunite–other clay minerals) is thought to be coeval with or postdate phyllic alteration. Minerals characteristic of advanced argillic alteration are present in the near surface. Phyllic alteration extends to greater depths compared to advanced argillic alteration. Early-potassic and late-potassic alteration are only observed in the deepest part of the system. Considerable overlap of phyllic alteration with both early-potassic and late-potassic alteration zones is observed.The hypogene mineralization contains 0.4–1.2% Cu and is spatially and temporally related to the late-potassic alteration event. Molybdenum concentration is typically <300ppm but positive anomalies (between 600 and 1200ppm) occur, and typically correlate with the zones of higher Cu grades. Silver and Au range up to 50ppm and 1ppm, respectively, and mostly occur in the deeper parts of the system. Individual assays of up to 18ppm Au and 274ppm Ag in the shallower part of the system are interpreted to be associated with areas of highly focused fluid flow (i.e., breccias and thick veins). A near-surface, discontinuous chalcocite blanket is represented by scattered Cu anomalies within the mixed oxide/sulfide zone and its discontinuous nature may reflect differential permeability along fractures and faults.In the deepest part of the system, an early generation of low-to-moderate density and salinity liquid+vapor inclusions with opaque daughter minerals is followed in time by halite-bearing inclusions that also contain opaque daughter minerals indicating that an early intermediate-density magmatic fluid evolved to a high-density, high-salinity mineralizing fluid. The increase in density and salinity of fluids with time observed in the deeper parts of the system may be the result of immiscibility (“boiling”) of the earlier magmatic fluids or may reflect the compositional evolution of fluids that exsolved from the magma. Trails of inclusions consisting of only vapor-rich inclusions are common in the shallow parts of the system, and are associated with advanced argillic alteration, suggesting that intense boiling (“flashing”) occurred at (or below) this level. Fluid inclusion assemblages consisting of coexisting vapor-rich and halite-bearing inclusions are observed in samples extending from the surface to the upper part of the late-potassic zone, indicating that fluid immiscibility occurred within this depth interval.