Abstract

Park Premier is a low-grade gold-rich porphyry copper system located about 3 km east of precious and base metal veins of the Park City district, Utah. The Park Premier stock consists of five small intrusions that were emplaced into coeval volcanic rocks at depths less than 1 km. Two of the intrusions are microaplitic granodiorite porphyries that developed strong hydrothermal alteration, stockwork quartz veins, and low-grade disseminated copper-gold mineralization. Early alteration of these intrusions included biotite replacement of hornblende, deposition of disseminated magnetite and actinolite, local sodic alteration of plagioclase, and formation of (1) quartz + magnetite + actinolite + or - Na plagioclase and (2) quartz + K feldspar + or - magnetite + or - chalcopyrite veins. Mineral compositions, fluid inclusion data, and inferred alteration reactions indicate that early alteration occurred at temperatures > or = 600 degrees C from hydrothermal fluids with unusually high salinities (> or = 82 wt % total salts) and low K/Na molar ratios (<0.25). These high-salinity brines probably formed by boiling of fluids released during crystallization of the microaplitic porphyries at high temperature (> or = 700 degrees C) and low pressure (< or = 200 bars). Quartz + pyrite + or - sericite veins and local alteration of high-temperature minerals to chlorite, sericite, calcite, and clay minerals occurred at much lower temperatures (< or = 270 degrees C) by the influx of dilute (0-3 equiv wt % NaCl), boiling meteoric(?) water.Large areas of the igneous wall rocks of the microaplitic porphyries are altered to a variety of hydrolytic assemblages, including albite + sericite, sericite, kaolinite-dickite, pyrophyllite, and alunite (all + quartz + or - pyrite). Albite + sericite, sericite, and local kaolinite-dickite alteration probably formed from chloride-dominated fluids derived from the microaplitic porphyries. Pyrophyllite and alunite alteration postdate all other types of alteration and formed along major northeast-trending fracture zones from fluids that were probably sulfate rich.The hydrothermal system at Park Premier contains several features that are unusual in other well-described porphyry copper systems but which may be common in gold-rich porphyry systems; they include: (1) early, extremely saline fluids that only migrated locally, (2) abundant magnetite, actinolite, and sodic plagioclase and relatively minor biotite, K feldspar, and sulfide minerals, (3) sparse quartz vein formation at temperatures < 500 degrees C, (4) little interaction of the microaplitic porphyries with dilute fluids until the intrusions cooled to temperatures < or = 270 degrees C, (5) early albitization of wall rocks adjacent to the microaplitic porphyries, and (6) relatively high Au contents and high Au/Cu ratios of the microaplitic porphyries. These differences probably arose primarily from the very shallow depths of emplacement, small size of the intrusions, and low(?) sulfur contents of the magmas.

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