Abstract

Mineralogic and fluid inclusion investigations conducted in the laboratory combined with in situ observations of temporal relationships among geologic features demonstrate that systematic changes in hydrothermal fluid characteristics occurred with time during evolution of mineralization and alteration of the granodiorite stock at the Santa Rita porphyry copper deposit. The earliest fluids to circulate are recorded in potassically altered (orthoclase- and biotite-stable) veins as vapor-rich H 2 O-CO 2 inclusions and hypersaline (30-70 equiv. wt % NaCl + or - KCl) inclusions which homogenize over a wide temperature range from 240 degrees to greater than 800 degrees C. The high-salinity fluid inclusions are complex chloride brines with significant concentrations of sodium, potassium, copper, iron, and sulfur, but sulfide minerals are not associated with the early potassic assemblage produced by this fluid. Oxygen isotope ratios of quartz in the orthoclase- and biotite-stable veins are identical to phenocryst quartz in the intrusion (8.2ppm). Later solutions circulated through newly formed fractures and older reopened veins, and are recorded in low-salinity (less than 15 equiv. wt % NaCl) fluid inclusions which homogenize primarily in the temperature range of 220 degrees to 360 degrees C. These hydrothermal fluids formed two distinct alteration assemblages as they reacted with potassically altered rock: chlorite is the dominant silicate gangue mineral of the earlier chalcopyrite-bearing, orthoclase- and clay-stable assemblages and sericite characterizes the later phyllic alteration assemblage with quartz and pyrite. Oxygen isotope ratios of quartz of the later assemblages (8.6-10.4ppm) are heavier than quartz of the earlier potassic assemblages.It is concluded that the earliest hydrothermal fluids to circulate within the stock localizing the orebody at Santa Rita could have been derived by phase separations of fluids exsolved from crystallizing magma into condensed, hypersaline liquids and H 2 O-CO 2 vapors. As these fluids moved through fractures, potassic alteration of rock occurred without concomitant deposition of sulfides. Later solutions of dominantly meteoric origin progressively formed hypogene copper mineralization associated with chlorite-stable alteration and then phyllic alteration.

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