Economic Geology Vol. 80, 1985, pp. 1328-1347 Evolution of Hydrothermal Fluid Characteristics at the SantaRita, New Mexico, Porphyry Copper Deposit T. JAMES REYNOLDS FLUID INC., P.O. Box 6873, Denver, Colorado 80206 AND RICHARD g. BEANE AMAX Exploration,Inc., 10 East Broadway, Tucson, Arizona 85701 Abstract Mineralogic and fluid inclusion investigations conducted in the laboratory combined with in situ observations of temporal relationships amonggeologic featuresdemonstrate 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) veinsas vapor-rich H20-CO2 inclusions and hypersaline (30-70 equiv. wt % NaC1___ KC1) inclusions which homogenize over a wide temperature range from 240ø to greater than 800øC. The high-salinity fluid inclusions are complex chloride brines with significant concentrations of sodium, potassium, copper, iron, andsulfur, but sulfide minerals are not associated with the earlypotassic assemblage produced by thisfluid. Oxygen isotope ratios of quartz in the orthoclase- and biotite-stable veinsare identicalto phenocryst quartz in the intrusion (8.2%0).Later solutions circulated through newly formed fracturesand older reopened veins, and are recorded in low-salinity (less than 15 equiv.wt % NaCI) fluid inclusions which homogenize primarilyin the temperature rangeof 220ø to 360øC. These hydrothermalfluids 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 quartzand pyrite. Oxygen isotope ratios of quartz of the later assemblages (8.6-10.4%0) are heavier than quartz of the earlier potassic assemblages. It is concluded that the earliest hydrothermal fluids to circulate within the stock localizing the orebodyat Santa Rita could have been derived by phaseseparations of fluidsexsolved from crystallizing magma into condensed, hypersaline liquids andH20-CO2 vapors. As these fluids movedthrough fractures, potassic alterationof rock occurred without concomitant deposition of sulfides. Later solutions of dominantly meteoricorigin progressively formed hypogene copper mineralization associated with chlorite-stable alteration andthen phyllic alteration. Introduction VARIOUS lines of evidencesuggest that, at different times, fluids of magmatic and meteoric origin cir- culated through intrusions related to formation of porphyry copper deposits. Field and laboratory studiesshow that during crystallization in shallow crustal environments, somesilicate magmas become saturated with and, consequently, exsolve an immis- cible H20-rich fluid phase. Mathematical models of fluid flow throughfractured,permeable plutons in- dicate that under such circumstances, the earliest hydrothermal fluidsto circulateare endogenous and that later fluids are largelyderivedfrom surrounding wall rocks (cf. Norton, 1978, 1982). Oxygen and hydrogenisotope studiesof hydrothermal biotites from several porphyry copper deposits throughout the western cordillera cluster around ratios measured from "normal" igneous biotite; the isotopic character of these hydrothermal micas does not vary with latitude(Sheppard et al., 1971; Taylor, 1974, 1979). From this it hasbeen concluded that hydrothermal biotite and attendantpotassic alteration in the por- phyry copper deposits formed from fluids either derived from magmaor equilibrated with igneous rock at magmatic temperatures. Hypogenesericites and clays from the porphyry copper deposits are •sO depleted, and the D/H ratios of these minerals in different deposits show systematic variation with latitude (Sheppard et al., 1971; Taylor, 1974). These factorsled to the association of phyllic alteration with a later influx of meteoric waters, derived from adjacent wall rocks,into the cooling, fracturedplu- ton. Because of the common spatial relation between hypogene mineralization andpotassic alteration (e.g., 0361-0128/85/429/1328-20 $ 2.50 1328