Fluid inclusions in porphyry and skarn ore at Santa Rita, New Mexico

Abstract

Fluid inclusions of apparent primary origin in quartz from both porphyry copper and skarn ore at Santa Rita are filled with two main types of liquid which differ markedly in salinity. Filling temperatures for both fluids are mostly in the range of 250 degrees to 450 degrees C but extend from about 200 degrees C to over 550 degrees C. The high salinity fluid generally contains 32 to >60 percent NaCl equivalent and was apparently saturated with NaCl below about 300 degrees C. The moderate salinity fluid, which is the most common type in most specimens, contains 1 to 20 percent NaCl equivalent. Many vapor-rich inclusions homogenizing to a vapor accompany the liquid-rich inclusions indicating that at least one of the fluids was condensing or boiling. High salinity, moderate salinity, and vapor-rich inclusions with a wide range of filling temperatures are intimately associated on a microscopic scale. Obvious secondary inclusions contain the same three types of fluids but tend to fill at lower temperatures. Data from critical behavior of two inclusions and from boiling of solutions containing 35 to 45 percent NaCl at 350 degrees to 450 degrees C indicate pressures of 120 to 520 bars.This same large range of temperature and salinity is shown by apparently primary inclusions in quartz from all major periods of deposition including: (1) early quartz veins associated with potassic alteration, (2) later quartz veins associated with sericitic and pyritic alteration, and (3) quartz-magnetite-sulfide veins cutting skarn. Filling temperatures in early quartz are no higher than in late quartz, both being predominantly in the range 250 degrees to 450 degrees C. In contrast, most inclusions in early garnet of the skarn are of the lower salinity type and fill to a liquid at 325 degrees to 425 degrees C. Necking down of inclusions and trapping of vapor-liquid mixtures may account for some or all of the high filling temperatures (>450 degrees C), but they cannot account for the range of salinity within each type of fluid. Trapping of solid NaCl along with liquid appears to explain some very high salinity inclusions that fill at temperatures 20 degrees to 165 degrees C lower than halite dissolves, although trapping at high pressures may account for some of these inclusions. Unrecognized secondary inclusions are a possibility, but the inclusions considered primary are clearly an earlier set of inclusions than the many obvious secondary inclusions. Also, the filling temperatures of primary inclusions are in reasonable agreement with temperatures estimated from oxygen isotopic fractionation and annite content of biotite. It is concluded that at least two major types of fluid, one with moderate and one with high salinity, deposited the quartz of the deposit and that at least the high salinity type was accompanied by vapor.The high salinity fluid probably formed as a condensate from a supercritical fluid emitted by an underlying magma and following a P-T path that intersected the two-phase field for the fluid, under conditions approaching hydrostatic pressure. Alternative origins for this fluid are boiling of lower salinity fluid which was heated by magmatic vapor or by encountering very hot rock. The lower salinity fluid can be explained as a similar supercritical fluid that remained near lithostatic pressure and followed a P-T path that did not intersect the two-phase field. Boiling and mixing with high salinity fluid and meteoric water may explain some occurrences of the lower salinity fluid. Fluctuation between hydrostatic and lithostatic conditions caused by recurrent episodes of emission of fluid from magma, fracturing to the surface, and finally choking of the fractures by quartz deposition can explain the intimate association of the two types of fluid, the large range in pressure, and the range in temperature. The large volume changes associated with condensation and/or boiling of hydrothermal fluids may be responsible for the extensive shattering at this and other porphyry copper deposits.