ORIGIN OF HYDROTHERMAL FLUIDS AT OLYMPIC DAM - PRELIMINARY-RESULTS FROM FLUID INCLUSIONS AND STABLE ISOTOPES

Fluid inclusions and stable isotope data provide evidence of the temperature, composition, and sources of hydrothermal fluids associated with formation of the Fe-rich breccias that host Cu-U-Au-Ag ore at Olympic Dam. Early magnetite associated with pyrite and siderite precipitated from fluids of high delta-O-18 content (approx 10 parts per thousand) at temperatures near 400-degrees-C, and magnetite in K feldspar-apatite veins at the nearby Acropolis deposit probably formed from similar fluids. In contrast, hematite associated with ore-bearing breccias formed at lower temperatures (200-degrees-400-degrees-C) from fluids of lower delta-O-18 content (< 9 parts per thousand). Fluid inclusion homogenization temperatures in quartz fragments within hematite breccias overlap with and are lower than those indicated by isotope geothermometry. Two-phase liquid plus vapor inclusions mainly homogenize at temperatures between 130-degrees and 280-degrees-C, with a mode at 170-degrees to 190-degrees-C. Quartz fragments within rare laminated barite clasts display a larger range of homogenization temperatures from 110-degrees to 420-degrees-C and may have preserved evidence of early, high-temperature hydrothermal events. Measurement of freezing point depressions in fluid inclusions provides evidence of widely varying salinities and complex solute chemistry. Indicated salinities vary from a minimum of 7.3 wt percent NaCl equiv (1.3 m) to a maximum of 23.7 wt percent NaCl equiv (4.2 m) in two-phase fluid inclusions, and up to 42 wt percent NaCl equiv in halite-saturated, three-phase inclusions. All the fluid inclusions studied displayed evidence of additional solutes in addition to NaCl, all required a high degree of supercooling to induce freezing and displayed initial ice melting at temperatures below -23-degrees-C, and many formed distinctive brown, granular ice. In addition, several inclusions also formed CO2 clathrates and other metastable solids. Qualitative assessment of these characteristics indicates fluids carrying multiple solutes probably including NaCl, KCl, CaCl2, FeCl2, CaF, and CO2. These preliminary results suggest that at least two sources of fluids, of contrasting temperature, composition, and oxygen isotpe characteristics, were involved in the formation of Olympic Dam. Early magnetite was deposited from hydrothermal fluids of deep-seated, possibly magmatic origin, but magnetite-bearing rocks are not significant hosts for copper-uranium mineralization. Later hematite-associated with ore-bearing breccias-was deposited by fluids of significantly lower temperature and lower delta-O-18 values. The geologic setting of the deposit (a fault-bounded depression at or near the paleosurface) implies that waters of surficial origin-seawater, closed basin water, or ground water-may have been involved in the later stages of hydrothermal activity that formed the hematite breccias. Although our preliminary isotopic data do not require such waters, they are consistent with this conclusion.