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Russian Geology and Geophysics

2011 year, number 11

A FLUID INCLUSION STUDY OF URANIUM AND COPPER MINERAL SYSTEMS IN THE MURPHY INLIER ( Northern Australia )

T.P. Mernagha and A.S. Wygralakb
aGeoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
bNorthern Territory Geological Survey, GPO Box 2901, Darwin, NT 0801, Australia
Keywords: Fluid inclusions, uranium and copper deposits, northern Australia
Pages: 1421-1435

Abstract

The Palaeoproterozoic Murphy Inlier is situated at the southern end of the McArthur Basin in northern Australia. The inlier contains over 50 uranium, copper, tin, and base metal occurrences. Fluid inclusion studies were carried out on samples of quartz veining from the uranium and copper deposits as well as from the basement rocks to determine the composition of the fluids and to investigate how uranium and copper were transported in these fluids. Four types of fluid inclusions were observed in this study: Type A - vapor-rich inclusions with ≥ 30 vol.% vapor, Type B - two-phase aqueous inclusions with ≤ 20 vol.% vapor, Type C - multiphase inclusions with one or more solid phases, and Type D - liquid-only inclusions.
At least three different fluids were identified in the Murphy Inlier. There is a CaCl2 ± LiCl-rich brine, a NaCl-rich brine, and a low-salinity fluid. The fluids can also be grouped into a high-temperature (typically homogenizing above 210 °C) population and a low-temperature (typically homogenizing below 240 °C) population. Depending on location, the high-temperature fluid may be enriched in CO2, N2, or CH4. In the uranium deposits, gas-rich inclusions are dominated by CO2 indicating that these fluids are relatively oxidized, while in the copper deposits both CO2 and CH4 are present, indicating that these fluids are more reduced. The low-temperature population of Type B inclusions has a mode of homogenization at 190 °C in the uranium deposits and a mode at 120 °C in the copper deposits. Similarly, Type C inclusions have a mode of homogenization at 235 °C in the uranium deposits and 170 °C in the copper deposits.
Variations in the composition of the inclusions suggest that at least two stages of fluid mixing occurred. Firstly, there was mixing between a CaCl2 ± LiCl-rich brine and a NaCl-rich brine to produce a fluid of intermediate composition. This fluid then mixed with a low-salinity fluid. Geochemical modeling has shown that both uranium and copper can be transported in the same fluid at high ? O 2 and moderate to high-chloride concentrations. In the proposed model for mineralization, uranium and copper were leached from the volcanics or sediments in the McArthur Basin and were simultaneously transported in the oxidized, Na-Ca-Li-bearing saline fluids which had undergone mixing within the basin. Uranium precipitated when this fluid was reduced, either by reaction with Fe-rich mafic volcanics, carbonaceous rocks, or by mixing with a CH4-bearing low-salinity fluid derived from the basement. Copper remained in the fluid until further changes in salinity, ? O 2, or pH occurred, most probably as a result of mixing with lower-salinity meteoric fluids. The fluid may have continued to cool to near-surface temperatures, as evidenced by the trapping of liquid-only fluid inclusions in some veins.