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.8 Maureen unconformity-related U-Mo-F deposit, Georgetown, QLD, Australia Hurtig Nicole C.*, Driesner Thomas*, Heinrich Christoph A.*, Wall Vic** and Matthison Ian** *Institute of Isotope Geochemistry and Mineral Resources, Clausiusstrasse 25, CH-8092 Zurich, Switzerland (nhurtig@student.ethz.ch) **Mega Georgetown Ltd Pty, c/- Taylor Wall & Associates, Ground Level, 67 St Pauls Tce, Spring Hill, QLD 4000, Australia Maureen deposit is situated about 290 km south-west of Cairns and 35 km by north-north-west of Georgetown. The general lithofacies can be subdivided into a Proterozoic metamorphic complex (Bain et al., 1978; Oversby et al., 1975), namely the Robert River Subgroup which is unconformably underlying the Maureen Volcanic Group of Carboniferous age, which in turn unconformably underlays the Mesozoic Gilbert River Formation. The Maureen Volcanic Group is subdivided into the basal Fiery Creek Formation, which is hosting the U-Mo-F mineralisation, the Lower Volcanic Unit and the Upper Volcanic Unit (Bain and Withnall, 1980; O'Rourke, 1975). The deposit has a calculated grade of 0.12 % U 3 O 8 , 13.1 % CaF 2 and 0.07 % molybdenum. The geological setting of Maureen deposit shows characteristics of unconformity-type uranium deposits. Uranium is supposed to be transported in the hexavalent state in oxidized basinal fluids. Along deeply rooted fractures and faults reduced basement fluids rise. Where mixing between these two fluids occurs uranium is reduced and precipitates. The aim of this study is to identify the transporting fluid of uranium and other relevant ore forming components and to find evidence for mixing of reduced and oxidized fluids. The precipitation of U, Mo and F was spatially related to the intersection of steeply dipping faults with the Precambrian – Palaeozoic unconformity and occurred preferentially in coarse grained and pebbly sandstone or conglomerate beds of the Fiery Creek Formation. Mineralised zones appear as elongated narrow bodies showing a close relationship to EW oriented structures. Mineral contours of U, Mo and fluorite are shown in Figure 2 along an EW cross section through the Southern mineralisation zone. Typical alteration assemblages include fluorite, dickite, chamosite and disseminated pyrite. In low to intermediate mineralized samples muscovite shows enrichment in Mg where it occurs in contact with chlorite. Such mineral assemblages show characteristics of hydrothermal deposits where the mineralizing fluids have compositions that are drastically different from the country rock assemblages (Komninou and Sverjensky, 1995). During early mineralisation, fluorite matrix replacement stays within narrow zones surrounding fractures. With progressive mineralisation grain replacement, veining and finally chemical brecciation dominate. Sulfide-rich mineral assemblages located in deeper parts of the deposit near the unconformity represent the reduced end member of the system, whereas assemblages containing anatase, trögerite and other oxides are supposed to represent the oxidized end member. Further constraints on mixing of oxidized basinal fluid with reduced basement fluids were obtained from fluid inclusion analysis. Figure 2 illustrates the result of LA-ICP-MS analysis and microthermometry on a fluid inclusion sample from Maureen deposit. This study combined field observations with fluid inclusion analysis and mineral geochemistry providing important insights into transport and precipitation processes of unconformity-related uranium deposits. REFERENCES Bain, J. H. C., Oversby, B. S., Withnall, I. W., and Mackenzie, D. E., 1978, Precambrian and Palaeozoic geology of the Georgetown region, Queensland.; Third Australian geological convention; excursions handbook, p. 1-27. Bain, J. H. C., and Withnall, I. W., 1980, Mineral deposits of the Georgetown region, Northeast Queensland.; The geology and geophysics of northeastern Australia, p. 129-148. Komninou, A., and Sverjensky, D. A., 1995, Pre-ore hydrothermal alteration in an unconformity-type uranium deposit.: Contributions to Mineralogy and Petrology, v. 121, p. 99-114. O'Rourke, P. J., 1975, Maureen Uranium Fluorine Molybdenum Prospect, Georgetown, Economic Geology of Australia and Papua New Guinea, 1. Metals: Victoria, The Australiasian Institute of Minin and Metallurgy, p. 764 - 769. Oversby, B. S., Palfreyman, W. D., Black, L. P., Cooper, J. A., and Bain, J. H. C., 1975, Georgetown, Yambo and Coen inliers; regional geology: Monograph Series Australasian Institute of Mining and Metallurgy, v. 5, p. 511-515. 21 Symposium 7: Geofluids and related mineralization: from shallow to deep
220 Symposium 7: Geofluids and related mineralization: from shallow to deep Figure 1. EW cross-section though the Southern mineralisation at Maureen deposit. Mineral contouring based on drill hole data provided by Mega Georgetown showing the distribution of U, Mo and Fluorite. The contour lines of U, Mo and F overlap, but differ in their extensi- ons. Fluorite forms an extensive halo surrounding high grade ore bodies. Figure 2. LA-ICP-MS signal of a saline 2 phase fluid inclusion (L3-type) showing corresponding peaks for U, Mo, As, Cu and Na. The uranium concentration is 11 ppm and Mo has a much higher concentration of 702 ppm, which is very unusual.
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