Hydrothermal processes at the Osborne Fe-Oxide-Cu-Au deposit, N.W. Queensland: integration of multiple micro-analytical data sets to trace ore fluid sources
Fisher, Louise (2007) Hydrothermal processes at the Osborne Fe-Oxide-Cu-Au deposit, N.W. Queensland: integration of multiple micro-analytical data sets to trace ore fluid sources. PhD thesis, James Cook University.
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Abstract
The Osborne mine exploits one of several significant iron oxide-copper gold (IOCG) deposits in the eastern part of the Proterozoic Mount Isa Inlier (Cloncurry District) in NW Queensland. Cu-Au bearing sulphides at Osborne are associated with volumes of massive, coarse grained quartz (silicification) which was precipitated both pre- and synore deposition. This extensive quartz is a feature unique to Osborne and make it ideal for a fluid inclusion study. The ore forming fluids were examined by several bulk and microanalytical techniques. The fluid history of the deposit has been evaluated by microscope petrography and the physical conditions under which the fluids were trapped, and their compositions were estimated using microthermometry, laser Raman spectroscopy, proton induced X-ray emission (PIXE) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The source of the fluids and their salinities were studied using combined noble gas and halogen analysis utilising a neutron activation-mass spectrometric method on bulk samples and PIXE analysis.
Petrographic studies show complex assemblages of fluid inclusions in which both pre and post-ore depositional ore fluids are identified. Primary multi-solid fluid inclusions (MS), some with a carbon dioxide component (CB), trapped at 340 - >600oC with ultrahigh salinity (<64 wt% NaCl equiv.), are correlated with the massive quartz precipitation event. High salinity (17-38 wt% NaCl equiv) pseudosecondary and secondary liquid-vapour ± halite fluid inclusions (LVD) were trapped at 105-292oC and lie on trails that emanate from chalcopyrite and are interpreted to relate to Cu-Au mineralisation and a second phase of quartz deposition. The decrease in temperature and salinity over the period of ore formation is attributed to fluid mixing.
Thermal and mechanical decrepitation of the fluid inclusion populations permitted semi-selective analysis of different fluid inclusion populations and a comparison of the noble gas and halogen composition of the ore fluids with those of the pegmatites. The halogen data for the Osborne deposit indicate multiple sources of salinity suggesting mixing between at least two components; the fluids with the highest values of Br/Cl = 3.8 ×10-3 and I/Cl = 27.4 × 10-6 are similar to bittern brine compositions and those with the lowest values of Br/Cl = 0.3×10-3 and I/Cl = 2.4 ×10-6 are similar to halite dissolution waters. Values for pegmatitic quartz hosted fluids fall within these ranges. The data are consistent with mixing between crustal fluids of diverse origin. 40Ar/36Ar values of <2000 and 36Ar concentrations of 1- 6 ppb are most similar to sedimentary formation waters but a metamorphic component, derived from devolatisation reactions during regional metamorphism, can not be excluded. The similar values obtained for samples of pegmatitic quartz support the presence and inclusion of ore fluids at the time of pegmatite anatexis. The moderately high 36Ar concentrations in the ore fluids and their low 40Ar/36Ar values preclude the involvement of magmatic fluids derived from Atype granites with a deep crust or mantle origin.
Compositional data obtained using PIXE and LA-ICP-MS shows significant compositional variation within single inclusion populations. A two order of magnitude range of Br/Cl ratios (0.2 – 18 ×10-3) correlates with noble gas and halogen data and indicates multiple sources of salinity. Low concentrations of copper in the high salinity ore fluids (Cu<<150ppm) suggest that changes in control factors of copper solubility were important in the formation of the deposit. At temperatures of 600oC, the Osborne ore fluids would be undersaturated with respect to chalcopyrite. Cooling, dilution and redox changes caused by interaction with host rocks and/or fluid mixing are interpreted to be the main controls on deposition.
Geochemical modelling of the ore forming processes, using HCh, suggests that a redox switch from hematite-stable conditions to magnetite-stable conditions could have triggered chalcopyrite precipitation during rock-buffered fluid mixing. Modelling suggests highest ore grades would be associated with pyrrhotite-bearing assemblages.
Data collected during this study indicate that cooling, dilution and redox changes caused by interaction with host rocks and/or fluid mixing are likely to have been the main controls on deposition at the Osborne deposit. Furthermore, halogen and noble gas data provide strong evidence that magmatic fluids are not a ubiquitous component of IOCG ore forming systems.
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