Comparison of decrepitation, microthermometric and compositional characteristics of fluid inclusions in barren and auriferous mesothermal quartz veins of the Cowra Creek Gold District, New South Wales, Australia.
Fluid inclusions in quartz from barren and Au-sulphide-bearing veins from the Cowra Creek Gold District, New South Wales, Australia, have been characterised based on their decrepitation behaviour and gas geochemistry. Inclusions in Au-sulphide veins, and some regional sulphide-bearing veins, show distinctly higher CO2/CH4 ratios compared to barren quartz-only and quartz-sulphide veins from the deposit environment. Additionally, the occurrence of low-temperature peaks in the acoustic decrepigrams from Au-sulphide veins that had previously been attributed to the presence of CO2-bearing fluid inclusions has been confirmed using optical techniques. This characteristic peak is absent from quartz-sulphide and quartz-only veins. These results suggest that the gas content of fluid inclusions can distinguish Au-bearing from barren quartz veins, and that the acoustic decrepitation technique may provide a rapid and simple means of identifying different generations of quartz, and potentially productive veins in mesothermal environments.
Our results support conclusions of previous studies concerning the close association of high-density, gas-bearing (in this case CO2) fluid inclusions with Au-bearing quartz veins. Moreover, acoustic decrepitometry has been shown to be an effective, rapid and inexpensive technique for detecting these inclusions. We have shown that fluid inclusion chemistry and decrepitation behaviour can be used to distinguish Au-bearing from barren quartz veins in an area containing numerous generations of quartz veins. But, why should an exploration geologist analyze fluid inclusions when simple assays will tell her unequivocally which veins carry gold? The answer lies in the fact that assay results can be misleading because of the nugget effect; Au grades man be spotty within one vein, with high grades in one portion and barren quartz a few metres away. Conversely, fluid inclusion assemblages are usually consistent over 10s to 100s of metres, both vertically and horizontally, in a single vein. Thus, Au grades at one sample locality along a vein may not be promising, but the fluid inclusion characteristics at that same location would tell the explorationist that the vein has potential and that Au is likely to be present at other locations along the vein.