Fluid inclusions: Their potential as an exploration tool for Archaean gold depositsSusan E. Ho
Recent advances in understanding precambrian gold
Geology dept and University Extension, University of Western Australia,
1987. Publication No. 11.
AbstractFluid inclusions in vein quartz from Western Australian Archaean gold deposits showing a variety of mineralisation styles indicate that gold deposition was from a distinctive, low salinity (typically < 2 wt % NaCl equiv.) H2O-CO2-rich (typically 12-25 mole % CO2) ore fluids with moderate densities (about 0.9 g/cc). Deposition occurred over a temperature range of 200o to 400oC, but typically between 250o and 350oC. Sporadic phase separation in some deposits is characterized by variable phase ratios in apparently contemporaneous CO2-rich inclusions. A lower pressure and/or temperature phase separation into CO2 and H2O-rich fluids was intermittent or late in the mineralisation history of some deposits. Fluid inclusion thermodynamic data indicate that the ore fluids were near-neutral to slightly alkaline, and were reducing. The presence of CH4 is characteristic of, and restricted to, fluid inclusions in veins adjacent or in proximity to carbonaceous meta-sedimentary rocks, indicating that it is a fluid-wallrock reaction product rather than a primary fluid component.
The low salinity and CO2-rich nature of the ore fluid appear to be characteristic for Archaean gold deposits, and also for younger gold-only deposits. Hence, there is the possibility of using fluid inclusions in vein quartz to evaluate the likelihood of mineralisation in a vein system, particularly in some weathered environments. Although visual examination is possible, decrepitometry has the greatest potential as a rapid guide to mineralized systems. However, more data on the nature of barren vein-systems are required. Where mineralisation is already confirmed, CH4 may be an important indicator of the presence of small, high-grade pods of gold +/- tellurides associated with carbonaceous sediments.
( Highlighting by reviewer)