Quantitive XRD of alteration at central Victorian gold deposits
Integration of Geochemistry & Semi-quantitative XRD Data
Hydrothermal alteration associated with central Victorian, sediment-hosted gold mineralisation has been well described in recent years (e.g. Bierlein et al. 2000; Arne et al. 2008). Alteration consists of sulphidic (pyrite, arsenopyrite, pyrrhotite), phyllic (sericite replacement of detrital albite) and carbonate (ankerite, siderite, ferroan calcite) haloes, some of which is cryptic in that it is not readily apparent in fresh diamond drill core. A number of studies have described the geochemical and mineralogical extent of these haloes away from mineralised structures, and have demonstrated the significant increase in the size of the deposit footprint where cryptic alteration is recognized.
Examples from Victorian Goldfields
Quantitative XRD analysis can be used to track the presence of ferroan carbonate minerals in meta-sedimentary rocks as mineralized structure are approached. The amount of chlorite in the meta-sedimentary rocks varies inversely with ferroan carbonate, either reflecting the replacement of metamorphic chlorite in the host rock or a change to an outer, more distal alteration halo. The key marker is the “cross over” in the proportions of chlorite to ferroan carbonate determined by quantitative XRD that can be tracked effectively using the Fe carbonate/chlorite ratio in the samples. Coincidently, this change occurs about 60 m away from the mineralized structures at both Ballarat East andFosterville, which are otherwise distinctly different styles of gold mineralization (quartz vein-hosted vs. disseminated). One geochemical marker that also reflects the inferred outer limit of the cryptic alteration zone is the arsenic (As) content of the samples, particularly that of disseminated pyrite crystals. A combined approach using both field portable XRD and XRF could effectively identify this alteration. The adjacent plotd show quantitative XRD and geochemical data for sandstone samples for two drill holes from the Ballarat East and Fosterville gold deposits in cnetralVictoria, Australia.
This case study is based on data taken from Bierlein, F.P., Arne, D.C., McKnight, S., Lu, J., Reeves, S., Besanko, J., Marek, J. and Cooke, D., 2000, Wallrock petrology and geochemistry in alteration haloes associated with mesothermal gold mineralisation, central Victoria, Australia. Economic Geology, 95, 283-312.
Summary of Quantitative XRD Data from Wallrock
Wallrock samples from both Ballarat and Fosterville show a decrease in chlorite and an increase in carbonate as mineralization is approached. The red vertical lines show the inferred extent of “cryptic” alteration adjacent to mineralized structures.