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Open File Report 57

Gold and associated elements in the regolith - dispersion processes and implications for exploration. P241A Final Report

Butt, C.R.M., Gray, D.J., Lintern, M.J. and Robertson, I.D.M.

This report summarises the results of the two-year continuation of the CSIRO-AMIRA Project P241 "Gold and associated elements in the regolith - dispersion processes and implications for exploration." The continuation had the principal objective of extending the research into some important topics identified during the original project. The data complement and expand those obtained previously and the discussion of each topic draws on the results and conclusions of each phase of the Project. The principal fields of research and the outcomes are as follows:

  1. Elemental dispersion in deep regoliths was investigated at Lights of Israel, Davyhurst and a background site at Mt Percy, Kalgoorlie, with a minor study at Mulgarrie. The data confirm that, at local to sub-regional scales, Au is the best indicator of Au mineralization. However, sampling and interpretation must account for the distribution of Au in the regolith, in particular, its accumulation in ferruginous and calcareous surface horizons, its depletion for 5 to 20 m below them, and further sub-horizontal enrichment at depth. The most suitable pathfinders suite is As, Sb, W, +/- Bi, Mo, Pb; Ba, K and Rb give expression to some alteration zones. These elements provide confirmatory evidence for mineralization where Au has been depleted or where the distribution is patchy or has little focus.
  2. The determination of bedrock lithology from its weathered counterpart is a major problem in regolith dominated terrain. The processes of rock weathering and procedures for geochemical, mineralogical and petrographic identification of different lithologies have been summarized in an "Atlas of Weathered Rocks" produced specifically for this Project. The Atlas illustrates the changes in fabric, mineralogy and composition that take place during weathering. It is far from an exhaustive catalogue of all lithologies, but demonstrates the amount of lithological information that can be obtained from chemical analysis and by careful observation, even at the hand lens scale.
  3. The close relationship between the distributions of Au and pedogenic carbonate in areas south of the Menzies Line was further confirmed by detailed orientation studies at Lights of Israel and Zuleika (Ore Banda), with a minor study at Mulgarrie. Zuleika and Mulgarrie represent sites where soil Au anomalies directly overlie mineralization concealed beneath barren palaeochannel sediments. Unfortunately, due to natural contamination from nearby outcropping mineralization, no unequivocal evidence could be obtained to relate the anomalies and the concealed deposits, although such a relationship has been demonstrated during routine exploration programs elsewhere. Further soil investigations were conducted north of the Menzies Line, where Au tends to be associated dominantly with ferruginous materials in residual soils. However, in hardpan at Granny Smith, Au is associated with lithorelics and the siliceous cement. There is no obvious association with Mn oxides. Gold enrichment occurs towards the base of the hardpan, overlying leached and possibly depleted saprolite. Secondary carbonate is present in the lower hardpan and upper saprolite; this has much less Au than pedogenic carbonates, although much of it is highly soluble.
  4. Laboratory investigations of the chemistry of Au in soil have led to an improved understanding of its associations with other constituents and the mechanisms of the formation of Au anomalies. Gold in carbonates is highly soluble and can be distinguished from that associated with Fe oxides by partial extraction. Gold and Ca are probably brought to the surface biochemically, being cycled via vegetation, but the Au carbonate association in soil is probably largely physical in origin, arising from precipitation via evaporative processes. Gold in soil is highly labile and, at least in part, as water-soluble complexes, so that under wet conditions can readily be redistributed.
  5. Hydrogeochemical data show that the clearest indication of Au mineralization is given by Au itself. Data interpretation depends upon knowledge of the dominant dispersion mechanism. Where Au is dissolved as a thiosulphate complex (Boags, Bottle Creek; Hornet, Mt Gibson), Au distribution matches that of mineralization. Where dispersion is by halide complexation (Panglo, Wollubar and parts of Mt Gibson), the distribution of dissolved Au is controlled by physico-chemical parameters unrelated to the presence of mineralization, in particular where a high Eh is maintained by Mn oxidation in acid groundwaters. Additionally, there is a strong antipathetic relationship between dissolved Au and Fe, due to Fe2+ acting as a reductant. The presence of other elements, such as As and Sb, may indicate sulphides associated with Au mineralization. However, groundwaters in some drainages have very high concentrations of base metals and REE unrelated to the presence of such mineralization, probably having been leached from country rock and concentrated by evaporation.
  6. The spectral characteristics of Fe oxides and clays in the 0.4 to 2.5 µm wavelengths may have use in distinguishing different minerals not always apparent by routine XRD analysis and, in field logging or remote sensing, for mapping regolith materials such as lateritic horizons, lag and exposed saprolite. Spectral identification and semi quantitative estimation of micas has potential for identifying alteration zones and micaceous lithologies, but practical application is limited to materials having over 20% mica. Similarly, poor discrimination of soil carbonates means that the presence of this important sample medium cannot be recognized.

Last updated: Thursday, January 06, 2000 09:07 AM


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