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Open File Report 91

Investigation of the hydrogeochemical dispersion of gold and other elements at Lawlers, Western Australia

Gray, D.J.

The hydrogeochemistry of the Lawlers district was investigated, with a view to understanding the interaction of groundwaters with mineralized rocks, and the potential for exploration, particularly in areas of extensive overburden. Groundwaters at Lawlers are neutral, with a similar Eh range to other neutral groundwaters. However, they differ from groundwaters investigated at other Yilgarn sites in having low salinities and marked divergence from sea water ratios, suggesting that the major ion concentrations are strongly controlled by local lithological or hydrological factors. Elements that appear to be controlled by mineral equilibration in some or all groundwaters are Ba (barite equilibration), Ca, Mg and HCO3 (carbonate), Mn (rhodochrosite), Sb [Sb(OH)3], Pb and V (Pb2V2O7).

The most marked feature of the Lawlers groundwaters is the extremely high Au concentrations, with the highest observed concentration of 40 µg/L being about ten times greater than any other previously reported values. These high concentrations are correlated with highly anomalous Co concentrations at Hidden Secret and in the Gt. Eastern/Caroline pits area but not at Genesis. Under the groundwater conditions observed at Lawlers, the most likely mechanism for dissolution is expected to be as the thiosulphate complex Au(S2O3)23-. The generation of thiosulphate requires oxidation of sulphides to occur under neutral conditions, with the acidity produced being neutralized by the dissolution of minerals such as primary carbonates. This is consistent with limited results at Hidden Secret suggesting that higher Au concentrations are correlated with increased Mg, Ca, HCO3 (from carbonate dissolution) and SO4 (from sulphide oxidation).

Most of the specific sites investigated showed specific groundwater element anomalies:

  1. Hidden Secret - Au, TDS, SO4, P, Mn, Fe, Co, Ni, Y, REE, W, As, Sb, I, U, and possibly Ba, Ga, Sc and Ge;
  2. Genesis - Au, Sc, As, and possibly Ni and Sb;
  3. Mineralization at Gt. Eastern-/-Caroline - Au, Co, and possibly SO4, Sc and As;
  4. Mineralization at Four Corners - SO4, Al, Ga, Mn, Fe, Co, Ni and Mo (though generally based on only one sample at the centre of the mineralized area);
  5. Granitic rocks at Gt. Eastern-/-Caroline - Al, Ga, Ba, Mn, Mo and U;
  6. Ultramafic rocks - Cr, Cs and Sr;
  7. Southern part of Four Corners - TDS, B, Sr, HCO3, I and U (possibly indicating a different groundwater system in this area).

The differences in the minor element 'signatures' for the groundwaters in areas of major Au mineralization (Genesis, Gt. Eastern/Caroline, Hidden Secret) may reflect differences in the lithologies of the three sites and/or the geochemistry of the Au mineralization. The Gt. Eastern/Caroline and Hidden Secret groundwaters have groundwater signatures similar to other mineralized sites elsewhere in the Yilgarn, with anomalies in a range of chalcophile and other pathfinder elements. In contrast, groundwaters at Genesis are anomalous in only Au, Sc and As, possibly due to the Genesis ore deposit being hosted in a quartzite unit with free-milling Au and disseminated arsenopyrite.

At the Four Corners area, which is covered with extensive transported overburden, there are anomalous concentrations of varying elements (but not Au), with size of the groundwater anomaly appearing to be less than 250 m diameter. The exception to this is SO4, which has a more disperse anomaly.

Groundwater at Lawlers appears to have exploration potential, particularly in the residual areas. In areas of extensive transported overburden, there is no observed groundwater Au response, though some elements (SO4, Ga, transition metals, Mo) may have potential as pathfinders for Au.

Last updated: Friday, July 21, 2000 04:24 PM


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