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Open File Report 97

Hydrogeochemical dispersion of gold and other elements at Baxter, Western Australia

Gray, D.J.

The hydrogeochemistry of the Baxter study area was investigated, with a view to understanding the interaction of groundwaters with mineralized rocks, and the potential for exploration, particularly in areas of overburden. Groundwaters at Baxter are neutral groundwaters, with a similar Eh range to other neutral groundwaters from the Yilgarn Craton. However, they differ from groundwaters investigated at sites in the southern Yilgarn in having very low salinities and marked divergence from sea water ratios, suggesting that the major ion concentrations are strongly controlled by local lithological or hydrological factors. The groundwaters at Baxter strongly resemble those at Lawlers, about 300 km to the SE in the north Yilgarn. Elements that appear to be controlled by mineral equilibration in some or all Baxter groundwaters are Ba (barite equilibration), Ca, Mg and HCO3 (carbonate), Pb and V (chervitite Pb2V2O7).

The Baxter groundwaters have extremely low Au concentrations, similarly to areas of overburden at Lawlers. This suggests that Au is not expected to be extensively chemically redistributed in the regolith under present-day conditions, and therefore that it will not be a useful pathfinder in groundwaters or in soils overlying extensive overburden. However, Sc, Mo, W and, possibly, Rb were observed to have greater groundwater concentrations in areas of Au mineralization and at this site are better pathfinders in groundwater than Au itself. This elemental suite is similar, though more limited, to those observed at other sites. These elements may have scope as target elements for selective extraction of soil or other regolith material. Other elements can be used in groundwaters to indicate underlying rocks or other geochemical features. For example, dissolved Cr concentrations correlate closely with the presence of ultramafic rocks at Baxter and elsewhere, even though the groundwaters are in contact with highly weathered lithologies. Dissolved As and, to a lesser degree, Ni correlate with a zone of As-enriched rocks to the SE of the study area.

Thus, results indicate exploration potential for groundwater in such environments, even where highly weathered rock are overlain by transported material. The elements enriched in mineralized groundwaters may also form part of a suite of elements that may yield a geochemical expression by selective extraction.

Last updated: Sunday, August 05, 2001 11:44:42


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