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Open File Report 93

Investigation of the hydrogeochemical dispersion of gold and other elements from mineralised zones at the Granny Smith Gold Deposit, Western Australia

Gray, D.J.

The hydrogeochemistry, and in particular, the usefulness of groundwater as an exploration medium, has been tested at the Granny Smith gold deposit. Two different sample treatment methods were used and compared:

No field treatment - samples filtered and acidified in the laboratory about 10 days later;
Samples filtered and acidified in the field.
The latter procedure is most commonly followed in groundwater investigations. Comparisons of the results indicated that agreement between the two methods was generally good, except for Fe, Al and, to a lesser extent, Au. However, given the importance of these elements in exploration and for understanding hydrogeochemical processes, it is recommended that the latter method be used.

Groundwaters at Granny Smith are generally neutral, with variable salinity. The waters appear to be depleted in Mg, K and Br, suggesting that halite (NaCl) is dissolving in the groundwater and displacing these ions. Additionally, there is a minor enrichment in Ca and SO4 in the less saline samples, which is possibly due to gypsum (CaSO4.2H2O) dissolution.

Although the hydrogeochemistry of the minor elements at Granny Smith is similar to that of other Yilgarn sites, dissolved Au concentrations at Granny Smith are very low and, indeed, are not much higher than sea water. The reasons for this are not understood, but the potential for dissolved Au alone as an exploration tool in this area is predicted to be poor.

One additional problem at this site is that the groundwater system appears to be highly stratified. Samples obtained via dewatering bores appear to be representative of a deep saline system, whereas those obtained using a bailing system appear to be sampling more shallow, fresher, groundwater. At Windich, mineralization occurs at depth, and the water samples, which were all obtained by bailer, do not appear to be from groundwater contacting mineralization. This may explain why the dissolved Au concentrations in the Windich groundwater samples are very low.

At the Goanna and Granny pits, the water samples are obtained from the dewatering bores, and appear to be in contact with primary Au mineralization. Samples in areas of high Au grade are enriched in Au (relative to Windich), As, Co, Mn, Mo, Ni, Pb, Sb, Tl and Zn, presumably reflecting the geochemistry of the Au mineralization. Arsenic, Mo, Sb and Tl are commonly associated with Au mineralization, whereas Pb and Zn enrichments may reflect associated galena (PbS) and sphalerite (ZnS), and Mn, Co and Ni have been observed to be enriched in mineralized groundwaters at other sites. Thus, this multi-element association may offer scope for exploration.

There is also some potential for elements which may be associated with mineralization, but which are soluble and tend to be readily dispersed, as a regional groundwater exploration tool. These possibly include As, I and Cs. However, further sampling distant from mineralization would be required to test this possibility.

The generally low Au concentrations in all of the groundwater samples, and the major inconsistency between results for Goanna and Granny and those for Windich, suggest that hydrogeochemistry would not be a very useful exploration method in this area. However, the reasons for these effects are only partially understood. Further work in similar, though hopefully less disturbed areas, may be of value in fully understanding the hydrogeochemistry of such deposits.

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


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