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Open File Report 153

Preliminary regolith studies at Earea Dam Gold Prospect, Gawler Craton, South Australia

M. J. Lintern

This CRC LEME Open File Report describes preliminary regolith studies at the historic (circa 1899) Earea Dam Goldfield in the Gawler Craton, South Australia. The study was conducted as part of the PIRSA Harris Greenstone Project a pilot study for the PIRSA-CRC LEME Central Gawler Gold Province Project. Previous case studies of Au in the Gawler Craton regolith have been summarised in The South Australian Regolith Project (Lintern, 2004) and this case study complements these as it provides an example of Au dispersion in areas where regolith is relatively thin.

The Earea Dam Goldfield is located 34 km west of Kingoonya on the Tarcoola-Glendambo road. A regolith landform map at 1:20000 scale was constructed to set the geochemical study into context. Although the area has experienced mining activity, the particular location chosen to investigate geochemical dispersal of elements was not considered significantly disturbed. The prospect is located in rolling hills of 30-40 m relief on lapped by shallow alluvium-colluvium. Common bedrock exposures in the hills are Kenella Gneiss and scarcer intrusions of mafic dykes. The hills are mantled with fine to cobbled-sized lag of angular bedrock. Elsewhere, bedrock and saprolite are mostly covered by a fine-grained red-brown soil partly of aeolian origin. Observations of costeans, pits and limited drill spoil indicate that the depth of weathering is limited to a few metres in much of the area. Calcrete is common in the erosional regime but its extent beneath, and within, the hardpanized (silicified) colluvium-alluvium was not determined. The thickness of transported cover varies from outcrop to 3 m in the vicinity of the mineralization. Sand dunes and playas to the south and east serve to mask the underlying regolith.

The distribution of gold and other elements was investigated within a part of the Earea Dam Goldfield known as Ian’s Mine (499444E 6585077N, AGD66), a 5 m by 20 m by 4 m deep mining slot with high grade ore hosted by quartz-hematite. Samples were taken from the slot and adjacent costeans, with a limited soil and auger survey downslope of the mineralization, followed by multi-element analyses.

Gold mineralization is associated with Ag, Bi, Cu, Sn, Te, U and W at Ian’s Mine. In the immediate vicinity, surficial dispersion is mostly mechanical, associated with mineralized bedrock fragments. Some high concentrations of Au are recorded in calcrete (maximum of 475 ppb) largely due to included detrital hematite-quartz fragments. Some chemical processes may have led to limited dispersion of Au in smectitic clays adjacent to the mineralized quartz-hematite veins. By 200 m downslope of Ian’s Mine, soil samples are close to background Au concentrations (2-3 ppb). Silver, and possibly W, are more widely dispersed than Au. Fine (<75 µm) and coarse (>2 mm) size fractions are generally richer in Au and pathfinders than intermediate size fractions for material from Ian’s Mine itself but only the fine fraction is richer in Au for soil from the depositional regime downslope of the mine. Calcrete has shown to be concentrated in Au in the erosional areas but its poor development in the surficial soils of the depositional areas precludes its use as a sampling medium in these areas.

In this environment, high-grade mineralization produces a limited geochemical dispersion halo for Au. Poorly-developed supergene Au, possibly due to the absence of a significant thickness of regolith within which it can develop, a resistant host rock (hematite-quartz), and the small, narrow characteristics of the veins may be possible explanations. Greater thicknesses of regolith, (with possible supergene redistribution) may have been present but have since been eroded. Exploring for more ore shoots of this type is expected to be difficult in this area unless very close spaced calcrete sampling (25 m grid) and/or transported-in situ interface sampling is adopted.

M.J. Lintern
Study Leader

May 2004



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