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Open File Report 102

The distribution of gold and other elements in surficial materials form the Higginsville palaeochannel gold deposits, Norseman, Western Australia

Lintern, M.J., Craig, M.A., Walsh, D.M. and Sheridan, N.C.

The CRCLEME-AMIRA Project "Exploration in Areas of Transported Overburden, Yilgarn Craton and Environs" (Project 409) has, as its principal objective, development of geochemical methods for mineral exploration in areas with substantial transported overburden, through investigations of the processes of geochemical dispersion from concealed mineralization. The Project has two main themes. One of these, 'Surface and sub-surface expression of concealed mineral deposits' is addressed by this report, which focuses on the soil geochemistry of palaeochannel Au deposits located 60 km south of Kambalda and 40 km north of Norseman.

The study area is located near Higginsville and encompasses the Mitchell palaeochannel and the Challenger-Swordsman palaeochannel Au deposits where the thicknesses of transported overburden and depth to mineralization vary from about 15 m to over 50 m. The study site is important since it provides several examples of a variably thick overburden, and extends the boundaries of Project 409 to an area well south of the Kalgoorlie study sites with which it can be contrasted. The great thicknesses of transported overburden are comparable to sites previously studied near Kalgoorlie but unlike many of them, the palaeochannels at Higginsville contain considerable economic deposits of Au, which are being extensively mined. It is considered that a detailed study of the nature of Au in surficial material from such an environment will enhance our understanding of the processes whereby Au may be enriched in the surficial environment in areas of substantially transported material.

The Higginsville palaeochannels are of considerable interest since it has been reported that at a number of locations there is a detectable surface expression of Au. One of the purposes of this research is to assess the validity of such reports by careful sampling and analysis and discuss the implications of the investigation for exploration in this area. The results indicate that:

Specific targeting of the calcareous horizon maximizes the probability of sampling the most consistently auriferous sample. In relict and erosional regimes, such sampling may accurately define drilling targets. However, in depositional regimes, the results indicate that there is no direct link with mineralization. Here, although a soil carbonate anomaly discretely overlies buried mineralization, the data suggest that it is derived from detrital Fe granules in the soil.
Separate sampling of ferruginous granules may provide a local source of the Au found in the carbonate horizon. Gold in ferruginous granules usually indicates that Au is being shed from relict or erosional areas within the catchment, hence these areas are most prospective. Like carbonate, ferruginous granules do not in themselves provide an indication of underlying palaeochannel mineralisation.

It is concluded that, in depositional areas examined in this study, sampling of calcareous material at best may indicate the potential of the area. It is suggested, therefore, that for such landscape regimes, wider sampling intervals should be used i.e. soil sampling is a regional tool, with a follow-up requirement that deep samples be collected including basal sands or ferruginous material in saprolite. Sampling of Fe granules or lag is a possible alternative, but the distribution of Au within them is more erratic. The most cost-effective sampling procedure is by power auger drilling and compositing the cuttings through the carbonate-rich horizon. Surficial soil sampling or drilling and routinely sampling at a specified depth without regard to the sample type may be inappropriate because Au anomalies may be overlooked.

Last updated: Sunday, August 05, 2001 13:47:52


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