How do we Distinguish a True Anomaly from a False Anomaly in Transported Overburden?

M.J. Lintern

Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, c/o PIRSA, GPO Box 2355, ADELAIDE, SA 5001, Australia.


As mineral explorers turn their attention towards exploring in areas of transported overburden, it is becoming increasingly important to establish whether or not inexpensive surficial sampling techniques can be used in lieu of deep drilling in such a terrain.  Many accounts in the open literature, company reports and anecdotal evidence claim that surface sampling can be used successfully to explore in areas with thick transported overburden.  However, independent verification of these case studies by rigorous scientific methods has not been routinely undertaken.  This is an unsatisfactory state of affairs since it has left the exploration industry in doubt as to whether they can confidently relinquish ground.  The example of Steinway (Yilgarn Craton, Western Australia) is used to show how an apparent surface expression to deeply buried Au mineralisation may not be all that it seems.


Steinway is a sub-economic Au deposit located 25 km south of Kalgoorlie, WA.  The central and northern parts of the area have a variable thickness (>20 m) of transported overburden of Cainozoic age consisting of partly consolidated clays, sands and silts.  Beneath the transported overburden, saprolite overlies a bedrock of mafic andesites, trachytes, porphyritic tuffs and black shales.  There are two types of mineralisation at Steinway: (i) saprolite-hosted supergene mineralisation located at ~30 to 40 m and (ii) primary mineralisation associated with quartz stockwork veining within mafic andesites.  The soil (0-1 m) overlying Steinway is anomalous in Au, with a maximum concentration of 150 ppb compared to a local threshold of 24 ppb.  This Au anomaly is one of the strongest in the area.  However, few anomalies have proved to have significant mineralisation beneath them.  Moreover, the Greenback Gold Deposit, located about 600 m to the west and, like Steinway, in depositional terrain, did not have a soil anomaly above it but has been mined.


A more detailed study of the nature and distribution of Au in the sediments and anomalous soils was undertaken to determine whether the Au at the surface was being sourced from the underlying mineralisation or elsewhere.  The study included the separation and analysis of the abundant, nearly black, vitreous, sub-rounded ferruginous granules, a few millimetres in size, from the bulk soil.  The principal results are summarised as follows:


1.      Gold is associated with both ferruginous granules and calcrete (pedogenic carbonate) in the soil.

2.      Some Au is highly soluble, and probably present in colloid particles or in a ‘chemical’ form.

3.      Some ferruginous granules have relict primary fabrics and extremely variable Au contents (<40–15000 ppb).


These results suggest that the ferruginous granules are the immediate source of Au in the soil, and that both the gold and the granules are derived by mechanical dispersion from upslope rather than from beneath.  The relatively-soluble Au in the calcrete is probably derived from either (i) the ferruginous granules, which have weathered and released Au that has migrated by capillarity to the calcrete horizon, or (ii) direct chemical dispersion from a similar upslope source as the ferruginous granules themselves.


Previous studies have shown that in relict and erosional regimes, sampling of the calcareous horizon may accurately define drilling targets.  However, in depositional regimes, such as Steinway, results indicate that there may be no causal link with underlying mineralisation and that, in certain depositional areas, sampling of soil including calcareous material, at best, may indicate the exploration potential of the (sub-)catchment.  It is suggested, therefore, that for such landscape regimes, wider sampling intervals could be used, with a follow-up requirement that deep samples be collected, including basal sediments and/or ferruginous saprolite.


This research has been the outcome of productive collaboration between CSIRO and the mineral industry through AMIRA, and the assistance and support of the sponsors of CSIRO-AMIRA Project 409 (1994–1997) and, in particular, Newcrest Ltd, are gratefully acknowledged.  CRC LEME is supported by the Australian Cooperative Research Centres Program. C.R.M. Butt and D.J. Gray are thanked for earlier comments on this abstract.