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Open File Report 92

Regolith geology and geochemistry Mt Magnet District - Geochemical orientation studies, Stellar and Quasar deposits

Robertson, I.D.M., King, J.D., Anand, R.R. and Butt, C.R.M.

Recent exploration under the lag-strewn colluvium-alluvium covered plain, mantling the Boogardie Synform, has located concealed Au mineralisation at Stellar and Quasar. The area is difficult to explore due to structural complexity in the basement, high geochemical backgrounds from numerous mineralised settings, a variably stripped, residual regolith beneath the transported overburden and scattered, high Au contents in the overburden. Pit exposures at Stellar and drilling at Quasar also revealed palaeochannel sediments hidden beneath the colluvium.

Regolith-landform relationships in the district were established by mapping the regolith of an area of 25 km2 around the mines and inspecting drill cuttings and pit exposures. A palaeochannel, filled with mega-mottled, grey clay and sandy clay, with some sepiolite towards the top and detrital, ferruginous, lateritic nodules, pisoliths and authigenic black granules towards the base, is exposed at Stellar. At Quasar, a similar palaeochannel occurs south-west of the pit and was detected by drilling. These fluvial channels appear to have been incised in already weathered basement and the sediments themselves have undergone post-depositional weathering. At Stellar, a clay-rich lateritic duricrust has formed by the weathering and later cementation of a partly transported unit overlying both the felsic and mafic-ultramafic bedrocks. Contemporary weathering of clays in the palaeochannel was probably responsible for the development of mega-mottles and some ferruginous granules. The colluvial-alluvial overburden was derived from dismantling the lateritic regolith. It contains lateritic nodules and pisoliths, probably from a proximal source, but is dominated by polymictic fragments, including BIF, ferruginous saprolite and saprolite, set in a silty-clay matrix. Its upper 2-3 m is silicified to red-brown hardpan.

Analytical data for a suite of 24 elements was provided by Hill 50 NL; the Ti and Zr data were unreliable and not used. Although geochemical backgrounds and thresholds have been established for the major regolith units, these must be regarded as of local application only because no data from distant background areas was available. Elevated concentrations of Cr, Ni, Cu, V and, possibly, Zn indicate mafic-ultramafic rocks. The saprolite and mottled zone have a significantly greater Cr/Fe ratio than the transported materials. Although the laterite has a high Cr/Fe ratio, Cr and Ni were ineffective for discriminating between laterites developed over felsic and mafic-ultramafic rocks. Regolith differentiation can be improved by canonical analysis; the most useful elements appear to be Al, Fe, Ni, Cr, Ga, Y, Zn, Th and Cu.

The Au mineralisation is poor in pathfinder elements such as As and Sb but there are weak anomalies in Bi and Pb. There is no correlation between the composition of the basement and the overlying colluvium, nor between nearly adjacent layers within the colluvium and its overlying lag. Unless partial extraction geochemistry can indicate basement-related anomalies in the colluvium-alluvium, this blanket must be penetrated to reach the weathered Archaean beneath. Comparison of sampling the top of the basement with that of the unconformity, or interface, between basement and colluvium, indicates lower order but broader and more consistent anomalies in the interface; this can be improved using additive indices of Bi, Pb and Zn. The interface sample is, therefore, the preferred sample medium, in this area of buried, stripped regolith, except where there are palaeochannels. Where there has been less regolith stripping and a buried laterite occurs, as at Stellar, this is the preferred sample medium.

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


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