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Open File Report 31

Petrography, mineralogy and geochemistry of soil and lag overlying the Lights of Israel Gold Mine, Davyhurst, Western Australia

Robertson, I.D.M. and Tenhaeff, M.F.J.

The Lights of Israel Mine Site lies in an erosional regime, where the weathered profile has been truncated to within the mottled zone. This study area, south of the Menzies Line, in an area of eucalypt woodland, where soil carbonates are common, provides a useful contrast with the arid environment of Beasley Creek to the northeast.

Samples of the lag and the generally thin, colluvial soil were collected along one traverse over the Lights of Israel Mine Site, prior to mining. The lag, the soil and its components have been examined petrographically, mineralogically and geochemically. The coarse fraction (>600 µm) consists of black, goethite- and hematite-rich nodules (some of which are magnetic), red to yellow, ferruginous clay granules, quartz fragments and scarce crystals of tourmaline and gossan fragments. All the ferruginous fragments are petrographically indistinguishable from the fine lag, which was formed by deflation of the top layers of soil. Some of the upper soil layers are rich in carbonates, contain crystals of pedogenic gypsum and curly crystals of halite.

The iron-rich fragments contain lithorelics, containing microscopic relics of layer silicates (smectites and kaolinite), set in, and largely replaced by massive, spongy or vesicular goethite. The clay-rich granules consist largely of hematite- or goethite-stained kaolinite and some include goethite-rich lithorelics. Very close to the mineralisation, fragments of gossan, showing pseudomorphs after fine-grained pyrite, were identified in the lag. The soil contains a significant, quartz-rich, component which is largely angular and glassy and appears to be largely of local derivation. It is most abundant in the 75-710 µm fraction, where it acts as a geochemical diluent. There is a very minor aeolian component, which becomes progressively more abundant in the finer fractions. The silty fraction (<75 µm) contains less quartz but more iron oxides and clay. Its contained <4 µm fraction is very clay rich. The sandy and silty fractions contain a trace of sharp, fresh crystal fragments of tourmaline, which have a composition indistinguishable from that of local veinlet tourmaline.

The complete soil and the >600, the <75 and the <4 µm fractions were analysed to assess their value as sampling media. The quartz-rich 75-710 µm fraction was discarded. Gold in the fine soil fractions is the best guide to mineralisation by far. Very weak and equivocal anomalies in As, Sb, Cu and Cd are probably also ore related. Maxima in K and Rb seem to indicate a phyllic alteration halo around the mineralisation. The elevated S background above the mineralisation is problematical; its isotopic composition is slightly heavier than that which would be expected from meteoric S.

Gold anomalies are best developed in the calcareous soil fractions, are small and lack any extensive dispersion halo. Apart from Au, the multi-element signature is weak and subtle and is likely to be overlooked by exploration. These conclusions contrast with those reached at Beasley Creek, north of the Menzies Line, where there is a strong multi-element signature and the best geochemical medium is the lag and the coarse, ferruginous fraction of the soil.

Last updated: Tuesday, January 04, 2000 03:59 PM


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