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Open File Report 10

Geochemistry, petrography and mineralogy of ferruginous lag overlying the Beasley Creek Gold Mine - Laverton WA

Robertson, I.D.M.

Two fractions (0.2 to 4.0 and 10 to 50 mm) of the black ferruginous lag that used to overlie the Beasley Creek gold mine have been studied physically, petrographically, mineralogically and geochemically. The fine lag was split into magnetic and non-magnetic components.

The coarse lag fraction consists of goethite and hematite with minor quantities of mica, kaolinite and quartz. It contains ferruginised lithorelics with remnant and pseudomorphed minerals and fabrics. These are related to both primary and authigenic features of the underlying saprolite. These relics occur as islands in several cycles of secondary goethite and hematite, which have obliterated much of the original fabric. In many instances, hematite can be shown to be a dehydration product of goethite. Later history of the coarse lag is shown by skins and nodules of ferruginous clay, which have undergone several cycles of solution, clay precipitation and permeation by iron-bearing solutions. Careful study of original fabrics could be used to aid the geological mapping of lag-covered areas.

In addition to black ferruginous nodules and red-brown and yellow-brown nodules, analogous to the coarse lag, the fine lag contains minor components of calcrete, quartz and a cellular ironstone, as well as very small quantities of silica-cemented red, aeolian sand and organic debris. The fine lag shows similar fabrics and components to the coarse lag but its finer and more fragmentory nature and wider dispersion make elucidation of the original rock type more difficult.

The orebody and its host, the weathered black phyllite, are depicted by positive anomalies in Au, As, Ba, Co, Cu, Mn, Mo (weak), Pb (possible), Sb, W and Zn in the lag. The cellular ironstone component is strongly anomalous in the target elements As, Au, Co, Cu, Mn, Sb, Se and Zn. Similar anomalies in Mg, Ca and Sr mark the occurrence of calcrete, which occurs near the hill crest and coincides with the orebody. Gold is the best indicator and shows strong anomalies (1000 ppb over a 10 ppb background) which are 600 - 900 m in width. Other elements show narrower dispersions. Superimposed on these broad gold anomalies are narrow, subsidiary peaks (10 000 ppb), specifically in the coarse lag, which accurately locate the ore. The wide dispersion of gold reflects chemical dispersion in the saprolite and lateritic duricrust, prior to mechanical dispersion of the lag on the surface. This is supported by the association of gold grains with vesicles and late goethite and even clay phases in the lag.

The fine lag is the most useful sampling medium and is best suited as a regional tool. The coarse lag, which is more tedious to collect, has use in follow-up work. No advantage is gained in analysis of the magnetic component, as it fails to give anomalies in Co, Cu, Se and Zn and the Au anomaly is not as distinct. This occurs because the important, non-magnetic, cellular ironstone or gossan component is excluded.


Geomorphology, petrography, maghemite, lag geochemistry, magnetic lag, mechanical dispersion, phyllite, duricrust, primary fabric, secondary fabric, Permian glacial, mica relic, accordion fabric, gold grain.

Last updated: Tuesday, January 04, 2000 10:26 AM


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