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Open File Report 19

Gold morphology and composition at Panglo, Eastern Goldfields, WA

Scott, K.M. and Davis, J.J.

Samples of the generally thin soil were collected along two traverses over the Beasley Creek Mine Site, prior to mining. The soil and its components have been examined petrographically, mineralogically and geochemically.

The coarse fraction (710-4000 µm) consists of black, goethite and hematite rich nodules (some of which are magnetic), red to yellow ferruginous clay granules, quartz fragments and scarce fragments of calcrete, hardpan and cellular ironstone. The 710-4000 µm fraction is petrographically indistinguishable from the fine lag which was formed by deflation of the top layers of soil. The fragments of cellular ironstone, which are probably gossanous, are slightly more abundant near the subcrop of the ore.

The black, goethite- and hematite-rich fragments contain lithorelics which have microscopic remnants of muscovite and pseudomorphs after 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. The soil also contains a significant, quartz-rich, wind-blown component, most abundant in the 75-710 µm fraction, which acts as a geochemical diluent. These subrounded, sand to silt-sized particles, which include a few grains of fresh microcline, are coated with red iron oxides. The silty fraction (<75 µm) contains less quartz but more iron oxides and day. The <4 µm fraction is very clay rich.

Sieving and clay sedimentation were used to separate the soil into the 710-4000, the <75 and the <4 µm fractions. The complete soil and its constituent size fractions were geochemically analysed to assess their value as sampling media. The 75-710 µm fraction has a significant component of aeolian sand and was discarded. The distributions of As, Au, Cd, Cu, Sb, Se, W and Zn are related to the occurrence of mineralisation, with anomalies centred over the subcrop of the shales hosting the ore. Maxima in Ca, Mg, P and Sr delineate the calcretes, the P peak being probably related to bone fragments from burrows under the calcrete. The phyllitic ore host rock is indicated by maxima in Ba and Mn and possibly by a decrease in Y. The explanation for a maximum in S over the ore and its host rock is problematical. The complete soil has clearly been diluted by wind-blown sandy material and it is less effective than its fractions as a sampling medium. The most effective medium is the ferruginous 710-4000 µm fraction, followed by the <4 µm clays. The <75 µm silt fraction also contains a significant wind-blown component and is the least effective size fraction.

Last updated: Tuesday, January 04, 2000 01:19 PM


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