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Open File Report 7

A mineralogical, geochemical and petrographic study of the rocks of drillhole BCD1 from the Beasley Creek Gold mine - Laverton, Western Australia

Robertson, I.D.M. and Gall, S.F.

The mineralogy, petrology and geochemistry of selected samples from DDH BCD1 were examined in detail to gain an overall understanding of the lithology, weathering characteristics and geochemistry of the Beasley Creek gold deposit (Laverton). Vertical drillhole BCD1 intersects the ore zone and footwall rocks to a depth of 100 m. The footwall consists of fresh, foliated basaltic amphibolites metamorphosed to the upper greenschist facies, consisting of a tremolite-albite-quartz assemblage. They contain at least one felsic unit and have been locally carbonated. There is a suggestion of hydrothermal alteration near the bottom of the hole where kaolinisation has taken place in the presence of fresh pyrite. Above this, pyrite is oxidised to goethite. Feldspars weather progressively to kaolinite and amphiboles to smectite. These rocks become upwardly friable and their foliation is cemented with clay, iron oxide and calcite.

The ore zone lies mainly within phyllitic rocks which are variably kaolinised and ferruginised. Garnet pseudomorphs indicate metamorphism to the quartz-albite-epidote-almandine subfacies. Progressive ferruginisation has replaced both minerals and fabrics, leaving only relict muscovite and shard-like quartz. Vesicles are lined with goethite, which becomes progressively poorer in Al, hematite and several Mn minerals including cryptomelane and lithiophorite. The upper part of the weathered profile has been subjected to calcrete formation where the rocks have been brecciated and set in a complex carbonate cement. Some of this has subsequently been dissolved leaving carbonate-lined vesicles.

The Ti/Zr ratio, together with Al and Cr are useful lithological indicators. Weathering has progressively depleted Mg, Ca and Sr and this is complete above 65 m. Mineralisation is marked by anomalous Pb, W, As and, to a lesser extent, by Be, Fe, Zn, Sb and Co.


Amphibolite, geomorphology, felsic schist, phyllite, mafic rock,

Last updated: Tuesday, January 04, 2000 10:14 AM


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