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Open File Report 36

Occurrence of gold in hardpan, Youanmi Mine

Gedeon, A.Z. and Butt, C.R.M.

Geochemical, mineralogical and petrological studies of the red-brown hardpan forming the surface horizon at the Youanmi gold mine were undertaken to determine the form and origin of the gold within it. The hardpan itself, apparently typical of the Wiluna Hardpan that occurs extensively in the Murchison District, is a silica-cemented unit across the transition from saprolite (in situ weathered bedrock) to colluvium (locally transported sheetwash deposit). Passing upwards, it consists of fractured saprolite, untransported saprolite blocks and poorly sorted colluvial debris cemented by a porous, red-brown matrix. The matrix contains apparently elastic clay, silt and sand-sized fragments and aggregates in a silica cement; translucent orange and clear silica (hyalite) forms a coating on the walls of fractures and voids. Despite the heterogeneity of the hardpan, the gold content appears to be fairly uniform within individual profiles. Four polished sections of hardpan were searched by scanning electron microscopy. Gold was found in only one section, in which four particles (1-3 microns) were located. These were all situated on open voids, which could be due to contamination during preparation or analysis of the samples, or to the late stage mobility of gold, either chemically or by physical illuviation. Gold probably also occurs throughout the hardpan matrix as very fine particles below the resolution of the scanning electron microscope.

Last updated: Thursday, January 06, 2000 08:10 AM


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