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Open File Report 5

Morphology and geochemistry of gold in a lateritic profile, Bardoc Mine, Western Australia

Freyssinet, P. and Butt, C.R.M.

The morphology and geochemistry of gold grains have been studied at different levels of the lateritic profile in the Zoroastrian Pit at Bardoc. At the bottom of the pit, 40% of the grains associated with the mineralised quartz veins were primary whereas, in the saprolite halo, only 17% were primary. The percentage decreases higher in the profile to only 4% in the mottled clay zone. However, in the ferruginous horizon, the proportion of residual, primary grains increases to 42%.

Several, different, secondary, gold morphologies have been observed, falling into four main categories: xenomorphic forms, euhedral crystals, flat pseudo-hexagonal crystals and irregular aggregates. Some grains are strongly corroded whereas others are quite well preserved and it can be assumed that there are several generations of secondary gold grains.

Electron microprobe analysis of polished sections indicates that primary gold contains 4-11% silver and that secondary gold is extremely pure. Secondary gold does, however, contain traces of iron, probably as micro-inclusions of iron oxide.

The observed gold distribution is probably the result of two mechanisms of chemical dispersion. Gold remobilisation first occurred during lateritization, principally in the ferruginous horizon, but dissolution was not complete. The second phase was during a later, arid period when gold was strongly dissolved by saline groundwaters and dispersed in the saprolite and mottled clay zone.

Last updated: Tuesday, January 04, 2000 10:09 AM


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