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
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