CRC LEME
Open File Report 16
ABSTRACT
Mineralogy and geochemistry of the Glasson Gold Deposit, Callion,
Yilgarn Block, WA
Llorca, S.M.
This report describes a mineralogical and geochemical study of
the weathered Glasson Au deposit and its country-rocks, in the Callion
area, Yilgarn Block, Western Australia. The fresh country rocks
are amphibole- and plagioclase-rich metabasalts. On weathering,
the amphibole and plagioclase are initially replaced by smectites,
opaline silica and kaolinite, and then, higher in the profile, by
kaolinite, opaline silica and goethite. As the amphibole and plagioclase
are weathered, Ca, Mg, Mn and some Si are leached; as, in turn,
the smectites are consumed, the remaining Mg, Co and some more Si
are leached away. This results in an increased relative concentration
of elements such as Fe, Al, Ti, Cr, Cu, Zr, Ga and Sc. Nickel and
Zn seem to accumulate in this part of the profile where smectites
disappear.
The fresh mineralization consists of quartz veins within a shear
zone containing additional amphibole, chlorite and mica, plus small
amounts of sulphides. Gold is present both as free Au in the quartz
and in the sulphides. In the overlying weathered zone, the sulphides
are replaced by mixtures of goethite and rutile. Amphibole and chlorite
are gradually replaced by smectites and kaolinite, passing upwards
to kaolinite and goethite. Mica is residual, though it is altered
to hydromica. Some halite has precipitated from groundwaters permeating
the shear zone.
Lateritization occurred under humid conditions during the Cretaceous
to mid Miocene. As the sulphides weathered, Cu, Zn, As, Au, Pb and
W were mobilized. These elements precipitated with the goethite,
both within the weathered sulphide zone and in the country-rock
(as far as 10-20 m away for As, Pb, Au). Although the Au included
in the sulphides was redistributed, Au in the quartz vein remained
in situ. Some of this Au was still protected by the quartz and the
Au located along the fractures which was attacked (its Ag leached),
and the Au was re-cemented virtually in situ by the goethite precipitated
in these fractures. Previously deposited secondary Au was also partially
re-cemented in the fractures within goethite. Chloride-rich solutions
that circulated along the quartz vein under the post-lateritisation
arid conditions mobilized some of the Au that had been so far protected
by the quartz. This Au was redeposited within the quartz vein and
within 20 cm of it.
During the Cretaceous to mid Miocene humid period, Au close to
the surface was remobilized and dispersed for up to 100 m from the
quartz vein, probably by complexing with humic acids derived from
soil organic matter. In the drier climates that followed, calcrete
formed close to the surface and Au was fixed within it. Finally,
mechanical dispersion at the surface caused As, Au, Pb and Cr enrichments
in the pisolitic cover as far as 100 m away.
In a context such as the one of the Glasson deposit, the best indicators
for Au mineralization are Au, As and, to a lesser extent, Pb and
W. Chromium seems a good indicator of the shear zones.
Last updated: Tuesday, January 04, 2000 12:53 PM
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