CRC LEME
Open File Report 104
ABSTRACT
Geochemical expression of concealed gold mineralization, Safari
Prospect, Mt Celia, Western Australia
Bristow, A.P.J., Lintern, M.J. and Butt, C.R.M.
The distribution and solubility of Au and the distribution of other
elements in the upper regolith and the nature and distribution of
regolith materials has been studied at and around the Safari deposit,
200 km NE of Kalgoorlie, Western Australia. The deposit has a resource
of 1.08 Mt @ 3.3 g/t Au associated with quartz veins within sheared
metavolcanic rocks (mainly quartz-chlorite-sericite schists) in
the southern extension of the Laverton Tectonic Zone. The mineralised
and country rocks are deeply weathered, and subsequently eroded
to the lower saprolite and, in places, fresh rock. They are now
covered by up to 9 m of colluvium/alluvium derived from eroding
greenstone and granite regolith several km to the ENE. Groundwater
at the deposit is approximately 40 m below surface. Much of the
regolith, and particularly the upper few metres, shows evidence
of widespread and intensive modification associated with arid conditions
(namely calcification from approximately 0.5-4.0 m below surface).
Samples of regolith (transported overburden and the upper two metres
of saprolite) from a drill traverse across the deposit, and selected
samples of primary mineralisation and vegetation have been examined
to determine element distributions, their relationship to regolith
evolution and their significance in exploration. Elements associated
with Au mineralisation are W, As, Zn, S, Pb, Si, heavy REE and possibly
Sb, though only Pb displays a direct correlation with Au. Of these,
(excluding Pb, S and Si, not analysed in the regolith), only Au
shows evidence of remobilisation in the regolith, the others being
closely confined to the mineralised zones in the weathered Archaean.
Plant materials were found to be ineffective in detecting mineralisation.
The distribution and solubility of Au in the regolith suggest that
Au has been dissolved from mineralisation in the top of the saprolite
and re-precipitated in the transported overburden close to the surface,
in association with carbonate minerals. Gold is strongly enriched
at the top of the saprolite (10-l000 ppb for 500 m peaking over
mineralisation), and solubility is generally high, although appears
lower close to mineralisation (<30% iodide soluble), increasing
with lateral distance from mineralisation (up to 80% iodide soluble),
suggesting secondary lateral dispersion around the unconformity.
In contrast, Au is anomalous (7-40 ppb), and highly soluble (>70%
iodide soluble) throughout the calcareous horizon, and close to
surface, for up to 800 m across strike, peaking directly over mineralisation,
suggesting dominantly vertical chemical dispersion. This may be
the result of two phases of Au dispersion: the first, prior to sedimentation,
resulted in lateral dispersion of Au down the relatively steep palaeo-surface;
the second, after sedimentation, resulted from solution of Au from
the carbonates in the upper saprolite and vertical dispersion by
capillary action and/or evapo-transpiration, resulting in evaporative
precipitation with carbonate 0.5-4 m from the surface.
It is suggested that a combination of a shallow overburden and
a source at the top of the saprolite have contributed to the formation
of Au anomalies in the transported overburden. These factors need
to be considered if carbonate sampling is to be applied to other
depositional landform regimes.
Last updated: Sunday, August 05, 2001 13:55:33
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