Open File Report 153
PREFACE AND EXECUTIVE SUMMARY
Preliminary regolith studies at Earea Dam Gold Prospect, Gawler
Craton, South Australia
M. J. Lintern
This CRC LEME Open File Report describes preliminary regolith studies
at the historic (circa 1899) Earea Dam Goldfield in the Gawler Craton,
South Australia. The study was conducted as part of the PIRSA Harris
Greenstone Project a pilot study for the PIRSA-CRC LEME Central
Gawler Gold Province Project. Previous case studies of Au in the
Gawler Craton regolith have been summarised in The South Australian
Regolith Project (Lintern, 2004) and this case study complements
these as it provides an example of Au dispersion in areas where
regolith is relatively thin.
The Earea Dam Goldfield is located 34 km west of Kingoonya on the
Tarcoola-Glendambo road. A regolith landform map at 1:20000 scale
was constructed to set the geochemical study into context. Although
the area has experienced mining activity, the particular location
chosen to investigate geochemical dispersal of elements was not
considered significantly disturbed. The prospect is located in rolling
hills of 30-40 m relief on lapped by shallow alluvium-colluvium.
Common bedrock exposures in the hills are Kenella Gneiss and scarcer
intrusions of mafic dykes. The hills are mantled with fine to cobbled-sized
lag of angular bedrock. Elsewhere, bedrock and saprolite are mostly
covered by a fine-grained red-brown soil partly of aeolian origin.
Observations of costeans, pits and limited drill spoil indicate
that the depth of weathering is limited to a few metres in much
of the area. Calcrete is common in the erosional regime but its
extent beneath, and within, the hardpanized (silicified) colluvium-alluvium
was not determined. The thickness of transported cover varies from
outcrop to 3 m in the vicinity of the mineralization. Sand dunes
and playas to the south and east serve to mask the underlying regolith.
The distribution of gold and other elements was investigated within
a part of the Earea Dam Goldfield known as Ian’s Mine (499444E
6585077N, AGD66), a 5 m by 20 m by 4 m deep mining slot with high
grade ore hosted by quartz-hematite. Samples were taken from the
slot and adjacent costeans, with a limited soil and auger survey
downslope of the mineralization, followed by multi-element analyses.
Gold mineralization is associated with Ag, Bi, Cu, Sn, Te, U and
W at Ian’s Mine. In the immediate vicinity, surficial dispersion
is mostly mechanical, associated with mineralized bedrock fragments.
Some high concentrations of Au are recorded in calcrete (maximum
of 475 ppb) largely due to included detrital hematite-quartz fragments.
Some chemical processes may have led to limited dispersion of Au
in smectitic clays adjacent to the mineralized quartz-hematite veins.
By 200 m downslope of Ian’s Mine, soil samples are close to
background Au concentrations (2-3 ppb). Silver, and possibly W,
are more widely dispersed than Au. Fine (<75 µm) and coarse
(>2 mm) size fractions are generally richer in Au and pathfinders
than intermediate size fractions for material from Ian’s Mine
itself but only the fine fraction is richer in Au for soil from
the depositional regime downslope of the mine. Calcrete has shown
to be concentrated in Au in the erosional areas but its poor development
in the surficial soils of the depositional areas precludes its use
as a sampling medium in these areas.
In this environment, high-grade mineralization produces a limited
geochemical dispersion halo for Au. Poorly-developed supergene Au,
possibly due to the absence of a significant thickness of regolith
within which it can develop, a resistant host rock (hematite-quartz),
and the small, narrow characteristics of the veins may be possible
explanations. Greater thicknesses of regolith, (with possible supergene
redistribution) may have been present but have since been eroded.
Exploring for more ore shoots of this type is expected to be difficult
in this area unless very close spaced calcrete sampling (25 m grid)
and/or transported-in situ interface sampling is adopted.