Open File Report 102
The distribution of gold and other elements in surficial materials
form the Higginsville palaeochannel gold deposits, Norseman, Western
Lintern, M.J., Craig, M.A., Walsh, D.M. and Sheridan, N.C.
The CRCLEME-AMIRA Project "Exploration in Areas of Transported
Overburden, Yilgarn Craton and Environs" (Project 409) has,
as its principal objective, development of geochemical methods for
mineral exploration in areas with substantial transported overburden,
through investigations of the processes of geochemical dispersion
from concealed mineralization. The Project has two main themes.
One of these, 'Surface and sub-surface expression of concealed mineral
deposits' is addressed by this report, which focuses on the soil
geochemistry of palaeochannel Au deposits located 60 km south of
Kambalda and 40 km north of Norseman.
The study area is located near Higginsville and encompasses the
Mitchell palaeochannel and the Challenger-Swordsman palaeochannel
Au deposits where the thicknesses of transported overburden and
depth to mineralization vary from about 15 m to over 50 m. The study
site is important since it provides several examples of a variably
thick overburden, and extends the boundaries of Project 409 to an
area well south of the Kalgoorlie study sites with which it can
be contrasted. The great thicknesses of transported overburden are
comparable to sites previously studied near Kalgoorlie but unlike
many of them, the palaeochannels at Higginsville contain considerable
economic deposits of Au, which are being extensively mined. It is
considered that a detailed study of the nature of Au in surficial
material from such an environment will enhance our understanding
of the processes whereby Au may be enriched in the surficial environment
in areas of substantially transported material.
The Higginsville palaeochannels are of considerable interest since
it has been reported that at a number of locations there is a detectable
surface expression of Au. One of the purposes of this research is
to assess the validity of such reports by careful sampling and analysis
and discuss the implications of the investigation for exploration
in this area. The results indicate that:
Specific targeting of the calcareous horizon maximizes the probability
of sampling the most consistently auriferous sample. In relict and
erosional regimes, such sampling may accurately define drilling
targets. However, in depositional regimes, the results indicate
that there is no direct link with mineralization. Here, although
a soil carbonate anomaly discretely overlies buried mineralization,
the data suggest that it is derived from detrital Fe granules in
Separate sampling of ferruginous granules may provide a local source
of the Au found in the carbonate horizon. Gold in ferruginous granules
usually indicates that Au is being shed from relict or erosional
areas within the catchment, hence these areas are most prospective.
Like carbonate, ferruginous granules do not in themselves provide
an indication of underlying palaeochannel mineralisation.
It is concluded that, in depositional areas examined in this study,
sampling of calcareous material at best may indicate the potential
of the area. It is suggested, therefore, that for such landscape
regimes, wider sampling intervals should be used i.e. soil sampling
is a regional tool, with a follow-up requirement that deep samples
be collected including basal sands or ferruginous material in saprolite.
Sampling of Fe granules or lag is a possible alternative, but the
distribution of Au within them is more erratic. The most cost-effective
sampling procedure is by power auger drilling and compositing the
cuttings through the carbonate-rich horizon. Surficial soil sampling
or drilling and routinely sampling at a specified depth without
regard to the sample type may be inappropriate because Au anomalies
may be overlooked.
Last updated: Sunday, August 05, 2001 13:47:52