CRC LEME
Open File Report 32
ABSTRACT
Geochemical and hydrogeochemical investigations of alluvium at
Mulgarrie, Western Australia
Gray, D.J.
An integrated geochemical, chemical and hydrogeochemical survey
of the Mulgarrie gold deposit was conducted by sampling RAB material,
pit samples and groundwaters in the south-east margin of a palaeodrainage
which overlies Au mineralization. The boundary between basement
and alluvium is marked by decreased Mg, Zn, Ni and Au in the alluvium.
Both basement and alluvium were identified as ultramafic by examining
Ti/Zr and Cr/Fe ratios. Close to the surface, samples appeared to
be depleted in Ti, relative to Zr, possibly due to organic acids
dissolving Ti.
Elements associated with the surface carbonates are Au, which has
been correlated with surface carbonates throughout the southern
Yilgarn, and Th. These two metals are similar in that they are ordinarily
highly insoluble but have enhanced mobilities in organic-rich horizons.
Gold associated with carbonate is highly soluble and would be expected
to have a high mobility in soil horizons. The soil enhancement could
be caused by the metals been taken up by deeply rooting plants and
then deposited on the soil surface.
There is a reduction in the magnitude of the surface Au anomaly
from outcropping mineralization towards the middle of the drainage.
This could be explained in terms of a lateral transport of Au, with
dilution with distance, or by an upward movement of Au from the
buried mineralization, with the magnitude of the soil Au anomaly
being lesser with greater depth to mineralization. Iodide extraction
has shown that surface Au close to the outcropping mineralization
has a lower solubility than surface Au near the centre of the palaeodrainage
(which has a similar extraction behaviour to Au in carbonate soils
at other sites in the Yilgarn). Such distinctions may well be significant
in understanding and utilizing exploration data.
Iron oxides control the geochemistry of a number of elements, particularly
V, Cr, As and to a lesser extent, Sb, and Mn oxides have a critical
role in the accumulation of Co, Ba and the REEs, and possibly Cu,
Mg, Ca, Zn, Ni and S. In addition, Au in Mn-rich zones appears to
be highly mobile, with only that Au which has been totally occluded
by other phases being retained. Gold mobilized from such Mn-rich
zones, either at Mulgarrie or at other sites, could be a source
for secondary deposits.
Present-day groundwaters at this site are very unreactive, possibly
because of low rates of sulphide oxidation. However, there are still
several elements which have anomalous concentrations, due either
to significant lithological enhancement (e.g., Ni and Cr with ultramafics)
or to enhancements related to mineralization (e.g., Mn, Co, Ba,
I and Ni), which are similar to groundwater enhancements observed
at other sites. In addition, Mulgarrie groundwaters have specific
depletions (K) or enrichments (Mg and SO42-) of some of the major
ions, which vary at a regional level, and may be useful in distinguishing
different lithological regions. These data suggest the potential
usefulness of groundwaters at a regional, lithological and exploration
scale.
At this site (contrary to observations elsewhere) groundwater geochemistry
did not "see through" the alluvium. This may be a consequence
of the low activity of the groundwater and/or the presence of adsorptive
phases such as Mn and Fe oxides. Additionally, groundwater at Mulgarrie
has very low concentrations of Au, probably due to the lack of a
means for Au mobilization, which at other sites involves either
acid-oxidizing conditions (Au chloride) or neutral sulphide weathering
(Au thiosulphate). The Au contents of Mulgarrie groundwater would
be classified as background at these other sites.
Last updated: Tuesday, January 04, 2000 04:02 PM
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