CRC LEME
Open File Report 33
ABSTRACT
Investigation of hydrogeochemical dispersion of gold and other
elements in the Wollubar Palaeodrainage, Western Australia
Gray, D.J.
The hydrogeochemistry, and the usefulness of groundwater as an
exploration medium, was investigated for a 30 km length of the Wollubar
palaeodrainage, an acid groundwater system that passes over mineralized
Archaean rocks. Limited sampling was also conducted at the Golden
Hope pit, about 1.5 km north of the palaeodrainage.
The Golden Hope groundwaters are very similar to deep waters from
other mineralized sites, having neutral pH, low to moderate Eh and
anomalous concentrations of Fe, SO4 (from sulphides), Mg, Ca, Sr
and HCO3 (from carbonates). These data, and speciation results indicating
groundwater equilibration with calcite, dolomite and magnesite,
suggest that sulphides are dissolving at the weathering front, with
the resultant acidity being neutralized by carbonate dissolution.
Other minerals that appear to be in equilibrium with some or all
of the Golden Hope groundwaters are gypsum (CaSO4.2H2O), barite
(BaSO4), amorphous alumina [Al(OH)3] and ferrihydrite [Fe(OH)3.nH2O].
In addition, the groundwaters at Golden Hope are enriched in Ga,
Mo, W, Ag, Hg, Tl, I, PO4 and Cs. These elements (possibly with
As, which was not determined) may also have value for exploration.
Most of these elements appear to be sulphide associated, so they
may not be directly related to Au. However, a system for easily
pinpointing sulphide enrichments may still have exploration value.
The groundwaters at Golden Hope are different from other mineralized
sites in that they are not Au rich. The reasons for this difference
are not clear, because in all known respects these groundwaters
should be just as effective in dissolving Au, as a thiosulphate
complex, as other sites previously investigated. If dissolved Au
is to be used as an exploration tool, it is critical to understand
why strong groundwater enrichments are only occurring at some sites.
Total salinity, K and Br data indicate that groundwaters in the
palaeochannel to the west and to the east of the main Boulder-Lefroy
shear are hydrochemically distinct. This is consistent with the
proposal that the two systems join near the shear and then flow
south into Lake Lefroy. The palaeodrainage system is acid, with
pH varying from near 6 at the northern part of the study area down
to 3 in the western arm. In general, results for Wollubar closely
matched observations at other sites with acid groundwaters, with
the major difference that the Wollubar groundwaters were Fe-rich,
and therefore tended to have lower Eh values. The mineral phases
that appear to be equilibrating with some or all of the groundwaters,
and the elements being controlled are:
- fluorite (F);
- gypsum (Ca);
- barite (Ba);
- amorphous silica, for pH <4 (Si);
- jurbanite, for pH <5 (Al);
- amorphous alumina, for pH >5 (Al);
- ferrihydrite, for pH >4.5 (Fe);
With the exception of Au, for which speciation analysis works poorly,
the minor elements were undersaturated with respect to their least
soluble mineral phase, indicating that dissolution has occurred
slowly and/or that concentration is being limited by other mechanisms
such as sorption on, or co-precipitation with, iron oxides. Most
metals, and particularly the base metals (other than the higher
charge ions Al, Sc, Cr and U), showed no clear relationship with
pH, possibly because their abundance was also affected by other
hydrogeochemical or lithological factors. The concentration of REE
is very high at Wollubar, both in the palaeodrainage and where acid
waters are directly contacting Archaean rocks, being at least 5
times greater than for any other known surface water or groundwater
in the world.
The palaeodrainage samples adjacent to the main Boulder-Lefroy
shear showed particularly anomalous characteristics, being enriched
in a similar "sulphide suite" as for the Golden Hope samples
(Ga, Fe, Mo, W, Ag, Hg and Tl), in acid soluble elements (Sc, Y,
REE and, relative to the observed pH, Al, Si and U) and also Au
and Pb. This may represent acid weathering of a similar mineralized
material to that at Golden Hope, and indicates that even high flow
palaeochannel groundwaters can have solution characteristics relating
to underlying mineralization.
Last updated: Tuesday, January 04, 2000 04:11 PM
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