Open File Report 49
Hydrogeochemistry of sulphide weathering at Boags Pit, Bottle
Creek, Western Australia
Hydrogeochemical sampling of the Boags Open Pit at Bottle Creek
was conducted by collecting groundwaters thought to be in contact
with zones where sulphide minerals were weathering in non-acid conditions.
The near neutral pH and the chemical characteristics of the two
water samples were consistent with this assumption. The elements
that are anomalously high in these samples, and their possible source
- Ca, Mg and HCO3-, derived from dissolution of carbonate minerals;
- SO42-, As, Cd and Sb, released during oxidation of sulphide,
arsenide and antimonide minerals;
- and (possibly) Br-, which have been observed to be extensively
enriched in sulphide environments;
- Au, dissolved as the S2O32- complex, as S2O32- is released during
neutral oxidation of sulphide.
Various mechanisms are postulated to dissipate these anomalies
as groundwater disperses from the weathering sulphide body:
- Ca, Mg, HCO3-, SO42- and Br- are already present in the saline
groundwater and the anomalies will be dissipated via dilution,
acidification, and precipitation of calcite, gypsum or Al sulphates.
Thus, any enrichment of these ions in groundwaters in the surrounding
strata tend to be subtle and not easily distinguished;
- As, Cd and Sb can be lost from solution via a variety of reactions
such as precipitation or adsorption;
- I- is highly soluble, with the major loss from solution being
via oxidation to I2 (Fuge, 1990). This reaction is expected to
be slow and, given the low background of I-; the I- anomaly is
expected to only be slowly diluted out and is commonly observed
in mineralized areas (Gray, 1990, 1991);
- The extent of the Au anomaly is highly dependent on solution
conditions. If conditions remain neutral or only weakly acidic
then the Au(S2O3)23- complex may have a moderate mobility. However,
if the waters are Fe-rich then oxidation in the upper part of
the regolith can lead to waters with pH values down to 3. Under
such conditions Au(S2O3)23- will be destabilized and Au precipitated.
If conditions are sufficiently oxidizing, then Au may dissolve
as the chloride complex. However, this is a secondary reaction,
which is not directly related to the initial sulphide weathering
reactions as observed at Boags.
Last updated: Thursday, January 06, 2000 08:49 AM