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Open File Report 49

Hydrogeochemistry of sulphide weathering at Boags Pit, Bottle Creek, Western Australia

Gray, D.J.

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 are:

  1. Ca, Mg and HCO3-, derived from dissolution of carbonate minerals;
  2. SO42-, As, Cd and Sb, released during oxidation of sulphide, arsenide and antimonide minerals;
  3. and (possibly) Br-, which have been observed to be extensively enriched in sulphide environments;
  4. 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:

  1. 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;
  2. As, Cd and Sb can be lost from solution via a variety of reactions such as precipitation or adsorption;
  3. 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);
  4. 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


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