Abstract

The hydrogeochemistry of the Yilgarn Craton and its margins has been extensively investigated, with particular emphasis on the chemistry of Au. Four groundwater regions have been delineated based on variations in salinity, acidity and oxidation potential: (1) Northern (N Yilgarn and margins) – Fresh and neutral, trending more saline in the valley axes; (2) Central – Neutral and brackish (commonly <1% TDS) to saline (about 3% TDS), trending to hypersaline (10–30% TDS) at the salt lakes, with common increases in salinity with depth; (3) Kalgoorlie – Commonly acid (pH 3–5), except where buffered by extremely alkaline materials (e.g. ultramafic rocks), and saline within the top part of the groundwater mass, trending more neutral (pH 5–7) and hypersaline at depth and within a few kilometres of salt lakes; and (4) Eastern (E Yilgarn and Officer Basin) – Saline to hypersaline, neutral to acid and reducing. Dissolved concentrations of many ions are low, due to the presence of lignites in the channel sediments.

These regional variations have major effects on the concentrations of many elements. Aluminium, Li, Y, REE and U are dominantly controlled by pH and thus have higher concentrations in acid groundwaters, such as those in the Kalgoorlie region. Dissolved concentrations of Mn, Co, Ni, Cu and Zn are less closely correlated with acidity, and show scope for lithological discrimination, but there is no apparent relationship with Au mineralization. Dissolved Cr shows an absolute correlation with ultramafic rocks, apparently irrespective of pH, possibly due to its presence as chromate (i.e. Cr6+ as CrO42−). Concentrations of As, Sb Mo, W and Bi are low in acid groundwaters, but are higher above pH 6.5, particularly in the Central region. Therefore, acid groundwaters (particularly in the Kalgoorlie district) will be poor media for the use of these elements as exploration pathfinders. Molybdenum differs from the other elements in this group in having significant concentrations in acid groundwaters, although lower than in neutral and alkaline groundwaters.

Dissolved Au is commonly the best pathfinder for Au mineralization. It occurs dominantly as halides (chloride and/or iodide) and has enhanced concentrations (to >1 ppb) under the acid/saline/oxidizing conditions common in the Kalgoorlie region, whereas concentrations in the northern Yilgarn are two orders of magnitude less. This implies that supergene Au remobilization should be considerably less in the northern Yilgarn than in the Kalgoorlie region. Additionally, the threshold dissolved Au concentration as used for Au exploration differs significantly between regions.

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