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Open File Report 21

Hydrogeochemistry in the Mt Gibson Gold District

Gray, D.J.

Research was conducted into the hydrogeochemistry of groundwaters within the Mt Gibson mine area and in the surrounding district. This work involved determination of field parameters such as pH and Eh, laboratory analysis of water samples for major and trace elements, isotope determinations (D and O18), computer speciation of analytical data, and statistical analysis of the water data.

The groundwater system is dominated by a northward saline drainage system. Groundwater flow along this drainage appears to be restrained by an underground sill about 7 km north of the mine area, resulting in highly saline groundwaters within the mine region. This saline groundwater appears to flow back, south into the mine area, at depth. Thus, the north section of the mine area has fresher waters (about 3% TDS) overlying hypersaline water (>13% TDS).

Based on the major element and isotope analyses, the mine groundwaters were resolved into a number of hydrogeochemically distinct water masses. In particular, the waters from drill hole sample sites 600 m west of the major area of supergene Au mineralization at Midway were identified as probably originating from contact with granitic rocks. The other mine groundwaters appear to be associated with mafic or ultramafic systems.

Waters within the Midway area showed highly anomalous characteristics, being high in dissolved Au, Fe, Mn, Co, Cd, Ba and I, and having low HCO3 concentrations. These observations are explained as being due to weathering of sulphide minerals. Down gradient of the Midway area, the groundwater becomes acidic, due to oxidation-hydrolysis of the dissolved Fe. This has led to major dissolution of many metals, particularly (in order from least to most enriched) Cd, Co, Ni, Zn, Cu, Cr, Al and Ag. This enrichment is related to the base affinity of the metals.

Soluble Au was only observed above the detection limit (0.05 µg/L) within the mineralized area. Two major anomalies were recognized: the first, within the Midway area, may represent dissolution of Au by thiosulphate; while a second, more localized anomaly, within the N2 pit area may represent dissolution by chloride.

On the basis of this work, soluble Au analyses could be used at this site to indicate areas of Au mineralization at both a district and a mine scale. Thus, measurements of dissolved Au may represent a useful adjunct to drilling during Au exploration, particularly with respect to buried mineralization.

Last updated: Tuesday, January 04, 2000 01:42 PM


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