David J. Gray

Hydrogeochemistry in the Yilgarn Craton

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.

Naturally occurring Cr6+ in shallow groundwaters of the Yilgarn Craton, Western Australia

Hydrogeochemical investigations of the Yilgarn Craton and its margins have been dominantly in areas of deep (20–100 m) regolith, within unconfined aquifers where water tables are commonly 10–60 m below surface. These groundwaters contacting weathered Archaean rocks have a surprising (on the basis of its normal aqueous chemistry) lack of correlation between Cr content and acidity. Trivalent Cr has a soluble chemistry similar to that of Al, being extremely insoluble under neutral conditions, with appreciable solubilities only below pH 4. In contrast, dissolved Cr concentrations in the studied Yilgarn groundwaters can be very high, with no pH relationship, and are strongly correlated with the presence of ultramafic rocks. Groundwaters in contact with fresh and weathered ultramafic rocks contain consistently high (0.01–0.43 mg l−1) dissolved Cr concentrations, whereas waters in contact with other lithologies have Cr concentrations below detection (< 0.005 mg l−1). This effect is highly robust at the Yilgarn sites tested and offers a potential method for recognizing the presence of ultramafic rocks, even where they are intensely weathered. If the dissolved Cr were present as Cr3+, those groundwaters with Cr concentration greater than 1 μg l−1 and pH above 6 would be strongly over-saturated with respect to secondary Cr oxides. Comparison of ICP-AES and spectrophotometric analyses indicate that Cr in these groundwaters is present as Cr6+ (i.e. CrO42−), which has a much higher solubility than Cr3+. A high oxidation state of dissolved Cr is also suggested by its highly anti-pathetic relationship with dissolved Fe, possibly due to the capacity of dissolved Fe2+ to reduce CrO42− to the less soluble Cr3+ ion. A similar anti-pathetic relationship is observed between dissolved Cr and Mn. In non-reducing (i.e. Fe– and Mn–poor) groundwaters, CrO42− will be relatively stable and potentially mobile. However, the mechanism by which CrO42− is released into groundwater is not known. These naturally occurring concentrations of dissolved Cr6+ are, in many cases, well above the WHO maximum concentration allowed in drinking water (0.05 mg l−1). In at least one area (Lawlers mining district, Western Australia), otherwise potable groundwaters cannot be used for human consumption due to concentrations of Cr6+ up to six times greater than that allowable.

Mineral associations of platinum and palladium in lateritic regolith, Ora Banda Sill, Western Australia

Abstract Investigation of the mineral phases hosting the platinum group elements (PGE) has been undertaken at Mt Carnage on the PGE-rich Ora Banda sill. Western Australia. The regolith is 40–60 m thick and PGE contents increase steadily towards the surface, with a total enrichment of three- to five-fold in the lateritic, ferruginous zone, giving maximum concentrations up to 2000 ppb total PGE in clay-rich duricrusts. The accumulations appear to be residual and are of the same order as other elements, such as Cr, Zr and Cu, that also appear to be immobile. Scanning electron microscope and electron microprobe investigations were, with minor exceptions, unable to distinguish separate mineral phases enriched in PGE. Particle size analysis indicated that most Pt and Pd are in the The location of Pt and Pd in secondary Fe oxides or resistate primary phases is consistent with their accumulation being residual.

Gold mass balance in the regolith, Mystery Zone, Mt Percy, Kalgoorlie, Western Australia

Mineralized rocks at the Mystery Zone gold deposit, Mt Percy, are overlain by an almost complete lateritic regolith over 60 m thick, within which there are significant variations in Au concentration. This includes an apparent Au depletion in the upper saprolite and clay horizons and enrichment in the surficial residuum and soil. Gold leaching up through the regolith has been quantified by mass balance calculations. In the lower saprolite, 55% of the Au has been leached from the porphyries, but Au is essentially immobile in ultramafic rocks. Higher in the regolith, leaching trends are similar over both lithologies, with 74–90% depletion in the clay saprolite, and 84–95% depletion in the plasmic and mottled clay horizons. Within the surficial lateritic residuum and carbonate, Au concentrations increase to significantly greater than bedrock grade, but the calculations show there is little, if any, absolute Au enrichment, compared to the fresh rock. Silica is progressively depleted through the regolith, with more leaching over fuchsitic ultramafic rocks. Aluminium tends to remain stable in the saprolite. However, Fe shows contrasting mobility during weathering of the different lithologies. For ultramafic rocks, there is a strong absolute Fe depletion in the clay saprolite, with minor depletion in the overlying mottled and plasmic clay. In contrast, within weathered porphyries, Fe is absolutely accumulated in the most regolith horizons, possibly representing reprecipitation of Fe dissolved from adjacent ultramafic rocks. Results from this investigation agree well with a previous Au mass balance study based on isovolumetric calculations. However, differences higher in the regolith may be due to partial collapse causing errors in earlier calculations.

Regional scale hydrogeochemical mapping of the northern Yilgarn Craton, Western Australia: a new technology for exploration in arid Australia

The northern Yilgarn Craton, with an extensive mineral exploration history and relatively fresh and neutral groundwaters, was selected to test the utility of regional hydrogeochemical mapping in Australia. The assembled data of 2509 groundwater samples (generally at 4–8 km spacing) are relatively unbiased, allowing robust statistical analysis such as testing sample types (flowing v. ‘stagnant’), contamination, and lithological controls on groundwater characteristics. Lithological indicators were developed to map underlying bedrock through cover. Areas with discrepancies between groundwater results and previous geological mapping were identified. Where these are areas previously discounted as prospective for mineral commodities, they may now be re-considered on this basis. Even in well explored parts of this region, this study identified new areas which may have prospective rocks overlain with a thin (<50 m) veneer of granitic material. A large background data set was produced that has significant benefits for lithological discrimination, mineral exploration, guiding human and livestock drinking water supplies and environmental management (e.g. mine closure). Groundwater chemistry can effectively map large-scale lithological changes in these semi-arid environments, and in turn can reduce uncertainly about the prospectivity of areas within the northern Yilgarn Craton. This should reduce the drilling and associated costs required to delineate a target. The methods and interpretation developed in this study will enhance mineral exploration into covered environments as much of the northern two thirds of Australia has similar groundwater environments. This methodology can be expanded into covered arid terrains worldwide. Additionally, this can be used as background to improve interpretation of other small scale studies. Improving the exploration potential of other more difficult regions of Australia will encourage industry exploration in Australia, and provide potential future economic gains.

Hydrogeochemical exploration for volcanic-hosted massive sulfide deposits in semi-arid Australia

ABSTRACT Research on hydrogeochemistry for mineral exploration for inland Australia includes development of weathering models and extensive mine-scale and regional groundwater data. Mineral saturation indices for groundwater, activity–activity plots and reaction modelling simulate weathering of volcanic-hosted massive sulfide (VHMS) deposits in deeply weathered environments. At 10 m or more below surface, dissolved O2 is very low and other solutes such as sulfate, carbonate and nitrate are more likely oxidants. Modelling indicates that these processes differ from oxic weathering of highly eroded terrains, and provide the framework to develop robust hydrogeochemical exploration procedures in covered terrains. Sulfide weathering potentially occurs in two or more phases that effect surrounding groundwaters in differing manners. Deeper oxidative alteration of sulfides (e.g. bornite to chalcopyrite), occurring tens to hundreds of metres below surface, uses sulfate and carbonate as oxidants, causing neutral to alkaline conditions. In this zone, only pyritic massive sulfides potentially generate acidic conditions. Thus, deep sulfide-rich rocks are indicated by sulfate-depleted groundwater. Closer to the surface, sulfides are oxidised to soluble sulfates by dissolved nitrate, with much less acid production than if dissolved oxygen was the main oxidant. Thus, in shallow groundwater, sulfides are indicated by sulfate enrichment and nitrate depletion. Elements are released from sulfides and wall rocks by acid or alkaline conditions. The derived FeS (pH–Eh + Fe + Mn) and AcidS (Li + Mo + Ba + Al) indices distinguish sulfide systems through tens of metres of cover. VHMS systems are distinguished from other non-economic sulfide deposits where there is little transported cover, using various dissolved elements, including Zn, Pb and Cu. Elsewhere, ‘patchiness’ and limited aerial extent of metal signals are due to adsorption effects, that intensify with depth. Other elements such as Mn and Co have lesser diminution effects, but are less selective indicators for VHMS. There is exploration potential for elements such as Pt or Ag. These varying sulfide indicators have moderate utility, even for large-scale (∼5 km spacing) sampling. Results indicate that hydrogeochemistry can add value to greenfields exploration for VHMS ore deposits in deeply weathered terrains. It is also moderately successful at indicating the presence of sulfide-rich systems (whether magmatic or hydrothermal) under >100 m cover, thus providing a rapid and cost-effective regional prospectivity tool for deeply buried terrains. Such mineral exploration tools will encourage exploration investment for more difficult regions of Australia and in other deeply weathered regions of the world.