Nathan Reid

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 3, gas and overview

Gases provide a known mechanism of metal migration through cover and a potential sampling medium to explore through cover that is under-utilised and under-studied. Understanding how metals move through transported cover and their link to buried deposits is critical information for successful mineral exploration in many regions of the world including the well-endowed Yilgarn Craton of Western Australia. Here we employ metal and hydrocarbon soil gas collection methods to successfully predict the location of the underlying North Miitel Ni ore body. Laboratory experiments to replicate soil moisture, hypergeometric evaluation and variable spacing tests were used to verify the gaseous Ni signature. Soil gas hydrocarbon analysis also reported an unqualified, but positive result. Integrating this study with previous research on soil, regolith, groundwater and vegetation chemistry in the study area enabled a model of anomaly formation to be derived explaining the observed results and the contributions of weathering, hydromorphic, biotic, aeolian and gaseous dispersion mechanisms operating at the North Miitel site. Weathering and hydromorphic dispersion are responsible for lateral and minor vertical Ni migration at depth, aeolian Ni is dispersed laterally near the road, whereas vegetation is cycling Ni in the shallow soils only. Results indicate a gaseous migration of Ni is responsible for vertical migration through cover at this site and provides a viable target for exploration through cover.

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.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 1, regolith and groundwater

Weathering and groundwater dispersion were studied at the North Miitel komatiite-hosted Ni sulphide deposit to understand the near-surface signature of this economically important mineralization style. Soil chemistry, regolith chemistry and groundwater chemistry show the secondary Ni enrichment is weak, limited primarily to the residual parent material and dispersed a few hundred metres laterally and only a few metres vertically above the water table. Partial extractions of soils did not enhance the visibility of the mineralisation to geochemistry. However, a subtle Ni anomaly was observed in the interface of the carbonate/clay accumulation at c. 30–40  cm depth. The results show weathering and hydromorphic dispersion by groundwater are responsible for the large geochemical halo related to mineralisation at a depth of 10–20 m. A second mechanism involving gases or vegetation is proposed for the subtle near-surface anomaly. To successfully explore through transported cover it is essential to understand how surface anomalies form, what metals move and which sample media will best represent the buried ore signature. Here, groundwater provides the largest multielement target (Ni, Co, Pt and Pd), residual regolith is most accurately able to locate the mineralisation using Ni, and Ni in the soil clay/carbonate interface presents the easiest to collect but subtle surface geochemical signature of the primary ore which is buried 400 m below surface.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 2, vegetation and organic soil

Transported regolith cover hinders exploration for economically important komatiite-hosted sulphide ore bodies in the Yilgarn Craton. Biogeochemistry offers a surface sampling technique to explore for these buried deposits that has not been extensively tested in this region. In this study, eucalypt trees and some other common plants were investigated at the North Miitel Ni deposit for their use in exploration targeting and to understand what role vegetation is taking in creating near surface anomalous geochemical signatures. Primary ore is deep (c. 400 m) with a thin, weakly-enriched (compared to primary mineralisation), in situ weathered zone 10–20 m below the surface, under a cover sequence of 5–10 m deep. This study investigated the efficacy of several sample media including leaves, bark, litter, and organic-rich soils in an orientation traverse and a more extensive (150 m - spaced, 9 km2) grid-based survey (footprint). The organic-rich soil and biogeochemistry were influenced by contamination from the haul road. Directly over mineralisation some elements showed anomalous concentrations but data were erratic, not statistically significant and, therefore, not useful for mineral exploration. High groundwater salinity and low pH, lack of supergene development, weak subsurface signature and aeolian contamination contribute to why Ni anomalies have not been readily indentified at the surface. Nickel is also essential to plant physiology and is actively absorbed in a controlled manner. Investigation of plant-soil-water interactions is valuable in understanding metal mobility in the environment. Here we refined the viable exploration techniques in this setting where Ni sulphide discoveries through cover are important for future economic and resource development.

Foliar gypsum formation and litter production in the desert shrub, Acacia bivenosa, influences sulfur and calcium biogeochemical cycling in arid habitats

AimsSome sulfur-accumulating species from arid habitats facilitate a little-understood foliar gypsum (CaSO4•2H2O) biomineralisation. This study seeks to increase our understanding of the ecophysiological and nutritional function of phytogenic gypsum, including how gypsum-formers influence soil S and Ca (S/Ca) cycling mineralogy and distribution.MethodsWe studied leaf composition and mineralogy (ICP-OES, SEM-EDXS) through leaf senescence and litter degradation in Acacia bivenosa DC, together with detailed soil profile analysis (composition, S chemistry & stable isotopes).ResultsAcacia bivenosa accumulated foliar gypsum even when growing in surface soils without high S/Ca concentrations, accreting tissue-encapsulated gypsum, which was relatively recalcitrant to degradation, within the litter beneath the crown. Though A. bivenosa regulated limiting or potentially harmful elements during leaf senescence, it did not remobilise S/Ca or preferentially accrete gypsum in senescing foliage to enhance S/Ca excretion with litter. Instead, A. bivenosa maintained high S concentrations through reabsorption from phytogenic accretion zones supplemented by alternative sources, most likely in the deeper regolith.ConclusionsAcacia bivenosa influences S/Ca cycling, mineralogy and spatial distribution with the soil environment by readily accumulating S/Ca, which it concentrates within the topsoil as phytogenic gypsum. These phytogenic accretion zones can provide a sink for S/Ca salts and other potential phytotoxins, which could assist with revegetating sulfate-saline substrates.

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.