R.R. Anand

Naturally occurring gold nanoparticles and nanoplates

During the weathering of gold deposits, exceptionally pure, <200 nm diameter, nanoparticulate gold plates (6 nm thick) are formed. The particles display controlled growth of both size and shape and signs of assembly to form belts and sheets. The gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event. The size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids. This represents the fi rst direct observation of colloidal nanoparticulate gold in nature, confi rming this as an active mechanism of gold transport during the weathering of gold deposits.

Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits.

Eucalyptus trees may translocate Au from mineral deposits and support the use of vegetation (biogeochemical) sampling in mineral exploration, particularly where thick sediments dominate. However, biogeochemistry has not been routinely adopted partly because biotic mechanisms of Au migration are poorly understood. For example, although Au has been previously measured in plant samples, there has been doubt as to whether it was truly absorbed rather than merely adsorbed on the plant surface as aeolian contamination. Here we show the first evidence of particulate Au within natural specimens of living biological tissue (not from laboratory experimentation). This observation conclusively demonstrates active biogeochemical adsorption of Au and provides insight into its behaviour in natural samples. The confirmation of biogeochemical adsorption of Au, and of a link with abiotic processes, promotes confidence in an emerging technique that may lead to future exploration success and maintain continuity of supply.

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.

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.

The dynamics of gold in regolith change with differing environmental conditions over time

Significant Au discoveries are becoming less common because the remaining prospective, underexplored areas are obscured by transported cover. At Moolart Well (Western Australia), secondary Au deposits hosted in transported pisolitic ferricrete and saprolite are overlain by younger transported cover. Here, we show how Au has been, and is being, dispersed and concentrated in these deposits and the overlying younger transported cover and biota during the evolution of the landscape. We identified coarse (>400 µm), Ag-rich, primary, angular Au accumulated residually along with some precipitated, Ag-poor ( Acacia aneura ) and termite mounds indicates active dispersion.

Soil-gas and weak partial soil extractions for nickel exploration through transported cover in Western Australia

Traditional, near surface geochemical techniques have been effective in mineral discovery but to explore in deeper transported cover (>10 m) and find new resources, as demanded by a growing global population, new and improved exploration methods are required. As mineral exploration transitions from exploring in residual outcropping terrains into deeper covered terrains, geochemical signatures are diluted and the ability to successfully discern subtle geochemical signatures is essential. At the ‘geochemically-blind’ North Miitel Ni deposit in Western Australia, we compared passive soil-gas hydrocarbons and weakly extracted elements from soil (10 – 20 cm depth) with both proving successful. The primary mineralization is >200 m below the surface, but a weak zone of secondary Ni enrichment occurs in the saprolite at 15 – 20 m depth. This enriched zone is covered by 10 – 15 m of transported cover. Minimum hypergeometric probability (MHP) statistics were used to evaluate the near surface geochemical signatures of c. 100 samples from three traverses over the mineralization. Soil samples from 10–20 cm were subjected to distilled water, 0.1 M cold hydroxylamine hydrochloride and aqua regia extractions. The water-extractable concentrations of Ni, Co, Mo, Sb, and Sn were successful in identifying mineralization (MHP <1%, type II error). The hydrocarbons, 2-methylbutane, pentane and 1-pentene were also successful (MHP <1%) at identifying the zone of mineralization using the Amplified Geochemical Imaging (AGI) passive soil-gas collectors. Of the techniques used, the water extraction performed the best using MHP classified accuracy to identify mineralization. The passive soil-gas data were the second most effective, and superior to the stronger partial extractions using hydroxylamine and aqua regia that did not identify the mineralized zone (MHP>>1%). The water extraction and the passive soil-gas showed a greater degree of variability than the stronger extractions. The results indicate that depth of sampling, interaction with organic carbon and potential mechanisms of metal migration greatly influence the geochemical anomaly near the surface. These mechanisms are evident as hydromorphic dispersion at depth, with potential capillarity and gaseous migration up through the profile above the water table. Integrating an understanding of metal migration mechanisms, genesis and evolution of target and pathfinder compounds (particularly hydrocarbons) related to deposit types will improve future exploration. Extending into much deeper cover (>20 m), the viability of passive soil-gas methods may become more relevant and warrant further study for mineral exploration.