Ravi R. Anand

Use of vegetation and soil in mineral exploration in areas of transported overburden, Yilgarn Craton, Western Australia: a contribution towards understanding metal transportation processes

Five sites were selected in the semi-arid northern Yilgarn Craton (Jaguar VHMS Cu-Zn-Ag, Moolart Well Au, Rumour Au, Gossan Hill VHMS Cu-Zn-Au and McGrath North Au deposits) to test the use of vegetation as an exploration sampling medium in areas of transported overburden. A variety of vegetation samples (litter, roots, bark, branch wood and phyllodes) were collected from Mulga (Acacia aneura) plants. In addition, the soil from 10–20 cm below surface was sampled and analysed following total, partial and selective digests to investigate any potential chemical signature of bedrock mineralization at surface. At all sites, selective extractions of the <250 μm fraction of soil show either no expression of buried mineralization or, where present, only a weak or ambiguous signature. In contrast, the vegetation survey shows a multi-element signature in different plant organs. Branches show very low anomaly contrasts or none at all. Phyllodes and bark generally show weak to moderate anomaly contrasts. Litter shows the greatest anomaly contrast of all plant material. There is also an enrichment in metals and an expression of the bedrock mineralization with good anomaly contrast in the uppermost soil horizon (0–4 cm) at Jaguar and this may indicate some mixing of litter and soil by bioturbation, and fixing on organic material, and Fe and/or Mn oxides. This contrasts with greater depths (10–20 cm) where no significant metal enrichment appears to have occurred, suggesting that most contained metals were fixed near-surface and do not percolate deeper into the profile. This is possibly due to slow decomposition of litter in arid terrains, with continuous loss of soil and fine litter particles by wind erosion and bush fires. Our data support the hypothesis that vegetation plays an important role in bringing metals to surface in areas with a semi-arid to arid climate and a low water-table.

Evolution, classification and use of ferruginous regolith materials in gold exploration, Yilgarn Craton, Western Australia

Ferruginous regolith materials are abundant and widespread in the Yilgarn Craton of Western Australia and have been successfully used as sample media in Au exploration. However, their formation has been complex. They are developed in residual and transported materials of various ages. Four general types are recognized: ferruginous duricrust and gravel, ferruginous mottles, ferruginous saprolite and iron segregations. Ferruginous duricrusts include lateritic residuum and ferricrete. Lateritic residuum has evolved by partial collapse of mottled or ferruginous regolith, involving local vertical and lateral (generally 10–50 m) movements after chemical wasting. Ferricretes are ferruginized sediments. Some are detrital clasts cemented by Fe oxides and others are authigenic pisoliths and nodules in sandy or clayey sediments. Ferruginous mottles are formed by accumulation of hematite and goethite in saprolite, residual clays or sediments. Ferruginous saprolite is formed by the uniform ferruginization of saprolite. Iron segregations form by the replacement and/or modification of sulphide-rich lithologies or as exotic accumulation of Fe oxides along preferred pathways, such as fractures, faults and lithological contacts within saprolite. Gold dispersion in the ferruginous zone of the profile is commonly considered to have a ‘mushroom’ shape formed by downward weathering, where the surficial Au halo is derived from the mineralization by a combination of chemical, residual and mechanical processes. In this study, appreciation of regolith–landform history provides an understanding of the nature of Au dispersion (or lack of it) in ferruginous materials. It has been possible to relate the mechanisms of dispersion of Au to the material being ferruginized, the environment of ferruginization and its position within a weathering profile. The distribution of Au at the micro-morphological scale provides evidence of a mobility related to the formation of particular facies of ferruginous duricrust. Accordingly, it is essential that careful attention is paid to characterization of potential ferruginous sample media and their location in the landscape in order to deduce their relationship to bedrock.

Bacterial communities associated with a mineral weathering profile at a sulphidic mine tailings dump in arid Western Australia.

We investigated bacterial community assemblages and functions down a hill slope contaminated by tailings from a volcanogenic massive sulphide mine in arid Western Australia. Weathering of waste rock, high in S and Fe, had resulted in a varying elemental dispersal down a face of the tailings hill. Bacterial community assemblage, characterised by PCR-DGGE fingerprinting, was significantly associated with electrical conductivity (E.C.) (ρ = 0.664; P < 0.01). Analysis of mobile salts showed that E.C. values were driven by ionic S, Zn, Cl and Al. The bacterial community assemblage was directly characterised across an E.C. gradient using an oligonucleotide microarray (PhyloChip). The dominant taxa at the site were Proteobacteria, Actinobacteria and Firmicutes; however, 37 phyla were detected. The most responsive taxa to variation in E.C. was Acidobacteria (negative correlation). Patterns of heterotrophic processes (BioLog analysis) were also best explained by variation in E.C. (ρ = 0.53; P < 0.01), showing a link between primary mineral weathering by lithotrophic bacteria and abiotic processes, and secondary biogeochemical processes by heterotrophic taxa. These data significantly broaden our knowledge of the bacteria present in metallomorphic ecosystems, establish that mobile phase elements are key drivers of community structure, and that primary biogeochemical cycling is directly influencing other geochemical interactions in the samples.

Distribution of Metals in the Termite Tumulitermes tumuli (Froggatt): Two Types of Malpighian Tubule Concretion Host Zn and Ca Mutually Exclusively

The aim of this study was to determine specific distribution of metals in the termite Tumulitermes tumuli (Froggatt) and identify specific organs within the termite that host elevated metals and therefore play an important role in the regulation and transfer of these back into the environment. Like other insects, termites bio-accumulate essential metals to reinforce cuticular structures and utilize storage detoxification for other metals including Ca, P, Mg and K. Previously, Mn and Zn have been found concentrated in mandible tips and are associated with increased hardness whereas Ca, P, Mg and K are accumulated in Malpighian tubules. Using high resolution Particle Induced X-Ray Emission (PIXE) mapping of whole termites and Scanning Electron Microscope (SEM) Energy Dispersive X-ray (EDX) spot analysis, localised accumulations of metals in the termite T. tumuli were identified. Tumulitermes tumuli was found to have proportionally high Mn concentrations in mandible tips. Malpighian tubules had significant enrichment of Zn (1.6%), Mg (4.9%), P (6.8%), Ca (2.7%) and K (2.4%). Synchrotron scanning X-ray Fluorescence Microprobe (XFM) mapping demonstrated two different concretion types defined by the mutually exclusive presence of Ca and Zn. In-situ SEM EDX realisation of these concretions is problematic due to the excitation volume caused by operating conditions required to detect minor amounts of Zn in the presence of significant amounts of Na. For this reason, previous researchers have not demonstrated this surprising finding.

Regolith evolution and geochemical dispersion in transported and residual regolith – Bronzewing gold deposit

Geochemical dispersion in the regolith was investigated at the Bronzewing gold deposit in a variety of geomorphological environments. A series of economic gold deposits are concealed by 5–35 m of sediments, and a thick residual profile about 100 m thick. Prior to mining, samples of regolith materials were collected from surface and drill cuttings, along several lines over the north–south trending, elongate zone of mineralization. These samples were examined in detail to determine element distributions, their relationship to regolith evolution and their significance to exploration. The residual materials (lateritic residuum and ferruginous saprolite) have Au, W and Cu anomalies in the vicinity of the deposit. Elements associated with the Au mineralization in the lateritic residuum of the Laterite Pit are Ag, Ba, Ce, W, Mo, As, Sb and Cu. Gold anomalies in lateritic residuum are less consistent in the Discovery Pit and, at both the Central and Discovery Pits, are enriched in Cu and W only close to the weathered primary mineralization. Three-dimensional modelling indicates that Au in saprolite is localized and largely residual. Geochemical dispersion from primary mineralization into the sediments is only minor, except immediately overlying the lateritic residuum or saprolite. This dispersion is, however, almost entirely mechanical and there appears to have been little, if any, post-depositional chemical dispersion. Ferruginous mottles developed in palaeochannel sediments have very low Au contents (<5 ppb) but are slightly enriched in Cu. There is a significant concentration of Au (up to 300 ppb) in authigenic smectites formed at the residual-transported interface. The deposits are blind to soil sampling (10–30 cm depth) using total and several partial extraction techniques. The results demonstrate that dispersion of ore-related elements at Bronzewing is mostly confined to the lateritic residuum and ferruginous saprolite and that sampling of these materials, even if buried beneath transported overburden, will readily locate deposits of this type. However, it is important to distinguish between residual and transported regolith, especially nodules and pisoliths. In the absence of residual materials, sampling of the lowermost sedimentary units (gravelly sediments), possibly including the unconformity, is recommended because it may show the geochemical dispersion that is wider than the primary anomaly in saprolite or bedrock. The results also provide strong support for the use of multi-element geochemistry, with elements such as Cu and W being enriched close to primary mineralization.

Source of anomalous gold concentrations in termite nests, Moolart Well, Western Australia: implications for exploration

The Moolart Well gold deposit lies in the Duketon Greenstone Belt in the Western Australian Goldfields in an area that has seen nearly 150 years of Au exploration with limited success due to the transported cover masking deposits. Here, the site displays no anomaly indicative of underlying mineralization within surface soils. Termites have the ability to burrow to the subsoil and contribute to the development of soil profiles through bioturbation. This study discusses the use of mounds formed by the termite Tumulitermes tumuli from a site where shallow ferruginised palaeochannel sediments with secondary Au enrichment overlays deeper primary mineralization. A series of samples from termite nests and surrounding soil were sampled along a transect from background areas to over mineralization. Various fractions of these samples were analysed with ICP-MS/AES. Tumulitermes tumuli is able to bring sub-surface mineralized material to the surface from 1 to 4 m depth. Termite mounds over mineralization and soil immediately adjacent to the mounds display an Au anomaly in both <250 and >250 µm fractions. Very high concentrations (>5000 ppb) were found in >2000 µm fractions in nests over mineralization as a result of vertical transport of anomalous pisolitic gravels by termites. These results suggest termite-driven local soil heterogeneity and termite mounds being a consistent geochemical and mineralogical sample medium for the discovery of ore deposits beneath weathered cover and shallow sediments.

Mineral hosts for gold and trace metals in regolith at Boddington gold deposit and Scuddles massive copper–zinc sulphide deposit, Western Australia: an LA-ICP-MS study

The association between trace elements and saprolite minerals has been investigated at the Boddington gold deposit and Scuddles massive Cu-Zn sulphide deposit in Western Australia by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe. The study provides new information about the occurrence and abundance of base metals, gold and their pathfinders in clay and ferruginous materials. Traces are predominantly located in vermiculite, interstratified clay minerals (chlorite–vermiculite and chlorite–smectite), goethite, hematite and titanium oxides. Although kaolinite is the main constitutent of saprolite, it retains insignificant amounts of trace elements and therefore dilutes geochemical anomalies. Anatase and rutile host abundant trace elements used as pathfinders for Au mineralization at Boddington (W, Bi and Mo) and base metal mineralization at Scuddles (Sb and Bi). As a result, the relative accumulation of these resistate minerals during weathering inflates pathfinder geochemical signatures. LA-ICP-MS compositional depth profiling obtained on conventional thin sections has proved to be an efficient tool to determine element–mineral associations in weathered material and to measure trace element concentrations. It allows a rapid detection of inclusions and highlights the distribution of trace elements between mineralogical phases.

Importance of 3-D regolith-landform control in areas of transported cover: implications for geochemical exploration

Two-dimensional (2-D) regolith-landform mapping has gained greater importance and wider acceptance and usage in the minerals industry over the last 20 years. This 2-D method is generally adequate in residual areas but is a poor indicator of the complexity of the deep regolith and thickness of transported cover in depositional environments. This paper highlights the importance of three-dimensional (3-D) regolith-landform control as an aid to: (a) identify major controls on metals dispersion; (b) assist anomaly interpretation; and (c) selection of suitable sample media in areas of transported cover from the Yandal greenstone belt (YGB; 260 × 40 km) of the Yilgarn Craton. It represents the first fully integrated study of the regolith on a single greenstone belt. This greenstone belt has been intensively drilled and its regolith units systematically logged. The YGB has less than 10% fresh and weathered rock outcrop and c. 90% of the area is covered by deep regolith consisting of weathered bedrock (locally up to 160 m) under sediments (up to 100 m thick). The application of 3-D mapping in the YGB is restricted to areas of precise and accurate subsurface regolith data-sets comprising c. 50,000 drill-holes covering c. 70% of the belt. The data-sets for the 3-D mapping comprise transported cover, oxidized saprolite, lower saprolite and bedrock and gold geochemistry. The regolith surfaces show that the variability of transported cover, the depth of the oxidized saprolite and the lower saprolite are controlled by the palaeorelief, lithology, mineralization and deformation. The nature and thickness of transported cover determines the suitability of a particular sampling medium. Thus, prototype derivative maps showing residual areas and areas with varying thickness of transported cover (e.g. <5 m, 5–20 m, >20 m) are required for devising a suitable sampling strategy program in depositional environments. Three-dimensional relationships between regolith and gold distribution both at regional and deposit/prospect scales have provided information that may indicate the size of geochemical dispersion and suggest sampling strategies in different transported cover domains. It also allows testing of possible vectors to mineralized sources for anomalies in transported materials. An important feature of the palaeosurface reconstructions below deep transported cover is its considerable relief which shows striking anomalies at a number of deposits and prospects in the YGB. This reinforces the importance of precisely defining this palaeosurface through 3-D mapping as it may host the best direct geochemical indicators of buried mineral systems in areas of deep cover.