Matthias Cornelius

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.

Geochemical mapping of the deeply weathered western Yilgarn Craton of Western Australia, using laterite geochemistry

Multi-element analysis of ferruginous nodules and pisoliths from lateritic residuum, derived lag and ferruginous gravel, selected from locally derived colluvium (laterite) sampled at an approximate 9 km interval (triangular grid) over the western Yilgarn Craton, shows regional geochemical trends, major lithologies and dispersion halos around significant bedrock mineralization. The sample density (one sample per 60–100 km2, depending on sample availability) and extent of the coverage (c. 400 000 km2, including large unsampled areas in drainage) mean that the data are potentially valuable for both exploration and environmental purposes. More than 3100 samples have been analysed for 53 elements by XRF, ICP-AES and ICP-MS, with selected samples also analysed for PGE. Elevated Au abundances in the NE of the survey area not only cluster around known gold deposits but extend beyond them, indicating the likelihood of more widespread mineralization in these areas. A chalcophile element index illustrates potential for Au and base metal mineralization in the westernmost part of the Yilgarn Craton, whereas a pegmatophile index shows a regional NW trend parallel to regional structures. Abundant chromium in granite-dominated areas might indicate mafic-ultramafic remnants (some with anomalous Au) beyond known greenstone belts. A newly discovered regional Hg anomaly trends NW for more than 500 km. Anomalous As, Bi, Mo and Sb along the southern margin of the craton may be related to Au mineralization. The spatial and geochemical consistency of the dataset means that it is well suited to multivariate statistics.

Laterite geochemistry for regional exploration surveys - a review, and sampling strategies

Numerous orientation studies and regional sampling campaigns by CSIRO and, more recently, the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), over the past 20 years, have demonstrated that residual lateritic gravels and duricrust are useful sampling media for regional, district and detailed exploration for gold, base metals and rare metals in the Yilgarn Craton. This paper focuses on regional exploration. A regional approach to geochemical sampling, using lateritic duricrust and lateritic gravels, based on regolith–landform models, is advocated. Sampling of areas of 10 000 km2 or more, or integrating several data sets, is intended to identify geochemical trends and detect large geochemical haloes several hundred square kilometres in size, arising from concealed ore deposits or mineralized districts. In unexplored or underexplored regions, sampling at 3 km spacing on a triangular grid is appropriate at the reconnaissance stage. Results of this study show that, even if the sampling pattern is opened up to 9 km spacing, major trends can still be identified. In relict landform regimes, follow-up sampling should progressively close the spacing through 3 km, 1 km, 300 m, and less, in areas of interest. The effectiveness of a broad scale approach to laterite sampling is demonstrated by using As and Ni data for the southwestern part of Western Australia, where regional geochemical trends are apparent even at a 9 km sample spacing. For regional sampling in semi-arid terrains, use of a helicopter is effective because it provides the geologist with the optimum view of the landscape for regolith–landform control, as well as flexibility to choose an appropriate site for sampling. Whereas lateritic residuum (ferruginous gravels and duricrust) represents the preferred sample medium for regional surveys, other ferruginous materials are also suitable, particularly at the follow-up stage. Assigning regolith codes to all sample media and categorizing each sample are essential. The codes become part of the geochemical database and allow valid statistical evaluation of data for specific regolith materials. In all cases, analysis of a multi-element suite, including target and pathfinder elements, is recommended. Multi-element analysis allows interpretation that can take into account variable characteristics due to sample matrix, landform position, weathering environment and gross bedrock lithology. Multi-element geochemistry also provides the opportunity for multi-commodity exploration.

Geochemistry and mineralogy of the regolith profile over the Aries kimberlite pipe, Western Australia

Residual regolith over the Aries kimberlite pipe, situated in the central Kimberley Block of Western Australia, consists of lateritic duricrust, mottled zone, upper saprolite, lower saprolite and saprock. Duricrust and mottled zone are dominated by goethite and kaolinite while saprolite consists mostly of smectite with minor amounts of mica and serpentine. Most kimberlite indicator minerals except chromite have been weathered. Kimberlitic rock fabric is preserved in the regolith up to the top of saprolite. Chromium, Ni, Co, Cu, Nb, Zr, Ti, P and rare earth elements (REE) are significantly enriched in the mottled zone and duricrust relative to the saprolite and fresh kimberlite. Vanadium, Sb, As, Mo and U, derived from the sediments and basaltic rocks around the pipe, are also enriched. Goethite partly hosts Cr, Ni, and Co, and Nb and REE are associated with anatase. The clay fractions of the mottled zone and saprolite contain significant quantities of Cr, Co, Cu, Ni, Zr, Ga, Ti, Nb, Zn and REE. A suite of six trace elements comprising those associated with ultramafic rocks (Cr and Co) and felsic rocks (Nb, La, Sm and P) can readily distinguish the kimberlitic laterite from that derived from ultramafic or felsic rocks.

Laterite geochemistry applied to diamond exploration in the Yilgarn Craton, Western Australia

This paper describes an exploration programme based on regional laterite sampling (1 to 9 km spacing) aimed at locating geochemical anomalies of kimberlite pipes. Orientation studies at the Aries pipe, Western Australia, and several pipes in the Coromandel area in the state of Minas Gerais, Brazil, demonstrated that lateritic residuum on kimberlite reflects the characteristic geochemical signature of the bedrock. Data interpretation by identifying and ranking geochemical anomalies of alkaline ultramafic rocks in regional multi-element datasets was done using univariate and multivariate statistics. These techniques can also test the datasets for other mineral occurrences, such as rare metals. Although this study did not locate a kimberlite pipe, regional geochemical sampling identified an area (46 Gate Road anomaly near Lake Brown) in the northern Merredin region where lateritic ferruginous materials appear to have formed on mafic–ultramafic alkaline bedrock, possibly an ultramafic lamprophyre that had not previously been recognized. Other regional geochemical features identified by this study, and yet to be followed up, include a Ta–Sn–Nb anomaly SE of Meekatharra, a Ni–Cu–Zn–Co anomaly in an area NE of Lake Barlee, an As–Cr–Sb–(Ni–Co–Mn–Mg) anomaly east of the Youanmi greenstone belt, and a Mg–Cr–Co anomaly in the very southern part of the survey area. The deeply weathered terrains of the Yilgarn Craton and adjacent Proterozoic belts in Western Australia present difficult environments for diamond exploration and to date no economic diamond pipes have been found in the Yilgarn Craton. With the exception of diamond and chromite, indicator minerals are destroyed through intense weathering, with very short dispersion trails of any remaining minerals in a dominantly low-relief landscape. The use of aeromagnetic data is limited by deep weathering causing noisy magnetic backgrounds in areas with preserved lateritic residuum due to contained maghemite. Additional problems arise from variably magnetic granites and gneisses, and greenstone remnants within them. Electromagnetic techniques are often adversely affected by highly conductive overburden. Laterite geochemistry applied within a regolith–landform framework has the potential to be a complementary technique for diamond exploration in deeply weathered terrain. It can be used to test geophysical targets in areas with a preserved lateritic mantle and to explore for pipes that may have obscure geophysical signatures. Orientation studies, using ferruginous gravels and duricrust formed on alkaline ultramafic and ultramafic rocks, show characteristic geochemical signatures with a halo about three times larger than the pipe itself due to mechanical and hydromorphic dispersion within the regolith.

Soil and biogeochemical signatures of the Aripuanã base metal deposit, Mato Grosso, Brazil

A geochemical survey was undertaken along two traverses at the Aripuanã base metal deposit in the Mato Grosso province of Brazil. Traverse 1 has residual and colluvial soils and intersects a 130-m-long gossan body near the top of a ridge. Traverse 2 intersects 10–20-m-wide base metal mineralization concealed by c. 5–10 m of transported cover. Humic (A-horizon) and clay-rich and reddish (B-horizon) samples were collected on both traverses; saprolite was sampled on Traverse 1. The compositions of these regolith samples were compared with those of litter and bark samples from Traverse 1, and litter samples from Traverse 2. The soil samples were split into <75-μm and 75-250-μm fractions and, with saprolite samples, analysed for 68 elements following multi-acid, aqua regia or fusion digests. Samples of litter and bark from the imbauba tree (Moraceae family, Cecropiagenus) were prepared by digesting dry tissue and ash in mixed acids. Humic A-horizon soil samples exhibit the greatest anomaly contrast of all sample media. Fine-grained samples (<75 μm) are preferable for regional exploration, whereas coarser material (75–250 μm) is more suitable for prospect-scale surveys seeking gossan outcrops. Litter samples have similar contrasts to those in B-horizon samples, and delineate the gossan zones along both traverses. Bark samples from trees over the gossan are all anomalous and may indicate mineralization beneath barren regolith. The element suite that best identifies mineralization in soil at Aripuanã comprises the target elements Cu, Pb, Zn and Au, and associated pathfinder elements In, Mo and Hg. In dry litter, the best anomaly contrast is exhibited by Cu, Pb, Zn, As, Mo, Sb and In. In dry bark, the best indicators are Pb, Mo and Sb. In depositional terrain, such as Traverse 2, both soil and litter appear to identify the location of mineralization but the number of samples is insufficient to exclude the possibility (at a probability >95%) of obtaining the same results by chance.

From tree-tops to massive sulphide in the Mabel Lake area, British Columbia, Canada

Glacial deposits, dense vegetation and rugged terrain pose challenges to conventional mineral exploration in the Mabel Lake area of southern British Columbia, where metamorphosed schists, quartzites and calcareous rocks of the Monashee sequence are known to host significant Pb-Zn deposits such as Kingfisher and Ruddock Creek. In 2006, the Geological Survey of Canada conducted a reconnaissance tree-top sampling programme to provide a biogeochemical dataset for c. 700 km2 of this region. Douglas-fir tree-top samples from 562 sample stations were collected from a helicopter, and analysis of the tissues indicated several Tl and Cd anomalies. Detailed follow-up sampling east of Mabel Lake using western hemlock bark confirmed a Tl anomaly extending c. 600 × 600 m. Data from these surveys were published but received little attention for several years until the area surrounding the main Tl anomaly was staked and detailed exploration commenced. Ah soil horizon sampling within the Tl anomaly identified multi-element anomalies including high concentrations of Mo and Zn. Indicator minerals from a few stream sediments included gahnite and goldmanite providing further evidence for the presence of base metal mineralization. Follow-up work that involved 391 m of excavations along 3 parallel trenches revealed a gossan-rich zone, 10–35 m wide and >50 m long with best concentrations of 8.98% Zn (over 3 m); 580 ppb Re; 1339 ppm Mo, and anomalous concentrations of Cd, Cu, Bi, Ni, Pb, Sn, Tl, V and W. Subsequently, a HeliTEM® electromagnetic survey was flown over c. 12 km2 of the contiguous claims and a small drilling programme has intersected Zn-rich massive sulphide. This grassroots study demonstrates the evolution of an exploration programme that started with a chronological order of: (a) reconnaissance biogeochemical survey; (b) detailed biogeochemistry; (c) Ah soil horizon geochemistry; (d) indicator mineral stream sediment survey; (e) trenching; (f) airborne geophysics; and (g) drilling that has identified significant base metal mineralization.