Raymond E. Smith

The mobility of the rare earth elements: Evidence and implications from selected terrains affected by burial metamorphism

The mobility of the rare earth element (REE) during hydrous burial metamorphism is described from three localities. Comparison of REE abundances in relict domains and metadomains from flood lavas in the Maddina Volcanics, Fortescue Group, Western Australia shows that, relative to the relict domains, REE may be strongly depleted in certain metadomains. Strong variations in La/Yb, La/Sm and Eu may also occur due to postcrystallization secondary processes. Similar comparisons in flood lavas from Mamainse Point in the Canadian Keweenawan Series show that REE are mobile and increase in abundance in metadomains. Spilites from U.S. Virgin Islands also show evidence for REE mobility during low grade burial metamorphism. In this case light REE (LREE) have been preferentially mobilized with heavy REE (HREE) remaining parallel or sub-parallel on chondrite normalized plots.REE analyses from these locations together with an evaluation of published work suggests that the mobility of REE can be described by: 1. gross REE and selective LREE enrichment; 2. REE movement around a primary mean; 3. gross REE depletion; 4. selective REE mobility. Failure to take into account REE mobility may lead to incorrect conclusions concerning petrogenetic models based on altered basic sequences. The coherence of REE with other key trace elements during alteration is also discussed.

Conceptual models in exploration geochemistry: Australia

Abstract This is the fourth in a series of “Conceptual Models” volumes reviewing geochemical exploration practice in different areas of the world. This volume provides an overview of geochemical exploration for base and precious metals in Australia. The subaerial weathering history of most of the continent probably dates from the Permian including, broadly, an early period of widespread lateritization followed by a late trend to aridity. This gave rise to the formation of a deep weathering profile which may subsequently have been partially or wholly eroded. It differs markedly, therefore, from the terrains described in previous volumes in the series. Consequently, emphasis has been placed throughout on the long weathering history, and the influence it has had on element dispersion and on the surface expression of mineralization in a wide range of sample media - ironstones, gossans, weathered bedrock, soils, transported overburden, stream sediments, waters, plants and atmospheres. The nature, use and geochemical characteristics, including multi-element data where possible, are described for each of these sample media. A set of eight idealized models has been derived using data from fifty-two case histories and previously published literature. The models are based on generalized landform settings, classified hierarchically according to: 1. (1) degree of preservation of the deep weathering profile, i.e. complete, partly stripped or absent; 2. (2) relief, i.e. low, moderate or high. A further model describes the landform situation within which calcrete uranium deposits are formed. The case histories present, for the most part, previously unpublished data and include examples not only of successful exploration but also situations where surface anomalies are spurious.

Pisolitic laterite geochemistry in the golden grove massive sulphide district, Western Australia

Abstract The two known massive Cu-Zn sulphide deposits in the steeply dipping strata of the Golden Grove District of Western Australia — the Gossan Hill Deposit and Scuddles Prospect — both have a multi-element secondary geochemical dispersion halo in the pisolitic layer of laterite of probable Tertiary age. The Gossan Hill geochemical anomaly is about 1.5 × 2 km and has a Bi-Sn-Mo-In-Sb-As-Cu-Au association. Mean compositions of pisolitic laterite samples range from 4 to 18 times background, with maxima for Bi, Sn and Mo reaching 90 to 100 times background. Relief of Gossan Hill, which rises some 80 m above the laterite plain, would have favoured dispersion during lateritization. Nevertheless at Scuddles, where the laterite surface has only a gentle slope across strike, an anomaly in pisolitic laterite measures 1 to 1.5 by 1.5 to 2.0 km. The anomalous association, Bi-Sn-Mo-Sb-As, is similar to that at Gossan Hill; the mean contents in pisolitic laterite samples from the Scuddles anomaly range from 2.3 to 8 times background, with maxima for Bi being 12 times background. At Gossan Hill, individual pisolites and the concretionary skins contain gossan fragments and cassiterite, indicating mechanical dispersion during interitization. However, anomalous Cu, Zn and As in goethites of the concretionary skins of pisolites and nodules from the centre of the anomaly indicate that there has also been hydromorphic dispersion. The geochemical characteristics of pisolitic laterite samples have been represented by empirically derived additive indices based on up to eleven chalcophile elements. One index is suggested as a method to increase anomaly size and simplify interpretation; a more specific index based on Bi, Sn and Se is suggested for defining more closely the actual centres of mineralization. For routine exploration of lateritic terrain where the dips of the host sequence are steep, 320-m spacing of samples on a triangular grid appears suitable for locating anomalies and generally defining their source. However, for reconnaissance exploration, the density of sampling could be significantly reduced for example to 1 km spacing.

Bridging the Yilgarn and Pilbara Blocks, western Australia

Abstract The main areas of Archaean on the Western Shield, in Australia, are the Yilgarn and the Pilbara Blocks. They are separated by the Median Belt with Lower Proterozoic and younger Precambrian deposits within which there are smaller Archaean inliers. From gravity data, these inliers can be grouped with one or the other of the main Archaean areas. The distribution of a common stress pattern, deduced from dykes of a defined swarm, indicates that the Yilgarn and Pilbara Blocks were contiguous at least in late Archaean time. Palaeogeographic reconstructions within the Median Belt suggest that during the Lower Proterozoic, a dome-shaped sialic Archaean crust acted as a substratum to the Hamersley and Nabberu Iron Provinces. A northwest-striking shelf is invoked to link the Hamersley and Nabberu Iron Provinces and there is a faster subsiding zone to the southwest. The Hamersley Group sedimentation, which is not radically different from that of some Archaean deposits, is not considered to have occurred in a barred basin separated from the Wyloo Trough, but over a submerged shelf.

Rare earth element investigation of the Cliefden Outcrop, N.S.W., Australia

The effect of low grade hydrous burial metamorphism (prehnite-pumpellyite facies) upon the rare earth elements (REE) has been studied by using samples from the Cliefden Outcrop, New South Wales. The REE, together with other reputedly ‘immobile’ elements, have been mobilised during the metamorphism. Although mobile, the REE have behaved remarkably coherently with little light rare earth (LREE) fractionation. This is reflected in the chondrite normalised patterns which are sub-parallel to parallel in shape. High correlations of REE with other elements can be used to predict the maximum likely variation of these elements in the studied outcrop. The high correlations do not necessarily mean that, for similarly metamorphosed terrains, crystallisation-differentiation processes have operated but may rather have resulted from strong geochemical coherence during post-crystallisation elemental redistribution. The REE do not appear to be strongly domain controlled within the Cliefden Outcrop.

The implications to exploration of chalcophile corridors in the Archaean Yilgarn Block, Western Australia, as revealed by laterite geochemistry

Abstract The regional distribution of the chalcophile and associated elements As, Sb, Bi, Mo, Ag, Sn and W in laterite over the Archaean Yilgarn Block of Western Australia is dominated by elongated trends within which chalcophile abundances are relatively high compared with general backgrounds. As seen from orientation studies, laterite geochemistry in Western Australia generally provides an imprint of geochemical patterns observed in bedrock, albeit with some degree of dispersion. These chalcophile trends in the lateritic duricrust materials imply the existence of chalcophile geochemical provinces in bedrock and mark out the spatial distribution of such provinces across the Precambrian craton. The chalcophile trends revealed by regional multi-element laterite geochemistry are typically 15–30 km wide and 50–100 km long, some extending to 150 km. There appears to be a broadly coincident spatial relationship between the chalcophile trends and the distribution of a variety of ore types, including gold and base-metal sulphide deposits. The name “chalcophile corridor” is herein proposed because the trends appear to be the geochemical expression of the gold exploration corridors recognized some years ago. Some of the chalcophile corridors appear to spatially link with major faults and regional shear zones. Whether the chalcophile associations in laterite are reflecting hydrothermal processes of rock alteration and mineralization along shear zones, or hydrothermal activity at Archaean volcanic centres or intrusions is presently an open question. Much more research needs to be directed at identifying in bedrock the specific rock types/structural associations that give rise to the anomalous chalcophile elements. The concept of chalcophile corridors appears to have important implications in mineral exploration because of the possibility of delineating the most prospective terrain at the reconnaissance stage of the exploration operations.

Dispersion into pisolitic laterite from the greenbushes mineralized Sn-Ta pegmatite system, Western Australia

Abstract A study of dispersion into pisolitic laterite duricrust in the Archaean Greeenbushes mineralized pegmatite district reveals a geochemical anomaly that measures some 20 km by 12 km centred on the ore deposits that, prior to commencement of mining operations, were concealed. The anomaly is broadest for As, Sn, Be and Sb, all of which show well-defined centres over the swarm of mineralized pegmatites. Coincident highs of Nb, Ta and B also define the 5 km by 1 km anomaly centre. Over the strongest part of the multi-element anomaly, the following levels in ppm are reached in the pisolitic laterite: As 1150, Sb 75, Sn 4200, Nb 75, Ta 75, W 30, Li 100, B 500 and Be 60. The immediate source for the dispersion anomaly is the swarm of soft, weathered pegmatites and their wall rocks, most of which occur within a 5 km by 1 km area, elongated along strike. The source, as judged from past production plus soft pegmatite reserves, was a mass of some 30–40 million tonnes with an approximate average grade of 220 ppm Sn, 25 ppm Nb and 30 ppm Ta. The geochemical expression of the multi-element anomaly can conveniently be expressed in a map of pegmatite-associated elements. For example, the index PEG-4*X which uses an empirically derived weighted linear combination 0.09As + 1.33Sb + Sn + 0.6Nb + Ta (where values for each element are in ppm) gives a broad consistent anomaly. Investigation of dispersion processes shows that mechanical dispersion of cassiterite and columbo-tantalite took place for distances of at least 5 km during laterite formation along gradients ranging from 5 m to 20 m per km. The occurrence of anomalous As in the skins of pisolites indicates hydromorphic dispersion during laterite formation, but mechanical dispersion of goethitic material with scavenged As has also taken place. The size of the geochemical dispersion anomaly, together with its contrast and consistency, shows that it is feasible in the Australian environment to explore for concealed mineral deposits using low-density surface sampling. Sample spacings of 3 km are being tested in an extension of this approach to exploration.

Multivariate statistical techniques applied to pisolitic laterite geochemistry at Golden Grove, Western Australia

Abstract Multivariate statistical procedures are applied to pisolitic laterite geochemistry in a study of the Golden Grove massive sulphide district. The objective is to optimize identification of geochemical anomalies caused by base metal mineral deposits. The statistical approach used in this paper depends upon geochemical data for appropriate reference groups (or training sets) being available. The target group consists of orientation data from pisolitic laterite about the Gossan Hill Cu-Zn massive sulphide deposit. A group representing background sequence was selected by combining three subareas in a geochemically quiet part of the prospective acid volcano-sedimentary sequence. A multi-element allocation procedure was set up using data from the reference groups. The exploration samples are then allocated, one sample at a time, to either one of the reference group categories, using the probability of group membership . A map showing the relative probability values for each sample site is the final product for interpretation, aided by ancillary use of an index of typicality . The allocation procedures were carried out using different element combinations, these being based on a procedure for subset selection to give maximum separation of reference groups, and on geochemical insight. Whilst many versions of the allocation procedure gave positive identification of the anomaly related to the blind Scuddles Cu-Zn deposit, allocation using only Cu, Pb, Zn and Ag did not. The results emphasize the importance of pathfinder elements in geochemical studies in weathered terrain. The allocation procedure using the most appropriate element combinations provided more positive identification of the main areas of known mineralization than had the previously used empirically derived methods of Smith and Perdrix (1983). The formal allocation procedure has the following additional advantages: results are not markedly affected by a very high value for any single element since robust procedures are incorporated into the analysis; better discrimination appears to be possible for weaker anomalies; separation of target from background can be optimized by formal calculations instead of by trial and error; and better suppression of background variation results.

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.

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.