A geological assessment of airborne electromagnetics for mineral exploration through deeply weathered profiles in the southeast Yilgarn Cratonic margin,Western Australia

Mineral exploration in regolith-dominated environments is challenging, requiring the development of new technical tools and approaches. When airborne electromagnetics (AEM) is combined with information on stratigraphy, mineralogy, geochemistry, drilling and landscape observations in a geological context, it becomes a powerful approach to describe the architecture of the regolith cover. This has significant implications for mineral exploration in any regolith-dominated terrain (RDT). This research presents two case studies of AEM data, integrated in a geological context for mineral exploration in the Yilgarn craton margin/Albany–Fraser Orogen (AFO).In one of the study sites presented (study site 1: Neale tenement), the availability of AEM data allowed for lateral and vertical extrapolation of the information contained in datasets at specific locations, thereby creating a 2D architectural model for the regolith cover. In addition, it was determined: (1) the total thickness of the regolith cover and its variability (between 2 m and ~ 65 m); (2) that low conductivity transported overburden and silcrete units, with a total thickness between ~ 5 and 45 m, is widely distributed, capping the upper saprolite; and (3) that the silcrete unit varies laterally from being completely cemented to permeable, and that these permeable areas (“windows”) coincide vertically with mineralogical/textural/moisture/salt content changes in the underlying saprolite, resulting in increased conductivity. This has been interpreted as resulting from more intense vertical weathering, and consequently a higher vertical geochemical dispersion of the basement signature towards surface. AEM has been used to assist in identifying and describing the lateral continuity of these “windows” in areas with no direct field observations. Surface geochemical sampling above these permeable areas may deliver more reliable geochemical basement signatures.In the second study site (Silver Lake tenement) the AEM data was strongly influenced by the high conductivity of the hypersaline groundwater. This had a significant effect on the AEM response, resulting in reduced depth penetration and reduced resolution of subtle conductivity contrasts between cover units. Despite this, the AEM data set, combined with geological observations in the area, was able to map the presence and extent of a buried palaeochannel network, the most significant architectural sedimentary feature in the cover. This interpretation allowed for a more efficient drilling campaign to be designed to sample the fresh basement rock suites in the area, by avoiding drilling into palaeochannels.Integrated and constrained by the geological context, the application of AEM conductivity models by geologists is envisioned as one of the most promising tools within the exploration geologist toolbox to understand the architecture of the cover.     

The imprinting of aridity upon a lateritic landscape: an illustration from southwestern Australia

The southwestern region of Australia contains the Yilgarn Craton that has been exposed to subaerial weathering since mid-Proterozoic. The gently undulating landscape experienced lateritic weathering so that today variably dissected, deep in situ isovolumetrically weathered regolith is widespread. Imposition of a more arid climate since the Miocene with the cessation of effective external drainage has resulted in substantial geochemical modification of the highly porous regolith. This vast pore volume acts as a reservoir for complex solutions that may be highly saline, extremely acid to alkaline and reducing. Diverse precipitates have formed in the regolith including widespread occurrence of silcrete, calcrete, dolocrete, ferricrete and gypcrete together with localised occurrences of pyrite, alunite, jarosite, barite, halite and other salts. Clearing of bush land for agriculture in the 20th century increased recharge so that rising chemically active groundwaters are damaging farmland and infrastructure throughout the region. To cite this article: B. Gilkes et al., C. R. Geoscience 335 (2003).     

Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area,Western Australia

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.     

Gold distribution and lithogeochemical discrimination of residual regolith and transported overburden,Minotaur gold deposit,Lake Lefroy,Western Australia

Abstract

The Yilgarn Craton of Western Australia hosts a number of Cenozoic paleochannels, which have been incised into the underlying Paleozoic and Precambrian rocks, filled with sediments and subsequently weathered. The paleochannels are of particular interest in mineral exploration as they may not only host placer-type deposits but also overlie significant supergene and primary mineralisation. Paleochannels also pose particular challenges during exploration as they mask underlying geochemical anomalies, including gold and pathfinder elements. This study investigates a method of distinguishing transported overburden from residual regolith utilising a combination of field and laboratory-based techniques. At the Minotaur deposit, the residual regolith and transported overburden are mineralogically similar, although the presence of biotite, chlorite and muscovite is more characteristic of the residual regolith. Geochemically, Zr, Ti, Co and Sc ratios form distinct groupings allowing more confident discrimination of transported overburden from the underlying residual regolith units. The presence of a distinct gold depletion zone at the redox front was also identified to be a feature of the Minotaur deposit, with Au enrichment occurring above (within transported overburden) and below (within saprolite) the redox front, similar to other gold deposits on Lake Lefroy. The lithological, stratigraphic and Au-enrichment characteristics at Minotaur have also been compared with regional studies and suggest the long-lived impact of paleovalleys on the depositional systems and Au distribution of Lake Lefroy. This work provides a basis for future studies of the region in particular the variation of depositional sequences within regional paleochannel networks.     

Great Victoria Desert: Development and sand provenance

Sands of the Great Victoria Desert, south‐central Australia, can be divided into three main groups on the basis of their physical and chemical characteristics (colour, grainsize parameters, mineralogy of heavy‐mineral suites, quartz oxygen isotopic composition, zircon U–Pb ages). The groups occupy the western, central and eastern Great Victoria Desert respectively, boundaries between them corresponding approximately to changes in the underlying rocks associated with the Yilgarn Craton to Officer Basin to Arckaringa Basin. Several lines of evidence suggest derivation of the sands mainly from local bedrock with very little subsequent aeolian transport. Ultimate protosources for the sands, each in order of importance, are: western Great Victoria Desert—Yilgarn Craton, Albany‐Fraser Orogen, Musgrave Complex; central Great Victoria Desert—Musgrave Complex; eastern Great Victoria Desert—Gawler and Curnamona Blocks, Adelaide Geosyncline, Musgrave Complex. Sediment from the Adelaide Geosyncline includes in addition an ‘exotic’ component from Palaeozoic sedimentary rocks probably derived mainly from Antarctica. Sediment transport of several hundred kilometres from these protosources to the sedimentary basins was dominantly by fluvial, not aeolian, means. Post‐Tertiary aeolian transport or reworking has been minimal, serving only to shape sand eroded from underlying sedimentary rocks or residual products of local basement weathering into the current dunes.     

Significance of weathering and regolith/landscape evolution for mineral exploration in the NE Albany-Fraser Orogen,Western Australia

The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.     

Proximal and remote spectroscopic characterisation of regolith in the Albany–Fraser Orogen (Western Australia)

Vast parts of the Australian continent are prospective for precious and base metal mineralisation, but exploration is hindered by extensive cover of often deeply reaching regolith. New operational exploration methods are required that can help to characterise the cover and provide information about bedrock signatures. This paper shows how mineral mapping information from a combination of satellite multispectral Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery and drill core hyperspectral profiling data (HyLogging™) can be used to unravel the regolith stratigraphy and to describe regional variations of regolith landforms, delivering important information for mineral exploration.The case study is located in the Neale tenements in the northeastern Albany–Fraser Orogen (Western Australia), which is prospective for Tropicana-style gold mineralisation. By interpretation of indicator minerals from hyperspectral drill hole logging data the regolith stratigraphy atop a metamorphic basement, comprising saprock, ferrugineous saprolite, kaolinitic saprolite, silcrete and transported cover, is recorded in cm-detail. Important mineralogical parameters extracted from the hyperspectral subsurface data and validated by XRD and FTIR, are 1) the abundance and type of iron oxides, 2) the abundance and crystallinity of kaolinite, 3) the abundance and composition of primary minerals, such as white mica, and 4) the abundance of quartz.The HyLogging™ data served as ground control points for mineral mapping information provided by CSIRO's ASTER Geoscience Products, which are a collection of mineral maps that highlight variations in the abundance, type or chemistry of selected mineral groups. Key ASTER Geoscience Products for regolith characterisation were the Ferric Oxide and AlOH abundance and composition images. The comparison of the surface with the subsurface data suggests three major different regolith landforms, including erosional, depositional and relict areas, which were used to generate a map showing transported versus relict and erosional areas. Erosional domains were mapped out in great detail, providing important information for exploration in saprolite dominated areas. Furthermore, source areas of transported material could be identified, which may help to understand the distribution of geochemical signatures collected during, for example, geochemical soil sampling projects.     

Airborne electromagnetics — Providing new perspectives on geomorphic process and landscape development in regolith-dominated terrains

In regolith-dominated terrains, the nature of contemporary processes and the surface distribution of regolith materials may be a poor guide to the character and history of regolith materials at depth. The nature of regolith materials at depth is often critical to unravelling the development of a landscape. Conventional mapping aids such as air photos, multispectral remote sensing and airborne radiometrics are not wholly adequate in this context, as they penetrate limited depths (<0.4 m). Airborne electromagnetics (AEM) on the other hand, has the potential to map regolith materials to considerable depths (>100m).The application of AEM to regolith mapping and its potential as a tool in geomorphology are illustrated by reference to an AEM survey flown at Lawlers in the Yilgarn Craton of Western Australia. At Lawlers, AEM identifies a palaeochannel that has no surface expression. It cannot be seen in images of the Landsat, airborne radiometric or airborne magnetic data. The disposition of this channel in the landscape, and in particular its association with ferruginous materials forming breakaways, suggest that inversion of relief has been a significant factor in the evolution of the Lawlers landscape.The AEM data at Lawlers have also been used to map the weathering front. The topography of the weathering front not only reflects the movement of water through the landscape in a general sense, but also reflects the influence of lithology and structure. Different lithologies are clearly weathering to different depths. Information on the nature of the weathering front is potentially an important constraint on models of groundwater flow, and by association, models of solute dispersion.     

Provenance of the Ordovician–lower Silurian Tumblagooda Sandstone,Western Australia

Detrital zircon U–Pb ages and heavy mineral assemblages provide conflicting evidence of the provenance of the Ordovician–lower Silurian Tumblagooda Sandstone, a fluvial to shallow marine, red-bed succession over 2000 m thick, within the northern Perth and Southern Carnarvon basins in Western Australia. Tourmaline composition indicates a main provenance from interior continental terranes dominated by ‘Li-poor granitoids, pegmatites and aplites’ and ‘Ca-poor metapelites, metapsammites and quartz-tourmaline rocks,’ akin to the Yilgarn Craton to the east of outcrop of the Tumblagooda Sandstone. Other possible source areas include orogens mostly to the south but lack tourmaline analyses for comparison. Taking into account the lack of garnets—a conspicuous component of the adjacent Proterozoic Northampton Inlier—the limited zircon data are compatible with the Albany–Fraser and Pinjarra orogens along the southern and western margins of Australia and/or terranes in or adjacent to East Africa and/or Antarctica, as ultimate source regions with a minor contribution from the Yilgarn Craton, as with other Phanerozoic strata in Western Australia. Whereas the textural and mineralogical maturity of the sandstone could be explained by derivation from such regions, it is more likely that the source was relatively local and that the sediment passed through several phases of reworking. The main source of ilmenite and hematite, by comparison, may have been mafic–ultramafic rocks and/or banded iron formations within the Archean Yilgarn Craton to the east or the Pilbara Craton to the northeast, mobilised by acidic meteoric waters. Iron oxides forming the earliest cements may have been derived from the oxidation of detrital hematite and ilmenite grains concentrated along some bedding laminae or transported in solution from beyond the zone of deposition. Whereas the detrital iron oxides most likely come from the craton to the east of outcrop of the Tumblagooda Sandstone, the sand grains appear to have originally come from a relatively local orogenic source.     

Ge/Si and Sr/Sr tracers of weathering reactions and hydrologic pathways in a tropical granitoid system

Ge/Si and ⁸⁷Sr/⁸⁶Sr data from primary and secondary minerals, soil waters, and stream waters in a tropical granitoid catchment quantitatively reflect mineral alteration reactions that occur at different levels within the bedrock–saprolite–soil zone. Near the bedrock–saprolite interface, plagioclase to kaolinite reaction yields low Ge/Si and 87Sr/86Sr. Higher in the regolith column, biotite weathering and kaolinite dissolution drive Ge/Si and 87Sr/86Sr to high values. Data from streams at base flow sample the bedrock–saprolite interface zone, while at high discharge solutes are derived from upper saprolite–soil zone. Coupled Ge/Si and 87Sr/86Sr can be effective tools for quantifying the importance of specific weathering reactions, and for geochemical hydrograph separation.     

Mineral deposit formation in Phanerozoic sedimentary basins of north-east Africa: the contribution of weathering

The intra- and epicontinental basins in north-east Africa (Egypt, Sudan) bear ample evidence of weathering processes repeatedly having contributed to the formation of mineral deposits throughout the Phanerozoic.The relict primary weathering mantle of Pan-African basement rocks consists of kaolinitic saprolite, laterite (in places bauxitic) and iron oxide crust. On the continent, the reaccumulation of eroded weathering-derived clay minerals (mainly kaolinite) occurred predominantly in fluvio-lacustrine environments, and floodplain and coastal plain deposits. Iron oxides, delivered from ferricretes, accumulated as oolitic ironstones in continental and marine sediments. Elements leached from weathering profiles accumulated in continental basins forming silcrete and alunite or in the marine environment contributing to the formation of attapulgite/saprolite and phosphorites.The Early Paleozoic Tawiga bauxitic laterite of northern Sudan gives a unique testimony of high latitude lateritic weathering under global greenhouse conditions. It formed in close spatial and temporal vicinity to the Late Ordovician glaciation in north Africa. The record of weathering products is essentially complete for the Late Cretaceous/Early Tertiary. From the continental sources in the south to the marine sinks in the north, an almost complete line of lateritic and laterite-derived deposits of bauxitic kaolin, kaolin, iron oxides and phosphates is well documented.     

Experimental investigation of silt formation by static breakage processes: the effect of temperature, moisture and salt on quartz dune sand and granitic regolith

ABSTRACT Weathering simulations carried out using a climatic cabinet have demonstrated that diurnal temperature and humidity variations typical of those occurring in warm desert environments are ineffective in causing static breakage of quartz dune sand and polymineralic regolith sand grains. Wetting and drying combined with temperature variations was also found to be a relatively ineffective weathering process. These results suggest that 'crack tip' processes are insignificant where mineral grains are not subject to static loading. Sodium sulphate weathering was found to cause slight damage to quartz dune sand grains and major damage to first cycle regolith grains. Feldspars and mica in the regolith sands were more susceptible to salt action than quartz. Salt weathering of the regolith sands produced substantial quantities of silt in the size range typically found in natural loess deposits.     

Gold exploration in the callion area,eastern goldfields,western Australia

The Callion gold-quartz veins are hosted by metabasalts, interflow sediments and acid metatuffs within the Archaean Norseman-Wiluna greenstone belt. The area has been subjected to deep lateritic weathering. Gold prospecting using soil geochemistry led to the discovery of a laterite Au deposit (geological reserves of 103,000 t with 1.86 g/t Au at 0.7 g/t cut-off) and subsequently to the discovery of the underlying quartz vein mineralization (geological reserves of 104,000 t with 7.9 g/t Au at 2 g/t cut-off). The quartz vein mineralization is blind, beneath 2–10 m of laterite cover. The vein, which can be traced along strike for 1 km, is mineralized over a strike length of 300 m. Weathering around the vein extends to a depth of 70 m below surface. Ore microscopy shows that the Au in the weathered vein was partially mobilized and deposited contemporaneously with Fe-hydroxides.Mining of the laterite and the top of the quartz vein enabled a detailed reconstruction to be made of the Au-grade distribution from the top of the laterite down to the saprolite. The grade distribution is mushroom-shaped, widening at the top of the mottled zone or base of the pisolithic zone. From the vein in the saprolite, the “mushroom”, defined by the 2.5 ppm Au contour, widens to 40 m, at least 5 times the original width. This zone represents the transition to the overlying wide soil anomaly which has peak Au values of 2 ppm, which are about 50% of those in the laterite in the vicinity of the vein.Magnetic methods were also used during the course of exploration and led to the discovery of another small mineralized vein.     

Process recognition in multi-element soil and stream-sediment geochemical data

Stream-sediment and soil geochemical data from the Upper and Lower Coastal Plains of South Carolina (USA) were studied to determine relationships between soils and stream sediments. From multi-element associations, characteristic compositions were determined for both media. Primary associations of elements reflect mineralogy, including heavy minerals, carbonates and clays, and the effects of groundwater. The effects of groundwater on element concentrations are more evident in soils than stream sediments. A “winnowing index” was created using ratios of Th to Al that revealed differing erosional and depositional environments. Both soils and stream sediments from the Upper and Lower Coastal Plains show derivation from similar materials and subsequent similar multi-element relationships, but have some distinct differences. In the Lower Coastal Plain, soils have high values of elements concentrated in heavy minerals (Ce, Y, Th) that grade into high values of elements concentrated into finer-grain-size, lower-density materials, primarily comprised of carbonates and feldspar minerals (Mg, Ca, Na, K, Al). These gradational trends in mineralogy and geochemistry are inferred to reflect reworking of materials during marine transgressions and regressions. Upper Coastal Plain stream-sediment geochemistry shows a higher winnowing index relative to soil geochemistry. A comparison of the 4 media (Upper Coastal Plain soils and stream sediments and Lower Coastal Plain soils and stream sediments) shows that Upper Coastal Plain stream sediments have a higher winnowing index and a higher concentration of elements contained within heavy minerals, whereas Lower Coastal Plain stream sediments show a strong correlation between elements typically contained within clays. It is not possible to calculate a functional relationship between stream sediment–soil compositions for all elements due to the complex history of weathering, deposition, reworking and re-deposition. However, depending on the spatial separation of the stream-sediment and soil samples, some elements are more highly correlated than others.     

Cenozoic Eucla Basin and associated palaeovalleys,southern Australia — Climatic and tectonic influences on landscape evolution,sedimentation and heavy mineral accumulation

The Eucla Basin including the vast Nullarbor Plain lies on the margins of the Yilgarn, Musgrave and Gawler cratons in southern Australia and owes its distinctive landscape to a unique set of interactions between eustatic, climatic and tectonic processes over the last ~ 50 Ma. Understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits, and the sediments derived from them contain remobilised gold, uranium, and heavy minerals. In particular, a remarkably preserved palaeoshoreline sequence along the north-eastern margin of the Eucla Basin is highly prospective for heavy mineral placer deposits. The record of marine, marginal marine, estuarine, fluvial and lacustrine environments, as constrained mainly by an extensive borehole dataset, reflects major depositional events during the Palaeocene–Early Eocene, Middle–Late Eocene, Oligocene–Early Miocene, Middle Miocene–Early Pliocene and Pliocene–Quaternary. These events reflect the key role of eustatic sea-level variation which, during highstands, inundated the craton margins, flooding palaeovalleys to up to 400 km inboard of the present coastline. However, a systematic eastward migration of the depocentre across the Eucla Basin during the Neogene, together with apparent flow reversals in a number of palaeovalley systems draining the Gawler Craton, suggest that the Eucla Basin has also been subject to differential vertical movements, expressed as a west-side up, east-side down tilting of ~ 100–200 m. This differential movement forms part of a broader north-down–southwest-up dynamic topographic tilting of the Australian continent associated with relatively fast (6–7 cm/yr) northward plate motion since fast spreading commenced in the Southern Ocean at ~ 43 Ma. We suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits, largely through multistage, eastward reworking of near-shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems.     

Hydrogeochemistry and weathering on the Yilgarn Block,Western Australia—ferrolysis and heavy metals in continental brines

Groundwater samples collected from salt lakes over the Archean Yilgarn Block of Western Australia, show the influence of climate on geochemical weathering patterns. Major element compositions in salt lake groundwaters, show little systematic trend, but there are marked differences in pH and Fe, Cu, and Pb, concentrations in the alkaline carbonated systems of the arid northeastern part, compared to concentrations of the same elements in groundwater systems of the Mediterranean-type climatic region of the “wheatbelt” in the southwestern part of the block. In this latter region, in addition to higher rainfall input, the groundwater systems have steeper hydraulic gradients, weathering bedrock is closer to the surface, and oxidation and hydrolysis of iron (ferrolysis) is evident in weathering profiles. A combination of vertical diffusion and horizontal groundwater flow is thought to be responsible for the development of ferrolysis and acidity in groundwaters in this region. Anomalously high concentrations of heavy metals, particularly lead, are observed in some of the acid-saline groundwaters. The formation of laterite, which is a ubiquitous feature of the Yilgam Block landscape, is a process probably closely linked to ferrolysis.     

The development of regolith exploration geochemistry in the tropics and sub-tropics

This essay traces the development of geochemical exploration from its early beginnings in the modern era during the 1930s, concentrating especially in its application to deeply weathered terrain in the tropics and sub-tropics. Following promising results obtained in temperate regions in North America and Europe, test orientation surveys were conducted to see whether similar procedures were applicable in the tropics, where conventional geological prospecting was largely precluded due to the extensive cover of a deep lateritic regolith and consequent lack of outcrop. After initial work in Sierra Leone and Nigeria, the emphasis transferred to East Africa in the 1950s and 1960s, aimed principally at Cu exploration. Many of the basic principles for exploration in dominantly residual, free-draining terrain were quickly established in this period. Exploration in terrains with more complex weathering histories, however, raised a number of difficulties due to leaching and secondary concentrations of elements, problems in selecting and identifying appropriate sample media, and extensive transported overburden. These were encountered especially in more arid regions in Australia and Africa during exploration for Ni and Au during the 1970s and 1980s. This led to a change in approach, placing weathering and geochemical dispersion in the context of regolith and landscape evolution –a return to the early concept of landscape geochemistry. The 3D expression of mineralization in the landscape is depicted as empirical conceptual models, that account for both relict features and active processes, and portray element associations, dispersion mechanisms and host materials. They also indicate suitable sample media, sampling intervals and procedures for analysis and interpretation.     

Solute geochemical mass-balances and mineral weathering rates in small watersheds: Methodology,recent advances,and future directions

Solute-based geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of a variety of geochemical processes at the Earth’s surface, including primary-mineral weathering and soil formation, and the quantitative contribution of these elemental transfer processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Weathering rates of individual minerals in watershed mass-balance studies are determined by solving a system of simultaneous linear geochemical mass-balance equations with constant (stoichiometric) coefficients. These equations relate the measured net fluxes to the (known) stoichiometries and (unknown) rates of weathering reactions for multiple minerals in the weathering profiles. Solving the system of equations requires petrologic, mineralogic, hydrologic, botanical, and aqueous geochemical data. The number of mineral-weathering rates that can be determined is limited by the number of elements for which solute mass-balance equations can be written. In addition to calculating mineral weathering rates, elemental transfer into or out of the biomass may also be calculated. Elemental uptake by aggrading forest vegetation can act as an intrawatershed sink for at least some mineral-derived cations, producing mineral weathering rates higher than would be estimated from solute fluxes alone; similarly, element release from decaying forest biomass can result in higher solute fluxes than are produced by weathering alone. The mathematics of, significant contributions from, role of biomass in, and recent advances in, watershed geochemical mass-balance methods are discussed using examples from the Appalachian headwaters watersheds of the Coweeta Hydrologic Laboratory in the southern Blue Ridge Physiographic Province of North Carolina, USA.