Ferruginous Saprolite Research Articles

Mineralogy and geochemistry of the Morro do Engenho lateritic nickel deposit, Goiás Alkaline Province, Brazil

The Morro do Engenho lateritic nickel deposit results from the action of weathering processes on the alkaline mafic-ultramafic rocks that make up the Cretaceous zoned intrusive body belonging to the Goiás Alkaline Province. Historically, the deposit is classified as a silicate one, and mineralization is present especially in phyllosilicates such as serpentine and chlorite. By identifying Ni-bearing phases and providing the lithogeochemical characterization of weathering profiles, this study aims to provide further insights into the metallogenetic processes and controls acting on the mineralization of the Morro do Engenho lateritic Ni deposit. The following methods were used: petrographic studies, SEM-EDS, XRD and lithogeochemical characterization of the weathering profiles developed on each of the parent rocks that make up the body. The Morro do Engenho intrusive massif is formed by a dunite core, surrounded by a peridotite/pyroxenite zone, an alkaline gabbro zone and more externally by a syenite-nepheline zone. The lateritic profile constitutes eight weathering horizons, stacked one upon another from the bedrock namely: (i) saprock, (ii) lower saprolite, (iii) ferruginous saprolite, (iv) ocher saprolite, (v) plasma zone, (vi) lateritic duriccrust, (vii) silcrete and (viii) topsoil. Most Ni mineralization occurs in the lower saprolite, ferruginous saprolite and ocher saprolite zones. The findings pointed to the presence of Ni in silicate mineral phases, represented by serpentine, chlorite, smectite, as well as in Fe and Mn oxyhydroxides. The depressed area, located between the two major topographic elevations, has the highest Ni enrichments, which according to mass balance calculations can account for up to 40%. Hydrodynamic factors, by dissolution/precipitation of silicates in saprolite horizons of dunite core, are suggested as possible agents in the remobilization of Ni, among other mobile elements, to topographically flat zones. The geometry of the Morro do Engenho deposit show that lateral redistribution by hydrodynamic factors could also account for the mineralization of areas peripheral to the dunite core, composed of pyroxenite and gabbro-derived lateritic profiles. The direction of regional groundwater flow, strongly influenced by topography, would also be an important factor in the remobilization of Ni, as well as the mechanical transport of lateritic materials. Silicate mineralization predominates in the Morro do Engenho deposit, concentrated in Mg hydrosilicates and clay minerals; oxide mineralization also occurs, mostly in Fe and Mn oxyhydroxides. The findings discussed in this paper can broaden the understanding of metallogenetic processes involved in the genesis of lateritic nickel deposits. .

Evolution of the Piauí Laterite, Brazil: Mineralogical, Geochemical and Geomicrobiological Mechanisms for Cobalt and Nickel Enrichment

The Piauí laterite (NE Brazil) was initially evaluated for Ni but also contains economic concentrations of Co. Our investigations aimed to characterise the Co enrichment within the deposit; by understanding the mineralogy we can better design mineral processing to target Co recovery. The laterite is heterogeneous on the mineralogical and lithological scale differing from the classic schematic profiles of nickel laterites, and while there is a clear transition from saprolite to more ferruginous units, the deposit also contains lateral and vertical variations that are associated with both the original intrusive complex and also the nature of fluid flow, redox cycling and fluctuating groundwater tables. The deposit is well described by the following six mineralogical and geochemical units: SAPFE, a clay bearing ferruginous saprolite; SAPSILFE, a silica dominated ferruginous saprolite; SAPMG, a green magnesium rich chlorite dominated saprolite; SAPAL, a white-green high aluminium, low magnesium saprolite; saprock, a serpentine and chlorite dominated saprolite and the serpentinite protolith. Not all of these units are ‘ore bearing’. Ni is concentrated in a range of nickeliferous phyllosilicates (0.1–25 wt%) including serpentines, talc and pimelite, goethite (up to 9 wt%), magnetite (2.8–14 wt%) and Mn oxy-hydroxides (0.35–19 wt%). Lower levels of Ni are present in ilmenites, chromites, chlorite and distinct small horizons of nickeliferous silica (up to 3 wt% Ni). With respect to Co, the only significant chemical correlation is with Mn, and Mn oxy-hydroxides contain up to 14 wt% Co. Cobalt is only present in goethite when Mn is also present, and these goethite grains contain an average of 0.19 wt% Co (up to a maximum of 0.65 wt%). The other main Co bearing minerals are magnetite (0.41–1.89 wt%), chlorite (up to 0.45 wt%) and ilmenite (up to 0.35 wt%). Chemically there are three types of Mn oxy-hydroxide, asbolane, asbolane-lithiophorite intermediates and romanechite. Spatially resolved X-ray absorption spectroscopy analysis suggests that the Co is present primarily as octahedrally bound Co3+ substituted directly into the MnO6 layers of the asbolane-lithiophorite intermediates. However significant levels of Co2+ are evident within the asbolane-lithiophorite intermediates, structurally bound along with Ni in the interlayer between successive MnO6 layers. The laterite microbial community contains prokaryotes and few fungi, with the highest abundance and diversity closest to ground level. Microorganisms capable of metal redox cycling were identified to be present, but microcosm experiments of different horizons within the deposit demonstrated that stimulated biogeochemical cycling did not contribute to Co mobilisation. Correlations between Co and Mn are likely to be a relic of parent rock weathering rather than due to biogeochemical processes; a conclusion that agrees well with the mineralogical associations.

Supergene gold in silcrete as a vector to the Scuddles volcanic massive sulfides, Western Australia

This study reports data on the first occurrence of economic supergene Au in a near-surface silcrete horizon over the Scuddles volcanic massive sulfide (VMS) deposit in the Golden Grove district, Western Australia. A deep weathering profile was developed on dacite, rhyodacite, siltstone, and breccia that host primary Cu, Zn, and Pb sulfides with Au-Ag ore. From the base, the weathering profile at Scuddles is subdivided into five main zones: (1) supergene sulfide enrichment zone; (2) supergene oxide enrichment zone; (3) ferruginous saprolite; (4) leached zone of kaolinitic saprolite and silcrete; and (5) lateritic zone of mottled clays, ferruginous duricrust, and gravels. Silcrete at Scuddles hosts supergene Au deposit that formed in two generations: the first is intimately associated with Ag halides during supergene enrichment of the primary VMS, and the second is associated with kaolinite in dissolution cavities inside Ag halides during lateritic weathering. These two Au generations imply more than one mechanism of Au remobilization and formation, multiple fluid pathways, and superimposed episodes of weathering under variable timing and climatic conditions. Gold grains are pure, nanocrystalline (up to 10 nm) and clustered together forming microcrystalline aggregates. A few Au grains are residual in silcrete with Ag-Sb-rich cores and Ag-poor rims possibly formed during dealloying of Ag and Sb. Chemically, Au in silcrete is associated with a multi-element concentration of Ag, I, Br, Cl, Sb, Sn, Bi, Hg, Mo, W, Te, and Ge. Gold and Ag in the supergene weathering profile were mobilized to silcrete as a halide complex under acidic and saline conditions generated during the oxidation of massive sulfides at depth. The precipitation of Au-Ag halides in the silcrete may have taken place in response to a rise in pH. Gold was likely remobilized with kaolinite from the surface lateritic zone, facilitated by decays of plant roots and bioturbation. The clustered spongy, cube-octahedral, platy (six-sided), dendritic-, and reniform-like morphologies of Au in cavities inside Ag halides may indicate biogenic-related processes in its precipitation. Recognizing Au-Ag-rich silcrete over the buried VMS at Scuddles highlights the significance of the silcrete in finding buried VMS, particularly if the gossan is absent.

Mineralogy and genesis of the Ni–Co lateritic regolith deposit of the Çaldağ area (Manisa, western Anatolia), Turkey

The Çaldağ Ni–Co deposit is characterized by a reddish brown oxide lateritic regolith, containing residual Ni deposit formed by the intense tropical weathering of peridotites. Nickel–Co ore is associated with transported ferricrete during the late Paleocene – middle Eocene, represented by colloform Fe oxides and residual lateritization during the Oligocene. The lateritic regoliths are developed over dunitic ultramafic rocks and consist mainly of smectite, berthierine, kaolinite, gypsum, pyrite, takovite, and pecoraite. These units were examined using polarized-light microscopy, X-ray diffraction, scanning and transmission electron microscopy, and geochemical methods. Mineralogical zonation from the base of the profile upwards has the following zones: ultramafic bedrocks, serpentinized ultramafic rocks (saprock), saprolite, carbonate- and sulfide-bearing zone, ferruginous saprolite zone, and silcrete. In addition, Fe oxides, smectite and opal-CT, and quartz increase towards the surface, whereas olivine, pyroxene, and serpentine decrease upwards in response to chemical weathering. Nickel–Co mineralization associated with Fe oxides and smectitic clays formed under wet and dry conditions, respectively, as a result of an increased Fe2O3+ Al2O3+ Ni + Co/MgO ratio. Field observations and mineralogical and geochemical analyses reveal that the smectite formed under basic conditions was controlled by multistage chemical weathering of ultramafic and volcanic rocks and by the concentrations of Si, Al, Fe, and Mg. Locally, concentrations of S in conjunction with Fe and Ca resulted in precipitation of goethite–hematite, gypsum, and pyrite in dissolution voids in association with smectite under acidic conditions that developed in a well-drained system.

Geochemical exploration for supergene copper oxide deposits, Mount Isa Inlier, NW Queensland, Australia

Extensive, variable, and generally thick regolith is a major challenge to mineral exploration in regolith-dominated terranes. The landscape of the Mount Isa Inlier, NW Queensland, Australia, is the product of several episodes of complex weathering, erosion and sedimentation. Weathering profiles were studied from more than 100 drill holes along three traverses crossing the Lady Annie, Mount Kelly and Anthill sulfide deposits to assess the nature, lateral extent and thickness of the supergene Cu oxide mineralization. A typical regolith profile consists, from the base up, of saprock, ferruginous saprolite (Cu-oxide zone), kaolinitic saprolite and silcrete bands (leached zone), and a pedolith of mottled, lateritic duricrust and soil zones. Oxidation of sulfides (primarily pyrite) in host rocks has created an acidic pH gradient, mobilizing Fe, Mn, Cu, Mo, Zn, Co, Ni, As, Bi, In, Sb, Be, P, U and Cd. The acidic pH is buffered by calcareous lithologies of the host rocks, particularly in the vicinity of the primary sulfides at Lady Annie and Mount Kelly resulting in formation of thick, mushroom-shaped Cu oxide halo, in which Cu may reach >15wt.%. The Cu values decrease to <500ppm in the bleached kaolinitic saprolite and silcrete. Conversely, siliceous bedrock lithologies at Anthill act as a weak buffer to the acidic fluids; thus, Fe, Cu and other mobile elements are laterally dispersed from the sulfide ore body and precipitated at different lithological boundaries where they are buffered by carbonates. Therefore, thick kaolinitic saprolite and silcrete are formed over sulfide mineralization, particularly along faults. Ferruginous duricrusts marginal to fault-controlled sulfide mineralization are anomalous in Cu, Zn, Cd, Sb and Be, and thus can be used as useful medium in geochemical exploration. Gold, Ag, Pb and As are residually concentrated in pedolith and kaolinitic saprolite over the primary sulfide mineralization. Thus, these elements can be used effectively as pathfinder elements, particularly in truncated landscape, where pedolith is eroded and the exposed kaolinitic saprolite is depleted in Cu. Ferruginous soil (850–300, 300–80 and <80μm size fractions), lag (>1.6 and >7.6mm) and termite mounds have been applied successfully as sampling media for Cu oxide exploration in Anthill. Soil and lag over sulfide mineralization are rich in Fe, As, Pb, Ag, Bi, In, V, U and Mo. The best response over sulfide mineralization is recorded in the >7.6mm ferruginous lag. However, broadest signature of well-defined Au, Bi and In anomalies are better detected using termite mounds than soil and lag. Successful use of soil in exploration for Cu oxide deposits in Mount Isa requires a genetic discrimination between residual and transported origin.

Geochemical exploration challenges in the regolith dominated Igarapé Bahia gold deposit, Carajás, Brazil

The Igarapé Bahia, situated in the Carajás Mineral Province, is a world-class example of a lateritic gold deposit. It has developed under tropical weathering conditions since at least the Eocene and resulted in a regolith cover of at least 100m thickness. The regolith is dominated by ~80m thick ferruginous saprolite containing gossan bodies that constitute the main Au ore. Above saprolite the regolith stratigraphy has been established considering two distinct domains. One composed of residual materials and the other transported materials deposited over palaeochannels. In the residual domain the ferruginous saprolite grades upwards into a fragmental duricrust, interpreted as a collapsed zone, and then into different types of ferruginous duricrusts. Over palaeochannel the ferruginous saprolite is truncated by poorly sorted ferruginous sediment of variable composition that grades upwards into the ferruginous duricrusts formed over transported materials. Lateritization took place during a marked period that transformed the colluvium of the residual domain, and the transported materials accumulated in the channel depressions, into the ferruginous duricrust units. A later bauxitization event has overprinted all duricrust types but has mostly affected the duricrusts over the palaeochannel forming gibbsitic nodules. All duricrusts were finally covered by a transported layer of latosol which flattened the whole landscape in the Carajás region. Gold shows a depletion trend across the regolith but is enriched in the fragmental duricrust below the ferruginous duricrust from which gold is leached. Gold is also chemically dispersed laterally into the fragmental duricrust, but lateral Au dispersion in the ferruginous duricrusts of the residual domain is probably also influenced by colluvial transport. Metals associated with Au mineralization (Cu, U, Mo, Pb, Ag, LREE, Sn, W, Bi, Sb and P) are generally depleted in the saprolite but most of them are still anomalous. The fragmental and ferruginous duricrusts are more leached but the tests performed to estimate the dispersion potential of metals contained in the ferruginous duricrust show that some metals are still significantly anomalous especially Au, Ag and Cu. However, if ferruginous duricrusts are used as an exploration sample media their environment of formation must be considered. Metal depletion is generally more advanced in the ferruginous duricrusts developed in the vicinities of palaeochannels as oppose to those developed in residual domain. On the contrary, Au over palaeochannel areas is enriched in the upper bauxitized ferruginous duricrusts and in their gibbsitic nodules as a result of lateral chemical transport that is more widespread than in the colluvium over residual domain. The latosol is highly depleted in most metals due to its transported nature. However, the nodular fractions of the latosol show the greatest dispersion potential especially for Au, Ag, W, U, Bi and Sn. It can incorporate magnetic nodules that bring a rich suit of metals associated to the magnetic gossans, and non-magnetic nodules, classified as concretion and pisolites, which bring metals enriched or dispersed in the ferruginous duricrusts. This suggests that Lag constitutes a promising sample medium for geochemical exploration in the lateritic terrains of the Carajás region.

Regolith mapping of deeply weathered terrain in savannah regions of the Birimian Lawra Greenstone Belt, Ghana

A regolith map for the Lawra Belt has been developed by categorising the regolith-landform units by processing and interpreting remote sensing data. Regolith landform units were extracted from Landsat band ratios 3/1 and 5/4 to map ferruginous saprolite and lags; band ratio 5/7 was used to identify residual regolith and band ratio 4/2 was employed to separate ferruginous units from non-ferruginous regolith. Additional regolith landform units' discrimination was provided by compiling and interpolating radiometric data particularly for Landsat TM poorly defined areas. SRTM images were used to mark out the extent of the alluvial plains. High topographical terrains were marked from DEM image to represent the residual areas. Regolith landform unit (RLU) map that showed residual (relict and erosional), ferruginous, and depositional domains of the Lawra Belt was developed by superimposing the extractions made from the remote sensed data. Interpretive map generated from the remote sensed image analysis was validated by first creating a non-genetic regolith map through ground truth survey. The non-genetic map based on spatial distributions of the different regolith mapping units were classified on genetic classes or regimes based on regolith-landform similarities to develop a genetic map. The interpretive and the genetic map were superimposed to develop the regolith map for the Lawra Belt. The inliers and outliers presenting compositional overlaps within broad regolith classes were rectified from the field mapping information. The combined approach of image analysis and the ground truth mapping grouped the regolith of Lawra Belt into ferruginous (F), relict (R), erosional (E) and depositional (D) regimes.

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 20-55m 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 (2-25m 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 (<20cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3m at the Atlantis prospect, 5km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (>2,000μm, 2,000-250μm, 250-53μm, 53-2μm and <2μm). 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 20cm 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.

Composition and Genesis of the Nickel-Chrome-Bearing Nontronite and Montmorillonite in Lateritized Ultramafic Rocks in the Muratdği Region (Uşak, Western Anatolia), Turkey

AbstractWidespread lateritized ultramafic rocks in the southern part of the Muratdağı region of Turkey constitute a significant source of Ni-Cr-bearing ore with economic potential. However, no mineralogical or geochemical characterizations of these important materials have been performed previously. The aim of the present study was to describe the mineralogy, geochemistry, and genesis of Ni-Cr-bearing smectite in garnierite and ferruginous saprolite associated with the lateritized ophiolite-related ultramafic rocks. The lateritic zones are well developed over serpentinized harzburgitic mantle peridotites. The lateritized units and related bedrocks were examined using polarized-light microscopy, X-ray diffraction, scanning and transmission electron microscopies, and chemical and isotopic methods. The garnierite-containing saprolites are enriched in smectite, Fe-(oxyhydr)oxide phases, and opal-CT. Micromorphological images revealed that flaky smectite and, locally, Fe-rich particles, alunite, gypsum, gibbsite, and sulfur crystals developed along the fractures and dissolution voids. The development of saprolite demonstrates chemical weathering. The presence of silicified and Fe-(oxyhydr)oxide phases associated with gypsum, alunite, and local native sulfur in vertical and/or subvertical fractures and fault infillings are indicative of hydrothermal processes along the extensional, tectonically related fault systems. Chemical weathering and hydrothermal processes, which probably started during the Oligocene and Miocene, led to the formation of nontronite, Fe-bearing montmorillonite, and local Fe-rich kaolinite. Nickel and Cr are concentrated significantly in the saprolite zone and are positively correlated with Fe2O3 content, which is controlled by the formation of nontronite, montmorillonite, and Fe-(oxyhydr)oxide phases. Nickel-Cr-bearing nontronite and montmorillonite precipitated from alkaline water as a result of the increasing (Fe2O3+Al2O3+Cr2O5+Ni+Co)/(MgO+SiO2) ratio under the control of both chemical weathering and hydrothermal processes. The Fe and Mg (associated with Ni and Cr) required for the formation of smectite were supplied by solutions from both chemical weathering and hydrothermal alteration of Ni-Cr-bearing olivine and pyroxene in the harzburgitic bedrock; the Al was supplied by schists, granite, and volcanic units.

Mineralogical and geochemical characteristics of a serpentinite-derived laterite profile from East Sulawesi, Indonesia: Implications for the lateritization process and Ni supergene enrichment in the tropical rainforest

To evaluate the lateritization process and supergene Ni enrichment in the tropical rainforest, a well developed laterite profile over the serpentinite in the Kolonodale area of East Sulawesi, Indonesia, has been investigated using field geology methods, mineralogical and geochemical techniques. Three lithostratigraphic horizons over the bedrock are distinguished from bottom to top: the saprolite horizon, the limonite horizon, and the ferruginous cap. In general, the profile is characterized by (1) a depth-related pH ranging from 5.56 to 8.56, with a higher value in the saprolite horizon and a lower value in the ferruginous cap, (2) a highly variable organic matter concentration from 1.11% to 4.82%, showing a increasing trend from bottom to top, (3) a progressive mineral assemblage transition from the silicate mineral dominant (mainly serpentine) to the Fe-oxyhydroxide dominant (mainly goethite), and (4) a typical laterite geochemical pattern with an increase of Fe, Al, Mn, Cr and Ti but a decrease of Mg, Ca, Na and K upward from the bedrock. The highest concentration of Ni (up to 11.53%NiO) occurs in the saprolite horizon, showing nearly 40times richer compared to the bedrock. The mineral evolution during the lateritization process shows various paths from the primary minerals to altered minerals, which is integrally affected by the nature of the primary minerals and environmental conditions. Garnierite, as a significant ore mineral formed by the secondary precipitation processes in the study profile, is identified as a mixture of talc- and serpentine-like phases. The mass-balance calculation reveals that there are diversified elemental behaviors during the serpentinite lateritization under the rainforest conditions. In particular, Ni, as the ore-forming element in the laterite profile, is associated closely with the pH environment, organic matter concentration and mineral evolution during the lateritization process.The findings of the present study support a four-stage evolutional model for the lateritization process: the ferruginous saprolite development (stage I), the limonite development (stage II), the silicate saprolite and ferruginous cap development (stage III), and the precipitation of secondary minerals (stage IV). Due to this multistage process, there is a progressive Ni-enrichment in the laterite profile.

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 (&lt;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.

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.

Geochemical exploration around the Harmony gold deposit, Peak Hill, Western Australia

Primary Au mineralization at Harmony is associated with sulphide-poor quartz veins in mafic and ultramafic rocks. Gold and W are the most significant indicators of mineralization in fresh rock. Elements associated with the phyllic alteration halo are K, Rb and Ba, and Eu, Lu and Yb; these increase in abundance and range near mineralization. Although As and Sb are elevated, their abundances are far lower than in sulphide-rich mineralization styles. The Harmony gold deposit was completely covered by a blanket of soil and colluvium. Drilling beneath this revealed a complex, partly eroded residual regolith, clay-rich valley fill sediments and colluvium. Sampling and analysis of the top of the basement (ferruginous saprolite, lateritic residuum and mottles washed from the mottled zone) on a 250 m sample spacing showed significant Au and W anomalies in the vicinity of the deposit. Gold dispersion in the ferruginous saprolite is restricted and requires close-spaced sampling (50 m). The unconformity between the stripped basement and colluvium was sampled at a 150 m spacing to capitalize on any mechanical or hydromorphic dispersion along the contact. Dispersion of Au, W, Ta and Nb along this interface, indicating mechanical down-slope migration, produced better and more consistent anomalies than the basement sampling in this stripped environment. There are some very weak Au anomalies in the fine soil fraction (&lt;75 μm), where the colluvial cover over the Harmony deposit was extremely thin (&lt;2 m). However, this cannot be relied upon to locate mineralization.

ALTERAÇÃO SUPERGÊNICA DO DEPÓSITO DE COBRE-OURO DO SALOBO, SERRA DOS CARJÁS-PA - ÊNFASE NO COMPORTAMENTO DO COBRE

The weathering mantle of the Salobo deposit was developed upon metagraywackes with interbedded lens of iron formations. The profile is of the truncated type and constituted, from the base to the top, of a transition horizon, a saprolitic horizon and colluvial deposits. The two former horizons show lateral, textural and mineralogical variations which reflect the heterogeneous nature of the parent rock. Thus, alteration of the barren graywackes resulted essentially in the formation of a saprolite rich in clay minerais (hydrobiotite, smectite, kaolinite). On the other hand, weathering of the iron formations, host-rocks of copper sulfides (bornite, calcocite, chalcopyrite), originated a gossan type profile composed from the base to the top of a supergene sulfide zone, an oxidized minerais zone and a ferruginous saprolite. The supergene sulfides are digenite, covellite and bornite (?) while cuprite, malachite, azurite and native copper constitute the oxidized copper assemblage. Within the saprolite, smectite and goethite are the main copper enriched minerais. The evolution of such supergene setting involved oxidization and dissolution of primary sulfides with mobilization of Cu and Fe as soluble sulfatos. The downward and lateral transfer of these metais caused the formation of new mineral associations according to the Cu/S ratio in the solutions and the ambient pH and Eh conditions.In the reducing zone the ionic exchange between copper sulfate and primary sulfides yielded mainly digenite and covellite, while in the water-table fluctuation zone and immediately above, in more oxidizing conditions, copper precipitated as cuprite and native metal. Besides, reaction of copper sulfate with calcite from veins that cuts iron formations resulted in the fixation of Cu as malachite and azurite in neutral to alkaline conditions. In the Carajas region, the lateritic mantle was formed during the Lower Tertiary, but, during the Upper Tertiary and Quaternary, underwent dissection due to regional uplift. Thus, in the Salobo area, the duricrust and the upper saprolite were removed. Each phase of lowering of the hydrostatic levei resulted in the deepenning of the weathering profile and alteration, in oxidizing conditions, of previously formed supergene oxides and sulfides. One parte of the released copper then underwent downward leaching and reprecipitation in deeper reconstituted cementation and oxidation zones, while another part was fixed by neoformed goethite and clay-minerals.

Origins and applications of size fractions of soils overlying the Beasley Creek gold deposit, Western Australia

Primary mineralisation at the Beasley Creek Au deposit, hosted in black shales within mafic and ultramafic rocks, was sulphide- and trace element-rich. The deposit subcrops beneath soil within a small window of deeply weathered Archaean basement in the northeast Yilgarn Craton of Western Australia, surrounded by extensive colluvial–alluvial plains. Regolith overlying the mineralisation and its host sequence is comprised of ferruginous saprolite and some duricrust, overlain by red–brown soil which is strewn with residual ferruginous lag. The colluvial–alluvial wash plains around the weathered basement window are mantled by a thicker red–brown soil, strewn with polymictic lag. Soil samples and surface lag were collected along two traverses across the mineralisation with the aim of determining the optimum geochemical sampling medium for exploration. The 710–4000 μm soil fraction consists largely of black goethite- and hematite-rich nodules, red to yellow ferruginous clay granules, minor quartz, calcrete and, close to subcropping mineralisation, scarce gossan fragments. In contrast, the 75–710 μm fraction, which appears to be largely aeolian in origin, consists mainly of red–brown hematite-coated sand grains, minor feldspar, and small, ferruginous granules. The largely aeolian 4–75 μm fraction is composed mostly of quartz, with minor kaolinite and Fe oxides. A clay-rich <4 μm fraction was removed from the <75 μm material by dispersion in water and sedimentation. The complete soil and the 710–4000, 4–75, and <4 μm fractions were compared as sampling media. Due to its dominant wind-blown quartz component, the 75–710 μm fraction was discarded. The 710–4000 μm fraction, and the fine lag derived from it, are the most effective geochemical sampling media. The distributions of As, Au, Cd, Cu, Sb, Se, W and Zn in this soil fraction are related to mineralisation. Gold gives a 600-m-wide anomaly of >20 ppb, locally reaching 200–300 ppb. The black shales, which host the mineralisation, are indicated by maxima in Ba and Mn. The 4–75 μm fraction is a significantly less effective sample medium than the <4 μm fraction due to dilution by aeolian silt. The <4 μm fraction indicated the host phyllite by an increase in sericite; the phyllites and duricrusts were indicated by an increase in kaolinite. Dispersions of Au, As, Cd and Cu in the <4 μm fraction show that it is also a useful sampling medium.

Ferruginous lag geochemistry on the Yilgarn Craton of Western Australia; practical aspects and limitations

Ferruginous lag, a surface veneer of ferruginous saprolite and/or lateritic nodules and pisoliths, left after removal of soil fines by wind and water, is common on mafic and ultramafic rocks on the Yilgarn Craton of Western Australia. Because it is rich in hematite, goethite and maghemite, which readily adsorb and incorporate indicator elements, it provides an excellent geochemical sampling medium for the detection of Au and base metal ore deposits. Mapping lag types may assist in understanding the underlying regolith; examination of preserved internal micro-fabrics assists in identifying the underlying rocks. Lag is sampled very easily by sweeping it from the surface. Sieving on site may be necessary. Both coarse (10–50 mm) and fine (0.5–10 mm) lag are effective sample media. The very nature of coarse lag invites representativeness problems, however, it is dispersed laterally less than fine lag and targets mineralisation more accurately. Fine lag tends to be more dispersed, has less intense but more consistent anomalies and is the preferred medium for initial surveys. Geochemical anomalies in lag are generally broader than in soil. Lag geochemistry is responsive to minor mineralisation, even where soil is diluted by aeolian material. Sampling is low in cost but pulverisation increases analytical costs in relation to soil fine fractions. Lag sampling is effective in lateritic areas, where broad-spaced sampling is possible, and in erosional areas, where close-spaced sampling is advisable, but it is not recommended in areas of sedimentary cover, where sub-surface sampling is required. Multi-element analysis of the whole lag sample for at least As, Au, Bi, Cd, Cu, Mn, Pb, Sb, W and Zn is recommended for Au exploration. Use of a magnetic fraction is not recommended if any reliance is to be placed on indicator elements (other than Au) which may be present in non-magnetic gossan chips. Analysis of the whole of the lag is preferred.

Lateritic nickel deposits of Brazil

Many nickel deposits are known in Brazil, accounting for about 350 · 106 tons of ore with an average of 1.5% Ni. All are of the lateritic type. These deposits are scattered throughout the country, being rarer in the Northeastern Region and in the South, below 25 °S latitude. They are mainly associated with mafic-ultramafic massifs of large dimensions and ultramafic alkaline complexes, and occur in climatic regions of contrasting seasons. The weathering profile developed over the fresh rock consists, from bottom to top, of the following horizons: altered rock, coarse saprolite, argillaceous saprolite, ferruginous saprolite and lateritic overburden. The thickness of each horizon varies from one deposit to another, the whole profile generally exceeding 20 m. The saprolitic horizons with inherited minerals (serpentine, chlorite) or neoformed minerals (smectites) constitute the silicated nickel ore and are thicker were climatic conditions are drier; the ferruginous upper horizons made up of iron oxide-hydroxides are more developed in more humid regions. In Brazil, the silicated ore generally prevails over the oxidized ore. The main Ni-bearing minerals are serpentine, smectite, garnierite and goethite. The lateritic nickel deposits of Brazil may be correlated with two erosion surfaces, corresponding to the Sul Americano (Lower Tertiary) and Velhas (Upper Tertiary) levelling cycles. The degree of dismantling of the higher and more ancient surface and the consequent development of the Velhas Surface control the position of the nickel accumulation in the landscape. Thus, the deposits may be found either in the lowlands or in the highlands, where they are always covered by a silcrete layer. The alteration profiles in the Brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world. However, they present two characteristic features: the silicated ore prevails over the oxidized ore, and a silicified layer covers the profies developed on the highlands.

O DEPÓSITO DE NÍQUEL DE JACUPIRANGA (SP): EVOLUÇÃO MINERALÓGICA E GEOQUÍMICA

The nickel deposit of Jacupiranga was formed by lateritic weathering of ultramafic rocks. The estimated reserves are 13 m. ton, 1.4% Ni. The alteration profile is very thick and consists from bottom to top: a) fresh rock: serpentinized dunite; b) altered rock and argillaceous saprolite: serpentine and smectite, 30 to 40 m thick; c) silicified ferruginous saprolite: quartz and goethite, up to 25 m thick; d) upper horizon: kaolinite and goethite, up to 10 m thick. The weathering evolution of the serpentinized dunite has led to a loss of Si and Mg and a relative accumulation of Fe and Al. In the silicified horizon absolute accumulation has occurred. The upper horizon seems to be only partially derived from the evolution of the dunite (too high content of Al). Nickel is mainly concentrated in the altered rock and in the argillaceous saprolite associated with serpentine and smectite. Furthermore it occurs as pimelite in the garnieritic veins that cut the altered rock. The content of Ni in these horizons ranges between 0.5% and 2% (up to 9% locally). In the ferruginous saprolite there is always less Ni (< 1%), probably associated with geothite. Three factors make difficult the exploitation of the ore deposit: a) the thick overburden; b) the irregularity of the Ni distribution; c) the high silica content of the ore.