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Tropical weathering control of Ni, Cu, Co, and platinum group element distributions at the O'Toole Ni-Cu sulphide deposit, Minas Gerais, Brazil
Abstract
The O'Toole nickel sulphide deposit is located in the Southern Plateau morphological and structural province of Minas Gerais, which has a humid tropical climate characterized by a mean temperature of 19°C and rainfall of 1600 mm. The primary ore reserve is 5.44 million tonnes at 2.72% Ni, 0.45% Cu, 0.06% Co, and 1.3 ppm combined platinum-group elements. Tropical weathering of the vertically dipping orebody has created gossans at the surface, an oxidized zone to a depth of about 20 m, and a zone of transition above fresh ore to a depth of up to 70 m. The weathering profile is similar mineralogically to the profiles at the Kambalda, Mt. Windarra, and Agnew Ni sulphide deposits in Western Australia, but differs in the depth extents of its mineralogically distinctive zones. These zones exhibit geochemically distinctive enrichments and depletions of various elements in this environment based on mass balance calculations and on the relative immobility of iridium. The work presented characterizes the geochemical behavior of Ni, Cu, Co, and platinum-group elements in the weathering of the O'Toole deposit.
... Prerequisites to the reliable use of surficial media geochemistry in precious metal exploration are a knowledge of the fundamental controls on element distribution and establishment of relevant background and anomalous levels in the various media of interest in different geological and climatic environments. Recent advances in analytical techniques and increased interest in the geochemistry of precious metals have prompted numerous investigations to obtain this information (e.g., Kaspar et al., 1972;Fuchs and Rose, 1974;Travis et al., 1976;Pogrebnyak et al., 1984;Riese and Aria, 1986;Gunn, 1988;McHugh, 1988;Schmitt, 1989;Bergeron and Choini~re, 1989;Taufen and Marchetto, 1989;Cook and Fletcher, 1990;Hall et al., 1990;Benedetti and Boulegue, 1990;Sudarikov and Abramov, 1989;Wood and Vlassopoulos, 1990;Coker et al., 1991;Salpeteur and Jezequel, 1992). However, data are still insufficient for a complete understanding of the processes involved in the redistribution of the precious metals in the surficial environment, resulting in restricted use of some of these media (in particular, lake sediments and natural waters) as routine exploration aids. ...
... However, data are still insufficient for a complete understanding of the processes involved in the redistribution of the precious metals in the surficial environment, resulting in restricted use of some of these media (in particular, lake sediments and natural waters) as routine exploration aids. Wood and Vlassopoulos (1990), in their preliminary study of PGE and Au dispersion surrounding the Lac Sheen Cu-Ni-Pt-Pd-Au prospect in Quebec, showed that the platinoid elements are mobile in surficial environments and that Pd was relatively more mobile than Pt, in accord with the findings of earlier investigations of other PGE occurrences (Fuchs and Rose, 1974;Travis et al., 1976;Riese and Aria, 1986;Taufen and Marchetto, 1989). They also indicated that, in the Lac Sheen case, Au and Pt-Pd were decoupled from each other in their behavior in the various media investigated. ...
... In constrast, fewer published data are available on Pt and Pd concentrations in overburden, and much of this relates to laterites and other products of tropical weathering (e.g. Travis et al., 1976;Bowles, 1986;Taufen and Marchetto, 1989). However, Fuchs and Rose (1974) measured Pt and Pd concentrations up to 120-150 ppb each in soils over the Stillwater Complex, Montana and Riese and Arp (1986) recorded even higher concentrations (up to 1010 ppb Pt and 725 ppb Pd) in a later study of these soils. ...
Analysis of lake waters, groundwaters, soils and lake sediments from the vicinity of the Lac Sheen Cu-Ni-PGE prospect (Temiscaminque region, Quebec) for Pt, Pd and Au indicates that weathering processes have mobilized Pt and Pd from their source in exposed lenses of sulphide-bearing amphibolite. Detection limits as low as 2.5 ng/L Au and Pd and 10 ng/L Pt for waters and 0.5 ppb Au and Pd and 2 ppb Pt for soils and sediments have been attained using Te co-precipitation combined with graphite furnace atomic absorption spectrophotometry. Gold contents of natural waters from the immediate vicinity of Lac Sheen showed a distinct anomaly ( < 2.5- 1 000 ng/L) relative to regional background ( < 2.5- 1 00 ng/L). Based on sampling in August and November 1989 and June 1990, no significant seasonal or water depth variations were detected, and groundwaters were not distinct from lake waters. Much of the Au appears to be present in true solution. Significant concentrations of Pt ( < 10- 1 50 ng/L) and Pd ( < 2.5-400 ng/L) were also detected, predominantly in dissolved form, in Lac Sheen waters and associated groundwaters. Platinum and Pd, unlike Au, exhibited mild and extreme seasonal variations, respectively. Concentrations in the soil samples were: Au ( < 0. 5-57 ppb); Pd ( < 0. 5-29 ppb); and Pt ( < 2-306 ppb). Anomalous concentrations of Pt, Pd, Cu and Ni in B-horizon soil clearly identify known mineralization. In addition to a weak Au anomaly near known mineralization, several other Au anomalies are irregularly scattered throughout the study area, perhaps reflecting the dispersion of this metal from several sources. Anomalous concentrations of Pd, and to a lesser extent Cu, exhibit a more systematic pattern and are found both down- and up-ice, whereas additional Pt anomalies are found only several hundred meters to the north of the known mineralization.
... The main commodities in this region are the well-known deposits of Fe, Au and Mn. Regarding the Au deposits, they have several sulfide mineral phases containing large metal content, mainly Cu, Pb, Sb and Zn, which have not economic importance (Pereira and Santos, 1983;Taufen and Marchetto, 1989; Ribeiro-Kwitko and de Oliveira, 2004;Vial et al., 2007). Nevertheless, in the southern part of the QF, characterized by the Dom Bosco synclinal structure, there are relevant sulfide occurrences, such as Bico de Pedra, showing Cu, Pb and Hg sulfides (chalcopyrite, galena and cinnabar, respectively; Borba, 1998), as well as Au occurrences and Bule Hill, which has Sb and Zn sulfide phases (stibnite and sphalerite, respectively). ...
A new methodology based on a portable X-ray fluorescence (pXRF) was applied in this work to analyze the geochemical dispersion halo of a sulfide mineralization along the Bule Stream sediments (Ouro Branco, Minas Gerais State, Brazil). The study area, located in the Iron Quadrangle (QF), has an important sulfide occurrence of antimony and zinc, which was explored between 1920s and 1930s. Stream sediment samples were collected along the Bule Stream and from some of its tributaries. Aluminum, Fe, Ca, Mg, Mn and Ti contents were obtained by the pXRF “mining mode” and quantified in percentage (%), while the results for Cu, Zn, Pb, As, Cr, Ag, Sb and Sr were carried out by the pXRF “soil mode”, quantified in parts per million (ppm). The statistical treatment of data by Spearman correlation based on geochemical data and some sediment features were discussed, such as the geochemical affinities between the analyzed elements and supergene processes which control the elemental dispersion. Aluminum, Ca and Mg contents were compared with that of the Upper Continental Crust (UCC), since the composition of the Bule Stream sediments should be related to the composition of the source rocks. The results indicate that the Al contents reflect the surrounding soil maturity. The concentrations of Ca and Mg increase in weathered resistant mineral phases, mainly dolomite. The Fe enrichment is associated with geological units containing this element, such as dolomite/ferruginous itabirites (BIF). The high Mn content is related to the QF unity with larger dolomite occurrence (Gandarela Formation). Secondary environmental characteristics can justify the enrichment of Zn and Cu in the study area. All types of rocks have significant concentration of Cr. The comparison of the elemental concentrations of Bule Stream sediments with the average concentration of the elements in the Earth's upper crust (UCC) reveals intense weathering rate of the parent rocks characterized by high maturity of the geological material, enrichment of Fe and Mn and most of the analyzed trace metals. The results of an enrichment factor with three different normalizers (Al representing clay minerals; Fe + Mn representing oxides/hydroxides and; Ca + Mg representing carbonates) indicate that the carbonate phase is the main conveyor of trace elements. APLICAÇÃO DA TÉCNICA DE pXRF NA ANÁLISE GEOQUÍMICA DE SEDIMENTOS FLUVIAIS – RIBEIRA BULE, ESTADO DE MINAS GERAIS, BRASIL ResumoUma nova metodologia baseada na fluorescência de raios X portátil (pXRF) foi aplicada neste trabalho para analisar o halo de dispersão geoquímica de uma mineralização de sulfeto ao longo dos sedimentos Córrego do Bule (Ouro Branco, Minas Gerais, Brasil). A área de estudo, localizada no Quadrilátero Ferrífero (QF), tem uma importante ocorrência de sulfeto de antimônio e zinco, que foi explorada entre as décadas de 1920 e 1930. Amostras de sedimentos fluviais foram coletadas ao longo do Córrego do Bule e em de alguns dos seus afluentes. Os teores de Al, Fe, Ca, Mg, Mn e Ti foram obtidos por pXRF em “mining mode” e quantificados em porcentagem (%), enquanto os resultados de Cu, Zn, Pb, As, Cr, Ag, Sb e Sr foram obtidos pelo pXRF em “soil mode” e foram quantificados em partes por milhão (ppm). Este trabalho analisa o tratamento estatístico dos dados por correlação de Spearman com base em dados geoquímicos e algumas características do sedimento, assim como as afinidades geoquímicas entre os elementos analisados e os processos supergênicos que controlam a dispersão dos elementos químicos. Os teores de Al, Ca e Mg foram comparados com os da Crosta Continental Superior (UCC), uma vez que a composição dos sedimentos do Córrego do Bule devem estar relacionados à composição das rochas geradoras. Os resultados indicam que o teor de Al reflete a maturidade do solo ao redor. As concentrações de Ca e Mg aumentam nas fases minerais resistentes às intempéries, principalmente dolomita. O enriquecimento de Fe está associado a unidades geológicas que contêm esse elemento, como o itabirito dolomítico e/ou dolomito ferruginoso (BIF). O alto teor de Mn está relacionado à unidade de QF com maior ocorrência de dolomita (Formação Gandarela). Características ambientais secundárias podem justificar o enriquecimento de Zn e Cu na área de estudo. Todos os tipos de rochas possuem concentração significativa de Cr. A comparação das concentrações elementares dos sedimentos da corrente de Bule com as concentrações médias dos elementos da Terra (UCC) revela intensa taxa de intemperismo das rochas-mãe caracterizadas pela alta maturidade do material geológico, enriquecimento de Fe e Mn e a maioria dos metais traço analisados. Os resultados de um fator de enriquecimento com três diferentes normalizadores (Al representando minerais argilosos; Fe + Mn representando óxidos / hidróxidos e; Ca + Mg representando carbonatos) indicam que a fase carbonatada é o principal transportador de elementos químicos.Palavras-chave: Geoquímica. Quadrilátero de Ferro. Sedimentos. Fluorescência Portátil de Raios X. Metais. Fatores de Enriquecimento.
... Supergene nickel concentrations in Brazil were reviewed by Melfi et al. (1980), Oliveira (1990) and Oliveira et al. (1992), highlighting the quite conformable ore-forming environments and distribution along tropical climate. Taufen and Marchetto (1989) describes Ni, Cu, Co and Platinum Group Elements enrichment over primary sulfide ore in O'Toole deposit with strong leaching of the sulfides and host rock, secondary sulfide crystallization and Ni depletion from the uppermost levels to the saprolite and bedrock leading to gossan formation similar to the profiles on the western Australian nickel fields . Faria Jr. (2011, 2015 pointed out towards important structural control on development of Ni-bearing supergene ores in Morro do Níquel and two other places along the MFGBin Bom Jesus da Penha (BJP), along expressive ultramafic rock lenses, and in Jacuí, formed over less representative bodies of serpentinite bedrock. ...
Southwestern São Francisco Craton makes limit with Brasília thrust-fold belt and involves rocks from Archean to formed during the Brasiliano-Pan Africano Neoproterozoic event, including a mafic-ultramafic belt (Morro do Ferro Greenstone Belt) hosted along the Archean counterpart. Tis greenstone belt hosts two-nickel deposits (Morro do Níquel and O'Toole, respectively silicate and sulfide types) and occurrences. Tis study applies an empirical-conceptual model for lateritic nickel deposits formation into geographic information systems with aerogeophysical data (magnetic and gamma-spectrometry) and digital elevation models (terrain relief and slope). Our contribution aims for nickel deposits favorability mapping using a simple mathematical operator over a supporting spatial database translating the conceptual exploration model into evidential layers for geological processes involved on deposit formation. Evidential layers constructed for identi-fication of elements pertaining the supergene nickel mineral system are given by analytic signal amplitude maps, thorium over potassium ratio images, and digital elevation models and slope maps, derived from shuttle radar topography mission digital elevation models. Evidential layers integration through binary layers algebraic sum identified efectively known deposits and occurrences with its outputs highlighting possibilities for unknown resources in this under-explored terrain.
... Supergene nickel concentrations in Brazil were reviewed by Melfi et al. (1980), Oliveira (1990) and Oliveira et al. (1992), highlighting the quite conformable ore-forming environments and distribution along tropical climate. Taufen and Marchetto (1989) describes Ni, Cu, Co and Platinum Group Elements enrichment over primary sulfide ore in O'Toole deposit with strong leaching of the sulfides and host rock, secondary sulfide crystallization and Ni depletion from the uppermost levels to the saprolite and bedrock leading to gossan formation similar to the profiles on the western Australian nickel fields . Faria Jr. (2011, 2015 pointed out towards important structural control on development of Ni-bearing supergene ores in Morro do Níquel and two other places along the MFGBin Bom Jesus da Penha (BJP), along expressive ultramafic rock lenses, and in Jacuí, formed over less representative bodies of serpentinite bedrock. ...
Southwestern São Francisco Craton makes limit with Brasília thrust-fold belt and involves rocks from Archean to formed during the Brasiliano-Pan Africano Neoproterozoic event, including a mafic-ultramafic belt (Morro do Ferro Greenstone Belt) hosted along the Archean counterpart. This greenstone belt hosts two-nickel deposits (Morro do Níquel and O'Toole, respectively silicate and sulfide types) and occurrences. This study applies an empirical-conceptual model for lateritic nickel deposits formation into geographic information systems with aerogeophysical data (magnetic and gamma-spectrometry) and digital elevation models (terrain relief and slope). Our contribution aims for nickel deposits favorability mapping using a simple mathematical operator over a supporting spatial database translating the conceptual exploration model into evidential layers for geological processes involved on deposit formation. Evidential layers constructed for identification of elements pertaining the supergene nickel mineral system are given by analytic signal amplitude maps, thorium over potassium ratio images, and digital elevation models and slope maps, derived from shuttle radar topography mission digital elevation models. Evidential layers integration through binary layers algebraic sum identified effectively known deposits and occurrences with its outputs highlighting possibilities for unknown resources in this under-explored terrain.
... The potential for mobility of palladium in surface environments is relevant to the use of geochemical methods of exploration (e.g., soil and biogeochemical surveys) for platinum-group element (PGE) ores and to the behavior of these elements as environmental contaminants. Several authors have interpreted field evidence in terms of the mobility of Pd in soils and laterites (Fuchs & Rose 1974, Kothny 1978, Bowles 1986, Riese & Arp 1986, Taufen & Marchetto 1989, Wood & Vlassopoulos 1990, Coker et al. 1991, Cook et al. 1992, Zang et al. 1992, Cook & Fletcher 1993, Bowles et al. 1994, Salpéteur et al. 1995. Other authors have demonstrated uptake of PGE by plants and have suggested their possible use in biogeochemical exploration surveys (Kothny 1978, 1979, 1987, Dissanayake & Kritsotakis 1984, Dunn 1986, Riese & Arp 1986, Dunn et al. 1989, Hall et al. 1990, Rencz & Hall 1992, Roger & Dunn 1993, Fletcher et al. 1995. ...
Acetate and oxalate are simple organic ligands that occur naturally in surface waters, groundwaters, and soils; they may play a role in the transport of palladium in such environments. Mass transfer of Pd in surface environments has implications for geochemical exploration as well as the environmental impact of the platinum-group elements. The complexation of divalent palladium and acetate was studied via measurement of the solubility of amorphous Pd(OH)2 in 1 molal NaClO4 as a function of concentration of acetate (0.0001 to 0.1 molal) at 25°C. From these data, the predominant Pd 2+ acetate species was determined to be Pd(ac)2 0 (where ac denotes the acetate anion), with a conditional stability-constant of log � 2 * = 9.3 ± 0.3. Palladium oxalate complexes were investigated using UV-visible spectrophotometry. Experiments were conducted at 25°C as a function of concen- tration of NaCl (0.1 to 2.0 molal) and oxalate (0.1 to 100 millimolal). The results demonstrate that oxalate-bearing Pd species become important at relatively low concentrations of oxalate. At 1 molal NaCl, the existence of an isosbestic point indicates an equilibrium between one oxalate-bearing Pd species and PdCl4 2- . At lower concentrations of NaCl, more than two Pd species may be present under the experimental conditions. Calculation of speciation as a function of pH in a model soil solution shows that Pd(ac)2 0 never exceeds 40% total Pd, even in the absence of oxalate and at acetate concentrations as high as 5 millimolal. At lower concentrations of acetate or in the presence of oxalate, the proportion of Pd(ac)2 0 is negligible. However, Pd(ox) 0 appears to be an important Pd 2+ species, predominating at low pH, with its field of predominance expanding with increasing concentration of
... Average grades of the ore reserves are estimated at 2.2% Ni, 0.4% Cu, 0.05% Co, and 1.2 ppm platinum-group elements + Au (Brenner et al., 1990). Tropical weathering of the ore body has created an alteration profile composed by gossans at the surface, an oxidized zone to a depth of about 20 m and a transition zone above fresh ore to a depth of up to 70 m (Taufen and Marchetto, 1989). The gossan samples were collected from trenches located just above the mineralized zone. ...
Two sets of gossan samples derived from the oxidation of sulphides of a NiCu and a PbZn deposit, respectively, have been investigated by Mossbauer spectroscopy. For the first set of samples all spectra consisted of one magnetically split sextet with parameters similar to those of goethite, except for one sample that presented an additional sextet typical of hematite. The magnitude of the magnetic hyperfine field in goethite was found to decrease linearly with increasing (Ni + Cu)/(Ni + Cu + Fe) mole fraction, suggesting that Ni and Cu could be structurally incorporated in the goethite lattice, in an analogous way to previously reported Al substitution. The PbZn gossan samples yielded spectra with two sextets that could be assigned to goethite and hematite. One sample, in addition, presented a doublet that was attributed to jarosite. No correlation was observed between the goethite hyperfine fields and the (Zn + Pb)/(Zn + Pb + Fe) mole fractions. Reasons for such behavior are discussed.
... Previously, Barnes et al. (1985) concluded that hydrothermal alteration preferentially mobilized Pd relative to the other PGEs. Studies have also demonstrated that this enhanced low-temperature Pd mobility is independent of climate (e.g., Taufen and Marchetto, 1989; Prichard and Lord, 1990; Salpeteur and Jezequel, 1992). This behavior of Pd has been quantified by experimental work (e.g., Wood, 1990; Bowles et al., 1994). ...
Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ∼0.1 (Ir, Ru) to ∼5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (∼30%) and fractional crystallization (∼45%).
Soils, lake sediments, groundwaters and lake waters in the vicinity of two Cu-Ni-Pt-Pd occurrences in Quebec (Lac Sheen and Lac Long - Lac Montauban), were sampled and analyzed for Pt, Pd and Au. In both areas the Pd content of lake waters ranges from 0.1 to 0.3 μg/L, and this appears to be in true solution. However, the suspended particulate fraction dominates in groundwaters, with total Pd ranging between 0.25 and 17 μg/L. In all waters, Pt and Au contents are generally below the detection limit of 25 ng/L, although Pt occasionally reaches higher values, especially in groundwaters. The Pt/Pd ratio in outcrop and boulder samples collected at Lac Sheen is nearly uniformly 1:1, but the soils are depleted in Pd relative to Pt, suggesting that Pd is preferentially mobilized in solution. This is consistent with the water analyses. The Pd contents of the lake sediments are very low (<3 ppb). The Pt contents are also relatively low, but sporadically attain high values (up to 200 ppb). The Au content in the lake sediments is also extremely variable, ranging from <1 ppb to 213 ppb, but with high values more common than is the case for Pt. The results of the study substantiate the idea that platinum-group elements are mobile in surficial environments, Pd being preferentially transported in solution. Furthermore, the sampling of lake water may offer some promise either as an alternative or a complement to sampling of lake sediments for reconnaissance exploration.
Platinum Group Elements (PGE) have been considered as being inert in surficial condition for a long time. However, the study of mobility of PGE during weathering of ultramafic-mafic complex and black shale indicates that PGE, released from the primary rocks due to weathering, are abnormally concentrated in soil, plant and water. The secondary enrichment of PGE, especially of Pt and Pd, in surficial media around complex could be used as a potential tool for exploring PGE deposits. And osmium in black shale, transferring during weathering, could contribute main importance to the increase of seawater 187 Os/188 Os since Cenozoic. While PGE emitted from automobile catalysis converters would have a negative effect on ecological environment and human health by their mobility.
To study the geochemical characteristics and evolution of PGE during the laterization of the ultramafic rock, the Kolonodale profile from Indonesia and Yuanjiang profile from China were selected as the comparative studying targets. Our work revealed that the PGE supergene enrichment are apparent in both two laterite profiles, and the enrichment factors relative to parent rocks are 5.42 (Kolonodale profile) and 6.52 (Yuanjiang profile) individually. This PGE enrichment effect is closely associated with the laterization degree of the weathering product. However, the vertical distribution pattern of PGE is different in these two cases, showing “dumb bell”-like shape in the Kolonodale profile and “inverted spoon”-like shape in the Yuanjiang profile. Geochemical evidence indicated that chromite minerals are the original source of PGE in the laterite crust, and the geochemical behavior of PGE is highly consistent with Fe and Cr, which show typical residual concentration during the laterization process. This mechanism led to PGE enrichment in the upper part of the laterite crust. Furthermore, climate constrained the laterization degree of the ultramafic rock, and then influenced the concentration and behavior of PGE during weathering. For the Kolonodale profile, PGE may have undergone more migration and redistribution in the laterite crust due to the higher laterization degree, leading to one more PGE concentration segment occurring in the lower part of the laterite crust. Our work further confirmed that Pd and Os are more mobile in the intensively weathered crust, and they may show relatively high differentiation with Pt, Ir, Ru and Rh. Except this, the internal differentiation of PGE was not significant in general during the ultramafic rock laterization.
Re-evaluation of theoretical calculations reaffirms that, at temperatures up to 300°C, significant solubility (>10 ppb) of Pt and Pd as chloride complexes is restricted to highly oxidizing and acidic conditions. This conclusion is supported by recently reported experimental solubility measurements. Experiments on the solubility of Pt and Pd in sodium hydroxide solutions at 25°C confirm that a change in species from Pd(OH)20 to Pd(OH)3- occurs at a pH of approximately 12, but apparently no such change in speciation occurs for Pt. The measured log β2 for Pt(OH)20 agrees very well with the previously predicted value, whereas predicted stability-constants for Pt(OH)3- and Pt(OH)42- should be considered only as provisional estimates. Hydroxide complexation is unlikely to contribute to the hydrothermal transport of Pt and Pd in most geological environments. Nevertheless, Pt, Pd and Au hydroxide complexes may be the dominant inorganic species of these metals in oxidized surficial waters such as seawater, lake water, and river water. -from Authors
Sub-economic - but well-developed - platinum-bearing mineralization was discovered and investigated during systematic exploration of the poorly exposed Rincón del Tigre Complex between 1991 and 1993. Two principal exploration methodologies were employed: (1) low (ppb)-level Pt, Pd and Au soil geochemistry and (2) petrological evaluation of the cumulate crystallization history. The technically successful outcome of the exploration, the minimal dispersion of Pt and Pd recorded in the surface environment and the continuity of the metals distribution profile observed between bedrock source and residual soils are all notable results of the soil geochemistry.
Palladium is one of the platinum group elements (PGEs) and it is present on the earth crust at very low concentration levels. The members of PGEs occur in nature as various mineral species, chiefly as antimonides, arsenides, bismuthides, sulfides, tellurides and in the native state (Reddi and Rao, 1999). The precious metals have some unique geochemical characteristics: they are very refractory, having high boiling points and a strong affinity for iron (siderophile) and for sulfur (chalcophile) (Goldschmidt 1954, Rösler and Lange 1972). They are normally present in silicate rocks at very low levels, often forming discrete phases, such as alloys and therefore, unlike the rare-earth elements, they do not appear to partition themselves extensively between silicate phases. It is therefore a critical task to select a representative natural sample mass for analysis, since the presence or absence of one precious metal grain in the analyzed aliquot could have a significant effect on the analysis, particularly if small sample masses (<5 g) are chosen. This inhomogeneity that is overall typical for the rock samples, might be taken into consideration by the analyst that generally are use to operate with samples of small size. In fact the homogeneity and the representativeness of the sample is the dominant factor to validate the analytical data on the real Pd distribution in environmental matrices (Hall et al. 1990; Reddi and Rao, 1999). These considerations, together with the difficulties that arise applying analytical procedure owing to the very low Pd concentration in the environmental matrices, contrast with the large body of information concerning other metals of environmental significance such as Cd, Cr, Hg, Ni, Pb etc. A significant collection of data on the environmental distribution of Pd and the consequent evaluation of the risks for human health and the environment from exposure to palladium are the results of the great effort of the Task Group on Environmental Health Criteria for Palladium (WHO, 2002). Pd concentration in surface fresh water is generally detectable in the range from 0.4 to 22 ng L-1 in fresh water and from 19 to 70 pg L-1 in salty water. Drinking-water samples usually contain extremely low levels of pal ladium (few data always at a concentration level 24 ng L-1 are available in literature). A limited number of data are also available for Pd in tissues of small aquatic invertebrates, different types of meat, fish, bread and plants. Pd in soil range from < 0.7 to 47 μg kg-1 (WHO, 2002). Soil samples were mainly collected from areas near major roads resulting enriched in this element. Pd concentration in sewage sludge ranges from 18 to 260 μg kg-1, while a very high concentration (4700 μg kg-1) has been found in sludge samples contaminated by wastes discharged from the local jewellery industry (WHO, 2002). Population is primarily exposed to palladium through different pathways that have different relevance: dental alloys, jewellery and metal refineries, food and emission from automobile catalytic converters (this one especially in the last decade). It is noteworthy to stress that at this time, at the relatively low concentration levels, metallic form of Pd and PGEs are considered to be inert respect to biological reactions (Ravindra et al. 2004). The human average dietary intake of palladium has been calculated to be up to 2 μg/day. Ambient air levels of palladium below 100 pg m-3 can be expected in urban areas where palladium catalysts are used. Therefore, the inhalative palladium uptake rate is very low. In roadside dust, soil and grass samples a slight accumulation of palladium has been detected, correlating with traffic density and distance from the road (Zereini and Alt, 2000; Barefoot, 1999; WHO, 2002; Ravindra et al. 2004). Worldwide demand for palladium in automobile catalysts rose from 23.5 tonnes in 1993 to 76.4 tonnes in 1996 (Cowley, 1997) up to 167 tonnes in 1999 (Ravindra et al. 2004). About 60% of the European gasoline cars present on the market in 1997 were equipped with palladium-based catalysts while in 2002 about 84% of the total Pd demand by application in Europe was employed for autocatalyst. The introduction of palladium in automotive catalytic converters in Europe during the early 1990s and the new catalyst introduced to satisfy the request of European legislation (Euro III and IV) have caused a growing demand for Pd. The consequence will be an increasing release and accumulation of this metal in the environment, paralleled by a reduction of Pt and Rh (Helmers et al. 1998; Hees, et al.1998; Schafer et al. 1999; Ravindra et al. 2004). The production and recycling of Pd-containing materials, as well as the use of Pd as a constituent of dental restorative alloys, may be a source for toxic and allergic reactions for the organisms (Begerow et al. 1999; Begerow and Dunemann, 2000). A critical evaluation of possible risks for human health can only be given if reliable analytical data are available (Messerschmidt et al. 2000). In fact, according to Boch et al. (2002), studies over the past 10 years have shown that the allergenic potential of palladium has been probably underestimated. For this reasons it is important to develop analytical methods suitable for relatively easy determination of traces and ultra traces levels of Pd in the environment, as well as in body fluids of living species (Messerschmidt et al. 2000). Until now Pd has been preferentially determined in solid matrices as soils, sludge and dust, where is present at relatively higher concentration, while the paucity of data relatively for water system is related to the low levels and at the presence of salt that cause not easily resolved interferences. (Hall et al. 1990; Hall and Pelchat, 1993). Terada et al. (1983) suggest that owing to these low concentrations in water, a preconcentration step is necessary. As an example, water insoluble chelating material (p-dimethylaminobenzylidenerhodanine on silica gel, or Thionalide) has been used for preconcentration of trace of palladium with a slow flow rate (2.5 mL/min).This have permitted the complete retention of palladium on the column. (Terada et al. 1980; Terada et al. 1983).
Carajás, in the state of Pará, eastern Amazon, is the most important mineral province in Brazil. Its high plateau landscape contrasts with the lowland domain of the Amazon region. It is developed on old lateritic formations and covered by dense tropical rain forest under a hot and humid tropical climate. The bedrock is made up of high-grade metamorphic terrain of Archean age, greenstone belts and post-tectonic granites. The laterite profiles are complex, partly transformed into yellow to red latosols on the plateau and colluvium on the slopes. The lateritic terrain hosts the gold rich gossans of the Igarapé Bahia and Águas Claras gold mines. Gold is found in red latosols, iron crust, gossans and colluvium. In order to investigate the capacity of these materials as geological sampling media and as indicators of primary ore, a multi-element geochemical survey was carried out at Águas Claras. Seventy-four samples were analysed for 27 elements by AAS (Au, Cr, Cu, Mn, Ni, Pb and Zn), XRF (Cl, W), OES (B, Be, Ga, Mo, Sc, Sn, Sr, V, Y, Zr and Bi), AA–HG (As, Hg, Se and Sb), ICP–AES (Te), and the mineralogy identified by XRD. The supergene materials are mainly kaolinite, quartz, hematite, maghemite, gibbsite and some goethite. Dravite, muscovite, wolframite and chromite are accessory minerals. In the latosols, As, Au, B, Cr, Cu, Ga, Mo, Pb, Se, Sn, V, W and Zr are concentrated by a factor greater than three times compared to crustal abundance levels, whist gossan and colluviums contain more than ten times the crustal average levels of As, Au, B, Mo, Sn and W. Multivariate R-mode analyses distinguish several associations according to the nature of the supergene materials. The As–Cu–Mn–Pb–B–Au–W–F cluster corresponds to primary signatures still preserved in latosols, iron crust and colluvium; and the Cr–V–Ga–Mo–Ni–Zn–Se–Sc–Hg–Sn–Cl–Y–Zr cluster reflects the general laterite evolution. The latosols preserve the geochemical signature and signal of underlying primary ore, and, together with the colluvium, they can be used as good sampling media.
Eighty-one argillaceous marine sediments collected from the southeastern coastal margin of the Japan Sea were analyzed for Pt and Pd by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation. The average Pt (1.2 ± 0.5 ppb) and Pd(1.6 ± 0.9 ppb) contents are generally similar to those of the published crustal abundances. The Pt or Pd is generally enriched in the fine sediment fraction, and positively correlated with Cu, Pb and water depth. The inverse correlation between the Pt or Pd content and redox potential of the sediment suggests that significant amounts of Pt and Pd are supplied in their dissolved forms, and reduced to their metallic states. It is concluded that the Pt and Pd distributions in the coastal marine sediments are controlled by the geological characteristics of the studied area, grain size and redox potential of sediments, water depths, and dissolved O2 concentration of sea water.
A 284 marine sediments of terrigenous, hemipelagic and pelagic origin were collected from the different environments, i.e., in the seven sea areas around the Japanese islands (terrigenous sediments), on the Mariana Ridge (hemipelagic sediments), and in the Central Pacific (pelagic sediments). These samples were analyzed together with 31 lake sediments and 41 marine shales for Pt and Pd by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation. The average abundances of Pt and Pd are markedly higher in the pelagic sediments than the terrigenous sediments, with the hemipelagic sediments showing intermediate abundance. Most of the samples analyzed in this study are richer in Pt than Pd, whereas siliceous ooze collected from the central Pacific high productivity zone was generally richer in Pd than Pt. Although the Pt and Pd abundances and water depths of the sampling locations are not correlated, there are clear negative correlations between the Pt and Pd abundances and the sedimentation rates in the studied regions. It is considered that most of the Pt and Pd in marine sediments is originally derived from weathering of crustal materials, and both the elements have been precipitated and accumulated as the elemental and oxide forms by the involvements in biological transport processes. The great enrichments of Pt and Pd in pelagic sediments may result from the very low sedimentation rates and strongly oxidizing environments. The different vertical distributions of Pt or Pd and organic C concentrations in the central Pacific cores suggests that the formation of organometallic complexes of Pt and Pd, and migration of both elements during the early diagenesis are not so significant except in very rare cases. The crustal abundances for Pt and Pd are tentatively estimated as 2.7 ppb and 1.9 ppb, respectively.
Solubility methods were employed to determine conditional equilibrium constants for the formation of hydroxide and mixed hydroxy–chloride complexes of Pd(II). Measurements were made over a temperature range of 25–85°C, a pH range from 0 to 12, and ionic strengths of 0.1, 0.2, 0.5 and 1.0 molal in both KCl and NaClO4 media. Several speciation models were fit to the data using nonlinear regression, and the model yielding the best fit with the fewest number of species was accepted for each temperature and ionic strength. The conditional equilibrium constants were then fit to a function of ionic strength and temperature (including a Debye–Hückel term) to facilitate interpolation and extrapolation to infinite dilution. The following species were found to be important in KCl solutions: PdCl42−, PdCl3(OH)2−, and Pd(OH)20. The relative proportions of the species are dependent on pH and ionic strength (chloride concentration). In perchlorate media the predominant species were Pd(OH)3−, Pd(OH)20, PdOH+ and Pd2+, depending on pH. Conditional stability constants determined in this study agree well with those reported in previous work for the simple chloride and hydroxide complexes, but our results suggest that mixed complexes may be more important than previously thought, and that PdCl3(OH)2− may be the dominant species in seawater, followed by Pd(OH)20.
Humic substances are complicated, naturally occurring organic compounds of high molecular weight. Owing to the presence of functional groups containing oxygen, and to a lesser extent, nitrogen and sulfur, these substances are capable of interacting with economically interesting metals in solution in a number of ways. Humic substances may: (1) form soluble complexes with metal ions; (2) cause precipitation of metal ions via reduction; (3) cause precipitation of metal ions via adsorption, ion exchange or other sorption processes (when humic substances themselves are insoluble); (4) modify the sorption behavior of mineral surfaces with respect to metal ions; (5) modify the behavior of colloidal particles of metals; and (6) affect the rates of geological processes. The ability to develop quantitative geochemical models of these interactions is important in the study of the secondary dispersion of most ore metals in the surficial environment and of primary ore-forming processes for a number of ore metals (e.g., Au, PGE, U, V). Available data suggest that humic substances are drastically altered, especially with respect to their ability to complex metals, at temperatures exceeding about 100°C, so their role is probably limited to low temperature environments. In this review, attention is focussed on the binding and reducing effects of humic substances on Au, PGE, U and V.
Fractionated basalts (4.5–8.9 wt.% MgO) from the islands of Malaita and Makira (San Cristobal) in the Solomon Islands represent outpourings of magma that formed the Ontong Java Plateau (OJP), SW Pacific. The origin of the OJP is explored by examining platinum-group element (PGE: Ru, Rh, Pd, Ir, Pt) abundances in these basalts. On Makira, mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) are occasionally intercalated within the stratigraphic sequence of the OJP—the (OIB)-type basalts could be representative of the plume tail sequence and the MORB-type flows may be an indication of heterogeneity within the OJP source. Partial melting models indicate that the MORB-type basalts were generated from a source dominated by upper mantle material. However, they suggest that the OJP basalts and the OIB-type basalts were not generated from an exclusively upper mantle source. Our illustrative modeling shows the range of PGE compositions in these plume-generated basalts are best generated from a hybrid source that incorporates 0–0.5 wt.% of outer core material, 50% depleted upper mantle and 50–49.5% lower (primitive) mantle.
Serra do Pirocaua is a plateau formed on mature lateritic rocks (Eocene), partially covered by earthy soil. Gold in the region around the serra (plateau) has been mined from the 17th Century to the present. Laterites extend not only over the entire plateau, but also in the neighboring flat and low-lying area. Outcrops of fresh rocks are rare and none has been found as yet on the plateau.Systematic geochemical and mineralogical studies were carried out on samples taken from crusts and surface soils of the plateau. Initial results show that these materials are practically identical both in terms of mineralogy as well as geochemistry indicating that the soils have a physical origin, that is, they are immature. Multi-element geochemical data have identified three main geochemical signatures both for the crusts as well as the soils, representative of three mineral groups: hematite + goethite, Al phosphates and anatase. A lithological map of the substrata was developed, based on the distribution of the anomalous levels of elements in each association. The data suggest that the substrata are formed of volcano-sedimentary rocks of mafic to intermediate composition and probably phosphorite beds.Both volcanic and non-volcanic mafic-ultramafic rocks may be present. Hydrothermal manifestations such as As-bearing tourmaline-quartz veins were suspected because of high levels of As and Au as well as dravite aggregates in Fe crusts and soils.
An electrochemical model for the weathering of massive sulphide deposits that have undergone conditions similar to those affecting the Kambalda nickel deposits is presented. Galvanic corrosion due to aeration of the top of the ore body near the water table results in a deep anodic reaction whereby primary sulphides undergo an oxidative transition to sulphur-rich violarite—pyrite ore and also a shallow oxidative anodic reaction where the violarite—pyrite ore is oxidised to sulphate and gossanous oxides. Electrons are conducted through the ore from the anodes to the cathode where dissolved oxygen radicals are reduced and the corrosion cells are completed by ionic transport through the groundwaters to the anodic regions that can be some 200 m deep in the case of deep weathering processes. The kinetics of the weathering are described in terms of resistance to the flow of corrosion currents in the electrochemical cells. The effect on the model of physical perturbations such as rising and falling water table, faulting, etc. is discussed.
Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile of the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co0.02Fe1.38Ni1.60 S4, releasing Fe2+ and Ni2+ ions into solution. Up to 43% of this Ni2+ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellar-textured violarite with an even higher iron content but lacking in cobalt (approximately Fe1.6Ni1.4S4). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe2+ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg2+ and Ni2+ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.
The O'Toole deposit is a Cu-Ni-Co sulfide lens hosted by ultrabasic rocks. The orebody has undergone postdeformational metamorphism
and shearing.The main sulfides of the O'Toole deposit are pyrrhotite (po), pentlandite (pn), and chalcopyrite (cp) in average
proportions (%) of 65po:30pn:5cp. Cobaltite-gersdorffite and sphalerite are the most frequent accessories minerals. Assay
data indicated the presence of platinum-group elements (PGE). Platinum-group minerals (PGM) were identified during microscopy
on ore from the O'Toole deposit. Mineral identifications were made and confirmed by electron microprobe.Kotulskite-melonite
followed by irarsite and sperrylite are the most abundant PGM. Other identified phases include OsAs 2 , OsAsS, OsRhAsS, RuTeAs, OsRuAs, OsAs 5 , and a complex Os-Re-As-Te-Fe-Y-Rh mineral. Platinum-group minerals occur frequently as inclusions in cobaltite-gersdorffite,
suggesting that the PGM formed first. Since pyrrhotite replaces cobaltite-gersdorffite indicating its later crystallization,
it can be deduced that the PGM formed during the first stages of magma crystallization, before sulfides and arsenides.Routine
microprobe analysis identified a rhenium sulfide (ReS 2 )--the second reported global occurrence.
Palladium and iridium show a strong positive correlation with nickel in Western Australian nickel sulfide deposits, and their
inertness during weathering, combined with depletion of the more mobile components, usually results in retention or enhancement
of the primary values in gossans and other oxidized derivatives.Precious-metal enrichment from background levels occurs in
some lateritized barren ultramafic rocks, but the low values in most lateritic products and gossans derived from non-nickeliferous
sulfide assemblages permit distinction from true nickel gossans. Material with 15 to 30 ppb Pd and 5 to 10 ppb Ir indicates
a possible minor original nickel sulfide component ([asymp]0.6% Ni), and at higher levels the former presence and approximate
grade of nickel sulfides can be confidently predicted from the precious-metal values.Pd-Ir analysis has particular application
to the recognition of an original nickel sulfide component in highly leached gossans with low Ni-Cu values, to the evaluation
of ferruginous, siliceous, and clay-rich lateritized ultramafic products enriched in Ni and Cu, and to the estimation of the
nickel content in the parent sulfide assemblage.
Lateritic weathering of massive and disseminated sulfides, consisting largely of pyrrhotite and pentlandite, has produced
a supergene pyrite-violarite assemblage that has been replaced by goethite, carbonates, and quartz toward the surface. In
the zone of supergene sulfides, iron released by the conversion of pyrrhotite to pyrite is fixed either as siderite or as
magnetite, depending on the local HCO 3 (super -) activity. The depth of supergene alteration is controlled by electronic conductivity in the massive sulfides and
by ground-water permeability in the disseminated sulfides. Supergene nickel enrichment of the sulfides is not economically
significant.The Oxide Zone is dominated by goethite, with carbonates toward the base and heavy silicification toward the surface.
Sulfide relict textures are well preserved. The depth of the Oxide Zone was controlled by Cenozoic water table fluctuations,
permeable fault zones, and the presence of sulfides. Major and minor elements in the Oxide Zone have been redistributed into
a pattern consistent with laterite weathering profiles in ultramafic rock. Nickel enrichment at about 23 meters from surface
is due to the presence of takovite, a green hydrocarbonate of nickel and aluminum.The development of the supergene profile
is attributed dominantly to an episode of lateritic weathering between the Eocene and middle Miocene epochs. Some subsequent
modifications have taken place, including silicification and erosion. It is unlikely that significant supergene alteration
is occurring at present.
An electrochemical study of nickel sulphide ore from three Kambalda shoots shows that the various parameters measured correlate both with depth and the degree of alteration of the ore. Eh and pH, respectively, vary from about −0.3 V (SHE) and 8–9 for the deeper primary ore to about +0.3 V (SHE) and 5–6 for the shallower weathered ore. Leaching and simulation experiments as well as resistivity measurements all give results that support the hypothesis that galvanic corrosion resulting from differential aeration is the major mechanism in weathering of the ore. The release of iron from the ore to solution, and its subsequent hydrolysis, could exert a major influence on the pH, as indicated by the reaction: Fe2+ + 3H2O → Fe(OH)3 + 3H+ + e−. The redox potential of the original mineral assemblage also plays an important role in pH control. Exploration and metallurgical consequences are discussed.
Major fluorspar districts are localized chiefly along and near continental rift zones and lineaments. More than 45 districts are cited along the Rio Grande graben of New Mexico and Colorado and along this rift projected northward through Colorado, Wyoming, Idaho, Montana, British Columbia, Yukon Territory, and Alaska and projected southward into Mexico. Other fluorspar districts similarly are associated with tensional rifts and lineaments in Illinois, Kentucky, Oregon, Nevada, Ontario, Nova Scotia, Newfoundland, Germany, France, Sardinia, U.S.S.R., India, Kenya, Tanzania, Malawi, and Mozambique. Evidently large volumes of fluorine leaked upward from the lower crust or mantle along and near the rifts. This association of major fluorspar districts and continental rifts and lineaments is regarded as a regional guide in the search for new fluorspar districts. The fluorspar may be accompanied by commercial quantities of barite, zinc, lead, molybdenum, uranium, thorium, niobium, tin, beryllium, or rare-earth minerals.
Recent chemical investigations by the author on 40 Ni ores, 103 true gossans and 69 false gossans (collected from 32 localities, including most producing nickel mines) have largely supported the findings of previous authors who have published on nickel gossans from the Western Australian Yilgarn Shield and have indicated that, in the early stages of gossan recognition, particular attention should be paid to “signatures” based on Cu, Ni, Zn, Mn and acid-soluble Cr (Cr(s)).Compared with ores at depth, gossans are typically depleted in platinoids (mainly Pd and to a lesser degree Pt). This may be explainable in terms of low-temperature Eh-pH relations for the system Pd-Pt-H2O-Cl. Furthermore, true gossans often contain a lower content of Cr(s) than false gossans. This probably occurs because (1) nickel sulphide ores are commonly depleted in total Cr, and hence Cr(s), with respect to their host ultramafic rocks and (2) under the highly acid conditions which prevail during the early stages of gossan formation, Cr(s) (mainly present in secondary ferrochromite rims on refractory chromite grains and in associated secondary silicates) is removed by leaching.
The weathering products of nickeliferous sulphides and their associated rocks in Western Australia Geochemical Explo-ration 1974
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Wilmshurst, J.R., 1975, The weathering products of nickeliferous sulphides and their associated rocks in Western Australia. In: I.L. Elliot and W.K. Fletcher (Editors), Geochemical Explo-ration 1974. Elsevier, Amsterdam, pp. 417-436.
Geologica, Petrologia e Prospecq~o Geoqufmica da Sequ~ncia Vulcano-Se-dimentar Morro de Ferro, Fortaleza de Minas (MG). (Tese de Mestrado-UNB; Masters The-sis
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Teixera, N.A., 1978. Geologica, Petrologia e Prospecq~o Geoqufmica da Sequ~ncia Vulcano-Se-dimentar Morro de Ferro, Fortaleza de Minas (MG). (Tese de Mestrado-UNB; Masters The-sis, Brasilia University, Brasilia (in Portuguese ).
O'Toole Ni sulphide deposit
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Cruz, F.F., DaCunha, C.A. and Moreira, S.S., 1986. O'Toole Ni sulphide deposit, Fortaleza de Minas, Brazil. BP Minera~o Intern. Rep. Geography of Brazil -Geografia do Brasil, Regi~o Sudeste, Vol. 3 Funda¢~o Instituto Brasileiro de Geografia e Estatistica -I.B.G.E. (in Portuguese).
Geochemical orientation survey of the Fortaleza de Minas O'Toole Ni deposit, southwestern Minas Gerais, Brazil
- Taufen
Taufen, P.M. and Brenner, T.L., 1987. Geochemical orientation survey of the Fortaleza de Minas O'Toole Ni deposit, southwestern Minas Gerais, Brazil. Geochim. Bras., 1: 1-18.
O'Toole Ni sulphide deposit, Fortaleza de Minas, Brazil
- Cruz
Palladium and iridium in the evaluation of nickel gossans in Western Australia
- Keays
The weathering products of nickeliferous sulphides and their associated rocks in Western Australia
- Wilmshurst
Geologica, Petrologia e Prospecção Geoquímica da Sequência Vulcano-Sedimentar Morro de Ferro, Fortaleza de Minas (MG)
- Teixera