Sulfide Orebodies Research Articles

Hydrothermal remobilisation around a deformed and remobilised komatiite-hosted Ni-Cu-(PGE) deposit, Sarah’s Find, Agnew Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Sarah’s Find nickel deposit, located 4.5 km north of the Mount Keith nickel mine, Western Australia, was chosen as a case study to investigate the nature and three-dimensional geometry of a geochemical halo created by the hydrothermal remobilisation of base metals and platinum group elements into the country rock surrounding a small massive Ni sulphide orebody. Portable and laboratory-based XRF analyses were carried out on samples from a shear zone localised along the basal komatiite-dacite contact that hosts the orebody. A geochemical halo was identified that extends along the shear zone up to 1780 m away from the massive sulphides, parallel to a prominent stretching lineation. Elevated Ni and Pd are associated with high As, Co, Cu and S. Palladium and Pt concentrations increase with proximity to massive sulphides (from 6 to 1190 ppb Pd). These anomalous concentrations reflect the presence of sulfarsenides and sulphides, either physically remobilised and forming veinlets close to the massive sulphides, or hydrothermally transported and redeposited within the foliation. In situ laser ablation ICP-MS indicates that Pd and Pt are hosted within these nickel sulfarsenides. This Ni-Co-As-Pd geochemical halo, observed around the Sarah’s Find ore body, is interpreted as forming syn deformation, by the circulation of As-rich hydrothermal fluids dissolving base metals, Pd and Pt from the orebody and redepositing them along the sheared footwall contact. Similar Ni-Co-Pd-Pt-As geochemical haloes could potentially exist around any magmatic nickel sulphide mineral system that has undergone a phase of arsenic metasomatism and may be a generally applicable proximity indicator for nickel sulphides in hydrothermally altered terranes.

Interpretation of gravity data using 2-D continuous wavelet transformation and 3-D inverse modeling

Recently the continuous wavelet transform has been proposed for interpretation of potential field anomalies. In this paper, we introduced a 2D wavelet based method that uses a new mother wavelet for determination of the location and the depth to the top and base of gravity anomaly. The new wavelet is the first horizontal derivatives of gravity anomaly of a buried cube with unit dimensions. The effectiveness of the proposed method is compared with Li and Oldenburg inversion algorithm and is demonstrated with synthetics and real gravity data. The real gravity data is taken over the Mobrun massive sulfide ore body in Noranda, Quebec, Canada. The obtained results of the 2D wavelet based algorithm and Li and Oldenburg inversion on the Mobrun ore body had desired similarities to the drill-hole depth information. In all of the inversion algorithms the model non-uniqueness is the challenging problem. Proposed method is based on a simple theory and there is no model non-uniqueness on it.

Effects of hydrous alteration on the distribution of base metals and platinum group elements within the Kevitsa magmatic nickel sulphide deposit

Platinum and nickel are commonly assumed to be immobile in most conditions, especially during low temperature hydrothermal alteration. However, only a small number of studies have rigorously tested this assumption. The Ni–Cu–(PGE) sulphide ore body hosted by the Kevitsa intrusion, northern Finland, provides a natural laboratory to study the behaviour of base metals and platinum group elements (PGE) during low temperature alteration. This ca. 2060Ma mafic–ultramafic intrusion, located in the Central Lapland greenstone belt, hosts disseminated Ni–Cu–(PGE) sulphide mineralisation in the middle part of the main ultramafic body. The mineralisation, which contains a range of Ni, Cu and PGE grades, is affected by three main alterations (serpentinisation, amphibolitisation and epidotisation), and is cross cut by various types of veins. The effect of the circulation of hydrothermal fluids on the distribution of base metals and PGE was studied at two different scales. Interrogation of an extensive deposit-wide assay database provided information on the deposit-scale (kilometre scale) effect of these different alteration styles, and a detailed study, involving laboratory X-ray fluorescence (XRF), portable XRF and micro-XRF mapping, of drill-core samples containing cm-scale cross-cutting veins provided information on the small scale (centimetre to decimetre scale) remobilisation of base metals and PGEs. Results show that the hydration and carbonation of the Kevitsa mineralised mafic–ultramafic intrusion did not significantly affect the distribution of Ni and PGE at scales larger than a few mm, and that Cu and Au are the only metals that are affected by small to large scale remobilisation from centimetre to kilometre scale.

Natural pollutants of Northern Lake Baikal

Hot and cold springs in the northwestern shore of Lake Baikal are investigated as natural pollutants of the lake, which can interfere with its chemistry. The study includes chemical analyses of lake water samples from different depths in the North Baikal basin, as well as waters of hot and cold onshore springs, and Baikal tributaries collected in 2007 through 2013. Thermal waters in the shore of Lake Baikal have much higher SO4, Cl, F, Na, and SiO2 concentrations than those in the lake water, as well as ten to hundred times greater enrichments in Li, Rb, Cs, Ga, As and W. Mineral water in cold springs is generally similar to the Baikal water with respect to total dissolved solids (TDS) and major- and trace-element chemistry. However, the Milky cold spring discovered in the lower reaches of the B. Cheremshan River within a few km from the lake stands out in its unusual Ca–Mg–SO4 composition, low pH (3.57) and high TDS (535 mg/l), and trace-element enrichments in Al, Mn, Ni, Co, Zn, Y, and REE. The concentrations of these elements are still higher, up to one or two orders of magnitude, in suspended solids abundant in the spring water. This water composition is typical rather of mine drainage in sulfide mineralization provinces and prompts the existence of a sulfide orebody in the area. Mn, Co, Ni, Zn, and Cd occur in the dissolved form, and their concentrations in stream water near the B. Cheremshan inlet into Baikal are only slightly lower than immediately downstream of the spring; REE and Y are ten times lower because of precipitation from the river water. However, all metals the spring water leaches from the surrounding rocks eventually end up in Lake Baikal. Although the spring has much greater enrichments relative to the Baikal water in a number of elements, it hardly can cause any significant change to the lake composition. Only local chemical anomalies of Mn, Al, Co, Ni, Zn, Cd, REE and Y can be expected in the bottom sediments where the lake uptakes the B. Cheremshan stream.

Building a 3D model of lithological contacts and near‐mine structures in the Kevitsa mining and exploration site, Northern Finland: constraints from 2D and 3D reflection seismic data

ABSTRACTThe Kevitsa mafic–ultramafic intrusion in Northern Finland hosts a large, disseminated nickel–copper sulphide ore body. The Kevitsa intrusion is an active mining and exploration site, for which we have built a 3D model of the main lithological contacts and near‐mine structures in the area. To build the 3D model, 2D and 3D reflection seismic data have been used together with borehole data and geological map of the area. The Kevitsa reflection seismic data reveal the internal architecture of the Kevitsa intrusion and the surrounding units. For example, the seismic data have uncovered a previously unknown, deeper continuation of the Kevitsa intrusion. Improved 3D knowledge of the basal contact of the intrusion provides an exploration target for contact‐type mineralization. Within the intrusion, a limited area of strong reflections is observed in the data. This has been associated with discontinuous, smaller‐scale magmatic layering that is thought to control the extent of the Kevitsa main mineralization. Thus, our 3D model of the extents of the internal reflectors can provide a framework for near‐mine and deep exploration of the main type of mineralization in the area. In addition to exploration, the original purpose of the 3D seismic survey was geotechnical planning of the Kevitsa open‐pit mine. Accordingly, the 3D seismic data were used to create a 3D model of the subsurface structures, with a focus on the vicinity of the mine. The interpreted structures reveal a complex pattern of fault and fracture zones, some of which will be important for slope stability and operational planning of the final stages of the mine.

Magmatic Cu–Ni sulfide mineralization of the Huangshannan mafic–untramafic intrusion, Eastern Tianshan, China

The Huangshannan Ni–Cu (–PGE) sulfide deposit, a new discovery from geological prospecting in Eastern Tianshan, is in a belt of magmatic Ni–Cu (–PGE) sulfide deposits along the southern margin of the Central Asian Orogenic Belt. The host intrusion of the Huangshannan deposit is composed of a layered ultramafic sequence and a massive gabbro–diorite unit. The major sulfide orebodies occur mainly within websterite and lherzolite in the layered ultramafic sequence. In-situ zircon U–Pb dating analyses yielded a crystallization age of 282.5±1.4Ma, similar to the ages of the Permian Tarim mantle plume. Samples from the Huangshannan intrusion are characterized by nearly flat rare earth elements patterns, negative Zr, Ti and Nb anomalies, arc-like Th/Yb and Nb/Yb ratios, and significantly lower rare earth element and immobile trace element contents than the Tarim basalts. These characteristics suggest that the Huangshannan intrusion was not generated from the Tarim mantle plume. The primary magma for the Huangshannan intrusion and its associated sulfide mineralization were formed from different pulses of picritic magma with different degrees of crustal contamination. The first pulse underwent an initial removal of 0.016% sulfide in the deep magma chamber. The evolved magma reached sulfide saturation again in the shallow magma chamber and formed sulfide ores in lherzolite. The second pulse of magma reached a level of 0.022% sulfide segregation at staging chamber before ascending up to the shallow magma chamber. In the shallow conduit system, this sulfide-unsaturated magma mixed with the first pulse of magma and with contamination from the country rocks, leading to the formation of sulfide ores in websterite. The third magma pulse from the deep chamber formed the unmineralized massive gabbro–diorite unit of the Huangshannan intrusion.

Formation of iron‐rich shelled structures by microbial communities

AbstractIn this paper, we describe the discovery and characterization of shelled structures that occur inside galleries of Pyrenees mines. The structures are formed by the mineralization of iron and zinc oxides, dominantly franklinite (ZnFe2O4) and poorly ordered goethite (α‐FeO(OH)). Subsurface oxidation and hydration of polymetallic sulfide orebodies produce solutions rich in dissolved metal cations including Fe2+/3+ and Zn2+. The microbially precipitated shell‐like structure grows by lateral or vertical stacking of thin laminae of iron oxide particles which are accreted mostly by fungal filaments. The resulting structures are composed of randomly oriented aggregates of needle‐like, uniform‐sized crystals, suggesting some biological control in the structure formation. Such structures are formed by the integration of two separated shells, following a complex process driven likely by different strategies of fungal microorganisms that produced the complex macrostructure.

A combined fluid inclusion and S–Pb isotope study of the Neoproterozoic Pingshui volcanogenic massive sulfide Cu–Zn deposit, Southeast China

The Pingshui Cu–Zn deposit is located in the Jiangshan–Shaoxing fault zone, which marks the Neoproterozoic suture zone between the Yangtze block and Cathaysia block in South China. It contains 0.45million tons of proven ore reserves with grades of 1.03wt.% Cu and 1.83wt.% Zn. This deposit is composed of stratiform, massive sulfide ore bodies, which contain more than 60vol.% sulfide minerals. These ore bodies are hosted in altered mafic and felsic rocks (spilites and keratophyres) of the bimodal volcanic suite that makes up the Neoproterozoic Pingshui Formation. Metallic minerals include pyrite, chalcopyrite, sphalerite, tennantite, tetrahedrite and magnetite, with minor galena. Gangue minerals are quartz, sericite, chlorite, calcite, gypsum, barite and jasper. Three distinct mineralogical zones are recognized in these massive sulfide ore bodies: a distal zone composed of sphalerite+pyrite+barite (zone I); an intermediate zone characterized by a pyrite+sphalerite+chalcopyrite assemblages (zone II); and a proximal zone containing chalcopyrite+pyrite+magnetite (zone III). A thin, layer of exhalative jaspilite overlies the sulfide ore bodies except in the proximal zone. The volcanic rocks of the Pingshui Formation are all highly altered spilites and keratophyres, but their trace element geochemistry suggests that they were generated by partial melting of the depleted mantle in an island arc setting. Homogenization temperatures of the primary fluid inclusions in quartz from massive sulfide ores are between 217 and 328°C, and their salinities range from 3.2 to 5.7wt.% NaCl equivalent. Raman spectroscopy of the fluid inclusions showed that water is the dominant component, with no other volatile components. Fluid inclusion data suggest that the ore-forming fluids were derived from circulating seawater. The δ34S values of pyrite from the massive sulfide ores range from −3.6‰ to +3.4‰, indicating that the sulfur was primarily leached from the arc volcanic rocks of the Pingshui Formation. Both pyrite from the massive sulfide ores and plagioclase from the spilites have similar lead isotope compositions, implying that the lead was also derived from the Pingshui Formation. The low lead contents of the massive sulfide ores and the geochemistry of their host rocks are similar to many VMS Cu–Zn deposits in Canada (e.g., Noranda) and thus can be classified as belonging to the bimodal-mafic subtype. The presence of magnetite and the absence of jaspilite and barite at the −505m level in the Pingshui deposit suggest that this level is most likely the central zone of the original lateral massive sulfide ore bodies. If this interpretation is correct, the deep part of the Pingshui Cu–Zn deposit may have significant exploration potential.

Krasnotur’insk Skarn copper ore field, Northern Urals: The U-Pb age of ore-controlling diorites and their place in the regional metallogeny

The Krasnotur’insk skarn copper ore field known from the theoretical works of Academician K.S. Korzhinskii is located in the western part of the Tagil volcanic zone (in the area of the town of Krasnotur’insk). The ore field is composed of layered Devonian (Emsian) volcanosedimentary rocks intruded by small plutons of quartz diorites, diorites, and gabbrodiorites. Widespread pre-ore and intra-ore dikes of similar composition control the abundance of the andradite skarns formed after limestones and the magnetitesulfide and sulfide ore bodies formed after skarns. The LA-ICP-MS U-Pb concordant age of zircon from the quartz diorite of the Vasil’evsko-Moskalevskii pluton calculated by 16 analyses (16 crystals) is 407.7 ± 1.6 Ma (MSWD = 1.5). Taking into account the geological and petrogeochemical similarity of diorites of small plutons and intra-ore dikes, it is assumed that this age corresponds to the period of formation of the ore-magmatic system of the Krasnotur’insk skarn copper ore field. It was probably formed somewhat earlier than the Auerbakh montzonitic pluton and the accompanying skarn magnetite deposits in the south.

Characteristics of Resistivity and Self-Potential Anomalies over Agbani Sandstone, Enugu State, Southeastern Nigeria

The characteristics of the Resistivity and Self-Potential (SP) anomalies over Agbani Sandstone have been carefully and painstakingly carried out. The study was aimed at investigating the possible rock types and their mineralogical potentials. Data was acquired using the high resolution versatile ABEM SAS 4000 resistivity meter, employing the profiling method. Datasets were analyzed using the Excel toolkits. Interpretation was basically qualitative. Based on the resistivity interpretation, Agbani Sandstone is laterally limited in extent while the mineralization potential is high as a result of the high negative SP anomalies. The negative SP values range is -200mV to -500mV. This is practically indicative of a sulphide orebodies – possibly pyrite (FeS2). Comparative profile plots show that the observed zones of sulphide orebodies are within the gradational contact of Agbani Sandstone with Awgu Shale. Stream sediment analysis and rock geochemical study are recommended. However, the study has shown that contact zones of sandstone deposits are possibly ore enrichment zones.

The Distribution of PGE and the Role of Arsenic as a Collector of PGE in the Spotted Quoll Nickel Ore Deposit in the Forrestania Greenstone Belt, Western Australia

The Spotted Quoll PGE-bearing Ni deposit in the Forrestania greenstone belt in the Archean Yilgarn block in Western Australia is a komatiite-associated massive sulfide orebody tectonically displaced from its original host. The brecciated ore contains clasts of quartz and garnet schist and is located along a shear zone overlain by banded iron formation (BIF) and underlain by BIF and quartz-biotite metasediments. The deformation of the ore has destroyed its magmatic textures and it has been sheared and recrystallized at amphibolite facies. Then the deformed ore has been subjected to a hydrothermal event that concentrated the PGE with Au and As, often at the edge of the Ni ore. The PGE are distributed between PGM and in solid solution in Ni sulfarsenides, and Pd also occurs in pentlandite. The PGM include sudburyite (PdSb), sperrylite (PtAs2), and irarsite (IrAsS). All six PGE and minor Au are hosted in gersdorffite (NiAsS). Two generations of gersdorffite have been recognized. A higher temperature magmatic euhedral Co-rich gersdorffite encloses Ir-, Pt- and Rh-bearing PGM surrounded by halos of Rh-, Ir-, and Os-rich gersdorffite. A lower temperature Ni-rich gersdorffite forms anhedral grains and rims on grains of nickeline (NiAs). In this low-temperature gersdorffite PGE are concentrated toward the mineral edges. Sudburyite and gold occur associated predominantly with nickeline. The PGE and gold are now predominantly associated with sulfarsenides that are the controlling factor for their distribution.

Extrusive Origin and Structural Modification of the Komatiitic Mount Keith Ultramafic Unit

Mount Keith is a large (up to 500 m thick, and >2 km long) serpentinized and talc-carbonate-altered dunite body. The geologic features of Mount Keith are similar to many of the ultramafic bodies in the Agnew-Wiluna belt and other greenstone belts within the eastern Yilgarn craton. Mount Keith hosts a large disseminated nickel sulfide orebody that is presently exploited by open pit. There are currently two hypotheses that explain the site of formation of these dunite bodies: either kilometer-scale komatiite lava channels, or subvolcanic sills intruded into the greenstone sequence are responsible. The intrusive model is essentially based on two pieces of geologic evidence: the presence of a single 20-m-scale and single 20-cm-scale ultramafic apophysis in overlying dacitic rocks; and the presence of dacite inclusions within the contact zone of the upper part of the Mount Keith ultramafic unit. New geologic data show that the western part of the Mount Keith ultramafic unit and the overlying dacite sequence are allochthonous and that the contact zone between the two comprises multiple anastomosing layer-parallel fault slices of both lithologies at a scale of meters to tens of meters. Within some of the fault blocks, primary igneous contacts are present between the top of the Mount Keith ultramafic unit and the overlying dacite. New evidence shows that both pyroxene and minor olivine spinifex-textured rocks are present along much of the upper preserved part of the Mount Keith ultramafic unit and pseudomorphs after pyroxene grains exist on a scale of 20 to 200 μ m that formed during emplacement and survived the cooling of the 500-m-thick Mount Keith ultramafic unit. Moreover, the evidence for a large-scale ultramafic intrusive apophysis is unsubstantiated, as this apophysis is shown to be a slice within a brittle/ductile fault. The reinterpretation of the small-scale apophysis is consistent with this being an ultramafic enclave within a younger dacite flow. Finally, the dacite exposed within centimeters to meters of the western ultramafic contact commonly retains delicate igneous textures and shows no evidence of thermal effects from the Mount Keith ultramafic unit. Modeling of the heat budget assuming the dimensions of the Mount Keith ultramafic unit shows that an intrusion of this size should have produced wholesale melting of roof rocks. Our findings indicate that the Western Dacite sequence was not present above the Mount Keith ultramafic unit during its emplacement and cooling. A model is favored for the extrusive origin of the Mount Keith ultramafic unit with construction of the olivine cumulate pile at the floor of an approximately 2-km-wide, intermittently sulfide-bearing, turbulent lava pathway. At the terminal stages of eruption magma flow decreased and finally ceased, allowing the lava to drain and cause the upper crust to collapse onto the top of the crystal pile, producing in places sharp juxtaposition of chilled margin rocks with coarse-grained olivine cumulates. In most places rapid flow and turbulent convection resulted in resorption of the crust, but local formation of stagnant ponds allowed survival of an olivine spinifex-textural profile. More extensive ponding of the magma allowed in situ fractionation and formation of gabbroic and pyroxenitic rocks derived from highly fractionated komatiite magma. The Mount Keith ultramafic unit is believed to be the product of extrusive magmatism from which an olivine cumulate pile developed by upward accretion at the floor of a major lava pathway; these rocks were then extensively modified by deformation.

An implementation of differential evolution algorithm for inversion of geoelectrical data

Abstract Differential evolution (DE), a population-based evolutionary algorithm (EA) has been implemented to invert self-potential (SP) and vertical electrical sounding (VES) data sets. The algorithm uses three operators including mutation, crossover and selection similar to genetic algorithm (GA). Mutation is the most important operator for the success of DE. Three commonly used mutation strategies including DE/best/1 (strategy 1), DE/rand/1 (strategy 2) and DE/rand-to-best/1 (strategy 3) were applied together with a binomial type crossover. Evolution cycle of DE was realized without boundary constraints. For the test studies performed with SP data, in addition to both noise-free and noisy synthetic data sets two field data sets observed over the sulfide ore body in the Malachite mine (Colorado) and over the ore bodies in the Neem-Ka Thana cooper belt (India) were considered. VES test studies were carried out using synthetically produced resistivity data representing a three-layered earth model and a field data set example from Gokceada (Turkey), which displays a seawater infiltration problem. Mutation strategies mentioned above were also extensively tested on both synthetic and field data sets in consideration. Of these, strategy 1 was found to be the most effective strategy for the parameter estimation by providing less computational cost together with a good accuracy. The solutions obtained by DE for the synthetic cases of SP were quite consistent with particle swarm optimization (PSO) which is a more widely used population-based optimization algorithm than DE in geophysics. Estimated parameters of SP and VES data were also compared with those obtained from Metropolis–Hastings (M–H) sampling algorithm based on simulated annealing (SA) without cooling to clarify uncertainties in the solutions. Comparison to the M–H algorithm shows that DE performs a fast approximate posterior sampling for the case of low-dimensional inverse geophysical problems.

Forward modelling and inversion of self-potential anomalies caused by 2D inclined sheets

Self-potential anomalies observed over sulfide ore bodies can be closely associated with electrochemical reactions and the ohmic potential drop within the rocks. Self-potential surveys based on laboratory measurements of electrochemical potentials allow us either to measure the amplitude of the anomalies generated by this mechanism or to determine the model parameters. In order to achieve these goals, two sheets of zinc and copper were joined together to simulate sheet-like ore bodies. Self-potential surveys were conducted over 684 electrodes with the purpose of revealing the influence of various angles of the sheet. In a laboratory experiment, four different inclinations were chosen to perform the forward modelling. The last part of this paper involves the inversion of measured data to recover the distribution of generated self-potential signals. The inversion results show a satisfactory agreement with the laboratory measured data. Finding the geometry of the buried source from the shape of the SP response is not intended as it is fixed in advance. The first aim of this paper is to show how the SP response is affected under the presence of a 2D conductive structure (sheet-like) in tank experiments. The second aim is to obtain one of the model parameters (coefficient M) using data regression.

The Horní Benešov ore deposit in the Devonian Šternberk-Horní Benešov Belt, Jeseníky Mts, Czech Republic. Part II: Fe-ore occurrences

The iron ore mineralizations at the Horni Benesov sulphidic deposit are located in the Devonian Sternberk - Horni Benesov Belt which is enveloped by the Lower Carboniferous flysch of the Culm Foreland Basin. The belt consists of metasedimentary and metavolcanic rocks. Horni Benesov is the only site in the Jeseniky Mountains, where the oxidic iron ores occur in close association with the sulphide orebodies of the VMS/SHMS type.

Unabated acid mine drainage from Cerro Rico de Potosí, Bolivia: uncommon constituents of concern impact the Rio Pilcomayo headwaters

Intensive mining and processing of the polymetallic sulfide ore body of Cerro Rico de Potosi (Bolivia) has occurred since 1545, leading to severe degradation of surface and subsurface waters, stream sediments, and soils at the headwaters of the economically vital, yet highly impacted, Rio Pilcomayo. Previous studies have documented extremely elevated concentrations of a limited suite of metals in local waterways from acid mine drainage (AMD), terrestrial and in-stream tailings, and ore processing plant discharges. However, contamination from a wider variety of ecotoxic metals/metalloids was considered likely due to the highly mineralized polymetallic nature of the ore body. To screen for this broader range of ecotoxic elements in AMD and receiving streams, data were gathered during two sampling events timed for the most extreme periods of the dry and wet seasons of one water-year. Concentrations of Ag, B, Ba, Mo, Sb, Se, Sn and V in AMD and receiving streams were greater than Bolivian discharge limits and receiving water body guidelines as well as international agricultural use standards. Locally, results indicate that contamination from mining Cerro Rico has a larger scope than previously thought and underscore the importance of remediation. Globally, the results raise the possibility that other mining regions could have unquantified hazards from overlooked ecotoxic elements and that screening for a broader range of contaminants may be warranted.

U–Pb dating constraints on the felsic and intermediate volcanic sequence of the nickel-sulphide bearing Cosmos succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Cosmos succession, within the Agnew-Wiluna greenstone belt of the Yilgarn Craton, consists of ultramafic, intermediate and felsic volcanic lithologies, and contains several komatiite-hosted nickel sulphide ore bodies. The volcanic succession that forms the stratigraphic footwall to the mineralised komatiite lavas consists of an intercalated sequence of fragmental and coherent extrusive lithologies, ranging from basaltic-andesites through to rhyolites. It also contains cross cutting felsic intrusions. The stratigraphic hangingwall consists largely of reworked volcanic-derived sedimentary rocks that include polymictic conglomerates containing granitic clasts. SIMS U–Pb dating was undertaken on zircons from nine felsic lithologies spanning the established stratigraphy. The footwall succession gives emplacement ages ranging from 2736±4 to 2724±3Ma. The overlying Cosmos Ultramafic sequence consists of two units; the basal ultramafic UMu1 package and the upper ultramafic UMu2 package, which are separated by a discontinuous felsic volcanic horizon that sits approximately 30–60m above the basal contact of the Cosmos Ultramafic sequence. This discontinuous felsic horizon is dated at 2685±8Ma and represents a break in accumulation of komatiite lavas and may be associated with a major hiatus. The succession also features two felsic intrusions emplaced at 2653±7Ma and 2670±6Ma, where the latter cross-cuts the upper contact of the UMu2 package. These results bracket the age of UMu1 package between 2724±3Ma and 2685±8Ma and that of UMu2 package between 2685±8Ma and 2670±6Ma. This stratigraphic division of the Cosmos ultramafic is further strengthened by the contrasting styles of mineralisation above and below the discontinuous felsic horizon and associated unconformity; UMu1 hosts several massive nickel sulphide deposits situated at the contact with the footwall volcanic succession, while UMu2 is typified by high-tenor disseminated nickel sulphide deposits situated just above the discontinuous felsic horizon. Our data indicates that the Cosmos volcanic succession was accumulated via punctuated volcanism spanning ∼50Ma that was followed by intrusive activity lasting for at least a further 32Ma. The results also imply that construction of this relatively small, mineralised greenstone succession lasted longer than anticipated on stratigraphic considerations alone, an outcome that may have implications for the timing of emplacement of other bi-modal volcanic successions in similar greenstone terranes.

Mobilization of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalt melts during low-grade metamorphism in the Alpi Apuane (Tuscany, Italy)

We report the discovery of an exceptional assemblage of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalts showing textural evidence for their mobilization as melts in the barite–pyrite–iron oxide orebodies of the Monte Arsiccio mine (Alpi Apuane, Tuscany, Italy). The relative abundance of rare thallium sulfosalts (including three new mineral species), their peculiar textural features within the orebodies (e.g., migration along matrix grain boundaries, drop-like internal textures, low interfacial angles between sulfosalts and matrix minerals), and the overall high thallium content in pyrite from the entire mining district (to ∼900 ppm) make the barite–pyrite–iron oxide deposits of the Alpi Apuane a reference locality for studying low-temperature sulfosalt melts in low-grade metamorphic complexes (greenschist facies). Our study reveals how sulfosalt melting during low-grade regional metamorphism controls the redistribution of economically valuable and environmentally critical elements such as thallium in sulfide orebodies containing significant amounts of low-melting-point chalcophile elements.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 2, vegetation and organic soil

Transported regolith cover hinders exploration for economically important komatiite-hosted sulphide ore bodies in the Yilgarn Craton. Biogeochemistry offers a surface sampling technique to explore for these buried deposits that has not been extensively tested in this region. In this study, eucalypt trees and some other common plants were investigated at the North Miitel Ni deposit for their use in exploration targeting and to understand what role vegetation is taking in creating near surface anomalous geochemical signatures. Primary ore is deep ( c. 400 m) with a thin, weakly-enriched (compared to primary mineralisation), in situ weathered zone 10–20 m below the surface, under a cover sequence of 5–10 m deep. This study investigated the efficacy of several sample media including leaves, bark, litter, and organic-rich soils in an orientation traverse and a more extensive (150 m - spaced, 9 km 2 ) grid-based survey (footprint). The organic-rich soil and biogeochemistry were influenced by contamination from the haul road. Directly over mineralisation some elements showed anomalous concentrations but data were erratic, not statistically significant and, therefore, not useful for mineral exploration. High groundwater salinity and low pH, lack of supergene development, weak subsurface signature and aeolian contamination contribute to why Ni anomalies have not been readily indentified at the surface. Nickel is also essential to plant physiology and is actively absorbed in a controlled manner. Investigation of plant-soil-water interactions is valuable in understanding metal mobility in the environment. Here we refined the viable exploration techniques in this setting where Ni sulphide discoveries through cover are important for future economic and resource development.

Supergene features and evolution of gossans capping massive sulphide deposits in the Iberian Pyrite Belt

Twelve massive sulphide deposits from the Iberian Pyrite Belt (IPB) show well-preserved iron caps, some of which were mined during the last century to recover precious metals (e.g., Tharsis, Rio Tinto, San Miguel). Field observations and correlation assays between the distinct mineral sequences at different deposits suggest that all the gossans were developed under similar conditions and have undergone the same geological events. All the gossans have a mushroom-like morphology in sharp contact with the underlying massive sulphide orebodies. In most cases these are located over an apparent supergene enrichment zone rich in secondary sulphides. Some gossans extend into tongues of alluvial heterolithic breccias consisting of eroded transported gossans displaced as far as several hundred meters away from their sources. The distribution of major minerals throughout the gossan profiles (goethite, hematite, quartz and jarosite) and the statistical analysis of the geochemical data distinguish three separate zones, with gradual contacts roughly parallel to the current topography: (1) the lower zone dominated by goethite and subordinate jarosite, with significant enrichment in S, As, P, Pb, Sn, Sb, Ag and Au; (2) the middle or principal zone dominated by goethite and lacking jarosite, which is depleted in S, and As, as well as heavy and precious metals; and (3) the upper zone near the surface, mainly composed of hematite and quartz with only weak anomalies in P, Pb and Sn. The origin and variations occurred in the profiles are explained by a three-stage process. This involves an initial acidic stage of gossan development centred on the oxidation of sulphides that lead to the formation of the first Fe-rich oxyhydroxides and sulphates (mainly goethite and jarosite, respectively). Over time, a progressive stage of maturity is reached progressively downwards through the gossan profile due to the intensification of the oxidation and leaching processes. The ongoing gossan formation produced alteration and reprecipitation of pre-existing oxyhydroxides, the loss of the majority of the previously sorbed heavy metals, and a major dilution of trace elements especially in the zones near the surface. The main results of this stage of formation are the production of heavy metal-depleted oxyhydroxides, most commonly goethite and hematite, and the disappearance of jarosite. Subsequently, local uplift of the gossanous rocks by neotectonic movements facilitated the rejuvenation of the oxidation of the ores. This final stage complicated the previously developed zonation with the formation of jarosite in mature areas. Possible major breaks in this gossan development ocurred in Messinian times (7–8Ma) and at the beginning of the Early Quaternary (1–2Ma?).