Ultramafic Formations Research Articles

МИНЕРАЛОГИЯ ЭЛЕМЕНТОВ ПЛАТИНОВОЙ ГРУППЫ В ЭКСПЛОЗИВНЫХ БРЕКЧИЯХ МЕСТОРОЖДЕНИЯ ПОПЕРЕЧНОЕ (МАЛЫЙ ХИНГАН, РОССИЯ)

The results of detailed study of the platinum-group minerals (PGM) from explosive breccias of the Poperechnoe Fe-Mn deposit (the Lesser Khingan Range, Russian Far East) are presented. Native PGMs are dominated by isoferroplatinum; rutheniridosmine, native iridium, platinum, and osmium are less com-mon. Micron-sized segregations of laurite, bowieite, сuproiridsite, cuprorhodsite, hollingworthite, as well as new mineral phases of (Ir,Rh,Os)7 (S,As)13 , (Rh,Ir,Ru) 7(S,As) 13, and Pd 3(Sb,As) were recognized as micro-inclusions in isoferroplatinum and on the surface of its grains. It is shown that the studied PGMs are derived from rocks of ultramafic formations: (1) vein pyroxenites, harzburgites, and dunites of metamorphic and cumulative complexes of the most depleted peridotite varieties of the suprasubduction wedge of island-arc ophiolites and (2) vein pyroxenites of cumulative high-pressure ultramafic complexes of the basement of the ensialic island arc and products of evolution of suprasubduction mantle melts. In terms of PGE contents, the compositions of isoferroplatinum from explosive breccias are divided into four groups: group I: isoferroplatinum of fluid-metamorphogenic genesis from harzburgite, group II: isoferroplatinum of fluid-metamorphogenic genesis from dunites and magmatogenic-fluid-metasomatic genesis from vein pyrox-enites; group III: isoferroplatinum of magmatogenic genesis from chromitites of dunites of the cumulative complex; and group IV: isoferroplatinum with an elevated Pd content, which is likely derived from the melt formed by explosive breccias. Three scenarios of PGM occurrence in the fluid-saturated andesite-dacite melt of explosive breccias at the Poperechnoe deposit are discussed: (1) directly from early ultramafic com-plexes in suprasubduction settings; (2) from the reservoir of ancient platinum placer deposits; and (3) from a mantle wedge above the Mesozoic subduction zone during the generation of initial island-arc melts. The first two petrogenetic models suggest «rejuvenation» of the 190Pt-4He age of isoferroplatinum grains as a result of the thermal effect of the andesite-dacite melt to the age of 125 ± 21 Ma. The third geodynamic scenario suggests that the Lower Cretaceous age of isoferroplatinum marks the processes of metamorphogenic-metasomatic transformation of the dunite-harzburgite mantle wedge and probably corresponds to the age of the subduction magmatism at the Poperechnoe Deposit in particular and within the Lesser Khingan terrane as a whole. PGM associations in andesite-dacite breccias of the Poperechnoe Deposit are a new type of potentially commercial noble-metal mineralization in the Russian Far East, while explosive breccias (fluidoliths) can be used as an exploration guide to discover PGE lode and placer deposits of volcanogenic-explosive genesis in the Russian Federation.

CO2 transport and carbonate precipitation in the coupled diffusion-reaction process during CO2 storage

During geological CO2 storage and utilization, CO2 plume migration forms widespread static retention regions including the dead-end fractures and pores in the host rock, where the mass transfer is dominated by diffusion. A series of packed column experiments with natural olivine were conducted to investigate how the coupled CO2 diffusion–reaction process influences the CO2 transport and the temporal-spatial evolution of carbonate precipitation in diffusion-limited zones. The carbonate precipitations were mainly magnesite, and some dolomite due to the combined effect of carbonate precipitation kinetics and the low Ca content of the solution with Mg enrichment. Compared with the pure diffusion in porous media, the geochemical gradients in the pore solution resulting from the coupled diffusion–reaction interaction promoted mass transfer and broad migration, which led to an increase in effective diffusion coefficients of CO2 by approximately-one order of magnitude. The free water layer and pressure mode could affect the distribution of carbonate precipitation and the overall sequestration capacity. In particular, the coupled CO2 diffusion–reaction interaction under pressure decay mode could accelerate the carbonation in the deep region in the water-saturated column. This study advances the fundamental understanding on the transport and reaction of injected CO2 for mafic and ultramafic formations.

Central Java regional geochemistry: influence of environmental geology and mineral occurrences

Stream sediments geochemical surveys were undertaken in Central Java, Indonesia as a part of the Geological Agency’s geochemical mapping project with support from several universities. A total of 1583 stream sediment samples were collected from Central Java and analyzed using X-Ray Fluoresces for 29 elements. Geochemical maps were processed by the GIS software. Based on the overlay between geological and geochemical maps, there are several elemental anomalies, for example, the ultramafic formation from Kebumen showing distinctive Chromium (Cr), Cobalt (Co), and Magnesium (Mg) anomalies. Meanwhile, the potassic igneous rocks from Muria Mountain have Cerium (Ce), Lanthanum (La), and Zirconium (Zr) anomalies. Elemental anomalies related to mineral resources and mining areas such as Arsenic (As), Copper (Cu), and Lead (Pb) are found in Trenggalek, Pacitan, and Kebumen; while Barium (Ba), Iron (Fe), and Manganese (Mn) are scattered along the southern coast of Central Java. Anthropogenic elements (Chlorine (Cl), Ce, and As) spread along the north coast of Central Java probably because of pollution by industrial, farming, and urban areas. Heavy metals pollutions that were identified previously in the Bengawan Solo River flow can be seen on this geochemical map which can convincingly delineate elements anomalies due to geological and environmental conditions.

Reactive transport experiments of coupled carbonation and serpentinization in a natural serpentinite. Implication for hydrogen production and carbon geological storage

Serpentinization and carbonation of ultramafic formations is a ubiquitous phenomenon, which deeply influences the biogeochemical cycles of water, hydrogen, carbon…while supporting the particular biosphere around the oceanic hydrothermal vents. Carbonation of peridotites and other mafic and ultramafic rocks is also a hot topic in the current energy landscape as the engineered sequestration of mineral CO2 in these formations could help reduce the atmospheric emissions and cope with climate change. In this study, we present two reactive percolation experiments performed on a natural serpentinite dredged from the ultraslow South-West Indian Oceanic Ridge. The serpentinite cores (length 3–4 cm and dia. 5.6 mm) were subjected for about 10 days to the continuous injection of a NaHCO3-saturated brine at respectively 160 °C and 280 °C. Petrographic and petrophysical results as well as outlet fluid compositions were compared to numerical batch simulations performed with the PHREEQC open software allowing to reconstruct the mineralogical evolution of both cores. The most striking observation is the fast and dramatic decrease of the permeability for both experiments principally due to the precipitation of carbonates. On the contrary, serpentine was found to be less impacting as it precipitates in low-flow zones, out of the main percolation paths. In total, about 5.6% of the total injected CO2 was retained in the core, at 280 °C. In the same time, hydrogen was consistently produced with a total recovered H2 corresponding to 0.8% of the maximum H2 possible. The global behavior of the cores is interpreted as the result from an interplay between interacting spatio-temporal lengthscales controlled by the Damköhler number.

Гипербазиты как фактор геодинамики по результатам исследований на Таймырском геофизическом полигоне

Seismic data indicate the widespread inversion origin of the largest geological structures of the Taimyr Peninsula and record repeated changes of the direction of tectonic movements. The inversion the Yenisei-Khatanga trough led to the formation of a system of megaarchs over the ancient depocenter containing massive hyperbasite intrusions at the Moho level, which are marked by the gravity maximum. These intrusive processes are an important factor that can initiate inversion and contribute significantly to its development under permanent compressional tectonic regime in the region.According to the geophysical data, the development of troughs in the north of Central and Western Siberia is associated with the formation of eclogites under the depocenters, at the Moho level. Consistent with the electrical resistivity anomalies, eclogites are plastic and overlap mantle hyperbasites of lower density that leads to gravitational buoying up of ultramafic formations. This, along with metamorphic and metasomatic processes, acts as an impulse for the uplift in the axial parts of the troughs. Simultaneously, expansion and acceleration of sagging, due to the displacement of plastic eclogites and crystal-mantle mixture from the penetration zone towards the peripheral parts of the sedimentary basin, result in the uncompensated sediment accumulation and the formation of the Neocomian clinoform complex.

Floristic study of an ultramafic formation in Sitio Magarwak, Sta. Lourdes, Puerto Princesa City, Palawan Island, Philippines

Abstract. De Castro ME, Carandang VI JS, Agoo EM. 2020. Floristic study of an ultramafic formation in Sitio Magarwak, Sta. Lourdes, Puerto Princesa City, Palawan Island, Philippines. Biodiversitas 21: 3769-3779. A study was done to assess the floristic composition of an ultramafic formation in Sitio Magarwak, Barangay Sta. Lourdes, Puerto Princesa City Palawan. A total of 52 trees and shrubs representing 30 families and 48 genera was recorded in the overstorey while the ground cover was dominated by 10 species mostly from families Poaceae and Zingiberaceae. The most dominant tree species include Dillenia monantha Merr., Xanthostemon speciosus Merr. and Schima wallichii ssp. oblata (Roxb.) Kurz. with Important Value Indices (IVIs) of 15.96%, 15.71%, and 14.20%, respectively. Level of endemicity was considerably high with 21 species classified as indigenous, 11 of which have their population restricted only on the island of Palawan. However, most of these endemics are now under threat based on the Updated List of Threatened Philippine Plants and their Categories (DAO No 2017-11) and the International Union for the Conservation of Nature (IUCN) Red List (2020-1). Two species are classified as ˈcritically endangeredˈ (Olea palawanensis Kiew and Guioa palawanica Welzen) while the two are ‘endangered’ (Kibatalia stenopetala Merr. and Nepenthes philippinensis Macfarl.). Findings from this study suggest that appropriate conservation measures must be put in place to ensure protection of the native flora’s remaining population from further destruction.

High Hexavalent Chromium Concentration in Groundwater from a Deep Aquifer in the Baiyangdian Basin of the North China Plain.

Hexavalent chromium (Cr(VI)) is known to occur naturally in shallow oxic groundwater, typically from aquifers associated with mafic and ultramafic formations, but information on the occurrence of Cr(VI) in deep groundwater from large sedimentary basins is limited. This study shows that groundwater from the Baiyangdian Lake Basin (BYB), home to the future second capital city of China, had high Cr concentration (>10 μg/L, up to 86 μg/L) in the deep aquifer (>150 m), while shallow groundwater had lower Cr concentration (<10 μg/L). Chromium occurred predominantly as Cr(VI) (>95%). Shallow groundwater was characterized by higher Mn and Fe concentrations relative to deep groundwater, likely indicating more reducing conditions. Sequential extraction experiments from aquifer sediments suggest that Cr(VI) may derive from silicate weathering and that Mn oxides in the aquifer play a major role in the formation of Cr(VI) in groundwater. Inverse correlations between Mn and Cr(VI) suggest that reductive dissolution of Mn oxides constrains Cr(VI) mobilization in the shallow groundwater, while oxic-suboxic conditions in the deep aquifer limit Mn solubility, which enhances oxidation of Cr(III) to Cr(VI) and promotes desorption of Cr(VI) under alkaline conditions. This study demonstrates the potential geogenic occurrence of high Cr(VI) concentration in deep groundwater from a nonmafic, large sedimentary basin containing Mn oxides in the aquifer sediments.

Geology of the Chernogorskoe Gem-Quality Scapolite Deposit (Central Pamirs, Tajikistan)

The Chernogorskoe gem-quality scapolite deposit, which is attributed to a new genetic type, is described. It is located in the Central Pamirs (Tajikistan), in the Sary-Dzhilga Formation of the Muzkol Group (PR1). Host rocks include metamorphosed mafic–ultramafic formations of the Kukurt complex, among which harzburgite was the first discovered. Albitite lenses host gem-quality scapolite mineralization. They were formed by the metasomatic replacement of nepheline syenite. This reaction was accompanied by a decrease in the volume of solid phases by 14% and the appearance of voids. Albitite bodies form a lenticular nesting structure, have crosscutting contacts, and make up amphibolite, which metasomatically replaces melanocratic gabbroid and partially altered harzburgite. Gem-quality scapolite is localized in 28 albitite lenses; the walls of their voids are encrusted with columnar purple scapolite. The sequence of processes at the studied deposit is as follows: formation of melanocratic gabbroids with small nepheline syenite bodies → metasomatic development of amphibolite after the former and albitite with voids after the latter → growth of columnar scapolite on voids walls → crystallization of gem-quality scapolite in voids.

Experimental evidence for chemo-mechanical coupling during carbon mineralization in ultramafic rocks

Storing carbon dioxide in the subsurface as carbonate minerals has the benefit of long-term stability and immobility. Ultramafic rock formations have been suggested as a potential reservoir for this type of storage due to the availability of cations to react with dissolved carbon dioxide and the fast reaction rates associated with minerals common in ultramafic formations; however, the rapid reactions have the potential to couple with the mechanical and hydraulic behavior of the rocks and little is known about the extent and mechanisms of this coupling. In this study, we argue that the dissolution of primary minerals and the precipitation of secondary minerals along pre-existing fractures in samples lead to reductions in both the apparent Young's modulus and shear strength of aggregates, accompanied by reduction in permeability. Hydrostatic and triaxial deformation experiments were run on dunite samples saturated with de-ionized water and carbon dioxide-rich solutions while stress, strain, permeability and pore fluid chemistry were monitored. Sample microstructures were examined after reaction and deformation using scanning electron microscopy (SEM). The results show that channelized dissolution and carbonate mineral precipitation in the samples saturated with carbon dioxide-rich solutions modify the structure of grain boundaries, leading to the observed reductions in stiffness, strength and permeability. A geochemical model was run to help interpret fluid chemical data, and we find that the apparent reaction rates in our experiments are faster than rates calculated from powder reactors, suggesting mechanically enhanced reaction rates. In conclusion, we find that chemo-mechanical coupling during carbon mineralization in dunites leads to substantial modification of mechanical and hydraulic behavior that needs to be accounted for in future modeling efforts of in situ carbon mineralization projects.

Fluid chemistry of the low temperature hyperalkaline hydrothermal system of Prony Bay (New Caledonia)

Abstract. The terrestrial hyperalkaline springs of Prony Bay (southern lagoon, New Caledonia) have been known since the nineteenth century, but a recent high-resolution bathymetric survey of the seafloor has revealed the existence of numerous submarine structures similar to the well-known Aiguille de Prony, which are also the location of high-pH fluid discharge into the lagoon. During the HYDROPRONY cruise (28 October to 13 November 2011), samples of waters, gases and concretions were collected by scuba divers at underwater vents. Four of these sampling sites are located in Prony Bay at depths up to 50 m. One (Bain des Japonais spring) is also in Prony Bay but uncovered at low tide and another (Rivière des Kaoris spring) is on land slightly above the seawater level at high tide. We report the chemical composition (Na, K, Ca, Mg, Cl, SO4, dissolved inorganic carbon, SiO2(aq)) of 45 water samples collected at six sites of high-pH water discharge, as well as the composition of gases. Temperatures reach 37 °C at the Bain des Japonais and 32 °C at the spring of the Kaoris. Gas bubbling was observed only at these two springs. The emitted gases contain between 12 and 30% of hydrogen in volume of dry gas, 6 to 14% of methane, and 56 to 72% of nitrogen, with trace amounts of carbon dioxide, ethane and propane. pH values and salinities of all the 45 collected water samples range from the seawater values (8.2 and 35 g L−1) to hyperalkaline freshwaters of the Ca-OH type (pH 11 and salinities as low as 0.3 g L−1) showing that the collected samples are always a mixture of a hyperalkaline fluid of meteoric origin and ambient seawater. Cl-normalized concentrations of dissolved major elements first show that the Bain des Japonais is distinct from the other sites. Water collected at this site are three component mixtures involving the high-pH fluid, the lagoon seawater and the river water from the nearby Rivière du Carénage. The chemical compositions of the hyperalkaline endmembers (at pH 11) are not significantly different from one site to the other although the sites are several kilometres away from each other and are located on different ultramafic substrata. The very low salinity of the hyperalkaline endmembers shows that seawater does not percolate through the ultramafic formation. Mixing of the hyperalkaline hydrothermal endmember with local seawater produces large ranges and very sharp gradients of pH, salinity and dissolved element concentrations. There is a major change in the composition of the water samples at a pH around 10, which delimitates the marine environment from the hyperalkaline environment. The redox potential evolves toward negative values at high pH indicative of the reducing conditions due to bubbling of the H2-rich gas. The calculation of the mineral saturation states carried out for the Na-K-Ca-Mg-Cl-SO4-DIC-SiO2-H2O system shows that this change is due to the onset of brucite formation. While the saturation state of the Ca carbonates over the whole pH range is typical of that found in a normal marine environment, Mg- and Mg-Ca carbonates (magnesite, hydromagnesite, huntite, dolomite) exhibit very large supersaturations with maximum values at a pH of around 10, very well marked for the Bain des Japonais, emphasizing the role of water mixing in mineral formation. The discharge of high-pH waters of meteoric origin into the lagoon marine environment makes the hydrothermal system of Prony Bay unique compared to other low temperature serpentinizing environments such as Oman (fully continental) or Lost City (fully marine).

Differences in gas venting from ultramafic-hosted warm springs: The example of oman and voltri ophiolites

Serpentinisation of mantle rocks, leading to natural venting of hydrogen and methane, has been reported to occur at the global scale, wherever fluids percolate in ultramafic formations. Here we compare gas composition from two on-land, low-temperature, and hyper-alkaline springs hosted on ultramafic rocks in the ophiolite massifs of the Sultanate of Oman and the Ligurian Alps (Voltri Group, Genoa region, Northern Italy). These two settings exhibit similar chemical and mineralogical features but show diverse styles of gas venting. Commonly to all hyper-alkaline springs, gases are characterised by relatively high N 2 , very low O 2 and CO 2 concentrations, and a strong enrichment in H 2 and CH 4 . The comparison between Oman and Liguria highlights a high variability of the H 2 /CH 4 ratios whereby the gas phase of the Oman Ophiolite is enriched in H 2 whereas being CH 4 enriched in the Voltri Ophiolite. These results combined with literature data define three groups that may reflect different stages of serpentinisation producing fluids from hydrogen-dominated to methane-dominated. The origin of these distinct groups might lie in the difference of the mineralogical composition of the rocks within which the fluids circulate, on the degree of alteration of the rocks and finally on the geological/metamorphic history of the ophiolite.

Initial Os-isotopic composition of Os-Ir-Ru alloys from ultramafic massifs of the Polar Siberia

This study firstly presents chemical and initial Os-isotopic compositions of Os-Ir-Ru minerals of two ultramafic formations of Polar Siberia, which are exemplified by Guli clinopyroxene-dunite massif of the Maimecha-Kotui Province and the Kunar dunite-harzburgite massif from the Chelyuskin ultramafic belt of the Taimyr Peninsula. The study employed a range of methods, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and laser ablation attached to an inductively coupled plasma mass spectrometry (LA MC-ICP-MS). The majority of platinum-group minerals (PGM) from the Guli massif are Os-(Ir-Ru) solid solutions or Os-rich minerals. At Kunar, minerals of Ru-Os-Ir system (i.e., osmium, ruthenium, iridium and rutheniridosmine) dominate the PGM assemblage. The ruthenium trend in the mineral compositions is due to the formation of these minerals under high pressures and temperatures at considerable depths. The 187Os/188Os values of Os-rich minerals from the Guli massif range from 0.12309 ± 0.00002 to 0.12606 ± 0.00003 (n = 168). The initial Os-isotopic composition of PGM from the central block of the Guli massif is characterized by the 187Os/188Os values, varying in the range 0.12404–0.12606. Osmiumrich minerals from the southwestern block of the Guli massif are characterized by the least “radiogenic” 187Os/188Os values (i.e., 0.12309–0.12341). Low relative to the chondritic universal reservoir (CHUR) 187Os/188Os values are indicative of a near-to-chondritic source of platinum-group elements (PGE). The most “productive” stage of PGM formation at Guli (n = 121) is recorded in the time interval of 545–615 Ma. The older model 187Os/188Os ages of osmium minerals are characteristic of the southwestern block of the Guli massif (e.g., 745–760 Ma). The results of the initial Os-isotopic composition for Os-rich alloys are consistent with a model, in which PGM were formed during multi-stage melt depletion events in the mantle. This agrees well with the suggestion that the Guli massif consists of heterogeneous blocks of ultramafic rocks. The 187Os/188Os ratio in the investigated PGM from the Kunar massif varies in a wider range (0.1094–0.1241, n = 28). For the dominant set of PGM samples (n = 25), regardless of their chemical composition, four groups of the initial osmium isotopic compositions can be estimated, with average 187Os/188Os values of 0.1217 ± 0.0002 (n = 7), 0.1223 ± 0.0002 (n = 7), 0.1230 ± 0.0002 (n = 6) and 0.1238 ± 0.0003 (n = 6), respectively. The average model Re-Os ages for the defined groups of the Kunar massif are consistent with Late Riphean age interval (e.g., 975 ± 42 Ma, 892 ± 42 Ma, 791 ± 28 Ma and 681 ± 42 Ma, respectively). Significant variations in the 187Os/188Os values and model ages for Ru-Os-Ir alloys at Kunar are close to those from other duniteharzburgite massifs of the Earth, pointing out for their prolonged multi-stage evolution within the upper mantle.

Influence of geology on arsenic concentrations in ground and surface water in central Lesvos, Greece

The occurrence of As was studied in groundwater used for human consumption and irrigation, in stream water and sediments and in water from thermal springs in the drainage basin of Kalloni Gulf, island of Lesvos, Greece, in order to investigate the potential influence of the geothermal field of Polichnitos-Lisvori on the ground and surface water systems of the area. Total dissolved As varied in the range <0.7-88.3 microg L(-1) in groundwater, 41.1-90.7 microg L(-1) in thermal spring water and 0.4-13.2 microg L(-1) in stream water, whereas As concentrations in stream sediments varied between 2.0-21.9 mg kg(-1). Four out of 31 groundwater samples exceeded the EC standard of 10 microg L(-1). The survey revealed an enrichment in both surface and groundwater hydrological systems in the northern part of the area (average concentrations of As in groundwater, stream water and stream sediment: 8.0 microg L(-1), 8.8 microg L(-1) and 15.0 mg kg(-1) respectively), in association with the volcanic bedrocks, while lower As concentrations were found in the eastern part (average concentrations in groundwater, stream water and stream sediment: 2.9 microg L(-1), 1.7 microg L(-1) and 5.9 mg kg(-1) respectively), which is dominated by ophiolitic ultramafic formations. The variation of As levels between the different parts of the study area suggests that local geology exerts a determinant influence on As geochemical behaviour. On the other hand, the geothermal activity manifested in the area of Polichnitos-Lisvori does not affect the presence of As in groundwater and streams.

MAGMATIC AND HYDROTHERMAL PLATINUM-GROUP MINERALS AND BASE-METAL SULFIDES IN THE BAULA COMPLEX, INDIA

The 3 × 0.5 km gabbro-hosted Precambrian Baula Complex, in Orissa, India, comprises a steeply dipping pyroxenite unit in tectonic contact to the east with a peridotite unit that contains three chromitite layers and that becomes pyroxene-rich eastward toward the top. The ultramafic formations are intruded by the Bangur Gabbro, the top of which entered a shear zone, forming a breccia zone. The breccia, 1 to 40 m thick and almost continuous along strike for 2000 m, shows extensive vertical and lateral variations. It is made up of ultramafic blocks of various sizes derived from the Baula Complex within a matrix of hydrothermally altered Bangur Gabbro. The brecciation process has affected one of the chromitite layers. Two categories of platinum-group-element (PGE) mineralization have been observed in the Baula Complex, both associated with the Bangur Gabbro. The first (type 1) occurs within a sulfide-free magmatic environment within the Bangur Gabbro near its contact with the peridotite, and is clearly formed within a magmatic environment. The PGE-bearing rock contains relics of dunite and chromitite extracted mechanically from the ultramafic formations by the intrusive gabbro. This mineralization is dominated by Pt (Pt/Pd in the range 8–9), is anomalous in Rh, and the platinum-group-mineral (PGM) assemblage is dominated by isoferroplatinum, braggite and malanite, with sperrylite and laurite, included in pyroxene and plagioclase. The second category (types 2A and 2B) is restricted to the hydrothermally altered matrix of the breccia zone. Type 2A has PGE associated with base-metal mineralization; the PGE assemblages are characterized by Pd (Pt/Pd 0.5), and the PGM are mainly sudburyite, minerals of the system Pd–Bi–Te–Sb, and sperrylite. Type 2B is not associated with base metals, the PGE are characterized by Pt (Pt/Pd in the range 2–3), and PGM phases of the Pt–Sb–As system (geversite, sperrylite) coexist with Pd antimony (mertieite-II). The PGE minerals form discrete grains, a) in the hydrous silicate matrix (commonly as clusters) without base-metal sulfides (BMS) or in the silicate matrix accompanying BMS trails, where they are in some cases adjacent to disseminated BMS, b) within ferrian chromite, c) within BMS in silicates, and d) within or adjacent to BMS in ferrian chromite. We present the results of electron-microprobe analyses of the PGM and BMS phases, and propose a model in which type-1 mineralization results from an orthomagmatic episode, whereas the type-2 mineralization is a result of hydromagmatic processes affecting the gabbroic matrix of the complex breccia zone.

Magnetic properties of mafics and ultramafics from Eastern Ghats

Magnetic properties, namely intensity of natural remanent magnetisation (NRM) and magnetic susceptibility, of mafic and ultramafic formations of Eastern Ghats belt in the Godavari Districts, Andhra Pradesh are studied. The formations sampled for the study are, iron-ore, chromite-ore, amphibolite, gabbro, charnockite and khondalite gneiss. Mineralogical differences in iron-ore and the degree of martitisation are reflected in magnetic susceptibility. NRM and susceptibility of chromite-ore are feeble. The gabbro exhibits strong magnetic properties because of its contiguity with iron formation. The Koenigsberger ratio is found to indicate relative palaeomagnetic stability.

The geology of the Kambalda nickel field, Western Australia

The Kambalda nickel field contains the major concentration of volcanic peridotitc-associated nickel ores in the Archean of Australia. It occupies an area of 400 km 2 underlain by two sequences of ultramafic, mafic, and felsic volcanics, and sedimentary rocks, which can be broadly correlated with previously erected regional stratigraphic sections. Most ores occur in the lower (Kambalda) sequence at the base of the Kambalda ultramafic rocks; the ultramafic rocks of the upper (Bluebush) sequence contain only minor mineralization. Four phases of deformation occurred over an interval of more than 300 m.y., resulting in a dominant north-northwest structural trend. Faulting was active during original volcanism and continued intermittently until after mafic-felsic dike intrusion. The ultramafic rocks have been serpentinized, and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism.The Kambalda ultramafic rocks are of variable thickness and in places exceed 1,000 m. The lower member of the ultramafic rocks, occupying approximately one-third of the sequence, comprises thicker high magnesium flows (i.e., generally > 36% volatile-free MgO) commonly separated by sulfidic quartz-albite sediments. Thinner, lower magnesium flow units characterize much of the upper member. The ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few, grouped, ribbonlike original orebodies. Eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt-ultramafic contact; these troughs are thought to be original volcanic depressions which focused later deformation. The remainder is hanging-wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow. Hanging-wall orebodies occasionally grade into stratigraphically equivalent interflow sediments.The ores contain varying proportions of massive, matrix, and disseminated sulfides. The hypogene mineral assemblage is pyrrhotite-pentlandite-pyrite-(chalcopyrite), but millerite is locally significant. The massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position. Metamorphism has caused recrystallization of the ores and some redistribution of elements. There is commonly marked variation of mineralogy and S/Ni ratios between contact and overlying hanging-wall ores, and within and between original ore surfaces within shoots, but the relative contributions of primary composition and metamorphism to this variation are uncertain.In ore environments, the footwall basalt-ultramafic contact is more deformed than in non-ore environments, the basal ultramafic flows are thicker and richer in magnesium, the trend toward lower magnesium flows upward through the ultramafic formation is less well developed, and there is better textural development and compositional differentiation within flows. Non-ore environments usually contain interflow sediments, whereas there is generally a sediment-free prism of ultramafic rocks above ore environments.Most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor. Volcanic exhalations probably contributed to minor ore in sulfidic sediments.