Ore Environment Research Articles

Hierarchical Microtubular Covalent Organic Frameworks Achieved by COF‐to‐COF Transformation

AbstractPore environment and aggregated structure play a vital role in determining the properties of porous materials, especially regarding the mass transfer. Reticular chemistry imparts covalent organic frameworks (COFs) with well‐aligned micro/mesopores, yet constructing hierarchical architectures remains a great challenge. Herein, we reported a COF‐to‐COF transformation methodology to prepare microtubular COFs. In this process, the C3‐symmetric guanidine units decomposed into C2‐symmetric hydrazine units, leading to the crystal transformation of COFs. Moreover, the aggregated structure and conversion degree varied with the reaction time, where the hollow tubular aggregates composed of mixed COF crystals could be obtained. Such hierarchical architecture leads to enhanced mass transfer properties, as proved by the adsorption measurement and chemical catalytic reactions. This self‐template strategy was successfully applied to another four COFs with different building units.

Fluid-rock interaction, skarn genesis, and hydrothermal alteration within an upper crustal fault zone (Island of Elba, Italy)

The Terranera magnetite-hematite-pyrite deposit of the Island of Elba (Italy) is an historical skarn deposit hosted by a fault zone of regional importance (Zuccale Fault) and by its hanging wall rocks.We combine field observations with petrographic data, electron probe microanalyses (EPMA), XRPD data, fluid inclusion microthermometry, and element imaging by Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry (LA-ICP-TOFMS) to define the ore-forming process at Terranera. We show that in this location the fault is made of four levels of mineralized fault rocks having distinct mineral compositions. In these levels, a mineral association made of diopside, clinozoisite, and other Mg-rich minerals is replaced by magnetite, hematite, pyrite, Mg-hornblende, clinochlore, and other Mg-rich phyllosilicates. This paragenesis is overprinted by goethite and clay minerals. Chlorite-quartz geothermometry and fluid inclusion microthermometry show that ore precipitation occurred at 350–180 °C from fluids of distinct bulk salinities, but goethite and clay mineral overprinting progressed at lower T.We propose that Terranera is a magnesian Fe skarn formed due to the interaction between distinct hydrothermal fluids and a dolomitic protolith, which was preserved within the fault zone. These fluids mixed and cooled during protolith metasomatism, causing ore precipitation due to oxidation and desulfidation. A very similar process was described in a large deposit of Elba (Rio Marina). Argillic alteration was widespread within the fault but met permanently intermediate sulfidation conditions. Trace element composition of hematite shows that Terranera has features that overlap those of skarn and epithermal deposits. In particular, elements that are typical of epithermal deposits (Sb, Ga, Ge, As) occur at mass fractions (50–200 μg/g) that are either unreported or not typical of hematite from skarn deposits. These features identify Terranera as formed in an ore environment that was transitional between that of a skarn and of an epithermal deposit. These features are shared by other historical deposits located at Elba and in the massive pyritic ore district of south Tuscany (e.g., Gavorrano, Fenice Capanne). This indicates that a similar environment might have occurred during the Neogene beyond Elba, in a much larger ore district of south Tuscany.

Nickel Deposit Depth Estimation Using Source Parameter Imaging and Euler Deconvolution of Aeromagnetic Data of Bakin Kogi and its Environs in Jema’a Local Government Area of Kaduna state

This study aimed to estimate the depth of Nickel deposit in Bakin Kogi – Dangoma region, which lies between latitude 901''and 9027'N and longitude 8000' and 8017' E. The study area is located in Jema’a local government area of Kaduna state. The data was gridded using Oasis Montaj® to produce Total magnetic Intensity Map (TMI) of the area, and further subjected to some filtration processes in order to obtain regional and residual map of the study area. Enhancement methods such as first vertical derivative and analytic signal methods were applied to the residual map to delineate the area with potential Nickel deposit. The quantitativemethod applied were source Parameter Imaging and Euler Deconvolution to determine the various depths of Nickel deposit. From the results of the TMI map and the residual map, the magnetic intensity of the study area was between 32976.5 to 33093.3 nT and 32963.6 to 33072.9 nT respectively with Bakin kogi-Dangoma region, having the highest magnetic intensity of about 33093.3nT and 33072.9 nT respectively. The study area has a high magnetic intensity due to nickel deposit because Nickel ore environment are notable with recognizable “magnetic stratigraphy”, prospective mafic-ultramafic contact, typified by strong magnetic contrast. Abrupt changes in the magnetic intensity infer position of the outer contact of a Nickel bearing zone. The results on the superimposed analytic map on the lineament map showed regional lineaments in Bakin kogi-Dangoma region trending NE -SW, ENE – WSW and NNE – SSW directions. The fractures and faults seen around the study area were strongly associated with minerals deposit. The results of Source Parameter Imaging (SPI) gave a depth of shallower magnetic sources ranges from 92.7 to 116.0 m with an average depth value of 104.35 m while the depth of deeper magnetic sources ranges from 651.2 to 976.2 m with an average depth of 813.7 m. The results obtained from 3D Euler deconvolution which was associated with dykes (SI = 1) anomaly produced at a shallower depth range of 100 - 700 m. Results obtained, shows a correspondence between the depths results obtained from the SPI and that of the Euler Deconvolution methods, indicating correlation in the magnetic depth estimation.

Corrosion failure of titanium tubes of a heat exchanger for the heating of dissolving lye

Corrosion of titanium heat exchangers in the processing of sylvinite ore is undesirable from economic, safety, and process sustainability perspectives. Triggered by an industrial case, we investigated the extent of corrosion during simulated contact with sylvinite ore (in dissolving lye) in relevant conditions. Detailed characterization of the failed tubes and corrosion products was carried out to understand the mechanism of failure. Corrosion of titanium (Grade 2) tubes was investigated at room temperature, 60, 70, 80, and 90 °C. After electrochemical and surface morphology analysis, we found that pitting corrosion of the titanium tube material sharply increased above 80 °C in the simulated sylvinite ore environment (pH 7.1). The failure analysis revealed extensive degradation by transgranular cracking through both the oxide and metal matrix, likely caused by a combination of the high temperature, pressure, possible vibrations, the build-up of lye deposits causing crevices, the high salt content of the lye, and possibly metal (copper, iron, zinc) impurities/deposits in or on the titanium metal, which can catalyze hydrogen evolution.

Rutile as a pathfinder for metals exploration

The use of detrital rutile as an indicator of mineralisation is contingent on the identification of trace element signatures that uniquely distinguish rutile grains derived from mineralisation from those sourced from barren rocks. In order to discriminate rutile sources, principal component analysis was applied to a total of 3149 LA-ICP-MS analyses of rutile from ore environments (Au, Au-Cu, pegmatites, carbonatites, lamprophyres, VHMS) and barren igneous and metamorphic rocks. Hierarchical clustering of the results of principal component analysis (PCA) determined Sb and Cr exhibit the most chemical variation in ore-derived rutile. Rutile sourced from Au deposits and enriched in Sb, can be distinguished from rutile from pegmatites, which are enriched in Nb, Ta and Sn. Discriminating rutile from barren rocks can be done primarily using W and Fe contents, which can also provide insight on source rocks based on metamorphic grade and degree of crustal input.Clustering all rutile chemistries based on PCA of 12 elements, identified nine chemically distinct groups from ore mineralisation, and barren igneous and metamorphic rocks, primarily using variation in W and Cr contents. Two groups are solely derived of rutile from Au ores (high versus low Sb rutile). Of the remaining seven groups, one group is comprised mainly of rutile from pegmatites (high Nb, Ta, Sn), four groups are comprised solely of rutile from barren rocks and two groups are a mix of ore and non-ore rutile. Rutile from Au ore, similar to the giant Kalgoorlie and Big Bell Au deposits, is distinctive compared to rutile from all other rocks and can be distinguished from other ores and barren environments using multi-element clustering of PCA analysis. Thus rutile chemistry of detrital rutile grains is a valuable tool in Au exploration.

An experimental assessment of the chalcophile behavior of F, Cl, Br and I: Implications for the fate of halogens during planetary accretion and the formation of magmatic ore deposits

The elemental and isotopic abundances of halogens (F, Cl, Br, I) are used to constrain planetary volatile loss and volatile delivery processes, but their behavior during magmatic differentiation in general, and sulfide liquid segregation in particular, is currently not well constrained. To test whether sulfide liquid segregation could affect halogen behavior during magmatic processes, we performed high-pressure experiments to systematically quantify the sulfide liquid – silicate melt partition coefficients (Dsulliq-silmelt values, defined as the ratio between the wt.% concentration of the halogen in the sulfide liquid and silicate melt, respectively) of F, Cl, Br and I at a pressure of 1 GPa and temperatures of 1683–1883 K.Results show that dry-polishing target surfaces is crucial for obtaining representative halogen concentrations of sulfide liquids. The results also show that no appreciable amounts of F partition into sulfide liquids, whereas Cl, Br and I behave increasingly chalcophile with increasing atomic radius (i.e., DFsulliq-silmelt <DClsulliq-silmelt <DBrsulliq-silmelt < DIsulliq-silmelt), presumably as a result of an increasingly covalent nature of Fe-halogen bonds with increasing radius. This results in I behaving chalcophile (DIsulliq-silmelt > 1) in several experiments. In contrast to previous observations, DCl/Brsulliq-silmelt was found to be <1. The DCl,Br,Isulliq-silmelt predominantly vary with sulfide liquid melt composition, showing an increase with increasing O in the sulfide liquid, which itself is correlated with more oxidizing conditions (i.e., higher fO2) or silicate melt FeO contents. The DCl,Br,Isulliq-silmelt values remain constant and/or potentially decrease again at the highest O concentrations of the sulfide liquids in this study (∼2.5 wt.% O).Results indicate that the magnitude of halogen depletions in the terrestrial, martian and lunar mantle are not strongly affected by segregation of sulfide liquids during their accretion, given the expected low modal abundance of sulfide liquids and/or relatively low DCl,Br,Isulliq-silmelt values. Core formation remains the most important process in establishing iodine depletion in the terrestrial mantle, whereas volatility-related loss seems most likely for F, Cl, Br and I, in case of the martian mantle. However, segregation of sulfide liquids during accretion could have resulted in a relative increase of the offset between the mantle depletions of the lighter and heavier halogens. The experimental results confirm the previously proposed feasibility of sulfide liquids as reservoirs for halogens in magmatic sulfide ore environments. As proposed by Mungall and Brenan (2003), fractional crystallization of these sulfide liquids in the absence of a silicate melt can lead to the formation of halide melts or fluids, consistent with the association between halide minerals and magmatic sulfide ores in some localities.

Bulk microanalysis of assemblages of small fluid inclusions by LA-ICP-MS: Methodology and application to orogenic gold systems

Microanalysis of individual fluid inclusions by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) is a powerful tool for reconstructing the composition of hydrothermal fluids, but it demands a sample quality that is unattainable in many cases. In orogenic gold deposits, the need for direct fluid microanalysis has been present for several decades, but due to the high fluid flux and prolonged hydrothermal and tectonic history that typifies these systems, most samples do not meet the criteria of fluid inclusion size and distribution that allow LA-ICP-MS analysis of individual isolated fluid inclusions. To overcome this difficulty, a method has been developed and tested, whereby areas of quartz densely populated with fluid inclusions (e.g., growth zones, secondary planes) are analyzed along a single continuous laser ablation profile; the generated signals are subsequently converted to time-slice datasets and plotted as element ratios in ternary diagrams to reconstruct specific major- and trace-element ratios. The estimated fluid compositions are in good agreement with previous analytical results of the same material (microthermometry, evaporate mound and conventional LA-ICP-MS) and are shown to be geologically viable on a boarder scale when compared to literature data from similar ore environments. The method has high spatial and chemical resolution, which allows the reconstruction of micro-scale fluid chemical changes, such as significant fluctuation in the relative element concentration (e.g., K, Rb, Ba, Sr and V versus Na, Li and As) during crystal growth, as observed in one of the test samples. The significance of this bulk LA protocol is that it allows the quick, easy and cost-effective microanalysis of samples that are typically inundated with fluid inclusions, such as those in orogenic and epithermal systems, which would otherwise not be amenable for further quantitative analysis to constrain fluid chemistry.

Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

The application of VSP in the Pilbara

The construction of geological and geotechnical models in typical Pilbara iron ore environments is vital to enable an optimized mine design for the life of the asset, while maintaining pit wall integrity and overall mine safety. Geotechnical assessments require the measurement of geomechanical properties, such as the triaxial shear, direct shear and unconfined compressive strength tests and pressure and shear wave velocities on diamond core samples. Ideally, these velocities would be measured in Reverse Circulation (RC) boreholes as their spatial density is far higher than diamond drilled holes. Unfortunately, despite its value, such data is seldom collected as a large proportion of the holes are above the water table, limiting the use of sonic-logging tools. Even if measurements are possible, damage to the borehole caused by drilling biases the resulting velocity measurements. This paper details the results of a trial using the vertical seismic profile method to directly measure in-situ seismic velocities in RC boreholes. The method was successful in determining the velocities of the formations through the entire length of the holes. The data in several boreholes was of sufficient quality for the application of more advanced processing methods, important for geological mapping and the processing and interpretation of surface seismic data. The success of this first trial has implications for future iron-ore developments in the Pilbara. The widespread acquisition of accurate seismic velocity data is likely to enable the creation of more accurate geotechnical models and could improve future development decisions.

Determination of formation density through RC rods in iron ore environments

Logging through drill rods and casing is a well-known challenge in the coal, oil and gas industries, and several techniques have been developed to obtain open-hole values using open-hole tools in cased-hole (C-thru). Due to the differences between conventional open-hole and iron ore environments and practices, simply applying the same methods can result in spurious data and unreliable findings. A revised tool design together with an improvement upon the standard oil and gas technique through single wall rods was required to ensure accurate results. Development of an appropriate compensation algorithm and response through Reverse Circulation (RC) rods was determined where multiple walls of steel and gaps of fluid and air separate the tool from the target formation.

Modelling microbial transport in simulated low-grade heap bioleaching systems: The biomass transport model

A model was developed to describe microbial growth and transport in the flowing bulk solution and ore-associated phases within a mineral bioleaching heap. The retention of micro-organisms was assumed to be a function of microbial transport between the ore surface and the bulk solution, as well as growth in each of these phases. Transient variations in the corresponding microbial concentrations are presented together with predicted microbial growth, transport and oxidation kinetics within the agglomerate-scale, whole ore environment. The transport model presented in this paper was developed under the assumption that the microbial concentration gradient between the identified phases was the driving force for microbial transport. Further the population balance model was super-imposed to account for available reaction surface. The model was able to predict the change in microbial concentrations in both the bulk solution and ore-associated phase. The resulting microbial transport rates to and from the ore-associated phase were found to be significantly lower than the maximum specific microbial growth rates presented, suggesting that microbial transport is not governed by the microbial concentration difference. These findings confirm the value of the modelling approach in which the population balance model is included, while demonstrating that concentration gradient as the driving force is not the main contributor to microbial transport.

Use of near Infrared Spectroscopy for the Identification of Coffinite and Uranothorite

Near infrared (NIR) spectroscopy was performed on a synthetic coffinite–uranothorite series and natural coffinite. The NIR spectra revealed a characteristic band at 1135 nm, allowing the identification of coffinite–uranothorite minerals within uranium ore environments. These minerals can be identified even in some complex mixtures (host rocks containing coffinite and several infrared reactive minerals such as clay minerals) or samples with low structural uranium amounts (e.g. Th0.8U0.2SiO4). Well-resolved on field spectra, the 1135 nm band could be used as a powerful tool for uranium ore exploration.

Anthropogenic waters in the Komsomolsk, Kavalerovskii, and Dalnegorsk mining areas of the Far East and their impact on the hydrosphere

Potentially high charge of wastewaters with toxic metals in mining areas depends on the composition of ores, mineralization of the ore environment, and anthropogenic processes that are responsible for contamination of pore solutions and mine, slime, and drainage waters. As a result, the concentrations of toxic elements, such as Cu, Zn, Pb, As, B, Fe, etc., in the surface and ground water considerably exceed background levels. Anthropogenic impact of mining systems made environmental situation in the examined regions stressed and critical.

Phosphorus Degradation Capability of Aspergillus terreus on Nigeria’s Agbaja Iron Ore

A microbial fungus - Aspergillus terreus was used to degrade phosphorus in Nigeria’s Agbaja iron ore in the laboratory. The ore was first crushed to very tiny particles, screened using Shital test kits and 1.00/0.50mm, 0.50/0.25mm and 0.25/0.125mm particle size fractions were selected for the experiment. The microbes, obtained from the nascent remains on the ore environment, were cultured, used to inoculate 1g of sterile ore samples in 250ml conical flasks containing 100ml of equally sterile malt extract broth media and left to stand. At weekly interval, the samples were removed, treated through series of chemical reactions and ammonium phospho-molybdate precipitate was obtained. This was back-titrated with 0.1 NHCl to determine the amount of phosphorus left in samples and consequently, the amount removed. Findings reveal that A. terreus is capable of degrading the ore samples. pH monitoring reveals that the P degradation process proceeded in a culture media of increasing acidity. It is recommended to further study the chemistry of the mixture of culture media, ore samples and microbes to find parameters that favour the degradation process. Key words: Ore, microbes, screening, broth, degradation, accumulation, inoculati

Facies architecture of an Archean komatiite-hosted Ni-sulphide ore deposit, Victor, Kambalda, Western Australia: implications for komatiite lava emplacement

Nickel sulphide mineralisation is located in structural embayments or troughs at the base of thick basal komatiite lava flows at the Victor ore body, Kambalda, Western Australia. Primary contact relations, and textural and vesicle distribution of the host komatiite lavas, particularly the development of coherent or quench fragmented margins is consistent with emplacement of komatiites under laminar flow conditions. However, the absence of sedimentary units beneath komatiites with coherent flow tops in the ore environment seems contradictory and suggests both turbulent and passive laminar emplacement, respectively. The presence of erosive basal contacts in the ore environment suggests the komatiite lava was initially turbulent and probably open channel fed. It is during this initial stage that both erosion and deposition of the NiS deposits occurred. As the flow evolved, widened and thickened, laminar flow conditions prevailed. The komatiites are inferred to have flowed in a laminar state, through the development of interior magma tubes beneath surface crusts, and to have grown endogenously.

Re Os ISOTOPIC STUDY OF KOMATIITIC VOLCANISM AND MAGMATIC SULFIDE FORMATION IN THE SOUTHERN ABITIBI GREENSTONE BELT, ONTARIO, CANADA

We have investigated the Re–Os isotope geochemistry of 2.7-Ga metakomatiitic flows and associated Ni–Cu sulfide deposits from Alexo, Texmont and Hart in the Abitibi greenstone belt of Ontario in order to refine the thermal erosion model and evaluate the superimposed effects of metamorphism and hydrothermal alteration on ore environments and non-ore environments. Although the geochemical characteristics of these komatiites have led to the belief that these lavas were uncontaminated, radiogenic Os isotopic compositions (γOs = +20.2 and +33.2) obtained from well-preserved komatiites and olivine separates suggest that the Alexo flow has been contaminated by crust-derived material. These data are compatible with the trace-element enrichment observed in melt inclusions trapped within olivine. Redistribution of Os and Re did occur at least at the mineral scale and most likely during the Grenville orogeny. Hydrothermal fluids were channeled along the contact between the komatiites and their basement, and were responsible for the remobilization of Re or Os (or both) within the sulfides at Alexo and Hart. Matrix and disseminated sulfides from Texmont are located within the pile of cumulates and seem to have escaped this localized alteration. Although the Abitibi sulfides have experienced various degrees of metamorphism (from prehnite–pumpellyite to low amphibolite facies), the initial Re–Os isotopic composition of the flows appears to have been preserved at the whole-rock scale. Re–Os isotopic heterogeneity of the Abitibi sulfides is best explained by variable R -factor of the sulfides. Re and Os concentrations and Os isotopic heterogeneity of the Abitibi sulfides are consistent with the current model of nickel sulfide formation, which implies that the assimilation of sulfidic sedimentary rocks was the trigger for sulfide saturation.

Element ratios in nickel sulphide exploration: vectoring towards ore environments

The use of element ratios in geochemical exploration can help to distinguish mineralised from barren geological complexes, predict deposit types, and assist in estimating and evaluating dispersion of ore indicator elements. Exploration at Kambalda Nickel Operations, Western Australia, has focused on developing a repetitive robust geochemical method that can vector towards Ni–Cu–PGE sulphide deposits. One method has been developed so as to utilise element ratios in a manner which are compatible with the genetic geological exploration model for komatiite-associated massive Ni sulphide deposits. A particularly successful element ratio is expressed as [(Ni/Cr) × (Cu/Zn)]. The [(Ni/Cr) × (Cu/Zn)] ratio is effective in vectoring towards channellized komatiitic environments. This paper presents key examples that demonstrate the power of the [(Ni/Cr) × (Cu/Zn)] ratio in fresh komatiite rock, its intensely weathered equivalent, and in surface materials. Analytical techniques employing weaker acid attacks (HCl) and finer size fractions from surface materials enhance ratio contrasts. Exploration implications are discussed. Komatiites associated with Ni sulphides are low in Cr compared to barren komatiites. Low Cr values persist along the flow from Ni sulphide mineralisation in channellized komatiites. Mineralised basal komatiites flows are enriched in Ni compared to barren flows. Thus, utilising a Ni : Cr ratio, potentially mineralised flows (high Ni, low Cr) can be distinguished from barren flows (low Ni, high Cr) with Ni : Cr ratio values typically >1. This Ni : Cr signature can be traced down-plunge, on the basal komatiite, for >14 km and through the komatiitic stratigraphy. In addition, this signature is, in part, preserved through the weathered komatiite and in surface material. It has successfully identified new Ni sulphide occurrences and highlighted significant areas of exploration interest. Sulphide-bearing sedimentary horizons of variable thickness occur at the base of, and within, the komatiitic successions. These meta-sedimentary rocks typically have three times the Zn concentration (1500 ppm) to that of Cu (500 ppm). However, because nickel sulphides have a greater concentration of Cu than Zn, the Cu : Zn ratio can assist in discriminating Ni sulphide-bearing zones from those with no Ni sulphides. Combining the Ni : Cr and Cu : Zn ratios has the effect of reducing the influence of particular lithologies (e.g. sulphidic meta-sedimentary rocks) and aids in the delineation of Ni sulphide mineralisation. Multiplying the ratios enhances the contrast of element ratios related to mineralisation and subdues the effect of lithologies. Utilisation of the [(Ni/Cr) × (Cu/Zn)] ratio increases contrast between background and mineralisation, and thus the ability to vector towards basal komatiite channellized environments through the distinctive behaviour of Ni, Cr, Cu and Zn.

Re-Os Isotopic Study of Komatiitic Volcanism and Magmatic Sulphide Formation in the Southern Abitibi Greenstone Belt, Canada

We have investigated the Re–Os isotope geochemistry of 2.7-Ga metakomatiitic flows and associated Ni–Cu sulfide deposits from Alexo, Texmont and Hart in the Abitibi greenstone belt of Ontario in order to refine the thermal erosion model and evaluat e the superimposed effects of metamorphism and hydrothermal alteration on ore environments and non-ore environments. Although the geochemical characteristics of these komatiites have led to the belief that these lavas were uncontaminated, radioge nic Os isotopic compositions (Os = +20.2 and +33.2) obtained from well-preserved komatiites and olivine separates suggest that the Alexo flow has been contaminated by crust-derived material. These data are compatible with the trace-element enrichment observed in melt inclusions trapped within olivine. Redistribution of Os and Re did occur at least at the mineral scale and most likely during the Grenville orogeny. Hydrothermal fluids were channeled along the contact between the komatiites and their basement, and were responsible for the remobilization of Re or Os (or both) within the sulfides at Alexo and Hart. Matrix and disseminate d sulfides from Texmont are located within the pile of cumulates and seem to have escaped this localized alteration. Although the Abitibi sulfides have experienced various degrees of metamorphism (from prehnite–pumpellyite to low amphibolite facies), the initial Re–Os isotopic composition of the flows appears to have been preserved at the whole-rock scale. Re–Os isotopic heterogeneity of the Abitibi sulfides is best explained by variable R-factor of the sulfides. Re and Os concentrations and Os isotopic heterogeneity of the Abitibi sulfides are consistent with the current model of nickel sulfide formation, which implies that the assimilation of sulfidic sedimentary rocks was the trigger for sulfide saturation.

Constraints on Magma Degassing beneath the Far Southeast Porphyry Cu-Au Deposit, Philippines

K-silicate alteration in the Far Southeast (FSE) porphyry Cu–Au INTRODUCTION deposit formed at ~1·4 Ma, concentric to dikes of quartz diorite Magmas become saturated with volatile components as porphyry. At ~2 km paleodepth the hydrothermal system consisted they ascend in the crust, and either erupt or crystallize of magmatic hypersaline liquid and vapor at 550°C and Ζ50 at depth. Saturated magmas that remain at depth exsolve MPa (lithostatic pressure). Advanced argillic alteration formed at an aqueous fluid that forms a hydrothermal system on the same time over the deposit at Ζ1 km paleodepth from acidic separation from the magma chamber (Burnham, 1979). condensates of the vapor. At [1·3 Ma, K-silicate alteration was Although part of the exsolved fluid (a low-density vapor) overprinted by 350°C magmatic liquid (~5 wt % NaCl equiv.) may discharge to the surface from passively degassing at hydrostatic pressure. Sericite alteration and much of the Cu–Au volcanoes (Hedenquist, 1995), part will remain at depth, mineralization formed at this later stage. Evolution of the magmatic and this dense, residual magmatic liquid may be asfluid composition was simulated with a magma-chamber cryssociated with hydrothermal mineralization in the portallization model. Homogeneous crystallization during early stage phyry ore environment (Hedenquist & Lowenstern, convection is assumed, whereas at 50 vol. % crystals the chamber 1994). becomes stagnant and crystallizes from rim to core over a narrow In this paper we use evidence from a porphyry ore crystallization interval. The model calculation, based on a magma deposit in the Philippines to determine the composition chamber with 2 km thickness at 6 km depth (150 MPa) and of the fluid that exsolved from the deeper parent magma. [800°C (saturated melt composition at 30 vol. % crystals, 5 The evolving composition of the magmatic fluid at the wt % H2O, 0·2 wt % Cl and dD of –40‰), can reproduce the depth of the ore body can then be used to place constraints chemical and isotopic compositions of the early and late magmatic on the nature of the evolved magma chamber far below fluids. The most critical factor controlling the compositional evolution the porphyry ore body. This approach differs from that of the model hydrothermal system is the transition from convective of other studies where a portion of the parent magma to stagnant magma-chamber crystallization. There is also a sharp chamber is available to study directly (e.g. at Yerington, decrease in the rate of fluid exsolution associated with this transition, Nevada; Dilles, 1987). which can account for the thermal collapse of the FSE porphyry The Far Southeast (FSE) porphyry Cu–Au deposit system, from K-silicate to sericite alteration. and overlying Lepanto epithermal Cu–Au ore body are

Geophysical surveys for strata-bound Outokumpu-type CU-CO-ZN deposits at Kylylahti, Eastern Finland

Wannaway, Mariners and Mt. Edwards are three of many type 1A komatiitic peridotite-hosted nickel sulphide deposits clustered in the Widgiemooltha area, approximately 560 km east of Perth. Magnetic susceptibility measurements show that the susceptibility of the ore is generally high but is extremely variable, from 4.5 ´ 10-3 SI units to 269 ´ 10-3 SI units in serpentinised environments, and 14.9 ´ 10-3 SI units in talc-carbonate altered environments, hangingwall ultramafic rocks have a generally high but variable susceptibility depending on the style of alteration. Serpentinised ultramafic rocks have a high susceptibility of 70 ´ 10-3 SI units, while talc-carbonate altered ultramafic rocks have a low susceptibility of 4.4 ´ 10-3 SI units due to the respective formation or destruction of magnetite. Footwall basaltic rocks have a consistently low susceptiblity average of 0.8 ´ 10-3 SI units. Susceptibility-temperature data indicate that ores with a high susceptibility contain monoclinic pyrrhotite and magnetite, and ores with a low susceptibility contain only small amounts of monoclinic pyrrhotite, minor magnetite and abundant, non-magnetic, hexagonal pyrrhotite. Components of remanent magnetisation in the ores and hangingwall ultramafic rocks lie in two dominant orientations. Remanent magnetisations held by pyrrhotite and magnetite at Wannaway and by pyrrhotite at Mt. Edwards are upwardly inclined toward the northwest. Vectors held by pyrrhotite at Mariners and magnetite at Wannaway are upwardly inclined toward the southeast. Two dimensional magnetic modelling of the Wannaway ore environment indicates that the anomaly over the Wannaway deposit is caused predominantly by the hangingwall ultramafic unit, with a small contribution from the near-surface hangingwall ore. Contact ore makes only a minor contribution to the anomaly due to the depth of the ore below the surface. Although the ore is significantly magnetic, it is difficult to directly detect on magnetic data due to the highly magnetic nature of the surrounding ore environment.