Massive Sulfide Research Articles

Origin of Fe‐Ca‐Metasomatism in Exhumed Mantle Rocks at the MARK Area (23°N, ODP Leg 153) and Implications on the Formation of Ultramafic‐Hosted Seafloor Massive Sulfide Deposits

AbstractAt Mid‐Ocean Ridges, hot, reduced, acidic, and metal‐rich fluids are responsible for the formation of ultramafic‐hosted seafloor massive sulfide deposits (UM‐SMSs), where mantle exhumation efficiently operates. As UM‐SMSs display great structural, mineralogical, and geochemical variabilities from site to site, a simple genetic model cannot be applied. Notably, fluid circulation and Fe‐Ca metasomatism are reported in ultramafic‐hosted hydrothermal deposits exposed in ophiolites, suggesting it might have genetic implications on the formation of mineralized systems. Similar Fe‐Ca metasomatism was reported in drilled mantle rocks at the Mid Atlantic Ridge Kane (MARK) area, offering access to the vertical dimension beneath an exhumed oceanic core complex to provide an integrative study of the nature and geometry of deep magmatic and hydrothermal processes. At MARK, mantle rocks underwent complex processes of melt‐rock and fluid‐rock interactions. Magma channeling and interactions with surrounding rocks enriched mantle silicates in Fe, Co, and Zn. There, subsequent hydrothermal alteration allowed to stabilize Fe‐rich silicates. Mineralogy and geochemistry of hydrothermal phases at MARK suggest mineral crystallization under temperatures from ∼830° down to 350°C during early mantle exhumation at a depth <6.5 km below seafloor, followed by serpentinization of the massif during progressive mantle denudation. Considering the lithological heterogeneity at (ultra)slow‐spreading ridges, metal enrichment in deep mantle rocks during melt‐rock interactions may be a widespread process. In ultramafic‐dominated environments where extensional tectonics allow fluid flows to these deep zones, fluids may leach and transport metals to the surface, accounting for metal entrapment in UM‐SMSs.

Cobalt Prospectivity Using a Conceptual Fuzzy Logic Overlay Method Enhanced with the Mineral Systems Approach

AbstractThis paper describes mineral prospectivity research conducted in Finland to predict favorable areas for cobalt exploration using the “fuzzy logic overlay” method in a GIS platform and public geodata of the Geological Survey of Finland. Cobalt occurs infrequently as a core product in mineral deposits. Therefore, we decided to construct separate conceptual mineral prospectivity models within the Northern Fennoscandian Shield, Finland, for four deposit types: (1) “Orthomagmatic Ni–Cu–Co sulfide deposits,” (2) “Outokumpu-type mantle peridotite-associated volcanogenic massive sulfide (VMS)-style Cu–Co–Zn–Ni–Ag–Au deposits,” (3) “Talvivaara black shale-hosted Ni–Zn–Cu–Co-type deposits” and (4) “Kuusamo-type (orogenic gold with atypical metal association) Au–Co–Cu–U–LREE deposits”. In addition, we created a model combining till geochemical data with data derived from bedrock drilling and mineral indications, including boulders and outcrops. The mineral prospectivity models were statistically tested with the “receiver operating characteristics” method using exploration drilling data from known mineral deposits as validation sites. In addition, the predictive performance of the models was evaluated by using success rate curves, where the number of previously identified deposits was compared with the area coverage of the predicted highly favorable areas. These results indicate that the knowledge-driven mineral prospectivity method using parameters derived from mineral systems models is effective in defining favorable exploration target areas at the regional scale. This study's innovation lies in its comprehension of the process of evaluating mineral prospectivity when the commodity of interest is not the primary commodity within the mineral system.

Boiling-induced extreme Cu isotope fractionation in sulfide minerals forming by active hydrothermal diffusers at the Aegean Kolumbo volcano: Evidence from in situ isotope analysis

Abstract We analyzed the first Cu isotopes in primary cupreous pyrite and orpiment, from modern CO2-degassing, seafloor massive sulfide diffuser vents (“KCO2Ds”), from the Kolumbo submarine volcano, Hellenic volcanic arc. Samples came from six KCO2Ds that are actively boiling. Pyrite comprises colloform pyrite-I and euhedral pyrite-II, which occur erratically distributed within the KCO2Ds and are contemporaneous with barite and spatially concurrent with the chalcopyrite that is lining narrow internal conduits, respectively. Orpiment occurs on the outer walls of the KCO2Ds with barite and stibnite. The δ65Cupyrite-I values show high variability, ranging from +2.93‰ to +6.38‰, whereas the δ65Cupyrite-II and δ65Cuchalcopyrite values vary from −0.94‰ to +0.25‰ and −0.45‰ to –0.09‰, respectively. The range of δ65Cuorpiment between +1.90‰ and +25.73‰ is the most extreme ever reported from any geological setting. Pyrite-I is concentrically layered, with a core comprising random crystallites, whereas the mantle crystallites have grain-size, shape, and orientation variability between layers. Pyrite-II forms aggregates of uniform euhedral pyrite crystals. Pyrite-I has higher concentrations of Cu (≤21,960 ppm) compared to pyrite-II (≤4963 ppm), and both have incompatible and volatile metal(loid)-rich composition and low Sb/Pb (<0.5) and Tl/Pb (<0.03) ratios. When combined with evidence for significant magmatic contributions at Kolumbo and geochemical and micro-textural evidence for recurrent intense boiling and/or flashing or gentle and/or non-boiling, the measured extreme δ65Cu values are consistent with transport of Cu by vapor that is preferentially enriched by heavy 65Cu and controlled by continuous Rayleigh distillation–type Cu fractionation. Boiling-induced Cu vapor transport can generate extreme Cu isotope fractionation.

Gold metallogeny in Iran; implications for gold exploration and conceptual modelling

Significant progress in the classification, definition and understanding of the main Au deposit types could significantly aid improvements in Au exploration. Because of the wide occurrence of Au in the central part of the Tethyan Eurasian Metallogenic Belt, Iranian structures composed of more than 17 zones (arcs and blocks) are considered as having one of the largest Au reserves in the Middle East. Without attempts at understanding the tectono-magmatic evolution of Iran and the geodynamic settings of Au deposition, the establishment of a reliable predictive exploration model for Au-type deposits in Iran and other parts of the world will be unsuccessful. By considering a total of 33 Au deposits and prospects in Iran, a mineralization sequence is revealed from orogenic and volcanogenic massive sulfide (VMS), orogenic, Carlin-type, epithermal/porphyry Cu–Au/skarn, epithermal/iron oxide–copper–gold (IOCG). The trend of deposition gradually changes in the SW–NE axis to intrusion-related, epithermal and porphyry Cu–Au deposits at the Urumieh–Dokhtar Magmatic Arc (UDMA) and post-arc magmatism. VMS deposits occur adjacent to the NE Zagros Fold and Thrust Belt, at the Sanandaj–Sirjan Zone. The Zagros Orogeny and associated post-collisional magmatism at UDMA host many porphyry, epithermal and intrusion-related Au deposits, with a major magmatism peak in the Miocene. This work reveals that orogenic and Carlin-type Au mineralization are linked genetically. After each associated subduction for Palaeotethys (286–215 Ma) and Neotethys (210–68 Ma) in Iran, VMS and orogenic Au-deposits are formed in the border of the subduction (±obduction) zone. The porphyry, intrusion-related, epithermal and IOCG mineralization are emplaced in appropriate formations and structures during collision and post-collision processes.

Fluid Inclusion Studies of Barite Disseminated in Hydrothermal Sediments of the Mohns Ridge

This article discusses the results of a fluid inclusion studies in barite collected at the Jan Mayen vent field area (Troll Wall and Perle and Bruse) and Loki’s Castle vent field on the Mohns Ridge segment of the Arctic Mid-Ocean Ridge. Three mafic-hosted volcanogenic massive sulfide deposits were examined within the active vent fields that adequately correspond to the geological settings of ultraslow-spreading ridges and P–T conditions. Hydrothermal sediments were investigated to determine the temperature and salinity of the fluids responsible for barite precipitation. The hydrothermal origin of the barite was confirmed by its morphology. Fluid inclusions are two-phase and homogenize into the liquid phase on heating at temperatures below 287 °C. The salt concentration in fluids trapped in inclusions is 2.6–4.4 wt.% NaCl eq. The crystallization temperatures varied from 276 °C to 119 °C and from 307 °C to 223 °C for the Jan Mayen and Loki’s Castle vent fields, respectively. The data obtained allowed us to confirm evidence of fluid phase separation in the hydrothermal systems and to expand our knowledge of the temperature and salinity of mineral fluids previously known from recent direct measurements during the cruises within the G.O. Sars research vessel. The fluid inclusions data obtained from barites emphasize the fluid features characteristic of volcanogenic massive sulfide deposits, the similarities and differences among the studied hydrothermal sites and allow comparisons with similar products from other active hydrothermal systems.

3D Multicomponent Self-Potential Inversion: Theory and Application to the Exploration of Seafloor Massive Sulfide Deposits on Mid-Ocean Ridges

The marine self-potential (SP) method is currently playing an increasing role in the exploration and resource evaluation of seafloor massive sulfide (SMS) deposits. SP surveys are conducted using autonomous underwater vehicles (AUV), which yield multicomponent electric field datasets. By comparing with the single-component electrical field data used to date, the inversion of these multicomponent data is expected to provide a more accurate description of the 3D structure of SMS deposits beneath the seafloor (like gradiometry in gravity surveys). We introduce an inversion algorithm specifically adapted to multicomponent SP data. A synthetic model demonstrates that the inversion of multicomponent datasets allows us to better recover the amplitude of the current density and the morphology of the ore bodies compared to using a single component of the electrical field. Next, we apply our approach to a multicomponent SP dataset collected during the DY58 oceanic cruise at the Yuhuang hydrothermal field on the Southwest Indian Ridge. Subsequently, we reconstruct the three-dimensional (3D) geometry of the SMS deposits beneath the seafloor. The AUV-based SP system with the collection of multicomponent SP data inversion appears to be a powerful tool in the exploration and evaluation of seafloor sulfide resource and, in the future, could be used in concert with induced polarization data.

Random forest classification for volcanogenic massive sulfide mineralization in the Rouyn-Noranda Area, Quebec

Random forest (RF) classification was applied to 37 predictor maps (vectors to mineralization) producing a Mineral Prospectivity Map (MPM) for volcanogenic massive sulfide (VMS) mineralization in the Noranda District, Abitibi subprovince, which is host to ∼ 20 VMS deposits and numerous subeconomic occurrences. The predictor maps were created using geological, geochemical, and geophysical data, and the known VMS deposits were used to train the RF classifier. The RF model was applied on two regions of interest (ROI) to investigate the effect of different sized ROIs on the results. Five sets of balanced and unbalanced training data were used in the experiments to examine the sensitivity of RF to different sets of training data. The probability/prospectivity maps showed very high success rate of classification with regard to training data particularly for the training sets with higher ratio of non-deposits/deposits. Accuracy assessment of classification maps using cross-validation also showed very high accuracy when assessed by the training data in all experiments again giving a higher accuracy for the training sets with higher ratios of non-deposits/deposits, however the later showed very low accuracy when applying k-fold cross-validation or when assessed by 15 VMS showings used as the test data indicating an overfitting problem with the unbalanced data. In most of the experiments, principal component 4 (PC4) of geochemical analysis, proximity to synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, lithology, and sericite alteration maps showed higher predictive power. The importance of individual variables changed when the ROI was changed to a smaller area suggesting that RF is sensitive to the study area.

Fe–Cu Isotope Characteristics and Geological Significance of the Yushui Seafloor Massive Sulfide Deposit in the Late Paleozoic Marine Depression, Eastern Guangdong Province

The Yong’an-Meixian Late Paleozoic Hercynian depression, located in western Fujian-eastern Guangdong, is an important metallogenic belt in China. The Yushui copper-polymetallic massive sulfide deposit from the middle part of the depression, with extremely high copper grades, has attracted considerable attention and research interest from geologists for years. In most previous research, the ore-forming material source and metallogenic process were inferred from indirect evidence (i.e., using H-O-C-S isotopic systematics as geochemical tracers). In this paper, the ore-forming process of the Yushui deposit has been studied directly by using nontraditional stable (Fe–Cu) isotopes for the first time, providing new evidence for the genesis of this deposit. The results show that there is a relatively negative Fe-Cu isotopic composition in the Yushui deposit, with δ56Fe values ranging from −0.519 to −1.063‰ and δ65Cu values ranging from −1.539 to −1.609‰, respectively. The fractionation of Fe isotopes is primarily attributed to hydrothermal leaching of the basement strata by ore-forming fluids, along with rapid precipitation of sulfides during the ore-forming process. On the other hand, the fractionation of Cu isotopes is probably controlled by the relatively low temperature of ore formation, sulfide precipitation and the involvement of organic matter in mineralization. Combining our findings with previous studies, the ore-forming materials of the Yushui deposit are likely derived from the basement ore-bearing strata (pre-Devonian strata) through leaching by hydrothermal fluids. Moreover, some of the ore bodies might have been locally overprinted by late-stage hydrothermal reworking and alteration.

Genesis of volcanic-hosted Cu vein deposits in the foreland fold-thrust belt, southeastern Tibetan Plateau: Insights from ore geology, C-O-S-Pb-Cu isotopes, and fluid inclusions at Wenyu

Copper deposits hosted by volcanic rocks can have different genetic types. Many copper vein deposits at the margin of the Cenozoic Lanping-Simao foreland fold-thrust belt in southwest China are controlled by thrust faults and hosted by Triassic volcanic rocks formed in association with the evolution of the Paleo-Tethys Ocean. The genesis of these copper deposits is poorly understood. Wenyu is a typical Cu deposit hosted in the Middle−Late Triassic basaltic-andesitic rocks and experienced three major hydrothermal stages characterized by mineral assemblages of calcite, anhydrite, and barite (Stage I); early quartz, chlorite, and epidote (Stage II); and Cu sulfides, galena, pyrite, hematite, and late quartz (Stage III). Consistency of the boiling temperature from fluid inclusions in the calcite and the temperature inferred from the C-O isotopes of the calcite, and the oxygen isotopes of the anhydrite and barite indicate that the stage I fluids probably have a δ13C value of ∼−4.5‰ and δ18O values in the range of ∼−3‰ to +4‰. Oxygen isotopes of the early quartz (δ18O = 15.1‰ to 17.4‰), D-O isotopes of chlorite and epidote, and the fluid inclusion data indicate that the stage II fluids have δ18O values of ∼2.4‰ to 5.0‰ and δ18D values of −68‰ to −53‰. Thus, the stage I and II fluids were inferred to be deep-seated brines with temperatures predominantly between 180 °C and 240 °C and salinities of 16−18 wt% NaCl eq. More significant meteoric water was involved during stage III based on the oxygen isotopes of the late quartz (δ18O = 6.2‰ to 9.5‰). The large variation in δ34S values (−21.3‰ to 1.6‰) in the sulfides, the sample- and grain-scale sulfur isotope fractionation, combined with the ore textural evidence that the sulfates and chlorite were replaced by Cu sulfides accompanied by hematite, indicate that the reduced sulfur responsible for sulfide precipitation was generated by inorganic reduction of the previously formed anhydrite and barite (δ34S = 6.3‰ to 8.6‰) or sulfates in the brines by oxidation of chlorite to hematite. Lead isotopes of galena (206Pb/204Pb = 18.67−18.87; 207Pb/204Pb = 15.68−15.72) and copper isotopes of Cu sulfides (δ65Cu = −0.34‰ to 0.51‰) indicate that the ore-forming metals (e.g., Cu and Pb) were derived from the ore-hosting Triassic volcanic rocks. The Wenyu copper deposit cannot be classified as a porphyry, epithermal, or volcanogenic massive sulfide deposit. It is believed that the deep-seated brines, such as the saline formation water in the Permian marine strata, migrated upward along the thrust faults into the Triassic basaltic-andesitic volcanic rocks, and leached metals from the volcanic rocks and precipitated Cu sulfides via inorganic sulfate reduction at Wenyu during the Cenozoic. Such mineralization processes may explain the genesis of the volcanic-hosted copper vein deposits in the Lanping-Simao foreland fold-thrust belt and other similar belts in the world.

Geochemical discrimination of pyrite in diverse ore deposit types through statistical analysis and machine learning techniques

Abstract Pyrite is a ubiquitous mineral in many ore deposits and sediments, and its trace element composition is mainly controlled by temperature, oxygen fugacity, pH, compositions of fluids, and host rock composition. A discriminant analysis (DA), based on multi-element compositions of pyrite, distinguishes iron oxide-apatite (IOA), iron oxide copper-gold (IOCG), skarn Cu-(Fe), porphyry Cu-Mo, orogenic Au, volcanic-hosted massive sulfide (VMS), sedimentary exhalative (SEDEX) deposits, and barren sedimentary pyrite. Testing of the DA classifier yields an accuracy of 98% for IOA, 96% for IOCG, 91% for skarn Cu-(Fe), 94% for porphyry Cu-Mo, 87% for orogenic Au, 84% for VMS, 96% for SEDEX, and 85% for barren sedimentary pyrite. Furthermore, neural network, support vector machine, and random forest, were performed for selecting the optimum classifier more accurately. In these three techniques, the support vector machine yielded the highest overall accuracy (98% for IOA, IOCG, skarn Cu-Fe, and porphyry Cu-Mo, and 97% for orogenic Au, VMS, SEDEX, and barren sedimentary pyrite) and thus is an appropriate technique in predicting pyrite types.

Magnetite Talks: Testing Machine Learning Models to Untangle Ore Deposit Classification—A Case Study in the Ossa-Morena Zone (Portugal, SW Iberia)

A comprehensive investigation into the application of machine learning algorithms for accurately classifying mineral deposit types is presented. The study specifically focuses on iron deposits in the Portuguese Ossa-Morena Zone, employing a limited dataset of trace element geochemistry from magnetites. The research aims to derive meaningful methodological and metallogenic conclusions from the obtained results. The findings demonstrate that the combination of a restricted dataset of trace element geochemistry from magnetites with diverse machine learning models serves as a reliable tool for achieving precise classifications of mineral deposit types. Among the machine learning methods evaluated, random forest, naïve Bayes, and multinomial logistic regression emerge as the most accurate classifiers, whereas the support vector machine, the k-nearest neighbour, and artificial neural networks exhibit lower performance scores. By integrating all literature-proposed classifications, and applying them to selected iron deposits, confident classifications were obtained. Alvito and Azenhas are reliably classified as skarns, whereas Monges, Serrinha, and Vale da Arca are classified as either porphyry or a Banded Iron Formation (BIF). Notably, the classification of Orada proves cryptic, encompassing both BIF and volcanogenic massive sulphide (VMS) deposit types. Moreover, the application of machine learning models to pertinent case studies offers valuable insights not only for classifying mineral deposit types but also for discerning mixed or complex origins. This approach provides meaningful results that can aid in the interpretation of mineral deposit types and may facilitate the identification of new mineral exploration targets. The research highlights the robustness of machine learning algorithms in interpreting magnetite data and underscores their potential significance in exploration projects.

Behavior of palladium during fractionation of sulfide liquid: New constraints from the Kalatongke Cu-Ni sulfide-bearing intrusion in the central Asian orogenic belt, NW China

The distribution of platinum-group elements (PGE; Os, Ir, Ru, Rh, Pt, and Pd) in base metal sulfides (BMS) of magmatic Ni-Cu-(PGE) sulfide deposits has been extensively studied. However, the origin of Pd in pentlandite and the role of Te, As, Bi, Sb, and Se (TABS) in collecting PGE during fractionation of sulfide liquid have not yet been well constrained. Representative massive sulfide ores of the Kalatongke Cu-Ni sulfide-bearing mafic intrusion in the central Asian orogenic belt (CAOB) were selected to identify platinum-group minerals (PGM) using a scanning electron microscope (SEM) and to investigate the distribution of PGE and TABS in the BMS using in situ laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS). The results show that concentrations of Os, Ir, Ru, Rh, and Pt in the BMS are all below or close to the detection limits and only pentlandite contains 0.1 to 3.6 ppm Pd. Three generations of pentlandite are recognized in the massive sulfide ores; i.e., coarse-grained, granular pentlandite that crystallized earlier from monosulfide solid solution (MSS) prior to crystallization of pyrrhotite, loop-textured pentlandite around pyrrhotite that crystallized after pyrrhotite, and pentlandite that crystallized from evolved Fe-rich MSS and is associated with fine-grained myrmekitic magnetite. Granular pentlandite contains higher Pd (1.4 to 3.6 ppm) than that of loop-textured pentlandite (0.8 to 1.7 ppm Pd) and pentlandite associated with myrmekitic magnetite (0.1 to 0.5 ppm Pd). Given that Pd is incompatible to MSS during fractionation of sulfide liquid and is preferentially concentrated to residual sulfide liquid, we propose that the distribution of Pd in pentlandite may be attributed to rapid diffusion of Pd from the residual sulfide liquid into MSS/pentlandite on cooling and that Pd concentrations in the three generations were likely controlled by their crystallization sequence from MSS. In addition, LA-ICP-MS elemental mapping for BMS shows that TABS are not preferentially concentrated in chalcopyrite and euhedral Ni-merenskyite [(Pd,Pt)(Te,Bi)2] grains are mainly enclosed within pyrrhotite (n = 187) rather than chalcopyrite (n = 19). The random distribution of Ni-merenskyite in the BMS is supportive of an idea that Pt and a portion of Pd were likely stabilized by the presence of Te and Bi ligands in sulfide liquid and Te-Bi-Pd-Pt associations were then randomly distributed in MSS and intermediate solid solution (ISS) during fractionation of sulfide liquid. This study shows that the formation of TABS-bearing PGM in the massive sulfide ores of the Kalatongke intrusion was not controlled by fractionation of sulfide liquid, providing a new case that the behavior of PGE is mainly controlled by TABS rather than their partitioning coefficients in sulfide liquid.

Mineralogy and trace element geochemistry of hydrothermal sulfides from the Ari vent field, Central Indian Ridge

The Ari vent field (AVF) is an ultramafic-hosted seafloor massive sulfide (SMS) deposit in the middle part of the Central Indian Ridge. In this paper, we describe the detailed mineralogy and geochemistry of hydrothermal sulfide samples from the AVF, which can be classified into Fe–Cu- and Cu-rich types based on the major sulfide minerals. Sulfide mineralisation of the former type comprises: (1) stage I, early deposition of magnetite, pyrrhotite, isocubanite, chalcopyrite, and subhedral–euhedral pyrite under high-temperature fluid conditions (> 335 °C); (2) stage II, deposition of colloform pyrite, sphalerite, galena, and electrum from low-temperature fluids (< 200 °C) during the later mineralisation stage; and (3) stage III, seawater alteration that caused the precipitation of uraninite and chalcocite. This indicates that the fluids in the AVF had decreasing temperature and ƒS2 and increasing ƒO2 as mineralisation proceeded. The Cu-rich sulfide samples have mineral assemblages and a paragenesis similar to those of the Fe–Cu-rich sulfide samples, but the higher proportion of isocubanite is indicative of relatively high-temperatures and reducing conditions during mineralisation. Bulk chemical compositions of the AVF sulfides are characterised by high U contents (up to 51.9 ppm) and a distinct Sn distribution (2.1–86.4 ppm) between the two different types of hydrothermal samples, which differ from those of other ultramafic-hosted sulfide deposits. The U content is controlled mainly by the precipitation of discrete uraninite grains (< 1 μm in size) on altered surfaces of pyrite and hematite. The oxidative alteration of Fe-bearing minerals caused the fixation of seawater-derived U. Laser ablation–inductively coupled plasma–mass spectrometry analysis showed that most trace elements occur in solid solution in the sulfide minerals, mainly controlled by the physicochemical conditions of the hydrothermal fluids (e.g. temperature, ƒS2, and ƒO2). In particular, a comparative analysis of other mid-ocean ridge systems shows that the ultramafic-hosted sphalerite and pyrite are more enriched in Sn as compared with those hosted by basaltic rocks. However, the Fe–Cu-rich sulfide samples of the AVF are Sn-poor (< 10.2 ppm), because pyrite is substantially depleted in Sn (mostly < 1 ppm) as compared with sphalerite, regardless of the effect of the ultramafic-hosted mineralisation. This indicates that in situ trace element analysis of sphalerite and pyrite, especially for Sn, can provide insights into the different hydrothermal mineralisation in basaltic- and ultramafic-hosted systems, which cannot necessarily be inferred from bulk analysis. Our comparison also suggests that the Sn contents of ultramafic-hosted SMS deposits would be a possible source of Sn for the ultramafic-hosted volcanogenic massive sulfide (UM-VMS) deposit. The δ34S values (+ 6.2 to + 8.5‰) of the pyrite record thermochemical sulfate reduction of seawater, which suggests that sulfur and most metals were predominantly leached from the associated host rocks with a contribution (29–40%) from reduced seawater sulfur. In conclusion, the AVF is a rock-dominated system that contains ultramafic-hosted mineralisation in the Central Indian Ridge.

Iron, boron, and sulfur isotope constraints on the ore-forming process of subseafloor replacement-style volcanogenic massive sulfide systems

The well-preserved Ashele subseafloor replacement-style volcanogenic massive sulfide (VMS) deposit in the Central Asian Orogenic Belt comprises two stages of Cu mineralization, i.e., early massive sulfides dominated by colloform and euhedral pyrite intergrown with chalcopyrite and sphalerite, which were replaced by late vein-dominated chlorite-chalcopyrite assemblages. In this study, a combined systematic Fe, B, and S isotope investigation was first applied to investigate the sulfide precipitation processes and the relative proportion of fluid sources in different alteration and mineralization stages of the Ashele deposit. Boron isotopes of Mg-rich tourmaline (δ11B from −5.57‰ to −2.73‰, average (avg.) −4.23‰) indicate significant seawater (∼19%) participated during the formation of massive sulfides. A two-component mixing model is used to estimate the contribution of seawater and igneous sulfur to the total sulfur budget, and the results show the increasing contribution of magmatic sulfur from the early (35%) to late (76%) stages. In addition, δ56Fe values of pyrite gradually increase from the massive ore (−0.46‰ to −0.02‰, avg. −0.24‰), quartz-pyrite (−0.09‰ to 0.07‰, avg. −0.01‰), chlorite-chalcopyrite-quartz-pyrite (0‰ to 0.21‰, avg. 0.08‰) to the quartz-sericite zone (−0.02‰ to 0.29‰, avg. 0.14‰), which is likely related to the different extent of isotopic exchange and formation temperature, and could be used in the exploration of VMS systems. The new two-stage ore-forming model shows that in the early stage, rapid mixing of hydrothermal fluid from underlying magma chamber with abundant cold seawater led to rapid deposition of pyrite and associated Cu mineralization under relatively oxidized condition, and long-term hydrothermal activities in relatively closed systems would promote the formation of upper massive ores, which resulted in an equilibrium system between pyrite, chalcopyrite, and associated fluid with wide ranges of δ56Fe in pyrite (−0.46‰ to −0.02‰) and chalcopyrite (−1.56‰ to −0.49‰). The late hydrothermal activities in relatively open system would contribute to stringer sulfides or stockworks underlying the massive ore in relatively reduced conditions with heavier Fe isotope compositions in pyrite (−0.09‰ to 0.29‰) and chalcopyrite (−0.60‰ to −0.04‰). Overall, our study demonstrates that the coupling of B, Fe, and S isotopes could be a useful tool to indicate long-term subseafloor infilling and replacement processes for subseafloor replacement-type VMS deposits, which are the prerequisite to form large-tonnage VMS deposits.

Hydrothermal alteration of basalts in the ultramafic-associated Tianxiu Vent Field, Carlsberg Ridge

The Tianxiu vent field (TVF) is the first active ultramafic-associated hydrothermal field to be discovered at the termination of a detachment fault on the slow-spreading Carlsberg Ridge. Most hydrothermal chimneys in the TVF are developed on the basaltic apron. This paper presents mineralogical, petrological, and geochemical data for altered basalts in and surrounding the TVF. The basalt samples can be classified into seafloor-weathered and hydrothermally altered (i.e., slightly saponite-altered, highly saponite-altered, and silicified) types. At the periphery of the TVF, the basalts have experienced seafloor weathering, which has resulted in slight mineralogical and geochemical changes. In the TVF, most basalts have interacted with diffuse low-temperature hydrothermal fluids, leading to variable degrees of saponite alteration. As the distance from the vents decreases, basaltic glass, olivine, and pyroxene are progressively replaced by saponite; Zn, Cu, Co, and U are enriched; and Ca and Mn are depleted. Magnesium, Fe, and Li have been lost due to olivine replacement by saponite, and have been gained due to pyroxene replacement by saponite. In the fluid flow conduits, the basalts have interacted with focused medium- to high-temperature fluids, which has led to the replacement of saponite by opal; depletion in Mg, Fe, Li, and rare earth elements; and enrichment in Zn, Cu, Co, and U. We propose a new hydrothermal alteration model for basalts in an ultramafic-associated vent field. Based on a comparison with mafic-hosted vent fields along mid-ocean ridges, we propose that: (1) the permeable host rocks promote lateral fluid flow, widespread mixing with cold seawater, and pervasive low-temperature hydrothermal alteration; and (2) H2S-poor reducing fluids reduce the solubility of U and inhibit the precipitation of Fe, leading to high Cu/Fe and Zn/Fe ratios, and high U concentrations. The hydrothermal alteration model for the TVF is applicable to similar vent fields and can guide exploration for seafloor massive sulfide deposits along slow-spreading ridges.

Phylogenesis of gold in polymetallic volcanogenic-sedimentary massive sulfide ores

The Au concentrational level of the ores of polymetallic volcanogenic-sedimentary massive sulfide deposits is predetermined by the geodynamic setting of their formation. The ores formed in a mid-ocean ridge setting are characterized by low Au grades. The ores formed under the island arc conditions possess higher Au contents. In the course of the further existence of the ore deposit, its ores experience repeated external influences: underwater halmyrolysis, thermal and dynamic metamorphism, hydrothermal-metasomatic transformations, and exogenous oxidation. These processes lead to the transfer of gold from a micro- or nano-sized form in the sulfides to the free form and to redistribution of Au over the ore field of the deposit. The heterogeneity of the ores in terms of the Au content and existence forms must be taken into account when preparing a feasibility study. The work is intended for geologists conducting mining and geological modeling, designing techniques for the ore extraction, and proposing ore processing technologies.

The genesis of metamorphosed Paleoproterozoic massive sulphide occurrences in central Colorado: geological, mineralogical and sulphur isotope constraints

AbstractPaleoproterozoic massive Cu-Zn±Pb±Au±Ag sulphide deposits metamorphosed to the middle-upper amphibolite facies in central-south Colorado formed in a volcanic arc setting on the edge of the Yavapai crustal province. Previously published U-Pb ages on spatially related granitoids range from ∼1.9 to ∼1.1 Ga, while Pb isotope studies on galena from massive sulphides suggest mineralization formed at around 1.8–1.7 Ga. Some deposits in the Dawson-Green Mountain trend (DGMT) and the Gunnison belt are composed of Cu-Zn-Au-(Pb-Ag) mineralization that were overprinted by later Au-(Ag-Cu-Bi-Se-Te) mineralization. Sulphide mineralization is spatially related to amphibolite and bimodal, mafic-felsic volcanic rocks (gabbro, amphibolite, rhyolite and dacite) and granitoids, but it occurs mostly in biotite-garnet-quartz±sillimanite±cordierite schists and gneisses, spatially related to nodular sillimanite rocks, and in some locations, exhalative rocks (iron formations, gahnite-rich rocks and quartz-garnetite). The major metallic minerals of the massive sulphides include chalcopyrite, sphalerite, pyrite, pyrrhotite, and magnetite, with minor galena and gahnite. Altered rocks intimately associated with mineralization primarily consist of various amphiboles (gedrite, tremolite and hornblende), gahnite, biotite, garnet, cordierite, carbonate and rare högbomite. The Zn/Cd ratios of sphalerite (44 to 307) in deposits in the DGMT fall within the range of global volcanogenic massive sulphide (VMS) deposits but overlap with sphalerite from sedimentary exhalative (Sedex) deposits. Sulphur isotope values of sulphides (δ34S = −3.3 to +6.5) suggest sulphur was largely derived from magmatic sources, and that variations in isotopic values resulting from thermochemical sulphate reduction are due to small differences in physicochemical conditions. The preferred genetic model is for the deposits to be bimodal-mafic (Gunnison) to mafic-siliciclastic VMS deposits (Cotopaxi, Cinderella-Bon Ton, DGMT).

Another Look at Marine Minerals

Abstract Deep-sea mining is taking another step closer to reality. Early leases for exploration in the central Pacific manganese nodule fields and elsewhere in the oceans are coming to an end, and contractors are faced with a choice—extend the licenses to continue exploration or apply to mine the deposits they have found. The first 15-year licenses were originally signed into effect by the International Seabed Authority (ISA) in 2001 and began to expire in 2016. With no operations in a position to commence mining and, more importantly, no regulations in place to allow it, most exploration licenses were simply renewed. Eight of the original licenses were extended for five more years, some twice, and new licenses have been granted. Today, there are 31 contracts for exploration: 19 for manganese nodules, 7 for sea-floor massive sulfides, and 5 for Co-rich crusts. The first contract for massive sulfide exploration expires in 2026; the first for Co-rich crusts expires in 2029. Meanwhile, there is strong interest from a number of countries in the mineral resource potential of their exclusive economic zones (EEZs), particularly Japan and Norway. Against this backdrop of rapidly shifting exploration activity, it may be time to take another look at marine minerals as a resource for the future. In a report entitled “The Future of the Ocean Economy by 2030,” the Organization for Economic Cooperation and Development (OECD) asked, “What new developments could result in a complete revision of offshore mineral potential?” For most parts of the oceans, the answer to this question is plagued by inadequate mapping and a lack of geologic knowledge as a basis for assessing the resources. However, new approaches to exploration are emerging, and recent discoveries, such as on the continental shelf and beneath the cover of sediment, are changing our view of the resource potential.

In Situ LA-ICP-MS U-Pb Geochronology, Sr-Nd-Hf Isotope and Trace Element Analysis of Volcanic Rocks from the Gacun Volcanic-Hosted Massive Sulfide Deposit in Sichuan, China

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.

Measurement of Dried Seafloor Massive Sulfides

We carried out spectral-induced polarization (SIP) measurements on a set of dried seafloor massive sulfide samples and compared the results with those obtained with the same samples fully saturated with NaCl solution. We find that the conductivity and polarizability are generally high for both dried and saturated samples, i.e., exhibiting phase shifts in the order of 100 s of mrad and imaginary conductivities up to 1 S/m. Depending on the particular sample, the polarizabilities of the dried samples remain as high as for the saturated samples or are slightly reduced. The high polarizability of dried samples and the fact that polarizability cannot be destroyed by drying are significant observations because most of the existing theories to explain the polarization of mineralized rock assume a pore space filled with an electrolyte. We also found that the often-used agar gel is unsuitable for coupling the dried samples to the electrodes because it releases water into the sample. Coupling with plasticine is a feasible alternative because significantly less fluid is absorbed by the sample when it is incorporated into the sample holder.