Volcanic-hosted Massive Sulfide Research Articles

The Fate of “Immobile” Ti in Hyaloclastites: An Evidence from Silica–Iron-Rich Sedimentary Rocks of the Urals Paleozoic Massive Sulfide Deposits

The formation of Paleozoic silica–iron-rich sedimentary rocks in the Urals volcanic-hosted massive sulfide (VHMS) deposits is considered a result of seafloor alteration of hyaloclastites mixed with calcareous/organic or sulfide material. These rocks host various Ti mineral phases pointing to the transformation of precursor metacolloidal TiO2 phases to disordered anatase during seafloor alteration of hyaloclastites, which was later converted to globules and clusters and further to diagenetic rutile. The LA-ICP-MS analysis showed that the Ti content of hyaloclasts partly replaced by finely dispersed Si–Fe aggregates increases to 540–2950 ppm and decreases (<5 ppm) in full Si–Fe pseudomorphs after hyaloclasts. LA-ICP-MS element mapping reveals the enrichment in V, U, Cr, W, Nb, Pb, and Th of the anatase globules and the local accumulation of Zr, Y, and REE on their periphery. Corrosive biogenic textures in the outer zones of some hyaloclasts and biomorphic aggregates in rocks contain anatase particles in assemblage with apatite indicating the biophilic properties of Ti. This work fills the knowledge gaps about Ti mobilization during low-temperature seafloor alteration of hyaloclastites in VHMS deposits.

Resistivity anomalies in the shallow and deep crust beneath West Tasmania from a broadband magnetotelluric 2D transect

West Tasmania has a complex geological history, including a Cambrian tectonic collision and related orogenesis, which formed ore deposits in a volcanic hosted massive sulfide (VHMS) setting. While the topography and dense vegetation covering the area presents challenges for on-ground investigations, the area is relatively well-studied such that 3D geological models are available. This contribution presents results of a broadband magnetotelluric (MT) 2D transect across the area (∼30 stations over a ∼ 80 km line, deployed in early 2016) that enables a combined interpretation of regional-scale geoelectric and geological structures. After accounting for noise, distortion and geoelectric strike; we produce 2D resistivity models using OCCAM2D inversion. We infer low resistivity anomalies in the shallow ( < 3 km ), mid (3–8 km) and deeper crust. Along this MT transect the shallow and mid crust low resistivity zones correspond to major crustal faults. Deep low resistivity features in the east of the transect suggest fluid pathways associated with metamorphism along the western boundary of the Tyennan block during the Cambrian Tyennan Orogeny, while others potentially represent the metasomatism of lower crustal rocks and fluid pathways converging into the mid crust.

Whole rock lithogeochemical analysis of the Mount Read Volcanics: a new tool for geochemical exploration

The Mount Read Volcanics (MRV) of western Tasmania is a belt of Middle to Late Cambrian submarine volcanics and volcaniclastics and one of the major base and precious metal provinces within Australia. Exploration success in the MRV has been in decline in recent years due to a variety of challenges including intensely altered rocks with complex stratigraphy and lithofacies variation, and a lack of accessible outcrop. As a result, local and regional mapping of the MRV is often restricted to Group-level stratigraphy, rendering geological mapping ineffective for identifying decametre-scale mine stratigraphy. Surface rock chip and drill hole samples were collected and analysed using commercially available geochemical assays to create a regional geochemical dataset of the MRV, covering the entire province and all major stratigraphic units. From these data, a lithogeochemical methodology was developed, utilizing Ti/Nb and V/Sc systematics that effectively characterizes the samples according to composition and magmatic processes. This method is shown to be robust to biases due to alteration and lithofacies, eliminating the need for highly selective sampling or extensive data filtering. To demonstrate the utility of this method in an exploration context, data from the K-lens of the Rosebery polymetallic volcanic-hosted massive sulfide (VHMS) deposit was used as a case study. Using Ti/Nb–V/Sc systematics, a rock composition classification was developed that allowed for discrimination of the footwall, hanging wall and host stratigraphy of the Rosebery mine sequence. A geochemical fingerprint for the footwall stratigraphy to mineralization was developed, and by correlating this signature to the regional unclassified samples, geochemical analogues for the Rosebery footwall were readily identified. This approach rapidly reduces the exploration search space and provides valuable information for future mineral exploration. The methodology is applicable in other terrains; however, appropriate orientation studies are required to investigate potential biases due to lithofacies, alteration and mineralogical variability.

The Ballynoe Stratiform Barite Deposit, Silvermines, County Tipperary, Ireland

The Ballynoe barite deposit is a conformable, mineralised horizon of Lower Carboniferous age overlying a diastem and mass faunal extinction demarking the transition from a quiet water environment to one of dynamic sedimentation. The geometry of the barite orebody correlates with the palaeotopography of the footwall, which acted as an important control over the lateral extent, thickness, and nature of the mineralisation. Sedimentary features within the barite horizon suggest that it was precipitated in the form of a cryptocrystalline mud which underwent major diagenetic modification resulting in extensive stylolitisation, recrystallisation, and remobilisation. There is abundant and compelling geological and isotopic evidence for early local exhalation from the presence of a hydrothermal vent fauna consisting of delicately pyritised worm tubes and haematised filaments of apparent microbial origin. The worm tubes are remarkably similar to examples from modern and ancient volcanic-hosted massive sulphide deposits, and the filamentous microfossils have similarities to modern Fe-oxidising bacteria. Strontium in the barite has an 87Sr/86Sr ratio indistinguishable from seawater between 350 and 344 Ma whilst oxygen isotopes from barite and chert suggest a diagenetic origin in equilibrium with such seawater around 60–70 °C. Fluid inclusion studies have shown that, in general, low temperature inclusions are very saline (20%–25%) whilst at higher homogenisation temperatures they are more dilute (9%–12%).

Determining the ore-forming processes of Dongshengmiao Zn-Pb-Cu deposit: Evidence from the linear discriminant analysis of pyrite geochemistry

The orogenic base metal deposits gain more and more attentions in recent years. The ore-forming fluids associated with these deposits are enriched in reduced sulfur and have low salinity. Since this kind of fluid has low affinity to transport base metals, pre-enrichment of sulfide should take place during the formation of orogenic base metal deposits. The Dongshengmiao Zn-Pb-Cu deposit, which is located in the northern margin of the North China Block, was considered as orogenic base metal deposit, and a two-stage mineralization process was proposed by previous researchers, but other researchers proposed that the Dongshengmiao is a typical sedimentary exhalative (SEDEX) deposit. Even though the tectonic environment, fluid inclusion, geochronology and isotopic geochemistry have been widely investigated in the Dongshengmiao, its genesis is still under debate, and new data or proof should be presented to decipher the genesis model of the Dongshengmiao deposit. In this study, linear discriminant analysis (LDA), one of the commonly used machine learning methods, was employed to discriminate the genesis of pyrite in the Dongshengmiao deposit based on the trace element concentrations analyzed by LA-ICP-MS. The LDA results demonstrate that nearly all the early-stage fine-grained pyrites (Py1) show the same geochemical features as the pyrites from volcanic hosted massive sulfide (VHMS), whereas most of the late-stage recrystallized pyrite (Py2) is of orogenic origin. The Mesoproterozoic submarine volcanic activities resulted in pre-enrichment of sulfide with VHMS geochemical signature in the rocks hosting Dongshengmiao deposit. The pre-enriched sulfides were subsequently remobilized during the subsequent metamorphism in the early Cretaceous, leading to the formation of giant an orogenic base metal deposit.

Origin of Archean Pb isotope variability through open-system Paleoarchean crustal anatexis

Abstract Lead isotopic data imply that thorium and uranium were fractionated from one another in Earth’s early history; however, the origin of this fractionation is poorly understood. We report new in situ Pb isotope data from orthoclase in 144 granites sampled across the Archean Yilgarn craton (Western Australia) to characterize its Pb isotope variability and evolution. Granite Pb isotope compositions reveal three Pb sources, a mantle-derived Pb reservoir and two crustal Pb reservoirs, distinguished by their implied source 232Th/238U (κPb). High-κPb granites reflect sources with high 232Th/238U (~4.7) and are largely co-located with Eoarchean–Paleoarchean crust. The Pb isotope compositions of most granites, and those of volcanic-hosted massive sulfide (VHMS) and gold ores, define a mixing array between a mantle Pb source and a Th-rich Eoarchean–Paleoarchean source. Pb isotope modeling indicates that the high-κPb source rocks experienced Th/U fractionation at ca. 3.3 Ga. As Th/U fractionation in the Yilgarn craton must have occurred before Earth’s atmosphere was oxygenated, subaerial weathering cannot explain the apparent differences in their geochemical behavior. Instead, the high Th/U source reflects Eoarchean–Paleoarchean rocks that experienced prior high-temperature metamorphism, partial melting, and melt loss in the presence of a Th-sequestering mineral like monazite. Archean Pb isotope variability thus has its origins in open-system high-temperature metamorphic processes responsible for the differentiation and stabilization of Earth’s continental crust.

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.

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.

Quaternary Seabed Sediment Characteristics as an indication of VMS-mineral deposits at the South Banda Basin and its surroundings, Indonesia

The South Banda back-arc basin is a morphotectonic environment that is part of the South Banda Basin, characterized by a flat morphology with depths ranging from 5000m to 6000m. The purpose of this paper is to analyze and relate morphometric aspects to the Quaternary seabed sediment deposits deposited in an active tectonic basin in the Banda Sea area, including information on the type of sediment and its distribution as well as its relation to volcanic hosted-massive sulfide (VHMS) or volcanogenic massive sulfide (VMS) mineral deposits. The seabed lithofacies in the Banda Arc area were deposited in several morphotectonic environments. The types of deposits that are deposited are pelagic, and hemipelagic deposits including turbidite and volcanic origin deposits. Volcanic deposits consist of gravel, sand, muddy sand, and silt which are classified very poorly to moderately well sorted. The grain composition is volcanic lithic, feldspar, augite, hypersthene, and enstatite which are commonly found in volcanic arcs and troughs. Groups of volcanic grains composed of pyroxene andesite, pumice, and tuffite or siliceous gravel, distributed in the northern and the middle of the volcanic arc area are interpreted to originate from the Banda volcanic complex and the Seram-Gorong islands.

Archean Pb isotope variability tracks crust-mantle fractionation, granite production, and ore deposit formation

Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition.

Magmatic evolution and metal systematics of back-arc volcanic rocks of north–east Japan and implications for deposition of massive sulphide Kuroko ores

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880–940 °C, and Fe–Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO2 rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition.

New insights into the petrogenesis of rhyolites and evolution of volcanic‐hosted massive sulphide mineralization from shallow borehole core, Betul Belt, Central India

Shallow borehole drilling has been conducted at the Sarni area of Betul Belt, Central India to understand the subsurface geology and nature of mineralization. The predominant lithounits are ferruginous quartzites, felsic volcanic rocks, and with minor intrusions of amphibolites. The encountered felsic volcanic rocks are rhyolites varying from pink (rich in K‐feldspar) to grey (rich in Na‐feldspar and amphiboles). The rhyolitic rocks have undergone intense hydrothermal alteration. The alteration zones are defined by the presence of biotite‐chlorite‐garnet‐gahnite; biotite‐sericite‐chlorite‐anthophyllite and anthophyllite‐actinolite‐tremolite‐carbonate mineral assemblages. Sulphide pits, highly leached zones with occasional bluish tinges, limonitic patches are present in the felsic volcanic rocks, followed by semi‐massive to disseminated sulphide mineralization indicative of Kuroku‐ type Zn–Cu–Pb volcanic‐hosted Massive Sulphide (VHMS) mineralization. Scanning electron microscope and energy‐dispersive x‐ray spectroscopy studies documented sphalerite, pyrite, chalcophyrite, arsenopyrite, pyrrhotite, and minor Au–Ag phases. The Sarni rhyolites exhibit chalcopyrite disease in sphalerite, replacement, and overgrowth ore‐textures. The high HFSE, low Zr/Y ratio, pronounced negative Eu anomalies characterize the felsic volcanic rocks as FIIIb type with high zircon saturation temperatures of 856–931°C. These rhyolites are formed at a back‐arc rift setting in an ensialic island‐arc tectonic regime. The synvolcanic structures facilitated the magma ascent to the surface. The hydrothermal circulation due to adequate heat flow in the back‐arc rift environment, in an episodic multicyclic process provided sufficient time for accumulation of sulphides from the seafloor, which was later deposited to form a VHMS mineralization. We also report tungsten (W) mineralization in the quartz veins of these felsic volcanic rocks. It is suggested that the felsic magma has interacted with metasedimentary sequences and assimilated W‐ore bearing fluids, and during remobilization in the later stages of tectonic activity enabled the deposition of tungsten in these quartz veins.

A mineralogical and geochemical application for determining hydrothermal alteration zones associated with volcanic-hosted massive sulphide deposits at Tasik Chini area, Peninsular Malaysia

The volcanic-hosted massive sulphide (VHMS) deposits of the Tasik Chini area, Peninsular Malaysia are hosted by Permian felsic volcanic rocks (∼ 290 Ma) of rhyolitic to rhyodacitic composition and comprise two deposits (Bukit Botol and Bukit Ketaya). Although the ores and the felsic volcanic host rocks of these deposits have metamorphosed to greenschist facies and were affected by a complex deformation history, the hydrothermal alteration zones associated with mineralization are preserved and show systematic mineralogical and geochemical changes with increasing proximity to the ore bodies. The geometry of alteration assemblages of the Bukit Botol and Bukit Ketaya deposits shows they occur as semi-conformable or stratabound zones around the ore lenses. According to petrography study and short wavelength infrared (SWIR) spectra measurements supported by X-Ray diffractometer (XRD), the Bukit Botol deposit is characterised by proximal quartz-sericite-pyrite and distal quartz-sericite alteration zones, whereas distal quartz-chlorite-sericite-pyrite-pyrophyllite ± kaolinite and proximal quartz-chlorite-pyrite ± carbonate ± pyrophyllite form the alteration assemblages of the Bukit Ketaya deposit. The geochemical data is coherent with the mineralogical characteristics, using the Ishikawa alteration (AI) and chlorite-carbonate-pyrite (CCPI) indexes, showing the important of chlorite and sericite formation in the ore proximal and ore distal zones. In addition, the molar elemental ratios of Na2O/Al2O3 versus K2O/Al2O3 and MgO/Al2O3 versus K2O/Al2O3 support that the abundance of muscovite (sericite) and chlorite controlled the intensity of alteration at the both deposits. The results of this study indicate that the combination of mineralogy and geochemistry studies help in defining specific alteration styles and related mineral geochemical variations of the VHMS deposits in Tasik Chini area.

Ecandrewsite (ZnTiO3) in Amphibolites, Sierras de Córdoba, Argentina: Mineral Chemistry and Comparison with Different Worldwide Paragenetic Occurrences

ABSTRACT Ecandrewsite (ZnTiO3) and other ilmenite-group minerals have been found in amphibolites of the Sierras de Córdoba basement, Argentina, in an area where zinc is a relatively widespread element in the associated metasedimentary and metaigneous sequences. Ilmenite group minerals occur as anhedral to subhedral, tens to a hundred micrometer-sized relic inclusions in titanite. Electron microprobe analyses reveal compositions along a discontinuous solid-solution trend ranging from manganoan ferroan ecandrewsite toward ilmenite s.s., passing through intermediate members such as ferroan manganoan ecandrewsite, zincian manganoan ilmenite, and manganoan ilmenite. Considering that thermodynamic constraints do exist for the solubility of ZnTiO3 in ilmenite under mid- to high-grade regional metamorphic conditions, we believe that ecandrewsite and Zn-rich ilmenite compositions were attained by metasomatic fluid–mineral reactions during retrograde regional metamorphism, i.e., after the centripetal replacement of protolithic Zn-bearing ilmenite group species by titanite. The original composition of the ilmenite group species might have been Zn-poor ilmenite; however, the attainment of ecandrewsite compositions possibly needed an external supply of zinc provided by the fluid. The variations of the zinc contents were controlled by the substitution of Fe by Zn + Mn in the absence of any type of regular zonation. This is the first worldwide report of ecandrewsite in amphibolites, which has so far been described in quartz-rich metasediments, quartz-gahnite exhalites, kyanitic schists, nepheline syenites, metamorphosed volcanic hosted massive sulfide (VHMS) mineralizations, and albitites. The presence of ecandrewsite in amphibolite, as has been proved for zincian ilmenite and gahnite in other metasedimentary sequences elsewhere in the world, could become another pathfinder or indicator mineral for Zn-enriched portions of the crust.

Lithospheric conductors reveal source regions of convergent margin mineral systems

The clean energy transition will require a vast increase in metal supply, yet new mineral deposit discoveries are declining, due in part to challenges associated with exploring under sedimentary and volcanic cover. Recently, several case studies have demonstrated links between lithospheric electrical conductors imaged using magnetotelluric (MT) data and mineral deposits, notably Iron Oxide Copper Gold (IOCG). Adoption of MT methods for exploration is therefore growing but the general applicability and relationship with many other deposit types remains untested. Here, we compile a global inventory of MT resistivity models from Australia, North and South America, and China and undertake the first quantitative assessment of the spatial association between conductors and three mineral deposit types commonly formed in convergent margin settings. We find that deposits formed early in an orogenic cycle such as volcanic hosted massive sulfide (VHMS) and copper porphyry deposits show weak to moderate correlations with conductors in the upper mantle. In contrast, deposits formed later in an orogenic cycle, such as orogenic gold, show strong correlations with mid-crustal conductors. These variations in resistivity response likely reflect mineralogical differences in the metal source regions of these mineral systems and suggest a metamorphic-fluid source for orogenic gold is significant. Our results indicate the resistivity structure of mineralized convergent margins strongly reflects late-stage processes and can be preserved for hundreds of millions of years. Discerning use of MT is therefore a powerful tool for mineral exploration.

Mineralogy and geochemistry of clastic sulfide ores from the Talgan VHMS deposit, South Urals, Russia: Signatures of diagenetic alteration

Diagenetically altered thin-layered sulfide ores from the Talgan volcanic-hosted massive sulfide deposit (South Urals) represent an intercalation of thin sulfide and hyaloclastite layers up to 3 and 0.5 cm thick. Their mineralogy and trace element, including rare earth elements (REE), and S, C and O isotopic geochemistry are considered in this work. They occur at the slopes of ore bodies and pinch outside them within hyaloclastites and their thickness increases towards the flanks of the ore bodies. The sulfide layers are mostly composed of authigenic pyrite, which forms zoned nodules, anhedral aggregates, euhedral crystals and pseudomorphs after pyrrhotite crystals. The pyrite nodules and anhedral aggregates host inclusions of authigenic chalcopyrite, sphalerite, galena, barite, cotunnite, native gold, stromeyerite, tellurobismutite and hessite. The trace element content of sulfide layers widely varies and depends on the amount of non-sulfide minerals (chlorite, illite, calcite, quartz, rutile, scheelite), which formed as a result of alteration of dacitic hyaloclastite components mixed with sulfide clasts. The sulfide layers are enriched in REEs (up to 561 ppm) due to the presence of REE-bearing minerals (bastnaesite, parisite, synchysite, galgenbergite, REE-bearing xenotime), which do not occur in massive sulfide ores. A narrow range of δ34S values of pyrite (1.41–3.27 ‰) from sulfide layers is explained by equilibrium or kinetic dissolution–precipitation processes of redeposition of sulfide ore clasts during diagenetic processes. The δ13C values of bulk samples of sulfide layers (–5.29 to –17.89 ‰) indicate that the authigenic carbonates formed from pore fluids circulated in former organic-rich hyaloclastite layers. The δ18O values of the same samples (+4.00 to + 8.36 ‰) are higher compared to seawater δ18O values due to diagenetic isotopic exchange between the hyaloclastitic components and pore fluids at elevated temperatures of burial process. The positive δ34S–Mn, δ13C–Mn, δ13C–As, δ13C–Sb, δ18O–Mn and δ18O–U correlations reflect organic matter consumption during the evolution of authigenic minerals. Data presented in this study helps to define the origin of diagenetically altered layered sulfide ores in volcanic-hosted massive sulfide deposits and are important for understanding diagenetic REE accumulations in sulfide-rich sediments of these deposits.

Testing the advantages of simultaneous in-situ Sm[sbnd]Nd, U[sbnd]Pb and elemental analysis of igneous monazite for petrochronological studies. An example from the late Archean, Penzance granite, Western Australia

Monazite has the ability to incorporate a broad range of trace elements (TE), including SmNd, in addition to being a reliable UPb geochronometer. Nonetheless, many current isotopic tracer studies mostly focus on laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Hf-in-zircon or whole rock isotope dilution SmNd. The conditions necessary to produce reliable Nd-isotopic data from whole rock analyses (i.e. absence of weathering and alteration) restricts the number of samples suitable for this type of study. Conversely, accessory phases such as monazite, apatite, allanite, epidote and titanite are more resistant to weathering and alteration and have a greater chance of preserving their original Nd-isotopic signature.The Archean Penzance granite, in the Yilgarn Craton, Western Australia, is a relatively unweathered high field strength elements (HFSE)-enriched granite that is associated with the Teutonic Bore volcanic-hosted massive sulfide (VHMS) Camp. Here we compare newly acquired LA split-stream (SS)-MC-ICPMS analyses in monazite from the Penzance granite to previously collected whole rock Nd-isotopes, SHRIMP ages in zircon and monazite and Hf-in-zircon of a specific outcrop. The ƐNd0 of monazite from the Penzance granite have a weighted mean value of +1.6 ± 0.2 both for the values calculated from the SHRIMP single population age and from individual LASS dates. This value is the same within uncertainty to the whole-rock ƐNd0 value of +1.99 ± 0.39, whereas ƐHf0 show similar juvenile values with mean value of +2.2 ± 0.5 (2 SE).These findings complement previous studies that argue that monazite has the potential to provide reliable time constrained isotopic and chemical data, which are comparable to, and at times superior than, Hf-in-zircon and whole-rock Nd-isotopes. Additionally, because monazite can record TE signatures that can be used to retrieve petrological information, this study proposes a LASS-MC-ICPMS routine that achieves simultaneous SmNd isotopes, U/Th-Pb dating and TE analyses in monazite, allowing for further petrological information to be obtained from a single analytical routine. This study also assesses the precision and accuracy in UPb ages in monazite reference materials, when additional masses are measured in the same routine. The application of the routine presented here facilitates the use of monazite as isotopic tracer in petrological and sedimentary studies.

Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals

Abstract Advanced argillic minerals, as defined, include alunite and anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, and topaz), and diaspore. One or more of these minerals form in five distinctly different geologic environments of hydrolytic alteration, with pH 4–5 to &amp;lt;1, most at depths &amp;lt;500 m. (1) Where an intrusion-related hydrothermal system, typical of that associated with porphyry Cu ± Au deposits, evolves to white-mica stability, continued ascent and cooling of the white-mica–stable liquid results in pyrophyllite (± diaspore) becoming stable near the base of the lithocap. (2) A well-understood hypogene environment of formation is vapor condensation near volcanic vents, where magmatic SO2 and HCl condense into local groundwater to produce H2SO4 and HCl-rich solutions with a pH of 1–1.5. Close to isochemical dissolution of the host rock occurs because of the high solubility of Al and Fe hydroxides at pH &amp;lt;2, except for the SiO2 component, which remains as a siliceous residue because of the relatively low solubility of SiO2. This residual quartz, commonly with a vuggy texture, is largely barren of metals because of the low metal content in high-temperature but low-pressure volcanic vapor. Rock dissolution causes the pH of the acidic solution to increase, such that alunite and kaolinite (or dickite or pyrophyllite at higher temperatures) become stable, forming a halo to the residual quartz. This initially barren residual quartz, which forms a lithocap horizon where permeable lithologic units are intersected by the feeder structure, may become mineralized if a subsequent white-mica–stable liquid ascends to this level and precipitates copper and gold. (3) Boiling of a hydrothermal liquid generates vapor with CO2 and H2S. Where the vapor condenses above the water table, atmospheric O2 in the vadose (unsaturated) zone causes oxidation of H2S to sulfuric acid, forming a steam-heated acid-sulfate solution with pH of 2–3. In this environment, kaolinite and alunite form in horizons above the water table at &amp;lt;100°C. Silica derived within the vadose zone will precipitate as amorphous silica at the water table, as the condensate follows the hydraulic gradient, causing opal replacement above and at the aquifer. (4) By contrast, where condensation of this vapor occurs below the water table, the CO2 in solution forms carbonic acid (H2CO3), leading to a pH of 4–5. This marginal carapace of condensate, with temperatures up to 150°–170°C, commonly acts as a diluent of the ascending parental NaCl liquid. This steam-heated liquid forms intermediate argillic alteration of clays, kaolinite, and Fe-Mn carbonates; this kaolinite, which can be present at depths of several hundreds of meters, can potentially be mistaken as having been caused by a steam-heated acid-sulfate or supergene overprint. (5) The final setting is supergene, caused by posthydrothermal weathering and oxidation of mainly pyrite, locally creating pH &amp;lt;1 liquid because of high concentrations of H2SO4 within the vadose zone and forming kaolinite, alunite, and Fe oxyhydroxides. This genetic framework of formation environments of advanced (and intermediate) argillic alteration provides the basis to interpret alteration mineralogy, in combination with alteration textures and morphology plus zonation, including the overprint of one alteration style on another. This framework can be used to help focus exploration for and assessment of hydrothermal ore deposits, including epithermal, porphyry, and volcanic-hosted massive sulfide.

Zinc, cadmium and sulphur isotopic compositions reveal biological activity during formation of a volcanic-hosted massive sulphide deposit

Zinc and Cd isotopes fractionate strongly within a range of hydrothermal systems; consequently, metal stable isotopes have been used to understand the formation of hydrothermal ore deposits. We investigated the Zn–Cd–S isotopic compositions of sulphides from the Carboniferous-Permian Xiaobaliang Cu–Au deposit, Inner Mongolia, China. This ancient seafloor hydrothermal system is the only known Cyprus-type volcanic-hosted massive sulphide (VHMS) deposit in the Central Asian Orogenic Belt. The S isotopic compositions of sulphide minerals are highly variable with δ34SCDT values ranging from −21.2‰ to 25.2‰. We infer that negative δ34SCDT values reflect mixing of volcanogenic S and S derived from bacterial sulphate reduction (BSR), and that positive δ34SCDT values reflect mixing of volcanogenic S with S derived from thermochemical sulphate reduction (TSR). Zinc isotopic compositions are relatively consistent at the deposit; δ66Zn values range from 0.10‰ to 0.70‰. These values are similar to those reported for modern seafloor sulphides. We infer that consistency of δ66Zn values relates to the Zn species ([ZnCl4]2−) present in the ore-forming fluids during sulphide precipitation. All of the sulphides are enriched in the light Cd isotopes; δ114/110Cd values range from −0.74‰ to −0.08‰. Zinc and Cd can be affected by biological activity, and we infer that biological activity caused the negative δ114/110Cd values at this deposit because: (1) biological processes (e.g., bacterial activity) enrich fluids in the light Cd isotopes, leading to the precipitation of sulphides with negative δ114/110Cd values; and (2) the Cd isotopic composition of ore-forming fluid is readily modified because the concentration of Cd is much lower than that of Zn in ore and basalt. Our results indicate that Cd isotopes might record ancient biological activity, and that organisms might play a more important role than previously thought in the formation of VHMS systems.