Volcanogenic Massive Sulfide Deposits Research Articles

Copper for the early oxhide ingots traced to the South Urals

Oxhide ingots are flat, rectangular slabs of metal, most commonly of copper, with protruding corners of varying styles. These Late Bronze Age artifacts were widespread across the Mediterranean for approximately half a millennium between the 16th and 11th centuries BC. Although they are generally known as essentially a Cypriot “commodity brand” by the late 15th century BCE, the origin of this ingot’s distinct shape has remained unknown. With the expansion of the lead-isotope database of Eurasian copper ores, the origin of the oldest chemically characterized oxide ingots may now be defined. The bimodal lead isotope compositions of 15 early, pillow-shaped oxhide ingots excavated in Crete are consistent with those of copper ores from the Southern Urals, which are known to have been actively mined in the Bronze Age. Together, Pb isotope and trace element compositions indicate that the majority of the ingots were cast from copper derived from Ural-type Cu-Zn volcanogenic massive sulfide deposits. However, two ingots enriched in Ni, As, and Co match Main Ural Fault-type ores that are hosted by ultramafic rocks.The characteristic design of the pillow-shaped ingot was likely related to the particulars of overland transport, facilitating their long-distance conveyance on pack animals. There is further support of a non-Cypriot origin of this form in the possible association between the words for “ingot”, “copper” and “hide” across languages of Western Asia and the Mediterranean. Thus, an eastern origin of the oxhide form for ingots is likely.

Characterization of a Metamorphosed Volcanic Stratigraphy and VMS Alteration Halos Using Rock Chip Petrography and Lithogeochemistry: A Case Study from King North, Yilgarn Craton, Western Australia

Despite countless advances in recent years, exploration for volcanogenic massive sulfide (VMS) deposits remains challenging. This is particularly the case in the Yilgarn Craton of Western Australia, where outcrop is limited, weathering is deep and extensive, and metamorphism is variable. At Erayinia in the southern Kurnalpi terrane, intercepts of VMS-style mineralization occur along ~35 km strike length of stratigraphy, and a small Zn (-Cu) deposit has been defined at King (2.15 Mt at 3.47% Zn). An extensive aircore and reverse circulation drilling campaign on the regional stratigraphy identified additional VMS targets, including the King North prospect. Through a combination of detailed rock chip logging, petrography (inc. SEM imaging), and lithogeochemistry, we have reconstructed the volcanic stratigraphy and alteration halos associated with the King North prospect. Hydrothermal alteration assemblages and geochemical characteristics at King North (Mg-Si-K enrichment, Na depletion, and high Sb, Tl, Eu/Eu*, alteration index, CCPI, and normative corundum abundance values) are consistent with an overturned VMS system. The overturned footwall stratigraphy at King North is dominated by metamorphosed volcanic rocks, namely the following: garnet amphibolite (tholeiitic, basaltic), biotite amphibolite (andesitic, calc-alkaline), chlorite–quartz schist (dacitic), and narrow horizons of muscovite–quartz schist (dacitic to rhyolitic, HFSE-enriched). The hanging-wall to the Zn-bearing sequence is characterized by quartz–albite schists (metasedimentary rocks) and thick sequences of amphibolite (calc-alkaline, basaltic andesite). An iron-rich unit (>25% Fe2O3) of chlorite–actinolite–quartz schist, interpreted as a meta-exhalite, is associated with significant Cu-Au mineralization, adjacent to a likely syn-volcanic fault. Extensive Mg metasomatism of the immediate felsic footwall is represented by muscovite–chlorite schist. Diamond drilling into the deep hanging-wall stratigraphy at both King North and King has also revealed the potential for additional, stacked VMS prospective horizons in the greenstone belt stratigraphy. The discovery of HFSE-enriched rhyolites, zones of muscovite–chlorite schist, presence of abundant sulfide-rich argillaceous metasedimentary rocks, and a second upper meta-exhalite horizon further expand the exploration potential of the King–King North region. Our combined petrographic and lithogeochemical approach demonstrates that complex volcanic lithologies and VMS alteration signatures can be established across variably metamorphosed greenstone belts. This has wider implications for more cost-effective exploration across the Yilgarn Craton, utilizing RC drilling to reconstruct the local geology and identify proximal halos, and limiting more costly diamond drilling to key areas of complex geology and deeper EM targets.

Trace Element and Sulfur Isotope Signatures of Volcanogenic Massive Sulfide (VMS) Mineralization: A Case Study from the Sunnhordland Area in SW Norway

The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences, mostly of volcanogenic massive sulfide (VMS) and orogeny Au types. The VMS mineralization is hosted by plutonic, volcanic and sedimentary lithologies of the Lower Ordovician ophiolitic complexes. This study presents new trace element and δ34S data from VMS deposits hosted by gabbro and basalt of the Lykling Ophiolite Complex and organic-rich sediments of the Langevåg Group. The Alsvågen gabbro-hosted VMS mineralization exhibits a significant Cu content (1.2 to >10 wt.%), with chalcopyrite and cubanite being the main Cu-bearing minerals. The enrichment of pyrite in Co, Se, and Te and the high Se/As and Se/Tl ratios indicate elevated formation temperatures, while the high Se/S ratio indicates a contribution of magmatic volatiles. The δ34S values of the sulfide phases also support a substantial influx of magmatic sulfur. Chalcopyrite from the Alsvågen VMS mineralization shows significant enrichment in Se, Ag, Zn, Cd and In, while pyrrhotite concentrates Ni and Co. The Lindøya basalt-hosted VMS mineralization consists mainly of pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V, and Tl. The δ34S values of sulfides and the Se/S ratio in pyrite suggest that sulfur was predominantly sourced from the host basalt. The Litlabø sediment-hosted VMS mineralization is also dominated by pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb, V and Tl. The δ34S values, which range from −19.7 to −15.7 ‰ VCDT, point to the bacterial reduction of marine sulfate as the main source of sulfur. Trace element characteristics of pyrite, especially the Tl, Sb, Se, As, Co and Ni concentrations, together with their mutual ratios, provide a solid basis for distinguishing gabbro-hosted VMS mineralization from basalt- and sediment-hosted types of VMS mineralization in the study area. The distinctive trace element features of pyrite, in conjunction with its sulfur isotope signature, have been identified as a robust tool for the discrimination of gabbro-, basalt- and sediment-hosted VMS mineralization.

Diversity, spatial distribution and evolution of inactive and weakly active hydrothermal deposits in the TAG hydrothermal field

Introduction: Although, there is an increasing focus on inactive or extinct seafloor massive sulfide (SMS) deposits driven by the possibility of marine mining, only few studies have been devoted to them so far. The Trans-Atlantic Geotraverse (TAG) hydrothermal field is probably one of the best-studied hydrothermal systems even if the relict SMS deposits known since the mid-1980s have not been thoroughly explored.Objectives: The main objective of this study was to describe the characteristics of these so-called inactive sites.Methods: During four different expeditions, we acquired high-resolution acoustic data and performed numerous human occupied vehicle (HOV) dive operations including extensive rock sampling and in-situ temperature measurements.Results and Discussion: We discovered thirteen new hydrothermal mounds including six large (i.e. > 5,000 m2) deposits making the TAG hydrothermal field one of the largest accumulation of hydrothermal materials (21.1 Mt) known on the seafloor. However, copper and zinc grades of the largest SMS deposits remain low (i.e. < 1.4 wt%) even compared to on-land volcanogenic massive sulfide deposits. Additionally, eight areas of diffuse hydrothermal fluid flow were identified challenging the presumed inactivity of these SMS deposits and, for the first time, emphasizing the importance of low temperature (LT) hydrothermal activity in whole the TAG field. Inactive and weakly active SMS deposits exhibit a large diversity of surface mineralization (e.g. sulfides, Fe-Mn mineralization, jasper) illustrating complexity of hydrothermal activities but also different ageing history. Several mounds no longer have visible sulfide chimneys and are covered by a widespread layer of manganese and iron oxyhydroxides attesting the longevity of diffuse fluid flow at specific locations even long after last high-temperature (HT) hydrothermal activity has ceased. This contrasts with SMS deposits that are devoid of extensive LT precipitates but characterized by standing or topped sulfide chimney indicating a relatively abrupt cessation of HT hydrothermal activity.Conclusion: Together these results allow us to propose evolution models to explain the diversity of active, weakly active and inactive SMS deposits in the TAG hydrothermal field.

Metallogeny and Mineral Exploration – Some Personal Reminiscences

The following personalised narrative aims to document the highlights of my involvement in some of the ground breaking developments in Economic Geology and their direct application to mineral exploration and discovery over the past half century. The story begins with my introduction to geology at secondary school and university, followed by doctoral research based on fieldwork in the Andes of South America. Then, as an employee of the Chilean Geological Survey, I got my introduction to porphyry copper deposits before returning to the UK to take up a post-doctoral research fellowship. This formative period concluded with my starting out as an independent geological consultant to the global exploration and mining industry. These early years happened to coincide with the plate tectonics revolution and its radical implications for metallogeny. I realised that porphyry copper and related deposits are integral parts of volcano-plutonic arcs generated during subduction of oceanic lithosphere, and volcanogenic massive sulphide deposits in ophiolite complexes must have formed at oceanic spreading centres. At approximately the same time, application of K-Ar dating to copper deposits led to definition of metallogenic belts and corresponding epochs in the Andes, and then established the timing of their economically important supergene oxidation and enrichment. Subsequently, using more modern and precise U-Pb zircon and Re-Os molybdenite methods, collaborative attempts were made to determine porphyry copper deposit lifespans and ages of various copper belts, deposits and prospects around the world, including the Zambian Copperbelt. The focus on porphyry copper deposits led first to an appreciation of the linkage between them and subaerial volcanism and the importance of potassic alteration as a major host of hypogene copper mineralisation, and then to geological characterisation of the increasing number of gold-rich examples. Appreciation of the importance of hydrothermal breccias in porphyry copper deposits, including recognition of mineralised diatremes, resulted in a classification scheme for breccias that may be extended to related deposit types. Extensive fieldwork showed that zones of advanced argillic alteration, termed lithocaps, constitute the shallow parts of porphyry copper systems. The role of tectonic uplift in both porphyry copper formation and subsequent supergene modification was also charted. The end result of this body of work was a porphyry copper model that can be used as a basic exploration guide. In response to a marked increase in the world gold price in the late 1970s, more effort was devoted to gold concentrations in magmatic arc terranes, commencing with epithermal gold deposits in the shallow lithocaps of porphyry copper systems. This led to an input to classification schemes for epithermal precious metal deposits and, eventually, to assignment of the three main epithermal types to specific tectono-magmatic settings. After years of speculation, porphyry gold deposits were recognised for the first time in northern Chile, followed by definition of a new gold deposit class in association with relatively reduced granitic intrusions. A magmatic-hydrothermal origin for Carlin-type gold deposits was steadfastly supported over many years, notwithstanding its unpopularity until relatively recently, culminating in a proposal for modern analogues. For decades, metallogenic provinces and corresponding epochs have been widely appreciated, but debate concerning their origin(s) persists. The nature of accompanying magmatism could well provide an adequate explanation for at least some provinces (e.g., tin, molybdenum and possibly silver), but precursor metal enrichment in the lowermost crust and/or subcontinental lithospheric mantle may well be required in the case of gold and copper provinces. The story concludes with brief commentary on mineral exploration, which has been my lifelong (pre)occupation. Requirements for success in mineral exploration are discussed, based primarily on familiarity with the circum-Pacific region, followed by analysis of the types of companies and individuals involved and the burgeoning challenges to the exploration process, which, if not remedied, will have dire consequences for future metal production and global plans for the energy transition. My involvement as a lone practitioner in both mineral exploration and metallogenic research probably says something about my independent character traits, but nonetheless has been largely unstructured and certainly unplanned. I have no hesitation in recommending a similar career path for any recent graduate who relishes adventure and is prepared to endure significant work-life imbalance.

The Zircon U-Pb Age, Hf Isotopes, and Lithogeochemistry of Ore-Bearing Rocks from the Archean Hongtoushan Volcanogenic Massive Sulfide Deposit in the North China Craton: Implications for Tectonic Setting

Volcanogenic massive sulfide (VMS) deposits are globally significant sources of metals. The Hongtoushan VMS deposit is the only large Archean Cu-Zn VMS deposit in the North China Craton, carrying substantial economic value. Significant deformation and metamorphism have made the tectonic setting of the Hongtoushan VMS deposit the subject of extensive debate. This study investigates the petrogenesis and chronology of the ore-bearing host rocks from the Hongtoushan Cu-Zn VMS deposit in the North China Craton. By utilizing whole-rock geochemical analyses and zircon dating, this research sheds light on the origin and evolution of the ore-bearing rocks within the deposit. The whole-rock geochemical analysis data indicate that the Hongtoushan ore-bearing rock series is mainly composed of amphibole plagioclase gneiss (basalt protolith) and biotite plagioclase gneiss (andesite and rhyolite protolith), suggesting a complete volcanic cycle from basic to medium-acidic volcanic rocks. The amphibole plagioclase gneiss has slight LREE enrichment patterns with unremarkable depletions of Nb, Ta, and Ti and belongs to contaminated ocean plateau basalt (OPB) in terms of composition, which is generally interpreted as being generated from the mantle plume head. Meanwhile, the biotite plagioclase gneiss has relatively steep LREE enrichment distribution patterns with remarkable negative Ta, Nb, and Ti anomalies and a wide range of Zr/Y ratios, indicating their classification as FI- and FII-type felsic rocks; they were likely formed through the fractional crystallization of basic magma combined with crustal assimilation. When combined with the zircon dating results, the ore-bearing host rocks of the Hongtoushan VMS deposit were generated via a continuous magmatic evolution process. The zircon dating of the host rocks indicates a formation age of between 2609 and 2503 Ma, with metamorphic events between 2540 and 2466 Ma, which is consistent with the 2.5 Ga-related global mantle plume event. Further research shows that the ore-bearing host rocks are more likely to have been formed in a mantle plume-related stretching environment, possibly a margin rift.

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.

Re-distribution and upgrade of metals induced by the superimposition of later magmatic fluids in the Archean Hongtoushan VMS deposit, NE China

The Hongtoushan copper-zinc deposit is a highly metamorphosed Archean volcanogenic massive sulfide deposit (VMS) in China. After metamorphism, the deposit underwent later fluid superimposition, which is of significance for the re-distribution of metals and upgrading ores. However, the hydrothermal modification mechanism and fluid source remain unclear due to the difficulties in accurately recognizing the mineral textures and assemblages formed by hydrothermal modification. In this study, the mineral texture was investigated to identify the modification mechanism of later fluid and in situ sulfur isotope and trace elements of chalcopyrite were tested to track the source of later fluid and metals. The dominant assemblage formed by superimposed fluid consisted of pyrite, chalcopyrite, sphalerite, minor galena, magnetite, calcite, anhydrite, and hydrothermal alteration minerals, i.e. sericite and chlorite. These minerals infilled the fractures in early pyrite porphyroblasts or precipitated along the grain boundary. Additionally, the pre-existing sulfides were corroded by these minerals. They exhibited replacement and porous textures, indicating that the superimposed fluids resulted in a dissolution-reprecipitation process and formed new sulfides. The hydrothermal metasomatism produced the sericitization of plagioclase and chlorization of biotite, which concurrently liberated silica to form silicification. These interactions suggested that the hydrothermal fluid is acid fluid. The chlorite geothermometer defined the formation temperature of 226–279°C, implying that the superimposed fluid has a moderate temperature. The chalcopyrite precipitated from superimposed fluid either coexisted with pyrite (Ccp 2a) or magnetite (Ccp 2b). The sulfur isotope of Ccp 2a defined a closing zero range (0.75–2.41‰), demonstrating the superimposed fluids were derived from magmatic fluids. In contrast, the Ccp 2b enriched heavy sulfur with a δ34S range of 4.10–8.63‰, which was attributed to the oxidation of fluids. Partial ferrous ions were oxidized to ferric iron, and ferrous and ferric iron precipitated as magnetite. The mineral assemblage changed from chalcopyrite + pyrite, followed by magnetite + chalcopyrite + pyrite, to magnetite + pyrrhotite, representing the local increase of the oxidation state of hydrothermal systems. During the oxidation process, the oxygen fugacity of the superimposed fluid decreased, whereas the pH increased. These results provide crucial evidence on the re-distribution pattern of metals and the post-hydrothermal modification process in the Hongtoushan Cu-Zn deposit.

Geology and structural setting of the undiscovered Amgififa volcanogenic massive sulfide (VMS) deposit in the southwestern part of the Haya Terrane, Arabian-Nubian Shield, Sudan

The knowledge about the Amgififa deposit in the Haya Terrane of the Arabian-Nubian Shield (ANS)in Sudan is limited due to the geological and structural features of this deposit are hidden beneath Quaternary deposits. This study reveals the characteristics of this deposit through systematic mapping (from trenches, drill holes, and artisanal mining pits), structural analysis, whole-rock geochemistry, and SHRIMP U-Pb zircon age dating. The results suggest that the deposit originated during the Neoproterozoic, with compelling evidence of bimodal–mafic volcanogenic massive sulfides (VMS) that evolved in a localextensional setting. Structural developments were associated with the NW-SE collisions between the Haya and Gebeit terranes, which occurred during the amalgamation of the ANS. A comparison between our study area and the Ariab mining area revealed several similarities, suggesting a regional correlation. In this study, we present a four-stage deformation model to provide insights into the geological history of the Amgififa deposit, which is affected by the tectonic evolution of the ANS. This study not only enhances our understanding of the tectonic chronology of the ANS in the study area, but also offers valuable insights into the mechanisms of regional mineralization. Notably, our study can strengthen the overall understanding of the Amgififa deposit in the ANS context and be used as a foundation for improving future mineral-exploration techniques in the region.

Geochemical Signatures of Mafic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Mafic Magmatism

Abstract Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific. Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

Contrasting Cu mineralization styles across the Dananhu Arc, Eastern Tianshan: insights from the geochronology, geochemistry, and Sr–Nd–Hf isotopes of Carboniferous arc rocks

ABSTRACT The tectonic regime is an important factor controlling the formation of ore deposits, and magmatic rocks generated in different tectonic regimes are characterized by contrasting ore affinities and mineralization styles. A series of early Carboniferous volcanogenic massive sulphide (VMS) deposits occur in the western segment of the Dananhu Arc in the Central Asian Orogenic Belt. In contrast, coeval magmatic rocks in the middle segment are devoid of VMS copper mineralization but host porphyry copper deposits. However, the relationship between tectonic setting and mineralization still needs to be clarified. Herein, we synthesize the new and existing geochemical data of the Carboniferous arc rocks of the Dananhu Arc, showing diverse arc magmatism and contrasting Cu mineralization styles in the western and middle segments. Magmatic rocks in the western segment are consistent with normal island arc origin. In contrast, magmatic rocks in the middle segment of the Dananhu Arc have adakite-like affinities. The magmatic rocks in the western segment were derived from the partial melting of the juvenile lower crust triggered by slab rollback in an extension setting. In contrast, the rocks in the middle segment originated from partial melting of the subducted slab that was triggered by flat subduction. The extension setting coupled with high temperature magmatic activity induced the circulation of the hydrothermal fluids, which were critical to the development of the Xiaorequanzi VMS deposit. Copper would become progressively enriched in the fractionating melt under the high–fO2 condition, and the compressional regime caused by flat subduction reduces the ‘leakage’ of hydrothermal fluids, which are favourable for the formation of the porphyry mineralization in the middle segment. It is, therefore, evident that variations in the tectono-magmatic setting across the Dananhu Arc played an important role in generating the diversity of Cu deposit types and variable ore affinities throughout the Eastern Tianshan.

Deposit type discrimination based on trace elements in sphalerite

Sphalerite can form in different geological environments and is the main ore mineral of most Pb-Zn deposits. Trace elements in sphalerite combined with machine learning (ML) methods have been used for deposit type discrimination. However, the performance of different multivariate and ML methods is not systematically compared. Here more than four thousand laser ablation inductively coupled plasma mass spectrometry data from more than one hundred deposits were compiled and investigated by different multivariate and ML methods. Unsupervised multivariate and ML methods including principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE), and supervised multivariate and ML methods including partial least squares-discriminant analysis (PLS-DA), logistic regression (LR), support vector machine (SVM), K-nearest neighbors (KNN), random forest (RF), naïve Bayes (NB), and neural network (NN) are used to classify deposit types. The models are constructed for five deposit types, Mississippi Valley-type (MVT), Volcanogenic Massive Sulfide (VMS), sedimentary exhalative (SEDEX), Epithermal, and Skarn deposits, based on twelve elements in sphalerite, Fe, Pb, Mn, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, and Sb.The PCA and PLS-DA methods show similar clustering results, where Epithermal, MVT, SEDEX, and Skarn deposits can be separated from other deposit types. The t-SNE method is only efficient in discriminating MVT, SEDEX, and Epithermal deposits. The KNN, RF, and NN methods show better performance than LR, SVM, and NB in terms of average accuracy and precision. The PCA, t-SNE, and PLS-DA methods have advantages in data visualization, whereas KNN, NN, and RF methods are more suitable for classification tasks. Feature importance analyses of the PLS-DA and RF methods show that the most important discriminant elements in the classification are Ge, Mn, In, Sn, Ga, and Sb. The models based on PCA, PLS-DA, KNN, NN, and RF are used to predict sphalerite samples with uncertain origins. All the models give similar classification results for deposit types, which are consistent with the ore-forming geological setting. The multivariate and ML models, including discrimination diagrams of PCA and PLS-DA, can be used as the first screening for the type of Pb-Zn mineralization in mineral exploration.

Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

Abstract The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.

In situ sulfur isotope analysis of pyrite from the Ezuri Kuroko‐type volcanogenic massive sulfide deposit, northeastern Japan: Contribution of microbial sulfate reduction to initial sulfide mineralization

AbstractVolcanogenic massive sulfide (VMS) deposits are ancient analogues of seafloor massive sulfide (SMS) deposits. The importance and contribution of microbial activity during initial mineralization has recently been recognized in both VMS and SMS deposits. Here, we report in situ sulfur isotope compositions (δ34S) of pyrite from the Ezuri Kuroko‐type VMS deposit in northeastern Japan as determined by secondary ion mass spectrometry. During the evolutionary process of sulfide mineralization, pyrite textures changed from framboidal to colloform to euhedral. Initial framboidal pyrite had highly negative δ34S values down to −31.8‰ (average ± 1SD = −18.8‰ ± 13.0‰; n = 21), banded colloform pyrite exhibited medium δ34S values (−11.7‰ ± 10.4‰; n = 8), and euhedral pyrite displayed the highest average δ34S value of +2.7‰ ± 1.6‰ (n = 5); thus, δ34S varied as different textures of pyrite were produced during mineralization. The maximum isotopic fractionation between framboidal pyrite and past seawater sulfate (δ34S ca. +20‰) is −52‰; such values can be produced only by microbial sulfate reduction (MSR) in an open system. Framboidal pyrite with a low δ34S value is observed at the centers of sulfide‐rich areas within polished sections and has often been replaced by later sulfide minerals (sphalerite, galena, and chalcopyrite); thus, our S isotope data from the Ezuri pyrite reveal that sulfur derived from MSR induced and acted as a nucleation point for later sulfide mineral growth. Combined with previously reported data, our results endorse the importance and universality of MSR‐derived sulfur during the initial mineralization stage of both VMS and SMS deposits.

Geology and Lithogeochemistry of the Paleoproterozoic Flambeau Cu-Zn-Au Volcanogenic Massive Sulfide Deposit, Wisconsin, USA

Abstract The Paleoproterozoic Flambeau Cu-Zn-Au deposit in northwestern Wisconsin is the only volcanogenic massive sulfide (VMS) deposit to reach production in the Pembine-Wausau terrane of the Penokean orogen, despite the discovery of more than 150 million tonnes of mineralization within 14 deposits since the 1970s. Renewed exploration interest in the region and need for critical minerals has highlighted the importance of the VMS deposits in the region and the need for an improved understanding of their geologic setting. This study examined and sampled 50 drill cores from the Flambeau deposit stored at the Wisconsin state core repository to determine the volcanic, hydrothermal, and tectonic setting of the deposit. The least altered units at the deposit are metadacite and quartz-augen schists that are interpreted to be rapidly emplaced felsic to intermediate crystal-rich tuffs and lapilli tuffs. There are also chlorite ± garnet schists that are interpreted to be mafic sills and/or small flows. The main altered unit is biotite ± andalusite ± white-mica schist that contains porphyroblasts of biotite and/or andalusite that replace the intermediate volcaniclastic units. The ore horizon contains massive sulfide, metachert, and semimassive mineralization within a quartz–white-mica schist. Stratigraphic relationships and chemostratigraphy structurally above and below the ore horizon indicate that the strata are overturned. Despite structural and metamorphic destruction of microtextural evidence, evidence for lateral mineralizing fluid flow in rapidly emplaced, permeable volcanic units, interlayered impermeable to semipermeable units, and macrotextural evidence for lit-par-lit replacement of tuffaceous layers indicate that the Flambeau ores are replacement-style mineralization. Trace element systematics reveal that mafic rock and intermediate rocks are calc-alkalic with low Ti/V values, characteristic of arc environments. Felsic rocks are calc-alkalic, depleted high field strength element (HFSE), FI- to FII-type rhyolites that are also consistent with an arc environment. These lithogeochemical characteristics are similar to those of modern mature oceanic-arc settings. This setting contrasts with other host rocks at nearby deposits (e.g., Lynne, Back Forty) that have petrogenetic evidence for the presence of older Archean basement (i.e., continental crust). Therefore, the geology of the Flambeau deposit highlights the variability of VMS-forming environments within the Penokean orogen and has important local and regional implications for other VMS deposits globally.

GEOLOGY AND STRATIGRAPHY OF HOD (ARTVİN) REGION, EASTERN PONTIDES, NE TURKEY

This study reports a revised geological map and a stratigraphic section of the Hod region. It represents a synthesis of new data and earlier work. New results gathered from field geological mapping and petrographic investigation have clarified the lithological boundaries, textural characteristics, the stratigraphical relationships between various rock formations and spatial distribution and behavior of structural elements. Hod region, located in the eastern part of the eastern Pontides (Artvin district), comprises Jurassic to Cretaceous volcanic and sedimentary sequences with a roughly north-north-east trending, steeply east dipping stratigraphy. These lithological units are Liassic Berta Formation, Dogger Kırgülü Formation and Hızarlıdere Formation, Malm-Lower Cretaceous Öğdem Formation and İnanlı Member, Lower Cretaceous Aşağımaden Formation, Turonian Çatak Formation, Santonian Kızılkaya Formation and Santonian-Campanian Evliyatepe Formation. Among these formations, intermediate to felsic volcanic and volcanosedimentary rocks of Kızılkaya Formation are significant as they host several volcanogenic massive sulfide (VMS) and epithermal deposits throughout eastern Pontides. Hod Maden Au-Cu deposit which represents one of the highest-grade gold and copper discoveries made globally in recent years is also located in Hod region. Therefore, the revised geological map and stratigraphy presented here are important contributions to the regional geology and essential tools for exploration geologists working in the region.

Reimagining Niggli Numbers for modern data applications in petrology and exploration geochemistry

This study seeks to refamiliarize the geochemical community with Niggli Numbers and explore how Paul Niggli's largely forgotten scheme for representing major elements as equivalent molecular numbers that are combined as groups, sums and ratios. This scheme provides simple and effective tools for tackling many modern geochemical problems – particularly to reduce complexity and degrees of freedom where there are requirements to handle large volumes of data. Niggli Numbers provide an alternative to ratio-based methods for the long-recognised problem of the constant sum and data closure in geochemistry and offer a means to represent minerals and their relationships in a straightforward and consistent chemical framework. The Niggli calculation scheme follows simple rules that are easily automated, is flexible enough to accommodate missing elements or partial analyses, and is appropriate for all rock types. Following a short review of the most useful Niggli binary diagrams and where common minerals plot in Niggli space, a series of case studies are presented to illustrate potential applications of the scheme in modern petrology and resource exploration. These cover the following: magmatic evolution of a layered intrusion such as the Bushveld Complex; contamination of magmas by country rocks; tracing hydrothermal alteration in volcanogenic massive sulfide and porphyry CuMo deposits; and chemostratigraphy of sedimentary sequences.

Ore genesis of the Tieshajie Cu deposit: Implications for Cu mineralization in the Qin-Hang Metallogenic Belt, South China

The Tieshajie Cu deposit, located in the northeastern part of the Qin-Hang Metallogenic Belt (QHMB), South China, has long been regarded as a representative Meso-Neoproterozoic volcanogenic massive sulfide (VMS) deposit. Here we present a hydrothermal titanite U-Pb age, Re-Os and in-situ S-Cu isotope data for chalcopyrite to constrain the timing and ore genesis of the Tieshajie deposit. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of titanite from the disseminated Cu ore yielded a weighted mean 206Pb/238U age of 160.1 ± 4.4 Ma. Chalcopyrite from different ore types has low 187Os/188Os (0.85–3.60) and 187Re/188Os (46.1–614.0) ratios, combined with initial 187Os/188Os (0.74–2.00), excluding a mantle source. A Re-Os isochron age (188 ± 30 Ma) for five chalcopyrite samples is consistent with the titanite U-Pb age within errors. Moreover, the variations in Cu isotope compositions (δ65Cu: −1.13 to +0.12 ‰) and δ34S values (+3.8 to +7.7 ‰) of chalcopyrite are inconsistent with those reported from the ancient VMS deposits in previous studies. Therefore, our results are indicative of a Late Jurassic magmatic-hydrothermal origin instead of a VMS origin for the Tieshajie deposit. In combination with previous studies, we propose that the Tieshajie Cu deposit belongs to the distal part of the Mid-Late Jurassic (170–150 Ma) porphyry-skarn Cu mineralization event in the QHMB, likely triggered by the subduction of the Paleo-Pacific plate during the Late Mesozoic. This study also has new insights into the genesis of Cu mineralization in the QHMB and further provides implications for future exploration.

Sulphide petrology and ore genesis of the stratabound Sheep Creek sediment-hosted Zn–Pb–Ag–Sn prospect, and U–Pb zircon constraints on the timing of magmatism in the northern Alaska Range

The Sheep Creek prospect is a stratabound Zn–Pb–Ag–Sn massive sulfide occurrence in the Bonnifield mining district, northern Alaska Range. The prospect is within a quartz–sericite–graphite–chlorite schist unit associated with Devonian carbonaceous and siliceous metasedimentary rocks. Volcanogenic massive sulfide (VMS) deposits in the district are hosted in felsic metavolcanic rocks (362 ± 2 Ma) associated with siliciclastic and carbonaceous sedimentary rocks that overlie the stratigraphic sequence hosting the Sheep Creek prospect. Felsic metaigneous rocks in underlying units are 372 ± 4 to 366 ± 4 Ma. Sheep Creek is atypical of the other sulfide deposits in the district in (1) having Sn grades up to 1.2%; (2) being contained in fine-grained, quartz-rich rocks and quartz–pebble conglomerate that likely originated as chert and chert-clast sediment, respectively; and (3) showing minimal evidence of volcanic components in the host rocks. Comparison of immobile trace-element proportions for graphitic and siliceous rocks from the Sheep Creek area with those for argillite associated with the Bonnifield VMS deposits indicates a continental volcanic-arc provenance for the former and a within-plate and passive margin provenance for the latter. In contrast to previously published interpretations, our data analysis supports a clastic-dominated (CD) rather than a VMS affinity for the Sheep Creek prospect. In our model, Zn–Pb–Ag–Sn mineralization formed by syngenetic or early diagenetic processes on or beneath the seafloor, possibly in the shallow-water environment of an outer continental shelf setting. Potential analogues are the Paleozoic CD deposits in the Canadian Selwyn Basin outboard of the Laurentian continental margin.

Soil geochemistry prospection of a mafic-clastic type volcanogenic massive sulfide deposit in Central Pontides: a case study from the Sarıseki deposit (Kastamonu, Türkiye)

Soil geochemistry prospection of a mafic-clastic type volcanogenic massive sulfide deposit in Central Pontides: a case study from the Sarıseki deposit (Kastamonu, Türkiye)