Skarn System Research Articles

Pyrite geochemistry and its implications for ore formation in reduced copper skarn systems: A case study of the Saibo deposit, Northwest China

Copper skarns are the main source of Cu worldwide, and they are classified into two types according to their redox state: oxidized and reduced. Numerous studies have investigated the mineralization processes of oxidized Cu skarn deposits in terms of pyrite trace element compositions. However, it remains unclear how the compositional and micro-textural features of pyrite are linked to specific physicochemical processes in reduced Cu skarn systems. The Saibo deposit in the northern part of the western Tianshan orogenic belt is a typical reduced Cu skarn deposit, characterized by abundant Ti-rich garnet, arsenopyrite, pyrrhotite, and CH4-rich inclusions. In this study, we examined pyrite geochemical and micro-textural features and related these features to Cu-forming processes at Saibo. Copper skarn mineralization was recognized to occur in the contact zone between the granodiorite porphyry and the Kusongmuqieke Group limestone as veins, bands, massive, and disseminations. Five stages were identified: (I) prograde skarn, (II) retrograde skarn, (III) quartz–pyrite–arsenopyrite (Py1), (IV) quartz–polymetallic sulfides (Py2), and (V) quart–calcite–polymetallic sulfides (Py3).Pb, Sb, Ag, Cu, Zn, and Bi were typically present in the host pyrite as micro- and nanoscale mineral inclusions, whereas Co, Ni, Se, Te, and As were mostly present as lattice substitutions for S and Fe. In situ δ34S values of Py1 to Py3b for the Cu skarn ore at Saibo defined a narrow range from 4.86 ‰ to 7.67 ‰, which was well consistent with the published S values of sulfide separates (0.2 to 9.1 ‰). These values are similar to those of Middle–Late Devonian granitoids, such as the Lamasu plagiogranite, and different from those of sediments. This indicates that the dominant source of sulfides in the Saibo deposit is the Middle–Late Devonian granodiorite porphyry. The overall increasing concentrations of As, Sb, Te, Pb, Au, and Bi, and decreasing concentrations of Co, Ni, and Se from Py1 to Py3b indicate that prior to Cu precipitation, ore-forming fluids were in low oxygen fugacity conditions that were associated with fluid-rock interactions and fluid boiling. Furthermore, the main controlling factors during Cu precipitation were sulfur loss, the increase in pH, and the decrease in temperatures of ore-forming fluids, which could be attributed to fluid-rock interactions, later-stage fluid boiling, and mixing with meteoric water, respectively. The PLS-DA results show that pyrite composition varies following skarn redox state. Pyrite from reduced skarns have in common high concentrations of Au, Bi, and Ni, while those from oxidized have in common high As, Sb, and Pb. Pyrite trace element composition is effective in discriminating different redox states of skarns.

Fluid inclusion LA-ICP-MS constraint on hydrothermal evolution of proximal cassiterite-bearing quartz veins in the giant Gejiu orefield: Implications for controls on metallogenic potential of granite-related skarn system

Sn and Cu are proposed to have their mineralization potential predetermined by their contents in initial fluids of granite-related magmatic-hydrothermal systems. However, it remains ambiguous whether the giant Sn-mineralized skarn system is applicable, and whether the Sn-Cu association in some deposits is predominantly determined by their initial metal contents. The Gejiu orefield is one of the most essential Sn-polymetallic districts worldwide, with proven resources of 3.27 million tons of tin, 3.25 million tons of copper, 4.29 million tons of lead and zinc, and >20 other metals with economic significance. Sn-polymetallic mineralization at Gejiu constitutes a composite skarn ore system that includes proximal skarn and related cassiterite-sulfide, greisen, and tourmaline-vein types. The Laochang Sn-polymetallic deposit hosts several largest skarn and cassiterite-sulfide orebodies in the eastern part of Gejiu. Recent exploitation at Laochang discovered Sn-mineralized quartz veins hosted in the concealed granite, providing a valuable opportunity to characterize the proximal magmatic-hydrothermal process of the mineralizing granitic system. Here, fluid inclusion analysis is carried out on these veins to discuss the fluid evolution, cassiterite precipitation mechanism and whether metal content in early proximal magmatic fluids determines the metal association and endowment in the deposit.Based on the paragenesis of ore and gangue minerals, three hydrothermal stages are distinguished, including quartz-tourmaline stage (Stage I), cassiterite-arsenopyrite-quartz stage (Stage II) and late sulfide stage (Stage III). Fluid evolution controlling vein formation is constrained by microthermometry and LA-ICP-MS analysis of four fluid inclusions generations successively entrapped in quartz and cassiterite. The fluids involved during vein formation show an interplay between single-sourced magmatic fluids and meteoric water. The intermediate-density single phase fluid recorded at stage I quartz is derived from initial fluids directly exsolving from granitic magma. At stage II, fluid immiscibility occurred and the separated brines were entrapped in quartz and early-formed cassiterite. Along with cassiterite precipitation, brines were mixed with low-salinity and cooler meteoric water, leading to entrapment of low-salinity aqueous fluid in outer growth zones of cassiterite at stage II. The constructed fluid evolution history suggests that fluid immiscibility may have facilitated the nucleation of cassiterite crystals at the onset of deposition while mixing of magmatic fluid with meteoric water likely dominate later cassiterite mineralization.Compared with the fluid dataset of barren and mineralized granitic systems worldwide, pre-ore fluids of the studied quartz veins are enriched in Sn, confirming that high Sn content in the initial magmatic fluid can serve as indicator to distinguish mineralized system. In contrast, although Cu mineralization is economically important in the Laochang deposit, predicted Cu contents in pre-ore proximal magmatic fluids are as low as those obtained from Cu-barren system. This implies introduction of Cu into the hydrothermal system from other sources.

Micro- to nano-sized solid inclusions in magnetite record skarn reactions

Abstract. Magnetite is a widespread ore mineral in skarn systems and usually hosts a wide variety of inclusions. Micro- to nano-sized solid inclusions in magnetite are unique tools to track the evolutionary processes of its host mineral and, subsequently, to constrain the timing of the mineralization event. In this study, we characterize micro- to nano-sized solid inclusions in magnetite from the La Víbora magnesian skarn (Málaga, Spain) using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM energy-dispersive X-ray spectrometry (EDS) analyses and compositional mapping expose two types of nano-inclusions oriented along the (111) of magnetite: type 1 includes dolomite, spinel, and Mg–Fe–Al silicate, and type 2 is made up of Mg–Fe–Al silicates enveloping the Mg-bearing amorphous silica phase. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and fast Fourier transform (FFT) patterns reveal that the majority of the solid inclusions display non-oriented matrices compared to the host magnetite, precluding the possibility of sub-solidus processes. Instead, these inclusions are thought to preserve skarn mineral assemblages that were entrapped during the growth of magnetite. However, the local supersaturation of fluids trapped in the boundary layer of crystallizing magnetite is evidenced by coherent lattice orientation of precipitated and host magnetite and by the occurrence of an Mg-bearing amorphous silica phase. Our findings reveal that skarn reactions observed at field and microscopic scales are also recorded in nano-sized inclusions within magnetite. These observations underscore the significance of micro- to nano-scale solid inclusions in magnetite to decipher overprinted skarn reactions as well as constraining the timing of Fe mineralization events in skarns.

The occurrence and enrichment of cobalt in skarn Pb-Zn deposits: A case study of the Niukutou Co-rich deposit, East Kunlun metallogenic belt, western China

The East Kunlun orogenic belt is one of the most important Co-rich polymetallic metallogenic belts in China. Niukutou, a typical Pb-Zn-(Fe) skarn deposit, is situated within the marble formations of the Ordovician-Silurian Qimantagh Group in this belt. Six distinct stages of mineralization have been identified: (1) prograde skarn stage, (2) hydrous mineral-oxide stage, (3) pyrrhotite stage, (4) chalcopyrite stage, (5) sphalerite-galena stage, and (6) carbonate stage. Co enrichment predominantly manifests at chalcopyrite stage, with two distinct Co minerals identified: glaucodot and cobaltite. Co is also present as isomorphous substitution in sulfides (mostly arsenopyrite) and as inclusions in chalcopyrite. Variations in As and S contents in arsenopyrite suggest a steady decrease in ore-forming fluid temperature from the pyrrhotite stage to sphalerite-galena stage. Variations in the Co, Ni, Se, As, Sb, Cu, Sn, Ag, and Zn contents in pyrite indicate fluctuations in temperature, oxygen fugacity, and fluid composition during ore formation. Sulfur isotope compositions (δ34S) through the pyrrhotite stage to the sphalerite-galena stages show a limited range from +3.4 to +7.0 ‰, suggesting a predominantly magmatic source. Whereas, the δ34S of pyrite and marcasite in carbonate stage exhibit significant variations from −22.5 to +22.7 ‰. We propose that this significant variation may be caused by mixing of sulfur from the surrounding rocks and the large fluctuations in oxygen fugacity due to the influx of meteoric water at the carbonate stage. Overall, Co in Pb-Zn skarn system may occur either as independent minerals or as lattice substitution in sulfides such as arsenopyrite. The deposition of Co-bearing minerals is related to decreases in temperature and fO2 during the chalcopyrite stage.

Mineral chemistry of the Geyer SW tin skarn deposit: understanding variable fluid/rock ratios and metal fluxes

AbstractThe Geyer tin skarn in the Erzgebirge, Germany, comprises an early skarnoid stage (stage I, ~ 320 Ma) and a younger metasomatic stage (stage II, ~ 305 Ma), but yet, the source and distribution of Sn and the physicochemical conditions of skarn alteration were not constrained. Our results illustrate that contact metamorphic skarnoids of stage I contain only little Sn. REE patterns and elevated concentrations of HFSE indicate that garnet, titanite and vesuvianite of stage I formed under rock-buffered conditions (low fluid/rock ratios). Prograde assemblages of stage II, in contrast, contain two generations of stanniferous garnet, titanite-malayaite and vesuvianite. Oscillation between rock-buffered and fluid-buffered conditions are marked by variable concentrations of HFSE, W, In, and Sn in metasomatic garnet. Trace and REE element signatures of minerals formed under high fluid/rock ratios appear to mimic the signature of the magmatic-hydrothermal fluid which gave rise to metasomatic skarn alteration. Concomitantly with lower fluid-rock ratio, tin was remobilized from Sn-rich silicates and re-precipitated as malayaite. Ingress of meteoric water and decreasing temperatures towards the end of stage II led to the formation of cassiterite, low-Sn amphibole, chlorite, and sulfide minerals. Minor and trace element compositions of cassiterite do not show much variation, even if host rock and gangue minerals vary significantly, suggesting a predominance of a magmatic-hydrothermal fluid and high fluid/rock ratios. The mineral chemistry of major skarn-forming minerals, hence, records the change in the fluid/rock ratio, and the arrival, distribution, and remobilization of tin by magmatic fluids in polyphase tin skarn systems.

Geology and genesis of the Aqishan Pb-Zn deposit, NW China: Insights from mineralogy, geochemistry, and in situ U-Pb geochronology

The unique ore-forming processes and the key factors responsible for formation of skarn deposits are still obscure, and challenges exist in the determination of timing of Pb-Zn skarns owing to lacking suitable mineral chronometers. Here we present detailed paragenesis, bulk geochemistry, in situ U-Pb dating of zircon and garnet, and garnet oxygen isotopes together with in situ zircon Hf-O isotopes from the newly discovered Aqishan Pb-Zn deposit in the southern Central Asian Orogenic Belt (CAOB), northwest China. This comprehensive data set revealed a Late Carboniferous subduction-related distal Pb-Zn skarn system associated with the granitic magmatism. Pre-ore stage garnets are generally subhedral to euhedral with oscillatory zoning and show slightly fractionated rare earth element patterns with positive Eu anomalies that point to an infiltration metasomatism origin under high water/rock ratios. The syn-ore stage sphalerite is typically enriched in Mn and Cd and has moderate Zn/Cd ratios (337–482), with a formation temperature of 265 °C to 383 °C, which indicate magmatic-hydrothermal signatures. The isocons defined by P2O5 decipher that the principal factors for skarn formation were elevated activities of Fe, Ca, and Si species, where remobilization of Pb metals, meanwhile, contributed to ore-forming budgets to mineralizing fluids. SIMS U-Pb dating of zircons from granite porphyry that occurs distal to the skarns and Pb-Zn orebodies shows that these intrusions emplaced at ca. 311.3–310.6 Ma, recording the subduction of the Paleo-Tianshan oceanic plate. Hydrothermal garnets in close textural association with Pb-Zn sulfides yield indistinguishable in situ LA-ICP-MS U-Pb ages of 310.5 ± 4.1 Ma. Whole-rock geochemistry and in situ zircon Hf-O isotopes (δ18O = 4.6‰–6.0‰) indicate that the granite porphyry was derived from partial melting of juvenile crust and influenced by subducted oceanic crust. Oxygen isotope compositions of garnets (δ18O = 8.0‰–9.0‰) demonstrate that the equilibrated ore fluids were inherited from fluid-rock interactions between a primary magmatic water and host tuff rocks. Our study highlights the application of garnets as a potential robust U-Pb geochronometer and isotopic tracer of ore fluids in skarn mineralizing systems in subduction-related arc environments.

Hosts of Sn in reduced deep-seated W skarn systems: A case study on the world-class scheelite skarn deposit, Zhuxi, South China

Hosts of Sn in reduced deep-seated W skarn systems: A case study on the world-class scheelite skarn deposit, Zhuxi, South China

Systematic trends in the substitution mechanisms of minor elements in cassiterite

A dataset of 4751 EPMA point analyses on cassiterite crystals from 18 localities spanning a range of different granite-related Sn-bearing mineral systems was investigated and compared with thermodynamic considerations from speciation and lattice strain models. The existence of previously inferred charge balanced coupled substitution mechanisms of Fe3+, Fe2+, Mn2+, Nb5+, Ta5+ and W6+ for Sn4+ are confirmed. The homo­valent substitution of Sn4+ by Ti4+ and Zr4+ is also confirmed, and the first evidence for homovalent substitution of Nb4+ and W4+ in natural cassiterite is presented. The preferred substitution mechanisms for Ti, Fe, Mn, Nb, Ta, W and Zr vary systematically as a function of the mineralising environment. Cassiterite precipitated from silicate liquid dominated systems (such as in pegmatites) primarily display a coupled Fe–(Nb,Ta) substitution mechanism with a 1:2 stoichiometry, whereas cassiterite precipitated in systems dominated by autometasomatic processes (such as greisens) more closely follow a 1:1 stoichiometry, as well as increased levels of Ti. Cassiterite precipitated solely from hydrothermal fluids (such as Sn–(W) quartz vein systems) exhibit the highest Ti concentrations, with lower contents of Fe–(Nb,Ta) coupled substitutions, instead displaying a switch to the incorporation of Fe3+ via FeO.OH stoichiometry. Hydrothermally precipitated cassiterite from skarn systems display the highest Fe3+ contents, and the lowest Nb, Ta and Ti contents. These systematic changes define a continuous trend in a Ti–(Fe + Mn)–(Nb,Ta) ternary diagram, allowing for classification of the primary source of alluvial cassiterite.

Pyrite geochemical fingerprinting on skarn ore-forming processes: A case study from the Huangshaping W–Sn–Cu–Pb–Zn deposit in the Nanling Range, South China

Pyrite geochemistry has been extensively used to reveal ore-forming processes in diverse ore deposits, but its application in skarn systems is not well understood. The Huangshaping deposit in the Nanling Range (South China) uniquely develops W–Sn–Pb–Zn and Cu–Pb–Zn skarn mineralization systems, and both two systems have formed multi-types of pyrite, which provides a good window to reveal the mineralization histories of different skarn systems using pyrite trace element geochemistry. In the Cu–Pb–Zn system, texturally homogeneous Py1 mainly occurs in calcite veins within the host rock, whereas texturally homogeneous Py2 in the calcite-sulfide stage mainly occurs in skarn orebodies. Py3 in the siderite-sulfide stage replaces Py2 and commonly develops abundant pores or fractures, resembling the “bird's eye” texture. In the W–Sn–Pb–Zn system, Py4 in calcite veins can be divided into the oscillatory-zoned Py4a and irregular Py4b under BSE, and Py4b commonly replaces Py4a as veins or overgrowth. Texturally homogeneous Py5 in the calcite-sulfide stage occurs in skarn orebodies and is replaced by Py6 with a “bird's eye” texture in the siderite-sulfide stage.In the Cu–Pb–Zn system, Py1 may have formed by fluid cooling during its ascent along the hydraulic fractures indicated by its enrichment of Co, Ni, As, Sb, and Tl, whereas Py2 is likely formed under higher temperature and pH conditions caused by intense fluid-rock interactions evidenced by its depletion of Co, Ni, As, Sb, and Tl as well as its enrichment of Zn and Ag. In the W–Sn–Pb–Zn system, Py4b has higher contents of Co, Ni, As, Sb, and Tl than Py4a and similar contents of Ag, Zn, and Mo with Py4a, suggesting that decreasing temperature may have controlled the formation of Py4b. Py5 is featured by the depletion of As, Sb, Tl, and Mo as well as the enrichment of Zn and Ag, indicating that Py5 is likely formed under higher temperature and pH conditions resulting from the intense fluid-rock interactions. Marcasite replaces pyrrhotite and then is replaced by pyrite may be an important precipitation mechanism for pyrite with a “bird's eye” texture (Py3 and Py6) in both two skarn systems at Huangshaping. This study demonstrates that fluid-rock interaction is an important mechanism for sulfide precipitation at Huangshaping, which shows that pyrite geochemistry has good potential to reveal mineralization histories in skarn systems.

High-Cd sphalerite in the Pangxidong Pb-Zn-Ag deposit (Yunkai Domian, South China): Insight for physicochemical condition of orogenic-type deposit

The Pangxidong Pb-Zn-Ag-(Au) deposit is one of the representative orogenic-type Au-bearing polymetallic deposits in the Yunkai Domian, Cathaysia Block, South China, and both its vein- and altered rocks-type orebodies are controlled by the regional NE-trending Bobai-Cenxi Fault and its subordinate faults. Metallogenic potential of critical metals, such as Cd, In, Ga and Ge, hosted in these orogenic-type polymetallic deposits, have never been evaluated. We, therefore, selected the Pangxidong to conduct a series of field survey, microscopic observation, electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasm mass spectrometer (LA-ICP-MS) investigation. We identified two generations of sphalerite with different mineral assemblages and mineralogical features, and both sphalerite generations have the relatively higher Cd, Cu, Ag and Au contents, against the lower Ga, Ge, In and Sn contents. Notably, an extremely elevated Cd content was detected in the earlier SpA (4869–47194 ppm, mean = 15976 ppm), against those in the later SpB (5629–9980 ppm, mean = 7224 ppm). Further temperature evaluation using GGIMFis geothermometer indicates that the earlier SpA (229–367 ℃, mean = 239 ℃) have a slightly higher crystallization temperature than the later SpB (228–240 ℃, mean = 233 ℃). Collectively, we highlight that the Pangxidong sphalerite have different minor and trace element associations and concentrations against most other Zn-Pb deposit styles such as MVT and skarn system, and therefore represents a novel sphalerite type from the orogenic-type deposit.

Copper skarn mineralization related to reduced monzodioritic magma at the Huanren deposit, northeastern China

Most known copper (Cu) skarns are associated with oxidized intrusions. In this contribution, we report a Cu skarn associated with a reduced monzodioritic intrusion at Huanren, northeastern China, which contains 0.41 Mt of Cu accompanied by economic concentrations of Zn, Pb, Fe, Mo, and Ag. Copper-polymetallic mineralization in the Huanren deposit is concentrated in skarns located between the contacts of the monzodiorite (SiO2 = 52−55 wt%) and the Cambrian carbonate rocks, with minor molybdenite-bearing veinlets/veins and dissimilated chalcopyrite mineralization hosted within the monzodiorite. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology indicates the monzodiorite crystallized at 125.4 ± 0.6 Ma (2σ). Isotope dilution (ID)-ICP-MS Re-Os molybdenite geochronology indicates mineralization at Huanren occurred at 125.3 ± 0.8 Ma (2σ). Whole-rock major- and trace-element and zircon Hf isotopic compositions suggest enriched (subduction metasomatized) lithospheric mantle−derived sources for the parental magma of the monzodiorite without significant crustal assimilation. Zircon trace-element and magmatic apatite major-element compositions indicate the reduced nature of the monzodiorite, as evidenced by low magmatic oxygen fugacity (fayalite-magnetite-quartz [FMQ] buffer = −1.09 ± 0.19) and negligible apatite SO3 contents (<0.05 wt%). A reduced magmatic-hydrothermal system at Huanren is also supported by the predominance of magmatic ilmenite over magnetite in the monzodiorite and by the presence of pyrrhotite and the absence of anhydrite and hematite in the ore. Chalcopyrite from the Huanren deposit has an average δ34S value of 4.34‰ ± 0.88‰ (1σ), which is clearly higher than values from most porphyry-skarn Cu ± Mo ± Au deposits. Accordingly, we suggest that interaction between (1) external oxidized fluids equilibrated with evaporites and (2) reduced Cl-bearing magmas and related exsolved fluids may have played a critical role in the formation of the Huanren Cu skarn by increasing the ability of fluids to scavenge Cu from the reduced magma and subsequently precipitate Cu in the carbonate rocks. This study defines a new type of Cu skarn and thereby opens new potential for Cu skarn exploration proximal to intrusive units previously deemed too reduced to be Cu fertile, especially in non-arc settings. Moreover, we conclude that the availability of Cl and S in magmatic-hydrothermal systems may be as critical as fO2 in facilitating the actual ore-forming event in Cu skarn systems.

Temperature as a major control on Cd enrichment in a skarn system: A case study of the Yiliu Pb-Zn-As deposit, South China

The Yiliu Pb-Zn-As deposit is located in the Nanling Metallogenic Belt (NMB), South China, representing an extremely As-enriched skarn system, but the endowment of critical metals, such as Cd and In, and their recording metallogenic physicochemical condition have never been investigated. To address these questions, we conducted a series of field survey, microscopic observation, electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasm mass spectrometer (LA-ICP-MS) studies. The lenticular orebodies are closely associated with the altered Devonian limestone and Yanshanian Baoshan and Benggangling granitic intrusions, and the massive and disseminated ores comprise arsenopyrite, pyrite, sphalerite, chalcopyrite, galena and tetrahedrite, in a decreasing order. We identified three generations of sphalerite, that is, Sp1, Sp2 and Sp3, with different colors and mineral assemblages. EPMA and LA-ICP-MS data show that two groups of minor and trace elements are hosted in the Yiliu sphalerites: (1) Fe, Cd, Mn and In mainly occurring as lattice-bound (solid-solution) or homogeneously distributed nanoparticles, and (2) Cu, Sn, Pb, Sb and Ag mainly occurring as micro-mineral inclusions. It is notable that a relatively high Cd (4814–10149 ppm, mean = 6133 ppm) against low In (10–301 ppm, mean = 67 ppm) concentration is identified in all the three sphalerite generations. Calculation of sphalerite crystallization temperatures by GGIMFis geothermometer show that a clearly decreasing trend from the earlier Sp1 (273–319 ℃; median = 296 ℃) to the later Sp2 (267–301 ℃; median = 286 ℃) and Sp3 (278–290 ℃; median = 282 ℃). And there is a decrease for Cd concentrations from Sp1 to Sp2 and Sp3, i.e., 4903–10149 ppm (median 6964 ppm) for Sp1, and 4814–7078 ppm (median 5435 ppm) for Sp2 and 5756–6894 ppm (median 6420 ppm) for Sp3, respectively. Additionally, the enrichment of Cd is also affected by the content of Fe whose enrichment is controlled by the relatively high temperature. Collectively, we deduce that the temperature acts as the major control for Cd enrichment in the sphalerite of the Yiliu Pb-Zn-As deposit, and the higher temperature facilitate Cd enrichment in a skarn system.

Polymetallic tungsten skarn mineralisation related to the Periadriatic intrusion at Lienzer Schlossberg, East Tyrol, Austria

Abstract A regional tungsten anomaly was discovered and explored in the Lienzer Schlossberg area that is part of the crystalline Austroalpine nappe complex of the Eastern Alps in the 1970/80-ties. Tungsten is present as scheelite, which occurs in steeply SSW dipping WNW-ENE oriented quartz veinlets and joints within the porphyritic dioritic to tonalitic/granodioritic host rocks of the Oligocene Lienz/Edenwald intrusion and in the exoskarn, which developed at the contact of the intrusion with Ca-rich lithologies. The skarn system is characterised by two stages: (1) a primary high-temperature calc-silicate stage characterised by grossular, diopside-hedenbergite, vesuvianite, wollastonite and Ca-plagioclase and (2) a retrograde skarn stage including scheelite-bearing massive sulphide ores (pyrrhotite, chalcopyrite, tremolite-actinolite, diopside-hedenbergite) and scheelite-rich fault rocks with calcite, sericite and chlorite. The two ore stages, the mineral paragenesis and composition is like that of a reduced tungsten skarn; i.e., garnet is grossular-rich, clinopyroxene is diopside-hedenbergite. Vesuvianite contains up to 1.8 mass % fluorine. Scheelite has been studied by cathodoluminescence (CL), electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometry. Three types of scheelite (Scheelite 1–3) are distinguished. In short-wave UV light, all types show blue fluorescence but CL revealed internal micro-textures in scheelite grains which are dominated by oscillatory zoning. Prolonged hydrothermal activity is indicated by dissolution-replacement and overgrowth textures affecting the primary zonation and trace element composition of scheelite. The distribution of rare earth elements (REE) in Scheelite 1 in tonalite-hosted quartz veinlets shows a convex (i.e., middle REE-enriched), heavy REE-depleted pattern with negative Eu anomalies (EuA ). The evolution of REE patterns from skarn-hosted Scheelite 2 and 3 illustrates a gradation of convex REE patterns with high ΣREE and distinct negative Eu anomalies to relatively flat REE patterns with small to no EuA . Scheelite at Lienzer Schlossberg has one of the highest ΣREE+Y contents of all scheelite-bearing ore settings in the Eastern Alps but incorporates minor Mo and the least Sr. High Na and Nb contents together with the positive correlation of REE+Y vs. Na+Nb+Ta suggests that the main exchange vectors for REE incorporation in scheelite is via a combined coupled substitution mechanism. Results of this study confirm the genetic similarity with reduced tungsten skarns and highlight the tungsten potential of this area.

Allanite geochronology in the Mount Isa Inlier

New allanite geochronology is presented for four mineral deposits in the Mount Isa Inlier (SWAN, Mary Kathleen, Koppany and Elaine Dorothy). The data indicate the occurrence of multiple phases of allanite formation: one at 1732 ± 8 Ma at Koppany, one at ca 1520 Ma at Mary Kathleen, Koppany and SWAN, and one at ca 1473 Ma at Mary Kathleen and Elaine Dorothy associated with vein calcite. In the Mary Kathleen–Koppany–Elaine Dorothy skarn system, there are chemical and textural differences between the three allanite generations; the ca 1520 Ma generation formed by dissolution–reprecipitation of 1732 Ma allanite, with no addition of externally sourced rare earth elements, as shown by the identical Nd isotopic compositions of these two populations. Allanite at SWAN yielded a 207Pb/206Pb ID-TIMS age of 1519.6 ± 1.1 Ma; its homogeneity, low common Pb contents and near-concordance make it a suitable reference material. Repeated remobilisation of rare earth elements in the Mary Kathleen system is surprising, given the widely accepted low solubility of these elements, but may have been restricted in spatial extent.

Geochemistry and geochronology of Beiya giant Au polymetallic deposit: Implications for multistage ore-fluid pulses in a skarn system

Garnet is one of the most typical minerals in skarn deposits, and its texture and composition is a proxy for the changing physicochemical conditions. Thus, mineralogical and geochemical characterization of different garnet types can provide important information on the nature and evolution of ore-forming fluids. The Beiya gold-polymetallic deposit is China’s largest skarn-type Au deposit with Au reserves of >380 t. Although several studies have addressed the skarn formation mechanism and nature of fluid evolution, a systematic study on the changes in the composition and texture of garnet from wall rocks to ore-related intrusion has not yet been attempted. In this work, we carry out a systematic investigation along a horizonal profile in the open pit of the Beiya deposit. Our results show that the garnet in the profile belongs to grossular-andradite solid solution series with all grains showing well-developed fine oscillatory zoning. The garnets are of two types: the first type (Grt I) formed within the endoskarn, is reddish to red with euhedral-subhedral megacryst structure (>10 mm) and has altered surfaces. The second type (Grt II) occur in the proximal exoskarn, is red to brown–red, euhedral-subhedral, with medium- to fine- grained structure (0.3–4 mm) and displaying oscillatory zoning. Unlike Grt I, hydrous minerals occur in the pores among Grt II garnets, such as calcite, quartz, epidote and chlorite. Also, calcite-chlorite vein, quartz vein and quartz-chalcopyrite-pyrite vein cuts the Grt II garnets. Grt I is dominantly grossular (Adr14.88-43.58Grs55.25-82.87), depleted in the large ion lithophile elements (LILE), relatively enriched in high field strength elements (HFSEs) and enriched in heavy rare earth elements (HREEs) with negative Eu anomalies. Grt II is mostly andradite (Adr62.18-98.91Grs0-36.22), depleted in the LILE and HFSE, enriched in light rare earth elements (LREEs) with slightly positive Eu anomalies. Here we selected Grt II for in-situ LA-ICP-MS U-Pb dating, since Grt I contains negligible uranium. The results yielded a lower intercept 206Pb/238U age of 35.3 ± 1.8 Ma (MSWD = 0.68, n = 23). The calculated oxygen isotopic compositions of hydrothermal fluids in equilibrium with two types of garnets range from 6.46 ‰ to 8.56 ‰ (average 7.89 ‰), which are within the range of values for magmatic fluids. We infer that the physicochemical conditions of the hydrothermal fluid changed from relatively reduced, nearly neutral, and low water/rock ratios to slightly oxidized, slightly acidic, and high water/rock ratios. The chlorine content in the fluid increased during garnet growth. The fluctuating compositions of garnets in the skarn suggest multistage ore-fluid activities and the fluid evolution provide insights into the ore-forming process.

Earlier stage, higher temperature, and deeper space facilitate indium precipitation in a skarn system, as exemplified by the Baoshan Pb-Zn polymetallic deposit, South China

Indium (In), one of the strategic critical metal elements, is preferred to be enriched in the skarn system, but the enrichment regularity in terms of temporal and spatial distribution in a specific skarn Pb-Zn deposit remains unclear. To address the problem, we conducted a systematic geological, mineralogical, and trace element geochemical investigation on the Baoshan In-rich Pb-Zn polymetallic deposit, Hunan Province. This deposit is situated in the central part of the famous Nanling W-Sn-Pb-Zn polymetallic metallogeny belt, South China. Three ore districts in the Baoshan deposit with distinct economic element associations are divided, including the Central District (dominated by Cu-Mo), West District (dominated by Pb-Zn), and North District (dominated by Pb-Zn). They comprise a typical skarn system, including the outcropped granodiorite porphyry, typical skarn mineral assemblages, and Pb-Zn-hosting Carboniferous limestone. We systematically sampled the representative Pb-Zn ores in two ore districts (West and North) and three underground levels (-270 m, −230 m, and −190 m). Four types of sphalerites with different mineral associations and mineralogical characteristics were identified, that is, Sp1a and Sp1b (formed in the early sulfide stage), as well as Sp2a and Sp2b (formed in the late sulfide stage). Our EPMA (Electron Probe Micro-Analyzer) and LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometer) analyses reveal the four types of sphalerite have distinct chemical compositions. Such as, the mean value of indium is 3524 ppm for Sp1a, 142 ppm for Sp1b, 48.03 ppm for Sp2a, and 87.98 ppm for Sp2b in a decreasing order, and a similar trend of calculated temperatures ranging from 336 to 135 °C using GGIMFis thermometers can be obtained. Further LA-ICP-MS trace elements mapping show that the core of Sp1a is relatively enriched in more In, Cu, Sn, and Ag contents. Spatially, the sphalerites in the deep contain a higher indium content (mean = 1581 ppm) than those in the shallow (107.20 ppm) in the vertical profile. In the planar, indium is more enriched in the West District (mean = 1090 ppm) than in the North District (60.92 ppm). The indium distribution regularity reflects that the metals-carrying magmatic hydrothermal fluids flow from Central District, through the West District, to the North District. Collectively, we conclude that indium prefers to be enriched in the earlier stages, higher temperature, and deeper space during the sphalerite crystallization in the Baoshan skarn system, and therefore highlight the deep space of Baoshan West District is a promising target for indium exploration. This new finding maybe shed light on the scientific understanding on indium enrichment and associated exploration strategies in the similar skarn Pb-Zn metallogenic systems.

Robust dating of Pb–Zn skarn systems by LA–ICP–MS garnet U–Pb geochronology

Lawu and Yaguila are two Pb–Zn skarn deposits in the eastern Nyainqêntanglha metallogenic belt, central Lhasa subterrane, Tibet. The genesis of Pb–Zn deposits in this belt and the crustal processes leading to their formation remain ill-constrained, mostly because of the lack of precise mineralization ages. We integrate textural information, geochemistry, and in-situ garnet U–Pb geochronology to constrain the timing and genesis of Pb–Zn mineralization. Garnets from the two deposits display oscillatory compositional zoning (And14Gr86 to And100) and contain variable U contents (0.07–5.3 ppm). Aluminum-rich garnet displays a chondrite-normalized rare earth element (REE) fractionation pattern in which LREEs are depleted relative to flattish HREE segments. In contrast, Fe-rich garnet shows LREE-enriched, relatively HREE-poor patterns with positive Eu-anomalies. Uranium concentration is correlated with total REE and Fe components in garnets, implying that U incorporation is largely controlled by coupled substitution mechanisms. The same garnets also contain measurable contents of other metals (up to hundreds of ppm), such as Sn, W, and In. The distribution and fractionation of major and trace elements in zoned garnets record periodic fluid pluses with different compositions during hydraulic fracturing. The new garnet U–Pb data show that the Yaguila deposit formed between 68.5 ± 3.4 and 65.0 ± 4.7 Ma, and the Lawu deposit formed at 54.6 ± 2.9 Ma. Within the geochronological framework of igneous rocks and Pb–Zn mineralization in the Nyainqêntanglha metallogenic belt, the new garnet ages suggest a magmatic-hydrothermal origin for related mineralization, with the causative magma likely derived from partial melting of ancient continental material due to rollback and subsequent breakoff of the Neo-Tethys oceanic slab during India–Asia continental collision. This study highlights the opportunities offered by garnet U–Pb dating for elucidating the formation age and ore genesis of base metal skarn systems.

Physicochemical evolution and mechanism of a skarn system: Insights from the world-class Mazraeh Cu deposit, NW Iran

The Mazraeh Cu skarn deposit, NW Iran, is situated at the lithological contact between an Oligo−Miocene granodiorite and Cretaceous limestones. The causative granodiorite has I-type medium- to high-K calc-alkaline composition that formed in a subduction arc setting. Metasomatism generated exoskarn and endoskarn characterized by a temporal and spatial mineral-chemistry zonation with massive garnet and garnet-pyroxene proximal and epidote-amphibole skarn at the distal zones of the exoskarn. Our data reveal a multi-stage evolution of the hydrothermal system. Accordingly, the prograde stage was formed by barren, oxidized, high-temperature and hypersaline magmatic fluids, which suggests that the parental magma was emplaced at low lithostatic pressure conditions (∼1.5 km depth). By contrast, during the retrograde stage the regime changed from lithostatic to hydrostatic conditions, resulting in a significant decrease in pressure and oxygen fugacity, boiling, cooling, and mixing, respectively. Dilution by meteoric fluids led to additional cooling and lower salinity of the ore fluids. Sulfur isotope compositions (δ34S = −2.8‰−1.3‰) of sulfides indicate a unique magmatic source of the sulfur. The δ18Ofluid and δDfluid values of the prograde stage fluid also indicate a magmatic origin, while their gradual depletions from the prograde to retrograde stage imply increased mixing with meteoric water. We propose that the fluid mixing scenario continued over the entire life cycle of the mineral system in an open hydrostatic regime as reflected in the oxygen isotope values. It is suggested that boiling and fluid mixing played a significant role in the formation of this giant Cu skarn deposit.

Apatite texture and composition in the Tonglushan porphyry-related skarn system, eastern China: Implications for mineral exploration

The composition of ‘indicator’ minerals is thought to provide a guide to the potential of magmatic arc systems to form porphyry Cu deposits, but whether this is also the case for endoskarn- and exoskarn-dominated systems remains unclear, despite their importance as a source of Cu, Fe and Au. In a first step to address this, we compare the texture, cathodoluminescence (CL) colour and composition of apatite between relatively fresh quartz monzodiorite (QMD) and porphyry-type-, endoskarn- and Fe-(Cu)-mineralised exoskarn components of the Tonglushan porphyry-skarn system of the Daye ore district, China. In the relatively fresh QMD, apatite luminesces yellow-green due to elevated Mn contents. However, where affected by potassic-sodic alteration, it shows green–blue CL thought to reflect partial removal of Mn and an associated increase in REE. Apatite in the endoskarn is more pervasively replaced and veined, and shows mid-blue luminescence due to relatively low Mn, Mg and Cl. The exoskarns contain apatite with variable grain shapes and navy blue-violet or bright to dark blue CL colours, caused by low Mn and elevated Ce, and with only small patches of pale yellow-green CL. Apatite is near absent in the limestone wall rocks and xenoliths and, therefore, where present in the exoskarns is interpreted to have precipitated from the same fluids as the Fe-(Cu) mineralization.Apatite CL colour and chemistry is indicative of the different styles of alteration and mineralisation in the Tonglushan system and provides insights into the composition of skarn-forming fluids. Our results offer a potentially effective method for utilising apatite as a porphyry and skarn deposit indicator mineral in a range of exploration materials including regolith and stream sediments.

In Situ LA-ICP-MS of Zoned Garnets from the Huanggang Skarn Iron–Tin Polymetallic Deposit, Southeastern Mongolia, Northern China

The Huanggang deposit is the most important and largest skarn Fe–Sn polymetallic deposit in the Southern Great Xing’an Range of Northeast China. Cassiterite, magnetite, and other metal minerals are related to the garnets within skarn systems. The zoned garnets from various skarn stages are able to record numerous geological and mineralizing processes including variations in physicochemical conditions and hydrothermal fluid evolution. In this contribution, we present the mineralogy, systematic major, trace, and rare earth element (REE) concentrations of zoned garnets from the Huanggang Fe–Sn polymetallic skarn deposit. The in situ analytical results of garnets in the prograde skarn stage from andradite core (Grt I) to grossular rim (Grt II) reveal that core sections were formed from a fluid that was generally LREE-rich, with relatively high ∑REE, high LREE/HREE ratios, and weak negative Eu anomalies, whereas rim sections were crystallized from a fluid that was typically HREE-rich, with relatively low ∑REE, low LREE/HREE ratios, and obviously negative Eu anomalies. The garnets of the retrograde skarn stage from Fe3+-rich andradite core (Grt III) to andradite rim (Grt IV) demonstrate that the core sections have a flat trend with high ∑REE and obvious negative Eu anomalies, whereas rim sections were formed from a fluid with relatively low ∑REE, HREE-rich and obviously negative Eu anomalies. The garnets from the prograde skarn stage principally display relatively lower U and higher Y and F concentrations than those from the retrograde skarn stage. Based on optical and textural characteristics, REE patterns, Eu anomalies, and trace element variations in zoned garnets, it can be shown that, during skarn formation, Huanggang hydrothermal fluids shifted from near-neutral pH, oxidizing conditions, and high W/R ratios with relatively low LREE/HREE ratios characteristics to acidic, reducing conditions, and low W/R ratios with relatively high LREE/HREE ratios characteristics. We infer that variations in fluid compositions and physicochemical conditions may exert major control on formation and evolution of garnets and skarn hydrothermal fluids.