Magmatic-hydrothermal Ore Research Articles

What controls Sn-W mineralization in granite batholith: A case study from the world-class Dulong Sn-Zn polymetallic ore deposit of SW China

Magmatic-hydrothermal ore deposits hosted in granitic batholiths are an important source of tungsten (W) and tin (Sn). However, it remains enigmatic whether and how the Sn-W mineralization is triggered by the spatially associated batholith because the Sn-W-bearing granitic rocks are more likely to occur as small intrusions. Here we used a combination of LA-ICP-MS cassiterite U-Pb dates, published whole-rock geochemical, and high-precision exploration data to evaluate a genetic link between the world-class Dulong Sn-Zn polymetallic ore deposit and its spatially associated batholith in SW China. Our new cassiterite U-Pb dates indicate that the Sn-Zn and Sn-Cu mineralization events were temporally associated with the emplacement of concealed granites and porphyritic dykes respectively, consistent with the spatial pattern between Sn-Zn-Cu mineralized zones and granitic rocks in the Dulong ore district as revealed by the high-precision exploration data. The compiled data presented in this study show that these granitic rocks throughout the batholith are of S-type and highly fractionated affinities, and their parent magmas might experience similar degrees of fractionation. Importantly, the low Nb/Ta ratios (most < ∼5) in granites of the Dulong ore district are markedly differentiated from highly variable values (∼2.0 to ∼ 9.4) in the batholith, which may be caused by the extensive sub-solidus interaction with late magmatic fluids. Furthermore, the granites of the Dulong ore district are significantly poorer in Sn-Zn contents than those of the Laojunshan batholith, suggesting the extensive Sn-Zn-rich fluids exsolved from highly evolved granitic magmas could lead to a significant decrease in Sn-Zn contents in consolidated granites and the formation of Sn-Zn ore deposit, due to the relatively high fluid/melt partition coefficients of Sn and Zn. In conjunction with evidence from previous numerical and mineralogical models, and the spatial pattern between Sn-Zn mineralized zones and apical parts of concealed pluton in the Dulong ore district as revealed by high-precision drill data, we propose a model involving highly evolved granitic magmas that formed batholith, apical portions of which as the emanative centers of Sn-Zn-rich fluids played a crucial role in the formation of the world-class ore deposit.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Low-temperature thermochronology of the Longmala - Mengya’a Pb-Zn deposits (southern Tibet): Implications for ore exhumation and preservation

The large Gangdese metallogenic belt, located in the southern part of the Tibetan Plateau, hosts a variety of magmatic-hydrothermal ore deposits. Previous works in the area primarily concerned the ore genesis, while the exhumation history of these ore deposit has received less attention. In this contribution we explore the multi-stage thermo-tectonic evolution of the Longmala and Mengya’a Pb-Zn deposits, two large magmatic-hydrothermal ore deposits in the central Lhasa sub-terrane. In more detail, the thermal history of granitoids and deformed Paleozoic sediments inside and on the periphery of the ore districts have been constrained by low-temperature thermochronological methods, including the zircon (U-Th)/He and apatite fission track thermochronology. Additionally, zircon U-Pb geochronology was conducted on granitic host rocks to determine their emplacement ages. Our thermochronological data and inverse thermal history modeling results reflect two phases of accelerated basement cooling during the Cenozoic (i.e., ∼46–41 Ma and ∼20–5 Ma), interpreted as exhumation phases. The ore body was exhumed to the near surface during the Miocene, which provides essential information for ore exploration in the region. We also explore the degrees of basement rock exhumation along the Gangdese metallogenic belt and their spatio-temporal relations to major ore deposit distributions. It is suggested that complex interactions between tectonic-structural activities and surface erosion occurred during the late Oligocene-Miocene (∼25–5 Ma), which resulting in widespread exhumation along the Gangdese batholith. During this period, the ore-bearing rocks were gradually (with medium to high cooling rates) exhumed to the near-surface from ∼3–4 km crustal depth, making these ore deposits feasible for commercial mining.

Different metal coordination in sub- and super-critical fluids: Do molybdenum(IV) chloride complexes contribute to mass transfer in magmatic systems?

Molybdenum (Mo) is extracted predominantly from magmatic-hydrothermal ore deposits, and knowledge of the thermodynamic properties of aqueous Mo complexes over a wide range in pressures (P) and temperatures (T) is required to predict the transport and deposition of Mo in ore-forming environments. These properties are usually derived from experimental studies conducted over a limited P-T range, with uncertainties increasing as temperature and pressure increase, because of inherent technical difficulties and decreasing number of studies. There is a discrepancy between numerous experimental studies at T < 400 °C and a handful of studies at magmatic-hydrothermal conditions. The low T (<400 °C) experiments suggested that Mo(VI) chloride complexes are unlikely to play a significant role in nature, whereas some studies in magmatic-hydrothermal conditions showed the predominance of Mo(VI) chloride complexes. In an attempt to resolve these inconsistencies, we investigated Mo(VI)-Cl complexations from 350 to 750 °C (500 to 2000 bar) using ab initio molecular dynamics (MD) simulations. The simulations reveal that in high-temperature magmatic-hydrothermal fluids (750 °C, 2000 bar), a tetrahedral Mo(VI) chloride complex, MoO2(OH)Cl(aq), becomes stable; this coordination differs from the octahedral Mo(VI)-chloride complexes identified in highly acidic sub-critical fluids. The ab initio thermodynamic integration method was employed to calculate the equilibrium constants of the reaction.MoO2(OH)Cl(aq) + H2O = MoO2(OH)2(aq) + HCl(aq)Thermodynamic modelling of the speciation using the calculated equilibrium constants shows that at lower temperatures (350 °C, 500 bar), Mo(VI)-chloro complexes are only important at highly acidic conditions (pH < 1), in accordance with previous studies. However, the MD results show that at higher temperatures, Mo-chloro-complexes may be important in transporting Mo in mildly acidic conditions up to the quartz-muscovite-K-feldspar buffered pH. Hence, Mo(VI) chloro-complexes could play an important role in the formation of magmatic-hydrothermal Mo deposits (e.g., Climax-type Mo deposits). We conclude that the mode of metal transport can change dramatically at high temperatures as a result of coordination changes caused by changes in the solvation properties of water; and that quantitative MD is an important tool to support the interpretation of scant experimental results available for magmatic-hydrothermal systems.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Chalcocite Cu2S solubility in aqueous sulfide and chloride fluids. Thermodynamic properties of copper(I) aqueous species and copper transport in hydrothermal systems

The most important Cu reserves, including ores of porphyry deposits, were formed by Cl- and S-bearing hydrothermal fluids transporting and depositing metals within the magmatic-hydrothermal ore systems. In order to determine the state of Cu in the fluids, we measured the solubility of chalcocite Cu2S at 350 and 450 °C, 500 and 1000 bar. The concentration of H2S varied from 0.08 to 0.4 m [mol⋅(kg H2O)−1], NaOH – from 0 to 0.2 m, NaCl + HCl – from 0.01 to 4 m. The solubility of chalcocite can be described by the reactions:0.5 Cu2S(cr) + 0.5 H2S(aq) = CuHS(aq) log K°CuHS = −3.99 ± 0.07 (350 °C/500 bar)−4.61 ± 0.09 (450 °C/500 bar), −4.26 ± 0.11 (450 °C/1000 bar);0.5 Cu2S(cr) + 0.5 H2S(aq) + HS− = Cu(HS)2− log K°Cu(HS)2- = −0.85 ± 0.08 (350 °C/500 bar)−0.65 ± 0.11 (450 °C/1000 bar);0.5 Cu2S(cr) + HCl(aq) = CuCl(aq) + 0.5 H2S(aq) log K°CuCl = −3.11 ± 0.70 (450 °C/1000 bar);0.5 Cu2S(cr) + HCl(aq) + Cl− = CuCl2− + 0.5 H2S(aq) log K°CuCl2- = 0.33 ± 0.19 (450 °C/1000 bar)These results, together with data of Rubtsova et al. (2023) on the coupled solubility of Cu(cr) and Ag in chloride fluids, demonstrated that a simple extended Debye-Hückel equation describe activity coefficients of charged species at any NaCl concentration up to the solubility limit at near- and supercritical PT-parameters; activity coefficients of neutral species can be calculated with Setchenov parameter equal to zero. The Cu2S(cr) solubility in sulfide-chloride-bearing fluids is consistent with the formation of Cu-Cl complexes which implies that mixed Cu-HS-Cl complexes can be neglected. The experimental values pooled with the reliable literature data were fitted to the HKF (Helgeson-Kirkham-Flowers) model equation of state (EoS). The obtained HKF EoS parameters allow to predict thermodynamic properties of CuHS(aq), Cu(HS)2−, and CuCl(aq) from 25 to 700 °C and from Psat. to 2000 bar, and up to 1000 °C/5000 bar for CuCl2−. The AD (Akinfiev and Diamond) EoS parameters were derived for CuCl(aq). The AD EoS can be used to calculate the CuCl(aq) Gibbs free energy in the low-density fluids (ρH2O < 0.35 g⋅cm−3) up to 700 °C. The obtained thermodynamic data imply that CuCl2− predominates in high-density ore-forming fluids (ρH2O > 0.35 g⋅cm−3) of total chloride content >0.1 wt% up to the NaCl saturation limit. The hydrosulfide complexes are the main forms of Cu occurrence in the fluid with concentration of dissolved chlorides <0.1 wt% equilibrated with silicate mineral assemblages (e.g., K-feldspar-muscovite-quartz, pH is near-neutral). However, their concentration in the presence of Cu-bearing sulfides is low (< 0.5 ppm at 500 °C/1500 bar) and decreases with temperature decreasing.

Geochemistry and origin of hydrothermal apatite in Carlin-type Au deposits, southwestern China (Gaolong deposit)

Apatite can record the evolution of magmatic–hydrothermal ore systems, but it is unclear whether it is suitable for investigating low-temperature, sediment-hosted hydrothermal ore systems. To fill the knowledge gap. we investigated the Gaolong Carlin-type Au deposit in the Guangxi Zhuang Autonomous Region, southwestern China (Youjiang Basin). Results show that the hydrothermal apatite occurs in quartz–sulfide ores, has an intact crystal form and homogeneous texture, and is F-rich (2.77–3.67 wt%) and Cl-poor (<0.01 wt%). The apatite is enriched in rare earth elements (REEs; ΣREE = 739–2481 ppm) and has middle-REE-enriched chondrite-normalized REE patterns with positive Eu anomalies (δEu = 1.35–2.00). These REE features were inherited from a reducing hydrothermal system in which anion complexation was dominant. The apatite has high Sr contents (up to 0. 9 wt%) and relatively uniform initial 87Sr/86Sr ratios (0.709055–0.709208), suggesting that Sr was leached or derived from the deep continental crust. In situ U–Pb dating of 156.8 ± 8.3 Ma for apatite shows that the Late Jurassic–Early Cretaceous was an important time for the formation of Carlin-type Au deposits in the Youjiang Basin. The Au mineralization was associated with far-field tectonism and deeply sourced hydrothermal ore-forming fluids. Hydrothermal apatite can be used to fingerprint low-temperature, sediment-hosted hydrothermal ore systems.

Tin-bearing magnetite with nanoscale Mg-Si defects: Evidence for the early stages of mineralization in a skarn system

Tin-bearing magnetite is reported from several types of magmatic-hydrothermal ore deposits. The question of whether tin is incorporated within solid solution, as Sn4+, or as nanoinclusions remains open, however. We report a micron- to nanoscale investigation of Sn (Mg, Si)-bearing magnetite from serpentinite in the Dulong Zn-Sn-In skarn, South China, with the dual aims of understanding the mechanisms involved in accommodating Sn and associated elements into the Fe-oxide, and the inferences that this carries for constraining the early stages of skarn formation. Magnetite preserves a range of textures that record the evolution of metasomatism during prograde growth of grain cores and retrograde rim replacement. Observations reveal the presence of chondrodite and sellaite (MgF2) as nanoscale inclusions preserved in magnetite. This implies initiation of the Dulong mineralizing system during a humite-bearing, magnesium skarn stage. Magnesium-Si defects, forming along (110) planes prior to Sn-enrichment, are recognized for the first time. Release of high volatile, F-rich fluids is interpreted to lead to precipitation of cassiterite inclusions along &amp;lt;111*&amp;gt; directions in magnetite.

Toward quantitative experiment using hydrothermal diamond anvil cell: Solubility of sylvite up to 1.6 GPa

Potassium chloride is a common component in geofluids and sylvite is a broadly observed daughter mineral in fluid inclusions along with halite, especially in skarn- and porphyry-type deposits. Interpretation of fluid inclusion microthermometric results requires knowledge of high pressure and temperature (P-T) solubility of salts, which is, however, incomprehensive for sylvite in the KCl-H2O system. In this study, the high P-T solubility of sylvite is determined within the compositional (salinity) range of 38–62 wt% KCl, using a new method that applies the three-dimensional reconstruction function of confocal laser fluorescence microscopy (CLFM) to hydrothermal diamond anvil cell (HDAC) experiments. This method is used to retrieve the volumes of phases (i.e., sylvite, aqueous solution and bubble) in the sample chamber of HDAC and thus to achieve quantitative analysis of sylvite solubility at elevated P-T. Combining experimental results in this study with those published in previous literature, the P-T dependence of the solubility of sylvite is retrieved and applied to data interpretation of microthermometric results. For inclusions containing both halite and sylvite daughter minerals, the commonly used approach for evaluating fluid salinity and P is to simplify the inclusions as the binary NaCl-H2O system. This simplification would lead to underestimation of fluid salinities by up to ~34% and homogenization P by up to 0.25 GPa, for fluid inclusions having relatively high T of daughter mineral dissolution, which are typical of magmatic-hydrothermal ore deposits.

The trace element chemistry of quartz in carbonatite-related REE deposits: Implication for REE exploration

Quartz is a common gangue mineral in many magmatic-hydrothermal ore deposits (e.g., porphyry, epithermal and skarn deposits) and its trace element composition was widely used to constrain ore genesis. Quartz is also a common phase in carbonatite-related rare earth element (REE) deposits, but its geochemical characteristics and significance for ore genesis have not previously been documented in this environment. In this contribution, we report in-situ LA-ICP-MS trace element compositions of quartz from seven carbonatite-related REE deposits, namely the Maoniuping, Lizhuang and Dalucao deposits in the Mianning-Dechang REE metallogenic belt, SW China and the Miaoya, Taipingzhen, Huangshuian and Huanglongpu deposits in the Qinling REE metallogenic belt, Central China. Rare earth element mineralization in these deposits is associated with well-developed hydrothermal systems in which the dominant REE minerals, bastnäsite, parisite, and monazite, are intergrown with aegirine-augite, arfvedsonite, calcite, fluorite, barite, and quartz. The results show that quartz grains from the carbonatite-related REE deposits are characterized by low budgets of trace elements, of which Al is generally less than 50 ppm whereas others are generally less than 10 ppm. In general, the trace element concentrations in quartz from different deposits are similar (the total concentration is commonly <50 ppm), and are distinguishable from those of quartz in other magmatic-hydrothermal systems. The low concentration of trace elements in carbonatite-related REE deposits is controlled by the unusual nature of carbonatitic magmas and their derived fluids, and thus can be used for defining the affinity of hydrothermal REE systems that are of unknown origin. Significantly, the trace element chemistry of quartz samples collected from a cross-section in the open pit of the Maoniuping REE deposit displays a systematic spatial variation such that the concentrations of Al, Ti, Na, and K decrease with increasing distance of the samples from carbonatites. This spatial variation correlates well with the variation of physio-chemical conditions during the evolution of ore-forming fluids. This implies that the geochemistry of quartz can be used as a vector detecting the concealed carbonatite intrusions in carbonatite-related REE deposits.

Sulfur and chlorine budgets control the ore fertility of arc magmas

Continental arc magmas supply the ore-forming element budget of most globally important porphyry-type ore deposits. However, the processes enabling certain arc segments to preferentially generate giant porphyry deposits remain highly debated. Here we evaluate the large-scale covariation of key ore-forming constituents in this setting by studying silicate melt inclusions in volcanic rocks from a fertile-to-barren segment of the Andean Southern Volcanic Zone (33–40 °S). We show that the north-to-south, fertile-to-barren gradient is characterized by a northward increase in S and Cl concentrations and a simultaneous decrease in Cu. Consequently, we suggest that the concentration of S and Cl rather than the concentration of ore metals regulates magmatic-hydrothermal ore fertility, and that the loss of volatiles prior to arrival in the upper crust impacts ore-forming potential more than magmatic sulfide saturation-related ore metal scavenging.

The Miwah high sulphidation epithermal Au–Ag deposit, Aceh, Indonesia: Dynamics of hydrothermal alteration and mineralisation interpreted from principal component analysis of lithogeochemical data

The principal component analysis (PCA) of whole-rock major and trace element data has unravelled some of the dominant geochemical processes and identified the most relevant component as an exploration vector for many types of magmatic-hydrothermal ore deposits. An important exception is the high-sulphidation (HS) epithermal class of deposits. Here, the results of a PCA of different segments of the Miwah HS epithermal Au–Ag deposit in Indonesia show that the geochemical signature of the deposit varies from a tight group of Au, Ag, Cu and As in PC1 for the predominantly hypogene main zone to a moderately correlated assemblage of Au, As, Cu and Sb in PC1 for the variably oxidised rocks along the southern periphery to an association of Au, Ni, Cr, Co and Mo in PC1 for the overlying rocks in the north. PC2 in all zones represents the largest variability of Pb and Zn and has moderate contributions from Au and As in the periphery and the highest Au contribution in the north. By contrast, PC3 in the main and peripheral zones, shows a high variability of Cu. Representatives of auriferous vuggy-massive silica rocks from the periphery are in equilibrium with hydrothermal fluids with δ18O values of +6.18, +7.48 and +9.28‰, whereas oxidised, silicified rocks from the analogous northern zone are in equilibrium with hydrothermal fluids with δ18O values of +8.0 and +8.6‰. The results of the above PCA, complemented by centred-log-ratio bivariate diagrams of grade-scaled variables, mineralogical relationships and oxygen isotopes, are interpreted to indicate that PC1 in the main zone is consistent with the deposition of Au and related metals from the condensation of magmatic gas mixtures during the early-stage silicification of protoliths. By contrast, PC2 is related to late-stage Pb- and Zn-rich fluids that migrated away from the main conduits. A hybrid process, intermediary between PC1 and PC2, leached Cr, Ni and Co from the adjacent ultramafic rocks and superimposed the metals on the Au + Cu mineralised rocks. Processes related to PC3 and other smaller PC occurred at later and lower temperature stages in which steam-heated oxidation led to the local destruction of sulphide and arsenide phases and the resulting acid fluids decoupled Au-Ag alloys and enargite to produce separate Au- and Ag-based minerals and covellite. The PCA of individual alteration types shows metal associations that mimic those of the main-stage mineralisation, thereby providing a useful exploration tool. The ability of PCA to separate Au and the associated metals from the Cr + Ni + Co assemblage can be used to identify areas that have been affected by hyperacid fluids originating from epithermal systems; hence, such areas become exploration targets not only in the vicinity of Miwah but also throughout Sumatra or elsewhere.

Evolution of the Tepeoba porphyry-skarn Cu-Mo-Au deposit, NW Turkey: New mineralogical and geochemical findings

This study recently investigates the evolution of the Tepeoba porphyry Cu-Mo-Au deposit (Cu ∼ 3 wt%, Mo ∼ 0.16 wt%, Au ∼ 10 ppm) with associated skarn prospects in the Biga Peninsula (NW Turkey) using the geochemical and mineralogical data. Ore-related alteration types which include biotite (±muscovite)-K-feldspar at the center of the mineralization zone, actinolite-albite alteration, and outer chlorite-epidote-calcite mineral facies, are observed in the Tepeoba mine area along the southern contact between the Karakaya Complex (mainly hornfels at the mine area) and porphyry Eybek intrusion. Porphyry style alteration pattern has been detected recognized in the porphyritic microgranite host rock. On the other hand, the calcareous and carbonate as well as the mineralized hornfels refer to the skarn style alteration pattern associated with a porphyritic felsic stock. Sulfide-rich stockwork and mineralized brittle brecciated, and fracture zones are associated with these hydrothermally altered wallrocks. Ore mineralogy includes chalcopyrite, molybdenite, pyrrhotite, pyrite, subordinate cubanite and native gold. These minerals are replaced by marcasite, hematite, and goethite in the oxidized cap rocks. EPMA data of the alteration and sulfide minerals revealed that Cu-, Fe-, Mo-sulfides partially replace biotite ± muscovite-K-feldspar/actinolite-albite and chlorite-epidote-calcite alteration facies. The estimated ore formation conditions returned wide range of temperatures of 546–471 °C (biotite thermometry) to 340–264 °C (chlorite thermometry), at which sulfides had most likely precipitated while the oxygen fugacity of the magmatic-hydrothermal ore fluids was gradually lowered and developed during the stage of retrograde metasomatic reactions. Chalcopyrite and cubanite was likely to have formed via un-mixing of intermediate solid solution (iss) under oxidizing hydrothermal conditions. Under these conditions, thiosulfide complexes could have been responsible for the transport and deposition of gold. This also refers to native gold, which could have been liberated from sulfide ores (i.e., pyrite) in the oxidation zone due to the processes of supergene enrichment.

Age and mineralization processes of decratonic lode gold deposits in the southern North China Craton: Constraints from trace elements, in-situ S-Pb isotopes and Rb-Sr geochronology of pyrite from the Chen’er gold deposit

The Chen’er gold deposit, with 27 tons of known Au reserves, is one of the typical gold deposits in the Xiaoqinling district along the southern part of the North China Craton. In order to better understand the gold precipitation time and mineralization process, we report in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) trace element analyses, elemental maps, and the Rb-Sr and S-Pb isotopic composition of sulfides from the Chen’er gold deposit with a view to gain insights into the mineralization time and process. Trace element analyses and elemental maps of pyrite show that the content of As is low and has no correlation with Au, indicating As is unlikely to have controlled the Au incorporation into pyrite. Gold mainly occurs as solid solutions in the Py Ⅰ and Py Ⅱ, and as Au-Ag and/or Au-Te sub-micrometer inclusions in Py III. The Rb-Sr isotopic data on sulfides from the Chen’er deposit yielded an isochron age of 129.2 ± 3.4 Ma, which overlaps with the previously reported 40Ar/39Ar age of hydrothermal mica, indicating that the gold mineralization occurred in the Early Cretaceous. The in-situ sulfur isotope values of sulfides from stage Ⅱ to stage III fluctuate significantly (the average value decreases from 4.95 to −0.24‰) and were affected by physicochemical parameters of the mineralization process, especially ƒO2. In addition, the δ34S values are in a narrow range (-1.34 to 5.24‰), indicating a magmatic source for the sulfur. In-situ Pb isotopic compositions of the pyrite and galena of stage Ⅱ and stage III are relatively constant with 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values in the range of 37.264–37.595, 15.364–15.423, 16.992–17.078, respectively, indicating a mixed mantle-crust origin. Based on the new geochronological, geochemical and isotopic data presented in this study, and combined with those from previous studies, we suggest that the lode gold deposits in the Xiaoqinling district are magmatic hydrothermal ore deposits, with metal and ore-forming fluid originating primarily from the mantle, coupled with the cratonic reactivation during the Early Cretaceous.

LA–ICP–MS U–Pb Dating, Elemental Mapping and In Situ Trace Element Analyses of Cassiterites from the Gejiu Tin Polymetallic Deposit, SW China: Constraints on the Timing of Mineralization and Precipitation Environment

As a major constituent in magmatic–hydrothermal ore deposits, cassiterites, with moderate amounts of U and low Pb, can be dated with U–Pb geochronology. The tetragonal lattice structure makes cassiterites capable of incorporating dozens of elements within its crystal lattice (e.g., Fe, Ti, W, Zr, Hf, Ta, Nb, Mn, Sc, V, and Sb). Variations of these elements record information of potential elemental substitution mechanisms and precipitation environments of cassiterites. In this study, we collected cassiterite grains from four different ore styles of the Gejiu tin polymetallic deposit to perform LA–ICP–MS U–Pb dating, multiple element mapping, and in situ trace element analysis on these cassiterites. Systematic U–Pb dating yielded Tera–Wasserburg lower intercepted ages at around 85 Ma, coinciding with zircon U–Pb ages of regional Late Yanshanian granitoids, within their respective analytical uncertainties. Such age coincidence, combined with the spatial association, suggests that tin mineralization may be genetically related to the Late Cretaceous granitic magmatism. Multielemental mapping shows that the distribution of Nb, Ta, and Ti in the cassiterite grains correlates well with the regular oscillatory zoning patterns in cathodoluminescence (CL) images. The relatively high Sb, Fe, W, Ga, and U concentrations control the dark luminescing domains in these cassiterite grains. The systematic variations in chemical compositions suggest that trace elements such as Sc, V, Fe, and Ga incorporate in cassiterites via coupled substitutions of Sc3+ + V5+ ↔ 2 (Sn, Ti)4+, Fe3+ + Ga5+ ↔ 2 (Sn, Ti)4+ and Fe3+ + OH– ↔ Sn4+ + O2– or Fe3+ + H+ ↔ Sn4+. The covariation of redox sensitive elements such as W, U, Fe, and Sb indicates that the ”tin-granite” type of cassiterites were formed under an oxidized state whereas cassiterites from skarn, massive sulfide, and oxidized ore styles were precipitated in a reducing environment.

Trace element and isotopic (S, Pb) constraints on the formation of the giant Chalukou porphyry Mo deposit, NE China

Abstract Porphyry-type Mo deposits have supplied most of the Mo to the world. However, the source of the Mo and the controls on its enrichment in such deposits is still a matter of great debate. In this study, we present in situ trace element and isotopic data for a giant porphyry Mo deposit (the Chalukou Mo deposit in NE China) and use these data to address these issues. Three primary paragenetic stages of mineralization were recognized at Chalukou: (Stage I) K-feldspar + quartz + minor pyrite (Py-I) + minor molybdenite (Mol-I); (Stage II) quartz + sericite + molybdenite (Mol-II) + pyrite (Py-II); (Stage III) quartz + chlorite + epidote + fluorite + pyrite (Py-III) + galena + sphalerite + minor chalcopyrite. The bulk of the molybdenite was deposited in Stage II. In situ S isotope analyses of the sulfide ores show that the δ34S values vary from –5.2 to +7.8‰ (mean = +2.9‰) and correspond to δ34SH2S values from –2.4 to +3.3‰ (mean = +1.1‰). These values are consistent with a magmatic source for the sulfur. In situ Pb isotope compositions of the sulfide ores are almost identical to those of the local Mesozoic granites and other magmatic-hydrothermal ore deposits in this region, suggesting a close genetic association between the Mo mineralization and felsic magmatism. Pyrite from the three stages of mineralization differs significantly in its trace element composition. The first generation, Py-I, has a high Cu content (8.7 ± 49.6 ppm; where the first value is the median and the second is the standard deviation) and Mo content (6.9 ± 3.8 ppm). Pyrite-II has the lowest Cu concentration (1.3 ± 2.1 ppm) and a relatively high Mo concentration (5 ± 128 ppm), and Py-III has a high Cu content (8.7 ± 37.1 ppm) but the lowest Mo content (0.05 ± 5.7 ppm). From this, we infer that pyrite recorded the chemical evolution in the Mo/Cu ratio of the ore fluid and that this ratio reached a maximum in Stage II, coinciding with the widespread saturation of the fluid in molybdenite. The evolution of the Mo/Cu ratio in pyrite implies that the fluid was undersaturated in chalcopyrite at the high temperature of Stage I, despite the Cu concentration of the fluid apparently being at its high level, and chalcopyrite only saturated later, at a lower temperature. Molybdenite, however, because of its lower solubility, saturated early (Stage I) and in the subsequent stage (Stage II) was supersaturated in the fluid. There is a significant enrichment of Mo in the syn-ore intrusions at Chalukou compared to the pre-ore monzogranite. The very low Sr/Y ratios for the Chalukou syn-ore intrusions, which are in sharp contrast to the high Sr/Y ratios of the pre-ore monzogranite and those of porphyries related to Cu deposits, suggest that fractional crystallization of plagioclase may have been a key factor in generating the syn-ore magmas. Molybdenum is a highly incompatible metal and will concentrate in the crust, and assimilation of old continental crust, therefore, may explain the Mo enrichment of the syn-ore intrusions and ultimately the formation of the giant Chalukou deposit.

Timing of magmatic-hydrothermal activity in the Variscan Orogenic Belt: LA-ICP-MS U–Pb geochronology of skarn-related garnet from the Schwarzenberg District, Erzgebirge

Here, we present in situ U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338–331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (~ 327–310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (~ 310–295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time–space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by ~ 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.

The Shepherd Mountain iron ore deposit in Southeast Missouri, USA – An extension of the Pilot Knob magmatic-hydrothermal ore system: Evidence from iron oxide chemistry

The Southeast Missouri Iron Metallogenic Province in the Midcontinent USA contains seven major and several minor IOA/IOCG-type deposits and a series of shallow vein-type deposits/prospects, all of which are spatially and temporally associated with early Mesoproterozoic (1500–1440 Ma) magmatism in the St. Francois Mountains terrane. One of the vein-type deposits is the Shepherd Mountain deposit, which consists of two northeast-trending ore veins dominated by magnetite and lesser amounts of hematite. Here we report the findings of a study that investigates the origin of the Shepherd Mountain deposit and a possible genetic link to the nearby (i.e., <5 km away) magmatic to magmatic-hydrothermal Pilot Knob ore system that comprises the massive-to-disseminated Pilot Knob Magnetite deposit and the overlying bedded and brecciated Pilot Knob Hematite deposit.Petrographic observations, whole-rock data and the trace element and Fe isotope composition of magnetite and hematite show that the Shepherd Mountain deposit formed from at least five pulses of magmatic-hydrothermal fluids with different compositions and physicochemical parameters. Integration of the data for the Shepherd Mountain deposit with new and published data from the Pilot Knob Magnetite and Pilot Knob Hematite deposits shows that the three deposits are genetically linked through two local faults. The Ironton and Pilot Knob faults provided fluid pathways that connected the Pilot Knob Magnetite deposit to the shallower Shepherd Mountain and Pilot Knob Hematite deposits. Consequently, we argue that the Shepherd Mountain and Pilot Knob Hematite deposits are near-surface extensions of the same magmatic to hydrothermal plumbing system that formed the Pilot Knob Magnetite deposit at depth.

The efficiency of copper extraction from magma bodies: Implications for mineralization potential and fluid-silicate melt partitioning of copper

Abstract Multiple factors may downgrade the mineralization potential of an intermediate-felsic intrusion, such as the commonly invoked inefficient fluid exsolution and lack of ore-forming species (metals and their ligands) in magmas. However, other factors may affect the mineralization potential of a magma body but have poorly understood roles in the formation of magmatic-hydrothermal ore deposits. Here, we present a comparison between two Cu mineralizing plutons and a Cu-poor, Fe mineralizing pluton in the Edong district. Efficient fluid exsolution and extraction occurred during the solidification of all three plutons, as evidenced by extensive skarn alteration around them. The results show that the oxidation state of the three plutons is similar (within a range of ~ΔNNO+0.9 to ΔNNO+2.5). A systematic comparison of the Cu contents of a certain suite of minerals of the three plutons shows that the Cu concentrations of all minerals in the Cu mineralizing plutons are lower than those of the Cu-poor Fe mineralizing pluton. This indicates that the Cu mineralizing plutons underwent more efficient copper extraction. Thus, igneous crystals with anomalously low Cu contents may potentially be used as a tool to identify Cu mineralizing magmatic units in a deposit with multiphase intrusions. We suggest that the inefficient copper extraction from plutons may be ascribed to the lack of reduced S species during fluid exsolution or different evolution paths of Cu and Cl during magma crystallization.

Rutile and Chlorite Geochemistry Constraints on the Formation of the Tuwu Porphyry Cu Deposit, Eastern Tianshan and Its Exploration Significance

The chemical composition of rutile has been used as an indicator in magmatic and metamorphic-related diagenetic systems, but rarely in porphyry-style ore systems. The Tuwu deposit (557 Mt at 0.58% Cu) is a large porphyry-style Cu mineralization in Eastern Tianshan, Xinjiang, with typical disseminated, stockwork mineralized veins hosted in tonalite and diorite porphyry, and to a lesser extent in volcanic rocks of the Qi’eshan Group. We first present determination of rutile minerals coupled with chlorite identified in mineralized porphyries from Tuwu to reveal their geochemical features, thus providing new insights into the ore-forming processes and metal exploration. Petrographic and BSE observations show that the rutile generally occurs as large crystals (30 to 80 µm), in association with hydrothermal quartz, chlorite, pyrite, and chalcopyrite. The rutile grains display V, Fe, and Sn enrichment and flat LREE-MREE patterns, indicating a hydrothermal origin. Titanium in rutile (TiO2) is suggested to be sourced from the breakdown and re-equilibration of primary magmatic biotite and Ti-magnetite, and substituted by Sn4+, high field strength elements (HFSE; e.g., Zr4+ and Hf4+), and minor Mo4+ under hydrothermal conditions. The extremely low Mo values (average 30 ppm) in rutile may be due to rutile formation postdating that of Mo sulfides (MoS2) formation in hydrothermal fluids. Chlorite analyses imply that the ore-forming fluids of the main stage were weakly oxidized (logfO2 = −28.5 to −22.1) and of intermediate temperatures (308 to 372 °C), consistent with previous fluid inclusion studies. In addition, Zr-in-rutile geothermometer yields overestimated temperatures (&gt;430 °C) as excess Zr is incorporated into rutile, which is likely caused by fast crystal growth or post crystallization modification by F-Cl-bearing fluid. Thus, application of this geothermometer to magmatic-hydrothermal ore systems is questionable. Based on the comparison of rutile characteristics of porphyry Cu with other types of ore deposits and barren rocks, we suggest that porphyry Cu-related rutile typically has larger grain size, is enriched in V (average 3408 ppm, compared to &lt;1500 ppm of barren rocks) and to a lesser extent in W and Sn (average 121 and 196 ppm, respectively), and has elevated Cr + V/Nb + Ta ratios. These distinctive signatures can be used as critical indicators of porphyry-style Cu mineralization and may serve as a valuable tool in mineral exploration.