Deposits In South China Research Articles

Hydrothermal alteration of allanite promotes the generation of ion-adsorption LREE deposits in South China

Hydrothermal alteration of allanite promotes the generation of ion-adsorption LREE deposits in South China

Hydrothermal alteration and element migration in the Egongtang uranium deposit, central Nanling Range, South China

AbstractHydrothermal alteration records fluid–rock interactions and can therefore be used to constrain element migrations during mineralization. Although hydrothermal alteration is widely developed in hydrothermal vein-type uranium deposits in South China, consideration of elemental mass changes during alteration has not been examined. The Egongtang uranium deposit in the central Nanling Range is mainly hosted by the Qingzhangshan granite in South China, and was strongly altered by K-feldspar, quartz, chlorite, illite, haematite, pyrite and carbonates. The alteration section can be divided into five horizontal zones: fresh granite (Zone V), a distal alkaline alteration zone (Zone IV), a chlorite-rich zone (Zone III), a close-to-ore sericite/illite alteration zone (Zone II) and a central mineralization zone with strong haematitization (Zone I). Whole-rock geochemistry of the altered samples indicates that from Zone IV to Zone I, the content of SiO2 and U increases significantly. The mass gains of SiO2, MgO and Fe2O3 were proportional to the concentration of U. The content of trace elements (such as Ba, K, La, Ce, Pr, Sr, P, Eu, etc.) gradually decreases from Zone V to Zone I. The rare earth elements manifest a decrease in light rare earth elements and a slight increase in heavy rare earth elements accordingly from Zone V to Zone I. This study shows that the ore materials of the Egongtang deposit were mainly derived from the Qingzhangshan granites. In the early alkali alterations, large amounts of U were partitioned into the fluids. In the ore-forming stage, ores precipitated accompanied by acid metasomatism such as chloritization, haematitization and carbonation.

Geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Cretaceous igneous rocks in the Zhilingtou area, SE China, and their geological significance

The Zhilingtou deposit is one of the typical porphyry Mo deposits in South China. Disseminated and veinlet Mo ores occur in the roof pendant of the granite porphyry or the contact between the granite porphyry and the Badu Group gneisses, and some orebodies are cut by adamellite dikes. Here we present zircon LA–ICP–MS U–Pb dating yielding crystallization ages of 97 ± 1 Ma for the adamellite and 110 ± 1 Ma for the graniteporphyry. The adamellite and granite porphyry are metaluminous-peraluminous and belong to the high-K calc-alkaline-shoshonite type. They are characterized by high K2O, K2O + Na2O, FeOT, FeOT/(FeOT + MgO) and zircon saturation temperatures (784–817 °C); low MgO, TiO2, P2O5, Sr/Y and La/Yb values; enrichment in trace elements Rb, Th, U, La, Ce and other large ion lithophile elements; and depletion in Ti, Nb and P and other high field strength elements, revealing a highly fractionated A2-type affinity, formed in an extensional tectonic setting. The zircon εHf(t) values of the adamellite and granite porphyry range from −6.3 to −3.2 and from −10.0 to −4.1, respectively, with two-stage Hf model ages (TDM2 (Hf)) of 1559–1367 Ma and 1802–1430 Ma, respectively. The εNd(t) values of the adamellite and granite porphyry range from −19.0 to −11.5 and from −7.5 to −6.2, respectively, with two-stage Nd model ages (TDM2 (Nd)) of 2435–1835 Ma and 1522–1413, respectively, suggesting that they had mixed sources and were mainly formed by partial melting of a late Paleoproterozoic basement in the lower crust. In combination with previous studies, it can be concluded that after the Early Cretaceous, the subduction orientation of the Izanagi plate changed from oblique to parallel with respect to the continental margin, the slab rollback resulted in large-scale lithospheric extension and thinning, asthenospheric upwelling, and the basaltic magma underwent underplating and induced crustal melting, and finally formed the A-type granitic intrusions and related Mo mineralization systems.

Mineral textures, mineral chemistry and S isotopes of sulphides from the Tianbaoshan Pb–Zn–Cu deposit in the Sichuan–Yunnan–Guizhou triangle: implications for mineralization process

AbstractThe carbonate-hosted Pb–Zn deposits in the Sichuan–Yunnan–Guizhou (SYG) triangle region are important Indosinian deposits in South China. The Tianbaoshan deposit is a typical large Pb–Zn deposit in the SYG area and occurs as pipe-like type, hosted by Sinian dolostone. It contains ∼26 Mt Zn–Pb ore (7.76–10.09 % Zn, 1.28–1.50 % Pb and 93.6 g t−1 Ag) and >0.1 Mt Cu ore (2.55 % Cu). In this study, the detailed mineral textures, mineral chemical and sulphur isotopic compositions of the various sulphides have been analysed to constrain the abnormal enrichment mechanism and mineralization relationship. Four mineralization stages have been recognized: Stage 1, minor early pyrite (Py1) with relics and infill of intergranular dolomite or quartz grains; Stage 2, Cu mineralization with coarse-grained, elliptical crystal chalcopyrite (Cp1); (3) Stage 3, Zn mineralization with dark fine-grained sphalerite (Sph1) and light coarse-grained sphalerite (Sph2); and (4) Stage 4, as represented by a quartz–calcite assemblage with galena, minor pyrite (Py2) and chalcopyrite (Cp2). The petrography of the sulphide minerals (Py1, Cp1, Sph1 and Sph2) demonstrates a mutual inclusion relationship. The nature of this relationship from core to rim and their similar sulphur isotope values (5.5–8.3 ‰) indicates a single sulphur source, suggesting that the different mineralization types are the result of different stages of a continuous hydrothermal system. Sphalerite geothermometer study suggests that sphalerite in the Tianbaoshan deposit formed in a low-temperature (<200 °C) hydrothermal system. The low concentrations of Mn and In, low In/Ge ratios and high Fe/Cd ratios in the sphalerite are consistent with those of Mississippi Valley-type (MVT) deposits, but different from those of magmatism-related deposits (e.g. epithermal, skarn and VMS deposits). The positive δ34S values for Py1 (5.1–7.9 ‰), Cp1 (5.1–7.2 ‰), Sph1 (4.7–7.4 ‰), Sph2 (3.9–8.7 ‰), Py2 (4.4–9.3 ‰) and Cp2 (5.0–6.8 ‰) indicate a sulphur source from thermochemical reduction of coeval seawater sulphate. Widely developed dissolved textures (caverns and breccias) with massive sulphide infillings and deformed host rock remnants suggest that replacement of host dolostones by ore fluids was volumetrically significant and the ore formed nearly simultaneously with the cavities. The Tianbaoshan deposit is a typical MVT deposit, which resulted from mixing of a H2S-rich fluid and a metal-rich fluid, with thermochemical sulphate reduction occurring before ore precipitation rather than during ore precipitation.

Zircon texture and composition fingerprint HREE enrichment in muscovite granite bedrock of the Dabu ion-adsorption REE deposit, South China

More than 90% of the global heavy rare earth elements (HREE) supply is currently from the ion-adsorption REE deposits in South China. However, the mechanism of HREE enrichment in granite bedrock of REE mineralized weathering crusts remains poorly understood. In this study, we present a comprehensive investigation on zircon texture and composition as a proxy to unravel the magmatic-hydrothermal processes which resulted in HREE enrichment of the Dabu muscovite granites. Our results show that zircon in the Dabu muscovite granites can be classified into three types based on morphology, internal structure and chemical compositions. The early-magmatic zircon grains (ZrnI) are prismatic crystals with bright oscillatory zoning, and crystallized early in the granitic magma. The late-magmatic zircon grains (Zrn-II) occur as murky euhedral or subhedral crystals or as overgrowth on ZrnI, and crystallized from a volatile-enriched residual melt in the late magmatic stage. The hydrothermal zircon (Zrn-III) is anhedral without oscillatory zoning, possibly formed via intense hydrothermal alteration of Zrn-II in the hydrothermal stage. Raman spectra show a decrease in degree of crystallinity from Zrn-I to Zrn-III. In-situ compositional and isotopic analyses suggest that zircon geochemistry is a good proxy for the magmatic-hydrothermal processes of the host granite. Zircon UPb dating yields similar ages of 155.3 ± 1.4 Ma (2σ) and 154.1 ± 1.0 Ma (2σ) for Zrn-I and Zrn-II, respectively. The decrease of Zr/Hf from Zrn-I to Zrn-III indicates that the Dabu muscovite granites underwent magmatic to hydrothermal evolution. Furthermore, the decreasing δ18O values of Zrn-I to Zrn-III suggest the increasing involvement of meteoric water in the fluids. Significantly, compared with Zrn-I and Zrn-II, Zrn-III has the highest U, Hf and REE, in particular HREE. The LREE/HREE ratios decrease with the increase of REE contents and the decrease of Zr/Hf ratios from early to late zircon of the granite bedrock of ion-adsorption HREE deposits, which is significantly different from those of carbonatite-alkaline related REE deposits. The significant increase of HREE concentrations in early to late zircon should be ascribed to the introduction of HREE-rich fluids during magma evolution. Therefore, we propose that the HREE-rich fluids from magma exsolution metasomatized the granites to achieve further enrichment of HREE, which is an important prerequisite for the formation of ion-adsorption HREE deposits. Accordingly, we argue that zircon composition can be used as a chemical indicator for the REE behaviors in magmatic-hydrothermal systems that are vital to the formation of REE deposits.

Origin and distribution of rare earth elements (REEs) in the soils of Meizhou City, southern China with high abundance of regolith-hosted REEs

Currently, regolith-hosted rare earth element (REE) deposits in South China are the major source of global REEs, particularly heavy REEs (HREEs). This type of REE resource contains abundant movable, bioavailable, and ion-exchangeable REEs and may act as an environmental hazard; however, little is known regarding REEs in soils from areas with abundant deposits. Here, we investigated the REEs in the soil, surface water and crops, collected from Meizhou City, Guangdong Province, South China, as a representative region with abundant regolith-hosted REEs. Mean REE concentration (ΣREE) in the soil (318 μg g−1) exceeded the local soil background (169 μg g−1). The LREE/HREE ratio ranged from 2.04 to 69.4 in soil samples, suggesting the enrichment of light REEs (LREEs) compared with HREEs. Most soil samples exhibited positive Ce anomalies and negative Eu anomalies. The highest ΣREE (1162 μg g−1) was detected in forest land, followed by garden land (793 μg g−1) and paddy field (519 μg g−1). According to the correlation analysis, pH as well as soil organic matter (SOM), Fe, and Mn content contributed to REE enrichment, while SOM and Mn content affected REE fractionation. REE enrichment in soil may be related to natural sources (e.g., parent materials) and anthropogenic activities (e.g., fertilizer application and coal combustion). The REE contamination of surface water was not serious. However, ΣREE in crops (7.08 μg g−1·dw) indicated that these elements migrate from soils to plants, producing potential adverse effects on crops and humans. These findings shed new light on the geochemical behavior of REEs in the environments with abundant regolith-hosted REEs and can serve as a reference to prevent and control REE contamination.

Indosinian magmatism and mineralization in the Banjiaoyuan tin deposit, middle Nanling Range, South China: Constraints from zircon and cassiterite U-Pb ages, geochemistry and Sr-Nd-Hf isotopic compositions

Indosinian W-Sn metallogeny associated with granites in South China has been reported in recent years by many researchers. However, the refined genetic relations between these deposits and the related granites remain unclear in most areas, and the tectonic setting of Indosinian magmatism and W-Sn mineralization are still widely controversial. The Banjiaoyuan tin deposit is located in the east vicinity of Yangmingshan pluton in the middle Nanling Range, South China. The granitic stocks in this deposit comprise two intrusive episodes, i.e., the first episode of medium-coarse-grained porphyritic tourmaline-biotite monzogranite (G1), and the second episode of micron-fine-grained tourmaline-muscovite monzogranite (G2). LA-ICP-MS zircon U-Pb dating gives weighted ages of 226.9 ± 1.6 Ma to 223.8 ± 1.9 Ma for G1, and 224.3 ± 2.0 Ma for G2, respectively. LA-MC-ICP-MS cassiterite U-Pb dating yields Tera-Wasserburg concordia age of 216.7 ± 2.4 Ma for tin mineralization. These ages indicate that both the granitic magmatism and related mineralization were initiated in the late Indosinian period. All the granites have high SiO2, Al2O3, P2O5 contents, and low TFeO, CaO, MgO, TiO2 contents, indicating that the granites are highly differentiated S-type granites, and furthermore, the high DI and Rb, K, U, Hf, Sm, low SI and Ba, Sr, Ti, TREE further show that the granites have undergone high extent of differentiation and evolution. High Rb/Nb and K/Nb ratios, low Nb/Ta and Zr/Hf ratios, widely varying CaO/Na2O ratios, low and uniform εNd(t) values, and dominantly negative and relatively dispersed εHf(t) values jointly indicate that the granites were derived from a heterogeneous mixture of calcium-poor mudstone and calcium-rich psammite (or metamorphic igneous rocks) in the crust. The known Indosinian W-Sn deposits in South China are more closely related to S-type granites than to I-type granites, suggesting that their ore-forming materials are mainly derived from the crust. In addition, individual positive εHf(t) values indicate that a small amount of newborn crustal components were involved in the formation of granitic magma. Compared with G1, G2 differs distinctly in petrological, chemical and isotopic characteristics, especially it is more richer in F content and contains magmatic cassiterite and wolframite crystals, suggesting that G2 have undergone higher extent of differentiation and evolution, and are much more closely related to the Sn-W mineralization than G1. The late Indosinian granites in this deposit were formed in post-collisional extensional tectonic environment, while the early Indosinian granites intruded in the Yangmingshan area were formed in syn-collision compression environment. Based on the statistics, the metallogenic ages of Indosinian W-Sn deposits in South China are between 212 and 230 Ma, and the petrogenetic ages of related granites are between 211 and 230 Ma, both age groups belong to the late Indosinian period, representing a regional large-scale granitic magma intrusion and mineralization event that initiated in post-collisional extensional tectonic environment.

HREE enrichment during magmatic evolution recorded by apatite: Implication for the ion-adsorption HREE mineralization in South China

Understanding the enrichment of heavy rare earth elements (HREE) and high HREE/LREE (light REE) ratios in the granites is critical to understand the ore-genesis of exclusive ion-adsorption HREE deposits in South China. In this study, we conducted geochronological, whole-rock and apatite geochemical, and isotopic studies on the Xinfeng pluton, of which only parts show high HREE/LREE ratios and are ore-related. Both the ore-related granite and ore-unrelated granite from the Xinfeng pluton are characterized by high SiO2 contents, the same Late Jurassic age (ca. 155 Ma), and enriched Nd isotopic compositions. However, the ore-related granite has lower TiO2, CaO, P2O5, Fe2O3, Ba, Sr, Eu, and LREE contents, lower La/Yb, Zr/Hf and Th/U ratios, higher Rb, Cs, Nb, Ta, Pb, and HREE contents, and higher Rb/Sr ratios than the ore-unrelated granite. These features indicate that the ore-unrelated and ore-related granite are cogenetic and their geochemical differences can be explained by magmatic differentiation. The primary igneous apatite from both the ore-unrelated granite and ore-related granite shows negligible hydrothermal alteration. Fractional crystallization of feldspar (∼20%) and allanite (< 0.3%) led to higher HREE contents and HREE/LREE ratios, lower Sr contents, and a more pronounced negative Eu anomaly in the apatite from the ore-related granite. This study highlights that the magma evolution via mineral fractional crystallization could achieve the relatively high HREE contents and HREE/LREE ratios in the granites, and favors the formation of exclusive ion-adsorption HREE deposits in South China.

Contrasting alteration textures and geochemistry of allanite from uranium-fertile and barren granites: Insights into granite-related U and ion-adsorption REE mineralization

AbstractAllanite is an important rare earth element (REE)-U-bearing mineral in granites, and it can act as a metal source for the formation of some hydrothermal uranium deposits and ion-adsorption REE deposits. To investigate the potential of allanite as a mineral probe of granite-related uranium mineralization processes and the formation of ion-adsorption REE deposits, we present textures, geochemistry, and in situ U-Pb isotope data for allanite from the fertile Changjiang granite associated with the Changjiang uranium ore field and barren Jiufeng granite in the Zhuguangshan batholith, South China. Alteration of allanite in the Changjiang granite is characterized by the altered domains with lower backscattered electron (BSE) intensities than the unaltered domains and replacement by other secondary minerals such as REE fluorocarbonates, calcite, fluorite, thorite, clay minerals, quartz, chlorite, and epidote. Crystals from the Jiufeng granite were partly replaced by the altered domains appearing darker in BSE images and minor REE fluorocarbonates. The darker domains of the Changjiang and Jiufeng allanite grains have higher Fe3+/(Fe3++Fe2+) ratios and U concentrations than those of the brighter domains, indicating that the alteration of allanite was probably related to more oxidized fluids. This study suggests that the Changjiang granite might have been subjected to the influx of F- and CO2-bearing fluids.The brighter domains of the Changjiang and Jiufeng allanite grains have weighted mean 207Pb-corrected 206Pb/238U ages of 156.7 ± 4.3 Ma and 161.6 ± 5.3 Ma, respectively, consistent with the corresponding zircon 206Pb/238U ages of 156.1 ± 1.4 Ma and 159.8 ± 1.8 Ma. The darker domains of the Changjiang allanite grains yield a weighted mean 206Pb/238U age of 141.4 ± 5.6 Ma, which overlaps within error the timing of a uranium mineralization event (~140 Ma) in the Changjiang uranium ore field and the age of a crustal extension event (140–135 Ma) in South China. The BSE images and elemental maps reveal that rare earth elements such as La and Ce have been released from the Changjiang allanites during alteration and were precipitated as REE-fluorocarbonates that are susceptible to chemical weathering, which sets the stage for the formation of an ion-adsorption REE deposit. Our study suggests that the regional crustal extension might have played an important role in the formation of both granite-related uranium and ion-adsorption REE deposits in South China, as it could have triggered alteration or breakdown of REE-U-bearing minerals in source rocks.

Characterization of nano-minerals and nanoparticles in supergene rare earth element mineralization related to chemical weathering of granites

AbstractIon adsorption-type rare earth element (REE) ore deposits in South China are a major source of heavy rare earth elements (HREE) around the world, which are of considerable economic and strategic significance. In these ores, REE is enriched in the clay minerals, specifically kaolinite and halloysite, which are derived from their parent granitoid by the weathering process. However, the mechanisms of supergene REE mineralization remain unclear. We investigated the nature and origin of supergene REE mineralization, based on a nanoscale study of a typical REE-mineralized granite regolith profile (ΣREEmax = 1201 ppm) in the Dazhou super-large, ion adsorption-type REE deposit, Guangxi Province, South China. Bulk mineralogical and geochemical analyses, coupled with novel nano-characterization techniques [i.e., hollow fiber flow field-flow fractionation inductively coupled plasma-mass spectrometry (HF5-ICP-MS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)], were used to determine the nature of the nano-minerals and nanoparticles in the regolith samples. X-ray diffraction and SEM-EDS analyses revealed that ion-adsorption clay minerals are dominated by platy-shaped kaolinite and rod-like halloysite (10 and 7 Å) within the regolith. The average clay mineral contents decreased from 38 to 15% from the fully weathered horizon to the semi-weathered horizon, whereas the proportion of halloysite increased in the clay mineral fraction in the deep horizons. The REE-bearing nanoparticles consist predominantly of macromolecules of organic matter (2–5 nm) and clay minerals (5–40 and 40–80 nm) according to the HF5-ICP-MS analysis. There is a close association between REE and Al contents in particles with sizes of 5–40 nm in the semi-weathered horizons and 40–80 nm in the highly weathered horizons, which indicates that nanoscale clay minerals (halloysite and kaolinite, respectively) are important REE carriers. In addition, nanoscale secondary REE mineral phases, including oxide, silicate, and phosphate, were identified by the SEM and TEM observations. These phases are typically adsorbed onto the surfaces of clay minerals, specifically rod-like halloysite, but have different occurrences in the regolith profile. Cesium-oxide (cerianite) and Ce-silicate (cerite) occur mainly in the upper horizon of the regolith profile, whereas low-crystallinity REE phosphates [rhabdophane-(La)] occur mainly in the lower horizon of the profile. Our results indicate that nano-minerals and nanoparticles affect REE enrichment and fractionation during granite weathering. Migration and accumulation of REE-bearing nano-minerals were caused by leaching and neoformation of REE-bearing nano-minerals during secondary precipitation. These processes contribute to the formation of supergene REE mineralization in granite regolith.

Genetic Relationship between Granite and Fluorite Mineralization in the Shuanghuajiang Fluorite Deposit, Northern Guangxi, South China: Evidence from Geochronology, REE, and Fluid Geochemistry

Hydrothermal vein-type fluorite deposits are the most important metallogenic type of fluorite deposits in South China, most of which are closely related to granitoid in space; however, the genetic relationship between granitoid and fluorite mineralization remains controversial. The Shuanghuajiang fluorite deposit in northern Guangxi of South China is a typical vein-type fluorite deposit hosted in a granite pluton, with the orebodies occurring within brittle faults. Zircon U-Pb dating of the hosting Xiangcaoping granite yields an emplacement age of 228.04 ± 0.76 Ma (MSWD = 0.072). Fluorite Sm-Nd dating yields an isochron age of 185 ± 18 Ma. The new age data indicate that the fluorite deposit was precipitated significantly later than the emplacement of the hosting Xiangcaoping granite pluton. The fluorite and granite exhibit similar rare earth element (REE) patterns with negative Eu anomalies, suggesting that fluorine (F) was derived from the granite. The fluorite fluid inclusions show a homogeneous temperature mainly ranging between 165 °C and 180 °C. Salinity is typically less than 1% NaCl eqv, while the δ18OV-SMOW and δDV-SMOW values are between −5.2‰–−6.1‰ and −17.35‰–−23.9‰, respectively. These indicate that the ore-forming fluids were a NaCl-H2O system with medium-low temperature and low salinity, which is typical for meteoric water. Given the combined evidence of geochronology, REE, and fluid geochemistry, the mineralization of the Shuanghuajiang fluorite deposit is unrelated to magmatic-hydrothermal activity but achieved via hydrothermal circulation and leaching mechanisms. Our study presents a genetic relationship between the fluorite deposit and granitoids based on an example of northern Guangxi, providing a better understanding of the genesis of hydrothermal vein-type fluorite deposits in granitoids outcropping areas.

In situ elemental and Sr-Nd isotopic compositions of hydrothermal apatite from the Shazhou U deposit in the Xiangshan complex: Implications for the origins of ore-forming fluids of volcanic related U deposits in South China

• In situ Sr-Nd isotopes of apatite are effective in tracing origins of ore forming fluids in the Shazhou U deposit. • The reducing U-poor fluid was derived from mantle-derived magma which formed the mafic dikes in wall rocks. • The oxidizing U-rich fluid originated from percolation of meteoric water in the adjacent red-bed basins. • The Late Cretaceous mafic magmatism and red-bed basins are two key factors controlling U mineralization in South China. The origin of ore-forming fluids of volcanic related U deposits remains controversial. This study reports in situ elemental and Sr-Nd isotopic compositions of apatite associated with U mineralization from the Shazhou volcanic related U deposit in South China, which provides a new perspective to constrain the origin and evolution of its ore-forming fluids. Hydrothermal apatites from the hematized rocks and U-mineralized rocks show distinct textures and chemical features, indicating two types of fluids involved in the U mineralization process. The hematitization fluid was oxidizing and enriched in U and Cl. A reducing fluid with lower Cl/F and U was introduced and mixed with the oxidizing hematitization fluid to cause U precipitation. Apatite from the hematized rocks has a lower 87 Sr/ 86 Sr (0.709246–0.714383) than that of apatite in U ore veins (0.710383–0.719318). The εNd(t) of apatite in the hematized rocks is from −7.6 to −9.4, whereas apatite in U ore veins has higher εNd(t) ranging from −1.2 to −8.4, close to that of mafic dikes intruded into the volcanic rocks. In situ Sr-Nd isotopes of apatite indicate the oxidizing fluid originated from the red-bed basins, whilst the origin of the reducing fluid was related to the mantle-derived mafic dikes. The reducing fluid has experienced assimilation by the metamorphic basement rocks to gain higher radiogenic strontium. This study demonstrates that in situ elemental and Sr-Nd isotopic compositions of apatite are effective in identifying different ore-forming hydrothermal fluids and constraining their origins and chemical evolution.

A general ore formation model for metasediment-hosted Sb-(Au-W) mineralization of the Woxi and Banxi deposits in South China

Metasediment-hosted Sb-(Au-W) deposits occur widely in the Jiangnan Orogen (South China), but their complicated metallogenic histories are still under debate. Ore formation models such as syngenetic (SEDEX), granitoid-related, and orogenic scenarios have been proposed by previous studies, but their metal provenance and fluid evolutionary processes are still uncertain. In this study, two typical Sb-(Au-W) deposits (Woxi and Banxi) from this region were studied by in-situ geochemical and geochronological analyses of sulfides to establish a general ore formation model for this type of deposit. Two stages of gold-related pyrite (Py1 and Py2) and one stage of stibnite in the Woxi Sb-(Au-W) deposit, and two stages of arsenopyrite (Apy1 and Apy2) and stibnite (Stb1 and Stb2) in the Banxi Sb deposit were differentiated. Compared to Py1, Py2 is depleted in V but enriched in Au, As, Bi, Pb and Ag, with more negative δ34Sv-CDT and lower Pb isotopic values. Compared with stibnite at Woxi, stibnite at Banxi has higher but more variable contents of most trace elements (e.g., Li, Mg, Al, Si, Cu, Pb, Co, Ni, Fe, Sr and Mn, except for As) and lower Pb isotopic ratios. At Banxi, Stb2 has lower Fe, Cu, Pb, Co, Ni, Sr and Mn contents than Stb1, and Apy2 has higher Co, Ni, Zr, Ti, and Nb contents than Apy1. Analyses of Re-Os isotopes in four samples yielded an isochron age of 439 ± 3.4 Ma (2σ) for Apy1, with an initial 187Os/188Os of 0.3373 ± 0.0009. The above results indicate that early-stage Au mineralization at Woxi may have been orogeny-related, and that ore metals were derived mainly from local epimetamorphic sedimentary rocks with limited input from magmatic fluids. In contrast, the late-stage Sb mineralization at Banxi may have been influenced by deep granitic magmatism. We propose that the common temporal and spatial mineralization sequence of the metasediment-hosted Sb-(Au-W) ores is: orogenic-type gold (early stage; at deep levels of the ore field) → orogenic + magmatic-hydrothermal W (middle stage; at middle levels) → magmatic-hydrothermal + meteoric water Sb (late stage; at shallow levels). Both local metasedimentary rocks and granite magmatism provide metals and ligands for mineralization, with magmatic fluids playing an essential role in late-stage ore formation. Metasediment-hosted Sb-(Au-W) ores in this region exhibit similarities to both orogenic-type and intrusion-related gold systems.

Vesuvianite: A potential U-Pb geochronometer for skarn mineralization---a case study of tungsten and tin deposits in South China

Vesuvianite is one kind of common U-bearing minerals in various skarns and skarn mineralization. As an early phase mineral, vesuvianite can be well preserved since it usually occurs at the front of skarn close to the marble and far away from the pluton, therefore, accurately determining its growth history can directly date hydrothermal activity in the skarn system, just like the zircon U-Pb geochronometer recording magmatic processes. In order to demonstrate reliability of the vesuvianite U-Pb geochronometer, we choose six vesuvianite samples from two skarn deposits in South China for the LA-ICPMS dating. Accordingly, the U-Pb dating of four vesuvianite samples from the Shizhuyuan W-Sn ore district yielded precise ages of 160.3 ± 1.2 Ma, 155.6 ± 2.6 Ma, 154.5 ± 3.1 Ma and 150.4 ± 3.1 Ma, respectively; these dates coincide well with the previously published ages of the relevant granite complex emplacement and hydrothermal mineralization in the region with the ages of ca. 160 Ma, ca. 155 Ma and ca. 150 Ma. The other two vesuvianite samples from the Furong Sn ore district yielded precise ages of 159.3 ± 1.8 Ma and149.1 ± 1.6 Ma，which are consistent with the multi-stage magmatic activities in the studied area with emplacement time of ca. 160 Ma and ca. 149 Ma. Theoretically, the site of the Ca2+ position in vesuvianite is slightly larger than that in intergrown grossularite, which gives the former a better ability to incorporate uranium. Therefore, vesuvianite with high uranium concentrations and relatively low common Pb is beneficial for obtaining precise results during the LA-ICPMS U-Pb dating, it is an ideal mineral for accurately determining the time of skarn formation and/or various skarn mineralization.

The Paleo-Tethys subduction and related mineralization in the SW Yangtze Block: Evidence from the Zhesang Carlin-type gold deposit

The Zhesang gold deposit is a Carlin-type gold deposit located in the “Golden Triangle” structural belt of Yunnan-Guizhou-Guangxi. Pyrite and arsenopyrite are important gold-bearing minerals of the deposit. In order to understand the typomorphic characteristics of pyrite and arsenopyrite and the enrichment mechanism of gold, morphological and structural features of these sulphides and the occurrence state of gold were investigated by using reflected light microscope and electron probes (EPMA). The results show that pyrites are characterized mainly by medium-fine broken pentagonal-dodecahedron crystal forms. Most of the arsenopyrites display needle and columnar crystal forms. Compositionally, pyrite is depleted in both Fe and S, whereas arsenopyrite is As-depleted but S-enriched. The w(Co)/w(Ni) of pyrite is between 1.731 and 3.215 (i.e., within the range of ratio of the hydrothermal pyrite, which is between 1 and 5). The crystal forms and compositional features of pyrite and arsenopyrite indicate that the Zhesang gold deposit is typical of medium-low temperature hydrothermal origin. The positive correlation between As and Au and the Au/As (mol%) ratio are lower than the solubility limit of Au in arsenic pyrite. Combined with previous studies, our data suggest that gold is likely to exist in the form of Au+ complex. Arsenic pyrite and arsenopyrite condensed Au in unsaturated ore-forming hydrothermal fluids relative to Au0 through surface voids and/or defects and the As content, forming a Zhesang Carlin-type gold deposit. Based on the age data of the Mesozoic polymetallic deposits in South China, it can be concluded that the age of the Zhesang gold deposit (215.3 ± 1.9 Ma), is consistent with the age of the main body of the Mesozoic polymetallic deposits (230–200 Ma) in South China Plate and the age of the granites (230–200 Ma) related to tungsten-tin mineralization in the Cathaysian Block. Therefore, we believe that the Zhesang gold deposit is the product of the subduction regime of the Paleo-Tethys Ocean in the Middle-Late Triassic.

Granitic magma evolution to magmatic-hydrothermal processes vital to the generation of HREEs ion-adsorption deposits: Constraints from zircon texture, U-Pb geochronology, and geochemistry

The key point for further prospecting of heavy rare earth elements (HREE) ion-adsorption deposit is to figure out the granites that could generate HREE ion-adsorption mineralization in weathering processes. In this study, we present a detailed study of zircons from granites associated with Zudong, Dabu, and Xinfeng HREE ion-adsorption deposits in South China. The zircons were studied with regards to their texture, crystallinity, U-Pb dating, and geochemistry. The zircons from these granites all can be subdivided into two types. The type-1 zircons show oscillatory zonation and have Th/U and Zr/Hf mass ratios of 0.4–1.0 and 30–50, respectively. These textural and geochemical features indicate crystallization in a fractionated magma. The type-2 zircons are unzoned, occasionally porous, and have a low crystallinity. They occasionally rim type-1 zircons. The type-2 zircons show significantly higher F, P, Hf, Th, U, and REE contents, but display lower ZrO2 and SiO2 contents and lower Th/U, Zr/Hf, La/Yb ratios than those of the type-1 zircons. These geochemical features are consistent with zircon formation in a volatile-HREE-rich magmatic-hydrothermal transition stage. Under these conditions, the HREEs were also hosted in volatile-rich REE mineral phases including synchysite-(Y), aeschynite-(Y), calcybeborosilite-(Y), and atelisite-(Y), which have been observed in these HREE deposits. These volatile-rich REE-mineral phases can easily be dissolved during weathering and release HREE3+ to generate ion-adsorption HREE deposits. Therefore, we conclude that the granitic magma progression to a volatile-HREE-rich magmatic-hydrothermal system is vital for the generation of HREE ion-adsorption deposits. The long-term Mesozoic extension of the South China favors the generation of highly fractionated granites and is thus important for the generation of HREE ion-adsorption deposits. Furthermore, the zircons generated in a volatile-rich environment could be used to determine the HREE ion-adsorption mineralization potential of granites.

The Jurassic Gangmei Mo-W deposit in Guangdong Province and its implication for Mo-W mineralization in South China

South China developed large-scale Mesozoic magmatism and polymetallic mineralization, especially Jurassic W-Sn and Mo-W mineralization. Compared with W-Sn deposits, Mo-W deposits in South China have received less attention. The recently discovered Gangmei Mo-W deposit, which occurred in Guangdong Province, is the southernmost Jurassic Mo-W mineralization in South China. Here, we report the molybdenite Re-Os isotopic age of the Gangmei deposit and compile the Mo-W- and W-Sn-associated deposits in South China and their Re concentrations in molybdenites, aiming at constraining the ore-forming age of the Gangmei deposit, the scope of Mo-W mineralization in South China and the possible controlling factors of different mineralization types. Four molybdenite samples from the Gangmei deposit were chosen for Re-Os isotopic dating. The Re-Os model ages vary from 162.6 ± 1.6 Ma to 164.1 ± 1.6 Ma with a weighted mean age of 163.1 ± 1.4 Ma and yield an isochron age of 162.2 ± 4.1 Ma (MSWD = 1.01), consistent with the emplacement age of the Gangmei intrusion, indicating a genetic relationship between magmatic activity and mineralization. This age also agrees well with the large-scale Jurassic Mo-W- and W-Sn-associated mineralization in South China, suggesting that the scope of Jurassic Mo-W-associated mineralization can reach the southernmost part of South China. Rhenium concentrations in molybdenites from Jurassic Mo-W-associated and W-Sn-associated deposits in South China suggest that their magma sources are different, and Mo-W-associated deposits may involve more mantle-derived materials. In addition, oxygen fugacity may be another factor controlling different types of mineralization in South China.

Mineral paragenesis in Paleozoic manganese ore deposits: Depositional versus post-depositional formation processes

Unlike modern Mn(IV)-dominated manganese nodules, ancient manganese deposits primarily contain Mn(II) (manganous) mineral phases. The general assumption is that these manganous minerals formed post-depositionally from a Mn(IV) precursor phase during diagenesis and metamorphism. However, recent studies have shown that aqueous Mn(II) may combine with either CO32–(aq) or H4SiO4(aq) to nucleate and precipitate as primary manganous phases within anoxic waters. These findings thus make it difficult to assess what comprised the primary mineral assemblage of ancient Mn deposits, which in turn, has implications for understanding the redox fabric of the past marine water column. Here, we investigate the paragenesis and oxidation state of various Mn minerals (oxides, carbonates, and silicates) from the late Devonian Xialei and middle to late Permian Zunyi manganese deposits in South China. Through characterization of the micro-scale distribution, composition, and speciation of different Mn-bearing mineral phases, we provide evidence to support an oxidative mechanism during Mn cycling at the time of deposition. We document the presence of a residual oxide phase (hausmannite; Mn3O4) that represents a primary to early formed Mn(III)-bearing oxide preserved in Mn(II)-carbonate nodules. Additionally, we observe spherulitic textures comprised of mixed-valence Mn(II,III,IV)-silicates enclosed in diagenetic Mn(II)-carbonate laminations. Our results allow for the construction of a paragenetic model for these deposits beginning with the reduction of a primary Mn(IV) phase and the subsequent precipitation of intermediate Mn(III)-oxide or -silicate mineral phases. Following continued exposure to reducing conditions during early diagenesis, Mn(III)-oxides were then replaced by Mn(II)-carbonates. Similarly, Mn(III)-silicates were transformed into various Mn(II)-silicates during late-stage diagenesis and metamorphism. This model suggests that the accumulation of various divalent Mn mineral phases in ancient sediments may, therefore, be indicative of deposition under a water column with a sufficient oxidizing potential to induce Mn(II) oxidation.

Sulfate-limited euxinic seawater facilitated Paleozoic massively bedded barite deposition

Existing models for massively bedded barite (MBB) deposits (e.g., sedimentary exhalative and diagenetic/ cold-seep) satisfy some geological and geochemical observations, but none explain the Paleozoic clustering of MBB deposits in Earth history. Here we bring seawater redox history into the picture and propose a sulfate-limited euxinic seawater (SLES) model where hydrothermally sourced Ba2+ accumulates as dissolved ion while other metal ions precipitate as insoluble metal sulfides. A subsequent encounter of this Ba2+-rich water mass with a sulfate-bearing one results in the deposition of MBB, thus physically separated from the accompanied metal sulfide deposits. We tested the SLES model using early Cambrian MBB deposits in South China through petrographic and isotope analyses. Syngenetic sphalerite and barite layers show a clear separation. A wide range of 87Sr/86Sr values (0.7082 to 0.7120) of the MBB supports a mixing of seawater and hydrothermal sources. A large range of δ34S values (32.2 to 61.1‰) of the MBB and the occurrence of mineral hyalophane supports an overall sulfate-limited but sulfate-concentration temporally and spatially heterogeneous ocean. Our model established the genetic link between MBB deposits and the accompanied metal sulfide deposits in the Paleozoic. The dearth of MBB deposits before and after the Paleozoic is due to widespread ferruginous oceans with little sulfate and complete oxygenated oceans with too much sulfate, respectively. The Paleozoic clustering of the MBB deposits is a consequence of a critical redox transition in Earth history.

The metallogeny of the Devonian sediment-hosted sulfide deposits, South China: A case study of the Huodehong deposit

The Devonian sedimentary rocks in South China host numerous sulfide deposits (e.g. the Siding and Beishan Pb-Zn sulfide deposits, and the Yingde iron sulfide deposits) that are distinctly characterized by a wide variation of δ34S values in sulfides (−28.7‰ to +42.8‰). Such signature, however, has not been investigated in depth and thus the metallogeny of these deposits remain poorly understood. This paper investigated the Huodehong deposit hosted by sedimentary rocks of the Middle Devonian in the western Yangtze Block, South China. A complex paragenesis has been defined: I) micro-grained pyrite (Py1) associated with organic materials; II) radial and crustiform pyrite (Py2) and marcasite (Mc1) aggregates; III) subhedral-anhedral pyrite (Py3), microcrystal colloform sphalerite (Sp1), and dendritic galena; and IV) coarse-grained euhedral sphalerite (Sp2) occurred as veinlets. The mineralogical evidence shows that the Py1 occurred as syn-sedimentary pyritic stromatolites, which is overprinted by cavity-filling hydrothermal Py2, Py3, Sp1, and Sp2. In situ S isotopes show that the δ34S values of Py1-Py3 and Sp1 range from –22.8‰ to −10.8‰. Such highly depleted heavy sulfur isotopes indicate that the reduced sulfur was mainly derived from bacterially reduced seawater sulfate in an open system. The Sp1 has similar δ34S values to its reworking Py1, implying that part of reduced sulfur was sourced from sedimentary pyrite. Such sedimentary pyrite might act as a geochemical barrier in accelerating sulfate reduction via the hydrolysis of Fe2+ resulting an increase in the H+ concentration and the formation of HSO4−. In addition, the Sp2 has slightly depleted heavy sulfur isotopes (δ34S = −9.3‰ to −1.1‰), implying the late input of heavy sulfur isotopes component. Hence, the highly heterogeneous sulfur isotopes at Huodehong likely resulted from bacterial sulfate reduction coupled with reworking sedimentary pyrite, and subordinate incorporation of 34S-rich sulfur. The reduced sulfur of the Huodehong deposit has multiple sources (i.e. sedimentary pyrite and sulfate-bearing evaporites), and the ore formation has experienced the epigenetic hydrothermal mineralization with mixture of metals from the metamorphic basement rocks. The outcomes will have a great implication for understanding the origin and exploration of the Devonian sulfide deposits in South China.