Ore genesis and hydrothermal evolution of the Huanggang skarn iron–tin polymetallic deposit,southern Great Xing'an Range: Evidence from fluid inclusions and isotope analyses

The southern Great Xing'an Range is one of the most important metallogenic belts in northern China, and contains numerous Pb–Zn–Ag–Cu–Sn–Fe–Mo deposits. The Huanggang iron–tin polymetallic skarn deposit is located in the Sn-polymetallic metallogenic sub-belt. Skarns and iron orebodies occur as lenses along the contact between granite plutons and the Lower Permian Huanggangliang Formation marble or Dashizhai Formation andesite. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity, i.e., skarn, oxide and sulfide stages, all contributed to the formation of the Huanggang deposit.The skarn stage is characterized by the formation of garnet and pyroxene, and high-temperature, hypersaline hydrothermal fluids with isotopic compositions that are similar to those of typical magmatic fluids. These fluids most likely were generated by the separation of brine from a silicate melt instead of being a product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by amphibole, chlorite, quartz and magnetite. The hydrothermal fluids of this stage are represented by L-type fluid inclusions that coexist with V-type inclusions with anomalously low δD values (approximately − 100 to − 116‰). The decrease in ore fluid δ¹⁸O_H2O values with time coincides with marked decreases in the fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, the ore fluid evolved by boiling of the magmatic brine. The sulfide stage is characterized by the development of sphalerite, chalcopyrite, fluorite, and calcite veins, and these veins cut across the skarns and orebodies. The fluids during this stage are represented by inclusions with a variable but continuous sequence of salinities, mainly low-salinity inclusions. These fluids yield the lowest δ¹⁸O_H2O values and moderate δD values ( − 1.6 to − 2.8‰ and − 101 to − 104‰, respectively). The data indicate that the sulfide stage fluids originated from the mixing of residual oxide-stage fluids with various amounts of meteoric water. Boiling occurred during this stage at low temperatures.The sulfur isotope (δ³⁴S) values of the sulfides are in a narrow range of − 6.70 to 4.50‰ (mean = − 1.01‰), and the oxygen isotope (δ¹⁸O) values of the magnetite are in a narrow range of 0.1 to 3.4‰. Both of these sets of values suggest that the ore-forming fluid is of magmatic origin. The lead isotope compositions of the ore (²⁰⁶Pb/²⁰⁴Pb = 18.252–18.345, ²⁰⁷Pb/²⁰⁴Pb = 15.511–15.607, and ²⁰⁸Pb/²⁰⁴Pb = 38.071–38.388) are consistent with those of K-feldspar granites (²⁰⁶Pb/²⁰⁴Pb = 18.183–18.495, ²⁰⁷Pb/²⁰⁴Pb = 15.448–15.602, ²⁰⁸Pb/²⁰⁴Pb = 37.877–38.325), but significantly differ from those of Permian marble (²⁰⁶Pb/²⁰⁴Pb = 18.367–18.449, ²⁰⁷Pb/²⁰⁴Pb = 15.676–15.695, ²⁰⁸Pb/²⁰⁴Pb = 38.469–38.465), which also suggests that the ore-forming fluid is of magmatic origin.     

Copper‐gold mineralisation in New Guinea: Numerical modelling of collision,fluid flow and intrusion‐related hydrothermal systems

Two‐ and three‐dimensional numerical modelling techniques, constrained by key geodynamic data, provide insights into the controls on development of porphyry‐related Cu–Au mineralisation in the Tertiary collision zone of New Guinea. Modelling shows that the creation of local dilation to facilitate magma emplacement can be caused by reactivation of arc‐normal transfer faults, where they cut the weakened fold belt. Additionally, dilation occurs where fluid overpressuring is caused by collision‐related, south‐directed fluid flow being localised into the more permeable units of the Mesozoic passive‐margin sedimentary succession. Rapid uplift and erosion, which may be a mechanism for magmatic fluid release in these systems, is shown to be greatest in the west of West Papua, where the stronger Australian crust acts as a buttress. Within the Papuan Fold Belt, uplift is greatest near the margins, where the weaker fold belt abuts the stronger crust and/or major faults have been reactivated. Increased orographically induced precipitation and erosion exposes the lower parts of the stratigraphy within or on the margins of these uplifted zones. On a smaller scale, 2–D coupled fluid‐flow ‐ thermal‐chemical modelling uses a scenario of fluid mixing to calculate metal precipitation distribution and magnitude around an individual intrusive complex. Modelling highlights the interdependence of the spatial permeability structure, the regional temperature gradient, and the geometry of the convection cells and how this impacts on the distribution of metal precipitation.     

The genesis of sandstone‐type uranium deposits in the Ordos Basin,NW China: constraints provided by fluid inclusions and stable isotopes

Quartz from sandstone‐type uranium deposits in the east part of the Ordos Basin contains abundant secondary fluid inclusions hosted along sealed fractures or in overgrowths. These inclusions consist mainly of water with NaCl, KCl, CO₂ (135–913 ppm) and trace amounts of CO (0.22–16.8 ppm), CH₄ (0.10–1.38 ppm) and [SO₄]^2? (0.35–111 ppm). Homogenization temperatures of the studied fluid inclusions range from 90 to 210°C, with salinities varying from 0.35 to 12.6 wt‐% (converted to NaCl wt%), implying multiple stages of thermal alteration. Although high U is associated with a high homogenization temperature in one case, overall U mineralization is not correlated with homogenization temperature nor with salinity. The H and O isotopic compositions of fluid inclusions show typical characteristics of formation water, with δ¹⁸O ranging from 9.8 to 12.3‰ and δD from 26.9 to ?48.6‰, indicating that these fluid inclusions are mixtures of magmatic and meteoric waters. The oxygen isotope ratios of carbonates in cement are systematically higher than those of the fluid inclusions. Limited fluid inclusion‐cement pairs show that the oxygen closely approaches equilibrium between water and aragonite at 150°C. Highly varied and overall negative δ¹³C in calcite from cement implies different degrees of biogenetic carbon involvement. Correlations between U in bulk rocks and trace components in fluid inclusions are lacking; however, high U contents are typically coupled with high [SO₄]^2?, implying pre‐enrichment of oxidized materials in the U mineralization layer. All these relationships can be plausibly interpreted to indicate that U (IV), [SO₄]^2? as well as Na, K were washed out from the overlying thick sandstone by oxidizing meteoric water, and then were reduced by reducing agents, such as CH₄ and petroleum, likely from underlying coal and petroleum deposits, and possibly also in situ microbes at low temperatures.     

Ore geology,fluid inclusions and four-stage hydrothermal mineralization of the Shangfanggou giant Mo–Fe deposit in Eastern Qinling,central China

The Eastern Qinling, Central China, containing more than 20 Mesozoic porphyry ± skarn systems, is the most important Mo province in the world. The Shangfanggou giant Mo deposit, Luanchuan County, Henan Province, is a porphyry-skarn system hosted in a lithologic association comprising carbonaceous sandstone, shale, carbonate and chert within the Neoproterozoic Luanchuan Group. Mo ores are mainly altered porphyry, skarn and hornfels, with minor altered gabbro. The mineralization process includes four stages, potassic alteration of the porphyry and skarnization of dolomite marble in stage 1, stockworks of quartz + molybdenite ± sulfide (stage 2), pyrite + quartz ± sulfides (stage 3), and carbonate ± quartz ± fluorite (stage 4), respectively. Mo mineralization was generally associated with strong silicification and/or phyllic alteration. The fluid inclusions in minerals include three compositional types, i.e., CO₂-bearing (C-type), aqueous (W-type) and daughter mineral-bearing (S-type). Minerals formed in stages 1 to 3 contain all the three types of FIs, but the stage 4 minerals only contain the W-type FIs. Oxides and Cu-phosphate are recognized as daughter minerals in S-type inclusions in minerals of stage 1, whereas the daughter sulfide and reducing gases such as CO, CH₄, H₂S and C₂H₆ can be observed in quartz of stages 2 and 3, suggesting that the ore-forming fluids were initially oxidizing and then evolved to reducing. Boiling fluid inclusion assemblages can be observed in minerals formed in stage 2 or earlier, but not in stage 3 or later. Fluid boiling caused CO₂ escape, oxygen fugacity decrease and rapid precipitation of ore minerals, and was a key factor causing Mo-mineralization at Shangfanggou. Data and interpretations presented in this contribution show that the fluids forming the Shangfanggou Mo deposit evolved from CO₂-rich, high-salinity hypothermal, to CO₂-poor, low-salinity epithermal (low-T). The Mo mineralization at the Shangfanggou deposit mainly occurred at depth of 6.6–7.0 km, deeper than the majority of porphyry systems in volcanic arcs, which resulted from a CO₂-rich magma–fluid system originating from partial melting of thickened lower crust. The Shangfanggou mineral system developed during 158–134 Ma when the Yangtze–North China continental collision began to evolve from compression to extension. Magmatic hydrothermal deposits developed in a continental collision regime are generally formed by CO₂-rich, high-salinity fluids.     

Isotope and rare earth element chemistry of carbonatite–alkaline complexes of Deccan volcanic province: implications to magmatic and alteration processes

Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the ⁴⁰Ar–³⁹Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial ⁸⁷Sr/⁸⁶Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of ¹³C-enriched subcontinental lithospheric mantle by the plume.     

Origin of Archean lamprophyre dykes,Superior Province,Canada: rare earth element and Nd−Sr isotopic evidence

Hornblende- and clinopyroxene-phyric lamprophyre dykes exposed in the Roaring River Complex, Superior Province are alkaline, nepheline-normative, basaltic compositions (>50 wt% SiO₂), that range from primitive to fractionated [Mg/(Mg + total Fe)=0.66–0.40; Ni=200–35 ppm], and which have high abundances of light rare earth elements (REE) [(Ce/Yb)_n=16–26, Ce_n=60–300; n = chondrite normalized], Sr (870–1,800 ppm), P₂O₅ (0.4–1.3 wt%), and Ba (150–900 ppm). Crystal fractionation of the lamprophyres produced coeval gabbro and clinopyroxenite cumulate bodies. A whole-rock Sm–Nd isochron for lamprophyres and gabbro-pyroxenite yields a crystallization age of 2,667±51 Ma Ma (I=0.50929±0.0004; _Nd = + 2.3 0.7). Whole-rock Sr isotope data are scattered, but suggest an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7012, similar to bulk Earth. The elevated levels of light REEs and Sr in the lamprophyres were not due to crustal contamination or mixing with contemporaneous monzodioritic magmas, but a result of partial melting of a mantle source which was enriched in these and other large-ion-lithophile elements (LILEs) shortly before melting. The lamprophyres were contemporaneous with mantle-derived, high-Mg, LILE-enriched monzodiorite to granodiorite of the Archean sanukitoid suite. Both suites have concave-downward light REE profiles, suggesting that depleted mantle was common to their source regions, but the higher light REE abundances, higher Ba/La ratios, and lower _Nd values (+1.3±0.3) of the parental monzodiorites suggest a more enriched source. The lamprophyres and high-Mg monzodiorites were derived from a mineralogically and compositionally heterogeneous, LILE-enriched mantle lithosphere that may have been part of a mantle wedge above a subducting plate in an arc environment.     

The alkaline porphyry associated Yao’an gold deposit,Yunnan, China: rare earth element and stable isotope evidence for magmatic-hydrothermal ore formation

The Yaoan vein-type gold deposit is located in the Ailaoshan-Jinshajiang alkaline intrusive belt, Yunnan Province, China, and is associated both in time and space with 33.5±1.0-Ma-old alkaline intrusions. The gold mineralization is associated with potassic wall-rock alteration. The REE distribution patterns of secondary K-feldspar are generally similar to those of the igneous perthite but with about seven times higher total REE abundances. The alteration is ascribed to a high-REE magmatic fluid derived from the Yaoan alkaline intrusive suite. The hydrothermal Yaoan gold deposit formed during two gold-bearing stages, i.e. a sulfide (pyrite) stage and a sulfide-oxide stage (pyrite-specularite). The REE abundance of early stage I pyrite is relatively high with strong enrichment in LREE, (La/Yb)_n of 40–290, generally positive Eu anomalies (Eu/Eu*=0.86–1.55), and REE patterns very similar to those of secondary feldspar. In contrast, the later stage II pyrite has much lower REE concentrations and lower (La/Yb)_n of 5.5–11.8, Eu/Eu* of 0.49–0.76, and flat chondrite-normalized spidergram patterns. The stage I pyrite has ³⁴S in the range of –2.2 to +3.2, and overlaps with regionally distributed pyrite in least-altered syenite porphyry. In contrast, stage II pyrite has much higher ³⁴S values between +7.8 and +16.5. Carbon isotope data for four samples from stage II revealed ¹³C (PDB) values between –6 and –8. These stable isotope and REE data suggest that magmatic fluids of the alkaline intrusions caused both potassic alteration and stage I sulfide mineralization. The system evolved from stage I to stage II mineralization by influx of meteoric fluids with relatively heavier sulfur, although calcite carbon isotope data suggest that the CO₂ remained dominantly of magmatic origin.Editorial handling: B. Lehmann     

Sm–Nd dating and rare earth element geochemistry of the hydrothermal calcites from Guling carbonate-hosted talc mineralization in the central Guangxi province,South China

Acta Geochimica - Many carbonate-hosted talc mineralization, which are widespread in South China, exclusively developed in Carboniferous dolomitic limestone with many siliceous bands and nodules,...     

Trace element composition of magnetite from the Xinqiao Fe–S(–Cu–Au) deposit,Tongling, Eastern China: constraints on fluid evolution and ore genesis

The Xinqiao deposit is one of several polymetallic deposits in the Tongling ore district. There are two types of mineralization in the Xinqiao: skarn-type and stratiform-type. The skarn-type mineralization is characterized by iron oxides such as magnetite and hematite, whereas stratiform-type mineralization is characterized by massive sulfides with small amounts of magnetite and hematite. We defined three types of ores within the stratiform-type mineralization by the mineral assemblages and ore structures. Type I ore is represented by magnetite crosscut by minor calcite veins. Type II is a network ore composed of magnetite and crosscutting pyrite. Type III is a massive ore containing calcite and hematite. Type I magnetite is characterized by highly variable trace element content, whereas Type II magnetite has consistently higher Si, Ti, V, and Nb. Type III magnetite contains more In, Sn, and As than the other two types. Fluid–rock interaction, oxygen fugacity (fO₂), and temperature (T) are the main factors controlling element variation between the different magnetite types. Type I magnetite was formed by more extensive fluid–rock interaction than the other two types at moderate fO₂ and T conditions. Type II magnetite is thought to have formed in relatively low fO₂ and high-T environments, and Type III in relatively high fO₂ and moderate-T environments. Ca?+?Al?+?Mn and Ti?+?V discrimination diagrams show that magnetite in the Xinqiao deposit is hydrothermal in origin and is possibly linked with skarn.     

Post-collisional carbonatite-hosted rare earth element mineralization in the Hongcheon area,central Gyeonggi massif,Korea: Ion microprobe monazite U‐Th‐Pb geochronology and Nd‐Sr isotope geochemistry

The Hongcheon area in the central Gyeonggi massif is a unique carbonatite locality in South Korea. The age and petrogenesis of this uncommon rock type and associated rare earth element (REE) mineralization still remain uncertain. The NNE trending, 20–50 m wide and ~ 2 km long Fe-REE ore bodies are hosted within a swarm of carbonatite dykes intruding Precambrian basement gneisses. The intrusive nature of the dykes, fenite alteration halos, exsolution intergrowths of constituent minerals and stable isotope data in the literature collectively attest to the ore formation by crystallization of carbonatite magma. The carbonatites are composed primarily of dolomite, ankerite, siderite, magnetite, monazite, apatite, strontianite and pyrite with subordinate quartz, barite, columbite, fergusonite and calcite. The principal carrier phase of REEs is monazite. The REE contents of monazite vary narrowly (TREO = 66.1–69.4 wt.%) irrespective of the textural occurrence. Although the monazite shows a sample-to-sample variation in La/Nd ratio, the textural varieties from each rock sample are similar with respect to this ratio. Thorium contents in monazite are consistently low (average = ca. 2500 ppm) with unusually high (average = ca. 2200) Th/U ratios. Sensitive high-resolution ion microprobe (SHRIMP) dating of monazite yielded a weighted mean ²⁰⁸Pb/²³²Th age of 232.9 ± 1.6 Ma, which agrees with a weighted mean ²⁰⁶Pb/²³⁸U age of 227.2 ± 8.3 Ma within uncertainties. This age, coupled with comparable intrusion ages documented for kimberlites and monzonite-syenite-gabbro-mangerite suite from central Korea, demonstrates the occurrence of mantle-derived alkaline igneous activities and associated REE mineralization following the North and South China collision. The intrusion of the Hongcheon carbonatite and potassic or ultrapotassic suite in central Korea may have resulted from the post-collisional detachment of the subducted slab and consequent upwelling of hot asthenosphere and melting of the overriding lithospheric mantle. Initial Nd‐Sr isotopic ranges of the Hongcheon carbonatite (ε_Nd = ca. − 26, ⁸⁷Sr/⁸⁶Sr = 0.703–0.706) and previous trace element data deny a petrogenetic linkage with the coeval silicate magmas. The metasomatism in the lithospheric mantle source of the Hongcheon carbonatite must have occurred in the distant past (> 1.7 Ga) to generate significantly negative ε_Nd values.     

Origin of the Okrouhlá Radouň episyenite-hosted uranium deposit,Bohemian Massif,Czech Republic: fluid inclusion and stable isotope constraints

The Okrouhlá Radouň shear zone hosted uranium deposit is developed along the contact of Variscan granites and high-grade metasedimentary rocks of the Moldanubian Zone of the Bohemian Massif. The pre-ore pervasive alteration of wall rocks is characterized by chloritization of mafic minerals, followed by albitization of feldspars and dissolution of quartz giving rise to episyenites. The subsequent fluid circulation led to precipitation of disseminated uraninite and coffinite, and later on, post-ore quartz and carbonate mineralization containing base metal sulfides. The fluid inclusion and stable isotope data suggest low homogenization temperatures (～50–140 °C during pre-ore albitization and post-ore carbonatization, up to 230 °C during pre-ore chloritization), variable fluid salinities (0–25 wt.% NaCl eq.), low fluid δ¹⁸O values (?10 to +2 ‰ V-SMOW), low fluid δ¹³C values (?9 to ?15 ‰ V-PDB), and highly variable ionic composition of the aqueous fluids (especially Na/Ca, Br/Cl, I/Cl, SO₄/Cl, NO₃/Cl ratios). The available data suggest participation of three fluid endmembers of primarily surficial origin during alteration and mineralization at the deposit: (1) local meteoric water, (2) Na–Ca–Cl basinal brines or shield brines, (3) SO₄–NO₃–Cl–(H)CO₃ playa-like fluids. Pre-ore albitization was caused by circulation of alkaline, oxidized, and Na-rich playa fluids, whereas basinal/shield brines and meteoric water were more important during the post-ore stage of alteration.     

Shear-zone hosted copper mineralisation of the Omitiomire deposit — Structural controls of fluid flow and mineralisation during subduction accretion in the Pan-African Damara Belt of Namibia

The Omitiomire copper deposit is a relatively recent discovery in the Pan-African Damara Belt of central Namibia. The deposit is situated in Mesoproterozoic gneisses and amphibolites of the Ekuja Dome overlain by amphibolite-grade metaturbidites of the Southern Zone accretionary prism that formed during northward subduction of the Kalahari Craton below the Congo Craton between ca. 580–520 Ma. Copper mineralisation is confined to an anastomosing system of shallowly-dipping, retrograde mylonitic shear zones within the Ekuja Dome. The shear zones are centred around a lithologically heterogeneous amphibolite-gneiss sequence. Mylonitisation and copper mineralisation are closely associated with the retrogression of particularly amphibolites and the partial or complete replacement of amphibolites by biotite–epidote and biotite–chlorite–epidote schists that host the chalcocite-dominated mineralisation.Deformation and mineralisation in the heterogeneous shear-zone system can be shown to describe a progression. Initial strain localization is confined to lithological (amphibolite-gneiss) contacts and associated quartz veining and fluid flow are preferentially developed around the margins of competent amphibolite units. Fluid infiltration and the retrogression of amphibolites to biotite–epidote schists leads to strain localization into the marginal schists that envelop amphibolites. Further veining and fluid flow are localised into the central parts of amphibolite units leading to the pervasive retrogression to biotite–epidote schists that dominate the central parts of the shear-zone system. Earlier quartz-vein generations appear as isoclinally folded and dismembered ribbons or boudins in mineralised schists. The clearly syntectonic introduction of the copper mineralisation is underlined by the intergrowth of chalcocite with the retrograde assemblages and chalcocite forming part of the mylonitic shear-zone fabric.3D modelling of drillhole data combined with limited surface exposure delineates a shallow east dipping, gently undulating ore body parallel to the regional gneissosity of the Ekuja Dome. The ore body comprises several mineralised lenses varying in thickness from 10 m to > 100 m. Prominent ore shoots are gently doubly plunging to the N and S and parallel to the regionally developed L > S fabric in the gneisses. Kinematic indicators in the mineralised shear zone system point to a top-to-the S sense of shear, parallel to the regional L fabric and parallel to the southverging transport recorded in the structurally overlying prism metasediments.The regional setting of the Omitiomire deposit, kinematics, and retrograde, but high-temperature overprint of original mineral assemblages in the mineralised shear zones indicate deformation and fluid flow during the expulsion of the basement gneisses during N-ward direction subduction of the Kalahari Craton below the Congo Craton. Lithological, geochronological, structural and P–T data suggest numerous similarities and, indeed, correlations between the Omitiomire-style copper mineralisation of the Damara Belt with the large copper deposits hosted by basement gneisses in the Domes Region of the Lufilian Arc in Zambia.     

Malinovka deposit—A new type of gold mineralization in Primorye,Russia: Geology,mineralogy, and genesis

Original data on the ore composition are obtained by studying samples from mining workings. In terms of mineral composition, gold-productive veinlets are subdivided into three types: sulfide–quartz, sulfide–carbonate–quartz, and sulfide. The wall-rock metasomatites and typomorphic features of the minerals are studied and the genetic problems of the ores and deposit are discussed. It is established that the Malinovka deposit is ascribed to the gold–tourmaline type (gold–quartz formation), which is identified for the first time in Primorye. In terms of many parameters, the ores of this type are close to those of the large deposits from the Transbaikalian and Amur regions (Klyuchevsky, Kariy, Kirov, Darasun, and others), which are genetically related to granitoid magmatism.     

Magmatic–hydrothermal processes in Sangdong W–Mo deposit,Korea: Study of fluid inclusions and 39Ar–40Ar geochronology

The Sangdong scheelite–molybdenite deposit in northeast South Korea consists of strata-bound orebodies in intercalated carbonate-rich layers in the Cambrian Myobong slate formation. Among them, the M1 layer hosts the main orebody below which lie layers of F1–F4 host footwall orebodies. Each layer was first skarnized with the formation of a wollastonite + garnet + pyroxene assemblage hosting minor disseminated scheelite. The central parts of the layers were subsequently crosscut by two series of quartz veining events hosting minor scheelite and major scheelite–molybdenite ores, respectively. The former veins associate amphibole–magnetite (amphibole) alteration, whereas the latter veins host quartz–biotite–muscovite (mica) alteration. Deep quartz veins with molybdenite mineralization are hosted in the Cambrian Jangsan quartzite formation beneath the Myobong formation. In the Sunbawi area, which is in close proximity to the Sangdong deposit, quartz veins with scheelite mineralization are hosted in Precambrian metamorphic basement. Three muscovite ³⁹Ar–⁴⁰Ar ages between 86.6 ± 0.2 and 87.2 ± 0.3 Ma were obtained from M1 and F2 orebodies from the Sangdong deposit and Sunbawi quartz veins. The Upper Cretaceous age of the orebodies is concordant with the published ages of the hidden Sangdong granite, 87.5 ± 4.5 Ma. This strongly suggests that the intrusion is causative for the Sangdong W–Mo ores and Sunbawi veins.Fluid inclusions in the quartz veins from the M1 and F2 orebodies, the deep quartz-molybdenite veins, and the Sunbawi veins are commonly liquid-rich aqueous inclusions having bubble sizes of 10–30 vol%, apparent salinities of 2–8 wt% NaCl eqv., and homogenization temperatures of 180–350 °C. The densities of the aqueous inclusions are 0.70–0.94 g/cm³. No indication of fluid phase separation was observed in the vein. To constrain the formation depth in the Sangdong deposit, fluid isochores are combined with Ti–in–quartz geothermometry, which suggests that the M1 and F2 orebodies were formed at depths of 1–3 km and 5–6 km below the paleosurface, respectively. The similarity of the Cs (cesium) concentrations and Rb/Sr ratios in the fluid inclusions of the respective orebodies indicate an origin from source magmas having similar degrees of fractionation and enrichment of incompatible elements such as W and Mo. High S concentrations in the fluids and possibly organic C in the sedimentary source likely promoted molybdenite precipitation in the Sangdong orebodies, whereas the scheelite deposition in the deep quartz–molybdenite veins hosted in the quartzite is limited by a lack of Ca and Fe in the hydrothermal fluids. The molybdenite deposition in the Sunbawi quartz–molybdenite veins hosted in the Precambrian metamorphic basement rocks was possibly limited by a lack of reducing agents such as organic C.     

Metallogeny of the Handagai skarn Fe–Cu deposit,northern Great Xing'an Range,NE China: Constraints on fluid inclusions and skarn genesis

The newly discovered Handagai skarn Fe–Cu deposit is located in the northern Great Xing'an Range of NE China and is hosted by the Ordovician Luohe Formation. The orebodies that form the deposit are generally concordant with the bedding within these sediments, and are spatially related to areas of skarn development. The Fe–Cu mineralization in this area records four stages of paragenesis, namely prograde skarn, retrograde skarn, quartz–sulfide, and quartz–carbonate stages. The Handagai deposit is a calcic skarn that is dominated by an andradite–diopside–epidote–actinolite assemblage. The mineralogy and geochemistry of the skarn indicate that it formed from a hydrothermal fluid that altered the carbonate units in this area to a garnet (And_42–95Grs_4–53) and pyroxene (Di_71–78Hd_22–29Jo_0–2) bearing skarn. The epidote within the skarn has an epidote end-member composition, with the chlorite in the skarn dominantly Fe-rich, indicating that these minerals formed in an Fe-rich environment. The petrographic, microthermometric, and Raman spectroscopic analysis of fluid inclusions within garnet, epidote, actinolite, quartz, and calcite precipitated at different stages of formation of the Handagai deposit indicate that mineralization-related fluid inclusions are either liquid-rich two-phase H₂O–NaCl (type I), gas-rich two-phase H₂O–NaCl (type II), three-phase (liquid + vapor + solid) H₂O–NaCl (type III), or CO₂–H₂O–NaCl inclusions (type IV). The early stages of mineralization are associated with all four types of inclusion, whereas the later stages of mineralization are only associated with type I and II inclusions. Inclusion homogenization temperatures vary between the four stages of mineralization (370 °C–530 °C and > 600 °C, 210 °C–290 °C, 190 °C–270 °C, and 150 °C–230 °C, from early to late, respectively), with salinities also varying between the earlier and later stages of mineralization (11–18 and > 45, 7–15, 6–9, and 3–7 wt.% NaCl equivalent (equiv.), respectively). The majority of the inclusions within the Handagai deposit have homogenization temperatures and salinities of 200 °C–350 °C and 4–14 wt.% NaCl equiv., respectively, indicating that this is a medium–high temperature and medium–low salinity type deposit. The fluid inclusions were trapped at pressures of 11 to 72 MPa, corresponding to depths of 0.4 to 2.9 km. The geology, mineralogy, geochemistry, and fluid inclusions microthermometry indicate that the Handagai deposit formed as a result of contact infiltration metasomatism, with the deposition of ore minerals resulting from a combination of factors that include boiling as a result of reduced pressure, cooling, and fluid mixing.     

Mechanism of mineralization in the Changjiang uranium ore field,South China: Evidence from fluid inclusions,hydrothermal alteration,and H–O isotopes

The Changjiang uranium ore field, which contains >10,000 tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O₂, and those of the late syn-ore and post-ore stage contain H₂ and CH₄. The fluid inclusions in quartz of the syn-ore stage include H₂O, H₂O–CO₂, and CO₂ types, and they occur in clusters or along trails. Homogenization temperatures (T_h) for the H₂O–CO₂ and two-phase H₂O inclusions range from 106 °C to >350 °C and cluster in two distinct groups for each type; salinities are lower than 10 wt% NaCl equiv. The ore-forming fluids underwent CO₂ effervescence or phase separation at ∼250 °C under a pressure of 1000–1100 bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe₂O₃/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe₂O₃/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe₂O₃/FeO ratios in the alteration zones. The δ¹⁸O_W–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δD_W–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO₂ effervescence.     

The motion of deformable ellipsoids in power-law viscous materials: Formulation and numerical implementation of a micromechanical approach applicable to flow partitioning and heterogeneous deformation in Earth’s lithosphere

Earth’s lithosphere is heterogeneous in rheology on a wide range of observation scales. When subjected to a tectonic deformation, the incurred flow field can vary significantly from one rheologically distinct element to another and the flow field in an individual element is generally different from the bulk averaged flow field. Kinematic and mechanical models for high-strain zones provide the relations between prescribed tectonic boundary conditions and the resulting bulk flow field. They do not determine how structures and fabrics observed on local and small scales form. To bridge the scale gap between the bulk flow field and minor structures, Eshelby’s formalism extended for general power-law viscous materials is shown to be a powerful means. This paper first gives a complete presentation of Eshelby’s formalism, from the classic elastic inclusion problem, to Newtonian viscous materials, and to the most general case of a power-law viscous inhomogeneity embedded in a general power-law viscous medium. The formulation is then implemented numerically. The implications and potential applications of the approach are discussed. It is concluded that the general Eshelby formalism together with the self-consistent method is a powerful and physically sound means to tackle large plastic deformation of Earth’s lithosphere.     

Origin and evolution of hydrothermal fluids in the Taochong iron deposit,Middle–Lower Yangtze Valley,Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (− 66‰ to − 94‰). The decrease in ore fluid δ¹⁸O_water with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ¹⁸O_water values (− 2.2‰ to 0.4‰) and a wide of range of δD values (− 40‰ to − 81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.     

Ore genesis of the Tianbaoshan carbonate-hosted Pb–Zn deposit,Southwest China: geologic and isotopic (C–H–O–S–Pb) evidence

The Tianbaoshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province, is located in the western Yangtze Block and contains 2.6 million tonnes of 10–15 wt.% Pb + Zn metals. Ore bodies occur as vein or tubular types and are hosted in Sinian (late Proterozoic) carbonate rocks and are structurally controlled by the SN-trending Anninghe tectonic belt and NW-trending concealed fractures. The deposits are simple in mineralogy, with sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, freibergite, and pyrargyrite as ore minerals and dolomite, calcite, and quartz as gangue minerals. These phases occur as massive, brecciated, veinlet, and dissemination in dolostone of the upper Sinian Dengying Formation. Hydrogen and oxygen isotope compositions of hydrothermal fluids range from –47.6 to –51.2‰ and –1.7 to +3.7‰, respectively. These data suggest that H₂O in hydrothermal fluids had a mixed origin of metamorphic and meteoric waters. Carbon and oxygen isotope compositions range from –6.5 to –4.9‰ and +19.3 to +20.2‰, respectively. These compositions plot in the field between mantle and marine carbonate rocks with a negative correlation, suggesting that CO₂ in the ore-forming fluids had multiple sources, including the Permian Emeishan flood basalts, Sinian-to-Permian marine carbonate rocks, and organic matters in Cambrian-to-Permian sedimentary rocks. Sulphur isotope compositions range from –0.4 to +9.6‰, significantly lower than Cambrian-to-Permian seawater sulphate (+15 to +35‰) and sulphate (+15 to +28‰) from evaporates in Cambrian-to-Permian strata, implicating that the S was derived from host-strata evaporates by thermal–chemical sulphate reduction. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios range from 18.110 to 18.596, 15.514 to 15.878, and 38.032 to 39.221, respectively, which plot in field of the upper crust Pb evolution curve, unlike those of Proterozoic basement rocks, Sinian dolostone, Devonian-to-Permian carbonate rocks, and the Permian Emeishan flood basalts, implying complex derivation of Pb metal in the ore-forming fluids. Geological and isotopic studies of the Tianbaoshan Pb–Zn deposit reveal that constituents in the hydrothermal fluids were derived from multiple sources and that fluid mixing was a possible metallogenic mechanism. The studied deposit is not distal magmatic–hydrothermal, sedimentary exhalative (SEDEX), or Mississippi Valley (MVT) types, rather, it represents a unique ore deposit type, named in this article the SYG type.