Scheelite is a major (W)-bearing mineral enriched in rare earth elements (REEs) and is widely used to investigate the mineralization process, fluid source and redox environment of W deposits. In the Weijia skarn-type W deposit in South China, scheelite was formed during the greisen, retrograde skarn, oxidation, and sulfide-quartz vein stages. Various characteristics of cathodoluminescence (CL) images of scheelite grains (e.g., light to dark color, core-rim texture and oscillatory zonation) have recorded their detailed crystallization process. Scheelite grains formed in the greisen and retrograde skarn stages have similar LREE patterns, and the gradual fractionation in HREEs may be a result of the precipitation of garnet (And38-70Gr29-52Pr1-3) in the prograde skarn stage. In addition, the similar REE patterns of scheelite from the greisen stage and the granite porphyry pluton suggest a genetic relationship. The oxygen isotopic compositions of scheelite and quartz are all plotted near the edge of the magmatic fluid field, confirming their derivation from the same magmatic hydrothermal fluid. Elemental mapping and spot analyses using an electron probe microanalyzer indicated that extensive isomorphic substitution of W by molybdenum (Mo) occurs in scheelite, and the darker color in the CL images corresponds to a higher Mo content and lower W content. The Mo content of scheelite gradually increased from the greisen stage (mean = 0.46%), through the retrograde stage (mean = 2.89%), to the oxidation stage (mean = 19.8%), and then dropped sharply in the sulfide-quartz vein stage (mean = 0.20%), indicating a change in oxygen fugacity. Moreover, scheelite formed in the sulfide-quartz vein stage has an oscillating Mo content, thus implying a slight or periodical change in oxygen fugacity during the same mineralization stage.
In recent years, the advancements in multi-collector inductively coupled plasma mass spectrometry (MC – ICP – MS) technology have significantly enhanced our understanding of the isotopic variations in tungsten (W) and tin (Sn) among natural samples. The application of W isotopic fractionation (δ186/184W NIST 3163) has emerged in tracing the material cycle associated with solid Earth evolution as an excellent indicator. Also, the findings of Sn isotopic fractionation (e.g. δ122/118Sn 3161a) in cassiterite have achieved initial success in revealing magmatic-hydrothermal processes. However, the mechanisms responsible for variations in W and Sn isotopes in the ore-forming processes remain to be fully addressed, and there is an urgent need to apply the W – Sn isotope systematics to geology (especially for ore deposit geology). In this contribution, the high-precision analytical methods for W and Sn isotopes, the significant variability in isotopic compositions, and the immense functionality of the tracing process are systematically summarized and reviewed. Conclusively, the prospects of the application of W and Sn isotopes in geology are listed, and it is pointed out that it is urgent to introduce W – Sn double isotope systematics into the study of ore deposits, apply the stable isotopic method for tracing the source of ore-forming materials and modelling the evolution of W and Sn isotopes in W – Sn metallogenic system. Focusing on the composition variations of W and Sn isotopes in magmatic-hydrothermal evolution is expected to explore the fractionation mechanism of those isotopes in the mineralization process from multiple perspectives, establishing the evolution model of W and Sn isotopes in the complex W – Sn metallogenic system, which can provide a fresh approach for in-depth understanding of the genesis of W – Sn mineralization, and then facilitating a new perspective for the study of large-scale W – Sn polymetallic mineralization.
Low-temperature Sb (Au–Hg) deposits in South China account for more than 50% of the world's Sb reserves, however, their genesis remains controversial. Here we report the first study that integrates U–Pb and Lu–Hf analysis by LA-(MC)-ICPMS and conventional (U–Th)/He analysis, all applied to single zircon crystals, in an attempt to constrain the origin and timing of world-class Sb (Au–Hg) deposits in Banxi (South China). Zircon separated from a quartz-stibnite ore and an altered country rock samples revealed similar U–Pb age spectra defining two major populations – Paleoproterozoic (~1900–2500 Ma) and Neoproterozoic (~770 Ma), which are characterized by variable εHf(t) values (–10.7 to 9.1 and –16.5 to 11.2, respectively) and Hf crustal model ages (TDMC) (2.48 to 3.24 Ga and 0.97 to 2.71 Ga, respectively). The U–Pb age and Hf isotopic features of the zircons are consistent with the Banxi Group in the region, indicating that the zircons involved in the low-temperature hydrothermal system were originally from the Banxi Group country rocks. Thirty-three mineralization-related zircon crystals yielded a mean (U–Th)/He age of 123.8 ± 3.8 Ma, which is interpreted to represent the timing of the latest low-temperature mineralization stage of the Banxi Sb deposit. The combined U–Pb, Lu–Hf and (U–Th)/He data suggest that Precambrian basement rocks were the major contributors to the low-temperature mineralization, and that Early Cretaceous (130–120 Ma) could be the most important ore-forming epoch for the Sb deposits in South China. This study also demonstrates the analytical feasibility of integrated U–Pb - Lu–Hf - (U–Th)/He "triple-dating", all applied to single zircon crystals. This approach reveals the full evolution of zircon, from its origin of the magmatic source, through its crystallization and low-temperature cooling. Although this study demonstrates the usefulness of this integrated approach in dating low-temperature mineralization, it has great potential for zircon provenance and other studies that may benefit from the large amount of information that can be extracted from single zircon crystals.
Multistage, skarn-type W mineralization has recently been discovered in the Tongshanling Cu–polymetallic ore field, South China. This contribution reports new results from cathodoluminescence, trace element, and Nd–O isotopic analyses of scheelite samples from Tongshanling, designed to better constrain the W mineralization process and origin of associated metals and metallogenic fluids. Three generations of scheelite were identified that can be attributed to three paragenetic phases: (i) an early prograde skarn stage dominated by massive garnet and diopside intergrown with fine-grained scheelite, (ii) an intermediate retrograde skarn stage characterized by an assemblage of abundant scheelite, medium-grained chlorite and epidote, and (iii) a late sulfide-quartz vein stage typified by coarse-grained, subhedral scheelite grains that are typically intergrown with chalcopyrite, sphalerite, and galena. Our data show that although WO3 and CaO concentrations are similar in scheelite of all three stages and rare earth element (REE) patterns vary, the ∑REE + Y values of scheelite markedly and progressively decrease from the prograde skarn (mean = 901 ppm) to retrograde skarn (70.1 ppm) to the sulfide-quartz vein (34.1 ppm) stages. This signature is consistent with either of the following scenarios: The coprecipitation of REE-enriched minerals (e.g., garnet and scheelite), a change in fluid oxygen fugacity change and/or minor distortions in crystal texture. The δ18OH2O values of fluids linked to the prograde skarn and sulfide-quartz vein stages (5.02‰–6.19‰) are consistent with a magmatic fluid input. The δ18OH2O values of fluids associated with the retrograde skarn stage (2.85‰–3.45‰), on the other hand, are indicative of fluid mixing, possibly reflecting dilution of magmatic fluids by meteoric waters. Interestingly, scheelite samples of the sulfide-quartz vein stage returned low εNd(t) values (−7.46 to − 7.07) and moderate two-stage Nd model ages (T2DM = 1526 –1557 Ma) that are identical to the whole rock Nd isotope composition of an adjacent Jurassic I-type granodiorite porphyry pluton. Overall, the results of our study indicate a close genetic relationship between this pluton and the W mineralization, a scenario that is at odds with the common association between I-type granodiorites and Cu–Pb–Zn mineralization in South China.
The Bakoshi-Gadanya area, with widespread Neoproterozoic granites, is located within the Pan-African Trans-Sahara Belt, sandwiched between the West African and the Sahara metacraton. The whole-rock geochemistry, Sr-Nd isotope systematics and LA-(MC)-ICP-MS U-Pb-Hf isotopes of zircon were analysed to shed light on the age and genesis of the Bakoshi-Gadanya granites in the Precambrian basement of the northern western Nigeria shield. The granites are silicic (SiO2: 72.8–77.2 wt.%), ferroan, high-K, calc-alkalic to alkali-calcic in composition, with A-type granite affinities. The ferroan calc-alkalic types range in age from 706.6 ± 1 Ma to 638.4 ± 1 Ma, and the alkali-calcic type is dated at 710 ± 1 Ma. Whole-rock Sr-Nd and zircon Hf isotope systematics have constrained the ferroan calc-alkalic granites to an enriched mantle lithospheric magma source, contaminated by Archaean-Palaeoproterozoic crustal components. On the other hand, the magma that crystallized the alkali-calcic Gadanya granite originated from an enriched mantle source, triggered by reactivation of the lithosphere-scale Kalangai fault during the post 750 Ma major collision between West African Craton and the Saharan metacraton. Both alkali-calcic and calc-alkalic ferroan granites were formed contemporaneously with the last major Pan-African syn-metamorphic deformation in northern western Nigerian shield; thus, they are syn-kinematic granites. Neoproterozoic magmatism at Bakoshi-Gadanya area is related to easterly dipping subduction at the margin of microcontinental blocks in northern western Nigeria shield. Zircon U-Pb-Hf data envisaged both early Neoproterozoic and Cryogenian juvenile magmatic additions in this part of western Nigeria shield.
Widespread, large-scale polymetallic W–Sn mineralization occurs throughout the Nanling Range (South China) dated 160–150 Ma, and related to widely developed coeval granitic magmatism. Although intense research has been carried out on these deposits, the relative contribution of ore-forming elements either from granites or from surrounding strata is still debated. In addition, the factors controlling the primary metallogenic element in any given skarn deposit (e.g., W-dominated or Sn-dominated) are still unclear. Here, we select three of the most significant skarn-deposits (i.e., Huangshaping W–Mo–Sn, Shizhuyuan W–Sn–Mo–Bi and Xianghualing Sn), and compare their whole-rock geochemistry with the composition of associated granites and strata. The contents of Si, Al and most trace elements in skarns are controlled by the parent granite, whereas their Fe, Ca, Mg, Mn, Ti, Sr and REE patterns are strongly influenced by the wall rock. Samples from the Huangshaping skarn vary substantially in elemental composition, probably indicating their varied protoliths. Strata at the Shizhuyuan deposit exerted a strong control during metasomatism, whereas this occurred to a lesser degree at Huangshaping and Xianghualing. This correlates with increasing magma differentiation and increasing reduction state of granitic magmas, which along with the degree of stratigraphic fluid circulation, exert the primary control on dominant metallogenic species. We propose that wall rock sediments played an important role in the formation of W–Sn polymetallic mineralization in South China.
Abstract Mesozoic intrusive rocks are extensively outcropped in the Cathaysia Block (CB), indicating that they underwent significant exhumation after being formed. However, tectonothermal evolution of the CB during Mesozoic–Cenozoic times is still poorly constrained and associated geodynamic mechanisms driving the regional exhumation remain elusive. Toward this end, we present first zircon and apatite (U‐Th)/He data of eight Mesozoic granitic plutons distributed across the intracontinental CB. Our new dating results are integrated with a compilation of regional low‐temperature thermochronological data to determine the CB evolution in a tectonic and topographic evolution framework. Zircon and apatite (U‐Th)/He central ages of the eight granitoids range from 146 to 30 and 82 to 31 Ma, respectively, implying a long‐lasting exhumation of the intracontinental CB. Inverse thermal modeling of the thermochronological data for the eight plutons indicates that the intracontinental CB underwent three exhumation phases at 150–110, 110–85, and 66–38 Ma, of which the former two exhumation phases were prolonged and significant. A compilation of regional thermochronological data reveals a propagating locus of fast exhumation phase from the intracontinental CB to the seaward epicontinental CB over time. Combined with other geological evidence, we infer that primary exhumation events of the CB resulted from changing subduction processes of the Paleo‐Pacific Plate, which include slab break‐off and foundering in the Late Jurassic, progressive slab retreat in the Early Cretaceous, and normal subduction in the Late Cretaceous, with minor exhumation events presumably triggered by the Paleogene opening of the South China Sea Basin.
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