Numerous large deposits are formed by multiple mineralization events, however, how to identify superimposed mineralization events is poorly understood. The Xiangdong deposit is a large quartz vein-hosted W–Sn deposit within the Dengfuxian composite granite in South China. Previous studies have suggested that there are two discrete W–Sn mineralization events in the deposit, whereas the ore-forming conditions and processes of the two mineralization events are still unclear. To address these ambiguities, this contribution characterized the microtextures, trace-element chemistry, and geochronology of cassiterite assemblages within W–Sn-rich quartz veins. Based on cathodoluminescence (CL) images, cassiterite crystals were classified into two types with textural differences — Cst 1 and Cst 2. Cassiterite 1 is subhedral to irregular and exhibits complex zonation, whereas Cst 2 is euhedral and exhibits oscillatory zonation. Trace-element concentrations and maps of cassiterite obtained via LA–ICP–MS indicate that Cst 1 has high Fe, W, U, and Sb, but low Nb, Ta, and Zr concentrations, whereas Cst 2 has high Nb, Ta, and Zr, but low Fe, W, U, and Sb concentrations. This indicates that Cst 2 formed from higher temperature and more oxidizing hydrothermal fluids than Cst 1. Laser ablation ICP–MS U–Pb geochronological results of Cst 1 and Cst 2 demonstrate they precipitated at 150.0 ± 2.6 Ma and 136.3 ± 5.5 Ma, respectively. This contribution demonstrates that integrated textural and compositional studies of cassiterite assemblages are critical to identifying hydrothermal events that superimposed W–Sn mineralization, and to constrain ore-forming mechanisms and the physicochemical conditions of the environment.
Abstract The Xiaoqinling gold province, located in the Neoarchean–Paleoproterozoic uplifted footwalls of the Xiaoqinling metamorphic core complex (XMCC), is one of China’s largest gold producers; however, achieving a consensus regarding their metallogenic model remains elusive. Scheelite is an indicator mineral that commonly occurs in lode gold deposits worldwide used to recognize deposit types and understand hydrothermal evolution and the origin of features. Xenotime, monazite, and rutile are common hydrothermal minerals in association with lode gold deposits worldwide. Here, we provide textual, in situ U-Pb geochronology of xenotime, monazite, and rutile, and in situ elemental and Sr-Nd isotopic compositions of scheelite within different stages from the large Yangzhaiyu lode gold deposit, aiming to elucidate its genesis and, for the first time, establish a holistic correlation between the lode gold mineralization and the evolution of the XMCC. Notably low εNd(t) values (−30.7 to −23.7), high 87Sr/86Sr ratios (0.72659–0.75914), and distinct rare earth elements, Sr, Mo, and As contents of scheelite confirm a metamorphic crustal source. Xenotime U-Pb dating and pre-ore (Stage I) scheelite reveal that ore-barren metamorphic fluids at ca. 140 Ma were oxidized with low Bi contents and buffered by greenschist facies metamorphism when the XMCC initiated. Monazite and rutile U-Pb dating combined with ore-stage scheelite geochemistry indicate a compositional shift in the more reduced auriferous metamorphic fluids, which dominated during major gold deposition periods (stages II and III) from 130 Ma to 120 Ma, characterized by significantly depleted Na and increased Bi contents. This resulted from the prograde greenschist-to-amphibolite metamorphism at mid-lower crustal depths as the result of the XMCC isostatic doming and the lithospheric mantle thinning after 130 Ma. This study highlights the crucial role of metamorphic core complexes in governing the timing, locations, and resources of the lode gold metallogenic system.
ABSTRACT Tourmaline minerals observed in different geologic environments show significant variations in terms of chemical compositions. Determination of tourmaline species gives useful petrogenetic information about both igneous and metamorphic environments. Microscopic, XRD, XRF, and confocal Raman spectroscopic features of tourmaline segregations that are 2–4 cm thick, dark-blue to black in color, and mostly fractured occurring in the Buldan pegmatite are reported. Under the microscope, tourmaline samples show indigo blue, light blue, and olive-brown pleochroism with a thin long columnar, bladed shape. They exhibit distinctive enrichments in Fe2O3 (7.83–10.16 wt%), V (245.0–591.0 ppm), Sn (70.1–147.3 ppm), W (1076.0–1887.0 ppm), U (1.2–18.2 ppm), and Th (9.6–28.0 ppm). In terms of geochemistry, the tourmaline samples are schorl with Fe/(Fe + Mg) ratios of 0.65–0.74 and Na/(Na + Ca) ratios of 0.88–0.93. Five characteristic bands of tourmaline samples are observed at 1050, 710, 370, 220–245, and 185 cm−1. Tourmaline segregations in the Buldan pegmatite are schorl in composition and are probably generated from Li-poor granitoids and their associated pegmatites.
Abstract To systematically quantify the production, consumption, and migration of methane, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance and constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10 −3 to 2.78 × 10 −1 and from –1.88 × 10 −1 to 3.97 mmol m −2 d −1 at the sulfate‐methane‐transition‐zone (SMTZ) and sediment‐seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1512–43,096 Tg Myr −1 and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.
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