ABSTRACT The Meyon gold deposit is located within the Mogok Metamorphic Belt (MMB), approximately 150 km southeast of Yangon, in the northwestern part of Mon State, Myanmar. Gold mineralization is hosted by Carboniferous metasedimentary rocks of the Mergui Group, including slate, argillite, phyllite, schist, and minor turbiditic greywacke. Granitic rocks in the study area are peraluminous, calc‐alkaline, S‐type granites emplaced synkinematically at mesozonal depths. Contact metamorphic rocks, including hornfels and skarn, are developed along granite‐sediment contacts, while felsic intrusions of rhyolitic to rhyodacitic composition are locally associated with mineralization. The deposit represents a structurally controlled hydrothermal system, where gold is concentrated within NNW–SSE‐trending shear zones and associated fractures, forming quartz‐sulfide veins accompanied by silicification, argillic, and propylitic alteration. Fluid inclusion assemblages record a systematic evolution from CO 2 ‐rich to mixed CO 2 ‐H 2 O and subsequently to H 2 O‐dominant fluids, reflecting progressive decompression and cooling during fluid ascent. Microthermometric and Raman spectroscopic data indicate homogenization temperatures of 240°C–370°C and low salinity (< 10 wt.% NaCl equivalent), consistent with mesothermal conditions. The coexistence of carbonic and aqueous inclusions provides clear evidence for fluid immiscibility, which is interpreted as a key mechanism for gold precipitation. Sulfur isotope compositions of pyrite (δ 34 S = −2.80‰ to +4.43‰) cluster near 0‰, indicating a dominantly magmatic–metamorphic sulfur source with minimal sedimentary contribution. These results demonstrate that gold mineralization resulted from the upward migration of CO 2 ‐rich fluids along shear zones, followed by pressure decrease, phase separation, and cooling. The study provides a well‐constrained genetic model and highlights the exploration potential of the Mergui Belt for orogenic gold deposits.
Miarolitic cavities (MCs) serve as key indicators of volatile phase exsolution during magmatic differentiation, providing critical insights into the transition between igneous and hydrothermal regimes. Despite their metallogenic significance, the mechanisms controlling the selective extraction of metal-bearing fluids remain poorly understood. This study investigates the microstructural and geochemical characteristics of chalcopyrite- and molybdenite-dominated MCs in the Hongyuan porphyry Mo-Cu deposit (NW China). The cavities exhibit sulfide cores surrounded by graphic quartz-feldspar, aplitic, and K-alteration halos. Mass balance calculations reveal enrichments in Cu, Fe, Na, and Si during the magmatic-hydrothermal transition. The term "sulfide-silicate bulb" refers to silica-saturated fluid pockets or bubbles that develop during hypogene copper mineralization in porphyry deposits. The central sulfide-silicate bulbs represent hypersolidus mineral assemblages, with differential Cu-Mo precipitation linked to quartz growth rates. Late-stage meteoric water alteration further influences metal precipitation. We propose a genetic model wherein sulfide-silicate bulbs act as the primary vehicles for the extraction, transport, and focused precipitation of Cu- and Mo-rich fluids. Our results highlight the critical role of quartz growth rates and meteoric fluid influx in controlling metal segregation, providing a new framework for understanding the spatial zonation in porphyry Cu-Mo systems.
The Sandu-Danzhai Au-Hg-Sb metallogenic belt is an essential constituent of the giant Mesozoic low-temperature metallogenic domain of the South China Craton. Although many sediment-hosted Au-Hg-Sb deposits have been discovered in this belt over decades, possible sources of ore-forming elements (e.g., Au-As-S) and fluids remain controversial. This study presents chemical compositions and S isotopes of pyrites and C-O isotopes of calcites collected from the Miaolong deposit in the central belt. Based on their petrographic features and Au-As compositions, three generations of pyrites are classified in the deposit: syngenetic/diagenetic Py1, pre-ore Py2, and ore Py3. From Py1 (As < 0.1%) to Py3 (As > 1%), the As, Au, and Cu concentrations increase gradually. This increase is greatest for As, indicating that As may be critical in the enrichment of Au and other trace elements. Au occurs in the form of lattice-bound Au+, and the low Au/As ratios of the pyrite (< 1:100) indicate that the ore fluids are undersaturated in Au. Moreover, in situ S isotope analyses show that Py1 exhibits the largest variation in delta S-34 values (-21.65 parts per thousand to 34.97 parts per thousand), consistent with a sedimentary origin. In contrast, Py2 and Py3 share similar and relatively homogeneous delta S-34 values (Py2 = 19.46 parts per thousand-26.28 parts per thousand, Py3 = 20.94 parts per thousand-24.81 parts per thousand). These positive delta S-34 values support an abiotic thermochemical sulfate reduction (TSR) of marine sulfate. As inferred from their C-O isotopes (delta O-18(V-SMOW) = 15.11 parts per thousand-24.43 parts per thousand, delta C-13(V-PDB) = -3.95 parts per thousand to -1.52 parts per thousand), the late-ore calcite veins are likely derived from the dissolution and recrystallization of Cambrian marine limestone. In summary, a metamorphic model is proposed for Au mineralization in the Miaolong deposit, in which deep-sourced metamorphic fluids from the Precambrian basement are essential in Au remobilization and deposition.
Myanmar hosts a rich diversity of ore deposits, ranging from numerous artisanal operations to several world-class mines. One of the important gold deposits is the Phayaung Taung mine, located in the northeastern part of the Mandalay region, central Myanmar. This gold deposit is hosted in the interbedded layers of quartzite and phyllite units of the Late Proterozoic Chaung Magyi Group in the Mergui Belt. Although the occurrence, geological setting, and ore mineralogy of the deposit have been previously studied, detailed Raman spectroscopy-based fluid inclusion investigations have not yet been conducted. Here, we present the results of the Raman analysis of CO2 bearing fluid inclusions and the pressure-depth relationships using the density and temperature range of the CO2 fluid inclusions. Raman analysis indicates a very minimum water content inside the bubbles in CO2 bearing fluid inclusions with density ranging from 288.82 to 969.68 kg/m3. Nitrogen (N2) is present only in trace amounts in the analyzed inclusions. The latest recrystallization entrapment pressures of CO2 bearing fluid inclusions were estimated at 7 MPa (approximate to 0.26 km) for low-density inclusions and 34 MPa (approximate to 1.28 km) for high-density inclusions. The low-pressure inclusions are interpreted to record uplift and deformation events associated with the Mergui Belt. This research classifies the Phayaung Taung deposit in the Mergui Belt as a mesozonal, structurally emplaced orogenic gold deposit, providing insights into its geological characteristics and formation processes.
Efficient manganese (Mn) removal from acidic mine drainage (AMD) is challenging due to its high solubility and slow oxidation kinetics. We investigate a mechanism that enables the efficient removal of Mn in a sand filter at an operational AMD treatment facility, with particular emphasis on naturally formed Mn oxides on sand grain surfaces (Mn-precipitated sand; MPS). Characterization of the filter media using x-ray fluorescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy identified a coating of poorly crystalline birnessite (delta-MnO2) that formed in situ on the MPS and is absent on unprecipitated sand (UNS). X-ray absorption near-edge structure spectroscopy confirmed that this coating is composed mainly of tetravalent Mn(IV). Batch experiments simulating filtration conditions were performed to compare Mn removal kinetics using MPS, UNS, and commercial Mn oxide powder from thickener mine water and pure water with 2 mg/L dissolved Mn2+. The MPS removed > 99% of the dissolved Mn within 3 h from both mine and pure water, whereas UNS only removed some Mn (< 20% after 96 h). Microbial inhibition tests using UV-irradiated and ampicillin-treated MPS in pure water confirmed that this rapid Mn removal was dominated by abiotic physicochemical processes. This abiotic, autocatalytic oxidation mechanism proved effective even at a neutral pH, achieving > 96% removal. These findings reveal that Mn removal is driven by abiotic autocatalysis on a naturally formed, Mn(IV)-rich birnessite coating. The high efficiency of this process at a neutral pH presents a significant opportunity to optimize AMD treatment by lowering operational pH and reducing chemical consumption and sludge production for a more sustainable remediation strategy.
