Granite-related uranium ore is an important uranium resource type in China and worldwide. Whether the uranium geochemical theory “U⁶⁺ oxidative migration and U⁴⁺ reductive precipitation” is applicable to the granite-related uranium mineralization theory has not been determined. Detailed field and petrographic work, as well as scanning electron microscopy energy spectrum analysis, are conducted in this study to analyze the relationship between uranium minerals and pyrite from different ore types and evaluate the mechanism for the precipitation and enrichment of uranium in the Mianhuakeng uranium deposit of northern Guangdong. Uranium ore bodies in the Mianhuakeng deposit generally occur as vein-filling or vein-disseminated types. Four different kinds of ores are recognized: fluorite, carbonate, siliceous, and reddening types. Despite differences in the mineral assemblages, veined ores share similar characteristics and show that uranium minerals (1) occur in the central part or periphery of vein-filling ores or in interphase arrangements with syn-ore fluorite, quartz, or calcite veins; (2) occur as veinlets or are disseminated in cataclastic altered granite; (3) are inlaid with gangue minerals, primarily calcite, fluorite, and microcrystalline quartz; and (4) are closely associated with pyrite in aggregates or relatively independent states, forming straight boundaries with syn-ore gangue minerals that have euhedral and intact crystals and show mosaic growth features. All these results indicate that both pyrite and uranium minerals are co-crystallized products of the ore-forming fluid. Combined with previous research suggesting that the reducing fluid was sourced from mantle, this study shows that decreased pressure and temperature, as well as changes in pH and the solubility (saturation) of changes, rather than the redox reaction, caused the uranium precipitation in the Mianhuakeng deposit.

Carbonaceous material (CM) is common in meta-sediments and is generally interpreted to be intimately associated with gold mineralization. For the Bumo deposit in Hainan Province, South China, CM is mainly hosted by greenschist facies—to amphibolite-facies metamophic rocks of the Paleo—to the Mesoproterozoic Baoban Group, and by auriferous veins which could be used as an important gold prospecting indicator. However, the genesis of CM and its relationship with gold mineralization are still unclear. From the field work and thin section observations two types of CM occur, i.e., layered and veinlet. The layered CM occurred in CM-bearing black shales, up to meters thick, and prevails in the deposit. More importantly, Au-bearing sulfides are commonly distributed along the boundary between the quartz veins and layered CM. In contrast, the veinlet CM, co-precipitated with native gold and sulfides, has the thickness of micro- to centi-meters, and these thin veins occur in quartz veins and hydrothermally altered rocks. In addition, layered CM has a stringy shape and laminate structure, while veinlet CM occurs as isometric particles based on the Scanning Electron Microscope (SEM) analysis. The Raman carbonaceous material geothermometer indicates that layered CM with a high maturity is formed at elevated temperatures of 400–550 °C, consistent with X-ray diffraction (XRD) analysis. In contrast, veinlet CM with a low maturity is formed at 200–350 °C and generally consistent with gold mineralization. In addition, layered CM has ^δ13C values ranging from −30 to −20%, demonstrating a biogenic origin. Consequently, it is interpreted that layered CM is formed by a pre-ore metamorphic event during Caledonian, and its reducing nature promotes gold precipitation via destabilization of aqueous Au complexes or facilitating sulfidation. Veinlet CM is of hydrothermal origin, and its precipitation modified the chemical conditions of ore fluids, leading to the destabilization of Au complexes, which therefore are favorable for mineralization.

Carbonaceous material (CM) is common in meta-sediments and is generally interpreted to be intimately associated with gold mineralization. For the Bumo deposit in Hainan Province, South China, CM is mainly hosted by greenschist facies- to amphibolite-facies metamophic rocks of the Paleo- to Mesoproterozoic Baoban Group and by auriferous veins and is used as an important gold prospecting indicator. However, the genesis of CM and the relationship with gold mineralization are still not clear. Field work and thin section observation indicates that two types of CM occur, i.e., layered and veinlet. Layered CM, up to meters thick, prevails in the deposit. More importantly, Au-bearing sulfides are commonly distributed along the boundary between the quartz veins and layered CM. In contrast, veinlet CM, co-precipitated with gold and sulfides, has the thickness of micro- to centi- meters, and these thin veins occur in quartz veins and hydrothermally altered rocks. Scanning Electron Microscope (SEM) analysis indicates that layered CM has a stringy shape and laminate structure, while veinlet CM occurs as isometric particles. Raman carbonaceous material geothermometer indicates that layered CM with high maturity is formed at elevated temperatures of 400 – 550°C, consistent with X-ray diffraction (XRD) analysis. In contrast, veinlet CM with low maturity is formed at 200 – 350°C, generally consistent with gold mineralization. In addition, layered CM has δ13C values ranging from -30 to -20%, demonstrating a biogenic origin. Consequently, it is interpreted that layered CM is formed by a pre-ore metamorphic event during Caledonian, and its reducing nature promotes gold precipitation via destabilization of aqueous Au bisulfide complexes or facilitating sulfidation. Veinlet CM is hydrothermal origin, and its precipitation modified the chemical conditions of ore fluids, leading to the destabilization of Au complexes and thus favorable for mineralization.

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