Abstract
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.
Highlights
Introduction nal affiliationsSince the 1980s, breakthroughs in uranium prospecting and scientific research on granite-related and volcanic-related mineralization, theories of uranium mineralization, including magma differentiation uranium mineralization theory, hot water extraction theory, epithermal fluid transformation theory, and continental weathering theory have enabled Chinese geologists to propose dual-mixed, hot spot-related [1], and complex crust-mantle interactions [2,3]
Based on the above-mentioned studies, the spatial relationship between uranium minerals and pyrite in the Mianhuakeng deposit can be identified as follows: uranium minerals occur either in the center or periphery of vein-filling ores; alternately, they are arranged with coprecipitated gangue minerals, or they occur as veinlet and disseminated structures in cataclastic altered granite
Combined with their intact crystals, characteristics of inlaid growth, and flat boundaries between them and other gangue minerals, these observations indicate that uranium minerals and pyrites were directly produced by the same ore-forming fluid during the same stage
Summary
The Changjiang uranium ore field is located in the Zhuguang batholith, which is situated between the southwest margin of the Fujian-Jiangxi post-Caledonian uplift and the southeastern margin of the Hunan-Guangxi-Guangdong Hercynian-Indosinian depression belt in the northern South China Block. Indosinian medium- to fine-grained porphyritic two-mica granite (γ5 1–3 ) (Figure 1) dated at 232 ± 4 Ma (zircon age SHRIMP U-Pb; [23]) is well developed in the middle and eastern areas of the Changjiang uranium ore field, whereas Early Yanshanian medium- to coarse-grained porphyritic two-mica granites (γ5 2–1 ) and biotite granite (γ5 2–2 ) (Figure 2) with intrusion ages between 164 and 155 Ma are the main components of the western and deep parts Both periods of granites were later intruded by NE-, NW-, and nearly E-W-trending mafic dikes (110−90 Ma; [23]) and a small amount of syenite. As a result of its representativeness of metallogenic characteristics, the Mianhuakeng deposit has become a focus of uranium metallogenic research
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