Abstract
The volcanic-related type U mineralization in South China has been associated with the crustal extension occurred since the late Cretaceous. Uranium deposits of this type are spatially related to the Cretaceous red-bed basins and synchronous mafic magmatism. However, the contribution of basinal brines or mantle-derived fluids to the U mineralization is not well understood. This study examines the role of different hydrothermal fluids involved in the U mineralization process and traces their sources by using LA-ICP-MS analysis of fluid inclusions from the Shazhou deposit in the Xiangshan district, which is one of the largest volcanic-related type U ore-fields in the world. Two types of fluids were involved in the U mineralization process. One fluid was of higher salinity, enriched in U, Mo, and Cl, such that having low Br/Cl (0.3–1.0 × 10-3) similar to the basinal brines sourced from meteoric water. The other fluid was of lower salinity, enriched in Th and Sr, yet depleted in U and Mo, with Br/Cl ratios of 1.5–2.5 × 10-3, overlapping the range of mantle-derived fluids associated with the mafic dikes intruded into the felsic wall rocks at the U mineralization age. The high-U fluid and the low-U fluid also show distinct Rb/Na, K/Na, and Rb/Cs, indicating their respective origins of basinal brines and mantle-related fluids. The chemical variation of ore-forming fluids at the Shazhou deposit was comparable to the scenario at the Zoujiashan deposit, as reported in previous studies, suggesting similar fluid evolution history related to U mineralization in the west and the north of the Xiangshan U orefield. Our study suggests in situ analysis of Br/Cl of fluid inclusions can be a useful proxy to discriminate multiple hydrothermal fluids associated with volcanic-related type U mineralization in South China and to individually trace their origins.
Published Version
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