A comprehensive review of hydrocarbons and genetic model of the sandstone-hosted Dongsheng uranium deposit, Ordos Basin, China

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• 作者：B. F. Cao ; G. P. Bai ; K. X. Zhang ; L. K. Zhang and B. He
• 刊名：Geofluids
• 出版年：2016
• 出版时间：August 2016
• 年：2016
• 卷：16
• 期：3
• 页码：624-650
• 全文大小：10237K
• ISSN：1468-8123

文摘

The Dongsheng uranium deposit, the largest in situ leach uranium mine in the Ordos Basin, geometrically forms a roll-front type deposit that is hosted in the Middle Jurassic Zhiluo Formation. The genesis of the mineralization, however, has long been a topic of great debate. Regional faults, epigenetic alterations in surface outcrops, natural oil seeps, and experimental findings support a reducing microenvironment during ore genesis. The bulk of the mineralization is coffinite. Based on thin-section petrography, some of the coffinite is intimately intergrown with authigenic pyrite (ore-stage pyrite) and is commonly juxtaposed with some late diagenetic sparry calcite (ore-stage calcite) in primary pores, suggesting simultaneous precipitation. Measured homogenization temperatures of greater than 100°C from fluid inclusions indicate circulation of low-temperature hydrothermal fluids in the ore zone. The carbon isotopic compositions of late calcite cement (δ¹³C_VPDB = −31.0 to −1.4‰) suggest that they were partly derived from sedimentary organic carbon, possibly from deep-seated petroleum fluids emanating from nearby faults. Hydrogen and oxygen isotope data from kaolinite cement (δD = −133 to −116‰ and δ¹⁸O_SMOW = 12.6–13.8‰) indicate that the mineralizing fluids differed from magmatic and metamorphic fluids and were more depleted in D (²H) than modern regional meteoric waters. Such a strongly negative hydrogen isotopic signature suggests that there has been selective modification of δD by CH₄±H₂S±H₂ fluids. Ore-stage pyrite lies within a very wide range of δ³⁴S (−39.2 to 26.9‰), suggesting that the pyrite has a complex origin and that bacterially mediated sulfate reduction cannot be precluded. Hydrocarbon migration and its role in uranium reduction and precipitation have here been unequivocally defined. Thus, a unifying model for uranium mineralization can be established: Early coupled bacterial uranium mineralization and hydrocarbon oxidation were followed by later recrystallization of ore phases in association with low-temperature hydrothermal solutions under hydrocarbon-induced reducing conditions.