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

摘要

The Dongsheng uranium deposit, the largest insitu 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 (δ13CVPDB=−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 δ18OSMOW=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.