ALBERT

Description: 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 (δ 13 C 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 δ 18 O SMOW = 12.6 to 13.8‰) indicate that the mineralizing fluids differed from magmatic and metamorphic fluids, and were more depleted in D ( 2 H) than modern regional meteoric waters. Such a strongly negative hydrogen isotopic signature suggests that there has been selective modification of δD by CH 4 ±H 2 S±H 2 fluids. Ore-stage pyrite lies within a very wide range of δ 34 S (-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. This article is protected by copyright. All rights reserved.