The genesis of sandstone-type uranium deposits in the Ordos Basin, NW China: constraints provided by fluid inclusions and stable isotopes

摘要

Quartz from sandstone-type uranium deposits in the east part of the Ordos Basin contains abundant secondary fluid inclusions hosted along sealed fractures or in overgrowths. These inclusions consist mainly of water with NaCl, KCl, CO_2 (135-913ppm) and trace amounts of CO (0.22-16.8ppm), CH_4 (0.10-1.38ppm) and [SO_4]~(2-) (0.35-111 ppm). Homogenization temperatures of the studied fluid inclusions range from 90 to 210 deg C, with salinities varying from 0.35 to 12.6 wt- percent (converted to NaCl wt percent ), implying multiple stages of thermal alteration. Although high U is associated with a high homogenization temperature in one case, overall U mineralization is not correlated with homogenization temperature nor with salinity. The H and O isotopic compositions of fluid inclusions show typical characteristics of formation water, with delta ~(18)O ranging from 9.8 to 12.3 percent o and delta D from 26.9 to -48.6 per thousand, indicating that these fluid inclusions are mixtures of magmatic and meteoric waters. The oxygen isotope ratios of carbonates in cement are systematically higher than those of the fluid inclusions. Limited fluid inclusion-cement pairs show that the oxygen closely approaches equilibrium between water and aragonite at 150 deg C. Highly varied and overall negative delta C in calcite from cement implies different degrees of biogenetic carbon involvement. Correlations between U in bulk rocks and trace components in fluid inclusions are lacking; however, high U contents are typically coupled with high [SO_4]~(2-), implying pre-enrichment of oxidized materials in the U mineralization layer. All these relationships can be plausibly interpreted to indicate that U (IV), [SO_4]~(2-) as well as Na, K were washed out from the overlying thick sandstone by oxidizing meteoric water, and then were reduced by reducing agents, such as CH_4 and petroleum, likely from underlying coal and petroleum deposits, and possibly also in situ microbes at low temperatures.