Abstract
Detrital ilmenite and its altered minerals are common in sandstone-hosted U deposits in the northern Ordos Basin, north China. Petrographic observation, SEM-EDS, EMPA, and LA-MC-ICP-MS were utilized to characterize the spatial relationship between altered ilmenite and the U minerals, and to investigate the U enrichment mechanism and alteration processes of ilmenite. Ilmenite was completely or partially altered to leucoxene and anatase along its rim and crack in ore-bearing sandstone. Framboidal and cement pyrite of BSR and TSR origin were identified around altered ilmenite. Two U phases closely related to altered ilmenite contain Ti-coffinite (I) and coffinite (II). These data indicate that ilmenite alteration and the associated processes of U enrichment can be divided into two stages. Stage one involves U pre-enrichment and adsorption, with stage two involving U enrichment via TiO2 (leucoxene and anatase) catalytic reduction. Ilmenite was altered into porous leucoxene that can adsorb U as uraniferous leucoxene and Ti-coffinite (I) and framboidal pyrite directly by reactions with H2S, produced by sulfate-reducing bacteria during synsedimentary and early diagenesis stages. Altered ilmenite can enrich U in the form of coffinite (II) through a catalytic reduction reaction which is triggered by β and γ radiation produced by previously adsorbed U during the uplift of ore-bearing bed in the Late Cretaceous period. In addition, cement pyrite can also reduce U6+ into U4+ which deposits on its surface in the form of coffinite (II). These results demonstrate a new mechanism, mediated by adsorption and catalytic reduction, to enrich U in sandstone-hosted U deposits.
Highlights
Oxidized Fe–Ti minerals have been widely reported in roll-type sandstone uranium (U) deposits located in south Texas, United States [1,2,3,4,5,6]
It is generally agreed that sulfate-reducing bacteria produce isotopically light reduced sulfur, whereas thermal sulfate reduction processes result in comparatively isotopic heavy sulfur [45,46,47,48]
The δ34S values of framboidal pyrite are negative and relatively homogeneous, indicating that it is potentially the product of sulfate-reducing bacteria (SRB), and the sulfur in framboidal pyrite is likely from the sediments [44,46,47]
Summary
Oxidized Fe–Ti minerals have been widely reported in roll-type sandstone uranium (U) deposits located in south Texas, United States [1,2,3,4,5,6]. Detrital ilmenite and its associated altered minerals have been widely identified in sandstone-hosted U deposits and U minerals in the basins of northern China. The U enrichment mechanism of altered ilmenite and its specific role in the process of U mineralization are still unclear, especially regarding the U enrichment of altered ilmenite Previous studies proposing this adsorption mechanism have been unable to adequately explain the presence of U minerals located in the proximity of altered ilmenite [10,11,12,13,14], which indicates that the relationship between altered ilmenite and U minerals present may be related to reduction. In this study, sandstonehosted U deposits located in the northern Ordos Basin of north China were selected to conduct investigations into the U enrichment mechanism of altered ilmenite in sandstonetype U deposits. The findings are significant in the context of mineralization mechanisms and the theory of sandstone-hosted U deposit genesis
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