Diagenetic and hydrothermal history of the host rock of the Imouraren uranium deposit (Tchirezrine 2 Formation - Tim Mersoï Basin, Niger)

Abstract

The Imouraren uraniferous deposit is located in NW Niger and is hosted in the fluvial Tchirezrine 2 Formation (Jurassic, Tim Mersoï Basin). High-energy facies range from medium to very coarse-grained sandstones; low-energy facies are composed of fine-grained sandstones and analcimolites. Detrital material, which mainly occurs in sandstones but also occurs to a lesser extent in analcimolites, consists of quartz, K-feldspars, albite and accessory minerals.Since their deposition, these sediments have undergone transformations which were induced by two main processes. Burial diagenesis comprises the compaction and crystallization of secondary minerals, which consist of analcimes, clay minerals (i.e., chlorite, kaolins, illite), and quartz and albite overgrowths. This paragenesis suggests a burial depth of approximately 2 km, with maximum temperatures ranging from 80 to 120 °C. An alteration stage, related to the circulation of fluids, induced the alteration of diagenetic minerals, such as chlorite. This fluid/rock interaction is responsible for the deposition of uranium and the neoformations of clay minerals, such as tosudite and montmorillonite, as well as hematite, harmotome, calcite and copper sulfides. The fluids involved were U–Cu–Ba–Mg–Ca-rich, which enabled the crystallization of new minerals, and oxidizing, which destabilized chlorite and facilitated the transport of uranium.