Sandstone-hosted uranium deposits amenable for exploitation by in situ leaching technologies

Sandstone uranium deposits represent uranium concentrations formed by low-temperature hydrothermal processes, usually of diagenetic to epigenetic origin. The deposits are commonly hosted in arkosic sandstone and are therefore referred to as sandstone-type uranium. Globally, this is the most abundant type of uranium mineralisation, containing approximately 28% of the world's uranium resources and including several giant deposits with resources exceeding 100 kt of uranium. The main uranium minerals are pitchblende and coffinite, and uranium is recovered from host rocks by conventional hydrometallurgical technologies using sulphuric acid or alkaline leach. Host sediments were deposited in many different geological environments including continental intracratonic basins, intermontane depressions, coastal-plains and palaeo-river channels. Mineralisation is mostly stratabound and localised in the permeable sandstone at the redox interfaces where oxidised uranium-rich fluids have intersected with relatively reduced basin lithologies. Sandstone-type uranium mineralisation can also be distributed along permeable fault zones cutting sedimentary sequences. Deposits are subdivided into four groups: roll front (rolltype), tabular, basal channel and tectonic-lithologic types. Many sandstone uranium deposits cannot be exploited by conventional mining technologies because of low grade and difficult geotechnical conditions created by the presence of poorly consolidated wallrocks and the location of ore bodies below the water table. However, because of high permeability of the host sediments and the favourable uranium mineralogy, these deposits are well-suited to exploitation by in situ leach (ISL) technique. ISL mining is defined as the process of uranium extraction from the host sandstone in situ by injecting the chemical leach solutions directly into the ore zone. The pregnant solutions containing leached uranium are transported to the surface through production wells. Uranium is recovered from the solutions to produce yellowcake.