Resource estimation of in situ leach uranium projects

Abstract

Sandstone-type uranium deposits contain approximately 28% of the world uranium resources. Many of these deposits are located below the water table in weakly lithified or non-consolidated sands, and therefore they can be exploited using in situ leach (ISL) technology. Such technology is based on dissolving uranium minerals directly in their host rocks (in situ) by reactive solutions that are injected through drill holes and then pumping the dissolved solution to the surface through some discharge drill holes. Uranium grade is determined by down-hole geophysics, in particular the prompt fission neutrons, or PFN, technique, coupled with sampling and assaying of the drill core. The drill grid which is used in Kazakhstan for definition of ISL uranium resources are as follows: (i) Measured: average 50×100 m (range from 25×50 to 50×100 m); (ii) Indicated: average 50×200 m (range from 50×100 to 50×200 m); and (iii) Inferred: average 50×400 m (range from 50×400 to 100×800 m). Estimation and reporting of uranium resources for ISL projects differ from hard rock mining projects in the need for quantitative estimation of the geotechnical and hydrogeological parameters which are specific for ISL technologies. The main parameters which need to be considered are as follows: (i) grade and geometry of mineralisation are estimated with accuracy sufficient for supporting the remote mining; (ii) if grade is estimated using the gamma logging technique secular disequilibrium should be studied and reported; (iii) hydrogeological confinement of the mineralised horizon; (iv) permeability of the mineralised horizon; (v) composition of the host rocks, in particular the carbonate content, in order to estimate if uranium mineralisation is amenable to dissolution by acid or alkaline solutions; (vi) groundwater flow; (vii) aquifer salinity; and (viii) rate of the in-situ dissolution of the uranium minerals. Hydrogeological and geotechnical information is obtained by testing the drill core samples and in the field, using the pump tests and the down-hole piezometers. Modifying factors for conversion resources to reserves are verified and corrected using field leach tests of uranium. This test is a strict requirement for feasibility studies of the ISL uranium projects in Kazakhstan.