Sandstone Hosted Deposits
Sandstone-hosted uranium deposits are typically contained within medium to coarse-grained and poorly sorted, fluvial or marginal-marine sandstone. The sandstone is typically over and under lain by impervious layers of siltstone or shale which allows groundwater to be focussed into the sandstone. The sandstones usually contain pyrite and organic matter which may be disseminated or form minor coal seams. The organic matter is important as it allows reducing conditions to be created in the host rock.
As uranium is mobile under oxidising conditions and precipitates under reducing conditions, the presence of a reducing environment is essential for the formation of uranium deposits in sandstone. Post-Silurian (approximately 450 Ma) continental sandstone within tropical to temperate latitudes (ie ±50° of the palaeo-equator) is considered a potentially favourable host because of the widespread development of land plants which began in the equatorial Silurian period.
Sandstone with a slight dip, such as on the margins of continental basins and coastal plains, is favourable for uranium mineralisation processes as the rate of groundwater movement and oxygen intake are low enough to preclude destruction of reducing environments and favour the precipitation of uranium minerals. Sandstone units with low dips may also provide a large surface areas for the capture and introduction of uraniferous groundwater into the host rocks.
Based on the shape of the uranium mineralisation and depositional or structural environment, sandstone-hosted uranium deposits may be further subdivided into three categories:
- Tabular deposits with mineralised zones parallel to the direction of groundwater flow. On a local scale the ore zones may cut across sedimentary features of the host sandstone.
- Roll-front deposits which tend to be crescent-shaped in cross-section. Uranium mineralisation may cut bedding and extend from the overlying to the underlying impervious mudstone/siltstone layers. The mineralised zone is convex down the hydraulic gradient.
Mineralisation usually has a diffuse boundary down-dip (into a reducing environment) and a sharp boundary up-dip.
Tectonic–lithologic deposits occur along permeable fault zones which cut sandstone mudstone sequences. Mineralisation tends to inter-finger into the permeable sandstone layers adjacent to the fault. Often there are a number of mineralised zones ‘stacked’ vertically on top of each other within sandstone units adjacent to the fault zone. Sandstone deposits contain a large proportion of the world’s known uranium resources, although they are commonly of low to medium grade (0.05% to 0.4% U3O8).
In each province or basin there are usually many small to medium-size deposits but the cumulative tonnage in the province or basin may be large. Major sandstone uranium provinces include the Powder River Basin in Wyoming, Colorado Plateau and Gulf Coastal Plain of the USA, and the Tim Mersoi Basin of Niger. Sandstone deposits comprise approximately 7% of Australia’s and 18% of the world’s total known uranium ‘Identified Resources’. The principal uranium minerals tend to be uraninite and coffinite which are readily extractable. Furthermore, due to the inherent permeability of the host rocks, the mineralisation may be amenable to low cost, in-situ leach mining methods.
Regalpoint’s Lake Gregory, Pollock Hills, Mt Walter and Lyons/Curbur projects belong to this class of deposit.