Paleovalley-related uranium: exploration criteria and case-studies from Australia and China

Abstract

Uranium exploration and mining in Australia and China are significant and increasingly important sectors of each country's respective mineral industry. Here we focus on similarities in the geology of paleovalley-related uranium mineralising systems, which can be used to refine strategies for exploration. Paleovalley-related uranium resources are developed as sandstone-hosted and surficial deposits within paleovalley-fills, deposited either on incised crystalline bedrock, or incised sedimentary cover that is often of similar age to the sediments that host the mineralisation. With respect to Sino-Australian examples, paleovalley-related uranium occurs mostly around the margins of Mesozoic and Cenozoic basins; often the mineralisation is hosted within sands contained within paleovalleys developed upon, or proximal to, Precambrian crystalline rock that contains primary uranium mineralisation. Numerous examples are known in Australia and China where granitic rocks of pre-Mesozoic age contain uranium in the range 10-100 ppm, concentrations significantly above the crustal average of 2.8 ppm uranium. Crystalline rocks with above-background levels of uranium that are incised by paleovalleys are often the inferred source of the sandstone-hosted uranium. In these regions, extensive fluvial systems developed particularly during Mesozoic and early Cenozoic times, eroded weathered basement rocks to physically disperse uranium into the cover sediments, with subsequent chemical remobilisation, dispersion and concentration of uranium occurring throughout the late Cenozoic. Paleovalley-related uranium deposits are typically hosted within unlithified sand, lignitic clays or calcreted channel-fill, deposited in continental or marginal marine environments. Sandstone-hosted uranium deposits in many cases contain impermeable clay layers, intercalated within the sedimentary sequence; impervious layers often form the upper and lower bounding strata of individual deposits. Uranium deposits are associated with reduced lithologies; mineralisation is developed where oxidising fluids (carrying dissolved U) were modified by interaction with reduced sands, the latter commonly containing pyrite and dispersed organic matter and/or seams of lignite. Mineralised bodies, often tabular or roll-shaped, formed along the interface between clay horizons and the sandy facies of paleovalley sediments. Geological, geophysical and geochemical features of the paleovalleys and related uranium deposits have been used to construct models to understand ore genesis and assist exploration for paleovalley-hosted uranium deposits. Australia and China remain highly prospective for the discovery of new paleovalley-related uranium deposits. The precise geometric definitions of the basin margin and paleovalley architecture are important in identifying exploration targets and improving the effectiveness of drilling. This requires the integration of various geoscientific data sets. Refinements in remote sensing, geophysical techniques and data processing, in combination with sedimentological and depositional interpretations, provide an efficient approach for outlining the principal drainage patterns and channel dimensions. To help reduce risk, an exploration strategy should combine these technologies with a detailed understanding of the physicochemical parameters of uranium reduction, mobilisation and preservation.