Abstract Thorium is the most abundant actinide in the Earth’s crust and has universally been considered one of the most immobile elements in natural aqueous systems. This view, however, is based almost exclusively on solubility data obtained at low temperature and their theoretical extrapolation to elevated temperature. The occurrence of hydrothermal deposits with high concentrations of Th challenges the Th immobility paradigm and strongly suggests that Th may be mobilized by some aqueous fluids. Here, we demonstrate experimentally that Th, indeed, is highly mobile at temperatures between 175 and 250 °C in sulfate-bearing aqueous fluids due to the formation of the highly stable Th(SO 4 ) 2 aqueous complex. The results of this study indicate that current models grossly underestimate the mobility of Th in hydrothermal fluids, and thus the behavior of Th in ore-forming systems and the nuclear fuel cycle needs to be re-evaluated.
The objective of this work was to investigate flow and transport in a layered, variably saturated system consisting of both fractured rock and sedimentary material during focused infiltration from the surface. Two tracer tests were performed using the Vadose Zone Research Park (VZRP) at the Idaho National Laboratory (INL). The first test occurred under quasi‐steady‐state conditions and the second was initiated in a much drier system and thus provided information regarding flow and transport under transient conditions. A one‐dimensional analytical model was used to fit breakthrough curves resulting from the two tracer tests. The results of this modeling provide insight into the nature of flow in the fractured basalt, surficial alluvium, and sedimentary interbeds that comprise the vadose zone of the eastern Snake River Plain. Flow through the fractured basalt is focused and preferential in nature, and multiple flow paths arise due to numerous fractures functioning as transmissive pathways in addition to flow splitting along geologic contacts. Flow velocities were significantly higher during the test with the wetter flow domain, presumably due to increases in hydraulic conductivity associated with higher water contents of the geologic materials. Perching was observed above the alluvium–basalt contact and above the lower boundary of a locally continuous sedimentary interbed. The perching behavior between the two contacts was fundamentally different; the perched layer above the alluvium–basalt contact was neither laterally extensive nor temporally persistent in the absence of infiltration from the surface. In contrast, the perched layer along the interbed was significantly thicker and gave rise to lateral flow over distances on the order of hundreds of meters. Vertical transport is shown to occur predominantly through the main bulk of the sedimentary material of the interbed; lateral flow appears to occur primarily in the fractured basalt directly above the interbed.
The Slide Mountain terrane (SMT) in southern British Columbia consists of rocks of continental and oceanic affinity that are juxtaposed with parautochthonous rocks of the North American continental margin. In southern British Columbia, SMT consists dominantly of fine‐grained quartzose clastic rocks, limestone and lesser amounts of conglomerate and volcanic rocks of the Carboniferous McHardy assemblage, and predominantly mafic volcanic rocks of the Permian Kaslo Group. U‐Pb ages of individual detrital zircons from the McHardy assemblage are 1.7 Ga to 3.1 Ga and are similar to published ages of zircons from sedimentary rocks of the adjacent Kootenay terrane and the North American continental margin. These data and the petrology of McHardy assemblage sandstones and conglomerate suggest Kootenay terrane and the North American miogeocline as sources for McHardy assemblage detritus. U‐Pb zircon ages of granitoid clasts within McHardy assemblage conglomerate indicate that Silurian granitic rocks also provided detritus to the SMT. Mafic volcanic, ultramafic, and sedimentary rocks of the Kaslo Group conformably overlie the McHardy assemblage. New geochemical data demonstrate that the Kaslo Group consists of light rare earth element depleted basalts. On the basis of geochemical and geologic data, we suggest that Kaslo Group volcanics were erupted within an ocean ridge proximal to the North American continental margin and probably represent the eastern (continental) margin of a Permian marginal basin. Lithologie, stratigraphie, and U‐Pb geochronologic data suggest that the SMT was deposited on autochthonous, distal miogeoclinal rocks of the adjacent western North American craton and in close proximity to an early Paleozoic arc terrane. We infer that correlative late Paleozoic basinal terranes in western North American were deposited in a similar tectonic setting.
