Mineralogical controls on metal distribution in stream sediment derived from the Caledonides of the Scottish Southern Uplands and English Lake District

Abstract

Stream sediment geochemistry provides an innovative method of assessing the basinal history of the Caledonian slate belts. Despite glaciation, the stream sediment geochemical patterns spatially mimic the outcrop of underlying bedrock lithologies. However, erosion from rock to sediment by fluvial processes may either increase or reduce an element’s abundance depending on the nature of its mineral host. An element held in heavy, resistate minerals will be concentrated, whereas one residing in unstable ferromagnesian minerals, which readily break down to clays during weathering, may be preferentially removed. Examples are provided from the Cr-Ni-V-Mg, base metal and Rb-Sr element suites. Primary and secondary bedrock patterns are recognized in the stream sediments. Primary patterns follow the original composition of the source bedrock, with steep gradients in the element distribution coinciding with lithostratigraphical boundaries. Such patterns also reveal subtle divisions within the established geological units for which the main compositional control was the nature of the ancient sedimentary provenance. Secondary patterns reflect remobilization of elements within the bedrock and so may cut across lithostratigraphical boundaries. These patterns (or their absence) are influenced by the thermal histories of the Caledonian basins, and so are indicative of the geotectonic regime in which the sedimentary sequences were originally deposited.