The relationship of siltstone geochemistry to sedimentary environments and uranium mineralisation in the Beaufort Group, Cape Province, South Africa

Abstract

The geochemistry of 1700 siltstone and mudstone samples collected from 550 outcrops of the Beaufort Group (Permian to Triassic) in the Cape Province of South Africa is described and discussed. The siltstones have a relatively uniform composition and only two elements do not have a normal population distribution. The regional elemental variations cannot be readily related to known stratigraphic and sedimentological divisions in the Beaufort Basin, except in a small number of instances. These include the enhancement of Fe and K and their associated elements in clay-rich deltaic sedimentary rocks of Late Ecca (Permian) age, the increase in some biozones of Sr, Y and Zr abundances, and the northward decrease of K and Rb. However, there is a marked contrast in the element associations between the eastern and western parts of the Beaufort Basin. An association dominated by Fe, and others considered to relate to the presence of K-feldspar and volcanic fragments, are prevalent in the western part, but these are replaced in the east by the association of Ca + Sr, U + As + Mo + Cu and Nb + Y + Th. The division corresponds with both a change from meandering-channel to a floodplain sedimentary environment, and to a lesser extent with contrasting source areas for sediment in the SW and SE. Four element associations contain U in the siltstones: U + As + Mo + Cu (metal-enriched shales), Al + K + Rb (K-feldspar), Ca + Mn + Ti (possible volcanic fragments), and U + Th. The first three of these associations occur in the area containing the bulk of the established uranium mineralisation in the SW of the Beaufort Basin, indicating that the presence of lithologies capable of providing uranium for the sedimentary ores may have influenced the location of the mineralisation. Although the sedimentology of the host lithology is an important control of mineralisation, the geochemistry of the fine-grained sediments in this basin offers a viable method of statistically selecting areas of greater potential for sedimentary uranium deposits.