An evaluation of geochemical parameters for tin exploration in soil covered areas in the central Bushveld complex

English: The thick soil cover over most of the felsic rocks of the Bushveld Complex, presents a major obstacle in the search for Sn deposits. In order to define those parameters that might reflect the presence of Sn mineralisation under such conditions, an orientation study was made of soil geochemical parameters in an area of known Sn mineralisation. The area chosen was the farm Vlaklaagte 221JR in the central Bushveld Complex where quartz-vein greisen type Sn mineralisation is known to exist. The soils overlying the mineralisation are latosols in which three periods of soiI formation can be identified. A pebble layer representing an unconformity in the soiI profiles, with transported soils on top and residual soils or palaeo-colluvium below, has a dominating influence on the mineralogical and geochemical dispersion patterns in the soils. A consequence is that the B horizon, to which most attention is usually paid during exploration, presents an entirely unsatisfactory sampling medium due to its development in either the transported or residual soil. Mineralogical and geochemical data were used to identify the various weathering cycles and the relationships between primary and secondary anomalies. Special attention was given to the behaviour of Sn, Cu, Mo, Rb, Sr and Ba, which were then used to establish possible guidelines for exploration. Where Sn is used as a pathfinder element the pebble layer yielded the best results as a sampling medium. A combination of 1) the Sn concentration in a bulk sample in the lower section of the lower pebble layer, 2) the ratio of the Sn concentrations (using bulk samples) between the lower section of the lower pebble layer and the residual soils and 3) the distribution patterns of cassiterite grain-size populatlons in the lower pebble layer (lower section) proved to be the most sensitive means for exploration (provided that the Sn is associated with cassiterite and that the pebble layer is in direct contact with the residual soils). Copper and Mo are also very sensitive pathfinder elements but their source is not necessarily the same as for Sn. They are, however, always closely associated with the Sn deposits and can therefore be used effectively in tracing associated mineralisation which wilI eventually lead to the Sn deposits. The most sensitive parameters proved to be a combination of the Cu or Mo concentrations In the siIt-clay fraction in the lower part of the C horizon and their concentrations in individual goethite pellets from the lower section of the lower pebble layer. Rubidium/Sr and Rb/Ba ratios are useful in identifying highly differentiated or metasomatic zones with which Sn deposits are associated. The silt-clay fraction in the lower part of the C horizon is recommended for the latter. The weathering cycles and the elemental dispersion patterns are found to be dependant on the following factors: The palaeo- and present climatic conditions. The topography (palaeo- and present). The primary minerals with which the elements are associated and the weatherabiIIty of the minerals. The predominant end minerals in the soils and their elemental association. The mobility of the elements under different Eh-pH conditions.