SURFACE GEOCHEMICAL TECHNIQUES IN GEMSTONE EXPLORATION AT THE ROCKLAND RUBY MINE, MANGARE AREA, SE KENYA

This study was undertaken to investigate the feasibility of surface ge ochemical techniques applied to exploration for coloured gemstones in the Mangare area, SE Kenya. At the study site, the Rockland (lately Jo hn Saul) Ruby Mine, gem corundum mineralization is found in a pegmatit e reef lining the boundaries of a major ultramafic body with gneisses. Overburden, 1 to 3 m thick, is represented by secondary limestone, gr avels and soils. The latter were subjected to bulk sampling and in sit u selective leach by the Method of Diffusion Extraction (MDI). Anomalo us Ni, Co and Cr individual and product values were returned from the bulk soil and MDI determinations over the concealed ultramafics (UM). The MDI anomaly gave greater definition of the UM body. Additional ano malies were identified in the calcareous soils which appear to have a superior ability to accumulate trace elements susceptible to MDI extra ction. However, this landscape effect can be eliminated by differentia l data treatment. In addition, the planar distribution of an artificia l factor, the Local Empiric, is shown to delineate the pegmatite zone, regardless of soil type. It is concluded that the most cost-effective approach would involve bulk sampling over a loose grid to define ultr amafic belts, followed by MDI or other specific techniques to accurate ly delineate the individual bodies and their margins. (C) 1997 Elsevie r Science B.V.