The Samburu - Marsabit Geological Mapping and Mineral Exploration Project was a joint Kenyan and British technical co-operation project, carried out by staff of the Mines and Geological Department, Ministry of Environment and Natural Resources, Kenya and staff from the British geological Survey. The first phase of the project commenced in 1980, and covered the area between 36° and 38° E and from the equator to 2° N. The second phase, carried out between 1984 and 1986 covered the area between 36° and 38° E and from 2° N to the Ethiopian border. Sampling was carried out concurrently with geological mapping and was largely constrained by the requirements of that exercise. Little or no sampling was done in areas previously mapped by other bodies. Sampling was mainly confined to areas underlain by basement rocks of the Mozambique Belt and was very sparse over most of the Tertiary and Quarternary volcanic cover.
Sampling densities varied considerably across the Phase 1 project area, but generally a stream sediment sample density of one per 4 to 8 km² and a panned concentrate density of one per 13 to 16 km¹ was achieved. In the Phase 2 area, which was mainly very arid, a few samples were collected from dry stream beds, as part of a helicopter survey of the area, to provide some idea of the geochemistry of the major lithological units.
Stream sediments were collected by combining grab samples from 5 to 10 points within a 10m radius of the selected site. If necessary the samples were dried before being sieved and the fine (-80 mesh B.S.) fraction retained for analysis. Heavy mineral concentrates were obtained by taking 2 to 5kg of material from the sample site and panning at the base camp, where water was available, or at the Mines and Geological Department headquarters at Nairobi.
The fine fraction from the stream sediment samples was analysed by atomic absorbtion spectroscopy (AAS), after digestion in hot 1:1 hydrochloric acid, for Ag, Ba, Co, Cu, Fe, Li, Mn, Mo, Ni, Pb, Sr and Zn. Initially the analyses were carried out in the laboratories of the Mines and Geological Department, but for a number of reasons it unfortunately became necessary to switch to Geosurvey International Ltd after approximately one quarter of the total number of samples had been processed. Precision of analysis was monitored at both laboratories by the submission of control samples and replicates. For the Mines and Geological Department the analytical precision generally varied between 20 and 40%, depending on the element and concentration, the results being worst at low concentrations. For the Geosurvey analyses the precision was generally less than 20%, except for Pb and Li. Analyses for Ag, Mo and Fe are not available from the British Geological Survey.
Pan Concentrates were analysed by X-ray fluorescence (XRF) at the Department of Geology, University of Nottingham. The elements determined were As, Ba, Bi, Co, Cr, Cu, Fe, La, Mn, Mo, Nb, Ni, Pb, Sn, Sr, Th, Ti, U, V, W, Zn, and Zr. Because of the small amounts of concentrate usually available it was not possible to monitor the precision of the XRF analyses using replicates, but the laboratory claims a figure of better than 8% at the 100 ppm level for all the elements determined.
Source of stream sediment analyses by 1:50 000 topographic sheet from Hackman et al. (1989) Phase 1:
Relationship between 1:50 000 map sheets and the laboratory used to analyse the geochemical exploration samples. M = Mines and Geological Department; G = Geosurvey International Limited
Number of samples (sediment + concentrate) collected in Phase 2 area: (Some of these analyses are reported in the sheet reports, but no complete dataset is available in the U.K.)
Phase 2: all sample analyses by Geosurvey International Ltd.
Sheet numbers for the Phase 1 area. 1:50,000 scale sheets are numbered and outlined in black and 1:250,000 scale sheets are numbered on line intersections.