About 10 weeks were spent in Colombia from January through March 1970 examining the progress of the phosphate investigations. Field work was done in the Sogamoso area, where detailed mapped by geologists of the Instituto Nacional de Investigaciones Geologico-Mineras has shown a large area where structures are gentle and not too complex, and as a consequence, the phosphorite beds are in a potentially minable situation. The beds in the Sogamoso area are in the sandy facies of the late Cretaceous miogeosyncline, and weathered surface rocks are deficient in CaO, so that a part of the P2 O5 in these rocks is present as wavellite. No unweathered samples have been collected, but it is thought that fresh material may contain some calcite. Drilling to confirm structure and to obtain unweathered samples started in March 1970. This area is promising economically.
Resources of phosphorite in the southern part of the country, in the Departments of Huila and Tolima, are not as promising as are the resources in the Sogamoso area. The southern area is at or close to the linebetween the sandy and the calcareous facies of the miogeosyncline, andthus much of the material contains excess CaO. The structure is more complex, and the phosphorite beds are thinner than in the Sogamoso area, and much of the area is covered by a thick section of continental Tertiary sediments.
A very short visit was made to the sulfur mine on Purace volcano. The ore is native sulfur impregnating very recent tuffs. Sulfur was deposited by fumaroles. Although known reserves are small, much of Purace volcano has not been explored, and there is potential for additional reserves. Other volcanoes in the southern part of Colombia should also be investigated to determine their potential as a source of sulfur.
A few days were spent examining the aluminous laterite deposits in the Department of Cauca. The deposits are formed from intense lateritic weathering of Plio-Pleistocene tuff beds. The tuffaceous material is completely destroyed in the weathering,leaving a soft, very fine grained, very wet clay-like material. On drying, aggregates of gibbsite are formed and these are stable. It is thought that Al2O3, Fe2O3, and a part of the SiO2 in the clayey material may be non-crystalline. Reserves of the material are very large, and further work is definitely indicated.
