- © 2006 Geological Society of South Africa
The pegmatite deposits situated in the centre of a triangular area between the settlements of Vioolsdrif, Henkries and Steinkopf in the lower Orange River region constitute the westernmost extension of the Northern Cape pegmatite belt. Aspects regarding the resource classification, geology, and the economic viability of these pegmatite deposits are considered here. Current ore deposit models are reviewed and the deposits are classified according to recognised classification schemes. Feldspar is regarded as the primary economic mineral for exploitation based on its abundance and the consistency in the demand for this product. A financial analysis demonstrates the exploitability of the deposits, based on the resources quantified and the analysis of the feldspar market. Other potentially economic mineral constituents, occurring in the pegmatite body, could only be considered when more information on their distribution and abundance is available.
Pegmatites of the study area belong to lithium-caesium-tantalum enriched group of rare element bearing granitic pegmatites (Ginsburg et al., 1979). Both homogenous and zoned varieties are present. The currently prevailing ore deposit model for the formation of these pegmatites regards the pegmatite body as the final stage of crystallization of a cooling granitic magma (Cerny, 1998a). Recent studies suggest that crystallization of the pegmatite body happens at unexpectedly fast rates during under-cooling conditions, in which the rate of cooling exceeds the rate of crystallization (London, 2005).
The pegmatites of the study area are mostly developed in granites and granodiorites of the Vioolsdrif Suite, in a late- to post-orogenic tectonic setting. An apparent age discrepancy makes it improbable that the source of the pegmatite-forming liquids was the Vioolsdrif Suite. It is proposed that the pegmatites were derived from a younger suite of plutonic granitic intrusions, currently located largely beneath the surface, and that the late magmatic fluids derived from these crystallizing granites in depth, were injected into shear related tension fissures.
It is estimated that the larger pegmatite bodies in the study area host a global resource of approximately 50 million tons of pegmatite ore, belonging to a resource category referred to as resources of undetermined economic significance. The markets for feldspar currently display an increasing price tendency and there exists a positive outlook for the future demand for this mineral product. The financial analysis suggests that, although mining operations on pegmatite bodies are highly sensitive to changes in commodity prices, revenue, and costs related to fuel consumption (including transport), their exploitation is feasible under the current market conditions.