| Abstract: | A regional stream sediment survey has been conducted in Sumatra north of 4°N. The survey area is approximately 38,000 km2 of mountainous terrain mostly covered by dense tropical rainforest. The overall sample density achieved by the survey was one per ten km2. However, the difficult terrain and thick vegetation blanket have meant that a uniform sample density was not obtained. The uneven sample density has presented problems with interpretation of the geochemical patterns and identification of anomalous areas for mineral exploration purposes.Two methods have been designed for identifying mineral exploration targets from the geochemical results found by the survey. The first method uses computerised probability calculations and the second a simple empirical formula. Both methods appear to give similar results.Northern Sumatra has very distinct geochemical patterns. Copper shows a linear zone of high values along the axial Barisan Mountain Range, which are derived from two main sources: the Sumatran ophiolites and copper-rich calc-alkaline intrusives. High chromium values also occur not only over the ophiolites, but also over placers derived from them. High lead values are grouped to the east of the linear copper zone and they border the oil and gas basins of the eastern coastal strip. High tin values occur west of the copper rich intrusives, but east of the present-day subduction zone off the west coast of Sumatra. The overall pattern fails to conform to the classic zonation of mineral deposits across a simple subduction system, perhaps because large transcurrent faults have sliced northern Sumatra into blocks and so brought blocks of different provenance into contact. This explanation is confirmed by the lithium distribution, which reveals two wholly distinct provinces astride the Sumatran Fault System. |
