Bulletin of the Geological Society of Malaysia, Volume 43, Dec. 1999, pp. 261 – 273
Geological Sciences, Dept. of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada
Abstract: Stream sediment geochemical surveys are a widely used method of mineral exploration in tropical rainforests worldwide. The basic premise of such surveys is that a stream sediment is representative of the products of weathering and erosion upstream of the sample site and can thus detect anomalous concentrations of metals released from a mineral deposit within the drainage basin. Ideally, representative samples must be collected from streams throughout the survey area at a sample density sufficient to detect anomalies related to mineral deposits. Representativity of the sample must then be maintained through all stages of sample processing and analysis. There are many situations in which these ideals cannot be achieved. For example, a flood plain may isolate the stream from the valley sides or supply of new sediment to a stream may be from a few point sources (e.g., landslides), the location and activity of which varies with time. Also, fluvial processes modify the composition of the sediment as it moves along the channel with important consequences for elements, such as gold and cassiterite, associated with heavy minerals.
Detailed sedimentological studies in the S. Petai, Malaysia have shown that sediments (SG-2.7) finer than about 100 pm tend to be rapidly swept downstream in suspension whenever sediment transport occurs. This causes both the greatest and most consistent enrichment of cassiterite to occur in the finer fractions of the sediments. Heavy mineral content of coarser size fractions becomes increasingly erratic because their accumulation is strongly dependent on local hydraulic conditions on the streambed. Based on the S. Petai studies, the relatively uniform enrichment of heavy minerals in the finer fractions (<100 pm) of the sediments best represents the geochemistry of the drainage basin as a whole. Use of fine fractions also reduces the nugget effect during sampling, so that these fractions typically give the most consistent anomalous dispersion trains.
Anthropogenic disturbance, by logging or agricultural activities, can also influence sources of sediment and rates of supply. For example, in Thailand increased soil erosion, caused by deforestation and planting of maize, has resulted in a gold anomaly being diluted to the extent that it is not detected reliably by conventional stream sediments. However, effects of disturbance are not always so severe and results from a mature rubber estate in Peninsular Malaysia suggest that conditions return to near-normal once a groundcover of vegetation has been reestablished.