Behaviour of gold in stream sediments, Huai Hin, Loei region, northeastern Thailand

Stream sediment sampling for gold exploration has encountered various problems: these include location and type of sample to be taken, determination of the appropriate sample size in view of gold particle sparsity, and the apparently erratic distribution of gold in stream sediments. Study of the behavior of gold in stream sediments could help to solve these problems and is needed to guide systematic exploration for gold in Thailand. The Huai Hin Laep, an intermittent third order stream in Loei region, northeastern Thailand, drains a hilly area underlain by highly weathered sandstones, shales, andesites, and tuffs, blanketed by residual lateritic and podzolic soils. The stream reach is approximately 8 km long with an average gradient of 0.008. The original mixed evergreen forest has been logged and cleared for agricultural purposes. Active stream sediment samples collected from point bars and pavements along the stream reach were processed to obtain 8 size fractions. Of these, five size fractions between 0.425 and 0.053 mm were separated into heavy and light mineral fractions, and analyzed for gold by fire assay-atomic absorption spectrophotometry. The -0.053 mm sediment fraction was split, pulverized and further split prior to analysis. The corresponding dry-sieved -0.150 mm sediment fraction was also processed and analyzed for gold. Results show that in both point-bar and pavement samples gold is concentrated in the heavy mineral fractions, whereas in all but six samples, the corresponding light fractions and the -0.053 mm fraction contain < 5 ppb gold. Similarly, thirteen out of the sixteen -0.150 mm sediment samples contain less than 5 ppb gold. Gold content is typically higher at pavement than at point-bar sites where gold concentrations are closely correlated with narrow stream channel, shallow channel depth, high flow velocity, coarse-grained sediment texture and high bed roughness, indicating that higher energy conditions favour accumulations of gold. Estimates of numbers of free gold particles suggest that analysis of heavy mineral concentrates (between 0.425 and 0.053 mm fraction) from a 40 kg -12 mm field sample from either point-bar or pavement site has a high chance of detecting anomalous gold. In contrast, the probability of reliably detecting gold in a 30 g analytical subsample is very low. With respect to mineral exploration, conventional stream sediment samples will usually fail to detect the gold anomaly in the Huai Hin Laep. This probably results from the dilution of the Au-rich heavy mineral fractions by the barren light minerals and large amounts of silt-clay. The presence of anomalous concentrations of gold would, however, be recognized through the use of field pan concentrates or heavy mineral separates. During regional surveys, samples for this purpose should be collected from either pavement sites or high energy point-bar sites characterized by a narrow channel, shallow depth, high flow velocity and large amount of coarse grained sediment along the lower reaches of the third order streams. Subsequently, detailed follow-up surveys should consist of more detailed sampling of either 40 kg -12 mm sediments or field pan concentrates from at least 20 kg of sediments along the stream. Anomalous concentrations of gold at the lower reaches of the stream may result from accumulation of gold by hydraulic processes rather than the location of gold mineralization. High gold concentrations at low energy sites characterized by slow flow velocity, low bed roughness and fine grained sediment texture may indicate proximity to the source of gold.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat