USE OF A SENSITIVE ANALYTICAL METHOD AND THE SILT CLAY (LESS-THAN-53 MU-M) FRACTION OF STREAM SEDIMENTS IN EXPLORATION FOR GOLD IN NORTHERNTHAILAND

Heavy mineral concentrates (SG> 3.3) from the Huai Hin Laep, a tributa ry of the Huai Kho Lo River in northern Thailand, contain strongly ano malous concentrations of gold. In contrast, the gold content Of the as sociated < 149 mu m and <53 mu m fractions of the sediment is generall y less than the 5 ppb detection limit obtained by a conventional fire assay-atomic absorption spectrometry method. To test for the presence of a gold anomaly at concentrations < 5 ppb, we have used an aqua-regi a digestion followed by an Amberlite XAD-8 column preconcentration tec hnique that, when used with a spectrometer that enables full display o f the analytical spectrum to optimize baseline analysis, gives a detec tion limit of 0.1 ppb Au. Gold content of the <53 mu m sediment fracti on ranges from 1.0 to 3.1 ppb compared to concentrations that typicall y range from 1000 to more than 100,000 ppb in the heavy mineral concen trates. However, despite gold concentrations in the sediment being sev eral orders of magnitude lower than those in the heavy mineral concent rates, the downstream dispersion patterns are similar, with gold conce ntrations increasing at high energy sites and downstream away from the assumed source. These results identify the presence of a greatly dilu ted gold anomaly in the <53 mu m fraction of the sediments and suggest that transport and deposition of this fine grained gold is controlled by the same sedimentological factors that control the behavior of gol d in the sand-size range, between 53 mu m and 425 mu m. Most important from an exploration standpoint, is that by using a sufficiently sensi tive analytical method, meaningful gold dispersion patterns can be rec ognized at concentrations below 5 ppb.