Morphology and geochemistry of placer gold from Wang River, Lampang Province, Northern Thailand

Wang River (Mae Nam Wang) in Lampang Province, Northern Thailand which is surrounded with many lode gold deposits has potential to develop as a placer gold deposit. Gold particles from 10 sites in Wang River range from fine to coarse (<100μm, up to 800μm), showing variable shape and surface texture. We have classified gold particles morphologically based on roundness, flatness, and outline using the SEM (Scanning Electron Microscope) method. Many gold particles are sub-rounded, thin, in complex form, and flattened by hammering resulting from fluvial transport. Some of the grains are deformed in terms of folding indicating a high degree of flattening. These shape parameters can provide information suggesting the distance of transport to the source of the placer gold. The assemblage of heavy minerals in this area consists of garnet, spinel, magnetite, ilmenite, rutile, tourmaline, and wüstite. Measured Au, Ag, Cu, and Hg contents in the gold grain by EPMA (Electron Probe Micro Analyzer) resulted in data that showed that gold occurs as electrum or high purity gold with high fineness (1000Au/Au+Ag) levels up to 951 as well as a minor amount of Cu and Hg. Normally, the zoning patterns of Au and Ag are common in placer gold and the high fineness in rim can be explained by either selective extraction of Ag or precipitation of Au. However, there is no significant difference in fineness between core and rim in gold grains in this study even though there are differences in composition among grains. Many cavities are also shown filling with silicate and/or clay mineral in microtextures. These geochemical signatures from microtextures are capable of providing genetic implication. Furthermore, the combination of results such as shape parameter, geochemical signature, and assemblages of heavy minerals can be a useful tool in exploration for gold.