Morphology and Geochemistry of Placer Gold from Wang River, Lampang Proince, Northern Thailand

Abstract

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 binocular microscope and the SEM (Scanning Electron Microscope) method. Many gold particles are sub-rounded, thin, 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. Surface texture is useful indicators of ancient transport conditions and depositional environment. In this sample, pits and cavities are generally found in most of grains especially showing mechanical/chemical abrasion. Many cavities are also shown filling with silicate, almuminosilicate, and clay minerals in microtextures. Some of grains show distinctive manganese oxide precipitation on gold surface. 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 usually occurs as electrum with as a minor amount of Cu and Hg. Normally, the zoning patterns of Au and Ag are common in placer gold and the composition change of placer gold from core to margin of placer gold in upper stream of Wang River, can be explained by selective extraction of Ag. These geochemical signatures 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.