Geological setting, nature of mineralisation, and fluid characteristics of the Wang Yai prospects, central Thailand

Abstract

The Wang Yai prospects are located along the Loei Foldbelt, central Thailand. They are a newly discovered resource and up until now have not been the subject of any scientific research. Eleven vein systems at Wang Yai were investigated in terms of vein textures, mineralogy, fluid inclusions and stable isotopes. The vein systems occur as either well-preserved outcrop or sparse sporadic float. The vein systems can be divided into two types. They include: (1.) mineralised veins which are characterised by quartz - chalcedony - calcite ± adularia and (2.) poorly mineralised veins characterised by chalcedony and opaline quartz. The vein systems are hosted in volcanic and volcanosedimentary units intruded by andesite and diorite. LA-ICP-MS U-Pb zircon geochronological studies have revealed the age of host rocks as Late Permian to Early Triassic. Volcanology studies of the host rocks indicate that the depositional environment was a submarine below wave base setting. Zr/Y vs Zr plot of least altered volcanic rocks have identified the andesitic sandstone and andesitic lithic breccia as having island arc affinities. The younger andesite and diorite intrusives were found to have continental arc affinities. These findings are consistent with interpretations of the volcanic setting at the nearby low-sulphidation Chatree deposit. Vein texture, mineralogy, alteration and paragenesis studies have identified zonation between gold-bearing vein systems and barren vein systems. Gold-bearing systems yield crystalline quartz, lattice-bladed quartz, pseudo-acicular quartz and pseudomorphs after adularia. Ore mineralogy in these systems is hosted in ginguro bands which comprise of electrum ± argentite ± pyrite assemblages. Barren vein systems yield chalcedony and opaline quartz assemblages with associated recrystallisation textures. Based on quartz textural analysis and ore mineralogy assemblages the gold-bearing veins are interpreted to have formed at levels deeper in the system than the barren vein systems. Fluid inclusion studies of quartz indicate salinities are between 1 to 5 wt % NaCl equiv. Fluid inclusion homogenisation data yields three different ranges. Crystalline gold-bearing quartz yields temperatures of 187 – 218 C°, comb quartz yields temperatures of 260 - 290 C°.and crustiform-colloform veins yield temperatures of 140 – 200 C°.Stable sulphur isotope studies of pyrite range between +3.82 and -0.04 per mil. These values indicate a magmatic source. Stable oxygen isotope values of quartz range from +11 to +17.5 per mil. Low oxygen isotope values range from +11.0 to +13.0 per mil and are associated with crystalline quartz gold bearing systems. High oxygen isotope values (+15.0 to +17.5 per mil) are associated with barren quartz chalcedony dominated systems. The volcanic setting, vein textures, mineralogy and alteration assemblages of the Wang Yai prospects are comparable with the nearby low-sulphidation epithermal Chatree deposit. The Wang Yai prospects appear to be at the top of the broader epithermal system and therefore potential for high grade mineralisation at depth exists.