Geological setting and mineralisation characteristics of the Long Chieng Track and Ban Houayxai deposits, Lao PDR

The Long Chieng Track (LCT) Au deposit and Ban Houayxai (BHX) Au-Ag deposits are located in the Phu Bia Contract Area (PBCA), at the confluence of the Loei and Truongson Fold Belts in the northern Lao PDR. The LCT deposit is hosted by an Early Carboniferous sedimentary sequence intruded by Early Permian intrusive rocks, whereas the BHX deposit occurs in the Early Permian volcano-sedimentary sequence. The igneous units occurring at LCT and BHX are geochemically characterised by calc-alkaline affinities, and yield Early Permian age as revealed by the LA-ICPMS U-Pb zircon geochronological study. Detailed mineralogical and paragenetic studies indicate that the LCT deposit has five mineralisation stages including single syn-mineralisation stage (i.e., stage 2), whereas seven mineralisation stages are recognised at the BHX deposit including three syn-mineralisation stages (i.e., Stages 1, 2 and 3). Alteration is extensively formed at LCT and BHX associated with the Au-rich vein systems, consisting mainly of sericite, chlorite, adularia and quartz assemblages. On the basis of core assay and the electrum fineness data, the vein system of the Stage 2 at LCT and the Stage 1 at BHXT are identified as Au-rich system, whereas the Stages 2 and 3 of BHX are recognised to be Ag-rich vein system. Gold at LCT occurs as electrum and native gold, while the occurrence of electrum, native silver and stephanite is found at BHX. These precious metals are commonly associated with sulphides including pyrite, sphalerite, chalcopyrite, galena and arsenopyrite. The LA-ICPMS analyses of trace elements in pyrites at the both LCT and BHX deposits reveal that the variable amount of gold is hosted in pyrite structure, closely associated with Ag, As, Sb, T1, Co and Pb elements. The obtained fluid inclusion data for the Au-rich vein system and Ag-rich vein system indicate discrete and different characteristics, having homogenisation temperature ranges from 175 to 215 and from 240 to 415°C respectively, and the salinity values are form 1.7 to 8.3 and 2.7 to 13.0 NaCl wt. % equiv. respectively. The isotope analyses of, sulphur of the sulphides in syn-mineralisation stages at LCT and BHX indicate that the sulphur has a magmatic source. The oxygen and hydrogen isotopic compositions of the LCT and BHX show a mixing nature of the magmatic water and meteoric water. The lead isotopic compositions suggest the metals were mainly derived from crustal source at the LCT and BHX deposits. The present integrated field, mineralogy, paragenesis, fluid inclusions and isotopic studies indicate that the Au-rich vein system occurring at the both LCT and BI-IX deposits are comparable to the deeper part of the low-sulphidation epithermal model of Buchanan (1981), whereas the Ag-rich structurally-hosted vein system at BHX may be comparable to the middle to deeper part of the Ag-rich Mexican epithermal model of Albinson et al. (2001). However, the presence of CO2 and CH4 gases in fliiid inclusions at LCT as .detected by Laser Raman spectrometric analysis cannot rule out the intrusive-related style for the LCT deposit.