Abstract:

The paper present gold and pyrite study result at the Me Xi gold deposit in Vinh Linh, Quang Tri, central Vietnam. Electron microprobe analysis of gold grains from ore Stage 2 sample yielded high gold fineness values (~845), implying that the transport of Au in ore fluids takes place as chloride complexes rather than bisulphite complexes and the gold transport occurred in high temperature (>300 oC). However, the wide range of the inferred gold fineness (IGF) values (268-953 with a mode of 850-953) calculated from the fire assay analysis may suggest various physico-chemical conditions of Au-bearing fluid. The low-moderate IGF may reflect the low-moderate temperature (<300 oC) of gold deposition and gold is transported by bisulphite complexes. The LA ICP-MS trace element investigation of pyrite shows very poor Au content (mostly <1 ppm) and no inclusion of free Au in all selected pyrite. However, it shows a positive correlation between the concentrations of Au and Ag, Cu, Sb, Tl, Pb, Bi, As, Ni and Co in pyrite 2B. The Stage 2 pyrite exhibit Ni-As-Ag-Sb-Pb-Cu-Bi rims as the concentrations of those metals in the rim of the late sub-stage pyrite 2B. The high concentrations those metals in Stage 2 may be partially leached from the organic-rich host sequence of shale and black shale by hydrothermal fluids with the convection of meteoric water.