Geochemistry of Au-bearing pyrite from the Sepon Mineral District, Laos DPR, Southeast Asia: Implications for ore genesis

The Sepon gold-copper district occurs along the Truong Son Foldbelt in a 10 km × 40 km east-trending corridor in southern Laos. This study has investigated the nature of gold in pyrite and ore-forming fluid characteristics using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Proton Induced X-ray Emission (PIXE) techniques. Gold is present as invisible gold (Au⁺¹) and as nanoparticles of gold (Au⁰) in both sedimentary-rock hosted gold and skarn Cu-Au deposits. Gold is present in an early syngenetic pyrite generation (pyrite 2B) with concentrations to 0.5 ppm. Post-sedimentation processes included formation of quartz veins which host pyrite 3B with concentrations to 1.8 ppm Au. The highest concentrations of gold were found in the rims of hydrothermal vein-hosted pyrite 4A and in skarn related pyrite, suggesting gold concentration at a later stage during the ore-forming processes and not during sedimentation.

As rhyodacite-porphyry is proximal to calcareous shale and dolomite that host ores, it is plausible that the emplacement of rhyodacite-porphyry introduced significant amount of hydrothermal fluids that circulated through fractures and favoured high grade (to 293 ppm Au) in pyrite 4A and pyrite SKN2. Gold is preferentially concentrated in As-rich zones and in areas that underwent brittle deformation, as evidenced by fracture-fill coarse-grained pyrite 4A, indicating a structural control to ore formation. The predominance of Se, Bi and Te in pyrite SKN1 and the elevated concentrations of Bi, Cu, Se, Sn and Mo in pyrite SKN2 raise the possibility of derivation from a magmatic source. The presence of primary hematite associated with the skarn suggests prevailing oxidising fluid conditions during Au mineralisation in pyrite SKN2.There is a striking chemical similarity between pyrite 4A core and py-SKN1 and rimmed zones of pyrite 4A and pyrite SKN2, which indicates possibility of two types of hydrothermal fluids. The As-rich fluids with elevated concentrations of Au, Se, Sb, Tl, and Cu recorded in pyrite 4A rims and pyrite SKN2. The As-poor fluids with elevated concentrations of Ni and Pb yielded in pyrite 4A cores and pyrite SKN1. The chemistry of pyrite has also shown that both ore-forming fluids are undersaturated in Au similar to Au-bearing pyrite in the Carlin-type deposits, Nevada, USA.