Arsenopyrite-Pyrite Association in an Orogenic Gold Ore: Tracing Mineralization History from Textures and Trace Elements

Abstract

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spot analysis and element mapping coupled with high-resolution focused ion beam-scanning electron microscopy (FIB-SEM) imaging has been performed on Au-hosting sulfides from a Proterozoic orogenic gold deposit, Tanami gold province, north-central Australia. Gold distribution patterns in the arsenopyrite-pyrite assemblage, which probably crystallized toward the end of the Au-forming event, suggest a process of initial scavenging of Au into arsenopyrite and subsequent remobilization of that Au following brittle fracture. Gold expelled from the sulfide lattice during the remobilization event is reconcentrated around the margins of the same arsenopyrite grains and within swarms of crosscutting microfractures. The distribution of Pb, Bi, and Ag closely mimic Au, indicating that these elements were also reconcentrated. Preservation of oscillatory zonation patterns for Co, Ni, Sb, Se, and Te in arsenopyrite imply, however, that no significant degree of sulfide recrystallization took place. Residual concentrations of invisible gold (in grain centers) are <5 ppm in arsenopyrite and <1 ppm in coexisting pyrite. The presence of submicron-sized pores is suggestive of a fluid-driven process rather than solid state diffusion. Micron-scale (2–10 μ m) remobilized gold is accompanied by fine particles (200 nm–2 μ m) and nanoparticles (<200 nm) of gold. The extensive variation of Au concentrations within single arsenopyrite grains underlines the significance of textures when using trace element data to understand how gold ores evolved.