Pyrite zonation and source of gold in the Pangjiahe orogenic gold deposit, West Qinling Orogen, central China

Abstract

The Pangjiahe deposit is an orogenic gold deposit located on the northwestern margin of the Fengtai Basin and has proven reserves of 38 t Au at an average grade of 6.3 g/t. Phyllite is widely distributed in the mining area with Triassic granite porphyry and diabase dykes intruded in faults crosscutting it. Gold mineralization is mainly hosted in phyllite rocks. The granite porphyry present in the deposit is locally mineralized when it shares the same space with ore-controlling faults. Pyrite always displays zoning textures: Py0 and Py1 are present in the core; rim Py2 replaces and/or overgrows Py0 and Py1; and Py3 comprises the outermost rim. Previously determined in situ δ34S values indicate that Py0, Py1, and Py2 and Py3 are diagenetic, magmatic hydrothermal, and hydrothermal ore stage in origin, respectively. Electron microprobe analysis (EMPA) mapping of the zoned pyrite (especially Py2) shows a negative correlation between sulfur and arsenic, indicating the substitution of sulfur by As1-. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping and spot analysis of Py2 and Py3 reveal two Au and As relationships. In the mineralized phyllite and granite porphyry, Au has strong positive correlations with As, Sb, Cu, and Ag, similar to other arsenian pyrites in hydrothermal ore deposits. However, in altered samples, Au and As display decoupled geochemical behavior, especially in Py2, which has the same levels of As, Sb, Cu, and Ag but 2–5 orders of magnitude lower Au. We think that the ore fluid reacted with altered samples located distal to the ore-controlling fault (distal to the ore fluid center), and the fluid may not have been sufficiently charged with those elements, especially Au. Because As, Sb, Cu, and Ag could maintain at the same level in the ore fluid after precipitation of pyrite enriched in those elements, but Au was sharply depleted. This was also noted in a published fluid inclusion study in Carlin-type gold deposits in Guizhou Province, which has similar zoned pyrite texture. Based on the similar δ34S values (8‰ and 10‰) and trace element enrichments (Au, As, Sb, Cu, and Ag) between hydrothermal ore-stage pyrite and the Devonian seafloor exhalation sequence in the Fengtai Basin, the source of the gold in the Pangjiahe deposit should be attributed to metamorphism of the Devonian seafloor exhalation sequence during the Triassic orogeny.