Implications of pyrite geochemistry for gold mineralisation and remobilisation in the Jiaodong gold district, northeast China

Abstract

The Jiaodong gold district of eastern China, the largest gold producing district in China, is located on the eastern margin of the North China Craton. It consists of three mineralisation belts: the western Zhao-Ye belt, the middle Qixia belt, and the eastern Muping–Rushan (Muru) belt. Over 85% of mineralisation is hosted in the Zhao-Ye belt, which is bordered by the mantle-tapping Tan Lu fault zone. Pyrite crystals from three deposits in the Zhao-Ye belt and three deposits in the Muru belt were studied using a combination of optical petrography, bulk pyrite geochemistry, and in-situ laser ablation ICP-MS. Results show that although mineralisation is broadly similar between the two belts, there are significant differences in ore and gangue mineral textures, pyrite geochemistry, and style of gold mineralisation.Texturally, pyrite grains from the Zhao-Ye belt are generally cubic and do not exhibit zoning. In contrast, Muru pyrite grains are more often pyritohedral, commonly exhibit well-defined concentric zoning, and display textures in ore and gangue minerals indicative of open space growth. Bulk pyrite geochemistry suggests a distinct enrichment in Pb, Bi, Au, Ag and Te in the Zhao-Ye belt, whereas the Muru belt pyrite is significantly enriched in As, Cu and Co. In situ pyrite geochemistry indicates that Au and As are variably correlated in the Zhao-Ye belt, typically only exhibiting correlation at low Au concentrations. Most gold occurs as visible electrum along pyrite fractures and grain boundaries, with a minor generation of invisible gold formed through As-facilitated uptake into pyrite. In the Muru belt, Au and As have a strong correlation and there is limited occurrence of gold particles, indicating that most gold in the Muru belt is invisible gold contained in the crystal structure of As-rich pyrite.The differences in style of gold mineralisation between the belts indicates an inherent difference in timing of gold introduction: in the Zhao-Ye belt, the visible electrum accounting for most of the gold endowment is formed post-pyrite, whereas the invisible gold in the Zhao-Ye and Muru belts is formed syn-pyrite. The heterogeneity in gold distribution in the Jiaodong district is attributed to melting of metallogenically fertile Archean crust at the base of the well-endowed Zhao-Ye belt, and the lack of a similarly fertile source region beneath the Muru belt.