Multistage mineralization in the Haoyaoerhudong gold deposit, Central Asian Orogenic Belt: Constraints from the sedimentary-diagenetic and hydrothermal sulfides and gold

• Main Authors: Hai-Dong Zhang, Jian-Chao Liu, Mostafa Fayek
• Format: Article
• Language: English
• Published: Elsevier 2021-03-01
• Series: Geoscience Frontiers
• Subjects: Multistage gold mineralization; Stage pyrites; Monazite U–Pb age; In-suite trace elements; Black shale
• Online Access: http://www.sciencedirect.com/science/article/pii/S1674987120301833

The Central Asian Orogenic Belt, as one of world-class gold economic belts, preserves a number of giant, large black shale-hosted gold deposits, while it is still debated for origin of sulfides and gold mainly due to lack of identification for multiple stages of sulfides. The Haoyaoerhudong gold deposit is hosted in a sequence of Mesoproterozoic carbonaceous and pyritic slate, phyllite, and schist that form a tight syncline along the north margin of the North China Craton. Detailed petrography of the host rocks and mineralization have defined five stages of pyrites. The earliest form of pyrite (Py1) occurs as fine-grained dispersed pyrite in black carbonaceous slate and medium- to coarse-grained disseminated pyrite in pyrite-rich layers, contains relative low gold and high arsenic content, indicating a syn-sedimentary or diagenetic in origin. Stage II pyrite (Py2) occurs with garnet and quartz inclusions and Py3 occurs as pyrite veins, contains higher gold and lower As content, and are interpreted to have formed from the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism. Stage IV pyrite (Py4) from the pyrite-quartz veins crosscut the metamorphic garnet, contains the highest gold concentrations and other trace elements, and is considered to have formed post-peak metamorphism. Abundant native gold, electrum, and maldonite occur as inclusions within Py4 and monazite and in fractures that crosscut garnet. While, Py5 with typical remobilized feature is thought to be a product of melting of former pyrites (Py1 to Py4) triggered by the large-scale Hercynian magmatism. The sedimentary/diagenetic Py1 have δ34S values that range from +12.4‰ to +16.2‰. Later generations of sulfides, including Py2 to Py5, and Ccp2 to Ccp3, have δ34S values from +9.5‰ to +12.7‰. Monazite with maldonite inclusions from quartz-pyrite veins yielded an intercept age of 341.3 ​± ​6.6 ​Ma, while coarse grained monazite associated biotite along fractures in the reefs yielded an intercept age of 254.6 ​± ​8.2 ​Ma.The paragenetic, textural, chemical, and isotopic data suggest three distinct gold producing episodes at Haoyaoerhudong gold deposit. Gold and arsenic were clearly initially concentrated in organic muds, and enriched along the structures of diagenetic arsenic-rich pyrite. Subsequently, accompanying metamorphism and deformation, gold was liberated from the dissolution of diagenetic pyrites to form the pyrite veins. Finally, accompanying transformation of pyrite into pyrrhotite, gold was released into the metamorphic fluids to become concentrated as native gold, electrum, and maldonite in pyrite-quart veins. Monazite with age of 341 ​Ma from quartz-pyrite veins suggests that the third major gold mineralizing event in Haoyaoerhudong occurred before the Hercynian magmatism, suggesting that the Haoyaoerhudong deposit is a typical orogenic gold deposit rather than intrusion-related deposit.