Breccia Evolution and Genesis of High Sulfidation Cu-(Au) Mineralization in the Zijinshan Lithocap, Fujian Province, Southeast China

The Cretaceous Zijinshan Au-Cu deposit (>300 metric tonnes Au, 2 Mt Cu) is hosted by the Zijinshan lithocap in the Zijinshan granite complex on the western margin of the coastal volcanic belt of South China. Hydrothermal alteration is zoned vertically over 1,200 m, starting from –200 m elevation with quartz-pyrophyllite and quartz-muscovite assemblages at depth, grading upward to quartz-dickite, quartz-alunite, massive quartz, and vuggy quartz alteration. Sulfides are also zoned vertically, with enargite mainly at depth (–200–600 m elevation), and digenite and covellite at shallower levels. The hypogene ores at Zijinshan are dominantly hosted in discordant breccias. Three main types are recognized based on field observations of breccia infill: (1) matrix-dominated breccias, (2) cement- and matrix-rich breccias, and (3) cement-dominated breccias. High-sulfidation-state mineral assemblages occur in the cements and altered clasts of these breccias, defining subfacies of cement-dominated and cement- and matrix-rich breccias, including quartz-pyrite-cemented (CM1), alunite- (CM2) and copper sulfide-cemented (CM3) breccias, and pyrite- and copper sulfide-cemented matrix-bearing (CMX1), and dickite-cemented matrix-bearing breccias (CMX2). Structural analyses show that the emplacement of matrix-dominated breccia dikes and cement-dominated and cement- and matrix-rich breccia veins were controlled by a series of syn-mineralization NW-trending normal oblique faults. The matrix-dominated dikes, and cement- and matrix-rich and cement-dominated breccia veins are interpreted as tectonic-hydrothermal breccias, although the relative influences of tectonic and hydraulic processes varied between breccia facies. A large (400 m diameter) matrix-dominated breccia body in the south of the deposit is interpreted as a diatreme. Matrix-dominated breccia dikes surround the diatreme and were emplaced prior to cement-dominated breccias, recording a transition from a predominance of attrition to dilation along the deposit-scale faults. Matrix-dominated breccias are common in the southern and central parts of the deposit, whereas cement-dominated breccias are located primarily in the north, with minimal superposition of the two breccia types. The cement-dominated breccias are generally associated with high copper grades. The spatial distribution of breccias implies that tectonic processes began with attrition along faults in the north, migrating to a predominance of dilation synchronous with main-stage mineralization in the deposit center. The complex sequence of brecciation at Zijinshan is intimately linked to the deformation history and was fundamental to the development of this giant high-sulfidation epithermal copper-gold deposit, controlling both the location and geometry of orebodies. The well-developed fracture mesh generated by faults at Zijinshan was critical to focusing fluid flow, resulting in a series of NW-trending mineralized veins and breccias, and facilitating the mineralization and formation of an extensive lithocap in an otherwise highly impermeable granite protolith.