Synextensional emplacement of porphyry Cu-Mo and epithermal mineralization: the Zijinshan District, Southeastern China

The Zijinshan district, Fujian Province, southeastern China, is a major mineral field that contains high- and intermediate-sulfidation epithermal Cu-Au-Ag and porphyry Cu-Mo deposits. This contribution discusses the structural geology and the tectonic regime prevalent during mineralization at Zijinshan. Statistically, the district is dominated by NW-striking fault systems that dip at moderate angles (40°–50°) to the northeast and southwest. They played a first-order role in controlling the emplacement of hydrothermal veins and breccias. Most NW-striking faults were active as oblique normal/strike-slip faults during mineralization, as evidenced by the geometry of syntectonic hydrothermal mineral fibers. NE-striking, steeply dipping faults also controlled the emplacement of hydrothermal veins and breccias, and these faults contain syntectonic mineral fibers indicating predominantly dextral strike-slip movements. The kinematic and dynamic analysis of fault-slip data shows that the predominant tectonic regime during mineralization was extensional, with subvertical σ₁ and NNE-to NNW-trending σ₃. Secondary clusters of subhorizontal σ₁ suggest that short periods of fault reactivation under a strike-slip regime occurred locally. Although the formation of porphyry Cu-Mo systems is commonly thought to be favored by compressive to transpressive tectonic regimes, which lead to a diminution in the rate of volcanic output and to the entrapment of magmas and volatiles in the upper crust, these results show that at least medium-sized porphyry Cu-Mo systems can form under extensional conditions in previously thickened continental crust. This has important exploration implications globally. Based on current geologic knowledge, the district appears to have a high exploration potential at depth, mainly for intermediate-sulfidation epithermal and porphyry-style mineralization.