Structural Evolution of the Golden Cross Epithermal Au-Ag Deposit, New Zealand

Abstract

Golden Cross, located in the Hauraki goldfield of New Zealand, is a classic example of a fault-fracture–hosted epithermal vein system. Gold-silver mineralization was mined in two areas: the open-pit stock-work veins and the underground Empire vein system, which are collectively referred to as the Empire zone. Open-pit stockwork veins are either parallel or subperpendicular to northeast-striking, moderately southeast dipping bedding. Bedding-perpendicular veins are mostly pure extensional structures, whereas bedding-parallel veins open in an extensional regime with a minor component of shear. Vein frequency across strike is typically ~15/m, and most veins are 0.01 to 0.2 m thick. The underground Empire vein system consists of the steeply west dipping Empire vein and a complex array of footwall veins. The 2- to 10-m-thick Empire vein strikes northeast, dips steeply (65°–85°) northwest, has a mineralized strike length of about 500 m, and a vertical extent of ~250 m. It is localized in a deflection of the Empire fault where the dip of the fault decreases. The footwall veins are low angle, gently west dipping veins up to 100 m long, hosted in extensional shear and pure extension fractures. They splay off the Empire vein at an average angle of 30°. Up to seven vein types record the multiple fault-fracture openings that provided highly permeable pathways for hydrothermal fluids in the Empire zone. The Empire zone was initially characterized by a pervasive network of thin barren sulfide veins, followed by two to five stages of electrum-bearing quartz veins. The most voluminous of these stages were quartz-adularia veins with well-developed colloform and crustiform bands and breccia textures. The final stage was massive coarsely crystalline calcite veins. Similarities in vein type, orientation, and crosscutting relationships between the Empire vein system and stockwork veins suggest that they formed under similar stress regimes and that mineralization in the two areas overlapped in time, at least in part. Host rocks at Golden Cross dip 50° to the southeast. If the vein system and bedding are rotated so that bedding is restored to horizontal, the Empire fault and Empire vein reflect a steeply southeast dipping normal fault. After rotation, the former footwall veins form a network of subvertical veins in the hanging wall of the Empire fault; they form an en echelonlike array similar to “horse tail” extensional fracture networks that commonly develop in normal faults in the near surface. Therefore, we infer that, before tilting, Empire zone fractures and veins formed in an extensional regime with σ1 subvertical, σ2 subhorizontal oriented northeast-south-west, and σ3 subhorizontal oriented northwest-southeast, consistent with the interpreted rift tectonics of the Coromandel volcanic zone during the late Miocene time of mineralization.