Genetic types, mineralization patterns and exploration implications of ore-hosting fractures in porphyry-type deposits

Fractures concentrating around shallow granitic plutons are the key deposition site for porphyry-type ores. Early studies have proposed various formation mechanisms for different types of fractures, but these mechanisms are yet to be well integrated with the porphyry ore-forming processes. Here, we present a synthetic study on the formation mechanism and influence factors of fracture network development in/around the ore-causative porphyries during their emplacement, cooling/solidification, and alteration. According to their order of appearance, the fractures in porphyry systems can be divided into three types: (1) intrusion-induced type, (2) magma cooling-related type (i.e., cooling fractures), and (3) magmatic fluid-related hydraulic type (i.e., hydraulic fractures). Intrusion-induced fractures are formed by the compression on wallrocks exerted by magma emplacement, and are mostly distributed in the wallrocks around the pluton apex. Cooling fractures are only developed within the porphyries during magma cooling and solidification. Hydraulic fractures are largely formed by magmatic-hydrothermal pressure-driven cracking. The occurrence, geometry, orientation, and ore grade of the porphyry-type orebodies are all controlled by these three fracture systems, which offers insights for ore prospecting: (1) Intrusion-induced fracture-controlled orebodies are bell-shaped overlying the porphyry bodies, with ore grade gradually dropping away from the porphyries. The veins are relatively straight, and generally comprise a steep- (major) and a shallow- (minor) dipping group. They are few-mm to few-cm wide, and several meters long; (2) Cooling fracture-controlled orebodies are also bell-shaped and commonly developed in the densely-fractured porphyry apex, with the ore grade decreasing away from the porphyry margin toward depth. The veins are generally few-tens of cm long and within a few-mm wide, and are homogeneously distributed with no preferred orientation; (3) Hydraulic fracture-controlled orebodies are generally developed as pipes in the axial part of the porphyries, and can vertically extend over 1000 m. The veins are mostly steep and few-mm to few-cm wide, and several to tens of meters long.