Abstract:

By reviewing the latest research progress of porphyry copper deposits (PCDs) and combining with the new data, this paper focuses on discussing the geodynamic background of non-arc PCDs in Chinese mainland, the origin of ore-forming magma, the evolution of magmatic-hydrothermal system, the sources of ore-forming metals (Cu, Au, Mo) and H₂O, and their enrichment process. Except for a few PCDs formed in island arcs in China, most PCDs were formed during the stages of tectonic transformation and crustal extension in a collisional orogenic setting, and the stages of lithosphere extension and collapse, in the edge and interior of a re-activated craton. These non-arc porphyries are mostly isolated or nearly even-spaced stocks or bosses, and are characterized by high K contents and adakitic compositions. The ore-forming magmas are mainly originated from the newly formed thickened mafic crust or delaminated ancient lower crust, and a few originated from subduction fluid/melt metasomatized lithospheric mantle. Intercontinental collision and subduction led to large-scale crustal thickening, and the triggers for crustal melting include slab tearing, slab break-off, lithosphere delamination, and asthenospheric mantle upwelling. Similar to the island arc porphyries, the non-arc porphyries are also rich in water (> 4 wt% H₂O) and f(O₂) (ΔFMQ ≥+2). We think H₂O does not come from the subducting slab, but mainly from the decomposition of amphibole in the newly formed lower crust or injection of H₂O-rich ultrapotassic melt. Cu (Au) mainly comes from the decomposition of Cu-bearing sulfides in the newly formed lower mafic crust, or from the metal-rich lithospheric mantle reacted with delaminated lower crust. In contrast, Mo mainly comes from the continental crust in high Mo abundance. Regardless of island arc or non-arc settings, ore-forming magmas are generally enriched with metals (Cu, Au, Mo). Formation of PCDs does not require the incipient magma abnormally rich in metals, but requires sulfides not being saturated and separated from magma before exsolution of magmatic fluid. Although shallowly emplaced porphyry (1-6 km) can dissolve ore-forming fluids, large-scale PCDs usually require ore-forming fluids to dissolve from the deep part (emplacement depth ≥6 km) and have continuous mafic melt input. Shallowly emplaced porphyry can separate and condense immiscible low-salinity vapor phase and high-salinity liquid phase, while the deep-seated magma chamber directly dissolves high-temperature and low-salinity metal-rich supercritical fluid. Both high-salinity liquid phase and low-density supercritical vapor-phase fluid can transport metals and form PCDs with large-scale hydrothermal alteration.

Key words: porphyry copper deposit, origin of ore-forming magma, melting of lower crust, water and metal source, enrichment mechanism, metallogenic setting