The role of magma chamber depth on porphyry mineralization: Insights from the late Jurassic Gutian porphyry Cu-Mo deposit in coastal SE China

Abstract

Porphyry deposits (less than5 km) contain the world’s major Cu and Mo reserves, as well as a significant amount of Au, and are genetically linked to porphyries originated from deep (8–15 km depth) upper crust magma chambers. The role of magma chamber depth in the porphyry ore formation remains an unresolved key issue. Here, we analyzed various rock-forming minerals to constrain the magmatic physicochemical conditions of three coeval and cogenetic intrusions which are difference in fertility for porphyry Cu-Mo formation from the Gutian porphyry Cu-Mo deposit. Our results show that the barren granodiorite, barren granodiorite porphyry (P1), and fertile granodiorite porphyry (P2) have similar crystallization temperatures, oxygen fugacities, and water and sulfur contents, and are water saturated. However, only the P2 intrusion that sourced from a deep magma chamber (8.6–11.4 km) generated porphyry Cu-Mo mineralization. It indicates that magma chamber depth exerted a major control on porphyry Cu-Mo formation. Moreover, the emplacement depth of the fertile P2 intrusion is similar to that of the barren granodiorite and P1 intrusion (i.e., 3.7 ± 0.5 km), indicating significant decompression during the emplacement of P2 magma. This dramatic pressure drop is recorded by the plagioclase XAn (mole fraction of anorthite [An]) value, triggering fluid exsolution and rapid mineral crystallization, as supported by the micro-/crypto-crystalline groundmass of the P2 intrusion. Our results highlight that deep magma chambers can accumulate a larger volume of magma (and thus metals) to provide sufficient ore-forming materials and can promote fluid exsolution that extracts metals from the magma.