Abstract

Genesis of porphyry Cu deposits remains controversial. A common view suggests that the efficient partitioning of Cu into a fluid exsolved from a calc-alkaline magma with normal Cu but elevated S concentrations is critical to ore formation. However, this view was recently challenged by two new ideas, which emphasize that the formation of porphyry Cu deposits requires anomalously Cu rich sources of either a mantle domain or intracrustal magmatic cumulate. Here we report data on an intracrustal magmatic hornblendite, genetically associated with the large Mujicun porphyry Cu deposit in North China craton. Petrographic observations, geochemical data, and P-T calculations indicate that the hornblendite formed at a depth of ~28 km by amphibole accumulation from a basaltic magma. These cumulates, despite containing traces of Cu-bearing sulfides, have Cu contents of only 17 to 60 ppm, close to primitive arc magmas. Their parental magma, initially not Cu enriched, evolved to ≥60 ppm Cu in resultant gabbronorite emplaced at ca. 8 km. This was followed by strong Cu depletion (<10 ppm) in the quartz-diorite porphyries associated with Cu mineralization. Most of the gabbronorites contain magmatic sulfide phases that have been partially dissolved by hydrothermal fluids. One sample contains sulfide droplets and coexisting silicate glass and fluid inclusions that were locally trapped in interstitial magmatic quartz. We conclude that the exsolution of a Cu-rich sulfide melt and its subsequent redissolution into aqueous fluids was a significant factor in the formation of the Mujicun porphyry Cu deposit, and that anomalous Cu enrichments in the mantle source regime or intracrustal cumulates were not required to form the ore deposit.

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