Lower-Crustal Magmatic Hornblendite in North China Craton: Insight into the Genesis of Porphyry Cu Deposits

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 (