Contribution of mafic melt to porphyry copper mineralization: evidence from Mount Pinatubo, Philippines, and Bingham Canyon, Utah, USA

Mount Pinatubo in the Philippines, known for its cataclysmic eruption in 1991, hosts several porphyry copper deposits and active geothermal systems. An underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption. The eruption caused purging of the sulphur-rich fluid from the dacite to the atmosphere and extensive fracturing. Similar events took place at Bingham Canyon, Utah, site of the largest copper and gold deposit in North America at 38 Ma. The Bingham Canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling. Ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma, which consolidated to form the Bingham stock and its underlying magma chamber. Injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock.