Genesis, geochemical evolution and metallogenic implications of magnetite: Perspective from the giant Cretaceous Atlas porphyry Cu–Au deposit (Cebu, Philippines)

Abstract The Atlas porphyry Cu–Au deposit (PCD) in central Cebu Island (Philippines) has a proven ore reserve of 1420 Mt @ 0.45% Cu, 0.24 g/t Au, 0.018 g/t Mo, and 1.8 g/t Ag. The PCD is characterized by four generations (Mag1 to Mag4) of magnetite, a mineral that can record the chemical evolution of the magmatic-hydrothermal system. The magmatic Mag1 is characterized by being euhedral with ilmenite lamellae formed by the oxy-exsolution process. The well-developed microporosity and 120° triple junction suggest that Mag2 has been modified by recrystallization or dissolution-reprecipitation process. The Mag3 (from the early-ore Stage I) is well-porous with almost no oxy-exsolution ilmenite lamellae. The Mag4 is from the late-ore Stage II in which the Mag4a has distinctive zoning formed by massive cavities, whereas the Mag4b has only a few pores. LA-ICP-MS trace element (esp. Ti and V) compositions indicate that the formation temperatures decrease from Mag1a to Mag3, then increase from Mag3 to Mag4a, and eventually decrease from Mag4a to Mag4b. On the contrary, oxygen fugacity (fO2) decreases from Mag1a to Mag3 but increases from Mag3 to Mag4, suggesting two magmatic-hydrothermal pulses driven by the Lutopan porphyry intrusion at Atlas. This is consistent with the development of two mineralization styles at Atlas, i.e., high fO2 magnetite mineralization and the subsequent giant Atlas porphyry Cu–Au mineralization.