Basic Information on Copper-Gold Deposits in the Philippine Islands

      The Philippine archipelago is renowned worldwide for its copper-gold deposits, including porphyry copper-gold deposits and epithermal gold deposits. The formation of these deposits is related to the subduction of present or ancient tectonic plates surrounding the archipelago. Most of these porphyry copper-gold deposits are spatially and temporally related to epithermal diorites and quartz diorites, with some porphyries also showing a relationship to volcanic or subvolcanic rocks. Notably, most of these porphyries exhibit adakite geochemical characteristics. In addition, a small number of porphyry copper-gold deposits are associated with alkaline monzonite-syenite. These felsic porphyries mostly exhibit adakite geochemical characteristics, namely high Sr content, high Sr/Y ratio, and low Y and Yb contents. Volcanic activity in the Philippine active zone is mostly related to subduction since the Cretaceous, with peak magmatic activity occurring during the Eocene, Miocene, and Pliocene-Quaternary periods. The still-active Cenozoic subduction has resulted in the widespread development of arcuate volcanic rocks, niobium-rich basalts, adakites, and large copper-gold deposits. Porphyry copper-gold deposits and epithermal gold deposits are widely distributed in the Philippine archipelago (Figure 1), particularly on Luzon, Negros, Cebu, and Mindanao. Their formation is attributed to contemporary (Luzon and Mindanao) or ancient (Negros and Cebu) subduction processes surrounding the archipelago.

     Figure 1. Map of tectonic-magmatic activity and distribution of major porphyry copper-gold deposits in the Philippines.

      The Philippines is renowned for its world-class copper-gold resources, with large porphyry copper-gold deposits and epithermal gold deposits being widely developed throughout the archipelago. These deposits are formed by modern or ancient subduction processes surrounding the archipelago. Most of these porphyry copper-gold deposits are spatially and temporally related to epithermal diorite porphyry and quartz diorite porphyry, and some are also related to volcanic-subvolcanic porphyry (such as andesitic porphyry and dacite porphyry). In addition, a small number of porphyry copper-gold deposits are associated with alkaline monzo-syenite. Interestingly, most of these felsic porphyries exhibit the geochemical characteristics of adakites, namely high Sr content, high Sr/Y ratio, and low Y and Yb content.

      Adakite genetic models mainly include: ① formation following partial melting of subducted oceanic crust, followed by subduction of seismic ridges or seamount chains, or slab retraction; ② formation from partial melting of the lower island arc crust of garnet-bearing amphibolite; ③ formation from the fractional crystallization of mantle-derived basaltic calc-alkaline island arc magma in the crust (amphibolite, apatite, ilmenite, western Luzon and southern Mindanao) or mantle (garnet, eastern Mindanao); ④ formation from the mixing of slab adakite melt and mantle-derived mafic melt. The diverse formation models of adakites in the Philippine Islands present challenges to exploring the genesis and source rocks of porphyry copper-gold deposits and associated ore-bearing porphyries.

      Different understandings of the genesis of adakites in the Philippine Islands have complicated the understanding of the genesis and origin of porphyry copper-gold deposits and their associated ore-bearing porphyries. For example, in the porphyry-high sulfide hydrothermal deposits in the Lepanto-Far Southeast region of northern Luzon, porphyry and ultrathermal mineralization reached their peak from the Late Oligocene to the Pliocene-Pleistocene, but the genesis of the ore-bearing porphyries or intrusions remains to be elucidated. Furthermore, early or nearly contemporaneous gabbro or diabase dikes exist within or near these deposits. These basic dikes exhibit arc geochemical characteristics and coexist closely with adakite porphyries. However, the genetic connection between these basic dikes and the ore-bearing porphyries has long been overlooked.