An overview on the origin of adakites/adakitic rocks and related porphyry Cu-Au mineralization, Northern Luzon, Philippines

Abstract The Philippine island arc is famous for the widely developed large porphyry and epithermal Cu-Au deposits and associated Cenozoic adakites/adakitic rocks. Northern Luzon is one of the most Cu-Au enriched areas in the Philippines, which develops Late-Oligocene to Pleistocene porphyry and epithermal Cu-Au deposits in the Central Cordillera Range and southernmost of Cagayan Valley Basin. In this paper, Cenozoic magmatic events and geological characteristics of typical porphyry and epithermal Cu-Au deposits in Northern Luzon have been overviewed and briefly summarized. Geochemical data of the mineralized or unmineralized igneous rocks from three period of magma events in the area have been revisited, to investigate the different geneses and reveal the connections to Cu-Au mineralization. The Pliocene-Pleistocene Cu-Au deposits in the Baguio and Mankayan districts of Central Cordillera are mostly related to dioritic to dacitic porphyries. These rocks are intermediate to felsic in compositions, displaying specific geochemical features of “transitional” adakite to typical adakite. They were probably generated by partial melting of the flat-subducted Scarborough Ridge beneath the Luzon Arc at quite shallow depths and low pressures. They also underwent a certain degree of apatite ± ilmenite fractionation during magma evolution. The slab-melting genesis of these Pliocene-Pleistocene porphyries gives rise to the intense Cu-Au mineralization in the Central Cordillera. The Late Oligocene Dinkidi alkalic porphyry Cu-Au deposit in the Didipio area, southernmost Cagayan Valley Basin is directly related to the highly evolved Dinkidi monzonite-syenite stock, which also shows adakitic geochemical features. However, it was not directly derived from slab melting, but generated by combination fractional crystallization of hornblende + apatite ± clinopyroxene ± titanite from an alkalic basaltic magma. The basaltic magma was probably produced by partial melting of a melt-metasomatized mantle in a backarc rift setting caused by slab rollback of the subducted South China Sea plate in the Late Oligocene. Slab melt metasomatism in the primary source of the Dinkidi stock combined with late stage Cu-Au enrichment during magma evolution at high oxygen fugacities facilitates the porphyry Cu-Au mineralization in the Dinkidi deposit. The Early Miocene Central Cordillera Diorite Complex (also called Ango Batholith) in the Central Cordillera range shows clear fractionation of hornblende ± clinopyroxene, evolving geochemically from non-adakitic basaltic magma to adakitic granodioritic magma. There is no noteworthy porphyry or epithermal Cu-Au deposit related to the Central Cordillera Diorite Complex, mostly because they are primarily derived from a less enriched mantle source metasomatized by limited slab fluids. The initial lack of Cu-Au in the primary source of the Central Cordillera Diorite Complex prevents further Cu-Au mineralization of the complex, although they have undergone high degree evolution.