Porphyry and epithermal deposits and 40Ar/39Ar geochronology of the Baguio District, Philippines

The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the Baguio district. The intrusive complex was emplaced in the early Miocene based on ⁴⁰Ar/³⁹Ar age determinations for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma).

Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15 and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern part of the district at about 1.5 Ma, based on secondary biotite ⁴⁰Ar/³⁹Ar ages of 1.48 ± 0.05 Ma and 1.47 ± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry Cu-Au prospects returned ⁴⁰Ar/³⁹Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively.

Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone (the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based on previous K-Ar dating.

Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted the development of northwest-trending faults in the upper plate that interacted with major arc-parallel north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz of gold and several million tonnes of copper over approximately 3 m.y.