Metallogenesis of Porphyry Cu Deposits of the Western Luzon Arc, Philippines: K‐Ar ages, SO₃ Contents of Microphenocrystic Apatite and Significance of Intrusive Rocks

Abstract

K-Ar ages of the following porphyry Cu deposits in the western Luzon arc are determined: Lobo-Boneng (10.5±0.4 Ma), Santo Niño (9.5±0.3 Ma), Black Mountain (2.1±0.1 Ma), Dizon (2.5±0.2 Ma) and Taysan (7.3±0.2 Ma). Microphenocrystic apatite in the late Cenozoic intermediate to silicic intrusions associated with porphyry Cu deposits in the western Luzon arc contains sulfur as SO 3 detectable by electron probe microanalyzer. Sulfur is supposed to have been accommodated dominantly as oxidized species in oxidizing hydrous magmas that generated porphyry Cu deposits. Likewise, such high SO 3 contents in microphenocrystic apatite are common characteristics of the intermediate to silicic magmatism of the western Luzon arc, from tonalitic rocks of the Luzon Central Cordillera of about 15 Ma to an active magmatism at Mount Pinatubo. Thus, the western Luzon arc has been generating porphyry Cu mineralization associated with oxidizing hydrous intermediate to silicic magmatism related to eastward subduction, since Miocene to the present day. Intermediate to silicic rocks since 15 Ma to present-day western Luzon arc generally show high whole-rock Sr/Y ratio ranging from 20 to 184. However, porphyry Cu deposit is not necessarily related to the rocks that show higher Sr/Y ratios compared to the other barren rocks in the western Luzon arc. The characteristics of the intermediate to silicic magma associated with porphyry Cu deposit are not attributed to the composition of the source material of the magma, but to the properties defined by the high activity of oxidized species of sulfur in the fluid phase that is encountered during the generation of intermediate to silicic magmas.

The genetic association of adakites and Cu–Au ore deposits

“…For example, some adakitic rocks do not have any deposits (Chiaradia et al 2004;Huang et al 2008), whereas some rocks without clear adakitic geochemical features are ore-bearing, for example some porphyry Cu and epithermal Au deposits in Ecuador (Chiaradia et al 2004) and small porphyry Cu deposits in the western Luzon (Imai 2002); in both cases calc-alkaline andesites are the host rocks. Therefore, it has been argued that high oxygen fugacity, rather than adakitic magma composition, is essential to the formation of porphyry Cu deposits (Imai 2002;Bissig et al 2003).…”

Is gold solubility subject to pressure variations in ascending arc magmas?

“…Recent (< 20 Ma) magmatism in the North Luzon arc is linked to the current subduction of the South China Sea plate along the Manila Trench, and includes a range of arc rocks (Sajona and Maury, 1998;Prouteau et al, 2000;Bellon and Yumul, 2001;Jégo et al, 2005). World-class Cu-Au deposits are spatially and temporally associated with this recent magmatic activity, as shown by the chain of porphyry copper deposits extending from Dizon in the south to Lepanto in the north, and the epithermal Au deposits from the Baguio district Imai, 2001;2002). In the Baguio district, there is an intimate association between intrusive rocks and mineralization (Santo Tomas II Cu-Au porphyry and Antamok and Acupan epithermal Au deposits, Imai, 2001;2002;Polvé et al, 2007).…”

“…World-class Cu-Au deposits are spatially and temporally associated with this recent magmatic activity, as shown by the chain of porphyry copper deposits extending from Dizon in the south to Lepanto in the north, and the epithermal Au deposits from the Baguio district Imai, 2001;2002). In the Baguio district, there is an intimate association between intrusive rocks and mineralization (Santo Tomas II Cu-Au porphyry and Antamok and Acupan epithermal Au deposits, Imai, 2001;2002;Polvé et al, 2007). The selected starting rocks are samples of this recent magmatic episode in North Luzon.…”

Petrochemistry of I-type magnetite-series granitoids of the northern Chile, Highland Valley, southern B. C., Canada, Erdenet mine, Mongolia, Dexing mine, China, Medet mine, Bulgaria, and Ani mine, Japan

“…The adakitic magmas are considered genetically related to Au and giant Cu porphyries in the Andes (Oyarzun et al, 2001;Martin et al, 2003;Reich et al, 2003), Philippines (Sajona and Maury, 1998), Tibet (Hou et al, 2009) and others. However, genetically related intrusives in a strict sense may not be adakites in the Philippines (Imai, 2002).…”