Geochemical and geochronological insights into the metal fertility of magmas associated with the Kennon Cu–Au porphyry deposit of Black Mountain in the Baguio Mineral District, North Luzon, Philippines.

Abstract

New geochronological, whole-rock, and zircon geochemical data for diorite and andesite porphyries from the Kennon Cu–Au porphyry deposit located in the Black Mountain intrusive complex, in the Baguio Mineral District in the Benguet Province, Northern Luzon, Philippines, are presented to evaluate ore-forming processes. The U–Pb geochronological data indicate that diorite and andesite porphyries were emplaced synchronously at ∼3.36 to 3.10 Ma, coinciding with the mineralization event driven by the fertile Pliocene magmatism related to the subduction of the Scarborough ridge along the Manila Trench. Whole-rock and zircon geochemistry show moderate to high degrees of magmatic fractionation, as reflected in Nb/Ta, Zr/Hf, and Rb/Sr ratios, along with zircon Dy/Yb, Hf, and Th/U values. Zircon trace element characteristics, including Ce⁴⁺/Ce³⁺ ratios (>300), Eu/Eu^⁎ (>0.4), and (Ce/Nd)/Y (>0.01), indicate high oxygen fugacity and water content, supported by the presence of magnetite, hornblende, and apatite in samples. Crystallization temperatures ranging from 726 to 835 °C and oxygen fugacity values (ΔFMQ +0.99 to +1.17) calculated from zircon trace element data fall within the typical range of intrusion-related Au deposits. Furthermore, the combination of the oxygen fugacity and whole-rock Fe content places it in the Cu field. These results demonstrate that high oxidation states and elevated water contents typically associated with primitive magmas can coexist with significant fractionation in mid-crustal, hydrous, oxidized magma chambers.