Whole-Rock Geochemistry of Host Rocks and K/Ar Age of Hydrothermal Mineral of the Co-O Epithermal Gold Deposit, Mindanao, Philippines

摘要

Whole-rock chemical composition of host rocks and recently acquired K/Ar age of hydrothermal mineral of the Co-O epithermal gold deposit in Mindanao Island of the Philippines are herein reported. Located along a Pliocene-Quaternary calc-alkaline magmatic zone at eastern Mindanao region, the Co-O gold deposit is of intermediate sulfidation epithermal Au (+Ag ± Cu ± Pb ± Zn) quartz vein type. Geological units in the area are probable Eocene to Oligocene basaltic-andesitic to andesitic volcanic flows and volcaniclastic rocks, Oligocene andesitic to dioritic stocks and dikes, a diatreme-maar complex, and an overlying sedimentary sequence. The mineralized quartz ± calcite veins are mainly hosted in the intrusive rocks and surrounding volcanic rocks that are hydrothermally altered generally to K-feldspar, chlorite and other clay minerals. Discrimination diagram using immobile elements such as Zr/TiO₂ vs. Nb/Y indicates that these rocks belong to sub-alkaline andesite and basaltic-andesite to basalt and alkali basalt in composition. The volcanic rocks plot in the island arc tholeiite and calc-alkaline fields of generally basaltic andesite and andesite composition, with a few in basalt and dacite. In the “alteration box plot”, the samples mostly plot within the least altered intermediate volcanic host rocks in the hydrothermal alteration field. Plots of each rock unit show a general chlorite-carbonate (-pyrite) alteration trend, with plots of volcanic rocks more dispersed and a few fallen outside the least altered box. K/Ar dating of hydrothermal minerals from the andesite porphyry and polymictic diatreme breccia samples yielded ages of 28.6 ± 0.9 Ma (Late Oligocene) and 31.7 ± 1.9 Ma (Early Oligocene), respectively. Age dating of these hydrothermally formed minerals gives the age of the hydrothermal activity associated with the mineralization. This suggests that the hydrothermal activity associated with the Co-O epithermal vein system transpired immediately after or during the Oligocene magmatism in a tectonic setting that produced island arc tholeiitic to calc-alkaline magmas, prior to drifting to its present location and accretion with the central and western parts of the Mindanao Island; in contrast to more prominent Miocene and Pliocene to Pleistocene mineralization ages along the Philippine archipelago.