Philippine Mineralization in Time, Space and Geology

Fernando G. Sajona, Ph.D.

Supervising Science Research Specialist (Ret.)
Mines and Geosciences Bureau

Abstract

The Philippine archipelago developed as allochtonous island arc units and oceanic and continental crust fragments that welded as a function of plate movements. Its geologic history is characterized by a rapid succession and repetitions of subduction, cessation of subduction, arc polarity reversal, marginal basin opening and closing, and arc-arc and arc-continent collision. These were accompanied by multiple phases of calc-alkaline magmatism with simultaneous faulting and, thus, vigorous crustal hydrothermal activity. These give rise to various types of mineralization, contributing to the Philippines’ current global status as having one of the densest distributions of metallic mineralization on a per area basis.

Primary mineral deposits in the Philippines can be broadly classified as:

1. Oceanic crust (i.e., ophiolite)-related. The ophiolitic belts at opposite edges of the archipelago host world-class chromite deposits (Acoje, Coto, Samar, Dinagat) and potentially economic VMS-type copper (Barlo, San Mariano) and disseminated platinum-bearing nickel sulfides (Acoje). Deeply weathered peridotites yield nickel laterite, the recent abnormal high prices of which briefly induced an exploration and mining rush. These ophiolites are dated from Late Cretaceous to Eocene.

    

2. Island arc hydrothermal. These deposits are temporally distributed into the following general groups:

1. Pre-Tertiary-Early Paleogene. VMS (e.g., Canatuan, Rapu-Rapu, Bagacay) and porphyry copper (e.g., Atlas, Talibon) deposits of this age imply that as early as Cretaceous-Paleogene, the proto-Philippine arc already consisted of mature island arc pieces probably lying out in the Pacific or near the Eurasian margin.

    

2. Oligocene-Early Miocene. Most of the known and economic mineralization of this age are medium to large (200-800 Mmt) exposed porphyry copper deposits. These usually have copper grades of 0.4-0.5%Cu, low or insignificant gold and sporadic recoverable molybdenum grades (e.g., Hinobaan, Sipalay, Tapian, San Antonio). One exception is the Dinkidi deposit with 21 Mmt of 0.65%Cu and 1.7 g/tAu. Other deposits with preserved epithermal systems are Pao-Yabbe, Bulawan, Aroroy, Nalesbitan and Larap. The Fe-skarn deposits of Bulacan, Rizal and Paracale are also of this age.

    

3. Late Miocene-Pleistocene. This mineral epoch follows an apparent Middle Miocene hiatus in magmatic activity and, therefore, accompanying mineralization. Porphyry copper deposits of this age are relatively gold-rich in general, probably due to overprinting by, or telescoping of, shallower gold mineralization. These deposits are mostly distributed in western Luzon (e.g., Batong-Buhay, FSE, Guinaoang, Pua, Sto. Tomas II) and eastern Mindanao (e.g., Boyungan, Amacan, Kingking). The Tampakan deposit in Cotabato is also of Pliocene. Abundant epithermal gold and gold-copper deposits (Enargite, Victoria, Antamok, Acupan, Pana-on, Placer, Siana, Co-O, Diwalwal, Masara, Boringot, Sibutad) are spatially, temporally and, perhaps for many, genetically related to porphyry copper mineralization.

    

Secondary enrichment of these primary deposits give rise to gossanous gold enrichment that often yield bonanza grades in vein deposits and bulk-mineable blankets atop VMS-type orebodies. Supergene copper development enriched some porphyry copper deposits but not in scales as significant as those in South America. Nickel laterization of peridotites in ophiolite belts yield substantial nickel resource mostly of low to moderate grade.

The Tertiary tectonic reconstruction of SE Asia by Hall et al. (2001) can be used to place and view the development and installation of the various Philippine mineral deposits in time and space.

In the study of the Philippine Fault’s role in controlling mineralization, it can be generalized that the fault itself does not seem to host significant mineralization. Nevertheless, it played a crucial role in preparing the ground by generating permeability and subsidiary branches that localized hydrothermal mineralization.