Philippine Mineralization in Time, Space and Geology
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:
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.
Island arc hydrothermal.
These deposits are temporally distributed
into the following general groups:
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.
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.
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
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.