INTRODUCTION

The complex framework of geology of the Philippines has long been recognized by Irving (1949), Ranneft (1960), Gervasio (1969), Bryner (1969), Bureau of Mines (1971), Wolfe (1971), among many others. Santos (1972) attempted to relate mineral deposit distribution to the geological features of the island arc configuration. He classified the deposits on the basis of their occurrence in the frontal arc or third arc. He grouped the archipelago into a mobile belt and a stable region. The former has high seismicity while the latter is relatively free from frequent earthquakes. Luzon, the central islands of Mindoro, Masbate, Panay, Negros, Cebu, Bohol, Leyte, Samar and other smaller islands comprised the mobile belt while Palawan and Sulu Sea belonged to the stable regions.

The frontal arc is formed as a result of collisions of oceanic plates. One plate or slab is pushed down or subjected under an opposite plate. Places where these plates collide (convergent plate margins) are marked by deep oceanic trenches. The sediment cover on the descending oceanic plate is partly carried beneath the leading edge of the upper slab (subverted). The region where all these processes occur is called subduction zone. The subcreted material and magma are recycled and maybe brought to the surface as plutons or intrusive masses and by a string of volcanoes on the overlying plate. All these tectonic events result in the formation of a trench-arc system.

GENERAL GEOLOGIC SETTING

The Philippines is probably one of the most highly mineralized countries in the world. This is due to the complex regional evolution that led to its present geological formation. Gervasio (1971) grouped most of the Philippine archipelago into a mobile belt (MB) which is characterized by high seismic activities and an aseismic region which is relatively free from crustal movement. The MB is composed of Luzon, Visayas island and Mindanao island. The seismic portion is located mainly in Palawan. The MB is dissected by the left lateral Philippine Fault zone from eastern Mindanao through northwestern Luzon. Recent geological and geophysical investigations revealed that the tectonic history of the Philippine archipelago involved geological processes related to convergent plate margins, island arcs, aphiolites, sedimentary basins and microcontinental arc collision. Luzon in particular, has subdirection zones in both the western and eastern sides. Other locations of shorter subdirection zones indicated as trenches on the sea floor are found off the coasts of Sulu, Cotobato, Negros and southwestern Palawan. Ludwig et al., 1967; Hayes and Ludwig, 1967; De Boer, J., et al., and McCabe R. and Cole, J.; Taylor and Hayes, 1980; Holloway, 1982; Lewis, S.D. and Hayes, D.E., 1983; Rangin and Silver, 1990.

The southern Palawan including Balabac is inferred to have collided during Miocene time with a microcontinental terrain formed by late Paleozoic metamorphic rocks and Mesozoic clastic sediments and limestone. The continental rocks are exposed in northern half of Palawan and Calamian islands up to the western half of Mindoro and Brusanga Peninsula in Panay.

The microcontinental plate was formed by rifting and south-south sea floor spreading from the Asian Continental margin during Oligocene time. Sea floor spreading stopped during middle Miocene time due to the collision of the north Palawan microcontinent or block and the westward moving island arc originally formed in the Philippine Sea plate during Eocene time. McCabe and Cole, 1987.

MINERAL DEPOSITS AND GEOLOGICAL ENVIRONMENT

The setting of the mineral deposits may be grouped into those associated with oceanic crust-mantle (ophiolite), island arc and continental rocks or terranes. The classification is based mainly on the fact that the Philippines is composed of these principal geologic environments. However, as a result of plate collisions and subduction island arc formation, the characteristic rock suite under these 3 geologic environments may be quite mixed up and interspersed or intercolated with each other. This occurrence is more pronounced specially along the plate collision margins or leading plate edges.

DEPOSITS RELATED TO OCEANIC-CRUST (OPHIOLITE)

According to Yumul et al., (1995) an ophiolite suite or sequence is composed of pillow basalts, dikes/ sells complexes, gabbros, mafic-ultramafic rocks and peridotes.

The minerals of chromium and nickel-cobalt including the platinium group metals as well as massive sulfides of copper, zinc and iron are commonly associated with the ophiolite complexes. The 'Geological Map of the Philippines" (Bureau of Mines, 1963), shows the widespread distribution of the mafic-ultramafic rocks in the country paralled to the oceanic trenches and at or near some sections of the Philippine Fault. Large ophiolitic complexes composed mainly of peridiotite-diorite are found in the Zambales mountain range, western part of Sierra Madre Range, southeastern Samar, Dinagat island, Surigao, Davao Oriental, northern Tawi-Tawi. Smaller ultramafic masses are distributed in the other parts of the islands. The "Metallogenic Provinces" map (BM; PAEC; 1972) indicates the location of the important chromium-nickel deposits in the ultramafic bodies. Santos (1986) noted the occurrence of the Dasol massive copper-zinc-pyrite deposit in ophiolitic rock sequence dormantly pillow lavas and metavolcanics in Pangasinan. Balce et al., (1981) suggest the paired subduction zone-magmatic belts in the archipelago as the best indication of collision of several trench-arc systems. An example is the Manila Trench-Central Luzon volcanic belt. They also observed occurrences of unpaired ophiolite and magmatic belts. Other trench-arc systems are described in the JICA-MMAJ-MGB (1990) report. Table 1 lists the major mineral deposits and their locations associated with ophiolite complexes.

Table 1. Major mineral deposits and their locations related to ophiolite complexes.

                    Minerals                                                      Locations

1.Massive and disseminated chromites                           Acoje, Sta.Cruz,    (metallurgical grade)with minor nickel                           Zambales          sulfides (pentlandite)

2.Disseminated chromite (refactory grade)                     Coto, Masinloc,                                                                                      Zambales

3.Massive and disseminated chromite                             Dinagat Island,                                                                                      Surigao

DEPOSITS RELATED TO VOLCANIC OR MAGMATIC ARC

Listed in Table 2 are the mineral deposits and locations related to volcanic/magmatic arc. Smaller deposit occurrences are also found associated with metamorphosed volcanics intruded by dioritic bodies. The main copper minerals are chalcopyrite, bormite and enargite-luzon to Lepanto mine, Mangkayan, Benguet. The sulfides contain small amounts of gold and silver and free gold.

Other ore minerals include bormite, sphalerite, molydenite and galena, among others.

Table 2. Significant mineral deposits and their locations related to volcanic/magmatic arc.