SUMMARY OF THE GEOLOGY AND MINERAL RESOURCES OF MINDORO ISLAND
Mindoro Island is one of the major islands of the Philippine archipelago covering a land area of 10,244 square kilometres or 102,440,000 hectares. It lies about 130 kilometers south of Manila across Verde Island Passage. It is bounded on the east by the Tablas Strait, on the south by the Senirara Island, and on the west by the Mindoro Strait.
The major physiographic feature of the island is the central mountain ranges. Its topography, which is controlled by lithology and partly by structures, is characteristically rugged and forms the physiographic divide between Occidental and Oriental Mindoro. Most peaks on this divide range from 900 meters to more than 2,400 meters in elevation. These peaks are Mount Burburungan (1,524 m), Mout Tandrac Peak (900m) and Mount Baco (2,488m). Major structures observed in the Island trend northwest to north-south.
Other prominent physiographic feature includes the 10 kilometers wide and 15 kilometers long Naujan Lake located on its northeastern section. Narrow, swampy coastal plain fringes the WSW and ENE portion of the island.
Northeast trending river systems namely: Bugsanga, Lumintao, Monpong, Amnay and Mamburao Rivers comprise some of the major drainages on the western portion of Mindoro Island while Magasawang Tubig, Pinamalayan and Bongabong River Systems comprise the major drainage lines in the eastern section of the island.
A. General Geology of Mindoro Island
Based from the Geological and Mineral Inventory Map of Mindoro Island (RP-Japan Project, JICA-MMAJ, 1984), the regional stratigraphy of the area comprises the following rock formations from oldest to youngest: a) the Halcon Metamorphics b) the Baco Group which is further subdivided into the Mansalay Formation and the Lumintao Formation c) the Mamburao Group d) the Sablayan Group, e) the Bongabong Group f) the Socorro Group, and g) the Quaternary Alluvial Deposits. Brief descriptions of the geology and its distributions within the province of Occidental and Oriental Mindoro are discussed below:
Pre-Jurassic to Jurassic Halcon Metamorphics - This is the oldest rock formation in the island. It comprises metaconglomerates, schists and phyllites which form the backbone of the northwest ranges. The rocks crop out around the northwestern coast and the areas from Puerto Galera to Mt. Halcon.
Jurrasic Baco Group – This Group is composed of volcanic and clastic rocks that slightly undergone metamorphism. It is divided into two formations: the lower Mansalay Formation comprising predominantly of clastic rocks such as shale, sandstone, and slate to phyllite; and the upper Lumintao Formation composing mainly of basalt with basaltic tuff, sandstone, shale and slate to phyllite. The Mansalay Formation is distributed in a long belt from Mamburao to Mansalay in a northwest-southeast direction, while Lumintao Formation crops out from Lumintao River to Mamburao on the western side of the island. Exposures were also found in the upper reaches of Pula River and Balete River west of Pinamalayan. Other major outcrops were observed in the middle reaches of Bongabong River and also in the western portion of Roxas in Oriental Mindoro.
Paleocene Mamburao Group – This Group is composed of basic volcanic rock, mainly basalt. Exposures of this rock are confined mainly along the lowland of Mamburao and Abra de Ilog.
Late Eocene to Late Miocene Sablayan Group - This is given to the clastic beds which predominantly include limestone, calcareous sandstone, calcareous mudstone with andesite and andesitic tuff. These rocks are extensively distributed from Sablayan to Bulalacao. Other exposures are found in the lower and upper reaches of Mamburao River, and also in the upper reaches of Magasawang Tubig River, the Banus River, the Sumagui River and the Tangon River in Oriental Mindoro
Pliocene Bongabong Group – The Bongabong Group consists mostly of Pliocence conglomerate, tuffaceous sandstone and mudstone-siltstone. This is scatteredly distributed in Rizal and Calintaan , Occidental Mindoro while major exposures are found from Bgy. Villacerveza in Victoria up to Bongabong River in Bongabong and Mansalay municipalities.
Quaternary Socorro Group –The groupis composed mainly of terrace gravel and sand deposits; tuffaceous silt and andesitic tuff; and limestone. Major outcrops are distributed in the lowland of Oriental Mindoro from Puerto Galera in the north to Roxas municipality in the south. It also covers portions San Jose, Magsaysay, Sablayan and Mamburao in Occidental Mindoro.
Quaternary Alluvium – These are detrital deposits formed on valleys by stream action and essentially made up of silt, sand, and gravel. They are extensively distributed along the shores, river channels, broad floodplains and delta of Occidental Mindoro and Oriental Mindoro.
Intrusive rocks consist of big Ultramafic complex, a small body of acidic to intermediate rocks such as granodiorite, quartz diorite and dioite porphyry, and small-scale basic rocks of dolerite and gabbro are also present.
The major geologic structure in Mindoro Island is the north-south trending Central Mindoro Fault that stretches about 100 kilometers from Mansalay to Puerto Galera (See attached map). Previous works by the Mines & Geosciences Bureau (MGB) are vague on the recent activity of the Central Mindoro Fault, although topographic expressions along its trace seem to suggest that it is an active fault. The Aglubang River Fault appears to branch out of the Central Mindoro Fault south of Alcate, Victoria.
The older pre-Jurassic to Jurassic formations are distributed in the central area of the island while younger formations accumulate zonally on both sides of the central mountain ranges showing an anticlinal structure.
The other major structure is the Lubang Fault that traverses immediately the northern portions of Mindoro Island. This fault trends east west and is located between Mindoro and the Luzon mainland.
B. Mineral Resources of Mindoro Island
Various kinds of mineral showings were observed in Mindoro Island. The metallic ore deposits include nickel and chromite deposits associated with ultramafic rocks, contact metasomatic-type iron deposits, vein-type copper deposits and placer gold. The non-metallic ore deposits include those of barite, jade, marble and coal.
(1) Placer gold originated from the Halcon metamorphics and the Bongabong Group is not related to the mineralization of post igneous activities. The occurrence of the deposits found in Pto. Galera and San Teodoro is not extensive and usually erratic. Methods used for recovering placer gold in the area include sluicing and panning.
(2) Copper mineralization is vein type and fissure-filling deposit developed in pre-Jurassic to Jurassic formation, and is related to Paleogene dioritic rocks intrusion. Geochemical soil survey verified that the mineralized zone located in the upper reaches of the Pula River was very limited and not worth exploiting. Other Cu prospects were reported in Abra de Ilog and Sablayan municipalities in Occidental Mindoro.
(3) Chromite mineralization occurs in the Ultramafic complex and is classified into two types, layered and massive. The complex ultramafic body is composed mainly of dunite and harzburgite. Massive to banded low-grade chromite deposits were observed in Ambil Island, Lubang, Occ. Mindoro. Other chromite prospects were found in Bgy. Mariil, Paluan, Occ. Mindoro; near Amnay and Pintin Rivers in Sablayan, along Alagag River in Sablayan; and in Banus River-Bansud River area in Bansud, Oriental Mindoro
(4) Iron deposits are the metasomatic deposits which are formed by Tertiary dioritic rocks which intruded into the Jurassic and Tertiary limestone beds. The Fe ores are made up predominantly of magnetite and subordinate amount of hematite with some traces of manganese. The ground magnetic survey carried out northern Mindoro Island showed three promising areas: Nagsabongan, Lasal and Lapa-ao Fe deposits in Abra de Ilog and Mamburao, Occidental Mindoro. Other major iron prospects were found in Pto. Galera, San Teodoro and Baco in Oriental Mindoro.
(5) Nickel ore occurs as laterite deposit within the municipality of Sablayan, Occidental Mindoro and the municipality of Victoria in Oriental Mindoro. There are two rock types recognized in the immediate vicinity of the laterite deposit, namely, the schist and ultramafic rocks. The schist which is the oldest rock consists mainly of quartz, sericite and chlorite. The ultramafic rocks consist of pridotite, dunite and layered gabbro in their order of decreasing abundance. They appear thrusted over the much older schist. The laterite deposit extensively covers almost all the gentle slopes and flat areas over the ultramafic rocks to which it is genetically associated. The results of exploration drilling showed a variable thickness of laterite ranging from 3 to 11 meters with average nickel content of 0.94%.
(6) Hosting the barite deposit is the Mansalay Formation located in the municipality of Mansalay in southeastern Mindoro Island. The deposit is of vein and fissure-filling type and is related to Tertiary dioritic rocks. A geochemical anomalous zone was found in the upper reaches of the Barac River, which is close to the Taoga working mine. This may be an indication of new ore deposit but is still considered small in scale from the geological point of view.
(7) Coal deposits occur in the Miocene-age Sablayan Group composed of alternating beds of calcareous sandstone/siltstone and calcareous mudstone with calorific values ranging from 11,100 to 12, 600 BTU/lb, which correspond to high-volatile C bituminous coal (American Standard of coal Classification). The observed deposits are found in the municipality of Bulalacao in Oriental mindoro