IV. GEOLOGY AND MINERAL RESOURCES OF PALAWAN PROVINCE
A. General Geology of Palawan
1. The Northern Palawan
Northern Palawan is constituted predominantly by Upper Paleozoic to lower Mesozoic sediments regionally metamorphosed and deformed mainly during Early Jurassic. This sequence was later intruded by intermediate plutonic rock (Kapoas granite) and subsequence overlain by relatively undeformed Upper Eocene to Recent sediments offshore. The northern parts of the main Palawan Island and the Calamian Group are composed of uplifted basement chert and limestone, which constitute in part.
The igneous bodies in the Northern Part are the Kapoas Diorite intruded at the Kapoas Peninsula and at east side of the Ulugan bay during the Oligocene time and the Manguao Volcanics exposed at the eastern side of the northern main Palawan Island in the Pleistocene time.
In the northern central part of the main Palawan Island , the basement consists of phyllitic rocks and mica schist. These basement rocks are covered by Tertiary Sedimentary rocks and thrusted over by the ultramafic rocks at Paly Island offshore east of the main Palawan Island.
The Northern Part is divided geologically into the northeastern zone and southwestern zone by the thrust which passes from the Kapoas Peninsula in the west side to the Calauag Bay in the east side.
a.) The northeastern zone
Outcrops of Guinlo Formation, which has been dated as Cretaceous were in the Busuanga Island and at the western side of the Calauag Bay in the northern part of Palawan Island. This formation, which unconformably overlies the Busuanga Formation, consists of massive quartzose sandstones with occurrences of mudstone and conglomerates.
The Bacuit Formation, which is exposed at the vicinity of El Nido is dated as Middle Permian in age by the studies of fossils found in some chert samples. (Fontaine, 1981, Wolfart et al., 1984.)
b.) The southwestern zone
The Caramay Schist and the Concepcion Phyllite forming the basement rocks in this zone are thrusted over the Panas Formation and Busuanga Formation at the western and northern sides respectively. The Caramay Schist and the Concepcion Phyllite cover almost the entire area. The Concepcion Phyllite which is derived from alternating sandstone and mudstone is mainly distributed throughout the southwestern portion of the Northern Part thrusted over the cherty sediments at the northeastern portion and the Panas Formation at the northwestern side.
The metamorphic rocks have been considered to be Carboniferous to Permian in age by MGB (1981), but it was pointed out recently that these metamorphic rocks could probably be derived from some Cretaceous or Paleogene turbidite formations (R. A. Santos, 1988, oral.).
The Caramay Schist consists of muscovite, schist, quartz mica schist and graphite schist and is also characterized by an almost mica free quartzite.
The Concepcion Phyllite consists of phyllite mudstone, siltstone, sandstone and met-sediment, and is also associated with conglomerate mudstone.
The Panas Formation is distributed along a narrow area from the eastern part of the Ulugan Bay to the eastern portion of Pandanan Bay. The Concepcion is thrusted over this formation.
This formation, which is assumed to be of Eocene age is inferred to have been a submarine fan deposit. It is composed of tightly folded, faulted and imbricated units of quartzo-feldspathic sandstone, mudstone and shale.
St. Paul’s Limestone is observed at the east side of Mt. St. Paul Bay with a northeast trend consisting of massive dark gray limestone and contains abundant algae, coralline detritus and foraminifera fossils which were dated as Late Oligocene to Middle Miocene age (BMG, 1981; UNDP, 1985).
The Igneous Activities
The Northern Part
Only three igneous bodies are known to occur in the Northern Part, namely, the Kapoas Intrusives, the Mangunao Volcanics and the Ultramafics of Paly island.
The Kapoas Intrusives are observed as several stocks trending NNE the east from the east side of El Nido through the Kapoas Peninsula to the east side of the Ulugan Bay in the vicinity of Stripe Peak. These stocks are dated Late Eocene to Early Oligocene by KAr method (UNDP, 1984; MMAJ, 1987).
The Mangua Volcanics are observed as volcanic flows at the southern part of Manguao Lake in the northeastern main Palawan Island.
The Ultramafics are observed at Paly Island which is located offshore at the northeast of the Northern part. These ultramafics are believed to be thrusted over the north Palawan block and contain chromite dissemination.
2. The Southern Palawan
The igneous rock in the Southern Part is composed of basalt extrusives, thrusted ultramafics and the associated gabbros on both rock units. Dikes of pyroxynite and gabbro are widespread throughout the abovementioned rock units.
The Espina Basalt which is believed to be part of the basement and is of Cretaceous are is exposed as a window like-inlier in an area north of Puerto Princesa and in many localities with in Central and South Palawan areas.
The Mt. Beaufort Ultramafics are mainly made-up of harzburgite with irregular patches and lenses of dunites. Chrome spinel/chromites are known to exist and to occur as disseminations in these rocks./ These ultramafics are presumed to be of Eocene age.
The Stavely Range Gabbro along with the Mt. Beaufort Ultramafics constitute the Palawan Ophiolite which is thrusted over the Early Oligocene Pandian Formation and the Eocene Panas Formation. The thrusting is inferred to have occurred throughout the Eocene to Middle Miocene, although the inception of this activity is thought to have started during the Paleocene.
The Basement Complex
The basement complex of the Southern Part consists of the Sulu Sea Mine Formation, the Tagboros Formation and the Espina Basalt. The Sulu Sea Mine Formation consists of chert, mudstone and conglomerate and the Tagboros Formation is composed of green siltstone with wacke and conglomerate. Aside from being exposed as window like-inliers at a small spot north of Puerto Princesa, these basement rocks are distributed east of Quezon, around the southwester portion of the mountain range of this part and in the Balabac Island.
These basement units are dated as Cretaceous in age.
The Panas Formation is a turbidite formation which is composed of shale, mudstone and alternating arkose sandstone and is distributed at the west side of the Ulugan Bay, the periphery of the Pulot area and at the southwestern part of this area.
The Panas Formation is associated with and partially alternates with the Sumbiling Limestone which is though to be a contemporaneous heteropic facies of the Panas Formation.
The Pandian Formation is basically made-up of arkose sandstone associated with shale and siltstone, and considered to be Lower Oligocene in age. It is distributed at the west coast of the southwestern side of Quezon, overlying the Panas Formation unconformably.
The Ransang Limestone composed of sandy and silty limestone is estimated to be Early Miocene age. It has a limited distribution at the eastern side of Quezon and at the eastern and northeastern portion of Rio Tuba.
The Isgod Formation is composed of alternating sandstone, mudstone and siltstone is associated with coralline limestone. It is distributed at the east and southern sides of Quezon and is dated to be to Middle Miocene in age.
The Alfonso XIII Formation consists basically of reefal limestone and overlies the Isgod Formation. This formation is distributed around the Aboabo area in the south and is found to be Late Miocene in age.
The Sayab Formation consisting of alternating sandstone and shale beds and is exposed at the southwester part of Rio Tuba. It is to be late Miocene in age. Similar to the Alfonso XIII Formation.
The Clarendon Formation consists of alternating sandstone to shale and is distributed at the southern part of Balabac Island and is found to be Pliocene in age.
The Iwahig Formation which is composed of conglomerate and limestone bodies is distributed from Puerto Princesa to Narra in southeastern coast and at the vicinity of Wangle in the southernmost portion of the main Palawan Island. It is found to be Pleistocene in age as based on paleontological studies.
B. Mineralization Resources of Palawan
In general, the mineralizations in the Palawan area is prior intense in the Southern Part.
1. The Northern Palawan
There are four kinds of mineralization in this region as follows:
a) Strata-bound manganese mineralization in cherty rocks at the eastern part of Busuanga Island. The manganese deposit, probably of psilomelane and pyrolusite occurs as stratiform masses. It is associated with jasper and quartz, exhibiting a brecciated structure and fragments of jasper cemented by white quartz.
The bedded ones are dark gray to steel gray with grayish brownish streaks friable and easily soils the finger. Botroydal surfaces in some parts of the orebody are common. (64. Lanka; 65. Dapdapan)*1
b) Vein type manganese mineralization in sedimentary rocks at the northern part of Culion Island. The deposits are within an area underlain by a bedded sequence of highly siliceous rocks, collectively designated as chert and to a minor extent by quartzite. The bedded manganese bodies are folded with the enclosing chert beds.
The manganese deposits occur in two types, namely, 1) as tabular bodies that lie in conformity with the bedding of the enclosing chert and 2) as residual concentration is soil. (4. Kabol-Kabol)
c) Chromite dissemination in ultramafics at Paly Island (66. Paly Is.)
d) Stibnite mineralization associated with quartz vein in mica schist at the northwestern part of Tinitian.
2. The Southern Palawan
There are four kinds of mineralization in this region as follows:
a) Chromite lenses in ultramafics (67. Richman; 68. Boyo; 62. Berong; 77, Trident, etc.)
b) Nickel-laterite mineralization which are weathering products of the ultramafics (26. Atlas mine; 71. Ibat-ong; 73. Bethlehem; 76. Santa Monica; 39. Pulute Range; 54, Rio Tuba, etc.)
c) Cyprus type massive sulfide copper mineralization. Massive sulfide deposits (Cyprus type) in Balabac Island, Brooke’s Point and south Palawan are major base metal mineralization essentially of copper within the region. (78. 79. 90. Barong Barong , A, B, C; 81. Males, etc.)
d) Sulfide bearing quartz vein type mineralization (1. Pulot, etc.)
1. Orthomagmatic chromite and associated deposits
The chromite deposits are typically podiform type with shapes ranging from tabular, lensoid to irregular. The texture of the ore ranges from massive, nodular (leopard), brecciated to disseminated. There area also placer deposits derived by weathering of the host rocks and mechanical concentration.
Nickel, cobalt and vanadium are often associated with these chromite deposits. This deposits are known to occur in several localities in Palawan as in Puerto Princesa (Boyo Mine, Richman Minesand Benguet Mine) in Narra ( Trident Mines, Olympic Mines and Romarao) in Quezon (Berong) in Brookes Point and in Bataraza.
2. Hydrothermal Vein
Deposits Hydrothermal vein deposits in central and northern Palawan yield mercury and antimony as the main products. Confined within the tectonic windows of Iratag ang Bacungan in Central Palawan, mercury mineralization is one of the more recent deposists in the region and cinnabar (Hgs) is the chief mercury mineral. Antimony mineralization in the region within the Lasgas , Iraan, Fabrics and Bolo-bolo areas in Palawan with the Caramay Schist as the host formation. The antimony sulfide (stibnite) occur in quartz veins associated silicified rocks.
3. Bedded Manganese Deposists
The manganese occurrence throughout the region is essentially stratigraphically controlled and associated with the pelagic chert units. Notable occurrences of manganese throughout the region are in Busuanga (San Nicolas and Coron) and Culion Islands (Kobol-kobol) of north Palawan: Sulu Sea Mines in Puerto Princesa of central Palawan; and in Nabas, Buruanga Peninsula.
4. Laterite deposits
Laterite is one of the most important sources for nickel worldwide. In the stable belt which consists of rifted continental margin, ultramafic complexes thrusted over the continental crest are very extensively distributed. And upon weathering, they alter to laterite and nickel is often concentrated under favorable conditions. Several significant concentrations of Ni of close to 1 % or higher in deposits, at Betlehem, Santa Monica, Ibatang in Narra; Pulute Range in Quezon and Rio Tuba in Bataraza all in Palawan.
The operating mines in the Southern Part are the Atlas Mine (ATLAS Corp.) and the Rio Tuba Mine (Rio Tuba Nickel Corp.). The details of these miners are as follows:
a) Atlas Mine (Also called Berong Point Mine)
The ores of this mine consist of chromite gravel in laterite. This mine is located 5 km southwest of Long Point in the northwestern coast.
This mine started operating at the end of 1985 and has shipped concentrates at a rate of 1,147 ton per month. The country rock is serpentinite, the alteration product of ultramafic rocks.
b) Rio Tuba
The deposit of this mine is a nickel-laterite type, a weathering product of ultramafic rocks. Rio Tuba Nickle Corporation has been operating since 1977 and has been producing 500,000 tons of concentrates per year (average grade: Ni: 3.3%; Co: 0.04% and moisture: 30%). It has a proven reserve of 9,600,000 tons with an average grade of 2.3% Ni and a stripping ratio of 1:1. At present, 550 personnel are employed in the mine. At the surface is a 5 m thick reddish black to reddish brown lateritic soil. Beneath of this soil is a 2-5 m thick mineralized zone.