Geology and Ore Deposit

The geological formations of the Rio Tuba Nickel deposit and its vicinities consist, in stratigraphic order, of the following:

Chert-spilite (spilitic basalt associated with chert)
Utramafics
Younger sedimentary rocks (clastics and limestome)

The following geologic and climatic conditions are considered favorable in the formation of iron rich laterites and nickel silicate deposit:

Presence of rocks rich in iron, for example, ultramafics.
Structure of the rocks which allows groundwater to circulate freely.
Tropical to sub-tropical climate.
Suitable amount of rainfall.
Suitable balance of weathering and rate of erosion.

The iron minerals of the laterite are mainly magnetite, hematite, goetite and limonite. Chromite is present but rare. Minor amount of nickel minerals occur in solid solutions with the heavy minerals such as magnetite and chromite. However, most of the nickel minerals are associated with the fine minerals such as limonite, goethite and clay minerals.

Geological History

The basement complex of Palawan Island consists of metamorphosed Paleozoic rocks. The deposition of flysch started during late Triassic extending through Jurassic. Folding movement took place at the close of Jurassic or early Cretaceous, with subsequent truncation of the flysh fold belt and later marine transgression accompanied by widespread submarine effusion. The spilitic basalt associated with chert found in the vicinity of Rio Tuba belongs to the sediments of this period.

This was followed by folding and development of north-east trending thrust faults. Along these thrust faults, bodies of the ultramafic complex, including gabbro, were emplaced. The peridotites, which are the original rocks of the weathered residual nickel deposit of Rio Tuba, belong to these intrusives.

The present northeast axial trend of Palawan was established by the intrusion of ultramafic complex during middle or later Cretaceous. During the Paleocene extending through the Pleistocene, the southern part of Palawan Island entered a molasse stage and sedimentation of coarse sediments and limestone took place. In the meantime, this area experienced several times of tectonic uplifts, especially at the close of the Miocene, a rise of the central range of Palawan Island commenced.

The younger sedimentary rocks in the vicinity of the Rio Tuba Nickel deposit are surmised, from the facies of these rocks, to have their origin in the Pliocene or Pleistocene. The rocks distributed over the hillside slopes and small valleys of the Guintalungan-Mangingidong Area are ultramafics out of the above-mentioned rocks. These ultramafics, comprising dunite, pyroxenite and other varieties of peridotites, are mostly serpentinized. Their texture varies from fine grained to coarse-grained. Boulders obtained from test pits show a considerable degree of weathering and contain olivine, pyroxene and sometimes chromite.

This property includes the Guintalungan ore deposit, the Mangingidong ore deposit and number of small laterite and a number of small laterite-distributed areas, the Guintalungan ore deposit being the largest of them all. These ore deposits are weathered residual deposits, which have been formed by the weathering and decomposition of the above-mentioned ultramafics, resulting in deposition of nickel.

One of the characteristics features of these ore deposits is that their ground surfaces are covered with laterite. In the general laterization process, the ingredients of the original rock are selectively shifted according to the weathering stages of ultramafics (ordinarily consisting of peridotites containing 0.2 to 0.3% nickel), and iron and nickel, which have been initially contained in the original rock are separated from each other and deposited. The laterization process is greatly affected by ground water. Because of carbon dioxide in the atmosphere and organic acids which have been generated as a result of the decomposition of plants, ground water, in general, shows acidity in the upper part of the laterite zone and alkalinity in its lower part. If the original rocks abound with fissures and shears, weathering progresses deeper along them, thereby forming bonanza. It is considered, however, that comparatively uniform weathering took place at the Rio Tuba nickel deposit.

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