Geology and Chromite Resources of Dinagat Island, Philippines

Arnulfo B.Santiago, MSc.

Independent Consultant Geologist

Abstract

The Dinagat Ophiolite is a complex tectono-stratigraphic terrane made up of imbricated oceanic crustal rocks and its emplacement is accompanied by block faulting and trusts. The Dinagat Ophiolite is composed of three major formational domains, a suite of pre-Cretaceous metamorphic basement schists or proto-arc (Sunga and Palaganas, 1986) of diverse volcanic and sedimentary composition, a Lower Cretaceous ophiolite (Dinagat Ophiolite), and a succession of Tertiary basic volcano-sedimentary polymicts, calcarenites and limestone formation.

The ultramafic part of the Dinagat Ophiolite is composed of a basal harzburgite tectonite zone which grades transitionally upward to a dunite tectonite zone. An upper mafic rock suite from gabbro-diabase complex to an upper pillow basalt piles overlies the ultramafic tectonites. A transition zone between the harzburgite tectonite and dunite tectonite zones is characterized by narrow interlayered harzburgite and dunite with the latter measuring up to 20 meters in breadth and envelopes metallurgical-grade chromite deposits. The occurrence of dunite gradually decreases down to the massive basal harzburgite tectonite layer which becomes massive, foliated and deformed. On the regional context of Dinagat Island, the dunite tectonite zone is in a complex transitional boundary with the harzburgite tectonite zone toward the east. The orientation of layering structures and the distribution of the various ultramafic units from the west to the east suggest that the ophiolite strikes northwesterly and dips southwesterly, and the chronological younging direction is from the east to the west.

The geometry of the chromite deposits is controlled mainly by the complex structural patterns and their host rocks. The chromite deposits in the ultramafic tectonites of the Dinagat Ophiolite Suite generally occur as massive, irregular-shaped and podiform bodies, as alternating banded masses within a 5-10 meters breadth, and as low grade nodular or disseminated-type deposits. The chromite grades average between 30% and as high as 65% chromic oxide, and chrome-iron ratio of 2.5 which classify the deposits under metallurgical-grade type.

North Dinagat is hosting a number of metallurgical-grade chromite deposits in a lower harzburgite-dunite tectonite transition zone including the Bel-At Kong-King prospects with an estimated but undisclosed resource of at least 0.5 million tons (Santiago, 1994), the Kromico mine which is estimated to contain about 1 million tons estimated from northwest trending and southwest steeply dipping podiform orebodies, the Sangay and Kalanugan River prospects with combined estimated potential resources of 2 million tons. The general trend of the deposits is northwesterly and their distribution is controlled by the northeasterly displacement of post mineral structures.

The Omasdang chromite ore bodies (Acoje mine) in North Dinagat and the Vel-Ore mine chromite deposits in Central Dinagat are northwesterly trending chromite deposits occurring within the upper harzburgite-dunite tectonite transition zone. The chromite bodies are characteristically highly sheared and occur in the form of random clustered disseminations and irregular nodular pods, and as massive, banded or lenticular forms within wide dunite masses. The disseminated or nodular type ore contains grades between 25% and 45% Cr₂O₃ while the massive type ore has > 45% Cr₂O₃. The Omasdang area contains about 7.29 million tons at 7.6 % chromic oxide.

A large portion of Dinagat Island remains unexplored for its chromite resources. It is hoped that this paper will open a new corridor for more in-depth scientific research on the nature and progress of the Dinagat Ophiolite which could lead to the discovery of more economic chromite resources.