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RECONNAISSANCE GEOLOGY OF THE ZAMBALES CHROMITE
DEPOSITS

George C. Bacuta, Jr.
Geologist, Geological Survey Division
Bureau of Mines

 

 

 

 

ABSTRACT


 

The Zambales chromites contributed 2,468,888 m. t. or 94.2% of metallurgical grade chromite and 12,016,490 m.t. or 99.9% of refractory grade chromite to the total chromite production in the Philippines during the period from 1946 to 1976. Major producers are the Acoje and Coto
Mines which respectively produce metallurgical grade chromite and refractory grade chromite. Among the world producers of chrome, the Philippines ranks as one of the six top producers in 1972 to 1976.

As of 1975, demonstrated ore reserves in the Philippines are figured at 2,292,920 m.t. of metallurgical grade chromite averaging 34.66% Cr2O3. Of these, 1,706,920 m.t. or 74.3% of metallurgical reserves and 8,397,988 m.t. or 96.9% of refractory reserves occur in Zambales. It is roughly estimated that the Philippines would be depleted of these demonstrated reserves not later than 1985 for metallurgical chromite and by 1990 or probably earlier for refractory chromite.

This paper presents general geological information about the geology and chromite deposits in the Zambales Ultramafic Complex. It incorporates field observations gathered by the author during reconnaissance geological mapping and mineral canvassing in the Zambales Range and those reported by past workers.

The Zambales Ultramafic Complex is exposed over an area of 3,372 sq kms at the center of the Zambales Range in Western Luzon. It is considered a part of an ophiolite suite and is generally believed to be an alpine-type complex. The major rock units in the complex are ultramafic and feldpathic rocks intruded by quartz veins and several kinds of dikes. The ultramafic rocks are saxonite, dunite and pyroxenite while the feldspathic rocks consist of norite, olivine gabbro, troctolite and anorthosite. Four alternating belts of peridotite and gabbro disposed along a northnortheastern trend diagonal to the north-south direction of the Zambales Range are recognized and are designated from west to east as the Acoje Ultramafic Belt, Middle Gabbro Belt, Coto Ultramafic Belt and Eastern Gabbro Belt. The contacts between the major rock units show gradational and intrusive relationships. lrregularity of primary structures is persistent.

The chromite deposits are generally distributed in peridotite close to the contact with gabbro. Metallurgical grade chromite is confined exclusively to the Acoje Ultramafic Belt; likewise, refractory grade chromite is confined to the Coto Ultramafic Belt. The chromite orebodies range from large layered, lensoid and podiform masses to small lenses, schlieren and blobs. Chromite is massive, disseminated and nodular in either dunitic or troctolitic gangue. The orebodies trend in many directions; parallel and crosswise orebodies had been recognized.

The genesis of rocks and ore and the tectonic emplacement of the ultramafic complex is not discussed by the author. However, ideas of various workers are explored. A study of the Zambales Ultramafic Complex using new methods in structural analyses and techniques presently adopted in investigations of peridotite gabbro complexes may provide better results that may explain the original genetic relationships between the different rock units in the complex and may illustrate the tectonic mechanisms involved in their emplacement.

 

 

 

 

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