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ULTRAMAFIC ROCKS AND CHROMITE DEPOSITS

Dr. Fred Seligmann
Professor of Geology
Department of Geology and Geography
College of Arts and Sciences
University of the Philippines

 

 

 

 

ABSTRACT ONLY



In the Zambales range, the chromite bearing ultramafic rocks are divided into two fundamental different groups. Beside the distinguishable petrographic texture indicating a kinematic and a younger static phase of recrystallization, further evidence is given by the different Al2O3 content of spinel phases within the ophiolite suite. The two diverse ultramafic petrographic provinces are: A) M-province (metallurgical ore province): Principally in the northern block of the NE - trending Lawis fault system (San Clemente-Masinloc) where orogenic ultramafic rocks are exposed. The ultramafics are characterized by A12O3 poor spinels. These minerals
reflect the lower pressure and temperature conditions of formation. B) R-province (refractory ore province): Mainly in the southern part of the Lawis fault system where AI2O3 rich spinels indicate a higher recrystallization temperature and variablae pressure condition compared with the M-province.

It might be that the Lawis fault system (LFS) represents a relict of a normal fault where the southern segment of the LFS had been raised up. Similar relationships are known from the Caribbean and Mediterranean island arc structures where refractory and metallurgical chromite occur in lenses and layers. Here the ore bodies are the result of a polymetamorphic
differentiation under variable p - t conditions. The Chromite ore deposits are principally controlled by B and S tectonic structures. The emplacement of ore bodies via gravity layering is a possibility, although an argument against this is the apparent absence of fluidal structures and zoned mafic minerals.
 

 

 

 

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