Genesis of Low-Grade Chromite Ore Deposits in Lateritic Soils from the Philippines

Abstract

Deep in-situ weathering of ultramafic rocks of different composition has caused the formation of limonitic soil in various parts of the Philippine Archipelago. The chromitiferous rocks consist of disseminations and minor segregations of chrome-spinels in dunite or harzburgite. The chrome-spinels have been preserved in the in-situ latentes because of their high resistance to weathering. As a result of the removal of Mg and Si from the ultramafics, chromite has been enriched in the lateritic soils by a factor ranging from 2 to more than 10. In some areas, soils of different ultramafic rocks are mixed due to alluvial processes which can cause further enrichments of chromite. These mixed soils often occur as transported soils above non-ultramafic rocks. The abundance of chromite and the content of magnetite, ilmenite and rutile in the limonitic laterite vary considerably according to the occurrence of the accessory opaque minerals of the different types of ultramafics and to local weathering conditions. Depending on the chemical composition of the chrome-spinels in the mafic and ultramafic rocks, chromite ore of metallurgical, chemical and refractory grade occurs in significant reserves in lateritic soils in the Philippines.