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GEOLOGY AND MINERALIZATION OF THE ATLAS COPPER DEPOSITS IN CEBU ISLAND, PHILIPPlNES

FRANCISCO A. MADAMBA

 

 

 

 

ABSTRACT



Paleocene diorite bodies, intruded into late Cretaceous meta-sediments and metavolcanics, hosted copper mineralization in the Atlas Mines in Cebu Island. The diorite bodies lie along a northnortheasterly trend, suggestive of intrusion along pre-existing structures.

The Atlas copper district lies within a 2-mile wide horst, bounded by two almost parallel east-northeast treading structures. Uplift of the horst resulted in the denudation and erosion of the OligoŚMiocene sedimentary cover and the development of the present geomorphology of the area.

The Lutopan and BigaŚBarot deposits are the two maior ore bodies. They are essentially large tonnage, low grade disseminated copper deposits. Northeast and northwest structures played major roles in the localization of the ore. At Biga, mineralization appears to be related to breccia pipes.

Chalcopyrite is the principal ore mineral. Bornite is minor. The precious metals and molybdenite occur as traces. Pyrite is ubiquitous. Magnetite is fairly abundant, sometimes reaching 5 per cent. These minerals occur mainly as fracture fillings with or without quartz, and fine disseminations.

The most important alteration is silicification, occuring chiefly as quartz veinlets, stringers and massive replacements. The mafic minerals in the host rocks are altered to secondary brown biotite and chlorite. Feldspar transformed to clays. Sericitization and pyritization accompanied silicification. Chlorite with minus epidote occur in the fringe zones and outside of the mineralized areas.
 

 

 

 

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