Bulletin of the Geological Society of Malaysia, Volume 16, Dec. 1983, pp. 215 – 239
Billiton Philippines. Inc. (P.O. Box 441, Manila, Philippines)
Abstract: The Philippines porphyry copper province includes a number of widely-separated sub-provinces, one of which is that of southwest Negros in the southwestern Visayas region. As in most, perhaps all other sub-provinces, porphyry mineralizalion here is related to late differentiates of a dioritic to quartz dioritic batholiths emplaced in possibly consanguineous andesitic volcanic rocks of island arc facies. At the present state of knowledge, southwest Negros seems to be distinguished from other Philippines porphyry sub-provinces by a number of geological criteria. Salient amongst these are:-
1) An apparent (K/Ar) Late Eocene to Oligocene age of the mineralized intrusive rocks.
2) Greater amounts of molybdenum than other Philippines porphyries, with lesser amounts of gold.
3) An atypical distribution of copper mineralization. The Sipalay orebody is the second largest in the Philippines and probably contains over one billion (109) tons of ore. The next biggest deposit is around one quarter of this size whilst all other known occurrences appear to be considerably smaller.
4) A marked NW-SE structural grain which has controlled both the emplacement of the intrusive and the disposition of the mineralization.
5) Offshore the sub-province is bordered by a sedimentary (forearc?) basin beyond which is the short, arcuate Negros Trench. Despite appearances these may be resurgent and/or aborted features unconnected with the generation of the copper porphyries.
Southwest Negros is thought to represent a fault-determined block of basement which has been subjected to extensive peneplanation, burial and later in Pliocene uplift by some 500 m.