The San Antonio porphyry copper orebody of Marcopper Mining Corporation is located at the eastern margin of a northwest trending diorite stock in the central portion of Marinduque Island. The orebody is hosted mainly by hornblende diorite porphyry and quartz diorite that intruded pre-Tertiary (?) to Oligocene sedimentary and volcanic rocks. Routine examination of rock outcrops around the vicinity of the Tapian orebody and subsequent shallow test diamond drilling disclosed a limited area of enriched copper sulphides below a soft weathered overburden. A follow-up IP survey delineated a broad northwest trending IP anomaly which greatly increased the potential reserves of the known mineralisation. Subsequent grid drilling within the anomaly developed geologic reserves of 200 million metric tons averaging 0.57 % Cu at a 0.4 % cut-off. Section 80N illustrates the position of the orebody relative to the associated IP anomalies. The body is at the western side of the strong and continuous IP response that is characterized by a moderately high PFE zone. The eastern side of the anomaly is marked by the prevalence of pyritic mineralisation in the intruded volcanics. The low resistivities at 800W to 400W are due to the overlying tailings pond.