The Santo Tomas II porphyry Cu-Au deposit, located approximately 20 km south of Baguio City, in the southern part of the Baguio mineral district, Benguet Province, northern Luzon, Philippines. It was an underground, block caving operation.
The deposit lies within the southern end of the Luzon Central Cordillera, which represents a Miocene volcanic arc. From the Miocene to Late Pliocene, repeated intrusions of diorite were emplaced into this arc, hosting much of the porphyry copper mineralisation in the region.
The quartz-diorite (tonalite) porphyry stock at Santo Tomas II is of Pliocene age, with K-Ar age dating of hydrothermal biotite estimated to be 1.5±0.4 Ma. It is part of a complex of four main intrusive phases, namely: (1) porphyritic hornblende quartz diorite; (2) hornblende quartz diorite porphyry; (3) ore-related hornblende andesite porphyry; and (4) post-ore clinopyroxene-bearing hornblende andesite porphyry. This complex intrudes volcanic country rocks of Oligocene to Miocene age.
The dimensions of the orebody are approximately 600 x 200 m at surface, extending to a depth in excess of 800 m. The host porphyry stock is strongly fractured along both NW-Se and north-south directions.
The dominant alteration in the ore zone comprises potassic (biotite and quartz) and propylitic (chlorite, epidote and calcite) assemblages, with a limited phyllic phase. Sericite and chlorite are less abundant, and K feldspar is absent. Biotite replaces hornblende and rarely occurs in veins. Quartz forms numerous small veinlets and is associated with sulphides. Chlorite replaces hydrothermal biotite.
Two main ore asemblages are recognised: (1) Bornite-chalcopyrite-magnetite, concetrated in the core of the potassic alteration, constituting 2 to 5% of the rocks, with a grade of 0.3 to 0.8% Cu. Chalcopyrite is dominant, and occurs in close (mainly myrmekitic) intergrowth with bornite (usually in a 1:10 to 3:10 ratio). Magnetite usually occurs as veinlets, frequently as 1 to 10% of the ore minerals, although local concentrations of up to 80 vol.% are known. (2) Chalcopyrite-pyrite, with 2 to 3% sulphides and minor hematite, in the outer part of the potassic alteration core and into the inner propylitic zone. Copper grades are similar to that in the bornite-chalcopyrite phase. The pyrite to chalcopyrite ration increases outwards from 1:10 in the potassic core to 4:10 in the propylitic assemblage, with chalcopyrite always dominant. Pyrrhotite occurs as small inclusions in the pyrite. Two types of pyrite are evident, euhedral crystals which are replaced by chalcopyrite, while irregular crystals are probably the result of pyritisation of chalcopyrite and pyrrhotite. Accessory sulphides include molybdenite intergrown with chalcopyrite in the pyrite-chalcopyrite assemblage; tetrahedrite-tennantite in both the bornite-chalcopyrite-magnetite and chalcopyrite-pyrite assemblages, occurring as small veinlets cutting both chalcopyrite and bornite. Minor sphalerite and galena are intergrown with chalcopyrite.
The total resource has been variously quoted at:
499 Mt @ 0.375% Cu, 0.7 g/t Au (USGS Porphyry dataset) and 328 Mt @ 0.3% Cu (Tarkian and Koopmann, 1995).
(Source: Porter GeoConsultancy, www.portergeo.com.au, 1997)
