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Santo Tomas II
Luzon, Philippines
Main commodities: Cu Au

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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).

The most recent source geological information used to prepare this decription was dated: 1997.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Bryner L  1969 - Ore deposits of the Philippines - An introduction to their geology: in    Econ. Geol.   v64 pp 644-666
Hollings P, Wolfe R, Cooke D R and Waters P J,  2011 - Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits and a Case for Slab Roll-Back?: in    Econ. Geol.   v.106 pp. 1257-1277
Tarkian M, Koopmann G  1995 - Platinum-group minerals in the Santo Tomas II (Philex) porphyry copper-gold deposit, Luzon Island, Philippines: in    Mineralium Deposita   v30 pp 39-47

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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