The Didipio gold-copper project is located in the north of Luzon Island in the Philippines, approximately 270 km NNE of Manila.
The Didipio orebody intrudes into the southern margin of a composite plutonic complex composed of monzonite to diorite intrusives. The Dinkidi deposit occurs as low grade disseminated and stockwork porphyry-type gold-copper mineralisation. Dinkidi has a resource of 121 million tonnes, grading 1.0 g/t Au and 0.4% Cu, for a contained content of 3.8 million ounces of gold and 470,000 tonnes of copper (for a total gold equivalent resource of 5.8 million ounces).
The Dinkidi development comprises four years of open cut mining and a minimum eleven years of underground sub-level caving operations. Annual design throughput is two million tonnes of processed ore . Governmental development approval to build the project was obtained on 13 October 2005.
The geology of the Didipio region is typical of an island arc setting, consisting of volcanic, volcaniclastic and sedimentary rocks intruded by porphyries of intermediate to felsic composition. The Didipio gold-copper deposit is hosted within the multiphase Dinkidi Stock which is, in turn, part of a larger alkalic intrusive body, the Didipio Igneous Complex.
The Didipio Igneous Complex consists of: (1) an early composite clinopyroxene-gabbro-diorite-monzodiorite pluton; (2) the Surong clinopyroxene to biotite monzonite pluton; (3) the Cu-Au mineralised Dinkidi Stock, which comprises an early equigranular biotite-monzonite stock (Tunja Monzonite), a thin, variably-textured clinopyroxene-syenite (the Balut Dyke), and a monzosyenite porphyry (Quan Porphyry) that grades, in its core, into a crystal-crowded leucocratic quartz syenite (Bufu Syenite); and (4) post-mineralisation andesite dykes.
Five main hydrothermal events are recognised in the Didipio region: (1) contact metamorphism and weak biotite-cordierite alteration associated with emplacement of the early diorite phase; (2) K-silicate magnetite-biotite alteration, and sub economic Cu-Au mineralisation associated with the emplacement of the Surong monzonite pluton; (3) intensely developed porphyry-style alteration and ore-grade Cu-Au mineralisation spatially and temporally associated with emplacement of the Dinkidi Stock; (4) an advanced argillic alteration assemblage which has overprinted the Didipio Igneous Complex and is associated with sub-economic high-sulphidation style Cu-Au mineralisation; (5) late-stage unmineralised zeolite-carbonate veins associated with post-mineral strike-slip faulting.
Emplacement of the Balut Dyke was associated with a calc-potassic style diopside-actinolite-K-feldspar-bornite alteration assemblage and associated vein stockwork. This quartz-free mineral assemblage is associated with high gold grades (2-8 g/t Au). Intrusion of the Bufu Syenite led to the formation of a quartz-sericite-calcite-chalcopyrite stockwork vein and alteration assemblage, which has overprinted the calc-potassic assemblage.
The quartz stockwork hosts the bulk of low grade mineralisation (1-2 g/t Au) at the Didipio Gold Copper project. A coarse-grained assemblage of quartz-actinolite-perthite (the ‘Bugoy Pegmatite’) formed as an apophysis on the Bufu Syenite, and was subsequently brecciated by late-stage faulting. Mineralisation is directly associated with veining which has been sub-divided into a peripheral stockwork style and a centrally located, higher temperature, series of narrow diopside-actinolite-K-feldspar-bornite veins.
Chalcopyrite and gold are the main economic minerals in the deposit. Chalcopyrite occurs as fine-grained disseminations, aggregates, fracture fillings and stockwork veins, particularly within the vein zone of alteration. Chalcopyrite can replace magnetite and is, in turn, replaced by bornite. Bornite occurs as alteration rims around and along fractures within chalcopyrite grains. The deposit is oxidised from the surface to a depth of between 15 m and 60 m, averaging 30 m. The oxide zone forms a blanket over the top of the deposit, and largely comprises silicification, clay and carbonate minerals, accompanied by secondary copper minerals including malachite and chrysocolla.
(Sources: Wolfe and Cooke 2004, Climax Mining Ltd 2008, OceanaGold Corp 2008)
