Supergene mineralisation of the Boyongan porphyry copper-gold deposit, Surigao del Norte, Philippines

Boyongan is a blind copper-gold porphyry deposit that was discovered by Anglo American Exploration (Philippines), Inc. in August 2000. It is located in Surigao del Norte, Philippines. Current inferred mineral resources for Boyongan are estimated at 219 million tonnes of combined oxide and sulfide material with an average grade of 0.51% copper and 0.74 grams of gold per tonne. Most of the high-grade mineral resource is within the oxide (supergene zone). Deep oxidation at Boyongan has developed a thick supergene enrichment profile (up to 600 meters) which has a complex supergene mineralogy, consisting of chalcocite, digenite, pseudo-covellite, native copper, cuprite, malachite, pseudo-malachite, azurite, chrysocolla, pseudo-chrysocolla, and pseudo-neotocite. Fine gold (<100˜í¬¿m) has been observed in goethite, chalcocite, chrysocolla, and malachite. Supergene mineralisation is associated with iron oxides (goethite with minor hematite) and clays (kaolinite, halloysite, illite and montmorillonite). Oxidation and the development of supergene minerals has been controlled mainly by fracturing and the availability of hypogene sulfides. The low pyrite content of hypogene mineralisation at Boyongan allowed supergene mineralisation to develop in-situ from near-neutral pH groundwaters. The initial stages of supergene mineralisation involved the replacement of hypogene sulfides such as chalcopyrite and bornite by chalcocite, digenite and pseudo-covellite. In some places, chalcocite replaced pyrite. Goethite formed during the weathering of pyrite, chalcopyrite, bornite and chalcocite. Copper that was released into solution precipitated as native copper, which has replaced chalcocite locally. Native copper was then oxidised to form cuprite, and also acicular and euhedral crystals of chalcotricite. Some cuprite may have precipitated directly from solution, and also where chalcocite reacted with oxygenated groundwaters. The final stages of supergene copper mineralisation at Boyongan produced copper carbonate (malachite, pseudo-malachite, azurite and pseudo-neotocite) and a copper silicate overprint (chrysocolla and pseudo-chrysocolla) onto earlier-formed copper oxides and sulfides. Copper generally has a more dispersed or erratic distribution than gold. Gold is restricted spatially to the early mineral intrusions. Copper grades in the cuprite-dominated zone in the west generally decrease with depth toward zones of patchy native copper. The copper carbonate (malachite-azurite)-dominated blanket above the cuprite zone contains both high grade copper and gold (>1% and >2 g/t, respectively). Chalcocite zones that have partially replaced hypogene copper sulfides have higher grades (>0.5% Cu and >1g/t Au) compared to zones of chalcocite replacing pyrite (<0.5% Cu and <0.5g/t Au). Chrysocolla and/or pseudo-chrysocolla is confined to zones that contain high copper and gold grades (>0.5% and >1 g/t, respectively). Isotopic compositions of malachite and azurite from Boyongan are consistent with deposition from ambient temperature (15¬¨‚àûC to 20¬¨‚àûC) meteoric water. These low temperatures are consistent with Boyongan being a low-sulfide porphyry system. Higher pyrite contents would probably have led to greater degrees of sulfide oxidation as well as higher groundwater temperatures. `˜í¬•^13C` values of malachite are consistent with an organic carbon (soil?) source suggesting that malachite may have formed when Boyongan was uplifted and exposed. `˜í¬•^13C` values of azurite are much higher, and could be derived from seawater, or by remobilisation of an inorganic carbon from carbonate wallrocks, or by sulfide oxidation by supergene-related bacteria above the water table.