An acid-sulphate-type lode gold deposit at Nalesbitan, Luzon, Philippines

Abstract

The Nalesbitan lode Au deposit in the Bicol peninsula of southeastern Luzon, Philippines, is of the acid-sulphate type, and originally contained a minimum of about 15 tonnes of Au. Mineralization is hosted by Pliocene andesitic volcanic rocks transected by a northwest-striking, steeply west-dipping, sinistral strike-slip fault zone, which is considered to be subsidiary to the major Philippine fault zone. The principal lode comprises two discrete structures which flared and coalesced upwards to produce a 1300-m-long body of Au-bearing rock ranging from 12 to 145 m wide. Flanking subsidiary lodes represent mainly parts of the principal lode that were detached and transported several hundred metres left-laterally by post-mineralization motion on the fault zone. The two component structures of the principal lode are marked by steeply west-dipping, linear bodies of hydrothermal breccia, which are transitional outwards to irregular patchy breccias and swarms of subparallel veins and veinlets. Breccias, veins and veinlets are composed of chalcedonic quartz and up to 50 vol.% pyrite of replacement origin, and are enveloped by advanced argillic alteration composed of quartz and kaolinite, subsidiary alunite and lesser amounts of sericite and diaspore. Illite increases in amount as the margins of the lode are approached, and is transitional outwards to a halo of intermediate argillic alteration. Gold in the Nalesbitan lodes is associated closely with high-sulphidation Cu and Cu-Fe sulphide assemblages containing, in order of decreasing abundance, chalcocite, bornite, covellite and chalcopyrite. Enargite accompanies these sulphides, especially in some of the subsidiary flanking lodes. At least 95% of the principal lode, to depths of at least 130 m, underwent complete supergene oxidation, a process which resulted in increased rock permeability and the overprinting of supergene kaolinite on intermediat argillic assemblages. The highest Au grades are centred on the hydrothermal breccias, which die out in the lower, bifurcated part of the principal lode. Reconnaissance fluid-inclusion measurements indicate that mineralization at Nalesbitan took place some 300–500 m beneath the paleosurface from intermittently boiling fluids at temperatures ranging from 223 to 255°C. The Nalesbitan deposit is believed to be centred on a minor dilational jog on the fault zone, where two closely spaced en-echelon fault strands may be represented by the two component structures of the principal lode. Repeated hydraulic fracturing caused by pressure reductions across the jog could have caused the brecciation and fluid ascent responsible for lode construction. Fluid boiling induced by the pressure reductions may have facilitated Au precipitation. The magmatic volatiles and metals required to form the Nalesbitan deposit are inferred to have been supplied from a subjacent porphyry Cu-bearing stock. Advanced argillic alteration is believed to have been more widespread and alunite-rich in shallower, now-eroded parts of the system, which also may have been Au-bearing. The age, geological setting, morphology, alteration characteristics and zoning pattern, mineralogy and conditions of formation of the Nalesbitan deposit are closely similar to those described for the Nansatsu-type Au deposits of southern Kyushu, Japan.