Geology of the Boyongan and Bayugo Porphyry Cu-Au Deposits: An Emerging Porphyry District in Northeast Mindanao, Philippines

Abstract

The Boyongan and Bayugo porphyry Cu-Au mineral deposits, discovered under postmineralization cover during the previous decade, are part of an emerging belt of intrusion-centered Au-rich Cu mineral deposits and prospects in the Surigao district of northeast Mindanao, Philippines. Since their formation in the early Pleistocene, exhumation and weathering of these deposits have led to the development of a 600-m-thick oxidation profile at Boyongan and a modest (30–70 m) oxidation profile at Bayugo. Debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the weathered deposits, preserving their supergene profiles. The mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages. Three premineralization diorite porphyry stocks and a silt-sand matrix breccia complex represent early stages of magmatism and brecciation. Significant Cu and Au introduction followed these events and occurred in association with small early-mineralization diorite porphyry stocks at Boyongan and Bayugo. Within the diorite complex, the two mineral deposits are spatially distinct, separated by approximately 1 km of premineralization diorites. Inter- and late-mineralization intrusions were emplaced as the magmatic-hydrothermal system waned. A characteristic progression of vein and K silicate alteration styles affected each of the synmineralization intrusions. Rare comb quartz unidirectional solidification textures (stage 0) mark the transition from magmatic to hydrothermal conditions. Quartz-poor wispy magnetite-biotite-K-feldspar veinlets characterize stage 1. Stage 2 veins consist of quartz with selvage and/or disseminated magnetite or biotite and K-feldspar halos. Stage 3 quartz veins have K-feldspar halos but generally lack magnetite and biotite. Stage 4 veins consist of massive bornite-chalcopyrite and chalcopyrite-pyrite with K-feldspar halos. Stage 3 quartz veins and stage 4 sulfide veins host Cu-Au mineralization of the greatest volumetric significance, reflecting the general paucity of sulfide in the earlier vein stages. Despite the simplicity of this sequence, detailed paragenetic reconstructions reveal that this characteristic progression of veining and K silicate alteration was repeated with the emplacement of each synmineralization intrusive event, revealing multiple magmatic-hydrothermal cycles of alteration and mineralization. At Boyongan and Bayugo, intense and pervasive illite alteration, in association with pyrite, chalcopyrite, and tetrahedrite-tennantite, developed in narrow structures crosscutting quartz-K-feldspar veins. Debris flows in the burial sequence above and adjacent to the Boyongan/Bayugo complex also contain abundant clasts with intense, pervasive illite and alunite-pyrophyllite-dickite-kaolinite alteration assemblages that have overprinted K silicate-style quartz veins. In both mineral deposits, Cu and Au are associated with intense quartz-vein stockworks composed primarily of K silicate stage 3 veins. Despite this association, not all of these quartz-vein stockworks contain Cu and Au to the same tenor. Quartz-vein stockworks in the eastern high grade of Boyongan have been intersected over a vertical interval of 800 m, having affected much of the early-mineralization stock. However, hypogene Cu grades exceed 0.5% by weight only in the upper 300 m of the stockwork (in the cupola of the early-mineralization stock). Superior grade development in high-grade zones at Boyongan, locally exceeding 2% Cu and 3 g/t Au, developed where fertile vein stages from two or more magmatic-hydrothermal cycles affected the same wall rock. The documented paragenetic relationships demonstrate multiple discrete cycles of K silicate-stage veining and alteration associated with each synmineralization intrusive event. All such events predate formation of feldspar-destructive illite-smectite-chlorite, illite-pyrite, and quartz-alunite-clay assemblages. Existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly, in a period of less than 200,000 years (2.3–2.1 Ma). Geologic and geochronological constraints on the life cycle of Boyongan and Bayugo describe an extremely dynamic history of emplacement, exhumation, weathering, and burial over a period of 2.3 m.y. The study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting.