Abstract
The quartz-fragment-rich breccia (QBX) in the Didipio alkalic porphyry deposit of Nueva Vizcaya, Philippines is a high-grade Cu-Au orebody dominantly composed of quartz fragments. The QBX can be subdivided into different facies, each characterized by a particular spatial distribution and mineral composition. This study utilized petrographic, electron-probe, scanning electron microscope, and fluid inclusion analyses on the samples from different QBX facies to provide new insights about mineralization styles, gold occurrence, and ore-forming conditions. Based on their respective mineral assemblages, two styles of mineralization were determined for the QBX: (1) porphyry and (2) epithermal.The porphyry-style mineralization is characterized by early-stage disseminated and vein-hosted sulfides and alteration consisting of chalcopyrite, bornite, chlorite, biotite, magnetite, quartz, K-feldspar, and actinolite. The assemblage formed from high temperature (420–440 °C), high salinity brines (22–49 wt% NaCl) possibly derived from a magmatic source. The porphyry-style mineralization occurs mainly as disseminations in quartz-sulfide veins within diorite wall rocks, Quan porphyry, and Bufu syenite. These altered intrusive bodies were eventually fragmented to form a low-grade (<5 ppm Au) distal facies (QBX-Monz, QBX-Dio) and a lower contact facies (QBX-Sy), above and below the breccia core, respectively. Gold occurs as Au-Ag-Cu-Te blebs (gold telluride) at phase boundaries in bornite-chalcopyrite symplectites.The epithermal-style mineralization is embodied by a late-stage gold mineralization event that precipitated chalcopyrite, pyrite, sphalerite, galena, tennantite, calcite, and Te- and Se-minerals. The later epithermal fluids were of relatively low temperature (300–320 °C) and low salinity (3–4 wt% NaCl), indicative of diluted hydrothermal fluids. The vein-hosted epithermal mineral assemblage is observed in clasts of the upper contact facies (QBX-FP) and in the matrix and cement of the moderate- to high-grade (∼10–83 ppm Au) central facies (QBX-Un, QBX-Act). In this paragenetic stage, gold occurs as native gold hosted in chalcopyrite, pyrite, and tellurides – within fractures or as inclusions – and as invisible gold in pyrite.The close relationship between the gold and chalcogenides supports the Ag-Bi-Te scavenger model, which suggests that the presence of Ag, Bi, Se, and Te in magmatic hydrothermal ore-forming fluids aids in concentrating Au at high temperatures and exsolving it at lower temperatures.
Published Version
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