Volatile exsolution at the Dinkidi Cu-Au porphyry deposit, Philippines: A melt-inclusion record of the initial ore-forming process

Immiscible phases derived from degassing silicate magmas are considered to be precursors of metal-bearing hydrothermal fluids in porphyry deposits. Th e development of melt-inclusion techniques provides a window into this crit ical period of porphyry formation, when the cooling, decompression, and cry stallization of silicate melts result in the formation of immiscible phases . The record of magmatic to hydrothermal evolution is presented using inclu sions in clinopyroxene phenocrysts from the syenitic Balut dike, one of the host-rock lithologies for mineralization at the Dinkidi Cu-Au porphyry dep osit, Philippines. Primary inclusions include silicate glass, multiphase ag gregates comprising salts, silicates, sulfates, carbonates, sulfides and ox ides, and highly saline aqueous fluids. Various analyses, including in situ laser ablation inductively coupled plasma-mass spectrometry of the multiph ase inclusions, determined elevated concentrations of Cl, S, As, Tl, K, Na, and a number of metals, including those that form ore-grade deposits (e.g. , Cu) and those that do not (e.g., Mo, Pb, Zn, and W) at the Dinkidi porphy ry deposit. Silicate melt and multiphase salt-rich inclusions in clinopyrox ene are interpreted as having originally formed as immiscible phases at mag matic temperatures.