The Behaviour of Base Metals in Arc-Type Magmatic-Hydrothermal Systems -- Insights from Merapi Volcano, Indonesia

Porphyry and high sulfidation epithermal ore-forming systems are genetically associated with calc-alkaline volcanism in subduction zones, and where erosion has not been too deep, the volcanic rocks are still commonly exposed in close proximity to the deposits. Most models for porphyry copper and high sulfidation epithermal gold systems include a shallow magmatic reservoir (the porphyry stock), an overlying hydrothermal cell, its alteration paragenesis and a stratovolcano. Some investigations also discuss the importance of underlying granitoid batholiths as feeders for porphyry stocks and their hydrothermal systems. Although it is commonly believed that the ores deposit during the waning stages of volcanism, given the time span over which these deposits form (tens of thousands to several million years) and the undeniable existence of hydrothermal systems beneath volcanoes, it is quite probable that their formation is initiated at times when volcanoes are still active. Although currently mined ore deposits are excellent places to focus research, subduction zone stratovolcanoes provide important windows on the magmatic-hydrothermal processes at play.;The research reported in Chapter 1 showed that injections of sulfide melt-saturated mafic magma into shallower, more evolved and more oxidized resident magma at Merapi volcano induced exsolution of a magmatic volatile phase from the mafic magma. This hydrothermal fluid dissolved the sulfide melt and became enriched in chalcophile (notably copper) and siderophile metals. An argument is presented that the overpressure generated by the exsolution of a fluid originating in this manner triggered an explosive eruption at Merapi volcano in 2006. This is supported by the observation that the metal content, particularly of copper, was higher in the volcanic gas sampled immediately after this eruption than during periods of quiescence and that metal ratios of the gas are remarkably similar to those of sulfide melt inclusions. In Chapter 2, it is shown that the mafic magma mixed poorly with the more felsic magma, that both magmas evolved via assimilation and fractional crystallization and, most importantly, that the magmatic volatile phase transferred base metals to the more felsic magma. In Chapter 3, the fluid inclusion and volcanic gas data are used to make inferences about the evolution of porphyry ore-forming systems and link mechanisms of ore-formation to those operative during the eruptive cycles of volcanoes. Finally, the thesis integrates the findings of this study into a model that provides new insights into the formation of porphyry copper deposits below stratovolcanoes.;This thesis describes an investigation of the magmatic-hydrothermal environment that resides beneath Merapi volcano, Indonesia. The research involved sampling and chemical analysis of minuscule aliquots of evolving silicate and sulfide melts trapped as inclusions at different times and in different locations in growing crystals subsequently ejected during eruptions. The research also involved sampling and analysis of fumarolic gases (and their precipitates) emitted at Merapi volcano during times of quiescence and eruptive activity, as well as compilation of published compositional data for fumarolic gases from other arc volcanoes. These gases are the surface equivalents of ore-forming magmatic-hydrothermal fluids. Finally the research involved compilation from the literature of compositional data for fluid inclusions (micron-scale droplets of magmatic volatile phases) trapped in gangue minerals in porphyry copper deposits. The focus of the research was the behaviour of copper, nickel, cobalt, zinc, lead and molybdenum in magmatic hydrothermal systems.