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In most porphyry systems the obscuring effects of hydrothermal processes and subsequent alteration, along with limited exposure of source rocks, preclude a detailed understanding of how and where metals and volatiles were derived. However, in this study we examine melt inclusions which have escaped alteration and that sampled all of the phases coexisting in the magma, in late-and postmineral rhyolitic units from the Río Blanco Cu-Mo deposit, Chile. These inclusions demonstrate the existence of a volatile-rich melt, the exsolution from it of an aqueous volatile-rich phase, initially as melt + vapor bubble emulsions, and the disruption of these emulsions into melt and primary magmatic fluids. Trapping of these emulsions may explain the occurrence of melt inclusions containing widely varying proportions of melt and aqueous fluid found at Río Blanco. We demonstrate the sequestering of metals into the exsolved volatile phases and the derivation from these of possible ore-forming hydrothermal fluids, with particular reference to the implications for metal transport. Melt inclusions show differences between adjacent comagmatic intrusions that may be directly related to the extent of mineralization of the respective bodies. In one of the Río Blanco postmineral rhyolite bodies melt inclusions show exsolution of the volatile-rich phase but only minor evidence of trapping of a metal-rich vapor. In contrast, inclusions from an adjacent late mineral rhyolite body show similar volatile phase exsolution but also provide evidence of ponding of metal-rich hydrothermal fluids during the final stages of cooling.

The Sur-Sur tourmaline breccia is located in the southeast part of the Río Blanco-Los Bronces porphyry copper-molybdenum deposit, central Chile. The breccia hosts approximately one-quarter of the total resource of 57 Mt of fine copper at Río Blanco. The breccia is hosted within, and contains altered clasts of, granodiorite from the 12 to 8 Ma San Francisco batholith, which intruded a sequence of Miocene volcanic and volcaniclastic rocks. A series of weakly mineralized to barren felsic porphyries cut the breccia and indicate a minimum age of approximately 6 Ma for mineralization at Sur-Sur.The Sur-Sur breccia dike is at least 3 km long, 0.2 km wide, and has a vertical extent of at least 1 km. The breccia has been cemented by early biotite and anhydrite at depth and by tourmaline and specularite at higher altitudes. These early-formed cements have been overgrown and in some cases replaced by chalcopyrite, magnetite, pyrite, and quartz. Mineralogical zonation in the breccia includes a transition from biotite cement and related biotite alteration upward to tourmaline cement and quartz-sericite-tourmaline alteration at approximately 3,000-m elevation. Iron-oxide minerals are also zoned, with a transition upward from a magnetite-dominated zone below 3,330 m to a specularite-dominated zone above 3,600 m. Pyrite is the dominant sulfide at altitudes above 4,000 m.Secondary liquid-rich, vapor-rich, and hypersaline fluid inclusions are preserved in quartz and tourmaline cement. Measured homogenization temperatures are mostly between 300º and 450ºC, and salinities range from 0 to 69 wt percent NaCl equiv. Sulfur isotope compositions of sulfide cement range from -4.1 to +2.7 per mil. The lowest δ 34 S(sulfide) values are in samples from between 3,700-and 4,000-m elevation, where they correspond to the highest copper grades in the tourmaline breccia. This high-grade zone also contains abundant specularite (locally replaced by magnetite). Modeling of sulfate-sulfide equilibrium indicate that approximately 150ºC of cooling over a vertical interval of 100 m would be required to account for the zonation of sulfide isotope compositions at Sur-Sur, making conductive cooling an unlikely ore-forming mechanism.Measured 206 Pb/ 204 Pb values of lead in anhydrite cement in the Sur-Sur tourmaline breccia and the Río Blanco magmatic breccia range from 17.558 to 18.479. 207 Pb/ 204 Pb values range from 15.534 to 15.623, and 208 Pb/ 204 Pb values range from 37.341 to 38.412. The lead in anhydrite is considerably less radiogenic than that indicated by values obtained previously for lead in sulfide ores and igneous host rocks at Río Blanco-Los Bronces. The source of lead in anhydrite must have been from rocks external to the main magmatichydrothermal system, probably the Precordilleran basement.A magmatic-hydrothermal explosion from a deep-seated crystallizing intrusion triggered breccia formation at Sur-Sur. Hydrostatic pressures catastrophically exceeded lithostatic load plus the tensile strength of the confining granodiorite, leading to wi...

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