Abstract

Dikes of fiuidization breccia composed largely of fragments of dacite lava are exposed over an area of about 3 km 2 in the central part of the Julcani volcanic center, central Peru. The dikes, which predate later quartz-alunite-pyrite and argillic alteration stages and economic enargite-tetrahedrite mineralization, exhibit a crude but definite radial pattern centered on the zone of later W-Au mineralization. Fragments of Paleozoic rock in dikes in outer areas of the pattern roughly constrain the location of an inferred early central vent structure 1.0 to 1.5 > or = km in diameter now buried beneath the dacite domes that host the breccia dikes.The microfragmental matrix of the dikes has been subject to strong quartz-tourmaline-pyrite alteration that did not affect dacite wall rock or larger dacite fragments within the dikes. The alteration was probably produced by fluids closely related to those that fluidized the dike material during emplacement.Fluid inclusions containing cubes of halite + or - sylvite are present in some quartz fragments within the breccia dikes and in phenocrystic quartz in dacite lava adjacent to the dikes. Estimated salinities require minimum trapping temperatures of about 260 degrees to 450 degrees C and show that fluid pressures during dike emplacement at times were appreciably greater than hydrostatic pressures. A few inclusions contain relatively large grains provisionally identified as hematite, anhydrite and/or gypsum, and carbonate, indicating the existence of solutions rich in these components.The high boron concentrations and highly saline fluid inclusions in the breccia dikes, the radial dike pattern, the high f (sub O 2 ) and f (sub S 2 ) of the magmas, and the general magmatic-hydrothermal history all suggest that the breccia dikes at Julcani were produced by the explosive release of magmatic fluids from a pluton crystallizing beneath the central part of the district.Areally restricted elongate zones of shattered dacite postdate the fiuidization breccia dikes. Tourmaline is absent and only minor particle rotation, rounding, and transport are evident. These zones probably resulted from the locally explosive formation of steam by the intrusion of late dikes into dacite lavas saturated with ground water.

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