Abstract

The Julcani district consists of at least six mineralized centers in a 16-km 2 late Miocene, dacitic to rhyolitic dome complex that is built on a approximately 3,000-m-thick Paleozoic-Mesozoic clastic sedimentary section. Mineralization occurred within 0.5 m.y. of the youngest dome formation and was prior to emplacement of an anhydrite-bearing dacite dike. Hydrothermal events began with formation of fracture-controlled acid sulfate alteration zones having vuggy silica cores and successive quartz-alunite and quartz-kaolinite envelopes. The acid sulfate alteration in the center of the district was overlapped and followed by the emplacement of radiating swarms of tourmaline-pyrite breccia dikelets cemented by quartz-containing saline fluid inclusions. Several centers of mineralization developed 0.5 to 2 km apart along reactivated regional structures after the emplacement of a dacite dike most likely related to an underlying intrusion. The ores consist of Ag-Cu-Pb-W-Bi-Au-bearing mineral assemblages that occur as fracture fillings within the domes and underlying tuffs. Mineralization reached within 200 to 300 m of the paleosurface. The ores are strongly zoned with a general succession of quartz + pyrite + wolfamite-, enargite-, tetrahedrite-, galena-, and barite-dominant assemblages grading outward east and west from the central zone. Late-stage siderite and/or botryoidal pyrite overprint all zones, even at the deepest levels.Filling temperatures and salinities for inclusion fluids in main-stage minerals range from 250 degrees to 325 degrees C and 8 and 19 wt percent NaCl equiv, respectively. Filling temperatures for inclusions in late-stage siderite range from 170 degrees to 225 degrees C and salinities average about 7.5 wt percent NaCl equiv. Temperatures tend to correlate with salinities and both tend to decrease in the fluids of successively younger and more distal minerals from wolframite to enargite to siderite.The calculated delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of the preore alunite-forming fluids are about -46 + or - 5 and 7 + or - 1 per mil, respectively. The delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of inclusion fluids in later main-stage wolframite, enargite, tetrahedrite, and galena and late-stage siderite show a linear trend, ranging from -60 to -130 and 4 to -18 per mil, respectively. Based on the composition of biotite phenocyrsts in glassy volcanic rocks that bracket ore deposition, primary magmatic fluids had delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of about -70 + or - 20 and 9.7 + or - 0.6 per mil, respectively.The fluids responsible for main-stage mineralization as well as preore acid sulfate and tourmaline breccia fluids were distinctly magmatic. These ore fluids, however, were highly exchanged (similar in isotopic composition to alunite-forming fluids) and were not derived directly from the magma but from the low water-rock environment between the fluid-rich carapace of the magma and the brittle-ductile transition of the overlying rocks. Ore deposition occurred when the exchanged magmatic fluids mixed with meteoric water at higher levels.

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