The Rodalquilar epithermal Au alunite deposit occurs within the Rodalquilar caldera complex in the Miocene Cabo de Gata volcanic field in southeastern Spain. The Rodalquilar caldera formed by eruption of the rhyolitic Cinto ash-flow tuff at about 11 Ma; continued resurgence of the core of the caldera resulted in structural doming and was followed by emplacement of large ring domes, eruption of the Lfizaras ash-flow tuff, and development of the smaller, nested Lomilla caldera. Hydrothermal circulation associated with the emplacement of shallow hornblende andesitic intrusions late in the evolution of the caldera led to formation of the epithermal deposits along fractures related to multistage caldera collapse and resurgence.Ore deposits within the Rodalquilar caldera complex consist of low-sulfidation Pb-Zn-(Cu-Ag-Au) quartz veins and the economically most important high-sulfidation Au-(Cu-Te-Sn) ores. The latter are enclosed in areas of acid sulfate alteration present on the east margin of the Lomilla caldera. Drilling indicates that hydrothermally altered rocks are present to depths of >900 m, with a gradational change with depth from silicic, to advanced argillic, to intermediate argillic, to sericitic zones; an envelope of propylitic alteration surrounds these zones. The sericitic zone is present at depths >400 m and occurs under the advanced argillic (stage i alunite, diaspore, zunyite, pyrophyllite) and silicic (vuggy silica and massive silicified rock) zones, which are well developed to present depths of •300 and 100 m, respectively. Vuggy silica and massive silicified rock are structurally controlled and spatially related to the Au deposits. K/Ar dating of stage i alunite and hydrothermal illitc indicates an age of mineralization of about 10.4 Ma.The results of paragenetic, fluid inclusion, and stable isotope studies indicate an evolution of the hydrothermal system, consisting of an early period of acidic wall-rock alteration and a late period of Au mineralization. A significant magmatic fluid component was present throughout, contributing acidity in the form of H2SO4 and HC1. Salinities in some samples of deep, hot (>400øC) fluids exceeded 40 wt percent NaC1 equiv, consistent with the presence of a magmatic brine. In addition, the O and H isotope ratios of hypogene alteration minerals (alunite, kaolinitc, sericite, and hydrothermal quartz) indicate that hydrothermal fluids (6•SO = 7 ___ 3%0, 6D = -20 ___ 10%o) during the main period of wall-rock alteration were dominantly magmatic in origin. The 634Szs was •9 per mil and H2S/SO4 of the bulk hydrothermal system during acidic alteration was •5. The 634S values of stage 1 alunite (22.3-31.0%o) and pyrite (0.3-8.0%0) in the advanced argillic zone reflect isotopic equilibrium between sulfate and sulfide at T --220 ø to 330øC, with the lower values corresponding with present-day surface samples. * Present address: Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, 305 Japan. 0361-0128/95/1704/795-2854.00 795 796 ,4RRIB,•S, JR. ET ,•L. Gold mineralization extend...

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Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain.We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly EuN/EuN* ≥0.4).We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S 6+ ) to SO2 (S 4+ ) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO2-rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.

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