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 Lazaras 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, Ito 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 1 alunite, diaspore, zunyite, pyrophyllite) and silicic (vuggy silica and massive silicified rock) zones, which are well developed to present depths of similar to 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 1 alunite and hydrothermal illite 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 HCl. Salinities in some samples of deep, hot (> 400 degrees C) fluids exceeded 40 wt percent NaCl equiv, consistent with the presence of a magmatic brine. In addition, the O and H isotope ratios of hypogene alteration minerals (alunite, kaolinite, sericite, and hydrothermal quartz) indicate that hydrothermal fluids (delta(18)O = 7 +/- 3 parts per thousand, delta D = -20 +/- 10 parts per thousand) during the main period of wall-rock alteration were dominantly magmatic in origin. The delta(34)S(Sigma S) was similar to 9 per mil and H2S/SO4 of the bulk hydrothermal system during acidic alteration was similar to 5. The delta(34)S values of stage 1 alunite (22.3-31.0 parts per thousand) and pyrite (0.3-8.0 parts per thousand) in the advanced argillic zone reflect isotopic equilibrium between sulfate and sulfide at T = 220 degrees to 330 degrees C, with the lower values corresponding with present-day surface samples.
