• journal_title：Economic Geology
• Contributor：Antonio Arribas ; Charles G. Cunningham ; James J. Rytuba ; Robert O. Rye ; William C. Kelly ; Melvin H. Podwysocki ; Edwin H. McKee ; Richard M. Tosdal
• Publisher：Society of Economic Geologists
• Date：1995-
• Format：text/html
• Language：en
• Identifier：10.2113/gsecongeo.90.4.795
• journal_abbrev：Economic Geology
• issn：0361-0128
• volume：90
• issue：4
• firstpage：795
• section：Articles

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, 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 1 alunite, diaspore, zunyite, pyrophyllite) and silicic (vuggy silica and massive silicified rock) zones, which are well developed to present depths of approximately 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 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 H ₂ SO ₄ 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, kaolinitc, sericite, and hydrothermal quartz) indicate that hydrothermal fluids (delta ¹⁸ O = 7 + or - 3ppm, delta D = -20 + or - 10ppm) during the main period of wall-rock alteration were dominantly magmatic in origin. The delta ³⁴ S (sub Sigma S) was approximately 9 per mil and H ₂ S/SO ₄ of the bulk hydrothermal system during acidic alteration was approximately 5. The delta ³⁴ S values of stage 1 alunite (22.3-31.0ppm) and pyrite (0.3-8.0ppm) 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. Gold mineralization extends to present depths of up to 100 m and is hosted by hydrothermal breccias and banded, black (pyrite-rich) chalcedonic quartz veins and open-space fillings. Calaverite, native Te, and pyrite disseminated in a chalcedonic matrix form the original Au ore in the high-grade breccias, which are generally weathered to an assemblage of native Au, Fe hydroxides, and secondary Te minerals, including tellurite, rodalquilarite, blakeite, and emmonsite. Pyrite is the dominant ore mineral in black chalcedony and native Au occurs as isolated grains (10 mu m) or intergrown with pyrite. Textural and O isotope relations suggest that black chalcedony (delta ¹⁸ O _fluid = < or =3ppm) precipitated originally as amorphous silica, at temperatures <180 degrees C from a mixture of O-exchanged meteoric and magmatic waters. The deposition of amorphous silica and the common association with hydrothermal breccias suggest that high grades of quartz supersaturation were attained by rapid ascent and cooling of the deep fluid initially at greater than hydrostatic pressures and temperatures probably >350 degrees C. The pressure and temperature drop associated with hydrothermal brecciation also led to deposition of Au with the silica. Strontium isotope systematics of ores, rocks, and alteration minerals indicate that Sr in the hydrothermal system (avg ⁸⁷ Sr/ ⁸⁶ Sr = 0.7136) was derived from the caldera fill sequence that hosts the mineralization and not from precaldera volcanic rocks or Miocene seawater. Other hydrothermal components, such as Eu and Ba, were also leached from the host rocks, whereas elements such as Te, Sn, Bi, Se, and possibly Au and Cu, were more likely derived by exsolution from the crystallizing magma.Oxidation of the primary sulfide Au mineralization between 4 and 3 Ma led to development of an intense su er ene acid sulfate alteration overprint consistent with the stable isotope systematics of stage 2 alunite (delta ³⁴ S = 4.1-10.4ppm; delta ¹⁸ O (sub SO ₄ -OH) = large and negative) and kaolinite formed during this episode. Lead and Sr isotope ratios of stage 2 alunite are not as homogeneous as those of hypogene alunite and are consistent with a variety of rock sources exposed to surficial runoff waters. The present surface is 200 to 300 m below the paleosurface at the time of mineralization at approximately 10 Ma.