Geology and geochemistry of the Rodalquilar gold-alunite deposit: Relationship to other hydrothermal ore deposits in southeastern Spain.

Abstract

Several calderas have been recognized within the Miocene Cabo de Gata volcanic field (SE Spain) including the Los Frailes, Rodalquilar, and Lomilla calderas. The Rodalquilar caldera originated by eruption of the rhyolitic Cinto ash-flow tuff at $\sim$11 Ma. Continued resurgence of the core of the caldera resulted in structural doming, emplacement of large ring domes, eruption of the Lazaras ash-flow tuff, and development of the smaller, nested Lomilla caldera. Gold-alunite deposits in the Rodalquilar caldera are closely associated with extensive sericitic, argillic, advanced argillic (quartz, alunite, diaspore, zunyite, pyrophyllite), and silicic (vuggy silica) alteration, typical of high-sulfidation-type deposits. K/Ar dating of hydrothermal alunite and sericite indicates an age of mineralization of avg. = 10.4 Ma. Stage 1 alunite, and coeval pyrite, is spatially associated with the mineralization and formed in a magmatic-hydrothermal environment from the disproportionation of SO$\sb2$ derived from a magma. $\delta\sp{34}$S values of Stage 1 alunite (22.3-31.0$\perthous)$ and pyrite (0.6-8.0$\perthous)$ reflect isotopic equilibrium between sulfate and sulfide at T = 220-330$\sp\circ$C. The ratio H$\sb2$S/SO$\sb4$ of the hydrothermal fluid was $\sim$5 and $\rm \delta\sp{34}S\sb{\Sigma S}$ $\sim9\perthous.$ O and H isotope ratios indicate that hydrothermal fluids at this stage ($\delta\sp{18}$O = 7 $\pm$ 3, $\delta$D = $-$20 $\pm$ 10$\perthous)$ were dominated by magmatic waters. Hypogene gold mineralization followed acid-sulfate alteration and is hosted by hydrothermal explosion breccias and banded black (pyrite-rich) chalcedony veins which occur within 100 m of the present-day surface. Black chalcedony precipitated originally as amorphous silica probably at temperatures of 120-180$\sp\circ$C from fluids of predominantly meteoric origin. Sr isotope systematics indicate that Sr in the hydrothermal system (avg. $\sp{87}$Sr/$\sp{86}$Sr = 0.7136) was derived from the caldera-fill sequence that hosts the mineralization and not from precaldera volcanic rock or Miocene seawater. Supergene oxidation of primary sulfides at $\sim$3.5 Ma resulted in acid-sulfate alteration, formation of Stage 2 alunite ($\delta\sp{34}$S = 5.6-10.2$\perthous),$ and remobilization and recrystallization of gold. Pb in the Rodalquilar deposit is derived from the host calc-alkalic igneous rocks. In contrast, Pb in most ore deposits in southeastern Spain is dominated by Pb from the pre-Permian metasedimentary sequence of the Betic basement.