Preliminary study of the ore deposits and hydrothermal alteration in the Rodalquilar caldera complex, southeastern Spain

Abstract

SUMMARYThe Rodalquilar gold-alunite deposits are the first documented example of calderarelated epithermal gold mineralization in Europe. Mineralization occurs in rhyolitic tuffs and domes of the Rodalquilar caldera complex in the Miocene Cabo de Gata volcanic field. Geologic constraints indicate a close temporal relationship between the mineralization and a late magmatic phase of the caldera cycle. A zone of extensive advanced argillically altered rocks trends from east to west through the caldera and is well delineated by Landsat Thematic Mapper data. Aeromagnetic and gravity data show that an east-west trending gravity and aeromagnetic high coincides with the distribution of the altered rock. The source of this geophysical anomaly is considered to be andesitic magma that intruded into the base of the volcanic pile and is regarded as the heat source responsible for hydrothermal circulation, niite and alunite K-Ar data indicate an age for the mineralization of 10.8 Ma. Recent drilling in the core of the system shows that hydrothermally altered rocks continue to depths of over 900 m and are characterized by assemblages typical of acid-sulfate epithermal systems. A deep sericitic zone, with chlorite present below about 500 m, grades upward into argillic, advanced argillic and silicic (vuggy silica) zones. Zonation of the alteration assemblages also occurs laterally; the advanced argillic alteration mineral assemblage is replaced by chlorite within the argillic zone at or near the ground surface.Ore deposits within the Rodalquilar complex consist of alunite, gold-alunite, and lead-zinc-silver-gold veins. The gold-alunite deposits are the most important economically. They are preferentially localized in ring and radial fractures of the Lomilla caldera nested within the Rodalquilar caldera. Gold extends over a vertical interval of about 200 m below the original paleosurface and changes at depth to a complex sulfide assemblage. Most gold was deposited in the highest part of the center of upwelling fluid following the peak of acidic alteration. Highest gold values, with a median near 8 g/t, occur in black, pyritic, banded chalcedony that fills preexisting open spaces and fractures within advanced argillically altered rocks. Hypogene alunite in the central core assemblage reaches depths of more than 300 m, is characterized by high sodium content, and is accompanied, below the zone of supergene oxidation, by kaolinite, zunyite, diaspore, pyrite and aluminum phosphate-sulfates. Stable isotope data indicate that this alunite formed in a magmatichydrothermal environment from the disproportionate of SO2 derived from a magma. The 6^4S data of coeval pyrite indicate the 834S of bulk sulfur in the system was ~7 per mil. Barren alunite-jarosite veins are extensive throughout the district to a depth of about 50 m. Stable isotope data from these alunites indicate that they were derived by the oxidation of H2S in a steam-heated environment that in some places was superimposed on the advanced argillic assemblage as the hydrotherma...

EnGeoMAP 2.0—Automated Hyperspectral Mineral Identification for the German EnMAP Space Mission

“…These two sites contain rock waste material, which is dominated by alunite, jarosite and kaolinite. The same alteration centers above have also been noticed and described in previous multispectral investigations [21] and furthermore in hyperspectral studies [20,49]. Figure 9 shows the spatio-spectral gradient as overlay to the material analysis results.…”

“…The two major alteration centers that have been described in literature [16,21] can be easily identified here, with the main alteration center in the central part of Figure 8. This has been the center of the gold mining operations near Rodalquilar.…”

“…This has been the center of the gold mining operations near Rodalquilar. The second prominent alteration center to the east of the image is situated around the Los Tollos deposit, where alunite was mined [21]. Large areas of rock dumps are situated to the east of the Consulta mine and to the northwest of the Cinto deposits, as also shown in Figure 1.…”

Development of a Decision Support System based on remote sensing and GIS techniques for gold-rich area identification in SE Spain

“…During the Miocene a very intense volcanic and subvolcanic activity developed in the region generating a set of hydrothermal systems. These processes produced large areas of hydrothermal alteration (epithermal), characterized by mineral assemblages made up of quartz-alunite and clay minerals (chlorite, montmorillonite, sericite), with which the main mineral deposits are associated (Arribas et al 1989, Rytuba et al 1990.…”

An ancient Ba-Sb-Ag-Fe-Hg-bearing hydrothermal system in SE Spain

“…Different references about the characteristics of the mineralizing fluids, paragenetic sequences, fluid inclusions, stable isotopes and many other aspects concerning these deposits can be found in Arribas et al (1989), Martínez-Frías et al (1989, Martínez-Frías (1991), Manteca and Ovejero (1992), Rodríguez (1993), , Morales (1994), Arribas et al (1995), Martínez-Frías (1997), Serna (1997), . Nevertheless, the source of these metals is still a point of some controversy.…”