EPITHERMAL Cu-Au MINERALIZATION IN THE PALAI-ISLICA DEPOSIT, ALMERIA, SOUTHEASTERN SPAIN: FLUID-INCLUSION EVIDENCE FOR MIXING OF FLUIDS AS A GUIDE TO GOLD MINERALIZATION

Abstract

The epithermal gold–copper-bearing deposit of Palai–Islica, near Carboneras, in Almeria, southeastern Spain, is hosted by volcanic rocks of Cabo de Gata – Cartagena volcanic belt (Betic Cordillera). Hydrothermal alteration occurs in the sequence: propylitic, sericitic, argillic (extending over a surface area of some 2.5 × 1.7 km 2 ), and silicic. Mineralization, consisting of quartz with sulfides (pyrite ± chalcopyrite ± other minor sulfides and native gold), appears in veins, stockworks and layers. Results of a petrographic study of the gold mineralization and a lithogeochemical study reveal a correlation between the position of the horizons bearing gold grains and the location of the geochemical anomalies, in such a way that different mineralized horizons can be defined, the most important being the “upper gold-bearing level” and the “lower gold-bearing level”. The precipitation of Au and other metals of economic interest within the mineralized horizons is systematically related to the presence of a fluid with a distinctive salinity, which interacts within a narrow interval of temperature with low-salinity fluids to produce the gold mineralization. In this sense, mineralized horizons systematically contain fluids of highly variable salinity (2–29.3 wt.% NaCl eq.) with a temperature varying some 25 to 50°C (approximately) in the interval of 217 to 315°C. In contrast, above and below the mineralized horizons, low-salinity fluids (6 ± 4 wt.% NaCl eq.) appear across a wide temperature range (≈120–400°C). The various characteristics of these fluids (due mainly to the isothermal mixing of fluids and a heterogeneous entrapment of the solutions) seem to have been responsible for the transport and precipitation of the metals studied. Gold could have been transported in the form of AuHS° or AuCl 2 − complexes and precipitated as a consequence of their destabilization owing to the abrupt change in physical and chemical conditions. Fluid-inclusion studies are shown to serve as a quick, inexpensive and very effective method for the exploration of new mineralized bodies within the gold-bearing deposit of Palai–Islica.