Alteración hidrotermal asociada a los yacimientos de sulfuros masivos de la Faja Pirítica Suribérica

Resumo

The Masa Valverde and Aznalcóllar-Los Frailes massive sulphide deposits are located in the SE sector of the Iberian Pyrite Belt (IPB). The deposits are hosted by a Carboniferous, Volcano-Sedimentary succession named Volcano-Sedimentary Complex (VSC). More specifically, both deposits occur at the top of the first felsic volcanic sequence, generally referred to as V1. Both deposits have a roughly lenticular morphology and overlie complex sequences of hydrothermally altered rocks. The host rocks are black shales, directly overlying volcaniclastic and coherent rocks of dacitic-rhyolitic composition. The mineralization consists mainly of pyrite and accessory amounts of chalcopyrite, sphalerite, galena, arsenopyrite and pyrrhotite. The aim of this study is to characterize the mineralogy, petrology and geochemistry of the footwall of the mineralization, integrating all the available data in order to distinguish alteration zones and their physicochemical characteristics. We finally suggest a plausible geological scenario for the genesis of the mineralization, prior to the Variscan deformation. Three alteration zones have been identified on the basis of their mineralogical, textural and geochemical characteristics. The innermost zone is characterized by a penetrative alteration that has destroyed most of the original volcanic textures. In this zone there are virtually no relict minerals. The mineral assemblage consists of chlorite + quartz + sericite ± carbonate ± sulphide. It is characterized by an extensive leaching of alkalis and an increase in the content of Fe and Mg. This zone, generally described as chloritic, is surrounded by another intense alteration area characterized by intense sericitization, sulphidation and silicification, in which sericite + chlorite ± quartz ± sulphide constitute the commonest assemblage. Here, both increase in Fe and loss of Mg are less significant than in the inner zone. The outer, peripheral alteration zone is characterized by sericitization and albitization with minor silicification. Geochemically, this zone is characterized by an enrichment in Na and K relative to the other zones. All these alteration facies are superimposed to regional hydrothermal alteration. The distribution thereof is approximately concentric and have an irregular development. Isotopic evidence obtained from carbon, oxygen and deuterium isotopic data, coupled with fluid inclusion study, indicate that the hydrothermal circulation related to the VMS formation was dominated by seawater in which temperature progressively increased from 200 to 350 °C. In cases, fluids could have been reached the boiling point. The available data are consistent with a model of convective hydrothermal circulation triggered by a mantle rise associated with cortical thinning. This would have led to melting of progressively more shallow crustal levels in an extensional/ transpressive geodynamic environment. Hydrothermal activity was marked by a progressive increase in temperature which stimulated leaching processes and triggered hydrothermal alteration, sustained along a protracted time span. Leaching was particularly intense in the inner zone, where even “immobile” elements were mobilized. In these chloritic zones, precipitation of hydrothermal zircon, significantly enriched in REE, has been described in detail. The hydrothermal regime evolved from a predominantly diffuse to focused, high-T hydrodynamic regime during the most intense hydrothermal stage. This evolution caused a progressive change in the VMS mineralogy and chemistry, evolving from polymetallic to cupriferous.