Geología y evolución hidrotermal de los sulfuros masivos de la faja pirítica ibérica, españa. Geology and hydrothermal evolution of massive sulphides of the iberian pyrite belt, spain

Resumen

The Iberian Pyrite Belt (IPB) is one of the largest and most important metallogenic provinces in the world, hosting more than 2500 Mt of pyrite-rich volcanogenic massive sulphides. The ore deposits are located in, at least, two different lithostratigraphic levels of the Volcanic Sedimentary Complex (Upper Devonian-Lower Mississippian), which fills a continental marine basin. Two different mineralization types can be distinguished: (1) In the northern zone, the sequence is dominated by calc-alkaline dome and crypto-dome complexes of rhyodacitic to andesitic composition which are interlayered with tholeiitic basalts and shale. Here, the mineralization has been interpreted as of the replacement-type and is preferentially hosted in the most reactive and permeable facies of the glass- and pumice-rich acid volcaniclastic rocks situated on top of a specific rhyodacitic unit. The most probable age of this unit is Lower Tournaisian. (2) In the southern domain, the sequence hosting the mineralization is dominated by shale covering or intruded by dome-like complexes with dacitic to rhyolitic composition with local shallow intrusions and lava flows of basalt. These massive sulphide ores are located in a stratigraphic horizon close to the Devonian-Carboniferous limit. The mineralization is exhalative and hosted by anoxic black shale. In the Rio Tinto mining district, located in the central part of the IPB, both types of mineralization, exhalative and replacement, coexist in close association with a felsic volcanic dome of Tournaisian age. In the Filón Sur area, the mineralization is (sub)-exhalative, being hosted by anoxic black shale of the Upper Sedimentary Unit. The replacement-type mineralization is hosted by glassy hyaloclastite located in the hanging wall of a rhyodacitic dome located in the top of the Felsic Unit. The geological and geochemical study of the northernmost IPB has allowed proposing a new stratigraphic sequence. The footwall includes a large lower andesitic unit followed by extrusive and intrusive basalt intermixed with sediments and two units of felsic volcanic domes on the top of the sequence. This sequence is highly modified by later tectonics, but it is basically similar to that observed in the Rio Tinto area, despite in the later case the andesitic unit is absent. The massive sulphides are spatially associated with the youngest felsic volcanic unit, a rhyodacite characterized by low Zr contents (i.e., below 200 µg/g). The underlying barren felsic rocks show higher Zr concentrations and have been interpreted as geochemically more evolved rocks which were likely emplaced at higher temperatures. This study reveals a great variability in the geology, geochemistry and thickness of the volcanic sequence, something that suggests a spatial evolution of the volcanism from north to south. In the northern sector there is a continuous volcanic activity characterized by the presence of a basal thick unit of andesite followed by basaltic lavas, sediments and large felsic volcanic domes. In the intermediate zone (Rio Tinto area), the volcanic activity is more discontinuous, it lacks of the lower andesite level and there are several packages of shale. In the southern area there are large amounts of shale and there are no mafic rocks in the basal part of the sequence. It is in this zone where the anoxic conditions required for the formation of the large massive sulphide ore bodies are found. The formation of the massive sulphides is related with the convective circulation of basinal fluids hosted and in equilibrium with the underlying continental sediments, the PQ Group, as the proposed 2D numerical model suggests. The model shows that the formation of the massive sulphides in the southern IPB was controlled by forced convective circulation associated to a period of tectonic extension. This model suggests that the heat could derive from an abnormal geothermal gradient without no involvement of a shallow magmatic heat source. Notwithstanding, the salinity of the fluid and the existence of tectonic structures that channelize the fluids and compartmentalize the basins were key factors for the ore forming processes. The model shows that the denser and hotter fluids (i.e., those with the best capability for metal transport) were the first to upflow and vent in the seafloor. The model also predicts that the duration of this event was shorter than 1 Ma, something consistent with the geochronology and the evolution of the basin. The study of the concentration and distribution of trace elements in the pyrite of the massive sulphides by LA ICPMS shows that the geochemistry of this mineral may help to differentiate between distal and proximal zones in the massive sulphide deposits, as well as to distinguish between the stockwork or feeder zone and the veins product of the late remobilization.