The Filon Norte orebody (Tharsis, Iberian Pyrite Belt): a proximal low-temperature shale-hosted massive sulphide in a thin-skinned tectonic belt

The Filón Norte orebody (Tharsis, Iberian Pyrite Belt) is one of the largest pyrite-rich massive sulphide deposits of the world. The present structure of the mineralization consists of an internally complex low-angle north-dipping thrust system of Variscan age. There are three major tectonic units separated by thick fault zones, each unit with its own lithologic and hydrothermal features. They are internally organized in a hinterland dipping duplex sequence with high-angle horses of competent rocks (igneous and detritic rocks and massive sulphides) bounded by phyllonites. The mineralization is within the Lower Unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation. The Intermediate Unit is made up of pervasively ankeritized shales and basalts (spilites). Here, hydrothermal breccias are abundant. The Upper Unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill. The tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody. Carbon, oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature (