THE FILON NORTE OREBODY (THARSIS, IBERIAN PYRITE BELT) - A PROXIMAL LOW-TEMPERATURE SHALE-HOSTED MASSIVE SULFIDE IN A THIN-SKINNED TECTONICBELT

The Filon Norte orebody (Tharsis, Iberian Pyrite Belt) is one of the l argest 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 ma jor tectonic units separated by thick fault zones, each unit with its own lithologic and hydrothermal features. They are internally organize d in a hinterland dipping duplex sequence with high-angle horses of co mpetent rocks (igneous and detritic rocks and massive sulphides) bound ed by phyllonites. The mineralization is within the Lower Unit and is composed of several stacked sheets of massive sulphides and shales hos ting a stockwork zone with no obvious zonation. The Intermediate Unit is made up of pervasively ankeritized shales and basalts (spilites). H ere, hydrothermal breccias are abundant. The Upper Unit is the less hy drothermally altered one and consists of silicified dacites and a diab ase sill. The tectonic reconstruction suggests that the sequence is in verted and the altered igneous rocks were originally below the orebody . Carbon, oxygen and sulphur isotopes in the massive sulphides and hyd rothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor to ok place at a low temperature (less than or similar to 150 degrees C) without indication, at least in the exposed section, of a high-tempera ture copper-rich event. Sporadic deep subsea-floor boiling is probably responsible for the formation of hydrothermal breccias and the wide e xtension of the stockwork. Its Co-Au enrichment is interpreted as bein g related with the superposition of some critical factors, such as the relationship with black shales, the low temperature of formation and the boiling of hydrothermal fluids. The present configuration and thic kness of the orebody is due to the tectonic stacking of a thin and ext ensive blanket (2-4 km(2)) of massive sulphides with low aspect ratio. They were formed by poorly focused venting of hot modified seawater e quilibrated with underlying rocks into the seafloor. Massive sulphide precipitation took place by hydrothermal fluid quenching, bacteriogeni c activity and particle settling in an unusual, restricted, euxinic an d shallow basin (brine pool?) with a low detritic input but with impor tant hydrothermal activity related to synsedimentary extensional fault ing. Resedimentation of sulphides seems to be of major importance and responsible for the observed well-mixed proximal and distal facies. Th e tectonic deformation is largely heterogeneous and has been mostly ch annelled along the phyllonitic (tectonized shales) deformation bands. Thus, sedimentary and diagenetic textures are relatively well-preserve d outside the deformation bands. In the massive sulphides, superimpose d Variscan recrystallization is not very important and only some early textures are replaced by metamorphic/tectonic ones. The stockwork is much more deformed than the massive sulphides. The deformation has a c ritical effect on the present morphology of the orebody and the distri bution of the ore minerals. This deposit is a typical example of the s heet-like, shale-hosted, anoxic, low temperature and Zn-rich massive s ulphides developed in a ensialic extensional basin.