THE VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS OF THE IBERIAN PYRITE BELT - REVIEW AND PREFACE TO THE THEMATIC ISSUE

The Iberian Pyrite Belt (IPB) has, over the past decade, been an area of renewed mining activity and scientific research that has resulted i n a wealth of new data and new geological and metallogenic concepts th at are succinctly presented in this Thematic Issue. The reason for thi s interest in the IPB, which forms part of the Hercynian orogenic belt . is that its Late Devonian to Middle Carboniferous rocks host a huge quantity of volcanic-hosted massive sulphide (VMS) mineralization (170 0 Mt of sulphides, totalling 14.6 Mt Cu, 13.0 Mt Pb, 34.9 Mt Zn, 46100 t Ag and 880 t Au). The mineralization and its environment display a number of typical signatures that can be related to the mineralogy and zoning of the sulphide orebodies, to the lead isotopes of the mineral ization, to the geochemical and mineralogical variations in the hydrot hermal alteration halos surrounding the orebodies, to the geochemical characteristics of the bimodal volcanics hosting the VMS, to the compl ex structural evolution during the Hercynian orogeny, to the presence of palaeofaults and synsedimentary structures that acted as channels a nd discharge traps for the metalliferous fluids, and to the gossans de veloped over VMS. Discriminant geological criteria have been deduced f or each domain which can be helpful in mineral exploration, complement ing the more traditional prospecting techniques. Although the question of the IPB's geodynamic setting is still under debate, any interpreta tion must now take into account some incontrovertible constraints: for example the geochemical characteristics of a large part of the basic lavas are comparable to those of mantle-derived basalts emplaced in ex tensional tectonic settings, and the associated acidic rocks were prod uced by melting of a basic crustal protolith at low-to medium-pressure s and a steep geothermal gradient, thus, the sulphide-bearing volcanos edimentary sequence differs strongly from recent are-related series. I t is considered here that the tectonic setting was extensional and epi continental and that it developed during the Hercynian plate convergen ce, that culminated in thin-skinned deformation and accretion of the S outh Portuguese terrane to the Iberian Paleozoic continental block.