Comment on the paper by Sanchez-España et al.: source and evolution of ore-forming hydrothermal fluids in the northern Iberian pyrite belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes (Mineralium Deposita 38: 519–537)

Abstract

The recent paper by Sanchez-Espana et al. (2003) presents new fluid inclusion (FI) and stable isotope data on several massive sulphide deposits of the northern Iberian Pyrite Belt (IPB). Using these results together with previously published data on some large deposits of the southern IPB, they concluded that the ore-forming fluids in the IPB were a mixture of highly evolved seawater, exchanged with the underlying sedimentary and magmatic rocks, and a significant deep fluid component, either of metamorphic or magmatic origin (the latter is the favoured conclusion). However, this conclusion essentially depends on the fundamental assumption that neither FIs nor stable isotope records were disturbed during the late Variscantectono-thermal event(s). Yet, according to the current knowledge on metamorphosed deposits (e.g., Marshall and Spry 2000; Marshall et al. 2000), FIs in such deposits generally do not preserve data pertinent to the pre-metamorphic ore stage, and the stable isotope data are usually difficult to interpret. Here, we would like (1) to discuss the possibility of finding pristine FIs in the IPB massive sulphide deposits that would have survived the late Variscan low-grade tectono-thermal event, (2) to present evidence for a significant, gold-depositing, post-kinematic hydrothermal stage in the IPB deposits, with the concomitant development of pervasive lateto post-kinematic fluid percolation through the ore bodies, and (3) to discuss the real significance of the (O, H) stable isotope data. Our discussion is mainly based on our study of the Tharsisstockwork (Marignac et al. 2003), with additional information from the La Zarza and Rio Tintostockworks (Diagana2001) and from the NevesCorvo deposit (Moura et al. 1997a, b). We conclude that all FI data and many stable isotope data are in fact a record of the late Variscan post-kinematic hydrothermal overprint and that only few data may be of significance for the early syn-sedimentary ore-forming event in the IPB massive sulphide deposits.