Constraints regarding gold deposition in episyenites: the Permian episyenites associated with the Villalcampo Shear Zone, central western Spain

The Villalcampo Shear Zone (around 307 Ma) shows second-order shear zones (420–390 °C) with gold ore bodies hosted by episyenites, which consist of albite episyenites (albitites) and albite-Kfs episyenites, both types with different contents in sulphides and gold. Mass transfer modelling supports higher fluid/rock ratios in albitites than in albite-Kfs episyenites. The study of worldwide barren and gold-bearing episyenites reveals abundant sulphides in the latter as a distinguishing feature. The electrochemical processes at the surface of sulphide would have enhanced gold precipitation, sulphides working as a gold trap. A complex fluid history occurred in gold ore bodies hosted by episyenites, although in essence, it was similar to quartz-sealed faults hosting late Variscan gold deposits: (a) an early fluid equilibrated with the metamorphic pile with sulphides or with a metamorphic fingerprint, resulted in a sulphide deposition and (b) a shallower fluid reservoir of meteoric origin provided gold deposition. In contrast to earlier claims regarding episyenite fluid flow, a down temperature and probably an upwards fluid flow are proposed for the episyenitization process, also in keeping with the early stages of fluid flow in quartz-sealed faults. Fluid inclusions in albite confirm that the striking coupled quartz leaching albitization processes occurred around 400 °C and 60 MPa, crosscutting the retrograde solubility field of silica and yielding a vuggy rock. Initially, albite, and later quartz and sulphide, filled the vugs from the same or a very similar fluid. Uraninites deposited during the albitization and probably the onset of the sulphide deposition afforded the same age (270 ± 12 Ma) as other Spanish episyenites, confirming a synchronous and a regional-scale process and ruling out a relationship with the granite cooling history (324 ± 11 Ma).