Ordovician volcanic rocks record rifting, Variscan metamorphism and gold mineralization processes (Truchas Syncline, NW Iberia, Spain)

Ordovician volcanic rocks outcrop in several locations of the NW Iberian Variscan belt. Their composition is mainly basaltic (with less acid types) and occur as volcanic-volcanoclastic layers within a shale-slate succession. This work focuses on volcanic and related rocks within a prominent Variscan structure, the Truchas Syncline. We studied field relations, petrography, mineralogy, geochemistry and conducted thermodynamic modelling to review the petrogenesis and establish the evolution of these volcanic rocks classified as within-plate alkaline basalts (high Ti/Y, Nb/Y and Nb/Yb). Crustal contamination is absent given the elevated Nb/La ratio (1–1.5). These features indicate low melting degrees of the upper mantle and a continental rifting environment. The finding of Ordovician orthid brachiopods in some of the volcanoclastic rocks suggests a shallow marine environment for the volcanic deposition. Variscan metamorphism occurred at lower greenschist conditions with chlorite-temperatures of 374 ± 6 °C. Quartz + carbonate veins indicate that H2O–CO2 metamorphic fluids traversed some volcanic rocks, reacting with Ca–Fe–Mg phases to produce carbonates (Mg-calcite–Fe-dolomite). For this event, T-XCO2 modelling indicates temperatures below 350–360 °C and fluid XCO2 between 0.10 and 0.45. Such fluids can be important carriers of Au and might explain gold deposits in adjacent quartzites. Metasomatic shales (Fe-chlorite + quartz) outcrop nearby and were derived from a mixed protholith of shales and minor volcanic components. Its geochemistry shows Fe enrichment and high peraluminosity. Variscan deformation further modified its geochemistry causing Si-depletions and relative increases of other elements (K, Na, Ti, Al, Rb, Sr, Ba and LREE) in shear zones domains.