Multi-isotope survey on the metallogenesis of the hydrothermal Co-Ni deposits in the Alpine Central Pyrenees of Spain

Abstract

Thrust systems, related to thin-skinned tectonics are the main structural control for the development of Co-Ni deposits (Co-Ni arsenides, diarsenides and sulpharsenides) in the Alpine Central Pyrenees. Co-Ni ores occur in the hanging wall of these thrusts, hosted either by carbonate (San Juan de Plan and Crescencia mines) or siliciclastic rocks (Solita Mine), whereas the footwall is formed by black shales. Previous studies have applied graphite geothermometry and crystal-chemical criteria (the presence of alloclasite and the extent of the cobaltite–gersdorffite solid-solution) to determine that the complex assemblages of the studied deposits were formed at temperatures between 300 °C and 600 °C. The possible sources of the mineralizing fluids and metals have been investigated with a multi-isotope approach. Stable isotope data (C, O) support a primary magmatic source for the ore-forming fluids although interaction with both the Variscan black shales and carbonate host-rocks is also evidenced. Nd and Sr isotope compositions suggest that these fluids reacted with basement siliciclastic and igneous lithologies (and to a lesser extent with carbonate rocks). This reaction scavenged sulfur from the Variscan basement as a whole (positive δ34S values) and specifically from the underlying local black shales (negative δ34S values) and local magmatic rocks (δ34S values close to 0‰). Metals were extracted also from the Variscan black shales underneath each deposit, as suggested by the close correspondence between the metal ratios in the Co-Ni deposits and in the underlying black shales.The ore-forming activity responsible for the studied Co-Ni deposits spans an age range (Upper Cretaceous, 86 ± 27 Ma) similar to that of other high temperature events, developed in a context of thinned crust in the Pyrenean realm (i.e. High T/low P metamorphism, mantle exhumation and alkaline magmatism). These processes promoted high temperature hydrothermalism and formation of ore deposits, prior to the tectonic inversion and thrusting during the compressive stage.