A Late Variscan tin province associated to the ilmenite-series granites of the Sardinian Batholith (Italy): The Sn and Mo mineralisation around the Monte Linas ferroan granite

The characterisation of the late Variscan intrusion of Monte Linas (southern Sardinia) allowed to firstly study some Sn-bearing ores of this area and their relationships with the surrounding intrusives. The Monte Linas pluton was emplaced at ca. 290 Ma at a shallow crustal level between allochthonous units and the foreland of the Sardinian Variscan belt. The pluton emplaced in a post-collisional regime into a previously exhumed low-grade basement, forming a coarse-grained monzogranite that is capped by an almost continuous sub-horizontal sheet of fine-grained rocks. The rocks within the pluton are ferroan F-bearing granites, belonging to the ilmenite-series. They have alumina saturation index values indicative of sub-aluminous to slightly peraluminous granitoids, as also indicated by the chemical composition of biotite within the pluton. The new data of this study indicate that the Monte Linas pluton formed from water-undersaturated magmas under low-fO2 conditions at temperatures > 850 °C. The later stages of magmatism were characterised by boiling and fluid expulsion at confining pressures of < 1 kbar, producing fayalite-bearing pegmatites, miarolitic facies and greisening. A wide variety of mineral deposits are associated with the Monte Linas pluton, including different types of Sn-bearing and Mo-bearing ores. Sn-bearing ores are represented by 1) Sn–As and Sn–Pb–Zn–Cu veins, and 2) “wrigglite” skarn Fe-Zn-Sn ores. Field and analytical studies, including fluid inclusion analyses, EMPA and SEM-EDS, allow to refer these deposits to the evolution of initial highly saline, hypothermal magmatic fluids, as confirmed by parageneses and fluid inclusion analyses of cassiterite in veins, which provide evidence of polyphase processes that started at temperatures close to 400 °C. The hydrothermal systems were initially characterised by low-fO2 and high-chlorine solutions that mobilised Sn and underwent rapid changes in physicochemical conditions that led to the deposition of cassiterite. In the “wrigglite” F-rich genetic environment, a role of fluoride as complexing agent for Sn can be inferred. The Monte Linas pluton is also characterised by Mo deposits, not typical in ilmenite series granites; they occur both as greisens and veins. Mo was mobilized as a result of rapid increases in fO2 within the magmatic system, and precipitated in a fS2-rich environment nearby the contact between the intrusion and the surrounding country rocks. The variety of Sn mineralising events around the Monte Linas pluton confirms the role of physicochemical characters of magmas and of magmatic processes in the genesis of tin deposits. The occurrence in southern Sardinia of a wide suite of ilmenite-series ferroan granites emplaced in similar geological contexts allows to consider the idea of a low-grade tin province and opens the way to further explorations.