Redox state of magmas and granite-related Mo mineralization: evidences from Late Variscan F- bearing granites from Southern Sardinia (Italy)

Redox state of magmas has been invocated to explain the metallogenic behaviour of intrusive magmatism (Ishihara, 1981). Thus, Sn deposits are linked to low-fO2, ilmenite series, while Mo deposits are better bracketed into high-fO2, magnetite series. In Variscan Europe Mo deposits are represented in a few districts, including Eastern Erzgebirge, France and Sardinia. In Sardinia, small Mo deposits are related to a post-collisional F-bearing granite suite dated close to 290 Ma by Re/Os on molybdenite (Boni et al., 2003) and 40Ar/39Ar on biotite (Dini et al., 2005). This late Variscan suite is dominated by metaluminous biotite leucogranites grading to hololeucratic microgranitic to granophyric varieties; amphibole biotite monzogranites are locally observed (Monte Sette Fratelli). They emplaced at shallow crustal levels and locally (Ogliastra) grade to felsic volcanics. Thermal effects of the intrusions are limited to narrow contact aureolas (andalusite zone) around the plutons. In SW Sardinia (Sulcis and Monte Linas intrusions), both Mo and Sn deposits occur. Magmatic bodies show a distinctive magmatic zonation, with medium-grained granites dimembered upward by thick, flat-lying fine-grained to porphyritic varieties, including fayalite-bearing pegmatite layers. Finegrained to porphyritic rocks suffered various degrees of greisening, and host Mo ores, related to numerous small endoand exo- quartz-muscovite greisens (e.g. Perda Lada), grading to quartz vein and stockwork systems (Perd'e Pibera, Su Seinargiu, Flumini Binu). Ores are commonly dominated by molybdenite, with subordinate pyrite, chalcopyrite and wolframite. The studied deposits are related to ilmenite rock -series, as evidenced by petrography (opaque contents of granites <1%; ilmenite>>magnetite), and confirmed by geochemical ratios (Rb/Sr, FeO/Fe2O3, K/Rb). This behavior seems to contrast with the Mo/magnetite series association, and is more coherent with the presence of Sn vein deposits in the same areas (Naitza et al. 2015). A possible explanation involve changes of physicochemical parameters (in particular, fO2 and HF activity) from magmatic processes during magma emplacement to greisening; variation of redox conditions are confirmed by discontinuous reverse zoning of plagioclase laths in fine-grained facies, indicating a progressive PH2O increase with magmatic evolution. Mineralization prevalently occurred in close system conditions, although in some areas (Oridda, Su Seinargiu, Flumini Binu) Mo ores are centered on bodies of porphyritic rocks, showing features close to porphyry-style mineralization (Fiori et al., 1986). Boni M., Stein H.J., Zimmerman A. & Villa I.M. 2003. Re-Os age for molybdenite from SW Sardinia (Italy): a comparison with 40Ar/39Ar dating of Variscan granitoids. In: Eliopulos K. et al., Eds., Mineral exploration and sustainable development. Millpress, Rotterdam, 247-250. Dini A., Di Vincenzo G., Ruggieri G., Rayner J. & Lattanzi P. 2005. Monte Ollasteddu, a new gold discovery in the Variscan basement of Sardinia (Italy): first isotopic (40Ar-39Ar, Pb) and fluid inclusion data. Miner. Deposita, 40, 337-346. Fiori M., Garbarino C., Padalino G. & Masi U. 1986. Chemical features of wallrocks from Mo-showings of Sardinia (Italy). Rend. Soc. It. Mineral. Petrol., 41, 25-39. Ishihara S. 1981. Granitoid series and mineralization. Economic Geology, 75th anniversary volume, 458-484. Naitza S., Secchi F., Oggiano G. & Cuccuru S. 2015. A