Geological, geochemical and mineralogical features of a sedimentary manganese ore in north-western Sardinia: Preliminary study

In north-western Sardinia a sedimentary manganese deposit occurs between the tertiary calcalkaline volcanic products and the overlying transgressive bioclastic limestones of the second Miocene sedimentary cycle. The manganese makes up the secondary cement of a conglomerate at the base of the transgression. Chemical, mineralogical and textural analyses carried out by the XRF, INAA, XRD, OM and SEM-EDX evidenced pyrolusite and hollandite as the main Mn-phases. As geochemistry is concerned, (La/Yb) ch points to a positive LREE and HREE fractionation. Moreover, the strongly positive Ba anomaly can be referred to its inclusion within the hollandite structure as well to the absorption exerted by Mn-oxyhydroxides.