Alteration of braunite ores from Eastern Liguria (Italy) during syntectonic veining processes: Mineralogy and fluid inclusions

The manganese ores of the Northern Apennine ophiolites (Val Gravegila, Eastern Liguria, Italy) occur within chert sequences, mainly as stratiform layers (0.1-1 m thick) or massive lenses (5-20 m thick). The ores originated in the Ligurian-Piedmont oceanic basin (Middle Callovian) during turbiditic resedimentation of hydrothermally Mn-enriched submarine muds. During Upper Cretaceous Lower Cenozoic orogenic events, the primary sedimentary oxide assemblages were completely recrystallized to braunite + quartz assemblages under prehnite-pumpellyte facies conditions (T = 275 +/- 25 degrees C; P = 2.5 +/- 0.5 kbar). This tectonic event induced mobilisation of Mn and Si, and thickening of the mineralised layers. A complex network of sigmoidal quartz veins formed at this stage. The interaction between the Mn mineralisation and the circulating fluids generated centimetric to decimetric reaction rims in the wall rock, where the braunite + quartz assemblage is replaced by Mn silicates (mainly bementite, johannsenite, parsettensite and rhodonite) and carbonates (mainly Mn-bearing calcite and rhodochrosite) with a zoned distribution. Mineral zoning points to an early interaction between the Mn mineralisation and H2O-CO2 fluids with high water activity. Two distinct types of fluids are present in syntectonic quartz veins, namely a low-salinity water-rich fluid, and a Mn-Ca-Na-Mg-Fe-bearing aqueous solution that in all probability represent a relict of an early fluid phase circulating during breakdown of braunite. The isochore distribution related to low-salinity fluids indicates that veining processes took place during the main tectono-metamorphic events, at peak P-T conditions.