Chemosynthetic precipitation of Mn-phases: the coated pebbles of the Scala Erre conglomerate (NW Sardinia, Italy)

The work focuses on a conglomerate, deriving from the dismantle of the Variscan basement of NW Sardinia, made up with quartz and minor metarenite clasts. The conglomerate was referred to an alluvial braided system and overlays a clayey Messinian palaeosoil. Noticeably the rounded pebbles are cemented by Mn oxides and even the loose, single, clasts are coated by a black patina that was submitted to mineralogical XRD and SEM- EDS and INAA chemical analyses. The coatings, similar to that of varnished rocks, have thickness variable from 20 up to 400 μm. Lithiophorite, birnessite and hollandite with traces of micas are the main mineral phases. The chemical analyses revealed a ΣREE in the range of 3000-5000 ppm and a strong LREE fractionation with (La/Yb)ch in the range of 7-20. In respect to the UCC composition (McLennan et al., 2006), Ni, Co and Zn are the trace elements that show higher enrichments, close to 100X, whereas Pb, Ba and Cu are less enriched, though their enrichment never drops below 10X. The SEM-EDS observations evidenced the foliated structure of phyllomanganates, such as birnessite and lithiophorite, and cocci colonies consisting of grape-like aggregates of spheroids diameter around 0.5 mμ that testify for chemosynthetic microbial activity. According with Lovley (1995) several bacteria act as catalyst able to control redox reactions involving metals oxides and organic matter. Among these Mn oxides and oxyhydroxides are the most common and reactive. The precipitation of insoluble Mn(IV) from the soluble Mn(II) occurs thanks to some enzymes and is very effective even in pH conditions not favourable to Mn oxidation (Tebo et al., 2005; Huelin et al., 2006). Presently in the study area such concretions neither affect pebbles in streams flowing from the Variscan basement nor coat pebbles in the shore of pods where this water accumulates. Mn contents of the pods water varies seasonally from 20 to 13000 ppb mostly controlled by pH, Eh and temperature conditions. The palaeoclimatic condition, perhaps of the savannah type, characterized by relatively humid seasons alternating with periodic droughts, favored the bacterial activity which explains both the manganese coating of the individual pebbles and the cementation of the finer-grained conglomerate by coalescence of the coating from one pebble to another.