Preliminary data on an anomalous chrome-spinel assemblage from Amanos Mountains serpentinites (Turkey)

The present work deals with an anomalous spinel assemblage detected within serpentinite rocks, whose peculiar textural and geochemical patterns can provide new information on spinel genesis and transformation within mantle peridotites and serpentinites.
   Spinel is an important accessory phase in mantle peridotites and serpentinites. It shows a wide range of compositions related to different processes and has been largely used as a petrogentic indicator. Due to its refractory character spinel has long been considered to reflect magmatic conditions of its formation, but in the last years studies pointed out changes in its composition also related to metasomatism, metamorphism and alteration.
   Spinel occurring as an accessory phase within mantle peridotites is dominated by a strong Cr-Al trend, from Al-rich spinel ss. in high-pressure lherzolite to Al-poor chromite in massive chromitite bodies. The main controls on magmatic spinel composition are Cr exchange with pyroxene and re-equilibration with olivine. During metasomatic and metamorphic events the most common change in spinel composition is a depletion in Al associated with enrichment in Fe and/or Cr. When this process affects Cr-rich spinel (chromespinel or chromite) it leads to the formation of ferritchromite, usually as alteration rims around chromite grains. Ferritchromite can further evolve to chrome-magnetite and magnetite, always associated with the formation of kammaererite (chromian-chlorite) in the silicate matrix.
   The spinels described in this study were found during exploration for chromite ore in the Amanos Mountains, about 15 km NE of Iskenderun, Southern Turkey. There ophiolite slices crop out below Mesozoic carbonates and cherts. Ophiolitic rocks are mainly composed of a serpentinite melange with some strongly tectonized gabbros. Serpentinites host several small chromitite lenses that underwent limited exploitation in the last century.
   All but one of the chromitite bodies detected show a massive to densely disseminated texture with 30 to 80% modal spinel and the composition of a typical chromite from podiform chromitite bodies within ophiolite peridotite. Alteration to ferritchromite is widespread even if it never completely obliterates primary spinel composition at the crystal cores.
   The exception to the described outline comes from a sample of chromitite which shares texture at the cm-scale with the other chromitites, i.e., millimetric euhedral spinel grains densely disseminated within a completely serpentinized matrix. Microscope observation shows a complex texture with strong zonation related to alteration of the primary spinel. The spinel crystals perfectly preserved their original shapes in spite of deep alteration and transformation occurred from rim to core. Primary spinel is preserved only as irregular core portions and is an Al-rich chrome-spinel with about 41 wt% Al₂O₃ and 27 wt% Cr₂O₃. The Al-rich core is surrounded by a porous rim of about 200-300 micron in thickness composed of Fe-rich chromite, with about 9 wt% Al₂O₃, 40 wt% Cr₂O₃ and 35 wt% FeO_tot. A second alteration rim, closer to the original crystal border, is highly porous and is made up of ferritchromite, with 5-7 wt% Al₂O₃, 30 wt% Cr₂O₃ and 31-35 wt% FeO_tot. Finally a thin rim, external and developed also in the fractures cutting the original crystal, consists of very anomalous Cr-magnetite. This Cr-magnetite shows a composition different from any spinels described in literature, with 35-37 wt% Cr₂O₃ and about 45 wt% FeO_tot, but still with 4-5 wt% Al₂O₃.
   The silicate matrix is mainly fine-grained chlorite that is found also in the porosity within the original spinel crystals. Chlorite Cr₂O₃ content (1-1.5 wt%) is lower than that of kammaererite usually associated to ferritchromite. Relics of serpentine are found only in the matrix.