Chromitite alteration in serpentinite mélanges of Nurali and Kalkan massif (Russia)

Studies on the genesis and evolution of Cr-mineralization in ophiolites usually do not consider the metamorphic processes that influenced the mineralization in later stages. Kalkan and Nurali chromitites from Urals mélanges exhibit different alteration. During post-serpentinization reactions Al diffuses out of magmatic spinel leaving behind a ferritchromite and promoting the formation of chlorite aureoles (Mellini, 2005). Kimball (1990) describes the alteration of spinel to ferritchromite and the formation of chlorite by the reaction: MgAl2O4 +4MgO+3SiO2 4H2O = (Mg5Al)(AlSi3O10)(OH)8 with MgO, SiO2 and H2O coming from the fluid. At Kalkan post-serpentinization processes led to the formation of chlorite and ferritchromite by reaction of fluid with spinel and serpentine, with MgO and SiO2 coming from serpentine. Only rare serpentine relicts testify the serpentinization stage. At Nurali no evidences of chlorite and ferritchromite formation have been observed and post-serpentinization processes led to partial reabsorption of chromite. Serpentinization did not affect chromite and led to the formation of magnetite as coronae around chromite crystals or as small scattered magnetite grains in the matrix. According to the mineral chemistry of the involved phases we propose the following reaction for ferritchromite formation: 2(Mg0.60Fe0.40)(Cr1.30Al0.70)O4 + 3/2(Mg2.57Al0.32Fe0.11)Si2O5(OH)4 + H2O + 1/12O2→ 7/6(Fe0.60Mg0.40)(Cr1.85Fe0.08Al0.07)O4 + 1/2(Mg9.18Fe0.34Al1.60Cr0.88) (Al2Si6)O20(OH)16. Magnetite formation during serpentinization without involving chromite can be described by the reaction: 4(Mg1.8Fe0.2)SiO4 + 2(Mg0.9Fe0.1)SiO3 + 6H2O + 1/6O2→ 3Mg3Si2O5(OH)4 + 1/3Fe3O4