Podiform chromitite : genesis and alteration

The renewed interest in chromite ore deposits is directly related to the increase of Cr price ruled by international market trends. The need of detailed knowledge of chemical features of chromitite ore bodies aims new projects focused on the understanding of chemical composition variation of such deposits. The present study, divided in two parts, has been focused on the understanding of the genesis and of the post-depositional alteration processes related to podiform chromitite ore bodies. The first part of the project focus on the major processes that influence chromitite genesis in Troodos ophiolite complex through the study of chromite chemical composition and on its related silicate inclusions that may preserve important information on parental magma source from which chromitite crystallized. Numerous silicate inclusions have been detected in chromite grains of different chromitite pods and in Cr-rich spinels of enveloping serpentinized host rock. The compositional features of silicate inclusions have been compared in term of major and REE elements with those of mineral matrix of fresh host rocks. Moreover chemical composition of selected included clinopyroxenes, were compared to experimental results of Gaetani et al. (2003) on hydrous peridotite partial melting REE contents of clinopyroxene inclusions of chromite of selected samples and in clinopyroxenes of the host rock have been measured in order to evaluate the role of a possible metasomatic process affecting chromite formation. Mineral REE concentrations were measured by means of Secondary Ion Mass Spectrometry (SIMS) using a Cameca IMS~4f ion microprobe at CNR-IGG (Pavia). The analysis of Troodos chromites associated to the study of silicate inclusions detected in chromitite pods as well as in host rock suggests a complex genesis probably related to an interaction between a hydrous boninitic melt, rich incompatible elements with a mantle lherzolite host rock. The second part of the study has been focused on post-depositional alteration processes that affected chromitite deposits of serpentinitic mélange in Kalkan ophiolite complex. In melange environments intense metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study we investigate Kalkan hosted melange chromitites through textural and chemical analyses and we propose a new, post-serpentinization, reaction for ferritchromite and chlorite formation on the basis of involved phases: 2(Mg0.60Fe0.40)(Cr1.30Al0.70)O4 + 3/2(Mg2.57 Al0.32Fe0.11)Si2O5(OH)4 + H2O+1/12O2→ 7/6(Mg0.40Fe0.60)(Cr1.85Fe0.08Al0.07)O4+ 1/2(Mg9.18Fe0.34Al1.60Cr0.88)(Al2Si6)O20(OH)16. Results of textural analyses fit well with proposed reaction and show that reaction usually proceeds very close to completion. Alteration of chromite into ferritchromite is controlled by initial chromite to serpentine ratio. In chromitites high ratios prevent complete transformation of chromite into ferritchromite.