Formation of PGE- and sulphide-bearing chromitites and associated anorthositic rocks in layered intrusion by infiltration of reactive, Cl-rich fluid

Abstract

The chromatographic model for chromitite formation previously presented is updated to include sulphide as a possible participating phase. In the model, chromite is precipitated at an upward-moving reaction front as a Cr-bearing (gabbro)norite protolith reacts with a Cl-rich fluid that becomes progressively undersaturated in pyroxene as it rises into hotter parts of the crystal pile, leaving an anorthositic residue. The liberation of Fe during the dissolution of orthopyroxene can drive concurrent sulphide saturation. If the reaction front encounters an orthopyroxenite the model produces a first-order approximation for the Merensky Reef section of anorthosite – chromite – orthopyroxenite. In addition, MELTS modelling shows that the isothermal addition of Cr 2 O 3 alone to a nearly solid (gabbro)norite assemblage increases the amount of both chromite and liquid at the expense of other silicate minerals owing to the liberation of Ca, Na, and Si, the latter two acting as fluxing agents. A generalized constitutional zone refining model is presented in which a variety of spinel-silicate layering types (e.g, Bushveld magnetite layers and Skaergaard trough banding) in that they can develop over time.