Abstract
Chromitite layers in mafic-ultramafic layered intrusions represent a special case of cumulate rock where chromite is the sole liquidus (cumulus) mineral. Different mechanisms for the formation of chromitite layers have been postulated. In order to provide geological and petrological constraints for these models, chromitites and hosting mafic and ultramafic rocks from the Bacuri, Niquelândia and Ipueira-Medrado layered complexes in Brazil were investigated. The Bacuri Complex (2·2 Ga) is a large layered intrusion of the Amazon Craton. Chromitite layers of the Bacuri mafic-ultramafic complex are restricted to a 30–120 m-thick sequence of ultramafic cumulates overlying mafic cumulates. Most of the chromite is concentrated in a several metres-thick chromitite layer (the main chromitite) located at the base of the ultramafic cumulates. Cryptic variation of olivine with stratigraphic height (from Fo89 at the base to Fo76 at the top) is consistent with extensive fractionation of the ultramafic sequence. The stratigraphic position of the main chromitite supports a model for its origin associated with a major new influx of primitive parental magma. The cause of chromite saturation must therefore result from changes in magma composition resulting from mixing new influxes of primitive magma with a fractionated magma resident in the magma chamber. The Niquelândia Complex (0·8 Ga) is a large layered intrusion in central Brazil. The Niquelaândia Complex has a thick (up to 4 km) Ultramafic Zone consisting mainly of dunite and interlayered harzburgite, websterite and several centimetres-thick discontinuous chromitite layers. A stratigraphic interval hosting relatively thicker and more consistent layers of chromitite occurs in the intermediate portion of the Ultramafic Zone. Compositional variation of olivine composition through this interval indicates that the magma located below the interval where chromitites occur was more primitive than the magma located above them. This feature is consistent with a model in which chromitites were formed by new influx of primitive magma, and mixing with slightly more fractionated resident magma. It is however intriguing to note that chromitite seams in the Niquelaândia Complex are mainly hosted by dunite. Different from chromitites observed in other large layered intrusions (e.g. Bushveld, Great Dyke), chromitite seams in the Niquelaândia Complex do not appear at the base of cyclic units. In the Niquelaândia Complex, the crystallisation of clinopyroxene as a cumulus mineral occurs immediately after the crystallisation of orthopyroxene. Extensive Cr depletion through clinopyroxene crystallisation is suggested as an explanation for the absence of chromitites at the base of cyclic units in the Niquelaândia Complex. The Ipueira-Medrado intrusion (2·0 Ga) is a small sill (7 km-long, 0·5 km-wide) consisting mainly of a basal Ultramafic Zone (∼250 m-thick) and an upper Mafic Zone (∼40 m-thick). At the upper part of the Ultramafic Zone a thick (5–8 m) and continuous chromitite layer (MCL) occurs. Cryptic variations of olivine suggest that the lowered sequence located below the MCL crystallised in a dynamic magma chamber undergoing frequent replenishment with primitive magma. Chromite compositions are very similar and show no systematic fractionation trends over the 5–8 m of the MCL, indicating that a continuous supply of primitive parental magma occurred during the time span of chromite crystallisation. Cryptic variation data for olivine rule out a model for the origin of the MCL from mixing new influxes of primitive parental magma with a fractionated magma resident in the magma chamber. In fact, the most primitive compositions of olivine occur in dunite and harzburgite located immediately below the MCL. Lithogeochemical and isotope data suggest that chromite crystallisation in the MCL of the Ipueira-Medrado sill was triggered by changes in physical–chemical parameters associated with crustal contamination. We propose that an increase in oxygen fugacity resulting from assimilation of carbonate-rich host rocks was the triggering factor leading to crystallisation of chromite as a sole cumulus phase in the Ipueira-Medrado sill. The unusually thick chromitite layer of the Ipueira-Medrado sill is interpreted to result from a large volume of magma passing through a conduit hosted by carbonate-rich crustal rocks. The examples of chromite deposits studied indicate that chromitite layers are associated with changes in the melt composition at stratigraphic horizons of the magma chamber. However, the specific trigger responsible for appropriate phase changes leading to chromite crystallisation may be different for each deposit.