Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy

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.

This paper reviews the distribution of platinum-group minerals in ophiolitic chromitites. Our data and literature data, obtained by in-situ investigation of polished sections and techniques of mechanical separation [hydroseparation (HS), or combining electric pulse disaggregation (EPD) plus HS], are contrasted. Finally, in-situ textural data are used as platform criteria to compare the different proposed models that attempt to explain the origin of the platinum-group mineral assemblages found in ophiolitic chromitites.

This paper presents whole-rock geochemical and mineral chemical data for chromitite layers of the Middle Group of the eastern Bushveld complex. It reports on compositional variations of chromite from the chromitite layers, and special attention is paid to the platinum-group element (PGE) content and mineralisation within the chromitite layers. The discussion is focused on possible mechanisms by which the association of PGE enrichment within the chromitite layers can be modelled. The MG chromitites from bottom (MG0) to top (MG4C) are characterised by progressive melt evolution, showing decreasing Mg# and Cr/(Cr+Fe) ratio and increasing Cr/(Cr+Al) ratio, TiO₂ and V concentrations. An increase in the Mg# together with a drop in the Cr/Cr+Al) and an increasing Cr/(Cr+Fe) ratio at the base of the MG4A chromitite strongly suggests the addition of hot and primitive magma. High Cu concentrations in the chromitite layers with coincident low S values suggest the application of Naldrett et al.³⁶ Fe-loss model during cooling. With the help of the La/Cu and Cu/S ratios it could be shown that Cu is concentrated neither in base metal sulphides nor in trapped silicate melt and thus the model is not applicable. Laser ablation inductively coupled plasma mass spectrometry studies of single chromite grains also have not revealed Cu to be in solid solution and thus another Cu concentrating phase must be found. The PGE patterns of the MG chromitite layers are very similar to the one of the UG2 suggesting that they derived from the same magma and the same style of mineralisation did apply. It furthermore implies the presence of one parental magma only for the entire Critical Zone. Due to low S contents in the MG chromitite layers, it seems unlikely that the PGE have been concentrated by base metal sulphides only. About a sixth of the platinum-group mineral observed is associated with As, Bi and Te, and therefore PGE concentration by the cluster model is favoured. Enrichment of the high-temperature PGE over the lower-temperature PGE in the lowermost MG chromitite layers and the MG4A is probably due to the presence of high temperatures of the chromitite forming melt, and thus temperature could play a role in the fractionation of the two PGE groups.

This paper is based on 465 new analyses of Ni, Cu, S and PGE from the 19 chromitite horizons between the LG-1 and UMG-2 from 6 sectors around the Bushveld Complex, along with microprobe analyses of representative samples of 41 chromites. Two trends in chromite composition, A and B, are distinguished on a plot of cation% Mg/(Mg + Fe²⁺) versus Cr/(Cr+Al). Trend A, that has a negative slope, is close to that predicted as the result of the reciprocal exchange substitution of Cr and Fe²⁺ for Mg and Al between spinel and liquid affecting the Mg-Fe²⁺ spinel-liquid K_d². Trend B, that has a positive slope and is defined primarily by the LG-5 to MG-2 chromitites, is the result of the progressive increase in the activity of Al₂O₃ as a result of the fractional crystallization of orthopyroxene. Overall, the average PGE concentrations in massive chromitite increase upward. The LG-1 to LG-4 chromities have low (Pt+Pd)/(Rh+Ru+Ir+Os) ratios (0·1 to 0·3), above which there is an abrupt jump to higher ratios in the LG-5 (0·9 to 10) and all overlying chromitites (also documented by Scoon and Teigler). The Pt/Ru and Pd/Ru ratios are very variable, but the Ru/Ir, Ru/Rh and Ru/Os ratios of all chromitites are relatively constant, indicating that Pt and Pd respond to different concentration mechanisms to the other PGE. Rh, Ru, Ir and Os were likely concentrated by chromite itself, probably as grains of laurite and alloys incorporated in growing chromite crystals, but the bulk of the Pt, Pd along with lesser proportions of the other PGE were concentrated by sulphide liquid. Most chromitites now have very low contents of S, but mineragraphic and chemical data support the suggestion of Naldrett and Lehmann that vacancies in chromite forming above 900°C were filled by Fe²⁺ derived from the destruction of interstitial sulphide liquid. Data on En composition through the Bushveld CriticalZone, indicate that the LG-1 to LG-4 chromitites formed at a stage when influxes of magma into the chamber were rapid and primitive, and overrode the effect of fractional crystallization, whereas above this, fractionation mostly overrode influxes of new magma. Irvine's model of mixing of resident magma with influxes of more primitive magma is invoked as the origin of the chromitite horizons. It is shown, using the equation for sulphur solubility and the programme MELTS, that influxes and mixing of fresh primitive magma from depth with that in the chamber (i.e. as envisaged for the LG-1 to LG-4) would not have caused sulphide immiscibility along with chromitite crystallisation, but that influxes and mixing of slower-ascending magma, that fractionated en route, could give rise to sulphide liquid segregating along with the chromitite (i.e. the scenario for the LG-5 and overlying chromitites). The modelling also shows that the more fractionated the magma in the chamber becomes, the more sulphide will form, accounting for the overall upward increase in Pt and Pd above the LG-5.

The Dalradian terrane in the north-west of Northern Ireland is prospective for orogenic vein-hosted gold mineralisation with important deposits at Curraghinalt and Cavanacaw. New geochemical and geophysical data from the DETI-funded Tellus project have been used, in conjunction with other spatial geoscience datasets, to map the distribution of prospectivity for this style of mineralisation over this terrane. A knowledge-based fuzzy logic modelling methodology using Arc Spatial Data Modeller was utilised. Four main groups of targets were identified, many close to known occurrences in the Lack – Curraghinalt zone and others in prospective areas identified by previous investigations. Additional targets are located along west-north-west trending linear zones at the eastern end of the Newtownstewart Basin and to the north of the Omagh-Kesh Basin. These zones may be related to major structures linked to a westward extension of the Curraghinalt lateral ramp which is regarded as an important control on the location of the Curraghinalt deposit.

Numerous economic deposits of high-grade iron ores occur in the Singhbhum-Orissa Craton, in Eastern India. The deposits are mainly located in the Jilling-Langalata, Noamundi and Joda areas which are part of the eastern limb of the regional horseshoe shaped synclinorium, where millimetre to centimetre scale Archaean Banded Iron Formation units have been converted to steel grey, iron rich fine grained powder, known as Blue Dust. Field observations and subsequent laboratory investigations indicate that in this region, the Blue Dust deposits occur as pockets or lenses of varying dimensions and are randomly oriented. However, in most cases the Blue Dust deposits are found above the Fe-rich primary host rock known as Banded Haematite Jasper. Mineralogical observations indicate that the Blue Dust is mainly composed of haematite, martite and goethite while quartz and kaolinite are the gangue minerals. Silica removal is the primary control of iron enrichment. Geochemical and field observations indicate that the Blue Dust in these deposits is regarded to be of supergene-modified hydrothermal origin. In the first stage, the early hydrothermal process affects the primary unaltered Banded Iron Formation by simultaneously oxidising magnetite to martite and replacing quartz with hydrous iron oxides. In the second stage, the supergene processes upgrade the hydrous iron oxides to fine grain microplaty haematite. The supergene process causes the leaching of remnant silica from hydrothermally upgraded iron ore under a suitable Eh and pH condition and leads to the formation of Blue Dust.