The composition of the magmas from which the chromites that form the massive chromite layers of the Stillwater, Great Dyke and Bushveld Complexes are of interest both to understand the economic importance of the resources in the layers (Cr and PGE), but also in understanding how these layers form. Magmas that have been suggested as parental to the intrusions are boninites or crustally contaminated komatiites. Another magma that could be considered in recognition of the continental setting of the Bushveld and Great Dyke is picrite associated with continental flood basalts. In order to investigate whether any of these magmas are suitable parental magmas for the chromites we have determined major and trace elements in komatiites of low metamorphic grade, boninites and chromites from low-Ti and high-Ti picrites of the Emeishan Provence.In order to test whether the chromites are in equilibrium with volcanic magmas we first modelled the major and minor element composition of the chromites that should have crystallized from the komatiite, boninites and picrite liquids using SpinMelt v2. The compositions are approximately correct. In terms of major and minor elements none of the chromites from the layered intrusions match boninite chromites. The Great Dyke chromites are similar to chromites from komatiites. The chromites the Bushveld are slightly more evolved with higher Ti contents and lower Cr# and resemble the chromites from the low-Ti picrites of Emeishan. The Stillwater chromites have similar Ti contents to the Emeishan low-Ti picrites, but have lower Cr#. Their compositions resemble chromite compositions reported from the North Atlantic Igneous Provence.Hafnium, Ta, Cu, Sn, Sc, Ti, Mn, Ni, Co, Mn, Ga, V and Zn were determined by LA-ICP-MS. To compare the composition of the chromites an estimate of their partition coefficients into chromite was made based on the concentrations of elements in komatiite chromite divided by element in komatiite. The elements were then arranged in order of compatibility and the chromites normalized to the median komatiite chromite. Podiform chromites from boninites are depleted in most elements and none of the layered intrusions chromites resemble them. The chromites from the Great Dyke have essentially flat patterns close to 1 times komatiite, but with negative Cu anomaly and a slight positive Sn anomaly. The Bushveld and Stillwater chromites are richer in Al, Ga, V and Ti than the komatiite chromite and are depleted in Cu. The patterns resemble the chromites form the low Ti-picrites form Sn to Zn, but differ from picrites from Hf to Cu. The picrites are enriched in Hf, Ta and Cu.The chromite compositions suggest that boninite magmas are not involved in forming the chromites from layered intrusions. The Great Dyke chromites appear to have a komatiitic affinity. The Bushveld and Stillwater chromites appear to have a low-Ti picrite affinity.

Bedrock in arctic and subarctic regions is covered by glacial deposits making the discovery of new mineral deposits difficult. Indicator mineral methods using glacial sediment have thus been developed for mineral exploration in such drift-covered areas. However, sulfide indicator minerals have been under utilized because it was thought that they would not survive glacial transport and post-depositional oxidation during soil formation. In this contribution we show that the 0.25-1 mm non-ferromagnetic heavy mineral concentrates of Quaternary till and esker samples from the Churchill province in northern Quebec, Canada, contain thousands of pyrite and chalcopyrite grains and a few sulfarsenide grains. Accordingly, sulfide minerals do survive glacial and glacio-fluvial transport, even in the relatively oxidizing environment of the eskers, and their presence indicates the potential presence of mineralized bedrock up ice. The study area is therefore ideal to test the use of sulfide mineral chemistry for mineral assessment and vectoring. The composition of the pyrite and chalcopyrite grains recovered from the glacial deposits have been determined by LA-ICP-MS and compared with known values for sulfides in magmatic and hydrothermal deposits. Although some elements (e.g. Ag, Cu, Zn, Pb, W, Ba, La, and Yb) are enriched in narrow rims on some sulfide grains, indicating their limited mobility during oxidation, most elements have not been mobilized and reflect initial sulfide compositions in bedrock sources. The binary diagram Co/Sb versus Se/As shows that most of the pyrite grains in surficial sediments are of magmatic origin although some are from hydrothermal sources. The hydrothermal pyrites are enriched in hydrothermal pathfinders (Au, Hg, Ag, TI, Pb, Zn, Cu, and Mo). The ternary diagram Se-Cd-Ni shows that chalcopyrites from both magmatic and hydrothermal deposits are present in glacial sediments. The high Cd/Zn ratios of the hydrothermal chalcopyrites are indicative of a high crystallization temperature, typical of metamorphosed VMS or SEDEX deposits. Integrated maps combining bedrock geology, glacial transport directions, sample locations, and sulfide grain compositions and populations can be used to delineate target sectors for mineral exploration. Here sulfides have been transported over similar to 100 km roughly towards north, from sources in the Rachel-Laporte Zone and the Labrador Trough, where metasedimentary/metavolcanic rocks and mafic/ultramafic intrusive rocks are favorable hosts for hydrothermal and magmatic mineralization, respectively.

The titanomagnetite of the Lac Doré Complex, an Archean layered intrusion that is located in the Abitibi greenstone belt in Québec (Canada), contains a wide variety of exsolution textures, which are the remnants of a complex cooling history. In the present study, we reconstitute the decomposition stages of the original solid solution in order to explain the formation of ilmenite, Al-spinel (hercynite and gahnite), and corundum exsolutions in magnetite. This was conducted through a detailed mineralogical and textural examination and in situ determination of mineral chemistry. Our investigation reveals two discrete types of ilmenite exsolutions, which are ascribed, respectively, to the oxidation of ulvöspinel at temperatures above and below the magnetite-ulvöspinel solvus. Exsolutions of Al-spinel result from either a decrease in the solubility of the (FeZn)Al2O4 components upon cooling, or local excesses of Al and Zn due to the removal of ulvöspinel during the early oxidation. The origin of corundum is ascribed to the oxidation of pre-existing hercynite exsolutions. The trace element composition of the titanomagnetite indicates stratigraphic reversals in Cr, Mg, Co, Ti, and Si and important changes in redox conditions. We interpret this as a direct consequence of a major event of magma chamber replenishment, which strongly influenced the distribution of exsolutions.

The incorporation of contaminating elements in vanadiferous magnetite concentrates produced from layered intrusions can lead to expensive complications during vanadium extraction and purification processes. These contaminants may be contained in magnetites, as well as in gangue minerals attached to magnetites after the ore was comminuted. To better understand the flow of contaminating elements incorporated into magnetite concentrates, this project investigates the implications of textural and chemical features of mineralogy on the metallurgical behavior of vanadium ores produced from the Lac Dore Complex in Chibougamau (Quebec). In situ quantitative mineralogical analysis will be combined with laboratory-scale comminution and magnetic separation tests to develop a framework for a geometallurgical model aiming to improve the process leading to the production of high-purity magnetite concentrates. Results from our preliminary petrographic study reveal that magnetite is preferentially associated with ilmenite, ferrochlorite and ferropargasite in response to magmatic and metamorphic processes. As the mineral abundance, attachments and composition vary greatly in the cumulate pile due to changes in rock type and metamorphic conditions, the chemical constancy of magnetite concentrates will inevitably be affected.

The positive correlations between whole-rock concentrations of Cr and Os, Ir and Ru (referred to collectively as iridium-like platinum-group elements, IPGE) in both sulphide-poor plutonic and volcanic rocks suggest that chromite contributes to the collection of these elements during the early stages of sulphide-undersaturated magma differentiation. However, it is not clear whether these correlations are the result of IPGE partitioning into chromite or whether other minerals are involved. Positive correlations between MgO and IPGE have been observed, suggesting that these elements could be incorporated into olivine as well as into chromite. Alternatively, given the siderophile nature of IPGE, they may crystallize as discrete minerals together with chromite, as suggested by the presence of platinum-group minerals in chromite-rich rocks such as mantle podiform and crustal stratiform chromitites. To investigate the effect of chromite crystallization on the distribution of IPGE and Rh in picritic magmas, we have determined the content of these elements in chromites from the Emeishan Large Igneous Province (southwestern China) by in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The time-resolved analysis signals are generally uniform, indicating that the IPGE and Rh are homogeneously distributed within the chromite crystals. The median concentrations are 30 ppb Os, 23 ppb Ir, 248 ppb Ru and 21 ppb Rh. The incorporation of Rh in chromites appears to be influenced by the oxidation state of the magma from which they crystallize because there is a positive correlation between the degree of inversion of the chromite structure [ expressed by Fe3+/(Cr + Al + Fe3+)] and the Rh content of the analyzed crystals. In contrast, the enrichments of Os, Ir and Ru in chromites appear to be controlled by other parameters such as temperature or the S content of the magma. Based on empirical calculations, we determined that partition coefficients between chromite and melt are higher for Ru (127) than for Os (23), Ir (27) and Rh (32). Despite these high partition coefficients, mass-balance calculations show that chromite does not account for all the IPGE and Rh concentrations in the rocks, with chromite accounting for maxima of 84, 49, 22 and 20% of the whole-rock Ru, Rh, Ir and Os budgets, respectively. In situ LA-ICP-MS and scanning electron microscopy analyses reveal the presence of micrometric-sized PGE-rich minerals, including laurite (RuS2), Os-Ir +/- Ru alloys, sperrylite (PtAs2) and Pt-Fe (+/- IPGE and Rh) alloys, in association with chromite crystals. The presence of these minerals may account for the balance of IPGE and Rh. Alternatively, if IPGE and Rh are compatible with olivine, the balance may be accounted for by the large amount of olivine crystals found in the Emeishan picrites. Based on numerical modelling, we conclude that chromite, olivine and platinum-group minerals all contribute to the collection of IPGE and Rh during the early stages of picritic magma differentiation. Also, we establish that the preferential incorporation of Ru into chromite is responsible for the negative Ru anomalies observed in the PGE patterns of the Emeishan flood basalts. On the other hand, the relative importance of olivine and platinum-group minerals in controlling IPGE and Rh remains uncertain and is a key subject for further investigation.