Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

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. In recognition of the continental setting of the Bushveld and Great Dyke another magma type that could be considered 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 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 picrites of 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. Podiform chromites from boninites are depleted in most trace elements and none of the intrusions chromites resemble them. The chromites from the Great Dyke have trace element concentrations similar to komatiite, but lower Cu and high Sn contents. The Bushveld and Stillwater chromites are richer in Al, Ga, V and Ti than the komatiite chromite and are depleted in Cu. The trace element concentrations patterns resemble the chromites form the low Ti-picrites except the picrites are richer 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 strong komatiitic affinity. The Bushveld and Stillwater chromites formed from magmas with larger continental crustal components.