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

Résumé

Major and trace element concentrations in whole-rock and cumulus and intercumulus minerals were determined in cumulate rocks from the Lower and the Lower Critical Zones of the Bushveld Complex, South-Africa. These new geochemical data are combined with microtextural observations to provide insights on the formation of the cumulates. The results are used to evaluate methods of calculation of parental liquid composition from which the cumulate rocks crystallized. Cumulus (orthopyroxene and olivine) and intercumulus (clinopyroxene and plagioclase) minerals have a relatively constant composition in terms of major and trace element throughout the Lower and the Lower Critical Zones suggesting that the minerals formed from a magma with a relatively constant composition and followed a similar crystallization history. The minerals are in most cases unzoned in terms of major and trace elements. However, Ti zonations are observed in pyroxene and are consistent with the steady increase in Ti concentrations in interstitial plagioclase with decreasing An. These features are interpreted to be the result of the crystallization of the trapped melt during closed system fractional crystallization without significant intercumulus melt migration as suggested by the absence of dihedral angle modification. Our calculations indicate that the rocks from the Lower and the Lower Critical Zones crystallized from a magma similar in composition to B1-type magma as suggested by previous authors. Our study illustrates that the combined analysis of whole-rock chemistry and cumulus and intercumulus minerals give useful results which allow the estimation of parental liquid composition from cumulate rocks for a wide range of elements. Moreover, our results indicate that the simple assumption of equilibrium between a melt and a mineral to calculate a parental magma can only be applied to pure adcumulate rocks. The methods of calculation presented in this study may be used to infer parental magma composition associated with Ni-Cu-PGE sulfide deposits and can potentially be used as an additional tool for mineral exploration to fingerprint prospective magma suites.