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

Résumé

The distribution of noble metals, Ni and Cu in mafic and ultramafic rocks is thought to be controlled by sulphides, chromite, olivine and platinum-group minerals (PGM). One method for presenting noble metal, Ni and Cu data focuses on the sulphide control by recalculating the data to 100% sulphides and presenting the data chondrite normalized. The relative importance of the influence of sulphides, chromite, olivine and PGM on the noble metals, Ni and Cu is examined here using two alternative methods.

Firstly, the metals can be plotted in the order: Ni, Os, Ir, Rn, Rh, Pt, Pd, Au and Cu and mantle normalized. The noble metals have a much higher partition coefficient into sulphides than Ni or Cu. Therefore, if sulphides have segregated from the magma or are retained in the mantle during partial melting, the magma and all rocks that subsequently form from it will be depleted in noble metals relative to Ni and Cu and the metal patterns will have an overall trough shape. Conversely any rocks containing these sulphides will be enriched in noble metals relative to Ni and Cu and the metal patterns will have an arch shape (characteristic of Pt reefs). Chromite-rich rocks tend to be enriched in the elements Os, Ir and Ru relative to the magma from which they form, therefore if chromite crystallizes from a magma or is retained in the mantle during partial melting, the magma and any rocks that subsequently form from it will be depleted in Os, Ir and Ru and the metal pattern will have a positive slope from Os to Pd, flat from Pd to Cu and have a positive Ni anomaly. The rocks containing cumulate chromite will be enriched in Os, Ir and Ru relative to the other elements and the metal patterns have an overall negative slope with a down turn at Ni (as seen in podiform chromitites). Olivine concentrates Ni and under some conditions Ir. Therefore if olivine crystallizes from a magma or is retained in the mantle during partial melting, Ni and Ir will be depleted in the magma and in any rocks that subsequently form from it. The metal patterns have an overall positive slope from Os to Pd, flat from Pd to Cu and no Ni anomaly. The cumulate will be enriched in Ni and Ir and tend to have a flat metal pattern (e.g. in dunites from komatiites and ophiolites).

A second approach to presenting noble metal, Ni and Cu data is suggested because while the effects of sulphide, chromite and olivine control can be seen on the metal patterns it is easier to distinguish these effects visually using metal ratio diagrams, Pd/Ir versus Ni/Cu and Ni/Pd versus Cu/Ir.

These two approaches are useful petrogenetic and exploration tools. Rocks which have trough-shaped metal patterns formed from magmas which were depleted in noble metals, possibly by sulphide segregation. These rocks do not make good exploration targets although there is the potential of a noble-metal deposit stratigraphically below them. Rocks which are not enriched or depleted in Ni and Cu relative to the noble metals formed from a magma that has not segregated sulphides and therefore a noble metal deposit could lie stratigraphically above these rocks.