Cumulus processes and the composition of magmatic ore deposits : examples from the Talnakh district, Russia

A remarkable feature of the Noril'sk-Talnakh area is the mineralogical and compositional zonation of the magmatic sulphide ore deposits. This zonation can be explained by fractional crystallization of sulphide liquid. The compositions of analyzed ore samples can be identified as belonging to a continuum ranging from evolved, residual liquids derived during the fractionation process to cumulates composed primarily of monosulphide solid solution with variable proportions of trapped liquid. Some massive-ore samples display chondrite-normalized platinum group element-gold patterns with negative platinum anomalies. This feature indicates that the dominant control on ore composition is not sulphide liquid-silicate liquid partitioning but is the composition of monosulphide solid solution derived from the crystallization of sulphide liquid. Massive-sulphide ores that have compositional features typical of sulphide cumulates are not limited to the Noril'sk-Talnakh area but occur in several districts in a range of geological settings