Progressive assimilation vs. fractional crystallization in sulfide magma conduits

Assimilation of crustal sulfur is widely considered as the main driver for the formation of massive sulfide orebodies, which concentrate metals in mantle-derived magmas. Over nearly 100 years of studying the giant sulfide deposits of Norilsk, a number of scenarios involving off-stage, on-stage or no crustal assimilation have been proposed. To resolve the ambiguity of the existing interpretations of the role of assimilation in the formation of Norilsk ores we examined co-variations between the sulfur isotope composition of sulfide ores and their metal grades and ratios based on our new data and an existing literature database, totaling over 1400 analyses. Massive ores of four ore-bearing intrusions of the Norilsk region are categorized into high-Rh, Cu-deficient, intermediate and Cu-rich types according to their metal ratios, which are controlled largely by the degree of fractional crystallization of sulfide liquids and efficient separation of residues from cumulates. The previously unrecognized high-Rh ore type with Cu/Ni < 2 and Pd/Rh < 10 represents the most geochemically primitive cumulates of the least contaminated protosulfide liquid entering the resident conduits. This is supported by their δ³⁴S values, which were analyzed in situ and appeared to be regularly 2‰-3‰ lower than the average δ³⁴S values for any other ore types in each specific deposit. The general negative correlation between enrichment in the heavier ³⁴S isotope and metal tenor in all massive and disseminated ores from the Norilsk deposits suggests that the dilution of metals in voluminous immiscible sulfides due to progressive assimilation of sulfates was only partly counterbalanced by the accumulation of metals in the residual liquid during fractional crystallization. The distinct ranges of PGE tenors and δ³⁴S values for each specific deposit are defined by the difference in the morphology of magma flow pathways through diverse lithologies. Although post-emplacement hydrothermal activity consequently influenced the sulfide associations and distribution, the observed lateral trends of the sulfur isotope variations are believed to indicate the directions of magma movement through changing wallrocks and strongly support on-stage contamination in the upper-crustal plumbing system of the initially PGE-enriched Norilsk-type sulfide-silicate magmas. This observation has a practical implication for targeting Cu-Ni-PGE chonolith-type mineralization in the Siberian trap province and worldwide.