Controls on the Fo and Ni Contents of Olivine in Sulfide-bearing Mafic/Ultramafic Intrusions: Principles, Modeling, and Examples from Voisey's Bay

Abstract

Both Ni and Mg are compatible in mafic minerals that form early during the fractional crystallization of mafic/ultramafic magma; thus, both decrease in abundance in the silicate magma, and hence in later-forming silicates as fractionation proceeds. The concentration of Ni in silicates such as olivine and the MgO/FeO ratio of the silicates are related to values in the magma from which the olivines are crystallized by coefficients, which have been experimentally determined and therefore can be used to infer information about the magma. If the magma is saturated in sulfide so that sulfide droplets are removed along with mafic silicates during fractionation, additional Ni will be removed in comparison with the sulfide-absent situation. This will be reflected in a more rapid decrease of Ni with Fo than if sulfides were not separated. Variations of Ni with Fo are examined in the light of model curves for the Voisey's Bay Intrusion that hosts a world-class Ni-Cu-Co sulfide deposit in Labrador, Canada. In the past, it has been a practice to compare the Ni and Fo contents of olivines from a given intrusion with the field determined by Simkin and Smith (1970) for a wide variety of igneous olivines to identify those that are Ni-depleted. The objective is that these are presumed to have come from sulfide-saturated, and therefore, economically-interesting magma. This study shows that this simple comparison can lead to errors. It is important to compare natural data with model curves that have been generated for, and reflect the cumulus mineralogy of each intrusion in question. Using this approach, the natural data can be closely duplicated by model curves, which, in some cases, place additional constraints on possible petrologic interpretations. For example, at Voisey's Bay, a period of sulfide-unsaturated fractionation can be shown to have been succeeded by the removal of a sulfide liquid plus silicate minerals, followed by a period of silicate crystallization.