Different populations of pyrrhotite and pentlandite inferred from major and trace elements, Outokumpu mining district, Eastern Finland

The Outokumpu deposit is an ultramafic-hosted VMS deposit in Eastern Finland. It is of Paleoproterozoic age and discussed as polygenetic in origin. Its primary enrichment in Cu-Zn-Co-Ni is of economic importance. The samples are from ore-rich formations and metaserpentinites retrieved from the Outokumpu Drilling Project, a borehole that crosscut the full sequence of the Outokumpu-Jormua ophiolite. The study utilizes the petrography and mineral chemistry (EPMA, LA-ICP-MS), to distinguish different pyrrhotite and pentlandite generations that might be associated with distinct potential magmatic, oceanic-alteration hydrothermal, or metamorphic-hydrothermal ore-forming processes. It also aims to establish probable relationships between the sample types. First, we distinguished monoclinic ferrimagnetic/fm and hexagonal antiferromagnetic/afm pyrrhotite (using magnetic ferrofluid) and correlated the more Fe-rich afm modification with silicates deposition and the less Fe-rich fm modification with fractures. Three populations are distinguished in terms of Fe/S ratio, ranging from typical ratios of monoclinic modifications (0.85-0.87) to nearly troilite compositions (~0.97). The last corresponds mainly to the ore samples, where the rest to the drillcore. The Ni/Co ratios show a corresponding clustering with the mentioned Fe/S ratios, whereas the enrichments in Pb, Bi and Sn show no correlation with population. Pentlandite shows different population characteristics for the Fe/S (i.e., ~0.85 and ~1.15), and Fe/(Ni+Co) (i.e., 0.6 and ~1.25) indices. The distinction between ore and drillcore samples is expressed in the first being more Fe-rich. The Ni/Co heterogeneity and higher values are attributed to be a result of the pentlandite exsolution from pyrrhotite, exclusively shown in the ore.