The petrology and genesis of the Paleoproterozoic mafic intrusion-hosted Co-Cu-Ni deposit at Hietakero, NW Finnish Lapland

Abstract Cobalt is a highly sought-after metal due to its economic importance in many hightech applications. It is mainly obtained from sedimentary-hosted Cu-Co deposits and magmatic Ni-Cu deposits as a by-product. We describe a recently discovered Co-enriched Cu-Ni deposit hosted by the Hietakero mafic intrusion in north-western Finnish Lapland. The intrusion contains gabbroic to pyroxenitic cumulates and was emplaced into a supracrustal strata composed mainly of mafic volcanic rocks, sulfur-bearing graphite schists and felsic volcanic interlayers. Hybrid rocks provide clear evidence for interaction of mafic magma with its country rocks. All these rocks have undergone intensive post-magmatic hydrothermal alteration by influx of H2O-, CO2- and Cl-bearing saline fluids, forming strongly scapolitized (±albitized) rocks and resulting in re-mobilization of sulfides and their metals. The Hietakero deposit is associated with pyroxenitic cumulates and hybrid rocks, with the sulfides (pyrrhotite, cobaltian pentlandite, chalcopyrite and pyrite) occurring as patches, brecciated to net-textured and vein to veinlet forms. Sulfur content is 6.1 wt.% on average, reaching 20 wt.% in some cases. Nickel and Cu tenors are low, but Co is elevated, resulting in very low Ni/Co of 2.6 compared to ‘classic’ Ni-Cu sulfide deposit globally. The average metal tenors are: 0.55 wt.% Ni, 1.07 wt.% Cu, and 0.23 wt.% Co. In-situ sulfur isotope analyses of sulfides from mineralized rocks yielded δ34S values from -2.0 to +4.5‰. In spite of strong post-magmatic alteration, we can conclude that the Hietakero cumulates were formed from a low-MgO and PGE-depleted basaltic magma, which developed in a staging chamber at a deeper crustal level. The magma produced compositionally different gabbroic to pyroxenitic rocks (i.e., high-Ti and low-Ti groups). Model calculations indicate that the relatively high Co tenor and low Ni/Co cannot be explained by an earlier phase of fractional crystallization or sulfide segregation. Instead, an external source of cobalt is needed, which is also consistent with the high Zn, Pb and Mo contents of the mineralized rocks. Based on sulfur isotope compositions, the associated black shales were not the primary source of sulfur, but sulfur was rather derived from a so far unrecognized, potentially Co-bearing contaminant with close to mantle-like δ34S. Our study reveals a new Co-Cu-Ni deposit type related to the widespread Paleoproterozoic mafic magmatism in Lapland