Role of sedimentary sulfates in the genesis of massive Ni-Cu-PGE deposits - constraints from the Sakatti Cu-Ni-PGE deposit, Northern Finland

The Paleoproterozoic (c. 2054 Ma) Sakatti Cu-Ni-PGE deposit is one of the most significant metal discoveries in Europe during the 21^st century with the resource of 44.4 Mt @ 1.9 wt.% Cu, 0.96 wt.% Ni, 0.05 wt.% Co, 0.64 g/t Pt, 0.49 g/t Pd and 0.33 g/t Au. The deposit lies in the municipality of Sodankylä, northern Finland, that is emerging as a Ni-Cu-PGE mining camp, already including the Kevitsa Ni-Cu-PGE mine (Boliden) and several subeconomic magmatic sulfide deposits. The Sakatti ore mainly consists of massive Ni- and Cu-dominated ore lenses (Cu/Ni < 0.5, Pd/Pt ≈ 4, and Cu/Ni ≈ 9, Pd/Pt ≈ 1, respectively). These massive ore shoots are surrounded by a veil of Cu-rich disseminated sulfides (Cu/Ni ≈ 7, Pd/Pt < 1) hosted by olivine cumulates and coarse-grained gabbronorites. In the upper parts of the deposit, massive ore lenses grade into Cu-rich stockwork vein ore (Cu/Ni > 100, Pd/Pt ≈ 2) that partly sits in olivine cumulates and partly in flanking komatiitic lavas. The sulfide deposit is underlain by tens of meters thick anhydrite-carbonate rock unit that represent a Paleoproterozoic meta-evaporite based on carbon and sulfur isotope values. Sulfate-bearing sediments are a legitimate source of sulfur for magmatic Ni-Cu-PGE deposits, as demonstrated by major Ni-Cu(-PGE) deposits, e.g., Noril‘sk-Talnakh (RUS), Bushveld and Uitkomst Complexes (RSA), Munali in Zambesi belt (ZMB) and Quill Creek Intrusive Complex in Slave Craton (CAN).

The ore and host rocks in Sakatti exhibit chaotic magma-sulfate interaction textures, salt crystal pseudomorphs, and crustal contamination trends that are evident in major and trace element geochemistry as well as in isotope geochemistry. The Sakatti deposit contains typical indicator minerals such as marialitic scapolite, albite, chlorapatite, pargasite and phlogopite-biotite resulting from the interaction of magma with sulfate evaporites and/or saline brines derived after dissolution of evaporites. Sulfur isotope ratio from different ore types is rather uniform (δ³⁴S_CDT c. 3-4 ‰) suggesting a mainly non-mantle source for sulfur and isotopically homogeneous sulfide phase, either due to homogeneous source composition or complete mixing of sulfur from multiple sources before its precipitation as sulfide. Assimilation of sulfate and conversion to a sulfide ore is a complex process likely facilitated by a co-existing fluid phase due to the degassing caused by assimilation. Recent melting experiments using Noril’sk picrite and sedimentary rocks have demonstrated how sulfate incorporation into mafic-ultramafic magmas by diffusion does not unequivocally lead to precipitation of sulfides but instead to a sulfate-rich magma, that further requires interaction with a reductant such as carbon-rich sediments, before sulfides may precipitate. Recognizing the role of sulfates in formation of a magmatic sulfide ores is detective work, as pristine sulfate evaporites tend to vanish from the geological record in structurally complex Precambrian metamorphic terrains, leaving only cryptic marks to the mineral deposits, including skarns, Cl-alterations, pseudomorphs, breccias and heat-resistant sulfates (e.g., anhydrite).