Isotope records of carbon, oxygen and sulfur reveal the role of sulfate evaporites in the formation of Sakatti Cu-Ni-PGE sulfide ore, Central Lapland Greenstone belt, Finland

Abstract

Stable isotope ratios of carbon, oxygen and sulfur have been used since the 1950́s to understand the formation of magmatic and hydrothermal ore deposits. They are particularly useful in tracing the origin and differentiation of phases involved in ore-forming processes. For many sulfide ore deposits, tracking the source of sulfur is important for decoding the genesis of a deposit and to understand the mineralizing systems regionally and globally. We studied the isotopic composition of carbon (δ¹³C), oxygen (δ¹⁸O) in wall rock carbonates, and sulfur (δ³³S and δ³⁴S) in sulfide ore and wall rock sulfates in the Paleoproterozoic Sakatti Cu-Ni-PGE sulfide deposit in the Central Lapland Greenstone Belt, northern Finland. The results suggest emplacement of a komatiitic olivine cumulate body into a c. 2.2–2.05 Ga evaporite sequence, which appears to have provided most of the crustal sulfur in the Sakatti sulfide ore body. Mechanisms of the incorporation of sulfate sulfur to mafic-ultramafic magmas have been recently advanced and recognized in many base-precious metal camps. Tracking the prior presence of evaporites can be cryptic, as they tend to be camouflaged in the geological record over lengthy time scales. Assimilation of sedimentary sulfur typically shifts sulfur isotope signatures away from mantle values towards the values of assimilated crustal sulfur. However, the shift in sulfur isotope signature of the resulting sulfides can be surprisingly subtle, only showing small deviations from mantle values. Sulfur isotopes can also be homogenized by the presence of coexisting fluids leading to a well-mixed δ³⁴S signature, as demonstrated by the limited range of the bulk of the δ³⁴S values (2-4 ‰) measured from the Sakatti deposit. The assimilation of evaporitic material might also play an important role in the formation of Cu (± Pd ± Au ± Ag)-dominated ores in many other magmatic Ni-Cu-PGE deposits.