Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

he Sakatti Cu-Ni-PGE deposit is situated in northern Finland and was discovered by Anglo American Exploration in 2009 (Halkoaho 2014). It is said to be one of the most significant discoveries in Finland for more than a generation (Maier et al. 2015). The mineralization comprises massive, disseminated and vein sulphides. A stockwork is formed by the sulphide veins, which contains exceptionally high grades of Cu, platinum-group elements (PGE) and Au in the shallow eastern part of the deposit. In contrast to the massive and disseminated sulphide mineralization, the mineralogy, mineral paragenesis and petrography of the stockwork zone is relatively poorly uninvestigated, and this study aimed at increasing knowledge of it.
Traditional light microscopy and automated scanning electron microscope (SEM)- based image analyses using a mineral liberation analyser (MLA) on 20 samples from 11 drill holes revealed a classic magmatic sulphide assemblage of chalcopyrite ± pyrrhotite, pentlandite and pyrite. Additionally, 1,133 platinum-group mineral (PGM) particles belonging almost exclusively to the moncheite (PtTe2) – merenskyite (PdTe2) – melonite (NiTe2) solid solution series were discovered (Fig. 1 a, b). Notably, almost two-thirds of the PGE-bearing phases consist of melonite. Some of the PGM particles contain inclusions of electrum (AgAu2) and muthmannite (AgAuTe2), which possess an average of 0.25 wt% Au. For the first time at Sakatti, one particle of irarsite (IrAsS) was encountered. The PGM display a trimodal particle area distribution, which is a result of an increasing mineralogical complexity of the particles relative to their sizes. Most of the PGM are hosted in inclusions (76%) (mainly in chalcopyrite), whereas minor amounts are located at grain boundaries (19%) and in cracks (5%).
An enrichment of PPGE relative to IPGE was recognized in the sulphide veins, which is in line with data on the massive and disseminated sulphide mineralization at Sakatti (Ahvenjärvi 2015). However, the di erence between maximum and minimum enrichments is more pronounced for the sulphide veins.
Element ratios calculated from geochemical data obtained by ICP-MS/OE analyses and displayed in various plots (e.g. Cu/Ir vs. Ni/Pd, Rh vs. Rh/Cu (Fig. 2)) indicate that the Cu-rich sulphide veins represent the magmatic crystallization products of a highly fractionated Cu-rich sulphide liquid enriched in Pt, Pd, Au, Ag, As, Bi, Pb, Se, Te and Zn, which separated from monosulphide solid solution (mss). Intermediate solid solution (iss) solidi ed from the Cu-rich sulphide liquid, exsolving chalcopyrite at <550 °C. Simultaneously, small volumes of intercumulus residual melt mainly contained the precious metals Bi and Te due to their incompatibility in iss. Solitary and composite PGM, as well as Au minerals crystallized rst from the residual melt (<600 °C), followed by a succession of various Bi, Ag and Pb tellurides (~540 °C), and nally sphalerite (ZnS) and galena (PbS). However, melonite crystallized in two stages: as large, solitary grains directly exsolved from Ni-bearing iss (~600 °C) shortly after the formation of moncheite and merenskyite from the residual melt, and during subsolidus exsolution (<600 °C) with precipitation on the surfaces of earlier formed PGM. Finally, slight remobilization of the PGM occurred at temperatures <300 °C, veri ed by minor amounts of Cl-bearing minerals and ragged particle shapes. Surprisingly, the geochemical data (Fig. 2), petrography and genetic concept of the sulphide veins at Sakatti proved to be very similar to the Cu-rich footwall veins at the McCreedy East deposit in Sudbury (Canada) (Naldrett et al. 1999, Dare et al. 2014).