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Genesis of sulphide vein mineralization at the Sakkatti Ni-Cu-PGE deposit, Finland
Fröhlich, F.; Siikaluoma, J.; Osbahr, I.; Gutzmer, J.
The Sakatti Ni-Cu-platinum-group element deposit is situated in northern Finland and comprises massive, disseminated, and
vein sulfide mineralization. A stockwork is formed by chalcopyrite-rich sulfide veins, which contain exceptionally high
platinum-group elements and Au grades. The mineralogy and geochemistry of this stockwork zone ore is documented in this
investigation. The results are used to develop the first robust genetic concept and its relationship to massive and
disseminated mineralization of the Sakatti deposit. This model is similar to that proposed for many Cu-rich magmatic sulfide
ores, most importantly the Cu-rich footwall veins described from the Sudbury Complex in Canada and the Cu-rich ore at
Noril’sk-Talnakh in Russia. Detailed petrographic studies using a sample suite from exploration drill core intersecting veinstyle
mineralization revealed a classic magmatic sulfide assemblage of chalcopyrite 6 pyrrhotite, pentlandite, and pyrite.
More than 1000 platinum-group mineral grains belonging almost exclusively to the moncheite (PtTe2) – merenskyite (PdTe2)
– melonite (NiTe2) solid solution series were identified in the studied samples. Notably, almost two thirds of the platinumgroup
element-bearing minerals consist of melonite. Some of the platinum-group minerals contain inclusions of Ag-rich gold
(AgAu2) and muthmannite (AuAgTe2). Most of the platinum-group minerals occur as inclusions in chalcopyrite, although a
few grains are located at base-metal sulfide grain boundaries and in fractures in base-metal sulfides. The whole-rock
compositions of the stockwork veins are Cu-rich and are interpreted to represent a fractionated Cu-rich sulfide liquid
enriched in Pt, Pd, Au, Ag, As, Bi, Pb, Se, Te, Zn, which separated from a monosulfide solid solution (mss). An intermediate
solid solution (iss) solidified from the Cu-rich sulfide liquid, recrystallizing chalcopyrite at,550 8C. Simultaneously, small
volumes of intercumulus residual melt contained mainly the precious metals, Bi, and Te due to their incompatibility in iss.
Solitary and composite platinum-group minerals as well as Au-minerals crystallized first from the residual melt (,600 8C),
followed by a succession of various Bi-, Ag-, and Pb-tellurides (~540 8C), and finally sphalerite and galena. Melonite
crystallized as mostly large, solitary grains exsolved directly from Ni-bearing intermediate solid solution (~600 C), shortly
after the formation of moncheite and merenskyite from the residual melt. Finally, remobilization of the platinum-group
minerals occurred at temperatures of,300 C, as suggested by the presence of minor amounts of Cl-bearing minerals and
ragged grain shapes. © 2021 Mineralogical Association of Canada. All rights reserved.
Keywords: Geochemistry; Mineral Liberation Analysis; Ni-Cu-PGE deposits; PGM; Sakatti
The Canadian Mineralogist 59(2021)6, 1485-1510