The lithogeochemistry of the Cu-rich disseminated ore and further geochemical characterization of the massive ore, Sakatti Cu-Ni-Co-PGE deposit, Central Lapland Greenstone Belt, Finland

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Title: The lithogeochemistry of the Cu-rich disseminated ore and further geochemical characterization of the massive ore, Sakatti Cu-Ni-Co-PGE deposit, Central Lapland Greenstone Belt, Finland
Author: Barron, Pelayo
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Thesis level: master's thesis
Abstract: Sakatti is one of the most significant magmatic Ni-Cu-PGE deposits discovered in the last decade. With a reported 44.4 Mt resource, is a polymetallic deposit with grades of: 1.90 % Cu, 0.96 % Ni, 1.40 g/t PGE; Anglo American ltd. report (2019). Sakatti is located within the Early Proterozoic Central Lapland Greenstone Belt (CLGB), Finland. The deposit is hosted by three ultramafic magma–derived olivine cumulate bodies: Main body, North-East body and South-West body. Very distinctive ore types can be recognized according to several academic studies: A) Massive ore containing an average 3.04 % Ni and 7 % Cu and showing a wide range in Ni/Cu (average of 1.42) and Pt/Pd of 0.98 (Ahvenjärvi 2015); B) Stockwork ore that is extremely copper-rich containing an average 0.68 % Ni and 26.17 % Cu, with Ni/Cu of 0.03 and Pt/Pd of 0.85 (Fröhlich 2016); C) Disseminated ore, where sulfides form a wide halo around the massive and stockwork ores, again being highly copper-dominated containing an average 0.07 % Ni and 0.61 % Cu. Ni/Cu and Pt/Pd values for disseminated ore are 0.13 and 1.83 respectively. The aim of this study is to shed some light on the genesis of the Sakatti´s disseminated ore and the massive sulfides from the NE and SW bodies. The disseminated mineralization was studied more in depth. Moreover, a comparison with other major Ni-Cu-(PGE) magmatic deposits in the world was done. Overall the disseminated ore seems to be dominated by a patchy texture with low connectivity but high wettability. Chalcopyrite is the predominant sulfide mineral and forms intergrowths with the texturally earlier pyrrhotite and pentlandite. These primary phases are widely altered to secondary phases like marcasite, millerite, violarite, pyrite, bornite, covellite and magnetite. Only a few platinum-group minerals (PGM) were found. They all are tellurides or bismuth-tellurides of which the merenskyite-moncheite-melonite series minerals are the most abundant. The chemical composition of the disseminated ore revealed compositional and fractionation similarities with both the massive and stockwork ores. When recalculated to 100 % sulfides and normalized to mantle values, the disseminated ore showed a moderate content in Ni, Co, IPGE and Rh close to the massive mineralization, and a higher enrichment in PPGE, Au and Cu with similar evolution patterns as the stockwork vein sulfides. This led to a fractionation path of the disseminated sulfide phase that seemed to be a mixture of the other two main ore types. Thus, it is suggested that the disseminated ore is formed by a combination of monosulfide solid solution (MSS) and intermediate solid solution (ISS), which originated from a sulfide melt genetically linked to the one that gave rise to the massive and stockwork ores. The massive sulfides from the NE and SW bodies show compositional similarities with the massive ore from the Main body that suggest a share origin and genesis. The Oktabr'sky, Noril'sk-Talnakh, disseminated ore, when normalized to mantle values, shows similar Ni, Co, PGE, Au and Cu distribution as the Sakatti´s disseminated sulfides. Moreover, the Oktabr'sky deposit seems to have similar S/Se vs Pt+Pd evolution trend to the one from Sakatti deposit.
Subject: Sakatti
Disseminated ore
Massive ore
Cu-Ni-Co-PGE magmatic deposit
Central Lapland Greenstone Belt.
Discipline: none

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