This thesis focuses on Archean (~2.8 Ga) (Tainiovaara, Vaara, Sarvisoaivi, Ruossakero, Hietaharju, Peura-aho) and Paleoproterozoic (~2.05 Ga) (Hotinvaara, Lomalampi) komatiite-hosted Ni-Cu-PGE deposits occurring in eastern and northern Finland, with an emphasis on their characterization, PGE content, and petrogenesis. Most of the studied deposits consist of disseminated sulfides (type II deposits), with some of them also containing massive sulfides (type I deposits) and/or post-magmatically modified sulfide concentrations (type IV and V deposits). Based on their metal contents, the Finnish deposits can be classified into two main groups: A) Ni-(Cu-PGE) deposits commonly related to komatiitic basalt to low-Mg komatiitic magmatism, as exemplified by the Vaara and Hietaharju deposits, and B) Ni-(Cu) deposits associated with high-Mg komatiites, such as the Ruossakero and Hotinvaara deposits. The deposits of the former group contain significant amounts of Ni, but are also enriched in Cu and PGE or, in rare cases, such as in the Lomalampi deposit, PGE are the most important metals. This deposit is also unique due to its high Pt/Pd (~2) compared to other deposits (~0.5). These kinds of PGE-enriched deposits are relatively rare among the komatiite-hosted sulfide deposits globally. Group B represents classic Ni-enriched deposits, which sometimes contain considerable amounts of Cu, but commonly only small amounts of PGE. The Finnish komatiite-related deposits that are associated with low-Mg komatiites and komatiitic basalts are commonly characterized by lower Ni/Cu, Ni/Co, (Ni+Cu)/(Pd+Pt), and Pd/Ir, and higher PGE and slightly higher Cu compared to the deposits related to high-Mg komatiites.
In general, the generation of komatiite-hosted sulfide deposits is linked to a high degree of mantle melting (>20%), which controls the distribution of chalcophile elements between the mantle residue and partial melts, and in many cases, the role of contamination and external sulfur has been important in the genesis of Ni-Cu-PGE ores. This thesis demonstrates that post-magmatic modification and the relatively high metamorphic grade of komatiitic rocks in Finland may limit the use of rare earth elements for the detection of a contamination signal. On the other hand, the Finnish Archean Ni-Cu-PGE deposits and their S-bearing country rocks record a varying degree of mass-independent fractionation of sulfur isotopes (MIF-S), demonstrating a significant role of the assimilation of external sulfur in ore formation, analogously with some other sulfide deposits in Canada and Australia. The PGE-enriched deposits that are associated with komatiitic basalts and low-Mg komatiites were commonly generated under a higher R factor (silicate/sulfide melt mass ratio), being up to 500 in the Lomalampi deposit, compared to high-Mg komatiitic Ni-(Cu) deposits. In some cases, such as the Vaara deposit, post-magmatic alteration processes have drastically upgraded the metal tenors as a result of S loss related to oxidation of the magmatic pyrrhotite-pentlandite-chalcopyrite assemblage to a secondary pyrrhotite-free pyrite-millerite-pentlandite-violarite-chalcopyrite assemblage.
Thick ultramafic cumulate sequences (e.g., channelized or conduit environments), and especially their Cr-poor olivine cumulates generated from high-Mg komatiites, are favorable targets for exploration, as indicated by several deposits in Finland. The occurrence of contamination signals, PGE depletion, and S-bearing country rocks significantly raises the exploration potential. In summary, Finnish komatiitic basalts to low-Mg komatiites are highly prospective for PGE-enriched sulfide deposits, and high-Mg komatiites for classical Ni-(Cu) deposits.
Viimeksi päivitetty: 5.2.2016