Petrology of platinum-group element mineralization in the Koillismaa intrusion, Finland

Abstract: The 2.44 Ga Koillismaa intrusion of the Koillismaa-Näränkävaara Layered Complex, in northeastern Finland, has been studied in order to assess the formation processes involved during platinum-group element (PGE) mineralization. The layered series is host to the reef-type mineralization called Rometölväs Reef, and the marginal series contains contact-type mineralization. This MSc-project, besides giving an in-depth review of PGE mineralization in layered intrusions, tries to elucidate the origin of PGE mineralization in the Koillismaa intrusion, via SEM-EDS mapping of platinum-group minerals (PGM) and δ34S in-situ laser ablation of base-metal sulfides (BMS). The sulfide assemblages of both the Marginal Series (MS) and the Rometölväs Reef (RT) are solely disseminated in character. Most BMS are heavily altered and display a speckled porous appearance and are distinctly associated with hydrosilicates. The dominant host-phase of the PGM are silicates (MS 81% and RT 76%), whereas sulfide-related grains are significantly less prevalent. The PGM of the Marginal Series were, in order of abundance, (1) merenskyite-moncheite-melonite (PdTe2-PtTe2-NiTe2), (2) sperrylite (PtAs2), (3) kotulskite-sobolevskite-sudburyite (PdTe-PdBi-PdSb), and (4) keithconnite-telluropalladinite (Pd3-xTe-Pd9Te4). In the Rometölväs Reef the PGM were; (1) merenskyite-moncheite-melonite, (2) sperrylite, (3) kotulskite-sobolevskite-sudburyite, and (4) PGE-alloy. The findings are similar to other coeval Fennoscandian PGE mineralizations, suggested to have formed in a low-temperature hydrothermal setting, but contrasts with many of the global PGE occurrences of which are dominated by PGM sulfides and Fe alloys. The δ34S data obtained from Rometölväs Reef (-0.40 to +1.80‰) and the Marginal Series (-0.94 to +2.19‰) suggests that crustal sulfur played no significant role in the generation of the PGE mineralization of the Koillismaa intrusion. The PGE mineralizations are proposed to have been generated by hydrothermal fluids, either in a late magmatic stage, or later-stage during the Svecofennian orogeny. The Rometölväs Reef is proposed to have formed from a primary metasomatic event, in association with microgabbronoritic bodies. On the other hand, the Marginal Series is suggested to have formed in an orthomagmatic setting due to sialic contamination during interaction with the footwall.