Re-Os isotope systematics and iron oxide compositions of the mafic-ultramafic Ni-Cu-PGE deposits in eastern and northern Finland

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium L6

Topic of the dissertation

Re-Os isotope systematics and iron oxide compositions of the mafic-ultramafic Ni-Cu-PGE deposits in eastern and northern Finland

Doctoral candidate

Master of Science Marko Moilanen

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Oulu Mining School

Subject of study

Geosciences

Opponent

Professor Tapani Rämö, University of Helsinki

Custos

Professor Kari Strand, University of Oulu

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Re-Os isotope systematics and iron oxide compositions of the mafic-ultramafic Ni-Cu-PGE deposits in eastern and northern Finland

Intrusive and extrusive rocks crystallised from primitive mantle-derived mafic-ultramafic magmas (e.g., komatiites, basalts, picrites) are important hosts for magmatic sulphide ore deposits of nickel, copper and platinum group elements (Ni-Cu-PGE). These metals together with cobalt (Co) are vital elements for modern society due to their use in electric cars, and especially in batteries, and therefore their demand has increased significantly over the last decade.

This PhD study comprises four research articles and is divided thematically into two parts. The first part focuses on the Re-Os isotope systematics of magmatic Ni-Cu-PGE deposits and their host rocks mostly from eastern and northern Finland and the second part on the trace element compositions of oxide phases in these deposits. Applying the Re-Os method to whole-rock and mineral separate samples, the following isochron ages were obtained for the studied Ni-Cu(-PGE) deposits: Re-Os isotope compositions indicate that the Ruossakero deposit is Archaean in age. The TRD model age of the Tulppio dunitic intrusion is 2680 ± 88 Ma, which is regarded as a minimum age for the ultramafic body. The Tainiovaara deposit also yielded an Archean Re-Os isochron age, 2753 ± 47 Ma. Similar Palaeoproterozoic Re-Os isochron ages were obtained for two deposits, 2063 ± 35 Ma for Sakatti and 2058 ± 93 Ma for Lomalampi, which are consistent with the age of the komatiitic-picritic volcanism in central Lapland. Most of the chromite separates from the studied Palaeoproterozoic and Archaean deposits show near-chondritic initial Os compositions, indicating that their Os is mostly derived from a mantle source with a long-term chondritic Re-Os isotope evolution. There are also indications that contamination with radiogenic Os is not reflected in the Os isotope signatures of the chromite minerals, but in contrast, two chromite samples from the Lomalampi and Tainiovaara deposits show moderately positive initial gamma-Os values, which indicates contamination with radiogenic Os. Sulphide and magnetite separates from the Sakatti deposit have radiogenic initial Os isotope compositions, suggesting a minor to moderate degree of contamination with crustal material. In contrast to these minerals, chromite separates yielded chondritic γOs values. This indicates that sulphur saturation took place after chromite crystallisation in the Sakatti ultramafic body. The study demonstrates that Re-Os isotopes can be used for dating, determining the source of Os and signs of contamination and post-magmatic alteration processes in magmatic Ni-Cu-PGE deposits.

In-situ trace element compositions were determined for iron oxides from the Ruossakero Ni-(Cu) deposit, Tulppio dunite and related Ni-PGE mineralisation, Hietaharju, Vaara, and Tainiovaara Ni-(Cu-PGE) deposits and Lomalampi PGE-(Ni-Cu) deposit. Based on their major element compositions from massive sulphide samples, oxides in these samples are both magnetite and Cr-magnetite. The abundances of individual trace elements in oxide minerals differ by two to three orders of magnitude as a reflection from the composition of the silicate or sulphide melt from which oxide grains were crystallised. Magnesium is lowest in magnetites from massive sulphide samples, suggesting that low Mg could be a diagnostic feature of an iron oxide phase crystallised together with Fe-rich MSS (monosulphide solid solution) in a microenvironment that was more or less isolated from the associated Mg-rich silicate liquid. The average nickel concentration in magnetite is a function of the average Ni tenor (in 100% sulphide fraction) of the associated sulphide phase for both magmatic and post-magmatic magnetite (e.g., oxidation product of sulphides). It was also discovered that it is possible to use iron oxide compositions in different discrimination diagrams and information on sulphide inclusions in oxide minerals (especially Ni-sulphide and/or PGE inclusions in magnetite) as an exploration tool and fingerprinting for magmatic Ni-Cu-PGE deposits. Especially in glaciated terrains in Fennoscandia, heavy mineral separates (e.g., magnetite and/or chromite) from till samples could be studied with this method.
Last updated: 30.11.2022