University of Oulu

M. Moilanen, E. Hanski, J. Konnunaho, S.-H. Yang, T. Törmänen, C. Li, L.M. Zhou, Re-Os isotope geochemistry of komatiite-hosted Ni-Cu-PGE deposits in Finland, Ore Geology Reviews, Volume 105, 2019, Pages 102-122, ISSN 0169-1368, https://doi.org/10.1016/j.oregeorev.2018.12.007

Re-Os isotope geochemistry of komatiite-hosted Ni-Cu-PGE deposits in Finland

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Author: Moilanen, M.1; Hanski, E.1; Konnunaho, J.2;
Organizations: 1Oulu Mining School, P.O. Box 3000, FI-90014 University of Oulu, Finland
2Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi, Finland
3Key Laboratory of Re-Os Isotope Geochemistry, Chinese Academy of Geological Sciences, Beijing 100037, China
Format: article
Version: accepted version
Access: embargoed
Persistent link: http://urn.fi/urn:nbn:fi-fe2019042913527
Language: English
Published: Elsevier, 2019
Publish Date: 2020-12-12
Description:

Abstract

We have studied the Re-Os isotope systematics of komatiite-related Ni-Cu-PGE sulfide deposits in Finland, including the Archean Ruossakero Ni-(Cu), Vaara Ni-(Cu-PGE), Tainiovaara Ni-(Cu-PGE) deposits and Ni-PGE sulfide mineralization related to a dunitic body at Tulppio, and the Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. We analyzed chromite separates from both mineralized and non-mineralized komatiitic rocks, bulk-sulfide separates, and country-rock black schist from one deposit, and for comparison, chromite separates from non-mineralized Archean komatiitic rocks from the Kovero and Kuhmo greenstone belts.

We have observed open-system behavior of sulfides in three deposits, with their Re-Os isotope compositions indicating resetting at ca. 1.83–1.87 Ga by metamorphic events related to the Svecofennian orogeny. Most of the chromite separates, especially those with >10 ppb Os, retain their primary isotope compositions with near-chondritic initial ¹⁸⁷Os/¹⁸⁸Os ratios, indicating that their Os was derived from a mantle source with a chondritic Re-Os isotope evolution. These include the Vaara deposit, which yielded a chromite Re-Os isochron age of 2719 ± 140 Ma and low initial γOs of −1.2 ± 0.02, in spite of the evidence for the presence of mass-independent S isotope fractionation in this deposit, suggesting a low-Os, sulfur-bearing contaminant. In the case of the Lomalampi deposit, chromites yield an isochron with an age of 2058 ± 93 Ma and initial γOs of +0.9 ± 0.02, consistent with earlier isotope data from Central Lapland komatiites. However, one chromite from a mineralized cumulate gave a γOs(2050 Ma) value of +24 ± 18, indicating a minor/moderate degree of crustal contamination involving material from country rock black schists. This is also supported by earlier S isotope data. In the Archean Tainiovaara deposit, chromite also suggests crustal contamination (γOs + 13 ± 1), but country rocks are Os-poor Archean gneisses, leaving the source of radiogenic Os unclear, though it is possible that contamination occurred at a greater depth. Our results also provide evidence that the Re-Os age of the Ruossakero deposit is ca. 2.9 Ga, with the deposit likely representing komatiitic magmatism that is older than the komatiitic magmatism elsewhere in Finland. Re-Os results also confirmed that the mineralized Tulppio Dunite is Archean in age.

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Series: Ore geology reviews
ISSN: 0169-1368
ISSN-E: 1872-7360
ISSN-L: 0169-1368
Volume: 105
Pages: 102 - 122
DOI: 10.1016/j.oregeorev.2018.12.007
OADOI: https://oadoi.org/10.1016/j.oregeorev.2018.12.007
Type of Publication: A1 Journal article – refereed
Field of Science: 1171 Geosciences
Subjects:
Funding: This work was supported by the Academy of Finland (grant 281859) and K.H. Renlund Foundation.
Academy of Finland Grant Number: 281859
Detailed Information: 281859 (Academy of Finland Funding decision)
Copyright information: © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/.
  https://creativecommons.org/licenses/by-nc-nd/4.0/