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Whole-rock and mineral compositional constraints on the magmatic evolution of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland

Luolavirta, Kirsi, Hanski, Eero, Maier, Wolfgang and Santaguida, Frank 2018. Whole-rock and mineral compositional constraints on the magmatic evolution of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland. Lithos 296-29 , pp. 37-53. 10.1016/j.lithos.2017.10.015

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Abstract

The ~ 2.06 Ga mafic-ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt. The lower ultramafic part of the intrusion hosts a large disseminated Ni-Cu-(PGE) sulfide deposit with Ni tenors ranging widely from < 4 wt% (uneconomic false ore and contact mineralization) to 4–7 wt% (regular ore) and up to 40 wt% (Ni-PGE ore). The stratigraphy of the ultramafic cumulates is divided into the basal pyroxenite–gabbro (Basal series), olivine pyroxenite (OLPX), pyroxenite, and plagioclase-bearing (ol) websterite (pOLWB), of which the latter occurs together with minor microgabbros in the ore-bearing domain of the intrusion. Around the ore domain, the ultramafic cumulate succession records a simple lithological stratigraphy and modest and predictable variations in whole-rock and mineral compositions. The ore-bearing domain, in contrast, is characterized by a complex internal architecture, variations in whole-rock and mineral compositions, and the presence of numerous inclusions and xenoliths. The OLPXs are mainly composed of cumulus olivine (Fo77–89) and clinopyroxene (Mg#81–92) with variable amounts of oikocrystic orthopyroxene (Mg#79–84). They comprise the bulk of the ultramafic cumulates and are the dominant host rocks to the sulfide ore. The host rocks to the regular and false ore type are mineralogically and compositionally similar (Fo~ 80–83, mostly) and show mildly LREE-enriched REE patterns (CeN/YbN ~ 2), characteristic for the bulk of the Kevitsa ultramafic cumulates. The abundance of orthopyroxene and magnetite is lowest in the host rocks to the Ni-PGE ore type, being in line with the mineral compositions of the silicates, which are the most primitive in the intrusion. However, it contrasts with the LREE-enriched nature of the ore type (CeN/YbN ~ 7), indicating significant involvement of crustal material in the magma. The contrasting intrusive stratigraphy in the different parts of the intrusion likely reflects different emplacement histories. It is proposed that the Kevitsa magma chamber was initially filled by stable continuous flow (“single” input) of basaltic magma followed by differentiation in an at least nearly closed system. In the following stage, new magma pulses were repeatedly emplaced into the interior of the intrusion in a dynamic (open) system forming the sulfide ore bodies. To gain the peculiar compositional and mineralogical characteristics of the Ni-PGE ore type, the related magma probably interacted with different country rocks en route to the Kevitsa magma chamber.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Elsevier
ISSN: 0024-4937
Date of Acceptance: 18 October 2017
Last Modified: 16 Feb 2018 10:32
URI: http://orca.cf.ac.uk/id/eprint/108266

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