JultikaUniversity of Oulu repository

Abstract

The 2.06 Ga old Kevitsa mafic-ultramafic intrusion hosts a magmatic, conduit-type Ni-Cu-(PGE) sulfide deposit. The intrusion is located in the Central Lapland Greenstone Belt (CLGB), in northern Finland. It is one of Finland’s largest open pit mine owned by Boliden with total mineral resources (inclusive mineral reserves) reaching 277.7 Mt by the end of 2019. The base and precious metal grades at Kevitsa comprise 0.25 g/t of nickel and 0.34 g/t of copper along with significant amounts of platinum ~0.19 g/t, palladium ~0.12 g/t, gold ~0.1 g/t and cobalt ~0.01 g/t. Ore types of the Kevitsa deposit are classified and grouped into three major categories : (i) the Normal ore accounting for about 90–95% of the global resources with average Ni and Cu metal content of 0.3 g/t and 0.42 g/t, respectively; (ii) the Ni-PGE ore with extremely high Ni content of > 10 g/t, lower Cu content but elevated platin and palladium concentrations, and (iii); the False ore with low Ni metal content < 0.1 g/t. All of the ores occur as disseminated sulfides throughout the ore body. Normal ore accounts for about 90–95% of the total resources, while the False ore is uneconomic.

In this study, the trace-element characteristics of the main silicate minerals, olivine, clinopyroxene and plagioclase, are utilized to provide insights into the genesis of the Kevitsa sulfide ore and ore types. In particular, the data is used to investigate the possible role of the black shales as a contaminant and source of incompatible trace-elements. Analysis were carried out using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), at GTK, Espoo. Results show that the plagioclase and clinopyroxene within the Ni-PGE ore type are significantly enriched in LREE, in comparison to the plagioclase and clinopyroxene within the Normal and False ore. The REE signatures of the Kevitsa silicate minerals are comparable to that of the black shales suggesting that Kevitsa magma may well have assimilated the black shale. It’s been approximated that about 10% on partial melting of the black shale is required to produce the LREE-enriched characteristics of the Ni-PGE ore type. Furthermore, no correlation is observed between the elevated Ni content of the silicate minerals in the Ni-PGE ore and their elevated LREE contents, indicating a different source of metals (Ni) and trace-elements to the magma. Lastly, based on the approximated of the oxidation state of europium in the silicates of the Normal and Ni-PGE ore, both ore types likely crystallized from a magma with similar oxygen fugacity.