Effects of hydrous alteration on the distribution of base metals and platinum group elements within the Kevitsa magmatic nickel sulphide deposit

- Study of the effect of low-temperature hydrothermal alteration on base-metals and PGEs.
- Use of the Kevitsa magmatic nickel sulphide deposit as a natural laboratory.
- Study combining a sample and deposit scale study of element fluxes.
- Results confirm a long standing assumption on the low mobility of Ni and PGEs in low-temperature hydrothermal fluids.

Platinum and nickel are commonly assumed to be immobile in most conditions, especially during low temperature hydrothermal alteration. However, only a small number of studies have rigorously tested this assumption. The Ni-Cu-(PGE) sulphide ore body hosted by the Kevitsa intrusion, northern Finland, provides a natural laboratory to study the behaviour of base metals and platinum group elements (PGE) during low temperature alteration. This ca. 2060 Ma mafic-ultramafic intrusion, located in the Central Lapland greenstone belt, hosts disseminated Ni-Cu-(PGE) sulphide mineralisation in the middle part of the main ultramafic body. The mineralisation, which contains a range of Ni, Cu and PGE grades, is affected by three main alterations (serpentinisation, amphibolitisation and epidotisation), and is cross cut by various types of veins. The effect of the circulation of hydrothermal fluids on the distribution of base metals and PGE was studied at two different scales. Interrogation of an extensive deposit-wide assay database provided information on the deposit-scale (kilometre scale) effect of these different alteration styles, and a detailed study, involving laboratory X-ray fluorescence (XRF), portable XRF and micro-XRF mapping, of drill-core samples containing cm-scale cross-cutting veins provided information on the small scale (centimetre to decimetre scale) remobilisation of base metals and PGEs. Results show that the hydration and carbonation of the Kevitsa mineralised mafic-ultramafic intrusion did not significantly affect the distribution of Ni and PGE at scales larger than a few mm, and that Cu and Au are the only metals that are affected by small to large scale remobilisation from centimetre to kilometre scale.