Mineralogy of the Paleoproterozoic Raja Au-Co prospect, northern Finland

Abstract

The Raja Au-Co prospect is located in the Paleoproterozoic Peräpohja Belt, northern Finland, within an approximately 10 x 10 km gold-enriched area (namely Rompas-Rajapalot) comprised of several gold-cobalt-bearing occurrences. The aim of this study was to describe the mineral assemblages and nature of gold in the Raja Au-Co prospect. Samples were collected from the drill cores representing mineralised and non-mineralised intervals. Silicate-, oxide-, and sulphide phases from polished thin sections were studied under transmitting and reflecting light using petrographic microscope. In addition, chemical compositions of silicates, oxides and sulphides were studied with an electron microprobe analyser (EPMA).

Samples from the mineralised intervals varied from intensely sericite-altered to chlorite-altered metasedimentary rocks. Generally, the mineralised area consists of rocks with albite-quartz matrix with anthophyllite-cordierite assemblages and high aluminum white mica. Only two gold grains were found from the studied samples, one within a chloritised amphibole and one associated with cobaltite. Co-bearing gersdorffite was identified in addition to the previously described Co-mineral phases (cobaltite, Co-pentlandite and linnaeite).

In terms of zoning, micas show compositional differences within and outside of the mineralisation, most notably with their aluminum content being higher in the mineralised rocks, consistent with the intense sericite alteration observed. Amphibole compositions ranges from Ca-amphiboles outside of the mineralisation to Mg-Fe-Mn amphiboles in the mineralised intervals. However,to fully characterise the zoning in the Raja Au-Co prospect, more detailed studies are needed.