A Paleoproterozoic Gold -Cobalt Resource in Northern Finland

Abstract

The Paleoproterozoic Peräpohja Belt in Finnish Lapland has a particularly long depositional history commencing at approximately 2.4 Ga and continuing through to approximately 1.9 Ga. It includes evidence for significant episodes in earth history, in particular, the “Great Oxidation Event” (GOE) and following this, a phenomenal era of deposition of carbonaceous material. Superimposed on the sequence created during this period are igneous, metamorphic, deformational and hydrothermal events leading to the complex array of compositionally diverse rocks.

Within the project area the protoliths to amphibolite facies rocks are interpreted to include thick sulphidic organic rocks mixed with sequence comprising clastic metasediments, marls, carbonates, metabasalts, komatiitic mafic volcanics and Mg-rich intrusives. Strong variations in redox state are evident within the isoclinally deformed package.

The gold-cobalt mineralisation is enclosed by an alteration halo that is distinctive, however varies depending on bulk composition of the protolith. In addition, both brittle and ductile structures are evident – a function of both the alteration mineral assemblages and the geometric relationship between the dominant planar structures of the poly-deformed amphibolite facies host rocks.

The shape of the Au-Co inferred mineral resources is dominated by the intersection of stratabound host rocks including muscovite-biotite schists (“K-Fe” type) and anthophyllite-chlorite rocks (“Mg-Fe” type) with a sub-vertical linear structural controls producing a high grade gold and cobalt envelope with a surprisingly constant orientation. Pyrrhotite, cobaltite, scheelite and native gold are the key non-silicate minerals within the resource, with subordinate cobalt pentlandite, linnaeite and pyrite. Silicate host minerals include muscovite/phengite, biotite, anthophyllite, chlorite, tourmaline and quartz. The structural hanging wall to the K-Fe type sulphidic high-grade Au-Co is dominated by altered rocks including a thick package of white to grey albitites which is in turn overlain by a grey-green-pinkish actinolite-chlorite-epidote-hematite calcschist. This alteration halo is in many ways similar to that found around classic porphyry copper systems.

The gold-cobalt mineralisation is a pyrrhotite-dominated system giving a characteristic geophysical response. It is conductive, chargeable and has a strongly negative remanent magnetic character. As more than 99% of the area is covered with till and fluvioglacial outwash material, these geophysical properties are particularly relevant to near-surface exploration.

Acknowledgement

I would like to acknowledge the assistance of the great team of Mawson geologists and fellow researchers Ferenc Molnár (Geological Survey of Finland, GTK) and Alan Butcher (GTK), Tony Prave (University of St Andrews) and Jukka Pekka Ranta (University of Oulu).

Biography

Nick Cook is an exploration geologist leading a Finnish team building gold-cobalt resources in Lapland. Nick has been exploring in this part of the world over the last seven years. Prior to an interlude running non-geological business, Nick was an exploration geologist working in eastern Australia in copper-gold and zinc-lead systems. Teaching at UNE (Armidale) coincided with a PhD on metamorphosed evaporites in the Olary Block, South Australia. With initial training from the University of Otago, Nick is a strong All Blacks supporter having watched the last 2 world cup finals in Windhoek and Lohijärvi and hoping for a third in Rovaniemi!