Cobalt enrichment at the Juomasuo and Hangaslampi polymetallic deposits, Kuusamo Schist Belt, Finland: a role for an orogenic gold fluid?

The Juomasuo (Co-Au) and Hangaslampi (Au-Co) deposits are located in the Kuusamo Schist Belt, part of the 1.9-1.8 Ga Svecokarelian Orogenic Belt, central Finland. The deposits are hosted by metasedimentary and mafic rocks, and structurally controlled by the F-2 Kayla-Konttiaho Antiform. Hydrothermal alteration is spatially zoned, from early albite through biotite and then chlorite and late muscovite alteration. Pipe-like chlorite-quartz-pyrrhotite bodies plunge parallel to the F-2 fold axis and are cut by tabular quartz-muscovite-pyrite gold lodes in the S-2 axial plane orientation. The chlorite alteration zone contains 10-40 vol.% pyrrhotite containing approximately 0.25 wt% Co but whole-rock Co grades are enhanced by the late-stage and heterogeneous enrichment of minor pyrite (up to 3% Co) and introduction of cobaltian pyrite and cobalt pentlandite. Cobaltite formed where muscovite replaces chlorite within metres of the gold lodes. Cobaltite and Co-enriched pyrite formed because Co and S were mobilized from pyrrhotite in relatively S-poor gold lodes by an As-rich gold ore fluid, during late-D-2. Potassium, cobalt, sulfur and arsenic were transported into the mainly chlorite-rich wallrocks, via veinlets and fractures, by the now depleted gold ore fluid. Approximately 5 vol.% pyrite in the gold lodes displays extremely variable cobalt contents, which is interpreted to reflect mobility of Co during formation of the gold lodes. The origin of the gold ore fluid is enigmatic but is most plausibly an orogenic gold fluid, despite the paucity of carbonate minerals in the gold lodes.