Hydrothermal co-enrichment of Au, As, Co, and Cu in orogenic deposits: insights from the Pohjanmaa belt

Abstract

The Huhta Au and Jouhineva Au-Co-Cu-Ag deposits lie ~ 3 km apart in the Pohjanmaa Belt, western Finland, in Paleoproterozoic metavolcano-sedimentary host rocks. Both deposits are spatially and genetically related to the Sievi shear zone but have different metal endowments. The main regional structural grain is characterized by D 2 -D 3 fold interference and NW-SE to NNW-SSE-trending D 3 shear zones. Huhta hosts auriferous D 3 quartz-arsenopyrite veins, while Jouhineva contains auriferous, Co-bearing D 3 arsenopyrite-actinolite-quartz, and D 4 quartz-chalcopyrite veins with native gold. Uranium–Pb titanite ages of ~ 1830 Ma in D 3 veins and alteration zones align with regional metamorphism. Gold mineralization postdates the intrusion of the nearby Rautio Batholith and peak metamorphism by ~ 50 m.y. The auriferous D 4 veins formed during progressive terrane exhumation, likely at ~ 1805 Ma, contemporaneously with orogenic gold deposits in greenstone belts further south. Accretionary and collisional tectonics during the Svecobaltic orogeny represent the regional control for deformation and hydrothermal activity. Lack of precision in the titanite U–Pb data does not allow resolution between the absolute timing of orogenic mineralization at Jouhineva and Huhta, however, structures, veins, and alteration assemblages are similar, except for cobaltite in Jouhineva. This indicates contemporaneous formation of typical and atypical orogenic gold deposits in the region. Cobalt enrichment restricted to Jouhineva may be explained by different pH and/or fO 2 during fluid-rock interaction or localized and compartmentalized fluid migration of different fluids in the Sievi shear zone system. Cobalt and Cu enrichment in Jouhineva are genetically and temporally unrelated, with the Cu-Au event being ~ 25 m.y. younger.

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