Evidence of gold related to Neoarchean alkaline magmatism in the Abitibi greenstone belt (Canada) from mineral parageneses and microscale trace element geochemistry on pyrite

Abstract

Most of Archean gold deposits are classified as orogenic gold systems (OGS), precipitated by metamorphic fluids during compressional deformation at convergent plate margins. However, Neoarchean gold deposits are also found spatially related to post-collisional alkaline intrusions such as syenites, suggesting a genetic link between gold and magmas. To bring novel evidence on this relation, ore parageneses related to Bachelor/O’Brien, Beattie, Douay and Golden Arrow gold deposits were studied together with trace elements distribution at micron-scale in Au-bearing pyrites. Further, the δ34Spyrite signature in the Beattie deposit were compared to epithermal Phanerozoic analogues in post-collisional tectonic settings. Except for Golden Arrow, results show a paragenetic assemblage of K-feldspar-albite-anhydrite-barite-celestite-apatite-rutile-calcite-dolomite which is also common in Phanerozoic magmatic-hydrothermal systems. Gold-bearing pyrites are characterized by a higher abundance of most trace elements compared to OGS. Positive Au-Ag-Bi-Te-Tl-W-anomalies – which are not observed in OGS – suggest deposition from highly oxidized fluids, while Au precipitation in OGS is mainly controlled by reduced fluids. The presence of highly oxidized environments is also confirmed by very low δ34Spyrite values ranging from −17.3‰ to −13.9‰ VCDT found at Beattie, in the range of those found in Phanerozoic deposits of Tuvatu and Emperor in Fiji. It is proposed that Archean gold related with late alkaline continental magmatism is deposited both at the top of porphyritic system and in epithermal environment, before the acme of deformation and metamorphism.