Bissa Gold mineralization in the Sabcé district, Goren belt (Burkina Faso, West African Craton): implication of arsenic in the concentration of visible and invisible gold in pyrites

The Bissa orogenic gold deposit is located at the contact between basic (basalt) and felsic (dacite) volcanic formations through intermediates (andesite) and graphite-intercalated sandstone-pelitic sedimentary formations. This green schist facies metamorphosed ensemble is hosted by the 15-km-long Sabcé shear zone within the Goren greenstone belt. Field investigations combined with fine microscopy enable us to define the mineralization host formations, deformation phases, and hydrothermal alteration in the Sabcé zone. The host formations (metabasalt, metasediments, metagabbro, metadacite) are affected by two deformation phases. The first deformation phase, D_1B, is a shear-type deformation that globally controls mineralization. It is taken up by the D_2B, which has a ductile-breaking deformation. The hydrothermal alteration with which the mineralization is associated is white quartz-mica-pyrite ± carbonate ± arsenopyrite, crystallization of which was manifested by veins/veinlets subparallel to parallel to D_1B. Various electron microprobe, scanning electron microscope, and LA-ICP-MS analyses show that visible gold is present in type I and II pyrites as inclusions or as microfracture fillers. LA-ICP-MS shows that in type I and II pyrites, where no visible gold is present, invisible gold is present in their distorted crystal structure as nanoparticles. The chemical association between Au-As controls the distribution of this couple in type I and II pyrites. This clearly shows that the limpid parts of arsenic-rich pyrites are where visible and invisible gold like to concentrate.