Multi-stage and multi-sourced fluid and gold in the formation of orogenic gold deposits in the world-class Mana district of Burkina Faso – Revealed by LA-ICP-MS analysis of pyrites and arsenopyrites

Abstract

Mana is a world-class Paleoproterozoic orogenic gold district (∼8.0 Moz) that includes five gold deposits (Fofina, Nyafé, Siou, Wona-Kona and Yaho) hosted in various rocks in the northern part of the Birimian Houndé greenstone belt, western Burkina Faso. The gold deposits recorded different styles of mineralization associated with sulfides during polyphase deformation events. Detailed petrographic and LA-ICP-MS analyses of pyrites and arsenopyrites from the main mineralization of each deposit provide new information on the potential gold and arsenic sources and timing of fluids responsible for mineralization at Mana. Four hydrothermal fluid events associated with two distinct source reservoirs and at least two hydrothermal systems were recognized. The first hydrothermal fluid event was generated during the Eoeburnean D1MD (MD for Mana district) shortening event (∼2172 Ma), evidence of which is preserved in the Fofina and Siou deposits, and occurred under greenschist metamorphic facies conditions. Economic gold deposits at Fofina are associated with metamorphic disseminated pyrite (Py1F) in black shale, zoned pyrite (Py2F) enriched in As, Ni, Cu, Ag, Sb, W, Au and Pb, and arsenopyrite in folded V1F quartz-carbonate veins. In contrast, the Siou deposit exhibits Au in porous pyrite cores (Py1S), which are associated with Ag, Bi, Te and W (±Sb) in V1S quartz veins hosted in the Siou granodiorite. These two trace element suites are interpreted as having been induced by a metamorphic source in the Fofina deposit and by a local contribution of a magmatic source in the Siou deposit. The second hydrothermal fluid event occurred during the Eburnean D3MD transpressional event (2113–2090 Ma) in the Nyafé deposit. This event is defined by Au-As-rich rims in hydrothermal pyrite (Py3N) and by the co-precipitation of Au-rich arsenopyrite in the V3N quartz veinlets (V3N) controlled by a second-order dextral shear zone. The third and main hydrothermal fluid event during the D4MD transpressional event (∼2090 Ma) is characterized by the development of strong silicification and by zoned pyrites (Py3WK and Py2Y) and Au-rich arsenopyrite in the Wona-Kona and Yaho deposits. However, the same late Au-As-rich rims in hydrothermal pyrite are observed in both deposits and in the Nyafé deposit, and these rims are interpreted to be related to the same economic gold source related to the Lower Birimian metamorphic reservoir, composed of black shale (diagenetic framboidal pyrite) and tholeiitic basalts. The diagenetic framboidal Py1WK in black shale (Lower Birimian group) is enriched in Au (median of 0.8 ppm), low in As (median of 70 ppm), with other trace elements (Ni, Cu, Zn, Ag, Sb and Pb). The last hydrothermal event is associated with free gold remobilization in late microfractures in the Wona-Kona and Siou deposit during the reactivation of major transcurrent shear zones during the D4MD event or the D5MD shortening event (as late as 2022 Ma). A binary diagram of As/Ag vs. Sb/Bi appears to be effective at separating the diagenetic, magmatic and metamorphic pyrites and could be used in orogenic gold exploration to ascertain potential gold endowment at the district scale.