Polyphase gold mineralization at the Yaou deposit, French Guiana

Abstract

Abstract The Yaou deposit, located in French Guiana within the Guiana Shield, is one of the most promising gold deposits of the regional Paleoproterozoic greenstone belt. It displays numerous quartz monzodiorite bodies aligned along a sinistral shear zone where a five-deformation phases model is established at the camp scale. The ductile D 1/2YA phase is responsible for the main penetrative foliation while the D 3YA phase is related to shearing. An intrusive event is identified as being pre to syn-D 3YA . The following phase D 4YA represents a brittle quartz-carbonate veining set hosted preferentially within intrusive bodies and along the shear zone. A local D 5YA brecciation event crosscuts the D 4YA veins. Among this deformation history, two auriferous events (D 3YA and D 4YA ) control the overall grade of the Yaou gold deposit. More specifically, most of the Au grade is associated with the main economic D 4YA veining event, where the gold is visible and linked to Py 4 within an ankerite/hematite rich alteration halo. At the microscopic scale, results of in situ analyses using LA-ICP-MS on pyrite show that metasediment-hosted Py 0 is a primary source of submicroscopic gold having a low contribution to the total endowment. Py 3 shows some gold content due to possible remobilization of Au D0YA . Gold in Py 4 is found as submicroscopic gold, as micro-inclusions and as infilling fractures in association with elements such as Te, Ag and Bi. Most contribution to the Au grade is from micro-inclusions and, to a lesser extent, from free and submicroscopic gold. The ore shoot locations are lithologically controlled for Au D0YA (metasedimentary unit-hosted), structurally controlled (shear zone-hosted) for Au D3YA and rheologically controlled for the Au D4YA (intrusion-hosted). The deposit is clearly polyphase both at the macroscopic and the microscopic scales, invisible gold is associated with As whereas visible gold is observed as inclusions in pyrite with high contents of Ag, Te and Bi. We define an early low-grade enrichment of Au D0YA to Au D3YA followed by a later high-grade event, Au D4YA supporting polyphase mineralization processes. This study confirms that orogenic gold deposits can be formed by remobilization and/or new gold inputs during multiple deformation, veining and hydrothermal events.