Structural control, magmatic-hydrothermal evolution and formation of hornfels-hosted, intrusion-related gold deposits: Insight from the Thaghassa deposit in Eastern Anti-Atlas, Morocco

Abstract

In the Moroccan Eastern Anti-Atlas, the Thaghassa intrusion-related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the Ikniwn granodiorite. Field studies reveal three tectono-magmatic stages controlling the formation of the deposit. i) The first stage refers to the top-to-the-south asymmetry and the syn-kinematic Ikniwn pluton emplacement controlled by a compressional or transpressional strain regime. ii) The second stage is characterized, from older to younger and further away from the intrusion, by: metatexite with leucocratic stromatic bands, aplo-pegmatite sills, intermediate veinlets composed of quartz, K-feldspar and muscovite, and then gold-bearing striped foliation-veins. All these features are assumed to have been emplaced during a large-scale ENE-WSW dextral shearing process that results from an ESE-WNW shortening direction during transtensive tectonics. We suggest that the progressive and continuous shearing was initiated since the aplo-pegmatite stage and achieved during the hydrothermal phase. The existence of intermediate veins characterized by quartz-rich core and apatite-muscovite-feldspar-rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character, at least until the onset of the hydrothermal process. iii) The late stage developed large volcanic dyke swarm and brittle faulting.Zircon U-Pb LA-ICP-MS dating yields a Concordia age of 563.5 ± 6.3 Ma for the Ikniwn granodiorite intrusion. The fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted: i) a hot aquo-carbonic (H2O-NaCl-CO2) fluid with N2 and CH4, evolving from vapour-rich N2 and CH4 inclusions for the magmatic stage (∼550 °C) to CH4-CO2 biphased inclusions for the ongoing hydrothermal stage (∼450–300 °C) and ii) an always secondary low T (∼200 °C) saline aqueous type (probably NaCl) free from volatiles, with very variable salinity. Eventually, we highlight that in the hornfels-hosted IRG deposits, fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks.