Anne-Sylvie André-Mayer

The Paleoproterozoic Copper‐Gold Deposit of Gaoua, Burkina Faso: Evidence for A Polyphased Mineralization

Due to increasing gold prices, gold exploration has intensified in the last 10 years, especially in the West African craton where many propects exist. Among these, the three copper-gold occurrences Dienemera, Gongondy and Mont Biri, of the Gaoua district (B2Gold), southern Burkina Faso, are thought to be part of a porphyric system (Sillitoe, 2007) but have not been so far subjected to detailed studies to confirm this hypothesis. This study presents mineralogical and fluid inclusion data for the mineralization and associated gangue phases that allow us to propose a metallogenic model for the formation of the Gaoua copper-gold system. In addition, the age of the mineralizing event was determined by Re-Os dating of sulfides.

Discussion: Age of the Zambian Copperbelt

Sillitoe et al. (2017) propose new robust Re-Os ages on molybdenite for 15 samples from seven deposits with both disseminated ore and ore in veins in the Zambian Copperbelt. The samples have been taken from twomain groups of rocks. Firstly, they analyzed molybdenite from several arenitehosted copper deposits (Chibuluma South, Mufulira; Selley et al. 2005) or from the siliciclastic footwall rocks (Chambishi South East). Secondly, samples originate from deposits characterized by metamorphic or hydrothermally altered rocks, consisting of garnet, clinozoisite, tremolite, and/or biotite (Lufubu South, Sentinel, Mujimbeji), or from actinolite-rich veinlets in upper Roan volcanic rocks (Konkola West). The arenite-hosted deposits in the Zambian Copperbelt post-date the most authigenic cement phases, and carbonaceous matter is present in both primary and secondary pores (Brandt et al. 1961; van Eden 1974), demonstrating their diagenetic origin and likely introduction as mobile hydrocarbons (Annels 1979). The geometry and relationship of the footwall and hanging wall sedimentary rocks to the ore zones indicate the ores are related to the geometry of subbasin bounding faults and terminate laterally against the margins of uplifted basement blocks (Selley et al. 2005). The characteristics of the arenite-hosted deposits, i.e., their occurrence and relative timing within the diagenetic paragenesis of the rocks, are comparable with hydrocarbon reservoir rocks and imply a late origin of the deposits (Selley et al. 2005), i.e., at least coinciding with basin inversion and the onset of Lufilian orogenesis. The dating of 540–490 Ma for these deposits confirms the age constraints based on earlier paragenetic studies and provides robust absolute ages. Metamorphism of the mineralized rocks at the different deposits analyzed by Sillitoe et al. (2017) is known to have taken place between 590 and 500 Ma (John et al. 2004; Rainaud et al. 2005; Eglinger et al. 2016; Turlin et al. 2016). Tremolite-actinolite alteration has been observed post-dating folding (Brems et al. 2009; Torremans 2016). So, a late Lufilian age for mineralization intimately associated with this mineralization is no surprise. In order to determine if an earlier phase of diagenetic mineralization occurred in the Zambian Copperbelt, the phases that are proposed as being characteristic for a hydrothermal diagenetic mineralization should be dated. The phases are nodules and lenses parallel to bedding in the Ore Shale mostly consisting of sulfides and carbonates (Annels 1974; Sweeney and Binda 1989). In some areas, the nodules and lenses exhibit chicken wire textures, indicative of the replacement of anhydrite (Sweeny et al. 1986). Fluid inclusion microthermometry of this first mineralizing phase reveals relatively low homogenization temperatures (100–180 °C) and Editorial handling: B. Lehmann