Summary:
Gold deposition at Assafou is considered to be of epigenetic origin, as supported by the hydrothermal alteration features of the sandstones (silicification, pyrite), and the structural control of the mineralisation (presence of the steeply dipping Assafou fault, sheared structures, quartz veins, and breccia crosscutting the sandstone).
Gold deposition was likely coeval with the activation (or reactivation) of the ‘Assafou structure’ during a Tardi or postTarkwaian south-southwest, northeast-southwest compression phase. Mineralising hydrothermal fluids preferentially invaded the sandstones rather than the basement rocks, due to their higher initial porosity/permeability and competency (Ciancaleoni, 2022).
Mineralisation
Gold mineralisation typically occurs as tiny (<1 mm) free gold grains, said grains are:
• disseminated within pervasively altered sandstones;
• distributed along micro-fractures, crosscutting altered sandstones;
• on the edges of pyrite, or filling cracks inside pyrite; and
• at the edge of quartz veins and breccia crosscutting the sandstones. Visible gold has also been observed within this type of occurrence.
Alteration is reflected by a silicification (±albitisation) of the sandstones and by the presence of sulphides (mainly pyrite, ±chalcopyrite and traces of galena) which are disseminated in clusters within the matrix and distributed along the sandstones bedding and oblique laminations. The more intense the silicification (and presence of pyrite), the more mineralised the sandstones tend to be. At least two generations of pyrite deposition are evidenced, and gold deposition appears to post-date pyrite crystallisation (gold filling cracks inside pyrite).
In mineralised zones, host-rock is featured by a light schistosity (primarily southwest dipping) and intense fracturing, forming a network of quartz filled (carbonates) fractures up to the development of breccia. These brittle structures are interpreted to be low-angle extension and extensional-shear fractures (primarily northwest dipping and, to a lesser extent southwest dipping), resulting from a sub-horizontal northeast-southwest compression.
Mineralising hydrothermal fluids preferentially invaded the sandstones rather than the basement rocks, due to their higher initial porosity/permeability and competency, absorbing strain in a more brittle manner. Additional to the structural control (induced by Tardi or post-Tarkwaian faults, fractures, and schistosity), hydrothermal fluid flow was also likely controlled by the primary sedimentary structures of the sandstones (coarser-grained units of higher primary porosity/permeability, laminations acting as physico-chemical barriers).