Yacouba Ouedraogo, PhD

The mineralization of gold, silver, and tellurium (Au-Ag-Te) in the orogenic Niou area is hosted by metamorphosed volcanic formations, such as metabasalts and metaandesites, in greenschist facies. These volcanic formations are cut by intermediate to basic intrusions, including metadiorites and metagabbro. The mineralization is an epigenetic type, exhibiting three stages of gold mineralization that are controlled by two episodes of shear deformation. These episodes have been described and documented in this deposit. Stage I is characterized by early pyrite mineralization, exhibiting chagrined, skeletal, and locally fractured grains ranging in size from 200 µm to 1 mm. These grains are disseminated along the S1Ni foliation, with low gold concentrations. This stage is associated with sericitic alteration and chloritization. Stage II is marked by remobilized mineralization invading Py1 pyrites, interpreted as Py2, exhibiting signs of partial dissolution at the margins. These pyrites display euhedral to sub euhedral textures. Notable inclusions of chalcopyrite have been identified in the cores and at the rim of theses pyrites, dispersed along the foliation plane structures and the S/C structures and micro-fold hinges. Stage III is characterized by the crystallization of Py3-type pyrite, which is concomitant with late silicification along the axes of the microfold axes. The distribution of gold within the sulfides can be categorized into the following forms: (i) Au, Au-Bi, Au-Te, and Au-Ag-Te present in the micropores of pyrites with a subhedral texture, (ii) as inclusions within large pyrite crystals in contact with chalcopyrite and pyrrhotite, and (iii) in fissural form in contact with pyrite and pyrrhotite. 

Sulfides have been observed to absorb gold in their lattices up to 0.18% for pyrite, 0.15% for chalcopyrite, and 0.10% for pyrrhotite. Metabasites (metabasalts and metaandesites) have been interpreted as the source of gold in the NGD.