THE GEOLOGY OF THE TAPARKO GOLD DEPOSIT, BIRIMIAN GREENSTONE-BELT, BURKINA-FASO, WEST-AFRICA

The Taparko gold deposit, located in the eastern branch of the Protero zoic Birimian Bouroum-Yalogo greenstone belt (Burkina Faso) consists o f a network of quartz veins developed in a N 170 degrees trending shea r zone (250 m wide, 4 km long) superimposed on the regional Birimian s tructural pattern. The quartz vein network is composed of: (a) a domin ant array of quartz veins (type 1), parallel to the shear zone and com prising strongly deformed dark quartz exhibiting foliation, layering, ribbon, tension gashes, etc.; (b) oblique and subparallel related vein s (type 2) of gray to white weakly deformed quartz crosscutting the do minant quartz veins resulting in breccia structures; and (c) shallow d ipping veins (type 3), cross-cutting veins types 1 and 2 and tilled by undeformed white buck structure quartz. Cross-cutting relationships a nd different quartz types in different veins and within individual vei ns imply a concomitant filling of the veins during the progressive def ormation. Initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 an d 2 veins and were deposited in two stages, with native gold being rel ated to second stage sulfides. Gold (and chalcopyrite) precipitated pr eferentially upon the surfaces of fractured pyrite grains in low-press ure sites (pressure shadow zones) around and/or within the sulfide gra ins (along subsequently annealed fractures). The formation of the Sout h Taparko deposit can be divided into a succession of events: (a) duri ng the first event, N 170 degrees-directed sinistral transcurrent shea ring resulted in a N 20 degrees mylonitic foliation and fractured rock which allowed H2O-, CO2- and SiO2-rich fluids to circulate and deposi t quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes a s viewed on a horizontal plane; and (c) during the third event, the si nistral transcurrent shearing evolved to sinistral reverse shearing an d the deformation style evolved correspondingly from ductile to brittl e-ductile. During the last phase of deformation gold nucleated and dep osited in low-pressure zones.