METALLOGENESIS OF THE LATE PAN-AFRICAN GOLD-BEARING EAST OUZZAL SHEARZONE (HOGGAR, ALGERIA)

In the W Hoggar (Algeria), the major transcurrent N-S East Ouzzal shea r zone (EOSZ) hosts several world-class gold deposits over a 100-km le ngth. The late Pan-African EOSZ separates two contrasting Precambrian domains: the Archaean In Ouzzal block to the west (orthogneisses with subordinate metasediments, reworked and granulitized in the c. 2 Ga Eb urnean event) and a Middle Proterozoic block to the east (again orthog neisses and metasediments, involved in the c. 600 Ma Pan-African event ). The EOSZ is a mylonite belt, 1-3 km wide, with a 50-m-wide ultramyl onite belt hosting numerous quartz veins and lenses (giant hydrotherma l quartz system) associated with a quartz-sericite-pyrite-carbonate (b eresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions. between 500 and 300 MPa, at 500 -300 degrees C, with aqueous-carbonic fluids derived both from underly ing devolatilized metamorphic rocks and a mantle source, as recorded b y stable (C, O) isotope data. No gold mineralization was associated wi th these typical mesothermal events. Following a pressure drop (to 130 MPa), related to the inception of extensional tectonics, the EOSZ was later percolated by a new set of hydrothermal fluids, evolved from ba sinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca-bearing brines (up to 25% wt. eq. NaCl), characterized by high delta D (-9 to + 18 parts per thousand range), mobilized by t he thermal energy released by the late Pan-African granite magmatism ( Taourirt granites). As demonstrated by Pb isotope data, the brines lea ched Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines i n the EOSZ at a depth of c. 5 km, due to mixing and cooling with desce nding diluted fluids.