Metallogenesis of the late Pan‐African gold‐bearing East Ouzzal shear zone (Hoggar, Algeria)
1996; Wiley; Volume: 14; Issue: 6 Linguagem: Inglês
10.1111/j.1525-1314.1996.00056.x
ISSN1525-1314
AutoresC. Marignac, A. SEMIANI, Serge Fourcade, Marie‐Christine Boiron, J. L. Joron, J. R. Kienast, J. J. Peucat,
Tópico(s)earthquake and tectonic studies
ResumoABSTRACT In the W Hoggar (Algeria), the major transcurrent N–S East Ouzzal shear zone (EOSZ) hosts several world‐class gold deposits over a 100‐km length. 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 Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses 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 ultramylonite belt hosting numerous quartz veins and lenses (giant hydrothermal quartz system) associated with a quartz‐sericite‐pyrite‐carbonate (beresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions, between 500 and 300 MPa, at 500–300°C, with aqueous‐carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source, as recorded by stable (C, O) isotope data. No gold mineralization was associated with 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 basinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca‐bearing brines (up to 25% wt. eq. NaCl), characterized by high δD (‐9 to + 18‰ range), mobilized by the thermal energy released by the late Pan‐African granite magmatism (Taourirt granites). As demonstrated by Pb isotope data, the brines leached Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines in the EOSZ at a depth of c. 5 km, due to mixing and cooling with descending diluted fluids.
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