Metallogenesis of the late Pan‐African gold‐bearing East Ouzzal shear zone (Hoggar, Algeria)

Abstract

A B S TR A CT 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-pyritecarbonate (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 dD (-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.

The Hoggar Gold and Rare Metals Metallogenic Province of the Pan-African Tuareg Shield (Central Sahara, South Algeria): An Early Cambrian Echo of the Late Ediacaran Murzukian Event?

“…In addition, a gold halo is developed within the alteration envelope (beresite, see below) of the quartz lodes, with contents varying from 1 ppb to 15 ppm (ORGM, unpublished data). This district is by far the best studied of all within the Hoggar province, being well described in several publications (e.g., Ferkous and Leblanc 1995;Semiani 1995;Marignac et al 1996;Ferkous and Monié 2002;and references therein).…”

“…Retrogressive metamorphism occurred concurrently with deformation and resulted in rehydration of earlier metamorphic rocks under low-grade conditions (biotite-chloriteactinolite-carbonate paragenesis). Syn-kinematic mafic bodies (mainly, gabbros) are intrusive into the mylonite, and stable isotope (O, C) studies showed that the early CO 2 -rich fluids percolating the shear-zone were of mantle derivation (Marignac et al 1996). Thus, the EOSZ is of lithospheric scale.…”

“…The latter are only gold-rich where crossing the ultramylonite belt, the richest being hosted by the Tirek syn-kinematic gabbro, whereas the former are only gold-rich where associated with either syn-kinematic mafic intrusions or boudinaged marbles. This strong lithological control on the location of rich ore shoots is likely related to a kind of pH effect on gold deposition (Marignac et al 1996). The gold-bearing veins are organized in dense swarms of subvertical veins (several 100 m in length, up to 1000 m), which are commonly irregular or boudinaged (lens-shaped veins), with thicknesses varying from a few centimeters to up to 3 m (Fig.…”

“…The gold mineralizing stage post-dated these events and was apparently coeval with extensional conditions (Marignac et al 1996). First, Fe-poor sphalerite (type I veins) or pyrite (type II veins), then galena and native gold, were successively deposited in a network of anastomosing veinlets that developed along intergranular joints in mylonitic and/or cataclastic quartz.…”

“…Alteration and mineralization (Ferkous and Leblanc 1995;Marignac et al 1996): The mineralized part of the EOSZ is characterized by a 20-m-wide alteration zone in the center of the ultramylonite belt, with a quartz-pyrite-sericite (phengite)-Fe-dolomite association (the so-called beresite) that overprints the mylonite foliation. Branching on the main N-S beresite belt, lesser N10°E-to N40°E-trending beresite belts also cut the mylonite foliation for lengths varying from 100 to 300 m.…”