ALBERT

Notes: 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.

Notes: During emplacement and cooling, the layered mafic–ultramafic Kettara intrusion (Jebilet, Morocco) underwent coeval effects of deformation and pervasive fluid infiltration at the scale of the intrusion. In the zones not affected by deformation, primary minerals (olivine, plagioclase, clinopyroxene) were partially or totally altered into Ca-amphibole, Mg-chlorite and CaAl-silicates. In the zones of active deformation (centimetre-scale shear zones), focused fluid flow transformed the metacumulates (peridotites and leucogabbros) into ultramylonites where insoluble primary minerals (ilmenite, spinel and apatite) persist in a Ca-amphibole-rich matrix. Mass-balance calculations indicate that shearing was accompanied by up to 200% volume gain; the ultramylonites being enriched in Si, Ca, Mg, and Fe, and depleted in Na and K. The gains in Ca and Mg and losses in Na and K are consistent with fluid flow in the direction of increasing temperature.When the intrusion had cooled to temperatures prevailing in the country rock (lower greenschist facies), deformation was still active along the shear zones. Intense intragranular fracturing in the shear zone walls and subsequent fluid infiltration allowed shear zones to thicken to metre-scale shear zones with time. The inner parts of the shear zones were transformed into chlorite-rich ultramylonites. In the shear zone walls, muscovite crystallized at the expense of Ca–Al silicates, while calcite and quartz were deposited in ‘en echelon’ veins. Mass-balance calculations indicate that formation of the chlorite-rich shear zones was accompanied by up to 60% volume loss near the centre of the shear zones; the ultramylonites being enriched in Fe and depleted in Si, Ca, Mg, Na and K while the shear zones walls are enriched in K and depleted in Ca and Si. The alteration observed in, and adjacent to the chlorite shear zones is consistent with an upward migrating regional fluid which flows laterally into the shear zone walls. Isotopic (Sr, O) signatures inferred for the fluid indicate it was deeply equilibrated with host lithologies.

Notes: The In Ouzzal Al–Mg granulites are found within sedimentary units deposited after 2.7 Ga, the whole association being metamorphosed under extreme temperature conditions (c. 1000 °C) at 2 Ga. The Al–Mg granulites are interlayered with other metasediments, including metapelites, quartzites and magnetite-bearing quartzites, forsterite-spinel marbles, and a few meta-igneous rocks (mainly pyroxenites). They do not occur at a specific position in the sedimentary suite, and they do not reflect any particular structural control.The major and trace element compositions of Al–Mg granulites (especially the high Cr, Ni, Co contents) show that their peculiar ‘refractory’ chemistry is more compatible with premetamorphic sedimentary characteristics rather than with metasomatic, metamorphic or partial melting processes. Sedimentary admixtures of a common mature detrital component coming from the weathering of the local acidic igneous crustal protoliths (normal pelitic component) with an extremely immature component derived from reworking of basic/ultrabasic lithologies (Al–Mg–Cr–Co–Ni–rich chloritic component) is consistent with the geochemistry of such rocks.As in other instances, the quartz-garnet oxygen isotopic thermometer here records an apparent temperature close to the peak metamorphism (c. 1000 °C). Although the persistence of pre-existing δ18O variations on a small scale during the metamorphism does not support a major pervasive fluid flow during metamorphism, it does not rule out the presence of syn- to post-metamorphic CO2. The low δ18O (c.+ 5 to + 6‰) of the most typical Al–Mg granulites indicate that the ‘chloritic component’ in these rocks was derived from hydrothermally altered mafic/ultramafic protoliths rather than dominantly from palaeosols. It is suggested that the presence of such Al–Mg–Cr–Co–Ni–rich sediments is indirect evidence for the presence of greenstone belts in the local crust of the In Ouzzal at 2.6–2.7 Ga.

Notes: The In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine-spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (δ13C=–0.8 to –4.2‰/PDB; δ18O = 7.9 to 18.9‰/SMOW) does not record a massive streaming of C-bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasomatic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is comparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic compositions of Nd (ɛNd(T)=–6.3 to –9.9), of Sr (87Sr/86Sr(T)= 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13C =–3.5 to –9.7‰), the fluid responsible for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle-derived magmas at depth. Intense interaction with the crust during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep-seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust.

Notes: The In Ouzzal granulitic massif is composed mainly of various meta-igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows.Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high-Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high-K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga.In spite of the high-grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle-derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift-related-granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening.The very high-temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet-Nd ages. In spite of the very high-temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite-bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.