Interpretar los resultados obtenidos por medio de los instrumentos que permitan definir funcionalidades detalladas de la herramienta

The Mauritanides belt that host the Guelb Moghrein Fe oxide-copper-gold-cobalt deposit is thought to be part of an extensive chain of Panafrican, Caledonian and Variscan orogenic belts dismembered by the Atlantic rifting. The southern part of the belt locally records a polyphase tectonothermal evolution, including Pan African I (ca. 650 Ma) and Pan African II (ca. 550 Ma) events, followed by a period of tectonic quiescence throughout most of the early and middle Paleozoic (Inglis et al. 2004; Thomas et al. 2004). Late Paleozoic (Variscan) collision of Gondwana and Laurentia resulted in eastward thrusting of previously tectonized Mauritanides on the West African basement (e.g. Lécorché et al. 1989; Dallmeyer et al. 1989). However, K/Ar and 40_Ar/39_{Ar dating by Clauer et al. (1991) near} Akjoujt revealed that in contrast to the southern Mauritanides, the supracrustal rocks in this area show no record of any Panafrican ages. Their 40_Ar/39_{Ar incremental release analyses of} hornblende and biotite from the crystalline basement suggest that the respective closure temperatures last occurred at 2400 and 1850 Ma, and that emplacement of the allochthonous cover sequence was between 301 and 312 Ma (Clauer et al. 1991). Martyn and Strickland (2004) found the widespread occurrence of banded iron formation to be more akin to Proterozoic or Archean settings and concluded that the Akjoujt supracrustal rocks resemble Archean greenstone belts on grounds of lithological make-up, metamorphic grade, style of deformation and hydrothermal alteration.

The IOCG mineralization at Guelb Moghrein has been previously interpreted to belong to either the Pan African or Variscian tectonic evolution of the regional rocks (Strickland and Martyn 2002; Kolb et al. 2006). In this study, the new chronologic results of the epigenetic mineralization show that the Guelb Moghrein deposit formed in the Late Archean-Early Proterozoic time (2492 ±9 Ma), and together with Salobo IOCG deposit in Brazil belong to the oldest known IOCG deposits worldwide. Thus, the results point to an age of the host rocks to the hydrothermal mineralization in excess of 2492 Ma. Therefore, it can be concluded that the supracrustal rocks in the Akjoujt area represent a volcano-sedimentary succession of Archean age that underwent deformation and peak metamorphism under amphibolite facies conditions prior to hydrothermal mineralization and retrograde upper greenschist facies metamorphism at 2492 Ma. A second deformation event coupled with hydrothermal fluid flow occurred at 1742 Ma under lower greenschist facies conditions (Figure 10.1). The final emplacement at the current position on the crystalline basement of the West African Craton by thrusting took place at ~300 Ma as a result of the collision of Gondwana and Laurentia (Lécorché et al. 1989; Dallmeyer et al. 1989; Martyn and Strickland 2004; Meyer et al. 2006).

110

10.2 Controls on mineralization and alteration

The iron oxide-copper-gold-cobalt mineralization at Guelb Moghrein is interpreted to be controlled by three main factors: contrast in competency, geochemistry of host rock and structure. The mineralization is epigenetic intimately associated with the brecciation of the metacarbonate host rock (Kolb et al. 2006). The ore is concentrated in breccia structures with the favorable contact between the Fe-Mg clinoamphibole-chlorite phyllonite and the metacarbonate body. Drill logging and three-dimensional modeling of the ore bodies show that these occur along (1) the sheared hanging wall and footwall contacts to the biotite- actinolite schist and, (2) in at least four discrete shear zones truncating the metacarbonate parallel to the D2shear sense (Section 4.2.3).

The mineralized breccias dip 25-35° to the SSW, approx. parallel to the S₂ foliation of the hanging wall and footwall rocks (biotite-actinolite schist; Section 4.1). Hence, a structural control of the Cu-Au-Co mineralization at the Guelb Moghrein deposit is provided by the D2 shear zones, which reactivated the lithological contact between metacarbonate, amphibolite, and Fe-Mg clinoamphibole-chlorite phyllonite in the footwall of the D2 thrust separating the Akjoujt Metabasalt unit from the Sainte Barbe Volcanic unit (Strickland and Martyn 2004; Kolb et al. 2006).

The emplacement of breccias is commonly associated with weaknesses in underlying rocks representing zones permeable enough for hydrothermal fluids to enter overlying lithologies (Gerdemann & Myers 1972). Features that controlled or triggered breccia formation that have been identified in other IOCG districts include intersection of highly permeable units with fault zones (e.g. Candelaria deposit) in others, dilational jogs (Olympic Dam), duplexes, splays on faults and shears (e.g. Carajas district and Monakoff deposit), folding (e.g. Tennant Creek district), or complex intercalation of high and low permeability units faults or thrusts. Other controlling features include extensional fractures created during compaction in lithologies that overly less compactable sediments (Ohle 1985), or impermeable zones that constitute a seal under which ascending fluids may be trapped long enough to initiate hydraulic fracturing and veining (Cox et al. 1986). At Guelb Moghrein, the rheological contrast between siderite and the Fe-Mg clinoamphibole-chlorite phyllonite at upper greenschist facies metamorphism is the key factor for the structural control of the epigenetic mineralization. These two different lithologies have a markedly different tensile strength, and a higher degree of fracturing is possible in the stronger units (metacarbonate) in the vicinity of the contacts, by cyclic fluid pressure fluctuations in adjacent shear zones. The Fe-Mg clinoamphibole-chlorite phyllonite adjacent to the core zones of the breccia is deformed by viscous deformation of chlorite and to a lesser extent of grunerite and contains small lenses of puzzle-like, sulfide-filled breccias (Section 5.3). The rheologically weak Fe- Mg clinoamphibole-chlorite phyllonite localized viscous shearing in the metacarbonate body and controlled the brittle deformation of siderite and, therefore, mineralization and

111 alteration in the breccia. Siderite is rheologically stronger at temperatures of about 400°C (c.f. French et al. 1971; Gotor et al. 2000) and, therefore, deformation results in the formation of monomictic breccias that are symmetrically developed in the hanging wall and in the footwall of the phyllonite. Similar to the damage zones in brittle faults, these breccia zones account for the bulk permeability of the D2 shear zones at Guelb Moghrein, hence, host the economic Fe oxide-Cu-Au-Co mineralization. Core zones of breccias contain up to 95% ore and grade into distal crackle breccia, where the siderite of metacarbonate displays incipient brecciation, to weakly fractured and less-mineralized at the margins. As such, the boundary of breccias tends to be gradational over 30 meters (Sections 4.2.3 and 5.2.1). Overprinting relationships of various vein generations in the massive breccias indicate repeated episodes of fracture formation. Mineralogic fabrics such as grunerite rosettes and idiomorphic crystal surfaces in veins suggest growth of these crystals in open or fluid-filled space, proving large porosity systems in the breccia zones.

The mineralized breccias at Guelb Moghrein show many elements typical of fluid- assisted (hydraulic) brecciation (Jébrak 1997) including, angular clasts in combination with in situ fracturing e.g. mosaic and crackle brecciation, high cement/clast ratios, and dilation along foliation/shear planes (Vearncombe et al. 1995; Caine et al. 1996; Jébrak 1997). The geometry of this system resembles the distribution of fault gouge and damage zones in brittle faults at lower temperatures (Jébrak 1997; Evans et al. 1997; Kolb et al. 2006). The development of open space in the mineralized breccia bodies can be attributed to the combined effects of hydraulic brecciation and siderite dissolution/recrystallization with both processes interacting and possibly enhancing one another in a positive feedback loop. Supporting arguments for hydrothermal dissolution of the siderite of the host metacarbonate by ascending hydrothermal fluids during brecciation include: a) the siderite clasts in the highly-strained, mineralized breccias have a rounded (pebble-like) shape and display significant signs of recrystallization (Sd2 clasts; Section 5.2.1); b) abundance of fine-grained siderite (Sd3) in the matrix; c) REE geochemistry (Section 5.2.2) and 13C-18O isotope analyses (Section 8.2) show that the hydrothermal siderites (Sd₂ and Sd₃) have inherited the geochemical signature of the pre-existing metacarbonate (Sd₁) host rock.

Creation of open space and fracturing may have been initiated by hydrofracturing, paving the way for siderite-undersaturated fluids to penetrate larger volumes of the host rock. Brecciation of host rock and Sd2/Sd3 hydrothermal siderite generations related to the main sulfide mineralization stage evidently took place by in-situ fracturing without significant rotation or shift of fragments (pebble and crackle breccias; Section 5.2). Drop in fluid pressure and subsequent ore precipitation is the likely consequence of the sudden increase in connectivity between fractures during hydraulic fracturing (Jébrak 1997; Cox et al. 2001). After each sequential failure event the fluid pressure gradually returns to pre- failure conditions and may overpressure with healing of the fault by hydrothermal precipitation. Overpressuring can induce fluid-generated fracturing at the layer interface in

112 the method envisaged by Sibson (1996). This, in turn, resulted in a renewed build-up in fluid pressure and new hydraulic brecciation during the main mineralization stage. This cyclic process explains the repeated brecciation and hydrothermal mineral precipitation in the breccia matrix at Guelb Moghrein. Therefore, brecciation of the metacarbonate during D2 can be considered as the necessary ground preparation that enabled pervasive fluid flow of the sulfur- and metal-rich, hydrothermal fluids and interaction with the carbonate-rich host rock that was necessary for ore precipitation.

Deformation during D2 was partitioned into brittle deformation in the metacarbonate and ductile deformation in the amphibolite of the Akjoujt Metabasalt unit. In the biotite- actinolite schist of the hanging wall and footwall rocks, fabric development and degree of biotite-chlorite-grunerite-calcite alteration increase toward the metacarbonate contact. Therefore, the development of porosities for hydrothermal fluid flow is strongly related to ductile strain in the Akjoujt metabasalts mainly accommodated by crystal-plastic deformation of hornblende, biotite, chlorite, and plagioclase. Hydrothermal fluid flow appears to have been concentrated along grain-scale and vein networks into the shear zone. The pervasive style of alteration suggests that a grain-scale, mesh-like porosity system was developed during D2 shearing. The evolution of shear zones that developed in crystalline basement rocks at intermediate crustal levels has been profoundly influenced by a mixture of intracrystalline plastic and cataclastic processes simultaneous with fluid influx (Kerrich et al. 1984; Eisenlohr et al. 1988; Gibson 1990; Cox et al. 2001). A model for the growth of the shear zone in the Akjoujt metabasalts involves: a) initial plastic deformation of relative strong, coarse-grained amphibolites; b) episodic influx of reactive fluids during period of fracturing and high fluid pressure; c) progressive alteration of feldspar and hornblende to biotite, chlorite, grunerite, actinolite, calcite and recrystallization of quartz to form relative weak, fine-grained matrix; d) concentrated deformation and focused fluid in the core of the most highly-strained part of the shear zone. However, in contrast to the brittle-deformed zones in the metacarbonate this alteration zone is not significantly mineralized.