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14 SEG NEWSLETTER No 91 • OCTOBER 2012 ... from 13 Mauritania: A Greenfields Exploration Opportunity in Northwestern Africa (Continued) corresponding to either magmatic or metamorphic sulfur, and the δD fluid values are typical of metamorphic waters. The combined oxygen and hydrogen isotope composition of the fluid plots in the field of metamorphic waters, leading Sakellaris and Meyer (2008) to conclude that the sulfur and ore fluid are of metamorphic origin and that the most likely sources of metal were the host metabasalt units. Numerous workers (Gunn et al., 2004; Marutani et al., 2005, and references therein) suggest that there is potential for Guelb Moghrein-type Cu-Au mineralization in the southern Mauritanides. They refer to specific prospects, including Kadiar, Indice 78, Guelb En Naadj, Diaguili, and Guidamaka as IOCG occurrences. Kadiar is one of the most significant Cu prospects in the Mauritanide belt and is currently being explored by OreCorp Mauritania SARL, which considers it a VMS target (OreCorp, 2012). The prospect consists of a siliceous gossan with anomalous Cu and Au values, which overlies a lens of silicified ankerite with malachite, chalcopyrite, and pyrite. Salpeteur (2005) notes that the only iron oxide present at Kadiar is hematite and that magnetite is absent, and thus he questions whether Kadiar should be considered an IOCG-type occurrence. The Indice 78 prospect, originally identified by soil sampling, consists of a 4.5- km-long zone of propylitic alteration containing disseminated copper sulfides and numerous quartz veins, which is localized along the contact between meta-andesite and metarhyolite (Gunn et al., 2004). Within the alteration zone, carbonate occurs in veinlets and as irregular, coarsely crystalline patches that led Gunn et al. (2004) to suggest that the Cu mineralization might be similar to Guelb Moghrein; however, they conclude that the mineralization is more likely of the VMS type. The Cu ± Au occurrences in the southern Mauritanides are all associated with accreted metamorphic rocks, which in - clude ultramafic bodies that mark suture zones. The lenses of intense carbonate alteration, silicification, and quartz veining, often with associated Cr-rich micas, are typical “listwaenite” found at many orogenic gold deposits. The southern Cu occurrences, however, such as Diaguili and Guidamaka, are also associated with jaspilite (red jasper) and hematite, and there are podiform chromite occurrences in the same area, features that are common in Cyprus-type VMS deposits. Finally, the age of the country rocks in the southern Mauritanides is Neopro - terozoic (Pitfield et al., 2004), in contrast with the Archean age of the Akjoujt area sequence. Based on the above, it is concluded that the Cu ± Au occurrences in the southern Mauritanides are not IOCG (Fernette, in press a; Taylor, in press a). URANIUM EXPLORATION Mauritania is the site of active uranium exploration and new discoveries are being made. There are currently at least six companies actively exploring for uranium in Mauritania. The country hosts 80 known uranium occurrences, mainly in the Paleoproterozoic part of the Rgueïbat Shield, which include both calcrete and granite-hosted types. In fact, much of the northern shield is anomalous in U as shown by widespread anomalies in airborne radiometric data. The granite-related occurrences are all within or proximal to major NW-trending fault zones. The anomalous granitic rocks provide the source for the U in the calcrete deposits (Fernette, in press b, and references therein). There are three granite-hosted and nine calcrete-type U deposits defined in the northern shield in Mauritania. The total combined resource of the deposits is 183.8 Mt, with an average grade of 310 ppm U 3 O 8 . The calcrete deposits total 138.3 Mt at 331 ppm U 3 O 8 versus 46.5 Mt at 248 ppm U 3 O 8 for the granite-hosted deposits. All of the deposits are held under license by either Forte Energy or Aura Energy. Forte Energy’s first exploration target was the Bir en Nar area (Fig. 1), where uranium mineralization was discovered by Cogema in the 1970s (Marot et al., 2003). A recent estimate for the Bir en Nar deposits includes an indicated and inferred resource of 1.33 Mt at 704 ppm U 3 O 8 . The granite-hosted deposits consist of three NW-trending mineralized zones that dip moderately to the northeast. “Red” alteration is common in mineralized granite, although anomalous U values are also present in unaltered granitic rock. Syenite and episyenite pods within the shear zone also host mineralization (Fernette, in press b). The A238 and A238 NW deposits are located 135 km northwest of Bir en Nar, along the same shear zone. The deposits were discovered by follow-up of an airborne radiometric anomaly that located uraniferous calcrete. Drilling intercepted uranium mineralization in a shear zone below the calcrete zone. Both deposits are located within cataclastic zones along the mylonitized contact between two granitoid bodies. A large area of unmineralized silica-hematite alteration and smaller areas of silicification occur to the northeast of the A238 deposit. The A238 NW deposit occurs on a splay of the main shear zone to the northwest of the A238 deposit (Fernette, in press b, and references therein). PHOSPHATE AND GYPSUM Phosphate deposits occur in outcrops of phosphatic limestones, mudstones, and sandstones of Eocene age exposed along the northern bank of the Senegal River, about 300 km from the coast. Two deposits have been extensively studied, one at the village of Bofal and another deposit at Loubboira. Reserves at Bofal are 70 Mt with an average thickness of 1.7 m, an average grade of 21% P 2 O 5 , and an average overburden of 8 m. In comparison, Loubboira has 29 Mt of reserves with an average thickness of 2 m, an average grade of 19% P 2 O 5 , and an average overburden of 7 m. These resources are open to the north at Bofal and to the southeast at Loubboira; total probable reserves could exceed 100 Mt (Gunn et al., 2004). Resources were also identified in several other zones to the south, which are less well known than the two principal areas. The deposits continue 100 km farther to the southeast into Senegal at Matam, where a deposit exceeding 36 Mt at 28.7% P 2 O 5 has been identified. On the opposite bank of the river from Matam, at Sivé in Mauritania, resources of about 150,000 t have been identified, where phosphate rock beds average 26–28% P 2 O 5 . These deposits have potential for use as direct application fertilizer and local farmers are extracting phosphate rock for this purpose (Gunn et al., 2004; Langer, in press). Additional occurrences of sedimentary phosphate deposits are widespread in the Taoudeni basin, although available information suggests those discovered to date are small and low grade (Gunn et al., 2004). Furthermore, their remote locations, far away from areas of
OCTOBER 2012 • No 91 SEG NEWSLETTER 15 farming and food production, suggest that they are unlikely to be high-priority exploration targets. The host rocks are mainly late Neoproterozoic. The potential of these strata to host economic phosphate deposits remains unknown, but the Neoproterozoic–Early Cambrian time period is an important depositional interval, with significant resources of this age occurring in several countries in West Africa (Langer, in press). Possibly one of the largest gypsum deposits in the world occurs at Sebkha N’Drhamcha, 50 to 100 km northnortheast of Nouakchott. Both bedded evaporate- and dune-type deposits are present. The bedded gypsum deposits crop out chiefly along the eastern flank of the Sebkha N’Drhamcha, an evaporative pan just inland of the coast that was cut off from the ocean by a coastal sand belt, which formed within the past 4,000 years (Pitfield et al., 2004). The thickness of the deposits is greatest toward the center of the sebkha, decreasing to the west and north. There are several hundred million tonnes of resources, of which about 140 Mt are proven reserves (Gunn et al., 2004). Resources identified in dune deposits occur about 55 km from the highway that connects Nouakchott and Akjoujt. Limited sampling of these deposits, which are secondary accumulations in both active and indurated dunes, indicated grades of 92–93% CaSO 4• 2H 2 O with about 1.5% SiO 2 (Pitfield et al., 2004). A provisional resource estimate suggested about 2 Mt in one small area of dunes. Extraction rates have been as high as 100,000 t/yr for plaster and cement production. The favorable location, close to the main road, together with the high grades and limited thickness of overburden make these deposits attractive for exploitation. They are capable of meeting Mauritania’s domestic requirements for plaster and derived products (Langer, in press). Pegmatites and pegmatitic veins are recognized in several areas of the Archean and Paleoproterozoic parts of the Rgueïbat Shield and are prospective for critical element and REE resources. More than 150 beryl pegmatites are known in the Khnefissat area of the Chami greenstone belt, and additional concentrations of pegmatites are known at Guelb Nich Sud in the Sebkhet Nich greenstone belt and in the northeastern part of the Amsaga complex (Gunn et al., 2004). Owing to the small size of these deposits, they are unlikely to be economic unless additional value can be gained by processing contained minerals for industrial use. Vein and stockwork deposits containing anomalous Th and REE, as well as the above mentioned U, are present particularly in the Paleoproterozoic rocks of the Rgueïbat Shield. The small mafic alkaline complex at Tabatanet is associated with magnetic and radiometric anomalies and consists of a probable vein-type deposit that extends for 500 m. The hyperalkaline granite at Tigsmat may have REE enrichments in associated placers, but appears to be of low economic potential. Two other areas, at el Mrhader and at el Hajar, have indications of resource potential based on geophysical data and high scintillometer readings (Taylor, in press b). Another possibility for U, Th, REE, and associated element enrichments exists in association with carbonatite complexes in Mauritania. Studies suggest the presence of a carbonatite at SEMSIYAT STRUCTURE Guelb er Richat (Woolley et al., 1984; O’Connor et al., 2005), also known as the Eye of Africa; it is a distinctive landmark visible from space and was used as such during the Apollo missions (Fig. 4). It is marked by a coincident high Th geophysical anomaly. The location of Guelb er Richat in an area of prominent ENE-trending structures that are coincident with two swarms of kimberlite intrusions suggests a deep-seated structure is present in the sub-Taoudeni cratonic basement, which could localize the emplacement of additional carbonatite (and kimberlite, see below) bodies. A feature referred to as the Semsiyat domal structure is also located along this lineament and may represent an additional carbonatite intrusion at depth. For the most part, most occurrences of Th, REE, and associated elements are poorly described, poorly understood, and underexplored. Diamond exploration was extensive in the late 1990s and was continuing at a lesser pace by Rio Tinto in the northeastern Rgueïbat Shield and northern margin of the Taoudeni basin at the time of the 2007 USGS field survey (Taleb Ould Abdival, Rio Tinto, oral commun., 2007). Gunn et al. (2004) discuss the Rex Diamond project area to the east and northeast of Zouerate. They also note that De Beers worked with Rex Diamond Mining Corp on the Akchar Permit area in the southwestern Rgueïbat Shield and that De Beers ceased active explora - tion in late 2002. Rex to page Diamond Mining 16 ... THE ADRAR ESCARPMENT REE, CRITICAL METALS, AND DIAMONDS IN ALKALINE ROCKS, CARBONATITES, AND KIMBERLITES Resources of U, Th, Nb, Ta, Be, rare earth elements (REE), and fluorite associated with alkaline rocks, pegmatites, and carbonatites are known in Mauritania and have been exploited in the past at the Bou Naga alkaline igneous complex. FIGURE 4. SRTM digital elevation model of the Guelb er Richat structure. Note smaller Semsiyat dome to the southwest.
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