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12 SEG NEWSLETTER No 91 • OCTOBER 2012 ... from 11 Mauritania: A Greenfields Exploration Opportunity in Northwestern Africa (Continued) into production in 2007, is located in the southwestern corner of the Rgueïbat Shield and is a world-class Paleoprotero - zoic(?) orogenic gold deposit. The producing Guelb Moghrein deposit occurs along a shear zone in Middle Archean rocks at the bend in the northern Mauritanides and is most commonly classified as an IOCG type of deposit (see next section). Both major deposits are surrounded by numerous prospects that show similar mineralization styles. Most of the Mesoarchean and Paleoproterozoic rocks of the Rgueïbat Shield in northern Mauritania are highgrade gneisses (e.g., Key et al., 2008; Schofield et al., 2012), which are not favorable for gold resources. However, the metamorphic grade in the shield appears to decrease to more favorable greenschist facies to the east and defines an area with potential for orogenic gold deposits in Paleoproterozoic rocks. A few small gold prospects (e.g., Conchita- Florence or Nsour, El Mheissat, Oued El Ma, and Bin-652), now being explored by Drake Resources Limited (Drake Resources Limited, 2012), are recognized north of latitude 21 o N, a region that includes most outcrops of ca. 2.1 Ga Birimian rocks of the shield. Drake Resources Limited reports high-grade gold samples at Conchita-Florence, adjacent to a 140-km-long shear zone system of anomalous gold termed the Hendrix shear zone. This zone also represents the only distinct belt of volcaniclastic rocks defined to date in the eastern part of the shield (e.g., Bradley et al., in press a). The high gold samples also have anomalous concentrations of Ag, Au, Mo, Sb, Te, and W, and are associated with outcropping gold-bearing quartz veins in greenschist facies metapelites. Several 40 Ar/ 39 Ar dates on hydrothermal sericite from prospects to the southeast along this trend (Bradley et al., in press b) suggest that these prospects are Paleoproterozoic in age. Further assessment of the region will require detailed geochemical maps, re - gional metamorphic maps, and evaluation of existing regional structure. If permissive areas exist for important gold resources in the Rgueïbat Shield, then such areas are likely to be broadly defined as those of relatively low metamorphic grade, mainly of greenschist facies. They will likely be newly recognized metasedimentary rock belts similar to those that host the Conchita-Florence occurrences. These favorable areas would include rocks of the Lower and Upper Birimian supracrustal rocks that are located east of the Zednes fault zone. In addition, the relatively NNW-trending major faults cutting the eastern half of the shield are likely to be particularly permissive hosts for gold ores where present in lower grade metamorphic domains; margins of igneous rocks shown along some of these faults in any lower grade exposures should be carefully examined (Goldfarb et al., in press). The far southwestern corner of the Rgueïbat Shield (lat. 20°–21° and long. 14°–16°) is the one area in the Meso - archean part of the shield with greenschist facies greenstone belts; the seven defined belts (Bradley et al., in press a) trend N-S in the west (e.g., Chami) and NE-SW in the east (e.g., Tirijit). The greenstone belts are exposed in the southern part of the Tasiast-Tijirit terrane of Schofield et al. (2012). They are located within a basement of ca. 3.1–3.0 Ga migmatitic gneisses that were metamorphosed at ca. 2.97 Ga; 0 20000 m Mapped BIF Rusty Aoueouat Chami Tasiast C-23 N’Derek the greenstone belts are likely of similar age to the gneisses. Younger, voluminous tonalities and granodiorites, which form the cores of dome-like features, surround the basement rocks and greenstone belts (Key et al., 2008) and are dated at ca. 2.95–2.87 and 2.69–2.65 Ga, respectively (Schofield et al., 2012). The Tasiast gold deposit (Fig. 3) is located along a N-S–trending shear system in the 70 × 15 km Chami greenstone belt. It has been mined since 2008, first by Red Back Mining and currently by Kinross Gold Corp. (2012). Kinross Gold Corp. reports a world-class reserve + resource of at least 21 Moz Au at 1.47 g/t. The mine produces just over 200,000 oz Au per year. The Tasiast deposit occurs as sheeted quartz-carbonate-albite-pyrrhotite veins, gold-bearing quartz-carbonate veinlets, and adjacent disseminated gold, hosted in an upper greenschist to lower amphibolite facies, magnetite-quartzite BIF and adjacent volcaniclastic rocks. The orebodies are present within two parallel zones that continue for 6 km along strike (Davis, Mapped Greenstone Belt Jurassic dikes Mesoarchean (?) dike Gold prospect Tijirit FIGURE 3. Reduced-to-pole aeromagnetic image of the southern Tasiast-Tijirit terrane, showing geophysical extensions under cover of Mesoarchean greenstone belts and locations of orogenic gold deposits and prospects. Intersections of greenstone belts and possible Mesoarchean (?) lineaments represent highly favorable targets for additional deposits. All linear NE-trending moderate amplitude anomalies relate to Jurassic dikes.
OCTOBER 2012 • No 91 SEG NEWSLETTER 13 2011). The brittle orebodies follow a series of reactivated thrusts that include the Tasiast and Piment fault zones. Anomalous W, As, Ag, and Sb are reported at the deposit. Altered rock includes biotite, sericite, and chlorite (Davis, 2011). Reported K-Ar ages on hydrothermal sericite of 1.85 and 1.50 Ga (Marutani et al., 2005) are unusual, as typically Mesoarchean rocks host Mesoarchean gold deposits and Paleo - proterozoic rocks host Paleoproterozoic deposits. A major NW-striking aeromagnetic lineament (Finn and Anderson, in press) intersects the Chami greenstone belt in the Tasiast mineralized area and may have important implications for explorationists (Goldfarb et al., in press; Fig. 3 and see below). Other prospective areas within a few tens of kilometers of the deposit could include the Rusty prospect to the northwest of Tasiast, where auriferous quartz veins are identified in folded BIF and mafic volcanic rocks. At prospect C-23, north of Tasiast and in the Chami greenstone belt, gold-bearing quartz veins occur along a sheared margin to a granodiorite body. To the south of Tasiast, in mainly sand covered areas, Drake Resources Limited reports gold anomalies from drill core from the southern part of the Chami greenstone belt. The above-mentioned aeromagnetic anomaly of Finn and Anderson (in press) also crosses the adjacent greenstone belts to the east and west of the Chami greenstone belt, and these locations are marked by additional small orogenic gold prospects. These intersections of greenstone belts and the lineament, which is perhaps an ancient craton margin boundary or fault parallel to the boundary just inside the ancient margin, are considered extremely favorable targets for new resources (Goldfarb et al., in press; Fig. 3). More detailed work in this southwestern part of the shield is likely to identify other large tonnage gold re - sources, because typically orogenic gold deposits do not occur as isolated single deposits, but rather as clusters of de - posits. Similar orogenic gold deposits are probably present at Tijirit, which is located in the Tijirit greenstone belt about 120 km ENE of the Tasiast deposit. Evidence for potential in this area is a NNE-trending shear zone containing auriferous quartz veins that continue for as much as 200 m along strike within basalt, gabbro, and schist (Goldfarb et al., in press). GUELB MOHGREIN AND THE POTENTIAL FOR ADDITIONAL IOCG The Guelb Moghrein Cu-Au deposit in the northern Mauritanides is considered by many to be an iron oxide coppergold (IOCG)-type deposit (Strickland and Martyn, 2001; Kolb et al., 2008; Kirschbaum, 2011). It shares many characteristics with IOCG deposits, including economic Cu+Au, structural control, abundant breccias in the ore zone, low Ti iron oxides, chalcopyrite and pyrrhotite as the dominant sulfides, and a lack of quartz veining. However, no other IOCG deposits have Fe-rich carbonate host rocks, so in that sense it is at least somewhat atypical of IOCG deposits (Kirschbaum, 2011) and there is a possibility that the mineralizing fluids are of metamorphic origin (Sakellaris and Meyer, 2008). Guelb Moghrein is located near Akjoujt in the Inchiri district in western Mauritania. In addition, there are 19 other copper and/or gold occurrences in the Inchiri district, including the El Khader gossan, where the goethite-rich cap is currently being considered as a source of iron ore by PT Bumi Resources Minerals Tbk. Ancient copper workings at Guelb Moghrein were noted by the French military in 1931 and sampled by the Bureau Minier in 1946. Attempts to mine the deposit started in the 1950s and continued until the early 1990s, when General Gold International recovered five t Au (Gunn et al., 2004). First Quantum Minerals Ltd started commercial production in 2006. Mine production in 2011 was 3.61 Mt at an average grade of 1.4% Cu, with an additional 1.1 t Au produced. Total proved and probable reserves as of December 31, 2011, were 3.2 Mt at 1.09% Cu and 0.79 g/t Au. Measured and indicated resources were 30.9 Mt of sulfide mineralization at 1.18% Cu and 0.77 g/t Au and 0.12 Mt of oxide mineralization at 1.58% Cu and 1.30 g/t Au (Fernette, in press a, and references therein). The Akjoujt area is situated at a major bend in the Mauritanide belt, where it turns westward to follow the southern edge of the Rgueïbat Shield and the rocks consist of a system of overlapping thrust sheets. The Akjoujt package is bounded to the north and east by a sole-thrust contact with the Archean Amsaga terrane of the Rgueïbat Shield (Gunn et al., 2004). Supracrustal rocks in the Akjoujt area consist of metabasalts and BIF, overlain with angular unconformity by the siliclastic (meta)sediments capped by a thick section of submarine basalts and dolerites. Partially migmatized leucogranite sheets and pegmatites, which occur in areas of overthrust basement in the Akjoujt area, are the only intrusive rocks (Martyn and Strickland, 2004). Geochronological studies suggest that whereas the host rocks at Guelb Moghrein are Archean, the ores are probably earliest Paleoproterozoic. Two types of hydrothermal monazite and xenotime associated with primary sulfide ore were dated by Meyer et al. (2006). Type I grains yielded an age of 2492 Ma, which was concluded to be the age of the mineralization. Type II grains showed an age of 1742 Ma, which was interpreted to indicate a later generation of hydrothermal fluid flow. Copper-gold mineralization at Guelb Moghrein occurs in discrete tabular breccia zones developed parallel to D 2 shear zones within metacarbonate rock composed of magnesite and dolomite. The mineralization and host metacarbonate lie within a thrust zone that separates metarhyolite from metabasalt (Strickland and Martyn, 2001; Kolb et al., 2006). The ore consists of carbonate breccia with a matrix of mainly Fe-Mg clinoamphibole and magnetite, with lesser chalcopyrite, pyrrhotite, and other sulfides. The massive sulfide-arsenide-gold mineralization formed at about 410°C and 2–3 kbars (Kolb et al., 2006). The metacarbonate host has been variously interpreted as a metasediment (Kolb et al., 2006; Kirschbaum, 2011) and as a hydrothermal alteration product (Martyn and Strickland, 2004). The wall rocks adjacent to the mineralized metacarbonate underwent albitization prior to and during greenschist to amphibolite grade metamorphism, and this was followed by a potassic alteration event (Kirschbaum, 2011). Factor analysis of rock geochemical data show a strong ore-related correlation of Ag, As, Bi, Co, Cu, Ge, In, Ni, Se, Sn, Te, and Zn (JICA, 2005). Reconnaissance fluid inclusion studies of quartz veins from the footwall greenschist at Guelb Moghrein showed poly phase inclusions containing halite crystals in gas-rich inclusions (JICA, 2005). Homogenization temperatures of the inclusions ranged from 240° to 320°C and salinities from 33 to 39 wt % NaCl equiv. Sakellaris and Meyer (2008) indicate sulfide δ 34 S values cluster near zero, to page 14 ...
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