Central Morocco

A Cordilleran Zoning Model for the Polymetallic W-AuPb-Zn-Ag Tighza-Jbel Aouam District (Central Morocco): Contribution from New He-Ar and U-Th-Pb Data Alain Cheilletz Ecole Nationale Supérieure de Géologie, GeoRessources UMR-CNRS 7359, Université de Lorraine, 2 rue du Doyen Marcel Roubault TSA 70 605, 54 518 Vandœuvre-lès-Nancy, cedex, France Magali Rossi, Leïla Tarrieu, Dominique Gasquet EDYTEM, UMR-CNRS 5204, Université Savoie Mont Blanc, 73376 Le Bourget du Lac cedex, France Hassan Bounajma, Tristan Mantoy, Lofti Ouazzani, Lahcen Ouchtouban Compagnie Minière de Touissit, Centre minier de Tighza, 54450 M’Rirt, Province de Khenifra, Maroc Etienne Deloule, Pete Burnard Centre de Recherches Pétrographiques et Géochimiques, UMR-CNRS 7358, Université de Lorraine, BP 20, 54501 Vandoeuvre-lès-Nancy, France Jean-Louis Paquette Laboratoire Magma et Volcans, UMR-CNRS-IRD 6524, Université Blaise Pascal, CNRS-UMR 6524, 63038 ClermontFerrand cedex, France Abstract. The W-Au, Pb-Zn-Ag, and Sb-Ba polymetallic ores of the Tighza-Jbel Aouam (central Meseta, Morocco) have been considered zoned magmatichydrothermal deposits, hosted in Palaeozoic rocks surrounding late-Variscan granites. The spatial distribution of the deposits was interpreted as zoning around a supposed hidden batholith. New dating of zircon and monazite allow revision of this model. The Tighza-Jbel Aouam W-Au mineralization formed at 295280 Ma and cuts granitic stocks dated at 320-300 Ma. It is related to a magmatic event only visible through a large hydrothermal biotitic alteration halo, thus suggesting the presence of a hidden batholith. Based on the occurrence of large veins, stockworks, sheeted veins, and disseminations hosted in skarns, the W-Au deposit is considered similar to a porphyry-type deposit. The currently mined Pb-Zn-Ag ores developed during an epithermal magmatic-related episode dated at 254 ± 16 Ma; it is disconnected from the W-Au deposit. The polymetallic district of Tighza-Jbel Aouam thus represents a Cordilleran mineralizing style related to pulses of calcalkaline magmatism. Late- Variscan and Permo-Triassic transpressive tectonics allowed localized magma emplacement and associated hydrothermal fluid circulation, and the formation of the polymetallic deposits. Keywords. W-Au-Pb-Zn-Ag, fluid sources, He-Ar, U-ThPb,zoned polymetallic district, Moroccan Meseta.

1 Introduction Many Pb-Zn-Ag-F deposits in Morocco as well as in Western Europe occur in veins that cut Paleozoic rocks, late Variscan intrusive granites, and overlying PermoTriassic to Jurassic sediments. In several occurrences, the spatial association of these veins with granites has been interpreted as proof of a genetic link between magmas and hydrothermal fluids, which produced the base-metal concentrations. By this model, the mineralizations are thus supposed to have an age of ca. 300 Ma.

Recent geochronological data have shown that some “Variscan” ore deposits from central Morocco are actually much younger (i.e., 205 ± 1 Ma for the El Hammam F deposit; Cheilletz et al. 2010). The genetic model of hydrothermal perigranitic Pb-Zn-Ag-F ore deposits in Morocco thus needs to be reconsidered, involving an update of the exploration strategy for these base and precious metals. In the Tighza-Jbel Aouam district (TJAD), fluid inclusion studies (Nerci 2006) previously suggested that the W-Au and Pb-Zn-Ag ores are associated with different types of fluids. This paper presents new He-Ar geochemical data for a better constraint on the sources of the mineralizing fluids. Furthermore, new geochronological investigations conducted on magmatic bodies (U/Pb on zircon) and hydrothermal mineralized Pb-Zn-Ag veins (Th/Pb on monazite) help to constrain the timing of the different magmatic and hydrothermal pulses in the TJAD (Tarrieu 2014) as already done for other deposits in Morocco (Levresse 2001; Pelleter et al. 2007).

2 Geological setting 2.1 Regional geology The Moroccan Meseta domain corresponds to the southwesternmost segment of the European Variscan belt (Michard et al. 2008). The Tighza-Jbel Aouam district (TJAD) belongs to the eastern part of the Meseta domain that comprises Paleozoic metasedimentary rocks cut by Variscan magmatic stocks and dikes (Fig. 1). Within the TJAD, mineralization is hosted in upper Visean limestone and schist, discordant on Ordovician siliceous schist and quartzite, Silurian graptolite-bearing black shale, and Devonian siliceous and marly limestone. These units are d efor me d into a succession of anticlines and synclines, SW-NE-trending, bordered by a major E-W Metallogeny of North and West Africa

1579

mega-shear corridor called the Tighza shear zone. All of these formations are metamorphosed t o very low- to low-grade greenschist facies. They are intruded by numerous, nearly parallel SW-NE- trending microgranite and microgranodiorite dikes and five small stocks ranging along a N-S strike (Izougarsa, Pitons de Tighza, Mispickel, Mine, Kaolin; Fig. 1). These stocks have a high-K calc-alkaline affinity like most Variscan granites, testifying to a strongly enriched mantle component (Giuliani et al. 1987; Gasquet et al. 1996). The three southernmost stocks are surrounded and altered t o various degrees by a large biotitic hydrothermal alteration halo associated with the W-Au mineralizing episode (Cheilletz 1984; Cheilletz and Isnard 1985). The gravimetric study conducted by El Dursi (2009) suggests that this hydrothermal alteration halo might be related to a hidden shallow intrusive pluton.

to a Bi-Te-Au-As (with löllingite) assemblage, and finally a sulphide-rich assemblage (arsenopyrite + pyrrhotite) with minor Au (Nerci 2006). The main veins are parallel to the E-W Tighza dextral shear zone that formed under a transpressive tectonic regime. The Pb-Zn-Ag mineralization occurs within several large veins with N25 to N75°E orientations that commonly present a “Y” shape, suggesting vein opening in a sinistral conjugate strike-slip system (Cheilletz 1984; Fig. 1) related to the Tighza shear zone. It is a sulphide-rich assemblage (galena + sphalerite) associated with a carbonate ± quartz gangue. The paragenetic sequence is divided into 4 stages: - S1 is uniformly barren and contains a siderite + quartz assemblage - S2 is the first mineralized sequence, having a galena ± sphalerite assemblage associated either with ankerite (Sidi Ahmed and Ighem Aousser, SA- IA veins) or calcite ± ankerite ± siderite ± red chalcedony (Signal vein). - S3 is also mineralized with a galena ± sphalerite assemblage, mainly associated with siderite (and quartz in SA-IA vein and with rare barite in Signal vein). - The late S4 sequence is barren and contains a calcite + quartz + pyrite assemblage (with some barite in Signal vein). Gangue carbonates contain numerous µm-size inclusions of monazite and xenotime that are strongly altered to synchysite, thus enhancing the REE content of the carbonates (ΣREE up to 4000 ppm in some samples; Tarrieu 2014). The Sb-Ba mineralization has only been documented by Agard et al. (1958). It occurs as l o c a l lenses of stibnite ± pyrite ± chalcopyrite.

3 Relative chronology

Figure 1. Geological map of the Tighza-Jbel Aouam polymetallic district, highlighting the mineralized veins. Mining activity focuses on the following veins: Signal, Structure 18, Sidi Ahmed, and Ighrem Aousser.

2.2 Mineralizations In the TJAD, three magmatic-hydrothermal systems have been identified (W-Au, Pb-Zn-Ag, and Sb-Ba) that together display a concentric base- and precious-metal zonation centred on the biotitic hydrothermal alteration halo (Agard et al. 1958; Cheilletz 1984). The W-Au mineralization is concentrated in the vicinity of the outcropping stocks, cutting the Mine Granite. It formed first as skarns, then as large veins, stockworks and sheeted veins, and finally as disseminations (Cheilletz 1984; Nerci 2006). A mineralization sequence is observed from an early WMo assemblage (wolframite + scheelite + molybdenite) 1580

Field observations allow the establishment of a relative chronology among the three main mineralization events and the magmatic pulses. Because the Sb-Ba mineralization lacks spatial relationships with the W-Au or the Pb- Zn-Ag occurrences, its timing remains unclear. However, Agard et al. (1958) suggested that the Pb-Zn-Ag veins postdate the Sb-Ba mineralization, based on other evidence elsewhere in Morocco. These authors also noted that the Sb-Ba veins cut some granitic dikes. The W-Au veins cut the outcropping magmatic stocks, but they are older and are displaced by the Pb-ZnAg mineralization (Fig. 2). As described above, the WAu mineralization is associated with the biotite-rich alteration halo affecting the stocks (Fig. 1), and is thus contemporaneous with a hidden magmatic event proposed by El Dursi (2009). Dike emplacement took place between the two main hydrothermal events as some of them cut the W-Au mineralization, and others are displaced by the Pb-Zn-Ag deposits. From the above observations, the following chronology is proposed: (1) dike emplacement, (2) SbBa veins, (3) emplacement of the currently outcropping stocks, (4) W-Au mineralization associated with a hidden pluton, (5) dike emplacement, and (6) Pb-Zn-Ag mineralization. The magmatic activity might have

MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 4

continued afterward, as demonstrated by the presence of a large Permo-Triassic volcanic field south of the TJAD.

5 U/Pb and Th/Pb geochronological data Zircons from magmatic rocks contain numerous and complex growth zones that are related to magmatic and/or later hydrothermal events. In situ U/Pb dating of such zircons thus allows dating of both types of events (e.g., Pelleter et al. 2007). As shown in Figure 4A, zircons from the TJAD granitic stocks and dikes show such zoning, suggesting that they record several magmatic and hydrothermal events. Some g r a i n s also have an inherited core (ages up to 420 Ma; Fig. 4A).

Figure 2. Underground field photograph showing the Pb-ZnAg veins cutting and d i s p l a c i n g the W-Au mineralization (Structure 18).

4 Fluid sources In order to give insights i n t o the mineralizing fluid sources, He-Ar analyses were performed on both the WAu and the Pb-Zn-Ag ores. The W-Au and Pb-Zn-Ag mineralization both have Ar isotopic values ranging between 299 and 328, which is consistent with meteoric Ar (40Ar/36Ar = 295.5; Fig. 3). The Pb-Zn-Ag ore has a very low 3He/4He signature (0.01-0.10) that is characteristic of a crustal fluid (Fig. 3). In contrast, the W-Au ore has a higher 3He/4He ratio, ranging from 1.0 to 1.8, the latter suggesting a small mantle component.

Figure 4. A. SEM image of a zircon grain showing an inherited core and multiple growth zones. B. SEM image of a dated monazite hosted in S4 calcite. C. Synthesis of all ages obtained for both magmatism and hydrothermalism in the TJAD (this study and literature data).

Figure 3. He and Ar isotopic compositions of W-Au and PbZn-Ag ore minerals. Mantle, crustal, and meteoric signatures are from Burnard et al. (1999) and references therein.

As suggested previously by Pb isotope and fluid inclusions studies (Nerci 2006; Tarrieu et al. 2011), the Pb-Zn-Ag mineralizing fluid resulted from mixing of meteoric and crustal fluids, whereas the W-Au mineralizing fluid derived from the mixing of meteoric, crustal, and mantle fluids. The W-Au and the Pb-Zn- Ag deposits are thus clearly unrelated genetically.

With the exception of some inherited cores and two very young dates, all individual ages are 320 to 240 Ma with a polymodal distribution (Fig. 4C). Three main periods can be distinguished: 320-300 Ma, 300-280 Ma, and 280-240 Ma. These periods are well correlated with intercept ages obtained for the stocks and one dike. The two older magmatic periods are well documented throughout Morocco, but the 280-240 Ma interval has only been evidenced on a single microdioritic dyke from the Jebilet massif, using the K-Ar chronometer on kaersutite (Gasquet et al. 1996). At a regional scale, this late magmatic pulse might represent the link between Variscan/late- Variscan granite emplacement and PermoTriassic volcanism related to rifting during formation of the Central Atlantic Magmatic Province (Gasquet et al. 1996; Verati et al. 2007).

Metallogeny of North and West Africa

1581

Two 10-15 µm-sized monazite crystals f r o m the Pb-Zn-Ag gangue carbonates were large enough to allow LA-ICPMS Th/Pb dating (Gasquet et al. 2010; Fig. 4B). These monazite grains give ages of 254 ± 16 Ma and 257 ± 48 Ma. The former, better-defined, age is assumed to date the Pb-Zn-Ag mineralizing event, which is contemporaneous with the youngest magmatic pulse (dikes; Fig. 4C).

6 Discussion and conclusion The Pb-Zn-Ag mineralization of the Tighza-Jbel Aouam district has been dated for the first time at 254 ± 16 Ma. Such a young age confirms field evidence and geochemical data, and indicates that the Pb-Zn-Ag event is not connected with the W-Au occurrences as suggested previously. The polymetallic Tighza-Jbel Aouam district thus developed during multiple injections of Cordilleran-type, calc-alkaline intrusives that produced hydrothermal pulses, each of which was associated with distinctive stages. The two main mineralizing episodes, W-Au then Pb-Zn-Ag, therefore could be related to two successively telescoped porphyry- then epithermal-style environments. This model leading to the base and precious metal zonation pattern in a polymetallic district has also been encountered in other giant districts (e.g., Morococha in central Peru; Catchpole 2011; Catchpole et al. 2015). In the TJAD, fluid flow and related polymetallic mineralization w e r e generated during a late-Variscan to Permo-Triassic transpressional regime (Michard et al. 2008), which favored emplacement of mantle- and crustally derived magmas, together with mixing of fluids of diverse origins. Fluid and magma emplacement were channeled by the Tighza fault, which played a major role in the district in localization and zonation of the polymetallic mineralization.

Acknowledgements This work was funded by the CMT mining company, INSU-CNRS, and the University of Savoie. It was realized in close partnership with geologists of Tighza mine, and benefitted from discussions with Prof. M. Bouabdellah of Oujda University.

References Agard J, Balcon JM, Morin P (1958) Etude géologique de la région minéralisée du Jebel Aouam (Maroc Central). Notes et Mémoires du Service Géologique du Maroc, 132, 127pp Burnard PG, Hu R, Turner G, Bi XW (1999) Mantle, crustal and atmospheric noble gases in Ailaoshan gold deposits, Yunnan Province, China. Geochim Cosmochim Acta 63:1595-1604 Catchpole HP (2011) Porphyry-related polymetallic mineralisation in the Morococha district, central Peru: mineralisation styles, timing and fluid evolution. PhD thesis, University of Genève, Switzerland Catchpole H, Kousmanov K, Putlitz B, Seo JH, Fontboté L (2015) Zoned b ase m etal m ineralization in a p orphyry s ystem: origin and evolution of mineralizing fluids in the Morococha district, Peru. Econ Geol 110:39-71

1582

Cheilletz A (1984) Contribution à la gîtologie du district polymétallique (W-Mo-Cu-Pb-Zn-Ag) du Djebel Aouam (Maroc Central) : application à la prospection des gisements de tungstène. PhD thesis, Institut National Polytechnique de Lorraine, France Cheilletz A, Isnard P (1985) Contribution à la prospection des gisements hydrothermaux de tungstène sur l’exemple du district polymétallique W-Pb-Zn-Ag du Jbel Aouam (Maroc Central). Min Deposita 20:220-230 Cheilletz A, Gasquet D, Filali F, Archibald DA, Nespolo M (2010) A Late Triassic 40Ar/39Ar age for the El Hamman high-REE fluorite deposit (Morocco): mineralization related to the CAMP? Min Deposita 45:323-329 El Dursi,K (2009) Minéralisations et circulations péri-granitiques : modélisation numérique couplée 2D/3D, applications au district minier de Tighza (Maroc Central). PhD thesis University of Orléans, France Gasquet D, Stussi JM, Nachit H (1996) Les granitoïdes hercyniens du Maroc dans le cadre de l’évolution géodynamique régionale. Bull Soc Géol France 167:517-528 Gasquet D, Bertrand JM, Paquette JL, Lehmann J, Ratzov G, De Ascenção Guedes R, Tiepolo M, Boullier AM, Scaillet S, Nomade S (2010) Miocene to Messinian deformation and hydrothermalism in the Lauzière massif (French W-Alps): new U-Th-Pb and argon ages. Bull Soc Géol France 181:227-241 Giulani G, Cheilletz A, Mechiche M (1987) Behaviour of REE during thermal metamorphism and hydrothermal infiltration associated with skarn and vein-type tungsten ore bodies in central Morocco. Chem Geol 64:279-294 Levresse G (2001) Contribution à l'établissement d'un modèle génétique des gisements d'Imiter (Ag-Hg), Bou Madine (PbZn-Cu-Ag-Au) et Bou Azzer (Co-Ni-As-Au-Ag) dans l'Anti Atlas marocain. PhD thesis, INPL, Nancy, France Michard A, Hoepffner C, Soulaimani A, Baidder L (2008) The Variscan belt. In: Michard A, Saddiqi O, Chalouan A, Frizon de Lamotte D (eds.) Continental Evolution: The Geology of Morocco. Lectures Notes in Earth Sciences 116, Springer Verlag, Berlin, Heidelberg Nerci K (2006) Les minéralisations aurifères du district polymétallique de Tighza (Maroc Central) : un exemple de mise en place périgranitique tardi-hercynienne. Thèse PhD thesis, Université du Québec à Montréal Pelleter E, Cheilletz A, Gasquet D, Mouttaqi A, Annich M, El Hakour A, Deloule E, Féraud G (2007) Hydrothermal zircons: a tool for ion microprobe U-Pb dating of gold mineralization (Tamlalt-Menhouhou gold deposit–Morocco). Chem Geol 245:135-161 Tarrieu L (2014) Nouvelles données minéralogiques, géochimiques et géochronologiques sur le gisement polymétallique de Tighza (Maroc Central) : contribution à la métallogénie des gisements de métaux de base en contexte post-collisionnel. PhD thesis, University of Grenoble, France Tarrieu L, Gasquet D, Rossi M, Cheilletz A, Deloule E, Bounajma H, Pisapia C, Deschamps P (2011) Hydrothermal Pb-Zn-Ag-F ore deposits in central Morocco: mineralogy and geochronology. Let's talk ore deposits. Proceed Eleventh Biennial SGA meeting, 1:282-284 Verati C, Rapaille C, Féraud G, Marzoli A, Bertrand H, Youbi N (2007) 40Ar/39Ar ages and duration of the Central Atlantic Magmatic Province volcanism in Morocco and Portugal and its relation to the Triassic–Jurassic boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 244:308-325 Watanabe Y (2002) 40Ar/39Ar geochronologic constraints on the timing of massive sulfide and vein-Type Pb-Zn mineralization in the Western Meseta of Morocco. Econ Geol 97:147-157

MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 4