The Granitoid-Related Tiouit Gold Deposit, Saghro Inlier, Eastern Anti-Atlas (Morocco): Neoproterozoic Mineralization by a Polyphase Late-Magmatic to Hydrothermal System

Abstract

The Tiouit gold deposit occurs in the central part of the Precambrian Saghro inlier in the eastern Anti-Atlas orogen. It is the second-largest gold producer in Morocco with a total production of ca. 1 Mt of ore at an average grade of 7.9 g/t Au, 67 g/t Ag, and 0.45 % Cu. Host rocks consist of metamorphosed sedimentary, volcanic, and volcaniclastic rocks of the Neoproterozoic Ouarzazate Supergroup locally intruded by the synto post-tectonic, I-type, calc-alkaline Ikniwn Granodiorite and Isk-n-Alla Granite. Auriferous mineralization is restricted to the southwestern part of the hydrothermally altered 690 ± 57 Ma Tiouit granodiorite, forming discontinuous sulphide veins and disseminations that trend N-S to N10°E and dip 10°-30°E to SE. Four mineralized zones are recognized, referred to as North, Central, South, and Northeast. The veins are up to 6 m wide, averaging 1.5-2 m, and extend along strike 40-300 m. Silicified halos containing up to 2-3 % disseminated pyrite are common near the veins; the highest gold and sulphide concentrations are confined to veins that exhibit the largest pyrititic halos. Primary mineral assemblages consist of varying amounts of intergrown galena, sphalerite, chalcopyrite, pyrite, electrum, and native gold, with subordinate arsenopyrite, argentite, sulphosalts (tetrahedrite-tennantite), and iron oxides plus quartz. Native gold occurs either as: (1) inclusions disseminated in sulphide grains, typically at boundaries between pyrite-chalcopyrite and arsenopyrite-galena-sphalerite-aikinite, interpreted to constitute the first generation of gold (Au-1); (2) anhedral fillings in pyrite, where gold (Au-2) is commonly accompanied by chalcopyrite; and (3) fillings of fractures in quartz ± chlorite ± hematite ± muscovite ± K-feldspar. Owing to the absence of radiometric age constraints, the inferred age for gold mineralization is attributed to emplacement and subsequent crystallization of the Ikniwn Granite. Mineralogical, textural, and geochemical data together with fluid inclusion measurements are consistent with phase separation of an H 2 O-rich fluid containing dissolved CO 2 , which ultimately exsolved an early high-salinity aqueous fluid from the Tiouit granitic magma. This early, high-salinity and CO 2 -rich fluid progressively evolved and mixed with larger volumes of colder, oxygenated, and more acidic solutions resulting in the precipitation of the Tiouit auriferous and base-metal mineralization.

Mapping of favourable mining areas in the Tiouit area by multispectral remote sensing and airborne gamma-ray spectrometry coupled with geochemical data (Eastern Anti-Atlas, Morocco)

“…It is marked by a reddish colouration, caused by the partial or total replacement of the ferro-magnesians (biotite and amphibole) by haematite. Haematization is more intense near gold mineralization, especially in the Southern sector, where it predominates, with silicification, in the so-called passage zones (Jettane 1995; Al Ansari and Sagon 1997; Chaker 1997; Benchekroun and Jettane 2004; Bouabdellah et al 2016). Chloritization: is very localized, and it is developed mainly in the Northern zone of Tiouit and on the mineralized veins wall.…”

“…Ferro-magnesian minerals are sometimes partially or entirely replaced by chlorite. This phenomenon is accompanied by pyritization, carbonate, and epidote veinlets and is interpreted as propylitization due to the formation of granophyre (Jettane 1995; Benchekroun and Jettane 2004; Bouabdellah et al 2016). In general, chloritization is well developed, especially on the wall of mineralized structures, unlike haematization, which would be more expressed on the roof. Intermediate zone: In this zone, chlorite and haematite are balanced.…”

“…It is located between the chlorite zone in the North and the haematite zone (centre, East, and South of Tiouit), and it is marked by the superposition of these two alteration phenomena making the ferromagnesians (biotite, amphibole) almost opaque and difficult to recognize (Jettane 1995; Chaker 1997). According to the work of El Hjouji (1990), at the level of the mineralized gangue, it shows an addition of chloritization and silicification, as well as an important sericitization and argilization (Jettane 1995; Al Ansari and Sagon 1997; Chaker 1997; Benchekroun and Jettane 2004; Bouabdellah et al 2016).…”

“…The mapping of the hydrothermal alteration zones (Figures 5 and 8) highlighted the main alteration zones, including a large hydrothermal alteration zone observed at the Tiouit gold mine associated with the Ikniouin granodiorite of the Lower PII. According to previous works (Jettane 1995;Al Ansari and Sagon 1997;Chaker 1997;Benchekroun and Jettane 2004;Bouabdellah et al 2016), several alterations affect its phenomena reflecting its various interactions with post-magmatic hydrothermal solutions.…”

“…All of the recognized gold structures that have been mined at Tiouit are located in an area of 6 km 2 Southwest of the host granodiorite. The gold veins filling is ensured, in variable proportions, by two main facies; a sulphidic facies relatively rich in gold (up to 300 g/t Au), and siliceous facies with free gold grains, of the order of 10 g/t Au (Benchekroun and Jettane 2004; Bouabdellah et al 2016).…”