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 syn- to 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 H₂O-rich fluid containing dissolved CO₂, which ultimately exsolved an early high-salinity aqueous fluid from the Tiouit granitic magma. This early, high-salinity and CO₂-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.