Re/Os Age Determination, Lead and Sulphur Isotope Constraints on the Origin of the Bouskour Cu–Pb–Zn Vein-Type Deposit (Eastern Anti-Atlas, Morocco) and Its Relationship to Neoproterozoic Granitic Magmatism

Abstract

The newly re-evaluated Bouskour deposit is a large, polymetallic vein-type system in the Precambrian Sidi Flah-Bouskour inlier of the eastern Anti-Atlas orogen. Resources are >53 Mt at 0.8 % Cu, of which 21 Mt has a higher grade of 1.3 % Cu and 9 g/t Ag. Host rocks are assigned to the Ediacaran Ouarzazate Group, and consist of a succession of mafic-ultramafic to felsic igneous rocks, both intrusive and extrusive, with ages ranging from 570 ± 5 to 557 ± 5 Ma, partly intruding Cryogenian basalt and andesite. Among these igneous bodies, intrusive felsic rocks are by far the most abundant lithologies, consisting of three elongate, NW-SE-trending, calc-alkaline intrusions referred to as: (1) Bouskour Granodiorite, (2) Bouskour Granite dated at 570 ± 5 Ma, and (3) amphibole-bearing Bouskour Granodiorite. An extensive dike system of red to white aphanitic rhyolite locally known as the “Bouskour rhyolitic dike swarm” intruded all older units at 564 ± 7 to 562 ± 5 Ma. Cu–Pb–Zn sulphide mineralization produced an array of five transtensional, N-S and ENE- to NNW-trending vein systems referred to as “Filon Principal,” “Filon Ouest,” “Filon I,” “Filon II,” and “Filon Camra.” The veins are up to 20 m wide and extend laterally from a few hundred meters to more than 10 km, spaced 50 to >1000 m apart, striking predominantly N160° E with steep dips (70° to ~90°). Characteristic are comb, cockade, laminated, breccia, and crack and seal textures, suggesting that episodic mechanisms were important in vein formation. Sulphide minerals consist predominantly of various proportions of chalcopyrite, bornite, galena, sphalerite, pyrite, and arsenopyrite, with subordinate tetrahedrite-tennantite, magnetite, cassiterite, and rutile. The hydrothermal alteration assemblage comprises sericite, epidote, chlorite, quartz, and carbonates (calcite, dolomite). Three successive stages of ore deposition are recognized. The earliest stage (I) is referred to as “Cu–Zn–(Fe–As–Co–Bi–Sn) stage” is followed by the main Cu–Pb–Zn sulphide stage (II), which accounts for most of the exploited sulphide ore at Bouskour. The latest stage (III) forms late fillings in calcite or quartz veins or within older stage I and II assemblages, and includes Au–Ag–Cu–(Zn–Pb) mineralization. New ¹⁸⁷Re/¹⁸⁸Os age dating of a single molybdenite crystal from stage II yields an age of 574.9 ± 2.4 Ma. This age coincides, within analytical uncertainty, with a SHRIMP U–Pb age on zircon from the Bouskour Granite. The age correspondence suggests a foremost role of granite-derived hydrothermal fluids in the genesis of this polymetallic vein system, consistent with sulphur and lead isotopic data. This timing of mineralization relates to the final, post-collision extensional stage of the Pan-African orogeny.