Cu–Mn–Fe alloys and Mn-rich amphiboles in ancient copper slags from the Jabal Samran area, Saudi Arabia: With synopsis on chemistry of Fe–Mn(III) oxyhydroxides in alteration zones

Abstract

In the Jabal Samran area (western Saudi Arabia), secondary copper mineralization in a NE-trending shear zone in which the arc metavolcanic host rocks (dacite–rhyodacite) show conjugate fractures and extensive hydrothermal alteration and bleaching. The zones contain frequent Fe–Mn(III) oxyhydroxides (FeOH–MnOH) that resulted from oxidation of pyrite and Mn-bearing silicates. In the bleached part, the groundmass is represented by Fe-bearing interstratified illite–smectite with up to 4.02wt% FeOt. FeOH–MnOH are pre-weathering phases formed by hydrothermal alteration in a submarine environment prior to uplifting. Five varieties of FeOH are distinguished, four of them are exclusively hydrothermal with ∼20wt% H2O whereas the fifth contains ∼31–33wt% H2O and might represent reworking of earlier hydrothermal FeOH phases by weathering. FeOH fills thin fractures in the form of veinlets and crenulated laminae or as a pseudomorph for pyrite, goethite and finally ferrihydrite, and this oxyhydroxide is characterized by positive correlation of Fe2O3 with SiO2 and Al2O3. On the other hand, MOH shows positive correlation between MnO2 and Al2O3 whereas it is negative between Fe2O3 and SiO2. Paratacamite is the most common secondary copper mineral that fills fractures and post-dates FeOH and MnOH. It is believed that Cl− in the structure of paratacamite represents inherited marine storage rather than from surfacial evaporates or meteoric water.The mineralogy of slags suggests a complicated mineral assemblage that includes native Cu prills, synthetic spinifixed Mn-rich amphiboles with 16.73wt% MnO, brown glass and Ca–Mn–Fe phase close to the olivine structure. EMPA indicate that the some Cu prills have either grey discontinuous boarder zone of S-rich Mn–Cu alloy (with up to 21.95wt% S and 19.45wt% Mn) or grey Cu–Mn–Fe alloy (with up to 15.9wt% Cu, 39. 12wt% Mn and 61.64wt% Fe). Mn in the Cu prills is expelled inward as Cu–Mn–Fe alloy inclusions whereas S is expelled outward as S-rich Mn–Cu alloy crust. Remains in the Samran smelter sites suggest the use of charcoal as a source of energy, quartzite as a flux and an air-cooling technique was used.