Skarn to Porphyry-Epithermal Transition in the Ouixane Fe District, Northeast Morocco: Interplay of Meteoric Water and Magmatic-Hydrothermal Fluids

Abstract

The Ouixane Fe skarn district in the northeastern Alpine Rifan belt is the highest grade iron district in Morocco with past production of 65 Mt of ore at >50 % Fe and estimated remaining reserves of 30 Mt grading 58 % Fe. Overall, the district consists of three major deposits, distributed in a 6 × 6 km zone along the northeastern part of the Beni Bou Ifrour Massif. Mineralization occurs either within the 7.58 ± 0.03 Ma Ouixane post-collisional, hornblende-biotite quartz-diorite porphyry and related dike swarms, or more importantly at contacts of the porphyry with a ~1,500-m-thick sequence of Upper Jurassic-Lower Cretaceous turbiditic and volcaniclastic sedimentary rocks. Igneous rocks have high-K, calc-alkaline to shoshonitic affinities and REE patterns that are consistent with emplacement in a typical arc setting. Concordance between the age of mineralization, which is thought to have occurred at 7.04 ± 0.47 Ma, and the 7.58 ± 0.03 Ma crystallization age of the Ouixane quartz-diorite porphyry constitutes strong evidence for a genetic relationship between Ouixane magnetite skarn mineralization and Late Neogene magmatism. Structural controls were important in focusing fluids and localizing the emplacement of late mineralizing phases. The ore zones have undergone strong post-ore displacement along steeply dipping, predominantly thrust faults, with significant remobilization of ore. Initial skarn formation started with isochemical reconversion of the hot rocks to marbles and hornfels and evolved through time into metasomatic replacements of carbonate precursors by calc-silicate, oxide, and sulphide minerals. High-temperature, early prograde mineral assemblages are dominated by pyroxene (Di₉₅Hd₅–Di₄₆Hd₅₃; max johannsenite content of 3 mol%) and garnet (Ad₉₈Gr₂–Ad₄₀Gr₆₀), with minor magnetite, K-feldspar, plagioclase, titanite, and scapolite. Low-temperature hydrous retrograde assemblages formed by nearly the complete replacement of the preexisting anhydrous skarn assemblage involve various proportions of amphibole, biotite, chlorite, epidote, quartz, calcite, barite, and sulphides. High Fe grades are spatially related to retrograde skarn zones that developed from carbonate precursors. A later potassic ± phyllic ± propylitic alteration overprints the calc-silicate mineral assemblages. The resulting alteration halos are spatially associated with porphyry-type and epithermal-style mineralization. The skarn mineral compositions indicate emplacement under high f _O2 conditions that shifted through time to a reduced environment. Such an evolutionary trend may record the temporal decrease of f _O2 and corresponding increase of pH and f_S2. Epithermal sulphide-rich mineralization occurred at progressively lower pressures, shallower depths, and lower temperatures, relative to iron-rich skarn mineralization. Indeed, fluid inclusion data together with oxygen isotopic compositions are consistent with the involvement of early high-temperature (347–600 °C), low CO₂ (<0.05 mol%), NaCl–KCl boiled magmatic brines, that mixed outward with increasing components of cooler (300–125 °C) and dilute (up to 7 wt% NaCl equiv) fluids probably of meteoric origin, which re-equilibrated with the early magmatic fluids. Late epithermal sulphide-rich veins formed at temperatures of 250–125 °C and pressures of <100 bars, corresponding to shallow depths of «1 km. The shift from oxidized to reduced fluid conditions is attributed to a transition in magma evolution from high-K calc-alkaline to shoshonitic compositions.