Two mineralizing events unraveled by trace element geochemistry of ore-related apatite from the polymetallic Cu-(Pb-Zn-Ba) Oumjrane deposit (Eastern Anti-Atlas, Morocco)

The Oumjrane copper deposit in eastern Anti-Atlas is one of the top three major copper producers in North Africa with an annual production exceeding 260,000 tons at an average grade of 1.4% Cu. Current inferred resources are estimated at 5 million metric tons grading 1.5% Cu. Host rocks consist of a succession of Ordovician carbonate-free siliciclastic metasediments comprising sandstone- and quartzite-dominated rocks with cherty beds and phosphate nodules, conformably overlain by Silurian organic-rich black-shales. In the mine area, high-grade ores were recovered from open pits and underground workings of three major roughly continuous, subparallel, and steeply dipping vein systems referred to as Bou N’Hass, Bou Kerzia, and Zagora Graben over a strike length exceeding 10 km. The veins extend vertically for as much as 300 m, and their widths vary from ~2 m to ~15 m averaging 4m. The paragenetic sequence comprises two main stages of alteration and related mineralization. The earliest stage, referred to as the main Cu-rich stage I, is economically the most productive consisting of ore-related quartz together with chalcopyrite and reddish to orange apatite (i.e., Ap-1). The later Pb-Zn-Ba-rich stage II consists of variable proportions of galena and sphalerite enclosed in a matrix of barite as the main gangue minerals. Texturally, this stage ends with the development of well-developed green apatite crystals (i.e., Ap-2) up to 12 cm across. In addition to its potential for U–Pb dating, the REE+Y chemistry of the paragenetically well-constrained ore-related apatite (i.e., Ap-1, Ap-2) could provide insights into fluid chemistry, fluid-rock conditions, and fluid-rock interaction processes that prevailed during ore deposition of both ore stages. In this respect, EMPA analyses show that Ap-1 is almost pure hydroxyapatite, (X_F=0.198-0.334, X_Cl=0.001-0.078, X_OH=0.642-0.795), whereas Ap-2 is fluorapatite (X_F=0.190-0.227, X_Cl=0.066-0.101, X_OH=0.672-0.743). More importantly, LA-ICP-MS analyses indicate that Ap-1 is characterized by ∑REE+Y contents ranging from 962 to 2435 ppm and shows bell-shaped PAAS-normalized patterns with prominent negative Y anomaly Conversely, Ap-2 yields on average the highest ΣREE+Y concentrations at 8473 ppm, and shows negative sloped pattern with no Eu anomaly. The Y/Ho values are variable, but are generally distributed between 15 and 18 for Ap-1, and 23 and 26 for Ap-2. Overall, these geochemical characteristics indicate that there are two temporally distinct mineralizing events. The earlier Zn-rich stage is thought to have occurred in response to hydrothermal circulation of basinal brines and extensive fluid-rock interaction with the phosphate-rich lithologies. Conversely, the late Pb-Zn-Ba stage is linked to mixing between basinal brines and mantle-derived fluids, likely derived from the synchronous Middle Jurassic–Lower Cretaceous alkaline magmatic series. Ongoing U-Pb geochronology of both apatite types is performed to constrain the timing of mineralization and to trace back the ore-forming source(s) and processe(s).