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)
Authors: Idbaroud Mohamed , Bouabdellah Mohammed , Levresse Gilles , Mouguina El Mostafa , Yans Johan , Maacha Lhou 
Published: February 2023

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&#8217;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&#8211;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, (XF=0.198-0.334, XCl=0.001-0.078, XOH=0.642-0.795), whereas Ap-2 is fluorapatite (XF=0.190-0.227, XCl=0.066-0.101, XOH=0.672-0.743). More importantly, LA-ICP-MS analyses indicate that Ap-1 is characterized by &#8721;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 &#931;REE+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&#8211;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).&#160;