Abstract
The Igoudrane mine with a total production of 700,000 t of ore grading 485 g/t Ag is currently one of the most productive mines in the Imiter district of the eastern Anti-Atlas in Morocco. The silver-rich ± base metal deposit occurs dominantly as vein- and hydrothermal breccia-hosted orebodies at the interface between the lower Ediacaran turbidites of the Saghro Group and the unconformably overlying, dominantly felsic volcanic, and volcaniclastic rocks of the late Ediacaran Ouarzazate Group. Higher-grade ores are lithologically hosted by the uppermost organic-rich black shale unit and structurally controlled by the intersection of subvertical NW- and NE-trending fault systems. Ore-related hydrothermal alteration includes, in order of decreasing abundance, carbonatization, silicification, sericitization, and chloritization. Three primary paragenetic stages of veining and associated silver ± base metal mineralization have been recognized: (1) early pyrite + quartz + Ag-bearing sulfides and sulfosalts; (2) main Ag-bearing sulfides and sulfosalts + calcite ± fluorite ± dolomite; and (3) late quartz + calcite + base-metal sulfides (galena, sphalerite, pyrite, chalcopyrite). Irrespective of the ore stage, the dominant Ag-bearing ore minerals are Ag-Hg amalgam, argentite, freibergite, acanthite, polybasite, pyrargyrite, and proustite. Fluid inclusion data show a trend of decreasing temperatures with time, from the main silver stage (Th = 180 ± 12 °C) to late base-metal stage (Th = 146 ± 7 °C), consistent with fluid mixing, cooling, and/or dilution. The coexistence of aqueous-rich and vapor-rich fluid inclusions together with variations in bulk salinity (NaCl + CaCl2) of the mineralizing fluids during the main silver stage, at similar temperatures, indicate that boiling and subsequent degassing occurred during the main ore-forming event due to a pressure decrease. Calculated δ18Ofluid values along with REE+Y and Sr isotope constraints suggest that the ore-forming fluids originated from a predominantly magmatic source, although incursion of meteoric waters during collapse of the hydrothermal system could have contributed to deposition. The post-ore, base-metal quartz-carbonate-dominated mineralization was deposited from dilute Ca-Na-Cl-bearing fluids at temperature below 150 °C. Overall, fluid–rock interaction with the black shales along major faults and thin permeable horizons, boiling-degassing—with subsequent fluid mixing, cooling, and/or dilution—were the main mechanisms of silver deposition.
Highlights
The world-class Imiter district of the Moroccan Anti-Atlas system consists of six closely distributed silver-rich mines referred to, from west to east, as Imiter 1, Imiter 2, Imiter Sud, B8 structure, Puits IV, and Igoudrane (Figure 1A,B); all of which are currently exploited and explored by Managem Group
The disagreement regarding the age of mineralization has created controversy over the sources of ore-forming fluids and metals, and both magmatic-epithermal model at 550 ± 3 Ma coincident with the emplacement of Neoproterozoic felsic intrusions [1,2,3,4] and deep circulation of basinal brines related to the opening of the Atlantic Ocean during the Permian-Triassic times [5] have been proposed
The samples plot in the field of metaluminous to peraluminous (Figure 6B) high-K calc-alkaline series (Figure 6C), and display subparallel similar slightly to moderately chondrite-normalized light rare earth elements (REE)-enriched patterns (La/SmCN = 1.11–7.03 and La/YbCN = 2.46–21.03) with pronounced negative Eu anomalies (Eu/Eu* = 0.16–0.97) (Figure 6D). This geochemical similarity and their stratigraphic relationships suggest that the different host rocks are petrogenetically related, with the felsic and mafic counterparts being potentially derived from the same differentiating parent magma
Summary
The world-class Imiter district of the Moroccan Anti-Atlas system consists of six closely distributed silver-rich mines referred to, from west to east, as Imiter 1, Imiter 2, Imiter Sud, B8 structure, Puits IV, and Igoudrane (Figure 1A,B); all of which are currently exploited and explored by Managem Group. The disagreement regarding the age of mineralization has created controversy over the sources of ore-forming fluids and metals, and both magmatic-epithermal model at 550 ± 3 Ma coincident with the emplacement of Neoproterozoic felsic intrusions [1,2,3,4] and deep circulation of basinal brines related to the opening of the Atlantic Ocean during the Permian-Triassic times [5] have been proposed The resolution of this controversy is crucial because the two proposed Minerals 2021, 11, x FOR PEER REVIEmWodels place Imiter orebodies in very different geologic settings that lead to contras3toinf 3g8 global exploration strategies. T[h6e,7i]n)s.eTthsheoinwssethseholowcasttiohne olofctahteioAnnotiftAhetlaAsnbtei-lAt wtlaitshbineltthwe firthaminetwheorfkraomf MewoororkccoofaMndorAocfrciocaacnodnAtinfreincat.c(Bon) tGinoeongtl.e(EBa)rGthopoigclteuEreasrthhowpiicntgurteheshsopwatiinalgdtihsetrsibpuattiioanl doifsttrhiebusitxiopnroinfctihpealsimx ipnreisncthipaatlfmorimnetshtehwatofrolrdm-cltahseswsiolvreldr--rcilcahssImsilivteerr-driicsthriIcmt.iter district
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