Internal structures and dating of non-sulphide Zn deposits 5 using rock magnetism: insights from the Moroccan High Atlas

International audienceThe renewal of interest in Zn-Pb non-sulphide ores has been induced by mineral processing improvement and leads to new exploration and mining projects in the world. Although the mineralogy is often precisely known, and despite several studies linking ore deposition to regional tectonics, absolute dating of non-sulphide stages is rare and structure of ore bodies was largely disregarded. Geochronological data from non-sulphide ores are essential to timely constrain alteration episodes and to insert supergene ore genesis in the climate and tectonic evolution of the metallogenic province. The access to internal organization of ore could reveal post-mineralization episodes related to supergene evolution. Thus, a rock magnetism study combining anisotropy of magnetic susceptibility (AMS) and palaeomagnetism was performed on four non-sulphide deposits from the Moroccan High Atlas. AMS generally shows similar horizontal magnetic fabrics for ores and the clayey and carbonaceous internal sediments filling karstic cavities. The palaeomagnetic directions of ores and internal sediments are compatible, and the calculated poles are consistent with the last 30 Ma of the Africa apparent polar wander path, with an upper age at 0.78 Ma. The proposed three-step scenario is placed within the evolution of the Moroccan High Atlas belt. Deposition of primary sulphides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation gave rise to the High Atlas fold-and-thrust belt and to regional uplift. Finally, Zn-Pb sulphides hosted in carbonates experienced oxidation under an arid climate to form karst-related Zn-Pb non-sulphide ores. These promising results pave the way for an efficient method to constrain the internal fabrics and age of Zn supergene deposits

Light rare-earth element mineralization in hydrothermal veins related to the Jbel Boho alkaline igneous complex, AntiAtlas/Morocco: The role of fluid-carbonate interactions in the deposition of synchysite-(Ce)

Highlights • The Jbel Boho alkaline igneous complex is characterized by LREE ore-bearing and ore-barren quartz-carbonate veins. • The mineralized veins occur around syenite showing synchysite-(Ce) as main LREE-host mineral. • Fluid inclusion study suggests the transport of LREE by highly saline fluids as Cl-complex under acidic pH conditions. • LREE mineralization results from LREE-bearing fluids neutralization with veins carbonate or by mixing with meteoric fluids. The world's largest mined rare-earth element deposits are associatedwith alkalinemagmatism,making it important to understand the mechanisms leading to magmatic and hydrothermal element enrichment.We present results from late-differentiation-stage hydrothermal veins of the Jbel Boho alkaline complex in the district of Bou Azzer in the Anti-Atlas of Morocco, which show high light-REE enrichments. The REE mineralized veins occur around a syenitic pluton at the centre of the complex and consist of quartz-jasper and quartz veinswhich contain at least twosilica generations. Only the second quartz generation is associatedwith LREE mineralization. The predominant REE-host mineral is the Ca-LREE-fluorcarbonate synchysite-(Ce), mainly present as anhedral crystals up to 200 μm. Somevery small anhedral rhabdophane-(Ce) grainswere also found in a synchysite-bearing quartz vein. Thermometric studies on mainly liquid-vapour-solid fluid inclusions in quartz crystals in the synchysitebearing veins suggest very high salinity (32 to 37 wt% NaCl equiv.) of the mineralizing fluid. Homogenization temperatures from 150° to 250 °C provide the minimum temperature conditions in which the quartz veins were formed. Based on recent experimental data, we propose a model for the formation of hydrothermal REE-Ca-F carbonate deposits in the veins, involving the transport of REE as chloride complexes at lowpH conditions in the presence of fluoride ions. The deposition of synchysite is proposed to result from neutralization of this fluid by mixing of hydrothermal ore fluids with carbonate-rich meteoric water or by interaction with already existing carbonates in the vein, As well as raising the pH, this interaction also provides the Ca+ and CO3 2– ions needed for REE precipitation as Ca-F carbonates. The barren veins show two types of mineralogy and REE patterns: quartz-carbonate veins with enrichment of LREE over HREE and iron-rich jasper-bearing veins with quite flat REE pattern and high HREE. The LREE/HREE fractionation in these veins seems to be controlled by an interplay of two factors: 1) a low activity of ligands like Cl, which favours LREE transport and (2) the mineralogical control, by which HREE having similar ionic radii to Fe2+ will be preferred over LREE

Magmatic evolution of the Jbel Boho alkaline complex in the Bou Azzer inlier (Anti-Atlas/Morocco) and its relation to REE mineralization

Highlights • The Jbel Boho complex is shown to have an alkaline, intraplate geochemical signature. • At least three magma generations are responsible for forming the extrusive-intrusive complex. • The highly evolved and LREE-rich rhyolitic dykes are associated with synchysite-(Ce) mineralization. Abstract The Jbel Boho complex (Anti-Atlas/Morocco) is an alkaline magmatic complex that was formed during the Precambrian-Cambrian transition, contemporaneous with the lower early Cambrian dolomite sequence. The complex consists of a volcanic sequence comprising basanites, trachyandesites, trachytes and rhyolites that is intruded by a syenitic pluton. Both the volcanic suite and the pluton are cut by later microsyenitic and rhyolitic dykes. Although all Jbel Boho magmas were probably ultimately derived from the same, intraplate or plume-like source, new geochemical evidence supports the concept of a minimum three principal magma generations having formed the complex. Whereas all volcanic rocks (first generation) are LREE enriched and appear to be formed by fractional crystallization of a mantle-derived magma, resulting in strong negative Eu anomalies in the more evolved rocks associated with low Zr/Hf and Nb/Ta values, the younger syenitic pluton displays almost no negative Eu anomaly and very high Zr/Hf and Nb/Ta. The syenite is considered to be formed by a second generation of melt and likely formed through partial melting of underplated mafic rocks. The syenitic pluton consists of two types of syenitic rocks; olivine syenite and quartz syenite. The presence of quartz and a strong positive Pb anomaly in the quartz syenite contrasts strongly with the negative Pb anomaly in the olivine syenite and suggests the latter results from crustal contamination of the former. The late dyke swarm (third generation of melt) comprises microsyenitic and subalkaline rhyolitic compositions. The strong decrease of the alkali elements, Zr/Hf and Nb/Ta and the high SiO2 contents in the rhyolitic dykes might be the result of mineral fractionation and addition of mineralizing fluids, allowing inter-element fractionation of even highly incompatible HFSE due to the presence of fluorine. The occurrence of fluorite in some volcanic rocks and the Ca-REE-F carbonate mineral synchysite in the dykes with very high LREE contents (Ce ∼720 ppm found in one rhyolitic dyke) suggest the fluorine-rich nature of this system and the role played by addition of mineralizing fluids. The REE mineralization expressed as synchysite-(Ce) is detected in a subalkaline rhyolitic dyke (with ΣLREE = 1750 ppm) associated with quartz, chlorite and occasionally with Fe-oxides. The synchysite mineralization is probably the result of REE transport by acidic hydrothermal fluids as chloride complex and their neutralization during fluid-rock interaction. The major tectonic change from compressive to extensional regime in the late Neoproterozoic induced the emplacement of voluminous volcaniclastic series of the Ediacran Ouarzazate Group. The alkaline, within-plate nature of the Jbel Boho igneous complex implies that this extensional setting continued during the early Cambrian

The Hajjar Regional Transpressive Shear Zone (Guemassa Massif, Morocco): Consequences on the Deformation of the Base-Metal Massive Sulfide Ore

The genesis of the base-metal massive sulfide deposits hosted within the Moroccan Hercynian Jebilet and Guemassa Massifs is still under debate. No consensus currently exists between the two models that have been proposed to explain the deposits, i.e., (1) syngenetic volcanogenic massive sulfide mineralization, and (2) synmetamorphic tectonic fluid-assisted epigenetic mineralization. Conversely, researchers agree that all Hercynian massive sulfide deposits in Morocco are deformed, even though 3D structural mapping at the deposit scale is still lacking. Therefore, while avoiding the use of a model-driven approach, the main aim of this contribution is to establish a first-order structural pattern and the controls of the Hajjar base metal deposit. We used a classical structural geology toolbox in surface and subsurface mining work to image finite strain at different levels. Our data demonstrate that: i) the Hajjar area is affected by a single foliation plane (not two) which developed during a single tectonic event encompassing a HT metamorphism. This syn-metamorphic deformation is not restricted to the Hajjar area, as it is widespread at the western Meseta scale, and it occurred during Late Carboniferous times; ii) the Hajjar ore deposit is hosted within a regional transpressive right-lateral NE-trending shear zone in which syn- to post-metamorphic ductile to brittle shear planes are responsible for significant inflexion (or virgation) of the foliation yielding an anastomosing pattern within the Hajjar shear zone. Again, this feature is not an exception, as various Late Carboniferous-Permian regional scale wrenching shear zones are recognized throughout the Hercynian Meseta orogenic segment. Finally, we present several lines of evidence emphasizing the role of deformation in terms of mechanical and fluid-assisted ore concentrations

Copper Mineralization in Adoudounian Cover of the Bou Azzer-El Graara (Anti Atlas, Morocco): Tectono-Stratigraphic Controls

International audienceCopper mineralization in sedimentary cover is well known in association with Pb and/or Zn in Sedex, Kupferschiefer, Mississippi Valley-Type, or Red bed deposits. In spite of great economic potential, the syngenetic versus epigenetic origin of widespread Cu occurrences in the Adoudounian sedimentary cover, in the Moroccan Anti Atlas, remains debated. Significant investigations that would provide critical information for mining exploration are lacking. Whatever their origin, these mineralizations do not correspond to classical sediment-hosted deposits, the ore being exclusively Cu concentrations in dolostone units. This study, based on field and microscopic observations, is focused on Cu mineralization hosted in the Adoudounian cover of the Bou Azzer-El Graara inlier. Two morphologic types of ore bodies exist. The first consists of stratabound lenses and the second type is vein networks along Variscan faults. Both types of mineralization have a stockwork texture and clearly postdate sedimentary fabrics, such as beddings and slumps; this suggests a common epigenetic origin for the both ore types. The proposed interpretation involves the upflow of Variscan fluid-driven interaction between a basement and its sedimentary cover

Multiscale Structural Analysis of Ediacaran–Cambrian Rocks on the Northeastern Edge of the Saghro Inlier (Eastern Anti-Atlas): Relevance of Post-Cambrian Deformation

The Ediacaran–Cambrian rocks on the northeastern edge of the Saghro inlier experienced polycyclic tectono-thermal events, which are reported here based on a multiscale structural analysis, from field measurements to fluid inclusion planes. Three striking populations were identified, cutting across both the Ediacaran and Cambrian formations. These tectonic structures were generated during four tectonic events. (i) E-W-striking structures that host ore mineralized bodies (sulfide, oxide, quartz, and barite). They display a polyphase tectonic history, caused by a dextral movement in response to a NW–SE-oriented shortening, leading to the formation of quartz gashes and veins. This tectonic event took place during the Neovariscan. These E–W-striking structures were subsequently reactivated during the Mesozoic time under a sinistral strike-slip regime as a result of NE–SW shortening syn-kinematic with barite mineralization. (ii) NE–SW-striking strike-slip structures (dextral or sinistral) crosscut the E–W-striking veins. These faults are related to the NW–SE-oriented shortening that occurred during the Neogene. (iii) The last tectonic episode, related to the N–S shortening, took place during the late Neogene to the Quaternary period. It resulted in NW–SE to N–S-striking structures that were related to dextral and sinistral strike-slip movements, which crosscut the preexisting E–W structures

Structural, mineralogical, and paleoflow velocity constraints on Hercynian tin mineralization: the Achmmach prospect of the Moroccan Central Massif

International audienceThe Achmmach tin mineralization (NE of theMoroccan Central Massif) is associated with tourmaline-richalteration halos, veins, and faults hosted in sandstones andmetapelites of the Upper Visean-Namurian. These depositsare reported to be late Hercynian in age and related to theemplacement of late-orogenic granite not outcropping in thestudied area. Structural and paragenetic studies of theAchmmach tin deposit were conducted in order to establisha general model of the mineralization. From field constraints,the late Hercynian phase is marked by a transition fromtranspression to extension with deformation conditions evolvingfrom ductile to brittle environments. The transpression(horizontal shortening direction roughly trending E-W) is coevalwith the emplacement of the first tourmaline halos alongseveral conjugated trends (N070, N020, and N120).Thereafter, a tourmaline-rich breccia formed in response tothe fracturing of early tourmaline-altered rocks.Subsequently, during the extensional phase, these structureswere reactivated as normal faults and breccias, allowing theformation of the main tin mineralization (cassiterite) associatedwith a wide variety of sulfides (arsenopyrite, chalcopyrite,sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition.The hydrothermal fluid flow velocity, calculated byapplying statistical measures on the tourmaline growth bands,varies with the lithology. Values are lower in metapelites andhigher in breccia. In the general evolution model proposedhere, tourmaline alteration makes the rock more competent,allowing for brittle fracturing and generation of open spacewhere the main Sn mineralization was precipitated

The Jbel Saghro Au(–Ag, Cu) and Ag–Hg Metallogenetic Province: Product of a Long-Lived Ediacaran Tectono-Magmatic Evolution in the Moroccan Anti-Atlas

The Jbel Saghro is interpreted as part of a long-lived silicic large igneous province. The area comprises two lithostructural complexes. The Lower Complex consists of folded metagreywackes and N070⁻090°E dextral shear zones, which roughly results from a NW⁻SE to NNW⁻SSE shortening direction related to a D1 transpressive tectonic stage. D1 is also combined with syntectonic plutons emplaced between ca. 615 and 575 Ma. The Upper Complex is defined by ash-flow caldera emplacements, thick and widespread ignimbrites, lavas and volcaniclastic sedimentary rocks with related intrusives that were emplaced in three main magmatic flare ups at ca. 575, 565 and 555 Ma. It lies unconformably on the Lower Complex units and was affected by a D2 trantensive tectonic stage. Between 550 and 540 Ma, the magmatic activity became slightly alkaline and of lower extent. Ore deposits show specific features, but remain controlled by the same structural setting: a NNW⁻SSE shortening direction related to both D1 and D2 stages. Porphyry Au(⁻Cu⁻Mo) and intrusion-related gold deposits were emplaced in an earlier stage between 580 and 565 Ma. Intermediate sulfidation epithermal deposits may have been emplaced during lull periods after the second and (or) the third flare-ups (560⁻550 Ma). Low sulfidation epithermal deposits were emplaced late during the felsic alkaline magmatic stage (550⁻520 Ma). The D2 stage, therefore, provided extensional structures that enabled fluid circulations and magmatic-hydrothermal ore forming processes

Structural control, magmatic-hydrothermal evolution and formation of hornfels-hosted, intrusion-related gold deposits: Insight from the Thaghassa deposit in Eastern Anti-Atlas, Morocco.

International audienceIn the Moroccan Eastern Anti-Atlas, the Thaghassa intrusion-related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the Ikniwn granodiorite. Fields studies reveal three tectono-magmatic stages controlling the formation of the deposit. i) The first stage refers to the top-to-the-south asymmetry and the syn-kinematic Ikniwn pluton emplacement controlled by a compressional or transpressional strain regime. ii) The second stage is characterized, from older to younger and further away from the intrusion, by: metatexite with leucocratic stromatic bands, aplo-pegmatite sills, intermediate veinlets composed of quartz, K-feldspar and muscovite, and then gold-bearing striped foliation-veins. All these features are assumed to have been emplaced during a large-scale ENE-WSW dextral shearing process that results from an ESE-WNW shortening direction during transtensive tectonics. We suggest that the progressive and continuous shearing was initiated since the aplo-pegmatite stage and achieved during the hydrothermal phase. The existence of intermediate veins characterized by quartz-rich core and apatite-muscovite-feldspar-rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character, at least until the onset of the hydrothermal process. iii) The late stage developed large volcanic dyke swarm and brittle faulting.Zircon U-Pb LA-ICP-MS dating yields a Concordia age of 563.5 ± 6.3 Ma for the Ikniwn granodiorite intrusion. The fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted: i) a hot aquo-carbonic (H2O-NaCl-CO2) fluid with N2 and CH4, evolving from vapour-rich N2 and CH4 inclusions for the magmatic stage (∼550°C) to CH4-CO2 biphased inclusions for the ongoing hydrothermal stage (∼450-300°C) and ii) an always secondary low T (∼200°C) saline aqueous type (probably NaCl) free from volatiles, with very variable salinity. Eventually, we highlight that in the hornfels-hosted IRG deposits, fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks