The Jebel Oust dome F–(Hg–Cu) vein–type deposit in the Zaghouan Fluorite District is located at the intersection zone of the north–south axis (NOSA) and the NW-trending Zaghouan Messinian Rift. ...Mineralization occurs in E–W and NW–NE-trending fault zones cutting Liassic to Tithonian Age limestones, intercalating between the Triassic salt (below) and the Bir M’Chergua Shale (above). The ores form banded veins (calcite and fluorite) and matrix breccia veins (calcite–fluorite–schwazite–cinnabar). The paragenetic sequence is formed by macrocrystalline sparry calcite (Cal1), followed by massive macrocrystalline fluorite, then by matrix breccia fluorite with Hg minerals (schwazite and cinnabar). Late veins are composed of onyx-like calcite (Cal2), with no fluorite or sulfide minerals. Both fluorites show halite-bearing inclusions (LVH), carbonic inclusion CO
2
gas, and aqueous inclusions (LV). They are also present in sparry calcite and onyx-like calcite. The fluid inclusion microphotometry, coupled with the paragenetic sequence, shows that the mineralization results from the mixing of at least two fluids: a high-salinity and high-temperature fluid (32 to 34 wt% NaCl eq. and Th from 225 to 282 °C) and low-salinity and low-temperature fluid (5 to 12.8 wt% NaCl eq. and Th between 85 and 144 °C). The fluorite ore is formed when the hot waters enriched with fluorine, Hg, Cu, and salt rise towards the calcium-rich Jurassic limestone and mix with low-temperature and low-salinity groundwaters. Fluorite derives its calcium from pre-deposited calcite and from the limestone wall rock. The high-salinity fluid acquires its salinity from underlying Triassic salt. The high temperatures are related to an abnormal high geothermal gradient with thermal heat due to crustal thinning and rifting during the Messinian extensional episode of the Alpine orogen.
The Khenchela massif, northeast Algeria, belongs to the eastern Saharan Atlas that extends northeasterly from the Aurès Chain, through the Mellegue mountains, to the Tunisian Atlas. This massif is ...characterized by sandstone and marl outcrops of the Lower Cretaceous that are overlain by limestone and marl of the Upper Cretaceous. The anticline structure was subjected to several NW–SE to WNW-ESE-directed major normal faults that split the massif into a number of mega-blocs. Triassic evaporitic lithologies crop out as diapirs at the northeastern and southwestern parts of the anticline. The Ain Mimoun region, located on the northern flank of the Khenchela anticline, is composed of Albian-Aptian sediments that host the barite veins of the so-called ‘Mizab barite deposit’. This deposit, which is currently exploited, consists of a dozen main veins composed mainly of barite with quartz, calcite, ankerite and rare base-metal sulphide mineralisation (galena, sphalerite, tennantite and chalcopyrite). The host sedimentary rocks were subjected to severe silicification and dolomitisation processes. Barite shows a number of aspects: banded, massive and stockwork; in all cases, barite shows tabular crystals of several millimeters in size. Microthermometric measurements carried out on primary two-phase fluid inclusions in barite crystals (barite I and barite II) and gangue quartz gave homogenization temperatures between 155 and 225 °C and salinities of 17.6 to 25.9% NaCl eq. The data show at least three types of mixing fluids that deposited the sulphate-sulphide mineralisation. δ
34
S values of barite are between + 17.9 and + 27.6‰ and the δ
34
S values of the associated sulphides vary between − 9.2 and + 3.0‰. These data indicate that the most likely source of sulphur is the sulphates (gypsum) of the Triassic evaporitic formation (δ
34
S ranging between + 11.5 and + 13.4‰). Thermochemical sulphate reduction is the most probable process by which H
2
S was produced, although relatively large negative δ
34
S values point to a possible minor contribution from the biogenic sulphate reduction process. Carbon and oxygen isotopic compositions of gangue carbonates (calcite and ankerite) and oxygen of gangue quartz indicate a common source of the mineralising fluid. Calculated δ
18
O
fluid
from quartz oxygen isotope data varies between + 6 and + 12‰, indicating deep-seated brines with minor contribution from a surface water component. The relatively hot and saline fluids have most likely migrated upward in several pulses during the Triassic halokinesis and compressive/distensive Atlassic (Eocene) and Alpine (Miocene) tectonic phases. The Mizab barite deposit exhibits geological, fluid inclusion and isotopic features similar to those of the peridiapiric mineralisation of northeastern Algeria and Tunisia.
•This is one of very few description of Tertiary oolitic ironstone of the Jebel Ank deposit.•Review the geological and mineralogical relationships and presents new spectroscopic and geochemical ...data.•We present and discuss the possible iron source(s).
The oolitic ironstones ore deposit of Jebel Ank (central Tunisia), is a simply folded stratiform ore body of about 2.5–8m thickness located in the upper part of the epicontinental Souar Formation (Late Eocene) and is covered by the continental Segui Formation (Mio-Pliocene). The deposit contains about 20Mt of ore with an average grade of 50% Fe. Generally, oolitic iron deposition occurs in shallow water lagoonal environments. The Jebel Ank deposit lies between two regional disconformities (Late Eocene and Miocene), and is evidence of a transitional stage at the end of regional regression before renewed transgression. The footwall of the oolitic iron ore-bearing bed consists of a fine-grained sandstone bed (10–20cm-thick) pinching out laterally westward into green clays. The hanging wall is composed of thin-bedded limestone and clay alternations (2–3.5m-thick).
Iron occurs in the form of cryptocrystalline goethite with limited Al-Fe substitution. The goethite contains around 48% Fe, 5% Al and up to 1.5% P. Jarosite, alunite and manganese minerals (cryptomelane, psilomelane and manjiorite) are supergene secondary minerals, probably related to descending surface fluids. These manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies (local cement, space filling and free centimeter sized nodules). Fe-oolites in the deposit are similar to those documented in many other oolitic ironstone deposits. The dominant Fe-oolite type (>90%) has a concentrically laminated cortex with no nucleus. The nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains.
Major elements in high grade samples (Fe2O3>65%) vary within a limited range and show higher concentrations of SiO2 (average 7.85%) and Al2O3 (average 5.1%), with minor TiO2, MnO, MgO, Na2O, K2O, and SO3 (less than 1%). PAAS-normalized trace elements of bulk samples and Fe-oolite generally show similar behavior, both are enriched in V, Co, Ni, Mo, As, Zn, and Y and are depleted in Cu, Rb, Zr, Nb, Ba, and Hf. Anomalous V, Cr, Ni, Zn, and REE-Y are correlated with goethite. PAAS-normalized REE-Y patterns of both bulk samples and Fe-oolite show slight HREE enrichment, positive Ce with negative Y anomalies.
The mineralogy (goethite and cryptomelane) along with the geochemistry (Si vs. Al; As+Cu+Mo+Pb+V+Zn vs. Ni+Co binary plots; Zn–Ni–Co triangular diagram, REE-Y content and patterns and Ce/Ce∗vs. Nd and Ce/Ce∗vs. YN/HoN binary plots) of the studied oolitic ironstone are congruent with a hydrogenetic type. While two possible sources of iron for Jebel Ank ironstone can be proposed: (i) submarine weathering of glauconite-rich sandstone and (ii) detrital iron from adjacent continental hinterland, the later is the more plausible source of iron, based on paleogeographic setting, the occurrence of fine sandstone underlying the iron level, occurrence of Mn-ores in the lower part of the Fe-ores succession, high phosphorous, zinc, ∑REE-Y concentrations and Y/Ho ratios, and low La/Ce ratios.
The Bou Dahar Pb–Zn–Ba (±Sr) is hosted in the Lower and Middle Liassic carbonate platform in the oriental High Atlas of Morocco. The paragenetic sequence includes ...quartz–pyrite–melnicovite–sphalerite–galena–calcite–barite±fluorite–±celestite. Fluid-inclusion studies were conducted on sphalerite (early mineralizing stage) and barite, and celestite (late mineralizing stage). These studies reveal two end-member fluids, a hot (~143°C) and saline fluid (~23wt.% NaCl eq.) and a cooler (<50°C) and diluted fluid (~5wt.% NaCl eq.). Based on fluid-inclusion and C–O–S isotope studies, a conclusion is reached that the Bou Dahar ore deposits were formed by the mixing of two fluid — a diluted, SO2−4-rich fluid, and an 18O-enriched basinal brine that carried Pb, Zn, and Ba. The sulfur required for the precipitation of sulfides was generated by the thermochemical sulfate reduction of dissolved sulfate (SO42−) of the Mesozoic seawaters, and delivered to the site of ore deposition. The sulfur of sulfate minerals was derived directly from these dissolved SO42. The Pb isotope compositions are homogenous with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.124 to 18.183, 15.630 to 15.634, and 38.325 to 38.440 respectively. This Pb isotope composition is indicative of an upper crust and orogene reservoirs as the source of lead and other metals. The emplacement of mineralization occurred during the Eocene–Miocene Alpine orogeny, and tectonic burial and compression were the driving forces behind the circulation of the orogenic-brines. These ore-forming fluids migrated, along thrusting regional E–W and NE–SW deep-seated faults, to the confined carbonate-Liassic reservoir.
•The carbonate reservoir developed during the Liassic period and fractured during the Eocene–Late Miocene orogeny. It was sealed with thick, mainly marls and argillaceous limestone of Middle and Late Jurassic.•The sulfur needed for the precipitation of the sulfide ore deposits was generated through TSR of dissolved Mesozoic sulfate away from the site of mineralization.•The post-Cretaceous Alpine orogeny triggered the remobilization and circulation of these metal-carrying fluids along E–W to NE–SW faulted zones toward the site of deposition.•At the site of deposition, the ore-forming fluids reacted with a subsurface cooler H2S-rich fluid to form sulfides. During the waning hydrothermal stage, the excess of unreduced sulfur reacted with barium and strontium to form barite and celestite.
A preliminary C–O stable isotopes geochemical characterization of several nonsulfide Zn-Pb Tunisian deposits has been carried out, in order to evidence the possible differences in their genesis. ...Nonsulfide ores were sampled from the following deposits: Ain Allegua, Jebel Ben Amara, Jebel Hallouf (Nappe Zone), Djebba, Bou Grine, Bou Jaber, Fedj el Adoum, Slata Fer (Diapir Zone), Jebel Ressas, Jebel Azreg, Mecella (North South Axis Zone), Jebel Trozza, Sekarna (Graben Zone). After mineralogical investigation of selected specimens, the C–O stable isotopic study was carried out on smithsonite, hydrozincite, cerussite and calcite. The data have shown that all the carbonate generations in the oxidized zones of Ain Allegua and Jebel Ben Amara (Nappe Zone), Bou Jaber, Bou Grine and Fedj el Adoum (Diapir Zone), Mecella and Jebel Azreg (North South Zone) have a supergene origin, whereas the carbonates sampled at Sekarna (Graben Zone) (and in limited part also at Bou Jaber) precipitated from thermal waters at moderately high temperature. Most weathering processes that controlled the supergene alteration of the Zn-Pb sulfide deposits in Tunisia had probably started in the middle to late Miocene interval and at the beginning of the Pliocene, both periods corresponding to two distinct tectonic pulses that produced the exhumation of sulfide ores, but the alteration and formation of oxidized minerals could have also continued through the Quaternary. The isotopic characteristics associated with the weathering processes in the sampled localities were controlled by the different locations of the sulfide protores within the tectonic and climatic zones of Tunisia during the late Tertiary and Quaternary.
The Late Paleocene–Early Eocene phosphorites of the Metlaoui Group in Tunisia are a world-class phosphate resource. We review the characteristics of phosphorites deposited in three areas: the ...Northern Basins; Eastern Basins; and Gafsa-Metlaoui Basin. Comprehensive new bulk rock elemental data are presented, together with complementary mineralogical and mineral chemical results. Carbonate fluorapatite (francolite) constitutes the dominant mineral phase in the deposits. Phosphorite samples are enriched in Cd, Sr, U, rare-earth elements and Y, together with environmentally diagnostic trace elements that provide detrital (Cr, Zr), productivity (Cu, Ni, Zn) and redox (Mo, V) proxies. Suboxic bottom-water conditions predominated, with suboxic to anoxic porewaters accompanying francolite precipitation. Phosphorite deposition occurred under increasingly arid climate conditions, accompanying global Paleocene–Eocene warming. The Northern Basins show the strongest Tethys Ocean influence, with surface seawater rare-earth element signatures consistently developed in the phosphorites. Bed-scale compositional variation indicates relatively unstable environmental conditions and episodes of sediment redeposition, with varying detrital supply and a relatively wet local climate. Glauconitic facies in the Northern Basins and the more isolated evaporite-associated phosphorites in the dryer Eastern Basins display the greatest diagenetic influences. The phosphorite – organic-rich marl – diatom-bearing porcelanite facies association in the Gafsa-Metlaoui Basin represents the classic coastal upwelling trinity. Modified Tethyan waters occurred within the Basin during phosphorite deposition, with decreasing marine productivity from NW to SE evidenced by systematically falling enrichment factors for Cu, Ni, Cd and Zn in the phosphorites. Productivity declined in concert with increasing basin isolation during the deposition of the commercial phosphorite beds in the latest Paleocene to earliest Eocene. This isolation trend was temporarily reversed during an episode of maximum flooding associated with the earliest Eocene Paleocene–Eocene Thermal Maximum (PETM).
•Tunisian Paleocene–Eocene phosphorites occur in three distinct basinal areas.•Deposits in each area have distinct geochemical signatures.•Commercial phosphorite deposits in the Gafsa-Metlaoui Basin are of Paleocene–Eocene boundary age.•The phosphorites accumulated in high productivity coastal upwelling environments.•Phosphorite deposition accompanied Paleocene-Eocene climate warming.
•First comprehensive mineralogical and geochemical study of major Mn–ores over northern and central Tunisia.•The results suggest different ore morphologies, structures, textures and ...lithogeochemistry.•Mn mineralization formed by multiple and complex processes and belongs to different genetic types.
Northern and central Tunisia hosts few small-scale stratiform and stratabound Mn deposits, low to medium in grade (Mn < 46%), which have a spatial distribution related to major reactivated NS- and NW-oriented faults. The Mn deposits can be broadly classified into sedimentary rock-hosted and karst-hosted. The ores occur in two different structural settings and are hosted by deposits of various age, lithology, size and grade. The geology shows that the Mn deposits are diachronic and were formed in continental and/or shallow marine environments. They commonly occur in close spatial association with Fe ores and are genetically not related to Pb-Zn-(Ba-Sr-F) deposits. Bulk Mn ores are characterized by high and variable Mn/Fe ratios and an enrichment in Pb, Zn, Ba and Sr. Shale-normalized trace element (TESN) patterns of all studied Mn ores show enrichment in the assemblage of Sr, Ba, Co, Cu, Mo, Pb, and Zn and depletion in high-field strength elements Zr, Nb and Th, which is characteristic for hydrothermal ore formation. Total rare earth element (REE) contents show considerable variations and REYSN patterns indicate a complex interplay of hydrothermal, marine and meteoric/supergene processes. LA-ICP-MS analysis of Mn oxides show considerable variations in TE contents and REYSN patterns due to differences in oxide mineralogy, fluid source(s), and their rate of precipitation and related scavenging processes. The TE and REY geochemistry of the Mn ores indicate a complex interplay of primary and secondary processes that were responsible for the formation of the Mn mineralization. In the Tamra Fe-Mn deposit (Nappes zone, polymetallic district and volcanosedimentary context), the Mn ores (stage 1 romanechite-hollandite) occur as porosity and/or microfracture fillings in the ferruginous matrix, as impregnation and massive concretions and also as late cavity fillings (stage 2 coronadite-chalcophanite). Mixing of hydrothermal and predominantly meteoric fluids induced the formation of typical supergene Mn ores. The stratiform Mn ores (pyrolusite) of Jebel Es Stah (southern Tunisian Atlas, Miocene alluvial system) occur as a syngenetic to early diagenetic oxide coating and cementation of detrital sand grains. REYSN patterns reflect the predominant influence of siliciclastic material as detritus in the Mn ore. Minor Mn ores (cryptomelane, hollandite) occurring as duricrust and fracture filling recorded in the Cenomanian-Turonian dolostone in central and southern Tunisian Atlas show flat REYSN patterns and negative CeSN anomalies and suggest remobilization of Mn under supergene conditions. In the Jebel Ank Fe-Mn deposit (southern Tunisian Atlas, hosted in a relatively restricted shallow basin), cryptomelane occurs as centimeter-sized irregular nodules in the late Eocene marls. Based on REY data, a primary hydrogenetic origin is suggested for Mn nodules. The formation of cryptomelane may have occurred at relatively late stages of weathering when local increases in pH and oxidizing conditions prevailed with the input of K+ from phyllosilicate-bearing layers. The stratabound Mn ores (stage 1 hollandite and stage 2 romanechite-coronadite) in the Jebel Aziza Mn deposit (southern Tunisian Atlas, shallow marine platform facies) correspond to karstic filling and stockwork type in the late Cenomanian-early Turonian dolostone. The Mn oxides in the deposit are epigenetic and their formation is related to changes in Eh and pH. The REYSN patterns for Mn ores and Mn oxides are between those of a mixed seawater-hydrothermal- and supergene-type Mn ore. The petrography, mineralogy, TE contents and REYSN patterns of the different Mn ores indicate interplay of hydrothermal, hydrogenetic and supergene weathering processes occurring during formation of the deposits. The data indicates different mineralization stages and different processes that lead to the low-temperature formation of Mn oxides typical of supergene deposits commonly found in North Africa.
Rare earth element (REE) enrichment in Algerian phosphorites has been evoked recently by a number of authors and are considered among the richest Paleocene-Eocene phosphorites worldwide. The Bled El ...Hadba deposit belongs to the giant Djebel Onk phosphorite complex in northeast Algeria. The deposit was subject to many geological studies that were focused on P2O5 contents for commercial purposes. The upper Thanetian phosphorite layer is about 30 m thick, and subdivided into three sub-layers (lower, main and upper sub-layer), based on P2O5 contents, where the main sub-layer is considered to be the richest. Despite this, few detailed geochemical investigations were carried out on this deposit thus far. In this work, major, trace and REE analyses were conducted on phosphate particles (pellets, coprolites and glauconites) using ‘in situ’ LA-ICP MS, and on whole-rock samples using XRF technique. The results show that the main sub-layer displays the highest whole-rock P2O5 contents, varying between 19.65 and 21.32 wt% compared to the lower (10.47–16.87 wt%) and upper (9.43–13.87 wt%) sub-layers. Among the phosphate particles, glauconites display the lowest P2O5 contents in the three sub-layers (17.45–19.35 wt%) compared to pellets (21.14–24.33 wt%) and coprolites (21.75–24.12 wt%), and largely the highest Al2O3, SiO2, MgO and Fe2O3(t). Glauconites also show higher ΣREE contents (764–2050 ppm) compared to pellets (221–910 ppm) and coprolites (214–909 ppm). Within the glauconite particles, the ΣREE, along with Al2O3, SiO2, MgO, and Fe2O3(t) contents, increase from core to the rim, whereas P2O5 contents decrease, which suggests that glauconitization postdates phosphatization processes. Also, the glauconitization process increase from the lower to the upper sub-layer; this is shown by the positive correlations between Al2O3 contents and those of MgO and SiO2 in one hand, and negative correlations between Al2O3 and P2O5 on the other hand. Ce, Eu, Y anomalies, along with La/Nd ratios and Nd contents, all point towards phosphatization under oxic conditions as a result of warm water upwelling, whereas glauconitization started under more reduced (sub-oxic) conditions; i.e., during early diagenesis, the peak of REE uptake from porewater, and under slow sedimentation rates.
Despite their relatively low P2O5 concentrations, the ΣREE contents of the Bled El Hadba phosphate particles record the highest concentrations in all Algerian and north African phosphorites. The main sub-layer is considered to be the most glauconite-rich phosphorite in Algeria. Therefore, more extensive REE analyses are recommended to better evaluate its economic potential in terms of critical raw materials.
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•The REE-enriched Bled El Hadba deposit belongs to the Dj. Onk phosphorite complex.•Phosphate particles were in situ LA-ICP-MS analyzed for their chemistry.•Glauconite particles are very enriched in REE compared to pellets and coprolites.•Phosphatization predates glauconitization processes.
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•The Aïn Allegua celestite-rich and Pb-Zn poor deposit (Allegua, Aguiba and Kef Ressas ore zones) is located at the north-eastern side of the SW-NE Dougas Triassic salt diapir in the ...Nappes zone of Tunisia.•Mineralization is hosted in a cap rock formant the cortical zone of the Triassic evaporite diapir.•The deposit shows five mineralizing stages with distinct mineral associations.•The Aïn Allegua deposit is salt diapir-related MVT deposit formed from basinal brines in a diapiric environment.•There is no obvious mixing with metamorphic and magmatic fluids and a magmatic heat source.
The celestite-rich and Pb-Zn-poor Aïn Allegua deposits are located at the northeastern extremity of the Dougas diapir in the Nappes zone of northern Tunisia. The Sr-Ba-Zn-Pb mineral system studied in this paper is enclosed in a Triassic breccia, labeled cap rock, located between the Triassic diapir and the surrounding Cretaceous to Lutetian marine sediments. It consists of evaporite dissolution float breccia where Triassic dolostone clasts are floating in a Triassic matrix of anhydrite and argillites. Five mineralizing stages are recognized, which are from early to late: celestite (stage 1); hydrothermal dolomite (stage 2); crosscutting galena-barite or sphalerite-galena-barite-calcite (stage 3); (4) fracture and karst filled by barite, celestite, and galena (stage 4); and late supergene mineralization (strontianite, smithsonite, and cerussite) (stage 5). The present study shows that the celestite and Pb-Zn mineralization formed from the circulation in the cap rock of at least three fluids: (1) fluid A (100–150 °C; 10 to 25 wt% NaCl eq.) deposited the celestite; (2) fluid B (125 to 185 °C; 29 to 34 wt% NaCl eq.) deposited the hydrothermal dolomite, and (3) fluid C (temperatures around 90 °C; 14 to 22 wt% NaCl eq.) deposited the calcite that accompanied sphalerite and galena precipitation. The δ34S values for sphalerite and galena fall within a narrow range from + 10.4‰ to + 18.9‰ (avr. 13‰) indicating a homogeneous source of sulfur. The δ34S values of celestite and barite are also relatively homogeneous from + 14.7 to + 23‰ (ave. + 16.5‰) and match those of Tunisian Triassic gypsum and Triassic marine sulfates (δ34S = +15‰). The temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate. The lead isotopes in galena show that Pb, and probably Zn and Ba were extracted from the upper crustal rocks. Part of the strontium is derived from the Triassic anhydrite and the other part is probably transported to the cap rock in formation waters. The tectonic setting, the carbonated nature of the host rocks, the epigenetic style of the mineralization, and the mineral associations, together with fluid inclusions data and sulfur and lead isotopes show that the Aïn Allegua deposit belongs to the diapir-related Mississippi Valley-type (MVT) deposits of the Eastern Maghreb salt diapirs province, that formed, as the most of the MVT deposits and districts in the world, from basinal brines with no mixing with metamorphic and/or magmatic fluid or involvement of a heat source from a concealed sill or magmatic intrusion.
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