Synmetamorphic Cu remobilization during the Pan-African orogeny: Microstructural, petrological and geochronological data on the kyanite-micaschists hosting the Cu(–U) Lumwana deposit in the Western Zambian Copperbelt of the Lufilian belt

International audienceThe Pan-African Lufilian orogenic belt hosts world-class Cu deposits. In the Congolese Copperbelt (DRC), Cu(-Co) deposits, are mostly hosted within evaporitic and siliciclastic Neoproterozoic metasedimentary rocks (Mines Subgroup) and are interpreted as syn- to latediagenetic deposits. In this paper, we present new data on Cu(-U) deposit hosted in metamorphic rocks of the internal zone of the Lufilian belt known as the Western Zambian Copperbelt in which a primary Cu mineralization is overprinted by a second syn-metamorphic Cu mineralizing event. This mineralizing event is synchronous with the Pan-African metamorphism affecting both the pre-Katanga basement and the Katanga metasedimentary sequence. Cu(-U) occurrences in the Western Zambian Copperbelt are hosted by kyanitemicaschists metamorphosed in the upper amphibolite facies. Mineral inclusions of graphite, micas and sulfides in kyanite porphyroblasts of the Cubearing kyanite-micaschists in the Lumwana Cu deposit point to a sedimentary protolith with relics of an inherited Cu stock. Based on petrologic, microstructural and geochronological evidence, we propose that this initial Cu-stock was remobilized during the Pan-African orogeny. Graphite, micas and sulfides preserved in a first generation of kyanite poikiloblasts (Ky1) define an inherited S0/1 foliation developed during the prograde part of the P-T path (D1 deformation-metamorphic stage) reaching HP-MT metamorphic conditions. Remobilization during the retrograde part of the P-T path is evidenced by chalcopyrite-pyrrhotite and chalcopyrite-bornite delineating a steep-dipping S2 schistosity and by chalcopyrite and bornite delineating a shallow-dipping S3 schistosity associated with top to the south kinematic criteria. This retrograde path is coeval with ductile deformation in the kyanite field as evidenced by a second generation of synkinematic kyanite porphyroblasts (Ky2) transposed in the S3 schistosity (Ky2-3), and is marked by progressive cooling from ca 620°C down to 580°C (rutile geothermometry). Syn-S2-3 metamorphic monazite grains yield U-Th-Pb ages ranging from ca. 540 to 500 Ma. Final retrogression and remobilization of Cu is marked by recrystallization of the sulfides in top to the north C3 shear bands associated with rutile crystals yielding temperatures from ca. 610 to 540°C. This final remobilization is younger than ca. 500 Ma (youngest U-Th- Pb age on syn-S3 recrystallized monazite). These data are consistent with successive Cu remobilization for more than 40 Ma during Pan-African reworking of sediment-hosted deposits either from the basement of the Katanga sedimentary sequence or from the Katanga sequence itself marked by burial (D1), syn-orogenic exhumation (D2), and post-orogenic exhumation during gravitational collapse (D3)

Internal vein texture and vein evolution of the epithermal Shila-Paula district, southern Peru.

The epithermal Shila-Paula Au–Ag district is characterized by numerous veins hosted in Tertiary volcanic rocks of the Western Cordillera (southern Peru). Field studies of the ore bodies reveal a systematic association of a main E–W vein with secondary N55–60°W veins—two directions that are also reflected by the orientation of fluid-inclusion planes in quartz crystals of the host rock. In areas where this pattern is not recognized, such as the Apacheta sector, vein emplacement seems to have been guided by regional N40°E and N40°W fractures. Two main vein-filling stages are identified. stage 1 is a quartz–adularia–pyrite–galena–sphalerite–chalcopyrite–electrum–Mn silicate–carbonate assemblage that fills the main E–W veins. stage 2, which contains most of the precious-metal mineralization, is divided into pre-bonanza and bonanza substages. The pre-bonanza substage consists of a quartz–adularia–carbonate assemblage that is observed within the secondary N45–60°W veins, in veinlets that cut the stage 1 assemblage, and in final open-space fillings. The two latter structures are finally filled by the bonanza substage characterized by a Fe-poor sphalerite–chalcopyrite–pyrite–galena–tennantite–tetrahedrite–polybasite–pearceite–electrum assemblage. The ore in the main veins is systematically brecciated, whereas the ore in the secondary veins and geodes is characteristic of open-space crystallization. Microthermometric measurements on sphalerite from both stages and on quartz and calcite from stage 2 indicate a salinity range of 0 to 15.5 wt% NaCl equivalent and homogenization temperatures bracketed between 200 and 330°C. Secondary CO2-, N2- and H2S-bearing fluid inclusions are also identified. The age of vein emplacement, based on 40Ar/39Ar ages obtained on adularia of different veins, is estimated at around 11 Ma, with some overlap between adularia of stage 1 (11.4±0.4 Ma) and of stage 2 (10.8±0.3 Ma). A three-phase tectonic model has been constructed to explain the vein formation. Phase 1 corresponds to the assumed development of E–W sinistral shear zones and associated N60°W cleavages under the effects of a NE–SW shortening direction that is recognized at Andean scale. These structures contain the stage 1 ore assemblage that was brecciated during ongoing deformation. Phase 2 is a reactivation of earlier structures under a NW–SE shortening direction that allowed the reopening of the preexisting schistosity and the formation of scarce N50°E-striking S2-cleavage planes filled by the stage 2 pre-bonanza minerals. Phase 3 coincides with the bonanza ore emplacement in the secondary N45–60°W veins and also in open-space in the core of the main E–W veins. Our combined tectonic, textural, mineralogical, fluid-inclusion, and geochronological study presents a complete model of vein formation in which the reactivation of previously formed tectonic structures plays a significant role in ore formation

Inclusions fluides et isotopes du soufre du gisement Cu–Au de Valea Morii (monts Apuseni, Roumanie) : un télescopage porphyre–épithermal neutre ? Fluid inclusions and sulphur isotopes of the Valea Morii ore deposit (Apuseni mountains, Romania): telescoping between porphyry and low-sulfidation epithermal ore deposits?

Le gisement Cu–Au de Valea Morii (monts Apuseni, Roumanie) montre la superposition d'un gisement de type porphyre (Cu±Au) et d'un gisement épithermal neutre (Au). Les données microthermométriques et isotopiques du soufre, obtenues dans les deux types de veines minéralisées, traduisent un télescopage entre ces deux minéralisations, qui peut être expliqué, soit par un phénomène d'érosion durant l'activité hydrothermale, soit par un effondrement de la partie supérieure du système hydrothermal. The Valea Morii Cu–Au ore deposit (Apuseni mountains, Romania) is characterised by a spatial association of a porphyry copper (Cu±Au) with a low-sulfidation epithermal deposit (Au). Microthermometric and sulphur isotopic data, obtained on both porphyry and epithermal veins, express a telescoping between these two types of mineralization, which can be explained either by an erosion phenomenon during the hydrothermal activity or by a collapse of the upper part of the hydrothermal system

Constraints on the ore fluids in the Sando Alcalde Ausingle bondAg epithermal deposit, southwestern Peru: fluid inclusions and stable isotope data ----- Contraintes sur les fluides minéralisateurs du gisement Ausingle bondAg de Sando Alcalde, Sud-Ouest du Pérou : données des inclusions fluides et des isotopes stables

The Sando Alcalde ore deposit (southwestern Peru) has been studied in order to characterize the physicochemical parameters of the ore fluids and to determine the fluid process (mixing or boiling) which involves the precious metal mineralization. Mineralogy, δ18Oquartz isotopic values and fluid inclusion data give arguments in favour of a boiling phenomenon. This conclusion corroborates fluid inclusion studies previously performed in this area on the low-sulphidation epithermal deposits of Arcata, Orcopampa and Apacheta, where boiling has been described as the main factor for ore depositio

A-type granites from the Pan-African orogenic belt in south-western Chad constrained using geochemistry, Sr-Nd isotopes and U-Pb geochronology

International audienceThe Zabili granitic pluton (SW Chad) exposed in the Mayo Kebbi massif is dominated by a coarse-grained hornblende biotite granite grading into a fine-grained biotite granite along its southern margin. Petrologic (micrographic intergrowth of quartz and alkali feldspars, granophyric microstructures, the presence of fluorite and bastnaesite as accessory minerals) and geochemical data (high silica, alkalis and Fe/Mg, depletions in CaO, MgO, TiO2; high Ga, Nb, Zr, Ga/Al, REE, depletions in Ba, Sr, Eu and compatible elements) indicate that this pluton consists of A-type granites crystallized from hot (apatite and zircon saturation temperatures ranging from 744 °C to 923 °C), extremely differentiated magmas. U-Pb zircon geochronology indicates that the magmas crystallized at 567 ± 10 Ma and reveals the presence of older Neoproterozoic xenocrystic zircons at 668 ± 5 Ma in both facies. Within the fine-grained biotite granite, discordant zircons with U-Pb and Pb-Pb ages ranging from Neoproterozoic to Archaean are also reported. The 668 ± 5 Ma old zircons are considered to derive from country-rocks while discordant zircons, characterized by angular shapes, internal fractures and inherited cores, are likely to represent multi-sources detrital crystals that have recorded at least one metamorphic event. Old pre-Neoproterozoic zircons are reported for the first time for rocks of the Mayo Kebbi massif and they attest to the contribution of an old basement (likely to be the Eastern Nigeria basement and/or the Congo craton) involved in a collisional event with a juvenile Neoproterozoic crust prior to the emplacement of the Zabili granitic pluton. Initial ɛNd values calculated for the Zabili pluton range from + 2.6 to + 7.0, the highest value recorded by one sample from the coarse-grained hornblende-biotite granite being close to the one of the depleted mantle at 570 Ma (ɛNd = + 7.4). Combining geochronology, Nd isotopes composition and geochemical modeling, leads us to suggest the following model for the origin of the Zabili granitic pluton: (i) contribution of juvenile magmas or partial melting of a juvenile basaltic protolith characterized by a short crustal residence time; (ii) interaction of granitic magmas with older continental materials as suggested by the presence of pre-Neoproterozoic zircons and lower initial ɛNd values of the fine-grained biotite granite; and (iii) fractional crystallization of feldspars and ferromagnesians to produce the observed geochemical features of sample GAB-B, from which an initial ɛNd value of + 7.0 has been calculated

Diagenetic origin of the stratiform Cu-Co deposit at Kamoto in the Central African Copperbelt

International audienceSediment-hosted stratiform ore deposits that underwent metamorphism and orogenesis are interpreted either to have a multiphase origin that might in part predate these events or to be exclusively related to fluid migration during the orogeny. This controversy concerns the formation of many world-class sediment-hosted ore deposits such as in the Central African Copperbelt. Here we present Re-Os dating results for disseminated and stratiform Cu-Co sulfide pseudomorphs after anhydrite in nodules and layers from the Copperbelt. Results demonstrate that at least some of the ores formed at around 800 Ma during rifting and basin development, and thus predate the Pan-African Lufilian orogeny. Younger Re-Os ages, i.e., between 682 +/- 28 and 230 +/- 36 Ma, may be due to the replacement of the early Cu-Co sulfides by younger sulfide phases. Alternatively, these younger ages may result from the disturbance or resetting of the Re-Os system during recrystallization/metamorphism or the transition of chalcocite from its high- to its low-temperature polymorph

Re-Os systematics of pyrite from the bolcana porphyry copper deposit, Apuseni Mountains, Romania.

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Record of the oldest vein-type gold mineralization throughout the Neoproterozoic Nubian shield: the Galat Sufar South gold deposit, NE Sudan

International audienceDespite the booming interest in Nubian shield for gold exploration, much more remains to be done to constrain geodynamical settings favorable for gold mineralization in a mineral system approach. Constraining pressure-temperature-timing-deformation (P-T-t-d) conditions relative to the formation of the structurally controlled Galat Sufar South (GSS) deposit is a case in point and represents a step forward in the understanding of the Pan-African gold mineral system along the westernmost Atmur-Delgo suture. The formation of the GSS gold deposit, hosted in an arc-related volcanosedimentary sequence metamorphosed and deformed by sheath folding, occurred at 738±16 Ma (in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of hydrothermal apatite coeval to gold mineralization). Additional chlorite-phengite-quartz-water multi-equilibria calculations in ore-hosting meta-volcanosediments suggest that the gold event occurred under lower amphibolite facies conditions (~7.2 kbar - 420 °C), probably during the tectonic accretion of the sedimentary wedge formed during Atmur-Delgo intra-oceanic subduction. To date, the GSS deposit is the oldest lode gold occurrence documented throughout the Nubian shield and the only one formed at regional peak metamorphism conditions in such a setting

Record of the oldest vein-type gold mineralization throughout the Neoproterozoic Nubian shield: the Galat Sufar South gold deposit, NE Sudan

International audienceDespite the booming interest in Nubian shield for gold exploration, much more remains to be done to constrain geodynamical settings favorable for gold mineralization in a mineral system approach. Constraining pressure-temperature-timing-deformation (P-T-t-d) conditions relative to the formation of the structurally controlled Galat Sufar South (GSS) deposit is a case in point and represents a step forward in the understanding of the Pan-African gold mineral system along the westernmost Atmur-Delgo suture. The formation of the GSS gold deposit, hosted in an arc-related volcanosedimentary sequence metamorphosed and deformed by sheath folding, occurred at 738±16 Ma (in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of hydrothermal apatite coeval to gold mineralization). Additional chlorite-phengite-quartz-water multi-equilibria calculations in ore-hosting meta-volcanosediments suggest that the gold event occurred under lower amphibolite facies conditions (~7.2 kbar - 420 °C), probably during the tectonic accretion of the sedimentary wedge formed during Atmur-Delgo intra-oceanic subduction. To date, the GSS deposit is the oldest lode gold occurrence documented throughout the Nubian shield and the only one formed at regional peak metamorphism conditions in such a setting