Gold mineralization in the Um El Tuyor area, South Eastern Desert, Egypt

This thesis reports petrographic, mineralogical and geochemical data that constrain the hydrothermal alteration and gold mineralization in the Um El Tuyor area, SE Egypt. In order to distinguish the characteristics and envisage the genesis of gold mineralization in the study area, insights into the geologic context, structural evolution and geochemistry of the country rocks are provided. The geochemical investigations have been integrated with field and petrographic relationships, along with Landsat (TM) imagery interpretations to better constraints on the tectonic setting of the basement complex in the study area. The particular geographic location near the intersection of two major high strain zones, namely the Allaqi-Heiani suture and Hamisana Shear Zone, played a crucial role in the deformation history of the Um Tuyor area. The basement complex cropping out in the Um El Tuyor area is part of the Neoproterozoic Allaqi-Heiani ophiolitic belt, comprising allochtonous ophiolitic thrust slices and detached sheets, island arc volcano-sedimentary-plutonic assemblages, and syn-orogenic and post-orogenic intrusions. The ophiolitic rocks exhibit field and geochemical characteristics that make them akin to the supra-subduction zone ophiolites, formed most likely in a back-arc basin. The island arc assemblage comprises mainly calc-alkaline metavolcanic-plutonic rocks and back-arc pelitic metasediments. Early calc-alkaline granite intrusions tapered along the foliation and thrust planes during the orogenic episodes, whereas less fractionated tholeiitic olivine gabbro and peraluminous monzogranite encompass a course of post-orogenic plutonism evolved in a within plate setting. An early period of crustal shortening (Dm) involved transportation and overriding of huge ophiolitic sheets from the north to south is manifested by major thrust faults and imbricate ophiolitic thrust slices. Regional folds and pervasive foliation cleavage signify a NE-SW compressional regime (D2) superimposed on the thrust fabrics. A third deformation increment is indicated by the presence of abundant NNW-trending major folds and left-lateral faults superimposed on the older structural fabrics (D3). D4 records an episode of transcurrent deformation yielded slip reactivation of the pre-existing NW-trending faults and formation of discrete shear zones, one of which accommodates gold mineralization in the study area. Finally, a weak shear strain (D5) is indicated by the intersecting fault and joint trends traversing the post-orogenic rocks. Regional metamorphism was coeval with deformation, and peaked under conditions of amphibolite facies during D2. Geothermobarometry calculations point to temperatures of 534-561oC and pressure of 5.26-6.20 kbar for the peak of the metamorphic path of Um El Tuyor basement. Gold is mainly confined to the quartz veins and less commonly to narrow domains of the next quartz-sericite alteration zones. Field, microscopic and microprobe observations suggest that hydrothermal alteration in the Um El Tuyor mine area was post-peak metamorphism, and syn-kinematic with local shearing. The main auriferous veins in the Um El Tuyor mine area consist of massive, partially recrystallized, or laminated quartz ±carbonate. The quartz-carbonate veins are essentially fault-fill bodies, which have been fractured and re-filled with milky to grey laminated quartz in later stages of the geothermal system. The laminated quartz veins contain narrow elongate slivers of the host pelitic rocks (composed essentially of chrorite-sericite-sulphides±graphite), assumed to have been peeled off and incorporated during incremental (crack-seal) vein growth. vii Gold occurs as inclusions or within the lattice in arsenopyrite and arsenian pyrite, commonly in association with subordinate sphalerite, chalcopyrite, and pyrrhotite in the auriferous quartz veins. Another, high fineness type of gold fills microfractures in sulphides and quartz, and/or occurs as dispersed blebs and globules in domains of pervasive alteration, particularly where sericite and carbonate are intergrown. In the mine area, a metal zonation extends from an inner Fe-As-Zn-Au ±Pb±Ag±Te bearing veins through an intermediate Fe-As-Cu rich wallrocks to a distal halo enriched in Fe-Cu-Co, and Ni. A three stage hydrothermal alteration model (initial, transitional and advanced) is proposed for the Um El Tuyor ore-forming hydrothermal system, on basis of the field and microscopic observations and the electron microprobe data. The initial stage involved hydrolysis of the wallrocks in presence of a near acid fluid, whose pH was buffered by the wallrock mineralogy. The transitional stage involved also hydrolysis reactions, but was dominated by carbonatization, sulphidation and redox reactions. The latter reduced the interacting fluids, particularly where the fluid : rock ratio was low and provided favourable conditions for gold deposition. It is interpreted that sulphidation has affected the gold solubility via changes in oxygen fugacity through redox reactions. The advanced stage was most likely a phase of intense sericitization (after chlorite), which consumed K+, liberated H+, and buffered the solution pH. Cation-exchange reactions were limited to the time when favoured Na+ activity and temperature conditions promoted deposition of albite. Sulphidation remained operating through this stage, and the un-buffered conditions were locally attained under high fluid/rock ratios. Compositional zoning of the auriferous pyrite and arsenopyrite crystals, along with the presence of patchy pyrrhotite and sphalerite inclusions in these crystals, and the absence of these features in pyrite and arsenopyrite of the late sulphide assemblage may imply incipient low oxygen fugacity conditions during gold deposition. Further, compositional zoning of the large arsenian pyrite crystals, from barren cores to auriferous margins, is considered as a function of redox reactions involving oxidation of Au and reduction of As. Fluid inclusion studies revealed heterogeneous entrapping of immiscible aqueous and carbonic fluids in the Um El Tuyor auriferous quartz veins. Criteria including the primary and secondary modes of occurrence of the carbonic inclusions, coexistence of inclusions with carbonic and aqueous fluids of variable relative proportions, densities, filling degrees, partial homogenization temperatures, and bulk compositions are considered evocative for phase separation as gold deposition mechanism in the Um El Tuyor auriferous quartz veins. The initially homogenous ore fluid was likely a low salinity aqueous-carbonic solution (±1-2 mol% CH4 or N2), which started to separate into two phases and precipitate gold when conditions attained ~340°C at ~1.5 kbar (at depth of ~6 km under lithostatic condition). Destabilization of gold-sulphur complexes through interplay of cooling, redox state variation, pH changes, and decrease in sulphur fugacity should have contributed in gold deposition in the auriferous veins. The entire gold-base metal mineralization and quartz veining event extended over conditions of 170-429oC at 0.9-2.1 kbar respectively, equivalent to depths of 3-8 km and compatible with crustal conditions of greenschist metamorphism and brittle–ductile transition. This wide range of pressure probably represents the total fluid pressure regime within the shear zone from the formation of the auriferous quartz veins to periods of continuous pressure decrease during uplift, including a sudden pressure decrease occurred as a consequence of incremental opening of the fissure, followed by filling by newly deposited quartz.Diese Dissertation beschreibt die petrographischen, mineralogischen und geochemischen Hintergründe, die zur Goldmineralisation und zur hydrothermalen Nebengesteinsalteration im Gebiet von Um El Tuyor, Südost Ägypten beigetragen haben. Um jedoch die genetischen Hintergründe und Charakteristika der Goldmineralisation im Untersuchungsgebiet näher zu verstehen, bedarf es zunächst einer eingehenden Analyse der geologischen Zusammenhänge, der strukturgeologischen Entwicklungsgeschichte und Geochemie der gesamten Nebengesteinsabfolgen. Die geochemischen Auswertungen wurden in die Feldarbeiten und die petrographischen Untersuchungen eingebunden, ebenso die Satellitenbildinterpretationen (TM-Szenen) zum besseren Verständnis der Tektonik des Grundgebirges im Untersuchungsgebiet. Die spezielle geographische Position des Um El Tuyor Gebietes im Umfeld der Überschneidung von zwei „high strain“ Suturzonen, nämlich des Allaqi-Heiani Gürtels und der von Süden kommenden Hamisana Zone trug wesentlich sowohl zu deren komplexen Deformationsgeschichte als auch Lagerstättengenese bei. Der anstehende Grundgebirgskomplex des Um El Tuyor Gebietes stellte sich als ein Teil des Neoproterozoischen Allaqi-Heiani Ophiolit Gürtels heraus, der aus allochtonen ophiolitischen Überschiebungspartien und abgesonderten Schuppen vulkano-sedimentärer und plutonischer Abfolgen vom Inselbogentyp sowie syn-und post-orogener Intrusionen besteht. Die ophiolitischen Gesteine lassen sich sowohl nach den Geländedaten als auch geochemisch als Supra-Subduktionszonen-Ophiolite eines Back-Arc Beckens deuten. Während des Pan-African (Spätes Proterozoikum) wurden diese Ophiolite transportiert, durch mittlere bis steile Verfaltung verkürzt und verschuppt wobei sie immature Inselbogenabfolgen mit zwischengeschalteten Back-Arc Metasedimenten überlagerten. Die Inselbogenabfolge besteht hauptsächlich aus kalk-alkalinen metavulkanisch-plutonischen Gesteinen und pelitischen Back-Arc Metasedimenten. Frühe kalk-alkaline Granitintrusionen drangen entlang der Faltungs- und Überschiebungflächen während der orogenen Episoden ein, wohingegen tektonisch weniger beanspruchte Olivin-Gabbros und Peraluminium-Mozogranite sich als postorogene Plutonite mit einer „Within plate“ Affinität darstellen. Eine frühe Krustenverkürzungsperiode (Dm) verursachte den Transport und die Überschiebung mächtiger ophiolitischer Decken von Nord nach Süd. Die Deformation ist Hauptüberschiebungsbahnen und durch verschuppte ohiolitische Splitter belegt. Regionale Störungen und Foliationsflächen, den Deckenbau überlagern deuten auf ein NE-SW verlaufendes Kompressionsregime (D2) hin.. Eine dritte Deformatiosphase (D3) wird durch zahlreiche NNW verlaufende größere Störungen und linkslaterale Verfaltungen repräsentiert, die älteren Strukturelemente überprägt. weist auf Eine Episode querverlaufender Deformation (D4), die zur Reaktivierung präexistenter NW-gerichtete Störungen und der Bildung diskreter Scherzonen führte, beherbergt einen Teil der Goldführung im Untersuchungsgebiet. Schließlich verursachte eine schwache Scherrichtung (D5) die Störungs- und Kluftrichtungen der post-orogenen Plutonite. Die Regionalmetamorphose fand gleichzeitig mit der Deformation statt und hatte während der D2-Phase unter amphibolitfaziellen Bedingungen ihren Höhepunkt. Geothermobarometrischen Berechnungen deuten auf Temperaturen von 534-561°C und Drücken von 5.26-6.20 kbar während dieses Metamorphose Maximums . Die Goldmineralisation ist im Wesentlichen auf Quarzgänge und gelegentlich auf schmale Bereiche der gangnahen Quarz-Serizit Alterationszonen beschränkt. Feldbeobachtungen, mikroskopische und Mikrosonden-Analysen ergaben, dass die hydrothermale Alteration im Gebiet der Um El Tuyor Mine nach dem Höhepunkt der Metamorphose und syn-kinematisch mit lokalen Zerscherungen stattfand. Die wichtigsten goldhaltigen Quarzgänge im Minengebiet von Um El Tuyor sind massiv, teilweise rekristallisiert oder lamelliert. Sie bestehen hauptsächlich aus Quarz-Karbonat-Gängen, die zerbrochen und mit milchigem bis grau lamellierten Quarz späterer Phasen des geothermalen Systems aufgefüllt sind. Die lamellierten Quarzgänge enthalten schmale, ausgelängte Einschlüsse des pelitischen Nebengesteins (bestehend vorwiegend aus Chlorit-Serizit-Sulfiden±Graphit), die vermutlich während des Aufspaltens der Gänge abgerissen und eingeschlossen wurden (Crack-seal Vorgang). Gold tritt in den goldführenden Gängen als Einschlüsse oder im Kristallgefüge in Arsenkies und arsenhaltigem Pyrit auf, gewöhnlich in Verbindung mit untergeordnet Zinkblende, Kupferkies und Magnetkies. Ein weiterer hochreiner Goldtyp füllt Mikrorisse in Sulfiden und Quarz und/oder tritt als disperse Bläschen und Kügelchen in Bereichen mehrfacher Alteration besonders bei Verwachsungen mit Serizit und Karbonat auf. Im Minengebiet lässt sich eine Zonierung vom Fe-As-Zn-Au±Pb±Ag±Te-haltigen Inneren der Gänge über ein Fe-As-Cu-haltiges Zwischenbereich und in einen distalen Rand mit Fe-Cu-Co und Ni beobachten. Ein dreistufiges Alterationsmodell (frühes, Übergang- und fortgeschrittenes Stadium) wird für das erzbringende Hydrothermalsystem von Um El Tuyor vorgeschlagen. Das frühe Stadium bewirkt eine Nebengesteinshydrolyse in Gegenwart eines schwach sauren Fluids, dessen pH durch die Nebengesteinsmineralogie gepuffert wird. Während des Übergangsstadiums kommt es zu ebenfalls Hydrolysereaktionen, es dominieren jedoch Karbonatisierung und Sulfidisierung neben Redoxreaktionen. Letztere reduzierten die Fluide vor allem dort, wo das Fluid/Gesteins Verhältnis niedrig war und stellten damit günstige Bedingungen zur Goldfällung dar. Es lässt sich folgern, dass die Sulfidisierung die Goldlöslichkeit über die Änderung der Sauerstofffugazität als Folge der Redoxreaktionen beeinflusst hat. Das fortgeschrittene Stadium stellte höchstwahrscheinlich eine Phase intensiver Serizitisierung nach Chlorit dar, die K+ verbrauchte, H+ erzeugte und den pH-Wert der Lösung erniedrigte. Kationenaustauschreaktionen waren auf die Zeiten günstiger aNa+/aK+ und Temperaturbedingungen beschränkt, die Albitbildung ermöglichten. Sulfidisierung fand weiterhin während dieses Stadiums statt und ungepufferte Bedingungen waren lokal bei hohen Fluid/Gesteins Verhältnissen wirksam. Chemische Zonierung der goldhaltigen Pyrit- und Arsenkieskristalle in Gegenwart kleiner Einschlüsse von Magnetkies und Zinkblende in diesen Kristallen und das Fehlen derartiger Verwachsungen im Pyrit und Arsenkies der späten Sulfidabfolge dürfte mit einer beginnenden niedrigeren Sauerstofffugazität während der Goldablagerung zu erklären sein. Weiterhin wird der chemische Übergang großer arsenführender Pyritkristalle von den tauben inneren Gangkernen zu den goldführenden Randbereichen als eine Funktion der Redoxreaktionen angesehen, die eine Oxidation der Goldkomplexe und Arsenreduktion bewirken. Untersuchungen der Flüssigkeitseinschlüsse ergaben heterogene Einschlüsse nicht mischbarer, wässriger und kabonischer (CO2-dominierter) Fluide in den goldführenden Quarzgängen von Um El Tuyor. Kriterien wie primäre und sekundäre Formen der karbonischen Einschlüsse, die Koexistenz von Einschlüssen mit karbonischen und wässrigen Fluiden in unterschiedlichen Verhältnissen, Dichten, Füllungsgrad, partialen Homogenisationstemperaturen und Zusammensetzungen weisen auf eine Phasentrennung als Mechanismus der Goldfällung in den goldführenden Quarzgängen von Um El Tuyor hin. Das ursprünglich homogene, erzbringende Fluid war eine gering salinare wässrig-karbonische Lösung (±1-2 mol% CH4 oder N2) das sich in zwei Phasen zu separieren und Gold auszuscheiden begann als die Druck-Temperaturbedingungen von ~340°C bei ~ 1.5 kbar erreicht wurden (bei Tiefen von ~6 km unter lithostatischen Gegebenheiten). Der Zerfall der Gold-Schwefel Komplexe durch das Wechselspiel von Abkühlung, Änderung des Redoxpotentials und pH-Wertes und Abnahme der Schwefelfugazität haben mit großer Wahrscheinlichkeit zur Goldmineralisation der Gänge geführt. Die gesamte Gold-Buntmetallmineralisation und die Bildung des Quarzgangsystems entstand bei Bedingungen von 170-429°C und 0.9-2.1 kbar, d.h. wahrscheinlich in einer Tiefe von 3-8 km und unter Bedingungen der Grünschiefermetamorphose im Übergang von duktiler zu spröder Deformation. Der weite Druckbereich steht vermutlich für das gesamte Fluiddruckregime innerhalb der Scherzone. Er beinhaltet die Bildung der goldhaltigen Quarzgänge während Perioden kontinuierlicher Druckabnahme als Folge des Aufstieges und ebenso plötzlichen Druckabfalls als Folge zunehmender Rissöffnungen, gefolgt von Verfüllungen neuerlich gebildeten Quarzes

Editorial for the Special Issue: Multispectral and Hyperspectral Remote Sensing Data for Mineral Exploration and Environmental Monitoring of Mined Areas

In recent decades, multispectral and hyperspectral remote sensing data provide un- precedented opportunities for the initial stages of mineral exploration and environmental hazard monitoring. Increasing demands for minerals because of industrialization and ex- ponential growth in population emphasize the necessity for replenishing exploited reserves by exploration of new potential zones of mineral deposits. Identification of host-rock lithologies, geologic structural features, and hydrothermal alteration mineral zones are the most conspicuous applications of multispectral and hyperspectral remote sensing satel- lite data for mineral exploration in the metallogenic provinces and frontier areas around the world

Vein-type gold formation during late extensional collapse of the Eastern Desert, Egypt: the Gidami deposit

Orogenic gold deposits, though construed to focused fluid flow during orogenesis, commonly post-date the main accretionary events. Several lines of evidence indicate that orogenic gold formation in the Arabian–Nubian Shield continued through the orogen collapse stage and associated rapid exhumation and thermal re-equilibration. The Gidami gold deposit in the Eastern Desert of Egypt is associated with post-foliation, brittle-ductile shear zones that deformed a weakly foliated tonalite-trondhjemite massif dated as ~ 704 Ma (U–Pb zircon age). Gold-sulfide quartz veins exhibit textural features indicative of repeated mylonitization, recrystallization, and muscovite crystallization. New $^{40}$Ar/$^{39}$Ar ages of muscovite flakes from the auriferous quartz veins and from the altered wallrock overlap within analytical uncertainty at ~ 583 Ma, which corresponds to the climax of extension-related wrenching and rapid exhumation in the region (~ 596 to 582 Ma). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data coupled with microtextural characteristics suggest that early formed pyrite generations experienced substantial fluid-mediated recrystallization, and that a set of metals was remobilized by later fluids. A late generation of fibrous pyrite, ubiquitous in microfractures, deposited while the veins re-opened and deformed. The occurrence of free gold particles along with a late-paragenetic assemblage of galena-sphalerite-chalcopyrite(± hessite ± cervelleite) was related to influx of low salinity, metalliferous H$_{2}$O-NaCl-CO$_{2}$-CH$_{4}$ fluids as indicated by the fluid inclusion laser Raman spectroscopy and microthermometry results. Au-mobilization and redeposition at T ≤ 350 °C and P ~ 1 to 1.7 kbar, triggered by intermittent fluid pluses and thermal re-equilibration, were most likely stimulated by extensional structures and within-plate magmatism. Coincident province- and deposit-scale pressure–temperature-time data highlight the pivotal role of the orogenic collapse tectonics in gold endowment in the Central Eastern Desert’s crust

Orogenic Gold in Transpression and Transtension Zones: Field and Remote Sensing Studies of the Barramiya–Mueilha Sector, Egypt

Multi-sensor satellite imagery data promote fast, cost-efficient regional geological mapping that constantly forms a criterion for successful gold exploration programs in harsh and inaccessible regions. The Barramiya–Mueilha sector in the Central Eastern Desert of Egypt contains several occurrences of shear/fault-associated gold-bearing quartz veins with consistently simple mineralogy and narrow hydrothermal alteration haloes. Gold-quartz veins and zones of carbonate alteration and listvenitization are widespread along the ENE–WSW Barramiya–Um Salatit and Dungash–Mueilha shear belts. These belts are characterized by heterogeneous shear fabrics and asymmetrical or overturned folds. Sentinel-1, Phased Array type L-band Synthetic Aperture Radar (PALSAR), Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), and Sentinel-2 are used herein to explicate the regional structural control of gold mineralization in the Barramiya–Mueilha sector. Feature-oriented Principal Components Selection (FPCS) applied to polarized backscatter ratio images of Sentinel-1 and PALSAR datasets show appreciable capability in tracing along the strike of regional structures and identification of potential dilation loci. The principal component analysis (PCA), band combination and band ratioing techniques are applied to the multispectral ASTER and Sentinel-2 datasets for lithological and hydrothermal alteration mapping. Ophiolites, island arc rocks, and Fe-oxides/hydroxides (ferrugination) and carbonate alteration zones are discriminated by using the PCA technique. Results of the band ratioing technique showed gossan, carbonate, and hydroxyl mineral assemblages in ductile shear zones, whereas irregular ferrugination zones are locally identified in the brittle shear zones. Gold occurrences are confined to major zones of fold superimposition and transpression along flexural planes in the foliated ophiolite-island arc belts. In the granitoid-gabbroid terranes, gold-quartz veins are rather controlled by fault and brittle shear zones. The uneven distribution of gold occurrences coupled with the variable recrystallization of the auriferous quartz veins suggests multistage gold mineralization in the area. Analysis of the host structures assessed by the remote sensing results denotes vein formation spanning the time–space from early transpression to late orogen collapse during the protracted tectonic evolution of the belt

Shear-Related Gold Ores in the Wadi Hodein Shear Belt, South Eastern Desert of Egypt: Analysis of Remote Sensing, Field and Structural Data

Space-borne multispectral and radar data were used to comprehensively map geologi- cal contacts, lithologies and structural elements controlling gold-bearing quartz veins in the Wadi Hodein area in Egypt. In this study, enhancement algorithms, band combinations, band math (BM), Principal Component Analysis (PCA), decorrelation stretch and mineralogical indices were applied to Landsat-8 OLI, ASTER and ALOS PALSAR following a pre-designed flow chart. Together with the field observations, the results of the image processing techniques were exported to the GIS environment and subsequently fused to generate a potentiality map. The Wadi Hodein shear belt is a ductile shear corridor developed in response to non-coaxial convergence and northward escape tectonics that accompanied the final stages of terrane accretion and cratonization (~680–600 Ma) in the northern part of the Arabian–Nubian Shield. The evolution of this shear belt encompassed a protracted ~E–W shortening and recurrent sinistral transpression as manifested by east-dipping thrusts and high-angle reverse shear zones. Gold-mineralized shear zones cut heterogeneously de- formed ophiolites and metavolcaniclastic rocks and attenuate in and around granodioritic intrusions. The gold mineralization event was evidently epigenetic in the metamorphic rocks and was likely attributed to rejuvenated tectonism and circulation of hot fluids during transpressional deformation. The superposition of the NW–SE folds by NNW-trending, kilometer scale tight and reclined folds shaped the overall framework of the Wadi Hodein belt. Shallow NNW- or SSE-plunging mineral and stretching lineations on steeply dipping shear planes depict a considerable simple shear component. The results of image processing complying with field observations and structural analysis suggest that the coincidence of shear zones, hydrothermal alteration and crosscutting dikes in the study area could be considered as a model criterion in exploration for new gold targets

Coeval calc-alkaline and alkaline Cadomian magmatism in the Bafq, central Iran: Insights into their petrogenesis

The Chah-Gaz and Mishdovan areas in the Bafq magmatic complex, central Iran, contain thick series of terrigenous sediments (the Rizu-Dezu complex), and arc-related calc-alkaline and alkaline igneous rocks. Geochemical analyses of igneous rocks from both areas indicate two distinct rock clans: (1) high-K, calc-alkaline-shoshonitic rocks with strong depletions in Nb, P, and Ti, and (2) an alkaline quartz gabbro-diorite, with trace element patterns resembling oceanic island basalts (OIB). New geochronological data reveal that magmatic rocks of both clans crystallized almost simultaneously, with zircon U[sbnd]Pb ages of 534 Ma and 537 Ma, respectively. The whole-rock Nd[sbnd]Sr isotopic data ((87Sr/86Sr)(I) = 0.7052 to 0.7064 and ɛNd(t) = +1.3 to +2.7) of alkaline quartz gabbro-diorite indicate an enriched OIB-like mantle source, while the high-K, calc-alkaline-shoshonitic rocks have εNd(t) = −5.5 to −7.6, clearly reflecting significant contributions from pre-existing Proterozoic basement. Apatite in both the Chah-Gaz and Mishdovan magmatic rocks is of magmatic origin, with light rare earth element (LREE) enrichment patterns. The low Sr/Y and Eu/Eu⁎ values in apatite demonstrate the non-adakitic character of the investigated rocks, while the moderately negative Eu anomaly and inverse correlation between δCe and δEu in the analyzed apatites may reflect reduced parental magmas. The geochemical and isotopic results presented here indicate that slab rollback and opening of an extensional basin could have initiated concurrent Cadomian arc-related calc-alkaline and rift-associated alkaline magmatism in the Chah-Gaz and Mishdovan areas

Polymetallic Sulfide–Quartz Vein System in the Koudiat Aïcha Massive Sulfide Deposit, Jebilet Massif, Morocco: Microanalytical and Fluid Inclusion Approaches

The Koudiat Aïcha Zn-Cu-Pb deposit (3–Mt ore @ 3 wt.% Zn, 1 wt.% Pb, 0.6 wt.% Cu) in the Jebilet massif (Morocco) comprises stratabound lenticular orebodies and crosscutting sulfide-bearing quartz ± carbonate veins in the lower Carboniferous Sarhlef volcano sedimentary succession. The veins are characterized by abundant pyrrhotite, sphalerite, subordinate chalcopyrite and galena and rare Ag and Au minerals. The stratabound massive sulfide ores are attributed to a “VMS” type, whereas the origin of the sulfide–quartz ± carbonate veins remains poorly understood. New mineralogical and microanalytical data (SEM, EPMA and LA-ICP-MS) combined with fluid inclusion results point to two-stage vein formation. The early stage involved C–H–O–N Variscan metamorphic fluids which percolated through fractures and shear zones and deposited pyrite at >400 °C, followed by the formation of pyrrhotite and sphalerite (300 ± 20 °C) in quartz veins and in banded and breccia ores. The pyrrhotite–sphalerite mineralization was overprinted by aqueous brines (34 to 38 wt% eq. NaCl + CaCl2) that precipitated carbonate and Cu-Pb sulfides (±Ag-Au) at ~180–210 °C through mixing with low-salinity fluids during tectonic reworking of early-formed structures and in late extension fractures. The latter ore fluids were similar to widspread post-Variscan evaporitic brines that circulated in the Central Jebilet. Overlapping or successive pulses of different ore fluids, i.e., metamorphic fluids and basinal brines, led to metal enrichment in the quartz–carbonate veins compared to the massive sulfide ores. These results underscore that even a single deposit may record several distinct mineralizing styles, such that the ultimate metal endowment may be cumulative over multiple stages

Mapping Listvenite Occurrences in the Damage Zones of Northern Victoria Land, Antarctica Using ASTER Satellite Remote Sensing Data

Listvenites normally form during hydrothermal/metasomatic alteration of mafic and ultramafic rocks and represent a key indicator for the occurrence of ore mineralizations in orogenic systems. Hydrothermal/metasomatic alteration mineral assemblages are one of the significant indicators for ore mineralizations in the damage zones of major tectonic boundaries, which can be detected using multispectral satellite remote sensing data. In this research, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) multispectral remote sensing data were used to detect listvenite occurrences and alteration mineral assemblages in the poorly exposed damage zones of the boundaries between the Wilson, Bowers and Robertson Bay terranes in Northern Victoria Land (NVL), Antarctica. Spectral information for detecting alteration mineral assemblages and listvenites were extracted at pixel and sub-pixel levels using the Principal Component Analysis (PCA)/Independent Component Analysis (ICA) fusion technique, Linear Spectral Unmixing (LSU) and Constrained Energy Minimization (CEM) algorithms. Mineralogical assemblages containing Fe 2+ , Fe 3+ , Fe-OH, Al-OH, Mg-OH and CO3 spectral absorption features were detected in the damage zones of the study area by implementing PCA/ICA fusion to visible and near infrared (VNIR) and shortwave infrared (SWIR) bands of ASTER. Silicate lithological groups were mapped and discriminated using PCA/ICA fusion to thermal infrared (TIR) bands of ASTER. Fraction images of prospective alteration minerals, including goethite, hematite, jarosite, biotite, kaolinite, muscovite, antigorite, serpentine, talc, actinolite, chlorite, epidote, calcite, dolomite and siderite and possible zones encompassing listvenite occurrences were produced using LSU and CEM algorithms to ASTER VNIR+SWIR spectral bands. Several potential zones for listvenite occurrences were identified, typically in association with mafic metavolcanic rocks (Glasgow Volcanics) in the Bowers Mountains.Comparison of the remote sensing results with geological investigations in the study area demonstrate invaluable implications of the remote sensing approach for mapping poorly exposed lithological units, detecting possible zones of listvenite occurrences and discriminating subpixel abundance of alteration mineral assemblages in the damage zones of the Wilson-Bowers and Bowers-Robertson Bay terrane boundaries and in intra-Bowers and Wilson terranes fault zones with high fluid flow. The satellite remote sensing approach developed in this research is explicitly pertinent to detecting key alteration mineral indicators for prospecting hydrothermal/metasomatic ore minerals in remote and inaccessible zones situated in other orogenic systems around the world

Controls on lode gold mineralization, Romite deposit, South Eastern Desert, Egypt

Field and remote sensing studies reveal that Au-bearing quartz ± carbonate lodes in Romite deposit, in the extreme South Eastern Desert of Egypt, are controlled by NNE-striking shear zones that splay from the ca. 660–550 Ma Hamisana Zone. Quartz in releasing bends with sinistral shear geometry and abundant boudinaged quartz-carbonate lodes with serrate ribboned fabrics suggest vein formation throughout a transpressive wrench system. Ubiquitous hydrothermal quartz, carbonate, and subordinate chlorite and sericite within the shear zones and as slivers in veins, indicate that gold deposition and hydrothermal alteration occurred under greenschist facies conditions. The Al (IV) in chlorite indicates a formation temperature of ∼300 °C, comparable with temperatures estimated from arsenopyrite composition for grains intimately associated with gold in quartz veins. The new geological and geochemical data indicate that splays off the Hamisana Zone are potential gold exploration targets. Quartz veins along the high order (2nd or 3rd) structures of this crustal-scale shear zone are favorable targets. In the Romite deposit and in surrounding areas, a Au-As-Cu-Sb-Co-Zn geochemical signature characterizes mineralized zones, and particularly rock chips with >1000 ppm As and high contents of Cu, Zn, and Co target the better mineralized areas. The carbonate δ13CPDB and δ18OSMOW isotope signatures preclude an organic source of the ore fluid, but metamorphic and magmatic sources are still valid candidates. The intense deformation and lack of magmatism in the deposit area argue for metamorphic dewatering of greenstone rocks as the most likely fluid source. The narrow ranges of δ13C (−4.6‰ to −3.1‰) and δ18O (11.9‰–13.7‰) in carbonate minerals in lodes imply a corresponding uniformity to the ambient temperature and δ13CCO2 (δ13CΣC) of the ore fluids. The calculated δ18OH2O values of 6.9‰–7.9‰ for ore fluids, based on δ18O values of vein quartz further suggest a likely metamorphic origin