Hassan M. Helmy

Composition and total-Pb model ages of monazite from high-grade paragneisses in the Abu Swayel area, southern eastern desert, Egypt

SummaryMonazites from high-grade metapelitic paragneisses from the southern Eastern Desert of Egypt (Abu Swayel area) were analysed with the electron microprobe mainly in an attempt to broadly constrain the metamorphic ages of the rocks by means of chemical Th(U)-Pb dating. Two samples were investigated, one showed weak signs of a greenschist facies overprint, the other one did not. For each sample, weighted average ages were calculated from long-time analyses of 18 (16) individual grains with a 5 μm beam placed in the grain centres. The average ages were almost the same (636± 10 Ma, 633±10 Ma). The monazites appeared chemically fairly uniform and homogeneous in both samples with ThO2 contents of ca. 3.3-4.5 wt.%, UO2 0.4-1.2 wt.%, La2O3 12–13 wt.%, Nd2O3 11-13 wt.%, Y2O3 1.8-2.6 wt.%. Some larger grains displayed a weak concentric zoning in the BSE image with increasing brightness near the rims. A microprobe traverse was laid across a zoned monazite from the slightly retrogressed sample. It was found that the U and Y contents were somewhat higher in the outer growth shell. The high Y contents at the rims argue for crystal growth under prograde temperature conditions and against a retrograde overgrowth. There appeared to be a tendency that the model ages become slightly younger towards the crystals rim (645±15 Ma in the core section versus 633±16 Ma in the rim section of the profile). However, the observed differences are interpreted as equivocal due to the limited resolution of EMP monazite dating. Clearly, the results do not support previous hypotheses, according to which Abu Swayel gneisses should belong to pre-Panafrican, mid-Proterozoic metamorphic sequences. Instead, the data accord with other 600-650 Ma metamorphic ages recently recognized near the contact of the East Sahara Craton and the Arabian Nubian Shield. The best interpretation is that high-grade metamorphism at that time occurred in connection with collisional crustal thickening, when a Panafrican terrane assembly was attached to the east Sahara Craton from the (present day) east. This event appears to be distinct from an earlier phase of high-grade regional metamorphism between ca. 700 and 750 Ma, which has been documented in other parts of the Arabian Nubian Shield.ZusammenfassungMonazite aus amphibolitfaziellen metapelitischen Paragneisen der Abu Swayel Region in Südägypten (SW Eastern Desert) wurden mit der Mikrosonde analysiert, unter anderem um die Metamorphosealter der Gesteine auf dem Weg einer chemischen Th(U)-Pb Datierung annäherungsweise zu bestimmen.Zwei Proben wurden untersucht, eine davon wies eine lei Überprägung unter grünschieferfaziellen Bedingungen auf. In jeder Probe wurden 18 bzw. 16 Monazite, jeweils in den Kornzentren, mit langer Zählzeit und mit auf 5 gm defokussiertem Strahl analysiert. Die aus den Analysen errechneten mittleren Th(U)-Pb Alter waren in beiden Proben praktisch gleich (636 10 Ma, 633 ± 10 Ma). Auch die Chemismen der Monazite erwiesen sich in beiden Proben als sehr ähnlich und ziemlich homogen (ThO2 3.3-4.5 wt.%, UO2 0.4-1.2 wt.%, La2O3 12-13 wt.%, Nd2O3 11–13 wt.%, Y2O3 1.8-2.6 wt.%). Nur einzelne größere Körner ließen im BSE-Bild einen ganz schwachen konzentrischen Zonarbau mit größerer Helligkeit am Rand erkennen. Durch ein solches Korn aus der leicht retrograden Probe wurde ein chemisches Profil gelegt urn den Zonarbau zu quantifizieren bzw. die Altershomogenität zu testen. Die Randzone wies dabei leicht erhöhte Gehalte an U and Y auf. Die hohen Y Werte sprechen für eine Kristallisation unter hohen Temperaturen und gegen ein retrogrades Weiterwachsen. Die chemischen Modellalter waren im Kornzentrum im Schnitt etwas höher als am Kornrand (645±15, 633±16 Ma). Allfällige Zeitdifferenzen im Wachstum oder Bleiverlusteffekte waren allerdings innerhalb des methodischen Fehlers nicht sicher auflösbar. Die untersuchten Paragneise wurden bisher meist als Reste eines mittelproterozoischen, prä-panafrikanischen Kristallins angesehen. Die chemischen Modellalter für die Monazite bestätigen diese Ansicht aber nicht, sondern passen vielmehr gut zu anderen Metamorphosealtern von etwa 600 bis 650 Ma, welche in der letzten Zeit mehrfach entlang der Grenzzone zwischen dem Ostsahara-Kraton und dem ArabischNubischen Schild dokumentiert wurden. Die wahrscheinlichste Erklärung ist, daß diese Regionalmetamorphose durch das Andocken panafrikanischer Terrane an den Kraton verursacht wurde.

Noble metal nanoclusters and nanoparticles precede mineral formation in magmatic sulphide melts

In low temperature aqueous solutions, it has long been recognized by in situ experiments that many minerals are preceded by crystalline nanometre-sized particles and non-crystalline nanophases. For magmatic systems, nanometre-sized precursors have not yet been demonstrated to exist, although the suggestion has been around for some time. Here we demonstrate by high temperature quench experiments that platinum and arsenic self-organize to nanoparticles, well before the melt has reached a Pt–As concentration at which discrete Pt arsenide minerals become stable phases. If all highly siderophile elements associate to nanophases in undersaturated melts, the distribution of the noble metals between silicate, sulphide and metal melts will be controlled by the surface properties of nano-associations, more so than by the chemical properties of the elements.

Mineralogy and fluid inclusion studies of the Au-Cu quartz veins in the Hamash area, South-Eastern Desert, Egypt

SummaryField relations, petrographic observations and fluid inclusion data are used to characterize the mineralizing fluids of gold-copper bearing quartz veins, which are spatially associated with a granite-porphyry, metavolcanics and metagabbro in the Hamash area, South Eastern Desert of Egypt. Four generations of genetically related quartz veins occur in the Hamash mine area. Two types of alteration are developed in vicinity of quartz veins; i.e., sericite-quartz-pyrite and chlorite-epidote-pyrite-sericite alteration. Fe-Cu sulfides in the veins were precipitated in two stages, early pyrite (PI) and chalcopyrite were altered to secondary chalcocite, bornite and digenite and a second generation of pyrite (PII and PIII). PI pyrite and quartz contain inclusions of gold as well as remobilized gold along cracks and microfractures. Two types of fluid inclusions are distinguished: 1) primary H2O-CO2-CH4-NaCl inclusions (type I) and 2) primary and secondary aqueous inclusions (type II). Type II is further subdivided by the inclusions occurrence within different vein types. Type I inclusions entrapped the endmembers of an unmixed fluid which consists of an aqueous phase and a CO2-rich gas phase, respectively. The entrapment conditions of approximately 250°C and 200 bars were estimated by intersecting the isochores of the two coexisting aqueous and CO2-rich fluids and indicate a shallow crustal level. The salinity of type IIa inclusions is generally low (< 9 wt.% NaCleq), they homogenize above 234°C and included heterogeneous and homogeneous fluids over a wide pressure range. Homogenization temperatures of type IIb inclusions vary between 102° and 284 °C, their assumed entrapping temperatures are 200°C and 110°C, respectively. The four generations of quartz veins are related with different inclusion types. The metals including primary gold were probably transported as bisulfide complexes and precipitated due to wallrock sulfidation, fluid mixing and phase separation. The Hamash Au-Cu mineralization shows a combination of porphyry- and epithermal-deposits characteristics.ZusammenfassungGeländebeziehungen, petrographische Beobachtungen und Untersuchungen an Flüussigkeitseinschlüssen erlauben die Charakterisierung der mineralisierenden Fluide in Gold-Kupfer-führenden Quarzgängen, die mit einem Porphyr-Granit und einem Gabbro im Hamash Gebiet der südöstlichen Wüste von Ägypten assozüert sind. Vier Generationen von Quarzgängen kommen im Gebiet der Hamash Mine vor. Zwei Alterationstypen sind in der Nähe der Quarzgänge ausgebildet: Serizit-Quarz-Pyrit und Chlorit-Epidot-Pyrit-Serizit Alteration. Die Fe-Cu-Sulfide in den Gängen wurden in zwei Etappen ausgefällt, früh kristallisierter Pyrit (PI) und Kupferkies wurden zu sekundärem Chalkosin, Bornit und Digenit sowie einer zweiten Generation von Pyrit (PII und PII) umgewandelt. PI Pyrit und Quarz enthalten sowohl Gold-Einschlüsse als auch remobilisiertes Gold entlang von Spalten und Mikrobrüchen. Zwei Typen von Flüssigkeitseinschlussen wurden unterschieden: 1) primäre H2O-CO2-CH4-NaCl Einschlusse (Typ 1), 2) primäre und selcundäre wäßrige Einschlusse (Typ II). Typ I Einschlusse schlossen die Endglieder eines entmischten Fluides ein, das aus einer wässrigen bzw. einer CO2-reichen Gasphase bestand. Die Einschlußedingungen von ungefähr 250°C und 200 bar wurden durch den Schnittpunkt der Isochoren der beiden koexistierenden Fluide bestimmt und zeigen einen seichten Krustenbereich an. Die Salinität von Typ Ila Einschlussen ist generell niedrig (<9 Gew.% NaCleq), sie homogenisieren uber 234 °C und schlossen heterogene und homogene Fluide über einen großen Druckbereich ein. Die Homogenisationstemperaturen von Typ IIb Einschlussen variieren zwischen 102° und 284°C, ihre angenommenen Einfangtemperaturen liegen bei 200 °C bzw 110 °C. Die vier Generationen von Quarzgängen stehen in Zusammenhang mit den verschiedenen Typen von Flussigkeitseinschlussen. Die Metalle, einschließlich des primaären Goldes, wurden wahrscheinlich als BisulphidKomplexe transportiert und auf Grund von Sulfidisierung der Nebengesteine, Fluidmischung und Phasenseparation ausgefällt. Die Hamash Au-Cu Mineralisation zeigt eine Kombination von Porphyr- und Epithermal-Lagerstätten Charakteristika.

Trace and REE element geochemistry of fluorite and its relation to uranium mineralizations, Gabal Gattar Area, Northern Eastern Desert, Egypt

Uranium mineralizations occur and form in a broad range of geologic setting and age, including magmatic to surfacial conditions, and there are numerous controls on their transportation and deposition, such as redox, pH, ligand concentration, complexation, and temperature. These temporal and spatial variations have caused a range of ore deposit mineral assemblages. Consequently, understanding their conditions of formation is still in its infancy. This research reports rare earth elements (REE) and trace elements of fluorite associated with hexavalent uranium mineralizations and tests of genetic models for the deposits. These data contribute to a better understanding of the variables controlling fluorite formation and uranium ore composition through understanding the evolution of these ore-forming hydrothermal systems. Fluorite in Gabal Gattar granite occurs as disseminations and/or thin veinlets and encrustations filling some uranium mineralized fissures and fractures along the northern margin of host granite mass. In the U-poor samples, fluorite forms well-developed large crystals that are commonly zoned. The zones are represented by alternating colorless and violet zones, and the outer zones are frequently dark violet. In the U-rich samples, fluorite is usually anhedral, unzoned, and has a dark violet color. The results of analysis of REE and trace element contents of fluorites using laser ablation inductively coupled plasma mass spectrometry indicate that total REE in the anhedral unzoned fluorite are elevated compared to the well developed zoned fluorite, and also total REE in dark violet zones of zoned fluorite are elevated with respect to the colorless zones. The fluorites and host granite are generally characterized by strongly negative Eu anomalies and slightly negative or chondritic Ce anomalies. Accordingly, REE patterns of the fluorite and host granite are roughly alike, indicating that the source of REE and trace elements of hydrothermal fluids is the host granite leached by fluids. Y/Y*, Ce/Ce,* and Eu/Eu* patterns show that fluorite clearly records the compositional evolution of the hydrothermal solutions that have transferred trace and REE from host granite during the fluid–wall rocks interactions. The high uranium contents of fluorite in Gabal Gattar granite suggest that parent fluids bearing fluorine have interacted with host granite to leach uranium from the accessory minerals of granite and tetravalent uranium minerals in reduced or weakly oxidized zones.