Peter Neumayr

Structural geology, single zircon ages and fluid inclusion studies of the Meatiq metamorphic core complex: Implications for Neoproterozoic tectonics in the Eastern Desert of Egypt

Abstract The Meatiq metamorphic core complex (MMCC) formed during the Precambrian as a result of multiple deformation and metamorphism in the Eastern Desert of Egypt. Structural, geochronologic, and fluid inclusion microthermometric analyses reveal a long formation/deformation history for the MMCC. This started with the break-up of Rodinia at ca. 800 Ma and continued until Pan-African collision at ca. 580 Ma. Between 800–780 Ma, rifting continued into sea floor spreading and oceanic crust formation. Synchronously, the Um Ba′anib granite intruded into an approximately 1.14 Ga old crust comprising migmatic amphibolites. Rifting was accompanied by the deposition of quartz- and mica-rich sediments. Between 660 Ma and 620 Ma, convergence between East and West Gondwanaland caused burial of sediments to a crustal depth of approximately 20 km and intrusion of calc-alkaline rocks. Subsequently, the meta-sediments were thrust across the Um Ba′anib granitoid. Deformation of both rock units took place under amphibolite-facies metamorphic conditions. Fluid inclusions with moderate density provide evidence for the retrograde stage of this metamorphic event. Continued oblique convergence between East and West Gondwanaland resulted in a transpressional regime with displacement partition. While nappe stacking continued in foreland domains, the MMCC was exhumed to a depth of 12–15 km in hinterland domains. Extension-related granitoids were emplaced between 620 and 580 Ma. Microthermometric analyses of fluid inclusions suggest a crustal depth of approximately 10–12 km for the transpressional event. Rapid exhumation was accompanied by detachment of the cover nappes and emplacement of syn-tectonic intrusions, which caused local contact metamorphism. Low-density fluid inclusions document high-T, low-P conditions for the contact metamorphism. The final stage of exhumation took place under brittle/ductile conditions at a crustal depth of approximately 3–6 km indicated by water-rich fluid inclusions. The age of this event is constrained by the intrusion of the late- to post-tectonic Arieki adamellite at approximately 580 Ma.

Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40 Ar/ 39 Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40 Ar/ 39 Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at � 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until � 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography. 2002 Elsevier Science Ltd. All rights reserved.

A late Neoproterozoic magmatic core complex in the Eastern Desert of Egypt: emplacement of granitoids in a wrench-tectonic setting

Abstract Core complexes within the Eastern Desert of Egypt evolved in the course of orogen-parallel extension during Neoproterozoic plate convergence. In the Sibai Core complex, more than 90% of the exposed surface consists of magmatic rocks. In this study four magmatic suites have been distinguished. They are geochemically characterised and their intrusion ages are determined using the Pb/Pb single zircon evaporation technique. Results show that all magmatic suites formed during Neoproterozoic tectonics. The oldest, calc-alkaline rocks of group (I) gave ages of 670–690 Ma and are related to subduction processes in an island-arc setting. Subsequent orogen-parallel extension was accompanied by the intrusion of group (II) and (III) granitoids yielding ages of 655 and 645 Ma, respectively. The younger, felsic granites of group (IV) intruded along pre-existing structures and exhibit variable composition. We relate emplacement of various granitoids within the Sibai to progressively evolving structures. Group (I) represents the deep level of an island arc and exhibits nappe stacking structures related to oblique convergence in the late Neoproterozoic. Group (II) granitoids have been emplaced at mid-crustal levels and portray transpression and orogen-parallel extrusion dynamics. Related structures are represented by an older set of sinistral shear zones. Granitoids of group (III) intruded upper crustal levels during exhumation of the Sibai Core complex. Their emplacement was controlled by the pull-apart regime of a younger set of sinistral shear zones and associated normal shear zones. Denudation due to ongoing orogen-parallel extension is evident from deposition of sediments within the adjacent intramontane Kareim molasse basin. The sediments were intruded by post-tectonic granitoids of group (IV). Close relations between core complex formation and ongoing magmatic activity define the Sibai as a magmatic core complex.

Five questions for fun and profit: A mineral system perspective on metallogenic epochs, provinces and magmatic hydrothermal Cu and Au deposits

We argue that the empirical basis of deposit classification schemes is an inhibitor of the adventurous conceptual thinking required to aid efficient mineral exploration. It ought to be possible to construct a limited number of process — based, scale-integrated, system models to describe mineral and hydrocarbon deposits, provinces and epochs. We suggest that the geodynamic controls on reservoir construction and focused fluid release may be a unifying theme across many classes of deposits, effecting the formation of mineral provinces and epochs. The common architectural, geodynamic and fluid reservoir characteristic of porphyry Cu-Au and orogenic Au classes of deposits serve to illustrate the concept. The “Five Question” description of mineral systems is utilized.

Mapping of hydrothermal alteration and geochemical gradients as a tool for conceptual targeting: St Ives Gold Camp, Western Australia

Camp- to deposit-scale alteration halos at the kilometrescale are documented in the St. Ives gold camp, the Yilgarn Craton, Western Australia. St. Ives has sulphide-oxide mineral footprints, which are interpreted to represent different hydrothermal fluids, a more reduced and a more oxidized fluid. Boundaries where reduced and oxidized fluid domains border each other are particularly suitable for gold precipitation, suggesting a redox control on gold mineralization. Oxidized zones can be identified using detailed gravity and aeromagnetic images as well as camp-scale, first-fresh-rock, multielement whole-rock geochemistry and PIMA data. Stable isotope variations also match well spatially with reduced and oxidized zones.

Characteristics and evolution of hydrothermal fluids from the Archean orogenic New Celebration gold deposits, Western Australia

The western segment of the Boulder Lefroy fault zone in the Eastern Goldfields province of Western Australia hosts the New Celebration gold deposits (∼20,000,000t @ 2.40g/t Au for ∼ 1,500,000 oz Au). Detailed petrography and microthermometry on fluid inclusions hosted in Early (Porphyry- and Mylonite-styles) and Late (Fracture-style) gold-related quartz and quartz-carbonate veins at New Celebration revealed a progression from early methane-rich fluids, which may represent deep-crustal fault-zone fluids, to low salinity, aqueouscarbonic gold-related fluids, to late, dense, highly saline brines.

Paleohydrologic evolution of the St. Ives gold camp

In the St. Ives gold camp in the Yilgarn Craton of Western Australia, several stages of hydrothermal alteration are pre-dating (epidote-calcite-magnetite 1-pyrite 1-chalcopyrite-quartz), synchronous with (plagioclase-carbonate-pyrite 1±Au±magnetite 2±hematite) and post-dating (quartz vein system-pyrite 2-chlorite) gold mineralization. Hydrothermal fluids are characterized throughout the alteration history to gain an in depth understanding of chemical processes which controlled gold precipitation and the site of gold ore bodies.