H. de Boorder

Late Proterozoic extensional collapse in the Arabian–Nubian Shield

A structural and petrological study of the Late Proterozoic rocks in the Wadi Kid area, Sinai, Egypt indicates the presence of an extensional metamorphic core complex in the northern Arabian–Nubian Shield. Gneissic domes throughout the Arabian–Nubian Shield resemble the core complex of the Wadi Kid area and as a result, they are interpreted as extensional metamorphic core complexes. The presence of a widespread phase of extension at the end of the Pan‐African period in the Arabian–Nubian Shield requires a new interpretation of the tectonic history of this shield. Three main tectonic phases are recognized in the Late Proterozoic of the Arabian–Nubian Shield. Ophiolites and island‐arc remnants are relicts of an oceanic phase, the oldest one. This phase was followed by arc‐accretion, well established in the Arabian–Nubian Shield from the presence of individual terranes bordered by sutures, which was responsible for lithospheric thickening. The Late Proterozoic ended with widespread NW–SE extension. The metamorphic core complexes, late‐orogenic extensional basins and large strike slip zones were formed during this phase. Similarity of the tectonic evolution of the Arabian–Nubian Shield with the Mesozoic and Early Cenozoic evolution of western North America lead us to conclude that gravitational instability at the final stages of the arc‐accretion phase caused the collapse and resulted in extension at the latest stages of the Pan‐African orogeny in the Arabian–Nubian Shield.

Late Cenozoic mineralization, orogenic collapse and slab detachment in the European Alpine Belt

Abstract It is known that mineralization in orogenic belts often occurs in extensional settings late in the collisional history, and without voluminous co-temporal magmatism. In search of the underlying geodynamic processes we consider the spatial and temporal distribution of Late Cenozoic hydrothermal mineralization in the European Alpine Belt and relate this to the distribution of heat in the lithosphere as inferred from seismic tomography. Specifically, we propose a hypothesis for mineralization in the European Alpine Belt involving increase in heat flow and fluid flow in response to tearing and detachment of lithosphere slabs and concomitant emplacement of the hot asthenosphere at lower crustal levels. We conclude that detailed knowledge of lithosphere/mantle structure and processes can help to define potentially mineralized zones within regions of orogenic collapse. Our approach pioneers the application of lithosphere and mantle tomography to metallogenesis and to earth resources exploration.

Structural controls of kimberlite and lamproite emplacement

Abstract In this article we review the regional and local structural controls on the emplacement, in the crustal environment, of kimberlites and lamproites after the generation of the magmas at depth. We find that there is good evidence that they are related to major deep faults or shears (mobile zones) that traverse the entire crust and may even traverse the lithosphere. Kimberlite and lamproite emplacement is favoured by transcurrent or extensional reactivation of these. The transcurrent reactivation may be a part of continental rifting. The extensional reactivation forms characteristic linear grabens (aulacogens). The emplacement of the kimberlites and lamproites appears to predate or, especially, postdate the peak tectonism, but this last point may be largely apparent because any sedimentation associated with the tectonism will tend to cover syn- or pre-tectonic intrusives. The kimberlites and lamproites within or adjacent to the mobile zones preferentially occur at intersections between conjugate zones or along internal and splay faults and shears which have a dilational or a conjugate orientation. The orientation of these depends upon kinematics of the transcurrent movement at the time of emplacement. Intrusives in the aulacogens are preferentially sited in the hanging wall of the major graben forming zone and tend to occur along oblique transfer structures which in turn are usually old basement shears or faults and may extend into the basement. The sedimentary cover may obscure the deep basement structures of aulacogens to such an extent that they may be seen only as widely spaced fault braids, fractures or joints in the cover. Diamondiferous kimberlites occur when the above structures form on Archean basement, whereas diamondiferous lamproites appear to be favoured by old reworked basement. In both cases the lithosphere should be abnormally thick.

Geodynamic setting of the mineral deposits of the Urals

The first-order geodynamic domains of the Uralide orogen constitute a relatively simple pattern across the orogenic belt. Continental rifting along the western margin is expressed by a system of Vendian-early Palaeozoic structures with shale-hosted siderite and magnesite and basalt-hosted base metals. It is superimposed on a Middle Riphean rift system with layered mafic-ultramafic complexes with chromite and ilmenite-titano-magnetite and subordinate ophiolite massifs with gold and magnetite. An oceanic spreading domain, immediately east of the Main Uralian Fault is associated with chromite, titano-magnetite and massive sulphide deposits (Dombarovsky or Cyprus type). Further east, bimodal volcanic associations in island arcs with oceanic crust have formed copper-zinc massive volcanic sulphide deposits (Uralian type). Subsequently, complex volcanic sulphide deposits are associated with belts of andesite-dacite and co-magmatic diorite (Baymak or Kuroko type). The eastern, destructive, margin of the orogen is characterized by magnetite and copper-magnetite skarns and porphyry systems. Relatively small plagiogranite to granodiorite complexes, related to oceanic crust, carry scheelite and gold. Calc-alkaline granitic massifs have formed associations of tungsten, tantalum and beryllium. This pattern was dissected by major thrusts and transcurrent shear zones. Increased fluid activity in the course of deformation, inferred to have been involved in complex multi-phase gold mineralization was most probably controlled by deep-reaching faults and shears. Although the recognition of the first-order domains represents a guideline for exploration, detailed structural geological studies of the kinematics of the major faults and shear zones are required in conjunction with radiometric dating of suitable fault- and ore-associated minerals, in association with on-going deep-reaching geophysical investigations.

Ductile reactivation of Proterozoic brittle fault rocks; an example from the Vestfold Hills, East Antarctica

Abstract Brittle fault rocks, usually in the form of minor pseudotachylyte veins have been reported from most high-grade metamorphic Archaean and Proterozoic terrains. In analyses of local crustal evolution such brittle structures are often ignored; brittle faults commonly reflect the very last phases of local deformation during final uplift of the basement to shallow crustal levels, and as such they are thought to be of minor or no importance in large-scale Archaean and Proterozoic tectonics. New data from Archaean and Proterozoic high grade terrains in Australia and Antarctica indicate that minor fault veins may in some cases represent significant early low-grade brittle events which have been overprinted and largely erased by subsequent phases of ductile deformation. This paper describes such a brittle event in the Vestfold Hills, East Antarctica, which may represent an important phase of mid-Proterozoic uplift and crustal extension.

Crustal architecture of the Donets Basin: tectonic implications for diamond and mercury-antimony mineralization

Abstract Kimberlite-like rocks and minor diamond finds are reported in the Precambrian Ukrainian Shield south of the Donets Basin. Prolific mercury-antimony mineralization occurs in Carboniferous quartz arenites within the Basin. The tectonic setting is examined on the basis of recent data compilations and ongoing research in the Ukraine and Voronezh shield areas and the Pripyat-Dnieper-Donets palaeorift. In the Donets region, a straightforward analogy of any diamond district with the Archangelsk province is not likely in the absence of a Proterozoic shear comparable with the White Sea-Belomorian Mobile Belt. A deep-reaching, NNW-striking lithosphere lineament is identified here as the Kharkov-Donetsk lineament. It transects the rift between the Donets and Dnieper basins. The structures involved in this lineament have controlled Palaeozoic sedimentation and the extent of Late Permian inversion of the Donets basin. During the inversion, the lineament and associated deep-reaching longitudinal structures provided pathways for the migration of mineralizing fluids from deep levels in the lower crust and upper mantle. The intersection, in the Kharkov area, of this lineament with a northeasterly striking lithosphere root should focus diamond exploration towards the northern shoulders of the rift. The extreme attenuation of the crust beneath the Donets Basin, relative to the western basins of the rift, is associated with crustal detachment and subsidence during and possibly after inversion, concomitant with emplacement of asthenospheric materials at higher levels. Together with the continued subsidence in the western Donets Basin, during the Late Permian inversion, this invokes a tectonic setting for the Hg Sb mineralization not unlike the orogenic-collapse-associated settings of Hg Sb deposits in western Europe. Further investigation of the geodynamics of the Donets Basin would benefit from deep reflection seismics, petrogenetic studies of magmatic products and their xenoliths, and satellite remote sensing analysis.

Ore deposits of central Asia: from Siberia to northwest China

The papers in this thematic issue provide up-to-date details on a range of ore systems across the Pamir (Tajikistan), Altai and Tien Shan regions (Siberia and northwest China), Russia Far East, including Fe–Pt ore deposits associated with the Siberian Traps, and a comprehensive review of Siberia’s mineral deposits. The giant Muruntau gold deposit in Uzbekistan is also discussed. Figure 1 shows the geographic position of the mineral deposits described in this issue. The first paper, Metallogeny of Siberia: tectonic, geological and metallogenic settings of selected significant deposits by Seltmann, Soloviev, Shatov, Pirajno, Naumov & Cherkasov, is also a first in that it provides details on a wide range of mineral systems across Siberia. Siberia is well endowed with mineral resources, such as Ni, Cu, PGE, Mo, W, U, Sn, Mn, Au, Ag, Pb, Ta– Nb and diamonds. These mineral resources are represented by an equally wide range of mineral deposit styles, including VHMS, SEDEX, porphyry, epithermal, intrusion-related, skarns, orthomagmatic, carbonatite and diamondiferous kimberlites. Some of the Siberian deposits are in the large to superlarge and world-class category (e.g. Norilsk, Udokan, Sukhoi Log, Olympiada and the Yakutian diamond-bearing kimberlites). The comprehensive review in this paper highlights the significance of thorough studies of mineral systems in contributing not only in the understanding of metallogenic processes, but also in providing useful vectors for exploration targeting. Ryabov & Lapkovsky’s paper Native iron (–platinum) ores from the Siberian Platform trap intrusions reports the results of the authors’ detailed study of native iron and Ni–Fe alloys (kamacite, tanite and awaruite) that are present in mafic–ultramafic intrusions of the Siberian traps and first discovered in the 1950s. The existence of a new Fe (–Pt) ore type is reported for the first time. These authors provide detailed descriptions of these unusual Fe occurrences, species, their mineralogy and geochemistry. The study of Siberian native iron shows wide variations in composition from Ni-free iron to awaruite and can be intergrown with a number of minerals. Globules of glass with a ‘liquid-inliquid’ structure are also found in native iron. Isotopic compositions, such as He/He, dCPDB, Sr/Sr, Rb/Sr of native iron and host rocks suggest contributions of continental-crust material in during ore formation. The Siberian native iron contains high concentrations of Cu, Ni, Co, Pt, Pd, Rh and Ge. Native iron and related Fe (–Pt) ores are modelled to have formed in feeder columns as a result of the interaction of tholeiite-basalt melts with hydrocarbon fluid reservoirs in the sedimentary rocks of the Siberian platform cover. The finding of Fe (–Pt) ores opens up new possibilities for discovering new types of ores in large igneous provinces, where hydrocarbons are present in the sedimentary cover rocks. Simonov, Gaskov & Kovyazin in Physico-chemical parameters from melt inclusions for the formation of the massive sulfide deposits in the Altai–Sayan Region, Central Asia describe their detailed studies of quartzhosted melt inclusions from rhyolitic dacite and andesite rocks hosting volcanogenic massive sulfide deposits in the Altai–Sayan region. The high-temperature (910– 11808C) of felsic melts is explained by degassing in open volcanic systems and heating by ascending basaltic magma. REE patterns of these melt inclusions display distinct negative Eu anomalies, which suggest extensive plagioclase fractionation. REE patterns are similar to those of melt inclusions of the Yaman–Kasy deposit (marginal sea setting) in the South Urals and to island arc rhyolites. Analyses of melt inclusions showed high values of Cu. Comparison of data on melt inclusions has shown that some physical and chemical parameters of the magmas that generate volcanogenic massive sulfide and porphyry Cu deposits can be similar within certain limits. Silver–antimony deposits of Central Asia: physicochemical model of formation and sources of mineralisation by Pavlova & Borovikov discusses the genesis of Ag–Sb deposits in southeast Pamir (Tajikistan), Talas in northern Tien Shan (Kyrgyzstan), southeast Altai and northwest Mongolia, and Verkhoyansk province (Yakutia). The Ag–Sb deposits of Central Asia have close spatial and temporal relationships with granitoid and alkaline mafic magmatism. The Ag–Sb mineral systems were formed in the temperature range 50–2808C, from Ag-specific reduced two-phase hydrothermal fluids containing a highly concentrated chloride solution and a