Deep-reaching fracture zones in the crystalline basement surrounding the West Congo System and their control of mineralization in Angola and Gabon

A framework of major, deep-reaching fracture zones in western Central Africa is inferred from airborne magnetometric and surface geological observations in Central Angola and Gabon. A correlation is proposed between these observations and the continental negative Bouguer anomaly. The minimum age of the inferred tectonic framework is probably Kibaran. Considerable portions are thought to have been reactivated, and others may have only originated at later stages. Its control of major structural units and the distribution of carbonatite complexes and kimberlite occurrence and of diamond and lead-zinc deposits is discussed

Deep-reaching fracture zones in the crystalline basement surrounding the West Congo System and their control of mineralization in Angola and Gabon

A framework of major, deep-reaching fracture zones in western Central Africa is inferred from airborne magnetometric and surface geological observations in Central Angola and Gabon. A correlation is proposed between these observations and the continental negative Bouguer anomaly. The minimum age of the inferred tectonic framework is probably Kibaran. Considerable portions are thought to have been reactivated, and others may have only originated at later stages. Its control of major structural units and the distribution of carbonatite complexes and kimberlite occurrence and of diamond and lead-zinc deposits is discussed

The Central European, Tarim and Siberian Large Igneous Provinces, Late Palaeozoic orogeny and coeval metallogeny

The formation of the Central European and Tarim Large Igneous Provinces (LIPs) in the Early Permian coincided with the demise of the Variscan and the Southern Tianshan orogens, respectively. The Early Triassic Siberian LIP was formed in the wake of the Western Altaid orogeny in the Late Permian. These processes coincided with the development of the majority of known Late Palaeozoic and Early Mesozoic hydrothermal and magmatic ore deposits in the corresponding orogenic domains. Nickel-copper (-PGE) deposits followed directly from the evolution of the (ultra-)mafic melts which make up the LIPs. In Western Siberia, the diverse assemblage of associated noble and base metals in the Noril?sk-Talnakh Ni-Cu (-PGE) deposits suggests these metals also had their source in the mantle domain from which the (ultra-)mafic melts were generated. The same metals variably found their way into the hydrothermal ore deposits in the defunct Variscan and Southern Tianshan orogenic domains. These ore deposits have traditionally been viewed as a result of orogenic processes. However, their ages, together with the timing and nature of their by then intracontinental tectonic control cause uncertainty concerning the role of the orogens. In view of their mantle sources in their association with the Siberian LIP, the mantle contribution to these Late Palaeozoic hydrothermal ore deposits in the orogenic belts may have been more significant than previously thought. An orogenic contribution to melting of mantle complexes and to mineralisation may have resided in the earlier modification of subcontinental mantle domains by subduction of oceanic lithosphere. In all three cases, the controlling tectonic setting of orogenic cessation, LIP formation and mineralisation was dominated by translithospheric strikeslip deformation, possibly in combination with orogenic collapse and lithosphere delamination. In view of their recurrence, the orogen-lip sequences were probably not fortuitous. The controlling strike-slip faults were principal elements of the lithosphere-scale dynamic framework that led to the amalgamation of Pangaea. At this scale, the exceptionally large volume of the Siberian LIP may, in addition to the strike-slip dissection of the lithosphere, have been related to extension in the continental lithosphere of the margin of the Supercontinent (cf. Gutiérrez-Alonso et al. 2008). The peripheral extension was associated with compression in its centre. The explanation of these Large Igneous Provinces does not require the concept of an active mantle plume because deep-reaching strike-slip deformation, orogenic collapse and lithosphere delamination involved in the destruction of the orogenic edifices can have caused decompression melting in large domains of the subcontinental mantle

Busting at the seams?

In Permo-Triassic Eurasia two transcontinental mantle events can be recognized: • the c. 295-280 Ma Tarim event, between western Europe and eastern Kazakhstan (c. 8,500 km apart) and • the c. 250-245 Ma Siberia event, between western Europe and northeastern Asia (c. 10,000 km apart). Were these events driven by deep mantle plumes, shallow mantle processes, or both, or something else

Spatial and temporal distribution of the orogenic gold deposits in the Late Palaeozoic Variscides and Southern Tianshan: How orogenic are they?

A principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes. The yardstick of the relation has resided, loosely, in the peak of metamorphism. Age estimates in the Variscides and Tianshan indicate that most orogenic ore deposits were formed in the course of the Late Carboniferous to Middle Triassic with a peak between 305 and 280 Ma. Their locations, settings and ages suggest an association with coeval, lithosphere-scale strike-slip deformation which played a role in the destruction of the orogenic system following crustal shortening. This destruction tends to invalidate the orogenic association of the orogenic ore deposits. Observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike-slip belts tapped into sub-crustal levels. Here, lithological composition had been modified by subduction during the stage of construction. Consequently, any orogenic element in the ore deposit models is, apart from location, only an element of inheritance of subduction-modified lithosphere features acquired during construction of the orogen. These features may, however, well have been crucial for the formation of the ore deposits. In view of the translithospheric extent of the strike-slip belts, fluids which contributed to the ore deposits may have come from at least the entire, in part metasomatized, lithosphere column. The ore deposits could probably not have been formed without these deep-reaching strike-slip systems. A model is proposed for the geological setting of the orogenicgolddeposits in the Variscides and the Southern Tianshan. It consists of plutono-metamorphic elements in the mantle, in the lower and in the middle crust, within a transpressional to transtensional tectonic framework of translithospheric faults, and associated domes and pull-apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics. The thermal engines of these systems were probably in localized upwellings of the asthenosphere, prompted and controlled by the lithosphere-scale deformation at the time. At an even larger scale, oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits. The thesis of specific, localized tectono-thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism. It suggests that the LatePalaeozoicgold(–antimony–mercury) ore deposits in the Variscides and the Southern Tianshan are more akin to the intrusion-related ore systems