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

Abstract

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 280Ma. 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 orogenic gold deposits 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 Late Palaeozoic gold(–antimony–mercury) ore deposits in the Variscides and the Southern Tianshan are more akin to the intrusion-related ore systems.