Paleozoic crustal growth and metallogeny of Central Asia: evidence from magmatic-hydrothermal ore systems of Central Kazakhstan

Abstract

The ore deposit spectrum of Central Kazakhstan reflects a range of lithospheric magmatism controlled by the polycyclic geodynamic evolution of an active continental margin which develops from back-arc oceanic settings (volcanic-hosted massive sulphide Cu–Au ore deposits) to subduction-controlled calcalkaline magmatism (copper porphyries) with subsequent stages of crust differentiation of low-degree partial melting and extended intramagmatic fractionation (molybdenum porphyries) on to continental rifting (peralkaline REE–Zr–Nb-rich systems). Mesothermal gold deposits mark transcrustal shear-zone controlled fluid conduits formed during arc slicing and thermal relaxation in a post-orogenic setting. The ore deposits characterize successive and spatially overlapping stages of crustal growth and consolidation. The general subduction setting of this range of ore deposits is reflected by calc-alkaline trends in the associated subalkaline, high-K felsic rocks with a wide range in silica contents. The predominant metal spectrum of Au, Cu and Mo indicates relatively oxidized magmatic-hydrothermal systems, in accordance with the metaluminous I-type nature of their igneous host rocks, i.e. oxidized igneous precursor material. Despite of presumably highly variable ratios of mantle/crust components in individual magmatic systems (gabbros to leucogranites), most rocks have very similar positive initial εNd values of 0 to +5.5, and depleted mantle model ages in the range of 500–800 Ma. The ubiquitous relatively young mantle extraction age is likely to characterize the lower crust of Central Kazakhstan. This basement is interpreted as back-arc oceanic crust that formed behind oceanwards drifting continental-margin slivers during the initial stages of the active continental margin evolution of the Late Precambrian Angara–Baltica supercontinent. The early back-arc oceanic crust was transformed into the present lower crust during the Paleozoic orogenic evolution, in which the back-arc crust was buried by magmatic arc and sedimentary material and recurrently affected by high-grade metamorphism, basaltic melt injection/underplating, and granitic melt extraction. Anorogenic rift-related Permian peralkaline riebeckite granites with REE–Zr–Nb mineralization have very high positive εNd of +5 to +8, which is probably inherited from young subcrustal lithosphere at the mantle/crust boundary that was affected by partial remelting due to asthenospheric upwelling during rifting.