Late Palaeozoic potassic igneous rocks of the Molo-Sarychat pluton in the eastern Kyrgyz Tien Shan: Geochemistry, U–Pb zircon geochronology and implications for related skarn-porphyry Mo-W-Cu-Au mineralization

The Molo-Sarychat multiphase pluton is situated along the fault systems of the ‘Main Structural Line of Tien Shan’ (known also as the ‘Nikolaev Line’). It comprises mafic to intermediate (monzodiorite, monzonite) and silicic (quartz monzonite, monzogranite and leucogranite-alaskite) rocks, followed by the late mafic (monzodiorite-porphyry to lamprophyre) dikes. Isotopic U-Pb zircon dating of quartz monzonite and monzogranite indicates their Early Permian age (ca. 293 to 286 Ma). The rocks appear to have been produced by remelting of a partially crystallized (mush) magma batch at deeper levels, which is evident by the presence of zircon antecrysts dated at some 306 to 320 Ma. These geochronological data are consistent with a post-collisional setting of the pluton that occurred after the cessation of subduction, which affected the Middle Tien Shan in the Late Palaeozoic. Geochemical signatures of the igneous rocks from the Molo-Sarychat pluton correspond to high-K calc-alkaline to shoshonitic series intrusions emplaced in a post-collisional setting. An initial shoshonitic magma was produced by a low-degree partial melting of the metasomatically enriched upper mantle, with amphibole fractionation in a deep (lower crustal) magma chamber. In this evolution, a generation, under the influence of mantle-supplied fluids and heat, granitic magmas in the crustal protolith can be suggested, with further mixing/mingling of the mantle-derived mafic (shoshonitic) magma and mantle-induced crustal granitic magma, followed by the magma fractionation and emplacement at shallower crustal levels. The skarn-porphyry Mo-Cu-W(-Au) mineralization associated with the Molo-Sarychat pluton complements the group of similar deposits associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The characteristic Mo-W-Cu-Au metal assemblage and the high endowment in W and particularly Mo can be related to the fertilization of subduction-modified subcontinental mantle in these metals, and its subsequent involvement in the magma generation in post-collisional setting. A certain role of crustal magma sources in the strong Mo endowment can be considered that would be consistent with some A-type granitoid affinity of the igneous rocks. The Early Permian age of these high-potassic intrusions supporting their post-collisional emplacement is remarkably similar to the age of hydrothermal alteration assemblages, previously reported for the super-large Kumtor Au deposit situated in a similar post-collisional setting nearby.