Postcollisional Molybdenum Porphyry Mineralization in the Middle Tien Shan: First Isotopic U–Pb Zircon Data for Rocks from the Molo-Sarychat Pluton (Eastern Kyrgyzstan)

The paper presents the first isotopic U–Pb study data (LA–ICP–MS method) of zircon from intrusive rocks of the Molo-Sarychat pluton situated along the deep-seated fault system of the “Nikolaev Line” in the eastern Kyrgyzstan. The intrusive rocks from this pluton belong to the high-potassic calc-alkaline to shoshonitic series. Intense Mo(–W–Cu–Au) (mainly molybdenum-porphyry) mineralization is spatially and genetically associated with this pluton. Together with the other Au, W and Cu deposits and occurrences, this mineralization is part of the extended metallogenic belt of Tien Shan; however, occurrences of molybdenum-porphyry mineralization are still rare in this belt. The concordant isotopic U–Pb ages of zircon autocrysts indicate the crystallization of quartz monzonite (293.3 ± 4.2 Ma) and monzogranite (286.6 ± 2.4 Ma) in the Early Permian. Zircon antecrysts dated at 306–320 Ma are also present. The crystallization age obtained corresponds to a post-collisional epoch of the development of this territory but the presence of the antecrysts expands the pluton emplacement to the Late Carboniferous-Early Permian, which, as a result, spanned over initially subduction-related and then post-collisional tectonic settings. Correspondingly, a post-collisional setting of the Mo(–W–Cu–Au) (molybdenum-porphyry) mineralization is established; it is related to the pluton studied and was formed after the emplacement of quartz monzonite (early stage) and monzogranite (late stage). Significant enrichment in Mo can be related to its progressing accumulation during magmatic differentiation causing the emplacement of quartz monzonite and especially monzogranite. These processes occurred under the more mature post-collisional tectonic regime, with possible formation of intermediate magma chambers in the Paleoproterozoic metamorphic rocks and ancient granitoids. The age dates determined for rocks from the Molo-Sarychat pluton are similar to those identified for the igneous and metasomatic rocks of the large Kumtor gold deposit that is also associated with the “Nikolaev Line.”