Geochemical fingerprints of the late Palaeozoic igneous rocks at the giant Muruntau gold deposit (Tien Shan, Uzbekistan) and implications for a metallogenic model

The Muruntau Au deposit, one of the world’s largest Au deposits, is localized within a sequence of tectonized and altered siliciclastic flysch rocks (intercalating sandstone, siltstone, and slate), locally enriched in carbonaceous and/or calcareous matter, and comprises a number of dikes and small intrusive stocks, together with larger granitoid pluton(s) at depth. Their emplacement alternated with multiple hydrothermal alteration events producing Au and associated W, Mo, and other mineralizations. The igneous rocks have been overprinted by hydrothermal alteration including that bearing Au mineralization. The igneous rocks at Muruntau form an extended series of varieties with evolving SiO2 content (from < 50 wt.% SiO2 to > 75 wt.% SiO2) corresponding to a multiphase differentiation trend and are characterized by a high K content typical of high-K calc-alkaline to shoshonitic series. The geochemical fingerprints further demonstrate the metaluminous to peraluminous I-type characteristics of the igneous rocks. The rocks have variable Fe2O3/FeO ratios corresponding to ilmenite- or magnetite-series intrusions. Most rocks plot in the post-collisional fields on tectonic discrimination diagrams, and the specific HFSE contents may be interpreted towards a complex tectonic setting that possibly involved an early subduction event and subsequent remelting of the subcontinental lithosphere in post-collisional arc environment. Separate compositional trends on geochemical diagrams may be indicative of magma differentiation in a series of separate deep to peripheral crustal magma chambers, with the multiple magmatic sources and their respectively different (separate) evolutional paths producing compositionally diverse intrusions (igneous suites ?). Overall, the compositional fields/trends identified for most igneous rocks at Muruntau are essentially coincident with the respective compositional varieties of igneous rocks from reduced intrusion-related Au deposits globally. In contrast, leucogranite-alaskite of a larger pluton at depth appears to be petrologically (with anatectic signatures ?) and geochemically diverse, and its affinity to granitic rocks at intrusion-related U deposits might be suggested.