Petrologic characteristics and Rb-Sr age dating of lamprophyre dikes of Tsagaan Tsahir Uul gold deposit, Mongolia

Abstract

Geochemical, mineralogical and geochronological investigations have been carried out on the lamprophyre dike in the Tsagaan Tsahir Uul gold deposit area, Bayankhongor, Central Mongolia. The lamprophyre is composed of olivine, amphibole, and clinopyroxene as phenocrysts with plagioclase, potassium feldspar, biotite, and amphibole as the groundmass. The amphibole is mostly pargasite; some grains are rich in titanium and constitute kaersutite. Pyroxene is slightly enriched in titanium. Although the olivine ranges from Fo_92.9 to Fo_84.6, a distinct compositional variation between the core and rim in a single grain is lacking. Biotite and phlogopite are dominant. Plagioclase is mainly andesine (An₄₅Ab₄₉Or₆) and oligoclase (An₁₂Ab₇₄Or₁₄), and potassium feldspar is orthoclase.
     The major oxides content of the lamprophyre has a good correlation with SiO₂. TiO₂, FeO^t, MgO, CaO, and MnO decrease, and Al₂O₃ and K₂O increase with increased SiO₂. Trace element composition is characterized by high values of Large Ion Lithophile Elements (LILE) (Ba, Rb, Sr) and also some compatible elements (Cr and Ni). The MORB normalized spider diagram shows slight depletion of Nb in contrast to Ba and Ce. The REE pattern shows LREE enrichment, and rapid decrease in HREE practically without an Eu anomaly. The Rb-Sr internal isochron yields an age of 248.61 ± 18.5 Ma (Zechstein, Late Permian) with ⁸⁷Sr/⁸⁶Sr initial ratio of 0.706096 ± 0.00002.
     Mineral assemblage and chemical characteristics are close to those of calc-alkaline lamprophyres (CALs). An Al₂O₃ versus TiO₂ tectonic discrimination diagram of the shoshonitic and potassic igneous rock shows that the Tsagaan Tsahir Uul lamprophyre might have originated within an intra-plate tectonic setting. The high-Ti content of the lamprophyre also suggests that the magma could have originated in an intra-continental tectonic setting. Small but similar intrusions emplaced during the Permian period are spatially and temporally associated with the main gold and copper mineralization in the Bayankhongor metallogenic belt. The obtained age of the lamprophyre dike (248.61 ± 18.5 Ma) demonstrates that its intrusion was a contemporary event. This was the youngest plutonic body in the Tsagaan Tsahir Uul area, which is close in age to the gold-bearing quartz veins. Although there is no direct genetic evidence associating the auriferous veins and the lamprophyre in the field, they occur together and possibly share a similar tectonic setting.