New data on the composition and age of gabbro-dolerites of the Oyu complex of the Yambpe Ridge, north-west Pai-Khoi

Research subject. This article presents data on the geological position of the Oyu hypabyssal complex. The aim of the study was to investigate the mineralogical, petrographic and petrological characteristics of dolerites deposited herein and to carry out their isotope dating.

Materials and methods. The material for research was obtained during the course of geological fieldwork across. Field research and sampling of the Oyu complex was carried out in the southern part of the Yamb-Pe ridge. The subsequent investigation was performed for 21 dolerite samples (petrographic description, XRF and ICP-MS methods) and 26 zircon grains isolated from a single dolerite sample for isotopic dating (SHRIMP-II method).

Results. The rocks of the Oyu complex are identified in the Lower Palaeozoic sequences of the Lemva zone of Pai-Khoi, where they form swarmof layered bodies and dykes with the thickness of the first meters–tens of meters and a length from 100 m to 1.5–2 km. The intrusions are located in the Middle-Late Ordovician sequences of the Khengur and Talbeytyvis formations and uncover in the Yamb-Pe structures and the right bank of the Bol’shaya Oyu river. The rocks of the complex have a distinct hypabyssal appearance, which is expressed in developed amygdaloidal, porphyry and glassy textures. According to petrographic data, two varieties of rocks – dolerites and leucodolerites – have been identified. Secondary changes have been described as propylitization and albitization. The rocks in the complex are enriched with SiO₂ (43.05–51.10 wt %) and correspond to the subalkaline series (sodium and potassium-sodium type of alkalinity). Oys dolerites are enriched in LILE (Rb, Ba, K, Sr), though being depleted in HFSE (Th, Nb, Ta, Ti, Zr, Hf). The total content of REE (La + Sm + Yb) is established to be 21.17 (13.57–33.65) ppm, with the distribution showing a downward trend and the Eu-anomaly being not pronounced. The isotope dating of zircons (SHRIMP II) has confirmed the absolute age of dolerites in the Oyu complex to be 313.2 ± 2.6 Ma.

Conclusions. The conducted comparison of the obtained data with those for gabbro and gabbrodolerites in the Khengur hypabyssal complex has revealed a paragenetic connection of the magmatic chambers of the Oyu and Khengur intrusions. A hypothesis has been proposed that links submarine hydrothermal processes with the intrusions of hypabyssal dolerite bodies. Such a hydrothermal activity is likely to have resulted in paleohydrothermal structures of "white smokers" and barite hydrothermal-sedimentary ore bodies, which are localized in the Lower-Middle Carbon Lemva formation.

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