Paleozoic development of OIB and see-mounts in the Turkestan Ocean within the Khaidarkan and Ulug-Too deposits, South Tianshan (STS)

The study area is situated in the Ulug-Tau and Khaidarkan gold-antimony-mercury deposits in the South Tienshan (STS). Together with Khadamzhai, Chauvai, and Abshyr deposits, they can be grouped into one ore province. The STS consists mainly of middle and late Paleozoic marine sedimentary rocks, which were deposited in the Turkestan Ocean and on the adjacent continental margins. They crop out along with subordinate metamorphic rocks, arc-related and intraplate volcanic suites, and ophiolites. Various lithologies were juxtaposed together in an accretionary prism during the late Carboniferous - early Permian closure of the Turkestan Ocean.

In the investigated area, Late Silurian to Devonian limestones of the Aktur carbonate platform cover both the shales of the Pulgon Formation (Fm.) (Zarhar-Say) and the basalts with gabbro bodies. Gabbro specimens were sampled for absolute age determination by amphibole 40Ar/39Ar geochronology. Volcanic rocks related to the basement of the Silurian-Carboniferous Akturian carbonate platform, part of the regional nappes of Osh-Uratyube, have been studied by Biske et al., (2019) and our group. The nappe sits on top of basaltic rocks of the Chonkoy Fm. and andesites, tuffs, and carbonate rocks of the Dedebulak Fm. In the latter unit, the volcanic suite forms the lower member which is overlain by Cambrian limestone and dolomite with intercalations of radiolarite (upper member). The volcanic rocks at the base of the Aktur carbonate platform succession indicate the Early Paleozoic geodynamic situation in the Turkestan Ocean as well as about the structure of the Khaidarkan and Ulug-Too gold-antimony-mercury deposits. The Ulug-Tau orefield is situated along the mélange zone at the base of the Aktur nappe.

Results of geochemical and geochronologic analyses (in progress) show that the basalts and basaltic andesites of the lower member of the Dedebulak Fm. formed in an island-arc setting. These volcanic rocks confirm the existence of Early Paleozoic island arcs in the Turkestan Ocean. In a later stage, those arcs possibly died out and were overlapped by carbonate platforms. For the Aktur carbonate platform, it can be assumed that it was detached from the Cambrian island arc basement during the Late Carboniferous and was added to the accretionary prism as the Aktur Nappe. The Cambrian island arc basement (Dedebulak Fm.) formed another thrust-sheet unit as part of the accretionary prism. Plastic Silurian shales and other sediments, primarily located between the Aktur carbonate platform sediments and the Cambrian island arc volcanic rocks, were incorporated into the polymictic mélange.