GRANITOIDS OF THE TAMIR (MO) ORE OCCURRENCE (WESTERN TRANSBAIKALIA): COMPOSITION, AGE AND PROBABLE SOURCES OF MELTS

This paper presents new data on the composition, age, and formation conditions of granitoids from the Tamir molybdenum ore occurrence. The object of study is located on the area of the Tamir volcano-tectonic structure and within the ChikoyIngoda structural-mineralogical zone of Western Transbaikalia. In terms of mineralogy, petro- and geochemical characteristics, the granitoids of the ore occurrence are correspond to fractionated granites. They have an ultra-acidic composition, increased alkalinity and iron content, moderate alumina content, and subrare-metal geochemical specialization. Two groups of leucogranite-porphyry have been identified. One group of dikes has an asymmetric REE spectrum similar to that of leucogranite host rocks with a predominance of light lanthanides over heavy ones ((La/Yb) (N) = 16.2-36.7), and the other group is characterized by a V-shaped distribution spectrum ((La/Yb) (N) = 0.4-1.7), with a reduced content of all REE, compared with dikes of the first type. According to UPb LA-ICP-MS dating of zircon, the formation of granitoids occurred in two stages - Late Triassic (237 Ma) and Early Jurassic (176 Ma). The formation of leucogranites host rocks (enclosing ore) and dikes of leucogranite-porphyries of the first type is associated with the early stage, and the formation of ore-bearing leucogranite-porphyries of the second type is associated with the late stage. The ore prospect of the object has not been fully established. It is likely that the flat-dipping, short and thin quartz-molybdenum veins of the ore occurrence uncovered by mine workings are the apophyses of the main steeply dipping veins to which ore bodies richer in molybdenum can be associated. In addition, in terms of the mineral composition of ore veinlets, the salt composition of solutions, and the temperature of formation, the Tamir ore occurrence is comparable to the Pervomaisky molybdenum deposit, the largest in the region. For leucogranites, the established values of the primary Sr-87/Sr-86 = 0.70401 ratio, and for the Early Jurassic leucogranite-porphyries is 0.70568. The study of the Nd isotope composition in granitoids showed that the rocks of both stages are characterized by close positive epsilon(Nd)(T) = 1.9-1.7, T-Nd(DM-2) = 0.87-0.83 Ga values, which indicates the absence of the Tamir volcanotectonic structures of rocks of the ancient continental crust and the granitoids formation due to the melting of the source of the corresponding Caledonian isotopic province.