Granitoid magmatism and metallogeny of the Lake Zone in Western Mongolia (by the example of the Bumbat-Hairhan area)

We present the results of a study of the geologic structure and age sequence of formation of the Late Neoproterozoic-Early Paleozoic plagiogranitoid and gabbroid associations in the Bumbat-Hairhan intrusive area of the Lake Zone in Western Mongolia. The petrogeochemical characteristics of the plagiogranitoids provide information about the conditions of formation of their parental melts at the island-arc and accretion-collision stages of the regional evolution. They also help to establish the main magma-generating sources as well as the major mechanisms of large-scale formation of granitoid melts and their relationship with ore generation processes. According to the trace-element and REE patterns and indicative ratios of these elements, the plagiogranitoids are subdivided into high-and low-alumina ones. Among the island-arc plagiogranitoids (551-524 Ma) of the Bumbat-Hairhan area, high-alumina varieties are the most widespread. They resulted from the partial melting of metabasites in equilibrium with garnet-containing restite at >= 15 kbar during their subsidence into the subduction zone. In geochemical features these plagiogranitoids are similar to high-Si adakites of different world regions. Island-arc low-alumina plagiogranitoids are scarcer. Their geochemical characteristics indicate that the parental melts were generated through the partial melting of metabasites in the lower part and/or in the basement of the island-arc system in equilibrium with amphibole-containing restite at <= 8 kbar. Plagiogranitoid associations of the accretion-collision stage (511-468 Ma) are the most widespread in the Bumbat-Hairhan area. They are subdivided into high-and low-alumina ones. According to the contents of trace elements and their indicative ratios, the low-alumina plagiogranitoids resulted from the partial melting of metabasites in equilibrium with plagioclase-containing restite at <= 8 kbar in the upper part of the collisional structure, and the high-alumina ones were generated through the melting of metabasites in the basement of thick crust in equilibrium with garnet-containing restite at >= 15 kbar. Geochronological studies in the Bumbat-Hairhan area revealed two stages of ore-generating processes spatially and temporally related to the formation of low-alumina plagiogranitoids. The early stage (518 +/- 5 Ma), development of vein Cu (Au) mineralization, coincided in time with the formation of island-arc low-alumina plagiogranitoids of the Darbi massif (similar to 524 Ma). The late stage (456 +/- 4 Ma), formation of porphyry Cu-Mo (Au) mineralization, was synchronous with the formation of the low-alumina plagiogranites at the accretion-collision stage (similar to 468 Ma). (C) 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.