Metasomatic processes in ore deposits in the mesozoic intraplate activization areas (Eastern Mongolia and the Transbaikal Region)

The Transbaikal Region and eastern Mongolia were influenced by intensive tectonic and magmatic activity in the Late Mesozoic. This activity was accompanied by hydrothermal deposition of various predominantly lithophilic ore metals and formation of a wide range of hydrothermally altered rocks from skarns to argillized ones. However, there are no metasomatic rocks formed under acidic or very acidic conditions or under a high activity of sulfur and oxygen (for instance, hydrothermal quartzites or products of solfatar argillization) in these areas. On the contrary, some products of neutral or slightly acidic solution activity, such as berezites (quartz + sericite + carbonate + pyrite rocks), hydromica, and argillized rocks are common. Such minerals as alunite, pyrophyllite, diaspore, and anhydrite are absent in altered rocks, and zones of acidic metasomatism are rather thin. Such conditions correspond to a pH no less than 3.5, loga(S2) no more than -11.5, and loga(O2) no more than -34. The K-Ar and Rb-Sr dating correspond to the time interval of J(3)-K-1, when most of the deposits were formed in the region. Granite-leucogranite and Li-F-granite complexes and siliceous volcanics have the same age. Granites are of the reduced type (S-type), and most of the hydrothermal ores in the Transbaikal Region probably relate to this subalkaline granitic magmatism. Hydrothermal solutions were produced by the initial magmatic chambers and had relatively low redox potential and sulfur activity. This is the reason why highly acidic metasomatic process did not develop. Similar varieties of metasomatic processes occur in some other intracontinental regions of the Late Mesozoic magmatic activity at the western branch of the Circum-Pacific ore belt. Thus, in northern and southeastern China, lithophilic ore metal deposits are common, hut metasomatic rocks of the ultraacid group are unknown.