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 te ctonic and magmatic activity in the Late Mesozoic. This activity was accomp anied by hydrothermal deposition of various predominantly lithophilic ore m etals and formation of a wide range of hydrothermally altered rocks from sk arns to argillized ones. However, there are no metasomatic rocks formed und er acidic or very acidic conditions or under a high activity of sulfur and oxygen (for instance, hydrothermal quartzites or products of solfatar argil lization) in these areas. On the contrary, some products of neutral or slig htly acidic solution activity, such as berezites (quartz + sericite + carbo nate + pyrite rocks), hydromica, and argillized rocks are common. Such mine rals as alunite, pyrophyllite, diaspore, and anhydrite are absent in altere d 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 inte rval of J(3)-K-1, when most of the deposits were formed in the region. Gran ite-leucogranite and Li-F-granite complexes and siliceous volcanics have th e same age. Granites are of the reduced type (S-type), and most of the hydr othermal ores in the Transbaikal Region probably relate to this subalkaline granitic magmatism. Hydrothermal solutions were produced by the initial ma gmatic 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 intracontine ntal regions of the Late Mesozoic magmatic activity at the western branch o f the Circum-Pacific ore belt. Thus, in northern and southeastern China, li thophilic ore metal deposits are common, hut metasomatic rocks of the ultra acid group are unknown.