Mineralogical Characteristics of Sapphire-Bearing Placers Associated with Cenozoic Alkali Basalts in Primorye and Northeastern China

The paper presents the results of a comparative analysis of sapphire-bearing placers in South Primorye (Shkotovo Plateau, Russia) and in the adjacent Heilongjiang Province (the Muling deposit, China). The placers are confined to intraplate alkali basalts with similar geochemical characteristics and age. It was found that the same minerals from placers in Russia and China are very similar, although with some differences, the most apparent of which being the predominance of fancy-colored gemstones and rubies among Muling corundums and blue-green-yellow sapphires (BGY sapphires) in the Primorye placers. The minerals of the megacryst assemblage, including gem corundum, bear traces of magmatic corrosion and have an affinity with the host alkali basaltic melt. Magma chambers of an alkali basaltic volcano provide the conditions necessary for the growth of large and transparent crystals. It is suggested that the sapphires formed from gaseous aluminum chloride in miarolitic cavities. Megacrysts are rapidly transported to the surface owing to the explosive eruptions of basaltic volcanoes. Further erosion of megacryst-bearing rocks and redeposition of minerals in alluvium lead to the formation of zircon- and sapphire-bearing placers. The publication emphasizes economic importance of placers of gem corundum and zircon in alkali basalts and, hence, the need for their further study.