GENESIS AND COMPOSITION OF LUDWIGITE-VONSENITE BORATE SERIES IN MAGNESIAN SKARN OF CENTRAL AND EAST ASIA

Abstract

The geochemistry of the postmagmatic evolution of Mg-Fe borates in metasomatically altered dolomite are considered using the example of deposits in Central and East Asia. It is shown that the ludwigite-vonsenite borate series is typomorphic of both abyssal magnesian skarn related to regional granitization in crystalline shields and hypabyssal contact aureoles of granitoids. These minerals are also known from 'genetically distinct rocks that are mineralogically similar to magnesian skarn and calciphyre. The peripheral portions of intrusions and successive phases of granitoids in contact with carbonate rocks are known to broadly vary in the major-element composition of the magma. The metasomatic zoning of the aureoles developing in dolomite and other magnesian carbonate rocks shows that boron-bearing skarn is related to the abyssal or hypabyssal T-P facies, including the spinel and occasionally gehlenite-monticellite facies. Under hypabyssal conditions, the overprint of calc skarn on magnesian skam containing early borate mineralization is accompanied by an increase in the content of the Fe-2(2+) Fe3+ [BO3]O-2 end member in the borates. This paper reports data on the classification and composition of borates of the ludwigite-vonsenite series and on various types of isomorphism in these minerals from skarn and calciphyre at the ore deposits in Central and East Asia. Limits are established for the isomorphic replacement of Mg by Fe2+ and Mn and of Fe3+ ion of borates by isovalent Al and Cr and heterovalent Ti, Sn, V and sb.The borates are demonstrated to be associated with iron mineralization and accompanied by tin, tungsten, rare-metal and sulfide ore mineralization. This predetermines the orientation of the exploration operations, evaluation of the ore quality, mode of mining, and the choice of processing methods applied to the polymineral ore related to the magnesian skarn.