GENESIS AND COMPOSITION OF ORE-FORMING MAGNESIAN BORATES, THEIR ANALOGUES, AND MODIFICATIONS

Abstract

The genesis and chemical composition of endogenic ores composed of ortho- and diborates are discussed with reference to Mg-skarn deposits of the hypabyssal and abyssal facies. The orebodies are either restricted to the contacts between the intrusions and dolomite or compose steeply dipping metasomatic bodies in dolomite away from contacts with the intrusions. Magnesian borates are the earliest minerals with respect to Mg-Fe borates of the ludwigite-vonsenite series and other borates replacing them (sakhaite, szaibelyite, and others). The composition of the borate ores is predetermined by the major-element chemistry of the replaced rocks and the physicochemical parameters of their formation. Except suanite and szaibelyite, which can crystallize in all depth facies, rock-forming kotoite, more rare jimboite, takedaite, nordskioldine, tusionite, and kurchatovite and late borates (sakhaite and borcarite) are abyssophobic minerals. Data are reported on the modeling of hydrothermal replacement of carbonates by ore-forming borates, and the processes of successive replacement of kotoite marbles, suanite, and kurchatovite by Mg-Fe borates, carbonatoborates and szaibelyite, are characterized. Magnesian and calc-magnesian borates are demonstrated to form ores at magnesian-skarn deposits in Russia, China, and other countries.