Mineral assemblages and chemistry of biotite of the Tyrnyaus ore deposit
Gramenitskiy E. N.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 1. P. 105-121
Compositions of biotite in the Tyrnyaus deposit (the North Caucasus) may be divided into 18 genetic groups related to distinguished magmatic (granitoids), metamorpic (hornfels, marbles) and metasomatic (ore-forming stage) series of rocks. All three series are characterized by the consequently increasing Fe/(Fe+Mg) ratio and the growth of Al content in the IV coordination. Biotite compositions of the granodiorite inclusions in granites of the Eldzhurta massif are formed a particular trend. The modified classification of biotite group minerals with the radical [Al2Si2O10]6– for the extreme part of eastonite—siderofillite row and the independant variation of the Al fraction in the octahedral coordination has been employed. Heterovalency substitutions in VI coordination, which compensate the replacement of Si by Al in tetrahedrons, explain distinctions between the trends. Al, Ti are of prime importance in the compensation, Fe oxidation and vacancies in the octahedron position («an trioctahedry extent») have the less degree. Proportions of common Al, Si, Mg and Fe, making possible the assignment of the particular biotite composition to facies of the relative alkalinity, present the basic degree of rocks. As well the Na fraction in the interlayered position decreases and the fluorine content in micas of metamorpic and metasomatic rocks increases in connection with the rock basic degree. Magmatic rocks of Tyrnyaus attribute to the low fluorine and high chlorine fluid-geochemical type.
Key words: biotite group, phlogopite, annite, siderofillite, eastonite, chemistry, trends, mineral assemblage, classification, Tyrnyaus deposit.