Endogenous transformations of kotoite in calciphyres at magnesian-skarn deposits of boron

Abstract

The succession of endogenic transformation of kotoite and accompanying minerals in marbles and calciphyres at hypabyssal magnesian-skarn deposits of boron is proved to be controlled by the anion composition of the hydrothermal solutions and the Mg mole fraction of the carbonate rocks. Early in the postmagmatic stage forming the B ore mineralization, the solutions contain Fe and F, as is reflected in the formation of kotoite and suanite in association with clinohumite, phlogopite, and Al-and Ti-bearing Mg-Fe borates, such as magnesiohulsite and magnesioludwigite. An increase in the F concentration of the hydrothermal solutions stimulates the formation of humites (from clinohumite to chondrodite and norbergite), which are replaced in the presence of kotoite by pertsevite (a hydroxofluorosilicateborate). In the calciphyres of Mount Brooks, Alaska, and at other deposits, the latter mineral is rock-forming in kotoite-clinohumite calciphyres, replaces this association, and is accompanied by fluoborite, nocerite, and fluorite. The further compositional transformations of the solutions with their enrichment in Cl result in the replacement of kotoite marbles and calciphyres and adjacent periclase marbles by sakhaite rocks with newly formed karlite. The neutralization of the hydrothermal solutions and their simultaneous cooling control the replacement of kotoite rocks by borcarite. The endogenic transformations of kotoite end with its szaibelyitization and/or brucitization and the simultaneous development of wightmanite in the marbles and the subsequent carbonatization of the borates. The research was carried out with the use of the author materials from boron deposits in Russia, Korea, Romania, England, France, and the United States.