Composition and Genesis of Endogenous Borates from the Pitkaranta Ore Field, Karelia

The borate mineralization of the Pitkaranta skarn field of Karelia is localized in metasomatically altered Proterozoic dolomites. In the contact aureole of rapakivi granites, the zoning of magnesian skarns includes spinel-diopside or fassaite skarns with syngenetic magnetite and spinel-forsterite calciphyres surrounded by periclase marbles, which confirms their hypabyssal genesis. Stringer-stockwork bodies developing in the brecciation zone at the roof show a primitive zoning consisting of an inner diopside and an outer forsterite calciphyre zone grading into a dolomitic marble. All these zones inherited the Ca/Mg ratio of the primary carbonate rocks. Rhythmically banded textures observed in the skarns and calciphyres of the deposits studied suggest their formation under thermodynamically disequilibrium conditions typical of hypabyssal metasomatites. Magnesium and magnesium-iron borates in marbles and calciphyres and beryllium borates in greisens were formed during the postmagmatic stage. Data are reported on the chemical composition and genesis of suanite, kotoite, ludwigite, hulsite, pertsevite, fluoborite, szaibelyite, and humites from the Hopunvaara, Klara, Lupikko, and Herberz deposits. The deficit of boron in magnesian borates is related to their endogenous hydration. Data on hambergites and berborite are given according to E.I. Nefedov.