Genesis and composition of endogenous borates in the skarns of the Eastern and Central Pyrenees

Abstract

This paper addresses the genesis and composition of endogenous borates and other minerals from the magnesian-skarn aureoles at the contacts between dolomites and Hercynian granitoid intrusions of the Eastern and Central Pyrenees (Querigut and Costabonne peak massifs in France, and the Monchi deposit in Spain). It was shown that these occurrences and other magnesian skarns in the Sierra Morena Range, Spain, genetically belong to the periclase depth facies: zoned metasomatic aureoles of dolomites of primitive structure complicated by the development of periclase marble zones. The near-contact zones of the intrusions are represented by the granitoids of increasing basicity and alkalinity, which indicates the assimilation of host rocks by overheated granitic magmas. The postmagmatic stage was marked by the formation of magnesium and magnesium-iron borates of diverse composition in calciphyres and marbles, replacement of forsterite by humites in the calciphyres, and the development of silicates of decreasing Mg mole fraction after pyroxene skarns; the latter is accompanied by magnesium migration into the outer zones of the aureole. It was determined that the studied deposits of France contain boron minerals (kotoite, suanite, and pertsevite) previously unknown for this region in association with fluoborite and late szaibelyite. In skarns of Spain, the high-Fe borates are represented by monomineral aggregates of fine-prismatic parallel oriented crystals of vonsenite or its coarse-grained masses. It is conceivable that sulfide-bearing magnesian skarns and calciphyres of the studied deposits contain magnesium hydroxysulfides: tochilinite (after pyrrhotite) and valleriite (after chalcopyrite).