Contrasting types of boron-bearing deposits in magnesian skarns from Romania

Abstract

Two types of boron-bearing deposits, displaying contrasting properties, were identified in five magnesian skarns from Romania, at Băiţa Bihor, Pietroasa, Cacova Ierii, Maşca Băişoara and Ocna de Fier. These should be defined as Fe-poor (Mg-B) and Fe-rich (Mg-Fe-B), respectively. The skarns are developed at the contact of igneous bodies of Upper Cretaceous age with dolomite sequences. They occur as metasomatic columns, since a roughly consistent zoning was found from core to rim in the skarn bodies. Borates (suanite, kotoite, ludwigite, szaibélyite, rarely fluoborite) occur in the outer skarn zones, whereas the inner zones typically contain magnesian silicates (e.g., forsterite, humite-group minerals, diopside, and phlogopite) and spinel. The Mg-B type of skarn deposit is typified by the abundance of suanite – szaibélyite and kotoite and by the scarcity of ludwigite and magnetite. The B-bearing portions of the Fe-rich skarns consist dominantly of bands of ludwigite alternating with magnetite. Small amounts of kotoite, forsterite and clinohumite are also present. The differentiation of the two types of deposits is further emphasized by the development of F-rich parageneses (i.e., fluoborite, fluorite, F-rich humites) only in the Fe-poor skarns. All B-bearing skarns in Romania experienced generally similar peak contact-metamorphic conditions of 400–650 °C and pressures up to 3 kbar. Borates have similar geochemical signatures with respect to minor elements as the contemporaneous silicates.