A new occurrence of hydrous andradite within pyroclastic rocks of the Sierra De Bahoruco, Dominican Republic

Abstract

Abstract The titanium-bearing (up to 3.31 wt% TiO2) hydrous andradite [Ca2.98Na(0.02)K(0.01)Fe2+ (0.01)]Σ=3.02[Fe3 + 1.25Al0.45Ti0.15Fe2+ 0.07Mg0.04V3+ 0.01Mn2+ 0.01]Σ=2.98Si2.43O9.72(OH)2.28 was identified within intensely altered and weathered pyroclastic rocks in the Sierra de Bahoruco region in the Dominican Republic. It forms small isolated euhedral crystals, infillings of fissures and fractures, and large polycrystalline aggregates, in close paragenesis with clinochlore and calcite. Other phases present in the secondary assemblage, formed as a result of hydrothermal and/or supergenic processes, include prehnite, pumpellyite, and natrolite. The formation of this secondary assemblage was a result of metasomatism under low temperature zeolite facies conditions. The formation of hydrous garnet required high-pH aqueous fluids enriched in Ca and Fe. The Ca needed for garnet crystallization might have derived from primary calcic plagioclases, diopside, and Eocene-Miocene limestones overlying the Cretaceous magmatic complex. Geothermometry on coexisting chlorite revealed that the metasomatic hydrous andradite was formed under low-temperature conditions (∼160 °C). Iron and Ti oxidation states in the andradite indicate that its formation occurred under relatively oxidizing conditions. The hydrous titanian garnet represents one of the latest phases of the pervasive calcium metasomatic event in the Sierra de Bahoruco area.

Keywords

• Hydrous andradite
• Low-temperature
• Metasomatism
• Las Filipinas Larimar Mine
• Dominican Republic