Contribution to the knowledge of the rocks hosting the blue pectolite (larimar) deposit in the SW part of the Dominican Republic

This paper presents the results of investigations on the mafic volcanics (basalts and dolerites) and pyroclastic (tuff) rocks from the Sierra de Bahoruco region, with a special attention to their alteration products. That area is known for its blue pectolite (larimar) deposit. The set of secondary minerals, formed from the parent rocks, is highly diversified and includes natrolite, pectolite, prehnite, calcite, serpentines, clay (chlorite, chlorite/smectite, celadonite), and SiO2 group minerals, as well as magnetite, ilmenite, titanite, rutile, pumpellyite, and garnets (with admixtures of Ti). The majority of those mineral phases were developed in hydrothermal processes, in an environment rich in sodium (Na+) and calcium (Ca2+). The parent effusive rocks were associated with deep tectonic rift zones. The pyroclastic rocks accompanying them seem to be genetically associated with a spilite- type assemblage, which underwent metamorphic changes in the conditions of a low -stage regional metamorphism characteristic of the zeolite facies (200-400 degrees C).