ORE PETROGRAPHIC ASPECTS OF RECENT BASALTIC ANDESITE FROM ARENAL VOLCANO, COSTA RICA

Arenal volcano is the youngest of the subduction related andesitic strato volcanoes in the Guanacaste Range of NW Costa Rica. Its recent eruptive cycle started in 1968 and is presently characterized by lava dome formation in one of the summit craters. Blocks of dome rocks are continuously tumbling down the N-NW slope of the volcano as the dome is apparently expanding and its crust is breaking off. Samples studied are from a block fan at the N base of the Arenal and the lava flow of 1525.

Arenal volcanites are holo- to hypocrystalline, plagioclase-phyric basaltic andesite rocks with pyroxene, clinoamphibole, and ore minerals. Magmatic ore mineralogy is dominated by magnetite of two generations. Older and larger magnetite is commonly associated with pyroxene phenocrystals. It may contain round inclusions of gold (2-7 microns dia.). Younger, smaller, and subhedral magnetite is an essential constituent of the groundmass. Partial to complete oxidation of magnetite in some rocks yielded hematite. It also formed by oxidative roasting of Fe-containing silicates.