Ore paragenesis of recent hydrothermal deposits at the Cocos-Nazca plate boundary (Galápagos Rift) at 85‡ 51' and 85‡ 55' W: Complex massive sulfide mineralizations, non-sulfidic mineralizations and mineralized basalts

Abstract

Complex massive sulfide ores (»black smokers«) as well as mineralized basalts and unconsolidated sediments were sampled during the German research expedition »Garimas 1« in the eastern portions of the Galapagos Rift at 85‡ 50' W longitude. The massive sulfide ores are characterized by a porous, layered, compositional texture with typical colloidal and/or gel textures. Local occurrences of organism remains (worm tubes), which are embedded in the ore, were also found. The composition of the massive sulfide ores exhibits a considerable range, from iron sulfide ores (pyrite, melnikovite-pyrite, marcasite) to copper-rich ores (chalcopyrite), and/or zinc-dominated ores (sphalerite, wurtzite, schalenblende). Additional new types of hydrothermal mineralizations were found, which consist primarily of manganese hydroxide, limonite or Fe-rich smectite (Fe-rich »protosmectite«) together with limonite and opaline silica. Hydrothermally altered basalts may be characterized by significant sulfide contents (primarily pyrite) and, locally, by hematite. Sediments in the vicinity of the »black smokers« frequently contain fragments of complex massive sulfide ores. With increasing distance from the »black smokers« these massive sulfide fragments exhibit complete replacement and pseudomorphism by limonite. Some replacement and pseudomorphism of microorganisms in unconsolidated sediments by limonitic material is also observed. In places, the unconsolidated sediments contain manganese hydroxide mineralizations of hydrothermal origin.