THE ORIGIN OF THE HYDROTHERMAL SULFIDE ORES IN THE AXIAL PARTS OF THEMID-ATLANTIC RIDGE

A comparison of geological structures and textural, mineralogical, and chemical features of ores and their morphologies and ages was carried out for the following modern sulfide edifices in the active hydrother mal fields of the axial zone of the Mid-Atlantic Rift: Broken Spur, Sn ake Pit, and the Transatlantic Geotraverse (TAG). The Broken Spur and Snake Pit ore occurrences were formed within a neovolcanic zone, where as the TAG active edifice is located beyond this zone. These ores typi cally have brecciated and concentrically zoned structures, which are i ndicative of multistage ore formation with erosion of edifices and pre cipitation of new minerals under changed physicochemical conditions. T he edifices vary in morphology from individual chimneys (Broken Spur) or complex joint chimneys and mounds (Snake Pit) to integrated mounds (TAG). The edifices were formed during different time intervals: Broke n Spur is the youngest, Snake Pit is older, and the ores in TAG have b een precipitated for the longest period. The sulfur isotope compositio n of sulfides becomes heavier from youngest to oldest edifices. The ra tes of ore deposition vary from 200 to 800 tons per year. It is sugges ted that the scale and morphology of an edifice are related to the dur ation of mineral-forming system activity and its cyclicity. A genetic model for sulfide edifices is proposed. It is concluded that a large v olume of sulfide ores tended to accumulate in hydrothermal fields situ ated outside the neovolcanic zone.