Physical and chemical processes of seafloor mineralization at mid-ocean ridges

Seafloor hydrothermal activity at mid-ocean ridges has a major impact on the chemistry of the oceans [Edmond et al., 1979] and has been responsible for extensive alteration of modem oceanic crust [Alt, this volume]. Massive sulfide deposits, together with metalliferous sediments and hydrothermally altered rocks, are an important record of this process. A fossil record of seafloor hydrothermal activity is also contained in much older deposits now preserved on land. Ancient massive sulfides as old as 3.5 billion years are known in the Pilbara region of Western Australia [Barley, 1992] and in the Barberton greenstone belt of South Africa [de Ronde et al., 1994], and many other examples in Archean and younger rocks can be cited [Franklin et al., 1981]. Detailed studies of the mineralogy and chemistry of these ancient deposits indicate that the physical and chemical processes of seafloor mineralization in the early oceans were fundamentally the same as those observed on the modern mid-ocean ridges, and direct comparisons can be made between the formation of modem and ancient sulfide deposits. In addition, because seafloor hydrothermal systems have been active since very early in the history of the Earth, the study of fossil sulfide deposits may be particularly relevant to arguments concerning the origins of life [Corliss et al., 1981; Corliss, 1990; Holm, 1992].