Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins: Comment and Reply

Abstract

34 S values within 2‰ of contemporaneous seawater sulfate, and (3) associated polymetallic sulfi de deposits. These criteria take into account spreading-ridge hydrothermal systems that can deposit barite in association with volcanogenic massive sulfi de deposits of Zn, Cu, and Pb. They do not apply to sedimentary-exhalative hydrothermal systems that can form both bedded barite deposits and the sediment-hosted massive sulfi de deposits of Zn, Pb, and Ag (±barite) that account for the bulk of the world’s known Zn and Pb reserves. Whereas spreading-ridge hydrothermal systems arise from seawater convection through hot volcanic rocks, sedimentary-exhalative hydrothermal systems arise from the ascent of basinal brines through sedimentary sequences into intracratonic or continental margin seas. There are barite deposits throughout the world that are clearly products of sedimentary-exhalative hydrothermal systems and show none of the criteria set forth by Torres et al. as diagnostic of a hydrothermal origin. To cite just a few examples, at Jason and Tom in western Canada, at Meggen and Rammelsberg in Germany, and at Red Dog in northwest Alaska, barite occurs both intergrown with sulfi de minerals and as barite-only deposits above or lateral to massive sulfi de bodies. At all these localities, the barite contains Sr that is more radiogenic than contemporaneous seawater Sr (Turner, 1991; Maynard et al., 1995; Ayuso et al., 2000) refl ecting scavenging of Sr and other elements from continental detritus in underlying sediments. The barites do not show δ 34