The proposal of Torres et al. (2003), that Paleozoic barite deposits in Nevada, Arkansas, and China formed at cold seafloor methane seeps, is an important addition to the large literature on the genesis of bedded barite. However, we would like to point out that Torres et al. use flawed criteria to exclude a hydrothermal origin for these and other barite deposits, and as a consequence, their larger conclusions are open to question. The diagnostic criteria set forth for hydrothermal barite are: (1) Sr that is less radiogenic than contemporaneous seawater Sr, (2) δ34S values within 2‰ of contemporaneous seawater sulfate, and (3) associated polymetallic sulfide deposits. These criteria take into account spreading-ridge hydrothermal systems that can deposit barite in association with volcanogenic massive sulfide 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 sulfide 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 sulfide minerals and as barite-only deposits above or lateral to massive sulfide 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) reflecting scavenging of Sr and other elements from continental detritus in underlying sediments. The barites do not show δ34S values within 2‰ of contemporaneous seawater sulfate, but rather can extend up to 30‰ higher (Goodfellow et al., 1993; Johnson et al., 2003). Barite-only deposits occur not only in close association with sulfide deposits but also far removed from them (tens of km), although commonly at the same stratigraphic horizons (e.g., Lydon et al., 1979).
We agree with Torres et al. that the presence of polymetallic sulfide deposits supports a hydrothermal origin for associated barite, but it is important to note that the absence of sulfides does not preclude a hydrothermal origin. Whether a hydrothermal fluid precipitates base metals, barium, or both can depend on the redox chemistry of the fluid (Lydon et al., 1979; Emsbo, 2000). Barite is more soluble in reduced fluids with low concentrations of sulfate, whereas base metals are only soluble in oxidized fluids with low concentrations of H2S. Thus, barite-only deposits are to be expected not only at cold seeps, as pointed out by Torres et al., but also at sites where hot, H2S-stable fluids are vented. We would also like to point out that methane venting is by no means restricted to cold seeps as Torres et al. seem to imply. Hydrothermal methane is known to be venting today on the modern seafloor (e.g., Simoneit et al., 1988; Canet et al., 2003), and methane expulsion is known to have been a key element of the sedimentary-exhalative hydrothermal system that formed the Red Dog sulfide + barite deposits (Johnson et al., 2003), which contains the largest zinc and barite resource ever discovered.
We believe that use of the hydrothermal-barite definition proposed by Torres et al. obscures relationships between hydrocarbon and brine-expulsion events and the thermal evolution of ancient basins. Our own work on this problem has led to the important discovery (Emsbo et al., 1999; Emsbo, 2000) that the Nevada barite belt, long considered a typical example of metal-free barite occurrences (e.g., Torres et al., 2003), also contains barite deposits that carry significant gold. As for the modern seafloor, we are intrigued by the Peru margin barite locality because it contains strikingly radiogenic Sr (Torres et al., 2003). This suggests to us an exciting possibility, namely that this locality may be the surface expression of deep-seated fluids (cf. Aquilina et al., 1997) that represent a nascent sedimentary-exhalative hydrothermal system.