Maynard and Okita (1991) divided ancient bedded barite deposits into two major types: (1) continental margin type, which lacks significant accumulations of base metals, such as the Nevada and Arkansas deposits; and (2) cratonic rift type, which contains significant Pb and Zn, such as the Meggen and Rammelsberg deposits in Germany, and Jason and Tom deposits in Canada. The tectonic setting of the second type should be broadly interpreted and may include outer miogeoclinal and marginal embayment settings, with a continental affinity, and commonly with more carbonates in the section than the continental margin–type deposits. Some localities with base-metal–rich deposits also contain distant barite-only occurrences at similar stratigraphic horizons.
The proposed tectonic settings for the “metal-bearing” and “metal-barren” deposits of Maynard and Okita (1991) were also shown to correlate with the strontium isotopic composition of the barite, as illustrated in Figure 1 (modified from Maynard et al., 1995). This figure also compares these Paleozoic deposits to two classes of modern barites. Strontium values for cratonic-type deposits are all significantly more radiogenic than seawater, while values for continental margin–type deposits coincide closely with modern cold seep barites.
Our postulated mechanism at cold seep sites was not intended to address the cratonic-type barite deposits or any districts that contain significant base metals. Our model pertains exclusively to continental margin–type, metal-barren barites such as those in Nevada and Arkansas, United States, in Sonora, Mexico, and in some Chinese deposits, as stated in our paper.
Among the previously postulated genetic mechanisms for continental margin–type barite was precipitation at or near sediment-covered oceanic ridges in hydrothermal vents; thus the criteria presented in our paper specifically addressed oceanic spreading systems. The range of isotopic composition measured in barite formed at modern oceanic spreading systems includes samples collected at white smokers, black smokers and sedimented ridges, all of which have clear nonradiogenic Sr isotope signatures (Figure 1, data from Paytan et al., 2002). Nevertheless, we agree with Emsbo and Johnson (2004) that by calling the barite precipitated at modern spreading ridges “hydrothermal barite,” we applied a usage of this term that is too limited, because ancient sedimentary-exhalative (cratonic-type) systems are also hydrothermal. There are no appropriate modern analogues to ancient sedimentary-exhalative cratonic hydrothermal deposits, and our comparisons of Paleozoic barren barite were made solely to modern well-documented examples. Thus our hydrothermal barite example is for modern ridge exhalative barites, which in no way changes the conclusions of our paper.
Emsbo and Johnson (2004) state that some Nevada barites are not metal-free and have been found to contain gold; we emphasize that these deposits do contain modest amounts of Fe and lesser Mn, but trivial amounts of Zn, Pb, and Ag. We further believe that the Sr isotopic signal of the Nevada deposits clearly eliminates a cratonic-type sedimentary-exhalative origin (Maynard et al., 1995), and that their lithologic and depositional framework supports a cold seep origin for these deposits (Torres et al., 2003). Emsbo and Johnson also suggest that modern cold seeps on the Peru margin may represent the expression of a “nascent sedimentary-exhalative hydrothermal system.” In the case of Peru, the radiogenic Sr data indeed reflect deep-seated fluids, but the cold seep waters are not associated with any recognized nascent hydrothermal activity or a sedimentary-exhalative tectonic setting (e.g., Kukowski and Pecher, 1999, and references therein). Instead, the radiogenic Sr results from fluid flow over a continental basement, transport through the accretionary margin sediment, and cold discharge at the seafloor, as discussed by Torres et al. (1996).