DID PALEOZOIC SEAWATER CHEMISTRY CONTROL THE FORMATION OF CARLIN-TYPE GOLD DEPOSITS?

Gold deposition in Carlin-type systems apparently occurred in response to sulfidation of ferrous iron in calcareous host rocks (Hofstra et al. 1991. Geology 19: 348-351). Should similar large Carlin-type systems occur in any environment where sulfide-rich hydrothermal fluids have interacted with carbonaceous or calcareous sedimentary rocks? Or, on the other hand, do the Cambrian through Mississippian host rocks in the Great Basin have characteristics that are unlikely to be repeated at other locations or in rocks of other ages? Carbonate rocks should be unlikely candidates to host large gold deposits formed by sulfidation of ferrous iron. Typical Phanerozoic carbonates contain ∼0.1% Fe. Where carbonate rocks contain significant amounts of organic matter, the iron is completely pyritized. Examples include the Cretaceous La Luna Formation which has average DOP (Fe_py/[Fe_py+Fe_{HCl soluble}]) values that approach unity (Mongenot et al. 1996. Sed. Geol. 103: 23-37). The capacity of most carbonate rocks to buffer sulfide solutions is, therefore, limited. Samples of the Roberts Mountains Formation contain from 0.1 to 2% iron (Mullens 1979. USGS OFR 79-753) and sufficient organic matter to have ensured a reducing diagenetic environment. The rocks have retained significant concentrations of labile iron. Preservation of labile iron in organic carbon-rich sediments requires that the value of [SO₄^2-]/[HS^-] in the sediment porewater be maintained at a low value. Low values of [SO₄^2-]/[HS^-] allow organisms that utilize carbon dioxide as the terminal electron acceptor to flourish relative to sulfate reducing bacteria. Low porewater sulfate concentrations can best be achieved by suppression of sulfate diffusion. Evidence from evaporite fluid inclusions (e.g., Horita et al. 1996. J. Sed. Res. 66: 956-964) and δ³⁴S data (Gill et al. in press. Palaeo³) indicate that the sulfate concentration of early Paleozoic seawater was much lower that its current value. Under such conditions sulfate diffusion would be suppressed relative to current rates and more labile iron would be preserved. This hypothesis suggests that the chemistry of seawater from the Cambrian through the Mississippian was responsible for preserving large amounts of labile iron and creating effective geochemical traps for auriferous hydrothermal solutions.