THE MIXING HYPOTHESIS AND THE ORIGIN OF MISSISSIPPI VALLEY-TYPE ORE DEPOSITS

摘要

Anderson (1991) proposed that methane generated from the heating of kerogen in the wall rocks of Mississippi Valley-type (MVT) deposits diffuses into the ore zone, reducing sulfate and precipitating the ore minerals. This hypothesis has been tested by calculating the diffusion of methane from wall rock into a flowing solution at 150°C. Methane pressures in the ore zone quickly reach a steady state which depends on the assumed methane pressure in the wall rock and the flow velocity. Calculated methane pressures in the ore solution are not sufficient to generate a gas phase, but other dissolved gases such as CO_2 may contribute sufficiently to make this happen. Sulfate reduction and sulfide precipitation will occur whether or not a gas phase forms, but many MVT ores have a tabular form or are located in the upper part of breccias, suggesting that there was a gas phase control. Recent experimental results (Chou and Burruss, 2007) offer an explanation for the fact that very light carbon isotopes are not commonly found in MVT deposits. The main obstacles to further development of these ideas is our lack of understanding of the thermochemical sulfate reduction reaction and the fact that little attention has been paid to possible sources of methane in the vicinity of ore deposits.