FLUID FLOW AND REACTION KINETICS IN THE FORMATION OF MVT DEPOSITS

A continuing problem in theories of Mississippi Valley-type (MVT) ore genesis is the origin of the sulfur in the ore minerals. Reduction of seawater sulfate to H₂S is commonly called upon, but because ore formation temperatures are too high for bacterial reduction and thermochemical sulfate reduction (TSR) is believed to be too slow, TSR is generally held to occur before ore deposition, with the H₂S held in a trap later intersected by a metal-bearing solution. Such mixing (the “mixing hypothesis”) should result in a high degree of supersaturation and hence fine-grained (e.g. colloform) textures, but the kinetics of the sulfate reduction reaction is not a genetic factor, and there are theoretically few limits to the amount of ore produced. Compilation of all available data, including some recent data of our own, shows that the overall TSR reaction has first order kinetics, and that at a typical MVT ore formation temperature of 150^oC, the rate constant lies between 10⁰ and 10^-4 y^-1, depending on experimental conditions. Data are not sufficient to establish the rate law, but it appears likely that the effects of pH and H₂S concentration cancel, leaving the reaction pseudo-first order in sulfate concentration. To what extent these measured rate constants apply to natural conditions remains problematic for several reasons.

An alternative to sulfate reduction is the release of H₂S from organic material. Our data for the release of H₂S from an organic-rich shale show that the process is zeroth order with a rate constant at 150^oC in the same range. A simple “continously stirred tank reactor” flow model shows that rate constants in the upper part of this range and Darcy flow rates of 1-10 m/y could form ore deposits in 10⁶ y but probably not in 10⁴ y. However, 1D reactive transport modeling shows that with the larger rate constants at 1 m/y all the zinc will precipitate as sphalerite within a few meters in the flow path. This is not considered reasonable, so that in this sense the larger rate constants may be too fast.

Although very slow, reaction rates for both processes are fast enough to generate small ore bodies, with H₂S generation taking place during ore formation. The ore deposits of Central Tennessee have several features such as large, well formed sphalerites in relatively small deposits, which suggest that one (or both) of these processes may be responsible.