Studies of base metal diffusion in experimental and natural systems

Abstract

Results of experimentation on the diffusion of zinc ions through carbonate rocks show that the system departs from ideal because of the chemical and physical influence of the matrix environment. The geology diffusion function (G), which is a measure of the diffusivity and reactivity of a diffusing ion, can be calculated from the proper diffusion equation for the system involved and approaches the true diffusion coefficient as the system approaches ideal conditions. The physical significance of G is difficult to assess, for it depends upon the total free surface of the matrix and the rate law or laws governing the reaction between the diffusion ion and the matrix, as well as temperature, concentration and the nature of the solution and matrix. When the duration of a nonideal diffusional activity is unknown, the factor (Gt) can be determined. By using a known or reasonably assumed G value for the natural system an estimate can be made for the period of the activity.In an effort to identify the parameters that controlled the distribution of metals in a carbonate host rock of epigenetic ore deposits, data were collected regarding wallrock alteration and structure, mineralization and base metal dispersion from three orebodies in the Eagle mine at Gilman, Colorado. In the immediate area of the mine dolomitization increased the permeability of the Leadville formation by a factor of 10 3 to 10 6 , with higher values progressively closer to ore. The shape and extent of the present orebodies at Gilman, and in particular the manto orebodies, are primarily influenced by solution channeling and increased rock permeability developed during dolomitization of the limestone.Wallrock dispersion curves for lead and zinc are similar to the experimental results reported and indicate that dispersion follows a nonideal diffusional system. In comparing sample traverses in the wallrock of the orebodies studied, both lead and zinc show decreasing wallrock penetration, and a decreasing period of wallrock metasomatism, with increasing distance from chimney orebodies that acted as feeder areas for the manto deposits.Assuming a value for G of 10 (super -6) cm 2 /sec to be correct within an order of magnitude, the maximum time span for the mineralizing epoch is calculated to be in the order of 10 5 years. This time span is only a fraction of that available for mineralization since early (?) Tertiary igneous activity, which pre-dates the ore. On the basis of accepting a nonideal diffusional mechanism for the dispersion of wallrock base metals, an estimate of the minimum concentration of lead and zinc in the ore fluid ( approximately 0.1%) indicates that a relatively short mineralizing period is not improbable.