The hydrology of ore formation in the Southeast Missouri district: Numerical models of topography-driven fluid flow during the Ouachita Orogeny

A growing body of evidence indicates a relationship in the genesis of the Pb-Zn-Cu ores of the Southeast Missouri district to sedimentary brines mobilized during Late Paleozoic tectonism along the Appalachian-Ouachita orogenic belt. An effective mechanism for this brine mobilization would have been the steep topographic gradients created in the Ozark region during the Ouachita orogeny The present study is an effort to reconstruct the paleohydrology of the flow system and explore its implications for ore formation. Transient finite element simulations of fluid flow, heat transport, and solute transport were used to describe the evolution of the fluid velocity, temperature, and salinity fields as a result of uplift and erosion of Ozark topography. A vigorous south to north flow regime vias predicted, characterized by strong recharge near the southern boundary of the Arkoma basin and strong discharge over the crest of the Ozark dome. Ground-water temperature and velocity were found to increase with time during the early stages of uplift, before reaching a maximum and thereafter declining to somewhat lower values that remained steady over time. Continuous meteoric recharge led to the development of a freshwater plume that gradually migrated through the flow system, displacing the more saline pore fluids that were present initially. The calculations showed further that the onset of fluid velocities and temperatures optimal for forming the ores to coincide very closely with the onset of low salinities unsuitable for forming the ores. Unless salinity was somehow replenished, a relatively short period of time, on the order of hundreds of thousands of years or perhaps less, was available for mineralization in South-east Missouri. Temperature variation in the Cambrian sediments was found to be minimal south of the Viburnum Trend after the earliest stages of uplift. Within the Viburnum Trend, however, a temperature gradient of about 25 degrees C over the length of the trend was predicted. The magnitude of the temperatures generated in the modeling also agreed well with the fluid inclusion data, matching most of the range of measured values. Once uplift had ceased, erosion would have gradually diminished topographic relief and hence, the driving mechanism for fluid flow and much of the heat transfer. The modeling results showed that ore-forming temperatures in Southeast Missouri would have been maintained for no more than a few million years once uplift had ceased, whereas ore-forming fluid velocities would have been maintained much longer, on the order of tens of millions of years.