Hydrology of the Western Arkoma basin and Ozark platform during the Ouachita orogeny: complications for Mississippi Valley-type ore formation in the Tri-State Zn-Pb district

Numerical groundwater modeling was used to investigate the role of fluid flow associated with uplift of the Arkoma basin during the closing stages of the Ouachita orogeny in forming the Mississippi Valley-type Zn-Pb ores of the Tri-State district. The model hydrostratigraphy was flexurally compensated to account for the restoration of Pennsylvanian-Permian sediments removed since the close of the orogeny in estimating the regional paleotopographic gradient. Estimates of the amount of Pennsylvanian-Permian sediment that has been removed by erosion vary widely. A thick and a thin endmember case were considered, and in both cases topography-driven fluid flow was shown to have been an important mechanism for groundwater motion, with a lesser component contributed during the early stages of uplift by overpressuring created by compaction in the deep portion of the Arkoma basin. The Pennsylvanian-Permian sediments and underlying Western Interior Plains confining system acted as thick capping aquitards that caused slow rates of groundwater flow over much of the profile. As a result, meteoric water infiltration initiated during uplift was slow to flush saline formation waters, allowing MVT ore-forming salinities to persist at Tri-State on the order of at least 10(0) Myr. The slow groundwater flow rates also caused heat transport to occur primarily by conduction rather than advection. Despite this, MVT ore-forming temperatures were still reached at Tri-State for both endmember cases of Pennsylvanian-Permian aquitard thickness, though much more readily in the thick aquitard case. Faults within the Tri-State district served as a regional fluid focusing mechanism and probably played a more important role in localizing mineralization than the window in the Ozark confining unit that occurs in the district. Fluids rising along these faults could have cooled by about 8-10 degrees C and as much as another 0.3 degrees C km(-1) as they flowed laterally northward. This temperature change alone however would not have been sufficient to precipitate the total mass of metal sulfide ore occurring at Tri-State.