Minor- and trace-element distributions in the Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence for possible multiple-basin fluid sources and pathways during lead-zinc mineralization

Abstract

The sources and flow paths of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri have historically been a subject of debate. This study presents geochemical data for epigenetic dolomites of the Bonneterre Dolomite directly above the Lamotte Sandstone aquifer and from the Viburnum Trend orebodies. Samples of epigenetic replacement dolomite were collected at the Bonneterre Dolomite-Lamotte Sandstone contact from 35 drill cores covering an area of more than 25,000 km 2 west of the St. Francois Mountains. There are distinct aerial trends in Fe, Mn, and Sr contents of the dolomite from values of 3.47 wt%, 0.38 wt%, and 27 ppm in the south to 0.82 wt%, 0.12 wt%, and 54 ppm in the north, respectively. In and near the Viburnum Trend lead-zinc subdistrict, the distributions of Fe, Mn, and Sr are reversed, from values of 0.50 wt%, 0.10 wt%, and 105 ppm in the south to 3.15 wt%, 0.55 wt%, and 40 ppm in the north, respectively. Fe and Mn contents of gangue dolomite cement in the Viburnum Trend orebodies show a similar south-to-north enrichment, ranging from 1.41 mole% FeCO 3 and 0.11 mole% MnCO 3 in the south to 2.34 mole% and 0.23 mole% in the north, respectively. On the basis of fractionation of minor and trace elements between fluids and dolomites, the data are interpreted to indicate a regional south-to-north flow of water from the Arkoma Basin through the Lamotte Sandstone. A second fluid-flow system, with a northern source (possibly the Illinois Basin), precipitated the late phases of gangue dolomite cement in the Viburnum Trend and may have been active during the earlier precipitation of the dolomite at the Lamotte-Bonneterre contact. The inferred fluid-flow paths suggest a more complex, multiple-basin fluid involvement than previously suggested for the Mississippi Valley-type Pb-Zn mineralization of southeast Missouri.