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Abstract

The Cherokee basin in southeastern Kansas is a shallow, cratonic downwarp adjacent to the Arkoma basin to the south and the Ozark uplift to the east. Nearby pre-Pennsylvanian carbonates of the Ozarks host several MVT lead-zinc deposits that provide evidence of late Paleozoic basinal fluid invasion. These carbonates constitute a prominent regional aquifer which extends into the Cherokee basin, where it is confined by Pennsylvanian-Permian mud-rich clastics and carbonates. The late-diagenetic assemblage in Pennsylvanian sandstones and limestones is characterized by a widespread dissolution event, baroque ankerite or Fe-dolomite, Ba-Sr sulfates and kaolinite. Such an assemblage normally forms under 3 to 4 km of burial, but the investigated strata have never been buried more than 1.8 km. This anomalous late diagenesis is interpreted to have been caused by regional fluid invasion and related advective heating which advanced the zone of extensive diagenesis towards the depths where alterations of the described type are not expected to be found.

Fluid inclusion studies indicate that the succession of latediagenetic fluids includes an earlier Na-Ca-brine with salinities up to 25 wt.% NaCl eq. and temperatures of 80-85°C or higher, followed by a later Na-rich brine with salinities of about 19-21 wt.% NaCl eq. and temperatures up to 150°C. Paragenetic trends defined by Ca-Mg-Fe, Mn, Sr, and stable isotopic composition of late-diagenetic carbonate cements indicate early replacement of preexisting carbonates followed by dissolution of framework grains and earlier replacive phases and precipitation of several generations of baroque carbonates. The overall progression from a more rock-dominated to a more waterdominated system corresponds to the identified succession of late-diagenetic fluids and may reflect the regional evolution of the flow system. Compositional differences between cements in sandstones and limestones reflect local lithologic control. Late carbonate cements in limestones utilized local sources of Mg and Ca, whereas cements in sandstones utilized local sources of Fe, Mn and 12C-enriched bicarbonate. The original chemistry of the incoming fluids was strongly modified by water/rock interaction with local components, therefore separation of local and regional signatures is a key,step in unraveling the evolution of basin-wide flow systems.

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