LATE PALEOZOIC WEATHERING AND RESIDUAL ACCUMULATION OF MINERALS ON ELK CREEK CARBONATITE, SOUTHEASTERN NEBRASKA, USA

The 0.5 Ga, 36 km² Elk Creek Carbonatite (ECC) in Johnson and Pawnee counties, Nebraska is overlain by an ancient weathering mantle that ranges in thickness from 0 to >15 m. Ferruginization extends even deeper into the parent beforsites (dolomite-carbonatites, with accessory fluorapatite and barite), although some of this effect may be related to hydrothermal alteration. Syenite, granite gneiss, and mafic rock bodies appear in a few cores of the ECC. The ECC regolith is overlain by Late Pennsylvanian cyclothems, specifically the shallow marine limestones of the Cherryvale Formation and/or Dewey Limestone.

In descending order, ECC weathering profiles consist of: (1) red, pervasively slickensided illitic clays (rare); (2) red, earthy to calcite-recemented, leached saprolite, with no significant relict rock fabric (common); and (3) syenite, gneiss, or mafic rock saprock (rare). Saprolite consists largely of hematite (a small fraction as pseudomorphs after magnetite) and lesser goethite, plus residual quartz, chlorite (clinochlore), fluorapatite, and barite. Illite is a minor weathering product. Residual K-feldspar, phlogopite, nepheline, anatase, bastnasite, and pyrochlore are also present in varying proportions. The P₂O₅ content in the few bulk samples of saprolite yet analyzed is as great as 14.1%, and shallow P₂O₅ anomalies (³50,000 ppm) mapped by Molycorp, Inc. in the 1980s are almost certainly related to ancient weathering, as is the localized enrichment of weathered materials with Nb₂O₅ (as much as 0.88%).

The comparatively thick sub-cyclothem regolith on the ECC contrasts with the very thin ancient regolith or stripped granitic terrain of the surrounding Nemaha Uplift. The widespread distribution of various residual minerals in supra-ECC regolith suggests that some transport by overland flow and mass wasting took place prior to burial. Present relief at the base of the Pennsylvanian System over the ECC exceeds 50 m. Although later structural effects cannot be completely discounted, variations in weathering-profile thickness and morphology indicate that there must have been appreciable paleorelief on the weathered surface of the ECC even prior to marine transgression.