Carbonate petrology and geochemistry of Pennsylvanian coal balls from the Kalo Formation of Iowa

Coal balls are carbonate and pyrite concretions enclosing uncompressed peat, primarily found in Pennsylvanian and earliest Permian paleotropical coals. Petrographic and microprobe analysis of coal balls from the mid-Moscovian (latest Atokan or earliest Desmoinesian) Williamson No. 3 Mine (Kalo Formation) from Lucas County, Iowa indicates that pyrite was the earliest mineral to form, followed by high-Mg calcite (average 13.9 mol% MgCO3), which is also high in Sr (700–1500 ppm), suggesting that the Kalo Formation coal balls formed in response to the incursion of marine water into the mire. The high-Mg calcite has an unusual growth habit, radiating arrays of bladed polycrystals, which look like distorted triangles with elongated apices when cut parallel to the basal plane. These high-Mg calcite polycrystals have low-Mg, non-ferroan calcite rims, consistent with diagenesis in meteoric water. The presence of early high-Mg and low-Mg, non-ferroan calcite in the Williamson No. 3 coal balls suggests that these coal balls formed in a hydrologically dynamic environment. Most of the remaining cements in the Kalo Formation coal balls are low-Mg ferroan calcite, consistent with burial diagenesis. These cements retain regions of low-Mg and high-Mg calcite, suggesting that they are neomorphs of the early high-Mg calcite cement. Fibers in the vascular bundle supports of Cordaites principalis leaves have narrow rims of dolomite that appear unique to this species. The presence of high-Mg calcite in the Williamson No. 3 coal balls supports the Stopes and Watson (Phil. Trans. Roy. Soc. London B (1909) 200:167–218) model of coal-ball formation as a result of incursions of marine water into coastal mires. It conflicts with stable isotopic data, which suggests that most coal-ball cements formed in meteoric water. However, in our samples, the most common cement is diagenetically altered, low-Mg ferroan calcite, which might yield ‘meteoric’ oxygen isotopic values. Excellent preservation of plant fossils in coal balls is not necessarily correlated with the presence of original cement.

► The earliest cement in coal balls from the Kalo Fm. is marine, high-Mg calcite.
► Early diagenetic pyrite and high-Mg calcite indicate a marine origin for coal balls.
► Coal balls with meteoric and marine calcite formed in hydrologically dynamic mires.
► Burial diagenesis has altered most coal-ball calcite to low-Mg, ferroan calcite.
► Diagenetically altered coal-ball cements may not retain original isotopic values.