PERMIAN IGNEOUS ACTIVITY IN SOUTHEASTERN ILLINOIS AND WESTERN KENTUCKY: IMPLICATIONS FOR TECTONISM AND ECONOMIC RESOURCES

Alkaline ultramafic dikes, sills, plugs and intrusive breccias occur along a NW-trending structural corridor that includes the Tolu Arch (anticline) in the Illinois-Kentucky Fluorspar district (IKFD) and the oil and coal fields of SE Illinois. The dikes were intruded along N- to NW-striking, steeply dipping extensional faults of modest displacement and length. Sill-dike-breccia intrusive complexes crosscut and locally domed strata as young as coal-bearing Pennsylvanian formations. Doming resulted in structural relief of ~1300 m at Hicks Dome (HD), ~80 m at Omaha Dome (OD), and >30 m at the recently recognized Cottage Magnetic Anomaly (CMA). Most dikes are ultramafic lamprophyres consisting of primary olivine, phlogopite, melilite, garnet, apatite, perovskite, and magnetite and secondary serpentine, chlorite, and carbonate. Intrusive breccias largely consist of sedimentary rock clasts, although those at HD and the Coefield Magnetic Anomaly (CfMA) contain phenocrysts or xenocrysts of phlogopite and amphibole.

Weakly altered phlogopite yielded ⁴⁰Ar/³⁹Ar vs. ⁴⁰Ar/³⁶Ar isochron dates of 271.0±0.3 Ma for a Davenport Mine dike, 267.8±1.3 Ma for a dike in the Hutson Mine area, and 269.4±0.4 Ma for a Hamp Mine (HD) intrusive breccia. These results are similar to most published K-Ar and Rb-Sr dates on igneous rocks in the region and a Sm-Nd date on fluorite (277.0±15.6 Ma). However, the Ar systematics for CfMA breccia phlogopite are disturbed; a coarse fraction yielded a plateau date of 1 Ga with Ar loss at 521 Ma, and a fine fraction yielded an apparent older date of 735 Ma and a younger date of 345 Ma. These results suggest the presence of Precambrian phlogopite that experienced partial Ar loss during breccia emplacement after 345 Ma.

Breccias at the HD and CfMA intrusive complexes host F-Zn-Pb-Ba-(Be, REE) and Zn-(Pb) resources, respectively. However, production of F, Zn, Pb and Ba in the IKFD has come almost exclusively from vein and stratabound replacement deposits related to NE-striking normal faults that offset the dikes and Tolu Arch. Petroleum has been produced from domed reservoir units at OD (>7M bbls) and is present as primary inclusions in fluorite and along fractures and faults exposed in underground fluorspar and coal mines.

Collectively, these data imply a Permian episode (~270 Ma) of alkaline ultramafic igneous activity related to NE-SW extension of the N. American plate and partial melting of the mantle. Late stage magmatic volatiles enriched in CO₂, F, REEs and possibly Zn and Pb were released explosively to create intrusive breccias and mineralize rocks of the IKFD, particularly along NE-striking faults. Petroleum migration since the Permian resulted in accumulations in traps that include structural domes formed by igneous intrusion. Consequently, sill-dike-breccia complexes represent viable exploration targets for both energy and mineral resources. Conversely, dikes negatively impact the exploitation of coal resources because their size, orientation, mechanical strength and metamorphic effects influence mine design and mining costs.