120-14 The Metalliferous Pennsylvanian Mecca Quarry Shale of the Illinois Basin: Deposition, Early Diagenesis, and U-Mo-V-Zn Enrichment

Abstract:

The Pennsylvanian black shales of the U.S. Midwest, such as the Mecca Quarry Shale (MQS) and Excello Shale, are rich in organic matter and various trace metals including uranium (U), molybdenum (Mo), vanadium (V), and zinc (Zn). The metalliferous MQS in the eastern (Indiana) part of the Illinois Basin was studied based on core description, trace element geochemical characteristics, X-ray diffraction (XRD), scanning electron microscopy, and organic petrography. Thirteen drill cores were studied, and 63 black shale samples were collected for mineralogical, geochemical, and petrographic analyses. The average total organic carbon (TOC) and total sulfur content is 14.38 wt % (maximum 45.35 wt %) and 2.64 wt % (maximum 5.29 wt %), respectively. U, Mo, V, and Zn contents of the MQS are all highly enriched compared to the concentrations in the Upper Continental Crust and range from 10.42 to 235 ppm (average 84.68 ppm), < 2 to 1,620 ppm (average 619.45 ppm), 233 to 2,770 ppm (average 1,241.65 ppm), and 58 to 5,480 ppm (average 1,412.58 ppm), respectively. Organic matter in the MQS includes alginite, bituminite, liptodetrinite, sporinite, vitrinite, inertinite, and solid bitumen, with bituminite being the dominant maceral. XRD analysis shows that the MQS is composed of illite (average 41.0%), quartz (average 24.1%), kaolinite (average 10.2%), plagioclase (average 6.3%), pyrite (average 4.5%), dolomite (average 3.3%), calcite (average 2.7%), K-feldspar (average 2.7%), chlorite (average 1.3%), and minor amounts of fluorapatite, ankerite, sphalerite, and siderite. From a geochemical proxy perspective, the U/Th, Ni/Co, and V/Cr ratios suggest deposition of the MQS under anoxic conditions, whereas stratigraphic context (directly above a coal bed), sedimentary texture (lenticular fabric), trace fossils, and maceral attributes (micrinized bituminite) collectively support deposition by oxygenated bottom currents in a shallow-water environment (probably several meters deep). This discrepancy between geochemical and sedimentological attributes clearly shows that redox-sensitive trace element ratios need to be combined with sedimentological and petrographic observations to arrive at more complete interpretations of black shale successions.

Based on multiple sources of evidence, we suggest that U-Mo-V-Zn enrichment in the metalliferous MQS was primarily controlled by deposition and early diagenesis. During early diagenesis, mineralization mainly occurred in the sulfate reduction zone where H₂S was present. Although contribution by hydrothermal fluids cannot be excluded, it lacks direct evidence.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-16898

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.