Date of Award
Master of Science
Hicks Dome is a mineralized intrusive center of alkaline ultramafic dikes, plugs and diatreme breccias that produced ~1200 m of structural doming of Paleozoic sedimentary rocks in southeastern Illinois. It is part of an igneous province referred to as the "Wauboukigou Alnöite Province" or "Middle Mississippi Alkaline Province". The 40Ar/39Ar dating of biotite and hornblende from igneous rocks throughout the region has consistently produced crystallization ages of 270 +/- 2 Ma (Early-Middle Permian). Hicks Dome lies near the intersection of two aulacogens, the Reelfoot Rift and the Rough Creek Graben. The igneous rocks in the Hicks Dome area range from dark-green, porphyritic dikes, through dikes with pelletal lapilli, to carbonate cemented breccias with country rock fragments and, locally, phenocrysts or xenocrysts and pelletal lapilli. Magmatic minerals include diopside, pargasite, phlogopite-biotite, titanomagnetite, ilmenite, and apatite, as well as carbonate (ferroan-dolomite - ankerite and calcite) of possible magmatic origin. This mineral assemblage only partly corresponds with other occurrences in the middle Mississippi Valley region igneous province as melilite, garnet, olivine, and perovskite were not identified in the Hicks Dome samples. The Hicks Dome igneous lithotype is defined here as a minette, a type of calc-alkaline lamprophyre. Evidence for this classification includes abundant Al-rich phlogopite-biotite, diopside, and pargasite, and the absence of melilite, garnet, and perovskite. Additionally, plotted Al2O3 vs. TiO2 compositions of biotite-phlogopite form a trend subparallel to that of minettes and alnöites, and Al vs. Ti values for most clinopyroxenes fall in the minette field. Some other igneous occurrences in the region are classified as types of ultramafic lamprophyres. For example, the North Hutson mine (Kentucky) and the Cottage Grove (Illinois) dikes are identified as alnöites and the Wildcat Hills dike (Illinois) is regarded as an aillikite. The Hicks Dome igneous suite contains 23 - 25 wt. % SiO2 and 13 - 21 wt. % CO2, whereas regional igneous rocks contain 33 - 36 wt. % SiO2 and 1 - 2 wt. % CO2. Such high CO2 values and inferred carbonate abundance is likely related to the elevated REE values at Hicks Dome. Finally, Hicks Dome samples show relative depletions in Ti, Nb, Ta, Zr, and Hf on a spider diagram compared to other regional igneous occurrences. These data indicate more variability among provincial igneous rocks than previously realized and perhaps two types of lamprophyres, one a calc-alkaline such as at Hicks Dome, and the other an alkaline ultramafic type. Rock samples from Hicks Dome and the surrounding region that were analyzed far exceed the silica cut-off to be considered a carbonatite (sensu stricto), yet a strong, consistent spatial association of abundant carbonate with igneous rocks throughout the region and similar patterns in LIL (large ion lithophyle), HFS (high field strength) and RE (rare earth) elements with carbonatites worldwide supports that magmatic carbon was part of the lamprophyre system at Hicks Dome. Carbon and oxygen isotope values for carbonate from Hicks Dome and other regional igneous samples do not plot within the field for primary carbonatite or the field for unaltered Mississippian limestone and instead form a broad linear trend between these two reference fields. The observed isotopic trend was possibly due to alteration of primary carbonatite by magmatic-hydrothermal and later low-temperature connate fluids. Parisite and/or synchysite, two REE fluorocarbonates of the bastnäsite group, and xenotime, a Y-(REE) phosphate, were identified in the Hicks Dome samples and occur as fibroradial aggregates in dissolution cavities in fluorite and carbonate and as anhedral crystals infilling microfractures and cleavage planes of phlogopite-biotite and paragasite. These textures clearly indicate the REE minerals are secondary phases most likely resulting from hydrothermal alteration-replacement by magmatic-hydrothermal fluids. A fertile (LREE enriched) lithospheric metasomatized mantle likely sourced the Hicks Dome carbonated alkaline silicate magma that, during emplacement, domed and brecciated the Paleozoic strata, expelled magmatic-hydrothermal fluids, and generated a F-REE mineralizing fluid with abundant carbonate. Igneous activity at Hicks Dome is genetically related to F-REE-base metal mineralization hosted by diatreme breccias, and likely contributed fluorine to the connate brines responsible for the F-base metal mineralization of the surrounding Illinois-Kentucky Fluorspar District (IKFD). Although resource estimates for the Hicks Dome area are beyond the scope of this work, an economic deposit of F, REEs, and/or base metals may be present at Hicks Dome and other igneous occurrences. The igneous lithotype, mineralogy, whole rock geochemistry, and regional geologic context support such a conclusion.
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