QiA Sc^^k-^ C550 INTRUSIVE BRECCIAS AT HICKS DOME Hardin County, Illinois J. C. Bradbury and J. W. Baxter 1992 Department of Energy and Natural Resources Circular 550 ILLINOIS STATE GEOLOGICAL SURVEY ILLINOIS STATE GEOLOGICAL SURVEY 3 3051 00002 8724 INTRUSIVE BRECCIAS AT HICKS DOME Hardin County, Illinois J. C. Bradbury and J. W. Baxter Circular 550 1992 ILLINOIS STATE GEOLOGICAL SURVEY Morris W. Leighton, Chief Natural Resources Building 615 East Peabody Drive Champaign, Illinois 61820 ACKNOWLEDGMENTS Conversations with many investigators have added to our understanding of the processes that combined to form the enigmatic structure known as Hicks Dome. We are grateful for the critical review of the manuscript by several colleagues at the Ilinois State Geological Survey, J. James Eidel, Jonathan H. Goodwin, Richard D. Harvey, and Randall E. Hughes. Careful review and sugges- tions by Timothy Hayes, U. S. Geological Survey, were especially helpful. - Graphic designer J. Hannah Editor - E. A. Latimer Typographer - D. Harding recycled paper ) printed on Printed by authority of the State of Illinois/1992/1000 CONTENTS ACKNOWLEDGMENTS ABSTRACT INTRODUCTION 1 Purpose and Scope of Study 1 Location and Setting 1 Methods of Investigation 1 REGIONAL SETTING AND STRUCTURAL HISTORY 3 ROCKS EXPOSED AT HICKS DOME 5 Sedimentary Rocks 5 Intrusive Rocks and Breccias 5 DISTRIBUTION AND CHARACTER OF THE BRECCIA BODIES 6 Central Area 7 Hamp locality 7 Hamp outcrop (?) 8 Pankey vent breccia 9 Rose Mine breccia 10 Rose Cemetery Breccia 11 New Albany Group Belt 11 Shatter breccias 11 Carbonatitic breccias 13 Grant Intrusive 14 Fort Payne Rim Shatter Breccias 16 Petrography 16 Robinson Ultramafic Dike 16 GEOCHEMISTRY OF THE BRECCIAS 17 Rock Composition Analysis 17 Vent and shatter breccias 17 Carbonatitic breccias 17 Trace Element Analyses 17 Fluorine (as Fluoride) Analysis 18 ORIGIN OF THE BRECCIAS 19 Shatter Breccias 19 Vent Breccias 20 Carbonatitic Breccias 20 Evidence from Cementation and Mineralization 20 Age of Brecciation 20 REFERENCES 22 TABLES 1 Strata at Hicks Dome 5 2 Summary of stratigraphic units intersected in Hamp well 7 3 Rock compositions 17 4 Trace element compositions 18 5 Fluoride analyses 19 FIGURES 1 Major structural features and igneous intrusive rocks, Illinois-Kentucky Fluorspar District iv 2 Geologic map of Hicks Dome 2 3 Relationship of Hicks Dome to Precambrian Rift Zones 3 4 Breccia occurrences at Hicks Dome 4 5 Shatter breccia dikes in the Fort Payne Formation 6 6 Texture of the Pankey Breccia 9 7 Barite mineralization in siliceous vent breccia at south margin of the Pankey Breccia 10 8 Texture of Rose Mine breccia 11 9 Calcareous shatter breccia 12 10 Sample site 17 12 11 Shatter breccia 12 12 Photomicrograph of thin section of carbonatitic (sideritic) breccia dike showing former 14 amphibole crystal 13 General texture of the Grant Intrusive, a carbonatitic breccia 15 14 Photomicrograph of thin section of Grant Intrusive 15 15 Silicified limestone breccia in the upper part of the Salem Limestone and up gully from the 15 Grant Intrusive fault igneous intrusions \) structural arch approximate boundary of Cretaceous sediments 15 mi j Figure 1 Major structural features and igneous intrusive rocks, Illinois-Kentucky Fluorspar District (modified from Nelson 1991, p. 236). IV ABSTRACT Within a 1- by 2-mile oval of Devonian and Lower Mis- silicified. Four breccia exposures and a core intersection sissippian exposures at Hicks Dome are several occur- have both igneous and sedimentary rock fragments in a rences of rocks, including intrusive breccia and one matrix of carbonate minerals that we believe were depos- ultramafic dike, genetically related to alkalic magmatic ited from a CC>2-rich gas exsolved from an alkaline activity. Additional breccias of less clear igneous affini- magma at depth; these breccias are classified as carbona- ties are more numerous. titic after Gold (1972). The breccias at Hicks Dome are exposed as dikes and Mineralization is largely confined to the central area other bodies of indeterminate shape in a central area, and of the dome and includes fluorite, barite, sphalerite, and chiefly as dikes in the dome's surrounding New Albany galena (most common and confined within breccias); shale and Fort Payne Formation outcrop belts. The brec- bertrandite (beryllium silicate) in two breccia bodies; and cia dikes exhibit a crudely radial arrangement around brockite (calcium thorium yttrium phosphate) and the dome, but most have strikes that fall within the major florencite (cerium aluminum phosphate) in a third brec- regional fracture trends. cia. Abnormal amounts of thorium, rare earths, and Breccias are divided on the basis of geometry and niobium were measured in breccia cuttings from a test composition into three types: shatter, vent, and carbona- well near the center of the dome. Vent and carbonatitic titic. Sliatter breccias, composed of fragments of the imme- breccias probably resulted from explosive release of diate wall rock, are commonly tabular, essentially gases heated by or derived from an alkalic magma at vertical bodies. Vent breccias occur in bodies of irregular depth. or indeterminate form and exhibit evidence of a large The ages of the breccia intrusion and mineralization amount of vertical displacement of some included frag- episodes are unknown, but they probably are in the ments. Many shatter and vent breccias are intensely interval from early Permian to early Late Cretaceous. INTRODUCTION east. Also in the oval is a central topographic high (hill), Purpose and Scope of Study designated Hicks Dome on the Karbers Ridge Quadran- Discovery of mineralized, radioactive breccia in an oil gle topographic map. Unless otherwise stated, the term test hole (the Hamp well ) near the center of Hicks Dome Hicks Dome is used in its more limited topographic sense (Brown et al. 1954) and subsequent exploration for radio- and conforms to local usage. Hicks Dome therefore in- active minerals focused attention on this structure in cludes the central hill, the valley that encircles the hill, southern Illinois (Bradbury et al. 1955, Trace 1960). The and the outer ridge that forms the periphery of the possible economic significance of Hicks Dome minerali- distinctive oval outcrop pattern. The outer ridge is nearly zation and the relationship of igneous activity at Hicks as high as the central hill. Dome to the deposition of fluorspar-lead-zinc-barite ore Hicks Dome, in a regional sense, comprises the north- bodies of the Illinois-Kentucky Fluorspar District have west end of the Tolu Arch, a poorly defined uplift that been of special interest. Field investigations by the Illi- traverses the fluorspar district from southeast to north- nois State Geological Survey revealed numerous addi- west (fig. 1). A broad belt of block faulting appears to tional occurrences of breccia, mostly as dikes around the offset the axial trend of the Tolu Arch. Farther to the periphery of the central area of the dome. This report southeast in Kentucky, the Tolu Arch becomes difficult discusses the occurrence of the known breccias and their to trace because of block faulting, but offset segments of petrography, chemical composition, mineralization, and the axis can be identified in several of the fault blocks origin. The data and interpretation presented should add (fig. 1 and Trace 1976, p. 69). to the understanding of this enigmatic domal structure. Methods of Investigation Location and Setting Detailed field mapping was conducted within the 1- by Hicks Dome is both a structural and topographic high 2-mile (1.6- by 3.2- km) oval area at the center of Hicks situated in Hardin County, Illinois, in the northwestern Dome. As bedrock outcrops are relatively rare downdip part of the Illinois-Kentucky Fluorspar District (fig. 1). from the escarpment, the boundary of the study area was Structurally, the area of Hicks Dome extends several chosen to be the top ridge of the Fort Payne Formation. miles from its center. Radial and concentric (ring) faults Samples were taken from all observed breccia out- that have been mapped in Mississippian strata approxi- crops and from clusters of boulders that collectively mately 3 miles (5 km) downdip from the apex attest to appeared to represent individual breccia occurrences. A the dome's formation by upward forces. The dome has previously known exposure of breccia, the Grant Intru- a central area where Devonian and Lower Mississippian sive located about 4,000 feet (1.2 km) south of the study sedimentary rocks are at the surface (fig. 2). The central area (fig. 2), was included in the study because it pro- area is oval and approximately 1 by 2 miles (1.6 by 3.2 vided relatively unweathered samples of a breccia type km); its long dimension is oriented northwest to south- encountered in the mapped area. Thin sections were QUATERNARY SYSTEM MISSISSIPPIAN AND DEVONIAN SYSTEM 1 alluvial deposits |s^\| New Albany Shale Group PENNSYLVANIAN SYSTEM DEVONIAN SYSTEM Middle Devonian Series R%%fl Tradewater Formation Y/\ Lower And shale, sandstone, and thin coal IGNEOUS AND BRECCIATED ROCK E^^j Caseyville Formation I breccias MISSISSIPPIAN SYSTEM + mafic dikes upper Pope Group, above top Hmgi —J-"' faults Hill f Glen Dean Limestone IHH lower Pope Group, below top ^^^ of Glen Dean Limestone Figure 2 Geologic map of Hicks Dome. ^ Ste. Genevieve-St. Louis Limestone Salem-Ullin Limestone J Fort Payne-Springville Formation made of relatively fresh samples and studied pet- sion spectrography. Fluorine (as fluoride) analyses were rographically. Badly weathered samples were gently carried out by fluoride ion-selective electrode. X-ray disaggregated and studied under binocular microscope diffraction studies supplemented these methods. The and/or by oil immersion. Fifty-seven samples were geochemical data were examined to identify suites of qualitatively scanned by emission spectrograph to deter- elements intrinsic to the depositional environment of the mine the elements present and their relative abundance.