Proceedings of the 47th Forum on the Geology of Industrial Minerals A Brief Overview of the Geology and Mineral Specimens from the Conco Mine: North Aurora, Kane County, Illinois Jared T. Freiburg Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign Thornton Quarry (photograph by Donald Mikulic) To cite this article: Freiburg, J.T., 2015, A brief overview of the geology and mineral specimens from the Conco Mine: North Aurora, Kane County, Illinois, in Z. Lasemi, ed., Proceedings of the 47th Forum on the Geology of Industrial Minerals: Illinois State Geological Survey, Circular 587. Full terms and conditions of use: http://isgs.illinois.edu/terms-use This article may be used only for the purposes of research, teaching, and/or private study. Commerical use or systematic download- ing (by robots or other automatic processes) is prohibited without explicit ISGS approval, unless otherwise noted. For more informa- tion, contact [email protected]. Please scroll down—article is on subsequent pages. ILLINOIS STATE GEOLOGICAL SURVEY Prairie Research Institute University of Illinois at Urbana-Champaign 615 E. Peabody Drive Champaign, Illinois 61820-6918 http://www.isgs.illinois.edu Copyright © 2015 University of Illinois Board of Trustees. All rights reserved. For permissions information, contact the Illinois State Geological Survey. A Brief Overview of the Geology and Mineral Specimens from the Conco Mine: North Aurora, Kane County, Illinois Jared T. Freiburg* Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign ABSTRACT sions. The operation was initially a of the Kankakee and Wisconsin Arches surface quarry, referred to as the Aurora (Figure 2). The northwest-trending Urban sprawl around the City of Chi- Quarry, and was owned by Conco-West- Kankakee Arch, a southeastern exten- cago has increased the demand for ern Stone Company. In 1991, Conco- sion of the Wisconsin Arch, began devel- construction aggregates in northeastern Western began underground mining opment at the close of the Canadian Illinois. Limited surface land available operations and named the underground (Early Ordovician) time period, which for quarry expansion and a cumbersome complex the Galena-Platteville Mine. In resulted in the separation of the Michi- surface permitting process have led to 2006, Lafarge North America purchased gan Basin to the northeast from the an increase in underground limestone the mine and renamed it the Conco Illinois Basin to the south (Collinson et and dolomite mines in the Chicago area. Mine. al. 1988). The region is host to local fold Since 1991, the Conco Mine in North and fault zones, including brittle faults Aurora has been mining limestone Deeper mining into the Paleozoic bed- that offset Phanerozoic sedimentary and dolomite underground. Previously rock at the Conco Mine has resulted stratigraphic contacts and monoclinal owned by Conco-Western and now in the discovery of trace amounts of folds that locally tilt the bedding (Mar- owned by Lafarge North America, the sphalerite mineralization along with shak and Paulsen 1996). Two dominant Conco Mine continues to prove that massive solution cavities lined with cal- trends of fold and fault zones throughout underground aggregate mining around cite crystals concentrically zoned with the midcontinent are recognized: (1) a large metropolis can be economical pyrite and marcasite. The discovery of east–west to northwest–southeast and and successful. An exciting discovery these mineral deposits has led to new (2) north–south to northeast–southwest. during expansion of the Conco Mine insights into Mississippi Valley-type Regionally and locally, major fault, frac- was the presence of massive solution (MVT) mineralization in northeastern ture, and joint systems trend northwest– cavities lined with large specimen- Illinois as well as the timing, composi- southeast and northeast–southwest. The quality minerals, such as calcite, pyrite, tion, and potential source of diagenetic Sandwich Fault Zone coincides with the and marcasite. The scientific impor- fluids responsible for alteration of the Kankakee Arch and follows this domi- tance of these minerals has been rec- bedrock. Along with these geologic nant regional northwest–southeast trend ognized, and the mining company has insights, spectacular mineral specimens (Figures 2 and 3). The Sandwich Fault allowed the author to extract specimens. have been preserved from the Conco Zone is approximately 84 mi (135 km) Research has been ongoing to interpret Mine. Through the generosity of the long and 0.6 to 1.9 mi (1 to 3 km) wide the diagenetic fluids responsible for car- mine operators, mineral specimens (Kolata and Graese 1983). On the north- bonate diagenesis of the limestone host have been extracted by the author for east side of the fault zone, Paleozoic rock and the precipitation of world-class scientific research and have been placed strata dip eastward into the Michigan mineral specimens. in museums and private collections. Basin. Conversely, on the southwestern The preservation of these specimens is side, the strata dip southward toward INTRODUCTION extremely important for future research, the Illinois Basin. The Conco Mine is for understanding the geologic environ- located approximately 22 mi (35 km) In northeastern Illinois, urban sprawl ment in which the third largest city in northeast of the Sandwich Fault Zone around the City of Chicago and the con- the United States is located, and for the just off the northern flank of the Aurora tinuous growth of the Chicago suburbs enjoyment of generations to come. Syncline (Figure 3). The Aurora Syncline have increased the demand for lime- trends northeastward across southern stone and dolomite aggregate. With lim- Kane County (Figure 3). It is approxi- ited land surface available for expansion GEOLOGIC SETTING mately 1.9 to 6.2 mi (3 to 10 km) wide and and a cumbersome surface permitting The Conco Mine is an underground has a maximum relief of approximately process, operators have begun convert- room-and-pillar mine in North Aurora, 98 ft (30 m; Nelson 1995). A recent mag- ing quarry operations into underground Illinois, located along the Fox River, netic anomaly map of Illinois compiled mining operations. The Conco Mine, approximately 40 mi (65 km) west of by the U.S. Geological Survey (Daniels et located in North Aurora, Illinois (Figure downtown Chicago (Figure 1). The mine al. 2008) suggests a scissored pattern of 1), is the result of one of these conver- lies immediately east of the intersection magnetic strength trending northwest– This article was previously presented at the 47th Forum on the Geology of Industrial Minerals, held May 15–17, 2011, in Champaign, Illinois. Page numbers shown are not final. *E-mail: [email protected] Illinois State Geological Survey Circular 587 1 Sources: Esri, DeLorme, NAVTEQ, USGS, Intermap, iPC, NRCAN, Esri Japan, METI, Esri China (Hong Kong), Esri (Thailand), TomTom, 2013 N Fox River I88 Figure 1 Map of the Chicagoland area and the location and an aerial photograph of the Conco Mine at the intersection of I-88 and the Fox River in North Aurora, Illinois. The arrow on the aerial photograph points to the mine’s decline. 2 Circular 587 Illinois State Geological Survey 0 100 mi N 0 200 km Wisconsin Arch Michigan Basin UMV -5000 0 ! 0 Kankakee 0 Arch Forest City Basin Illinois Basi Mississippi River Arch n 0 -2500 0 -2500 -5000 0 0 0 -2500 Ozark Uplift -2500 -2500 0 ati Arch Mississippi Embayment Cincinn Elevation (ft) of the top of the Upper Mississippi Valley -2500 Galena Group or equivalents Zinc-Lead Mining District (UMV) Conco Mine, North Aurora, Major regional faults and folds ! Kane County, Illinois Figure 2 Major basins, faults, and folds in the north-central United States, and the location of the Conco Mine and the Upper Mississippi Valley Zinc-Lead Mining District (modified from Freiburg et al. 2012). southeast. The Aurora Syncline sits in a a combination of these (Sumner 1976). be suspected (W. John Nelson, personal magnetic low and is parallel to a mag- Within the mine, small vertical faults communication, 2011). with centimeter-scale offset are evident netic high to the northwest and south- The bedrock stratigraphy northeast of within fractures, along with trace slick- east. The magnetic high to the northwest the Sandwich Fault, on the southwest ensides on horizontal clay-rich bedding is followed by a magnetic low to the margin of the Michigan Basin, consists planes. The most prominent features are north, similar to the magnetic low of the of Precambrian igneous basement rock fractures with no apparent offset inter- Aurora Syncline (Figure 3). These mag- unconformably overlain by a sequence secting massive solution cavities. With netic anomalies may reflect deep crustal of Cambrian to Silurian sedimentary little to no vertical offset apparent on rock petrology, basement structures deposits composed of sandstone, lime- these fractures yet development of mas- such as a horst and graben en echelon to stone, dolomite, and shale. Devonian sive solution structures, strike-slip fault- the southern tip of the Sandwich Fault, and younger strata covered the area but significant deposits of iron sulfides, or ing or transtensional movement might Illinois State Geological Survey Circular 587 3 BOONE MCHENRY LAKE COOK KANE DE KALB DUPAGE Conco Mine Sandwich Fault Zone ! 358 283 Aurora Syncline 234 195 er 162 iv R s 133 e n 105 i a l 81 P s 58 KENDALL e 35 D 13 –9 –30 –52 –74 WILL –97 Sandwich Fault Zone –120 LA SALLE –145 –172 –201 –234 GRUNDY –273 –322 0 10 20 mi –397 N 0 10 20 30 km nanoteslas KANKAKEE Figure 3 Magnetic anomaly map of structures in northeastern Illinois (Nelson 1995; Daniels et al. 2008) and the location of the Conco Mine. Map courtesy of the U.S. Geological Survey. were eroded after Pennsylvanian times in river valleys, where bedrock is typi- 1978) and the uplift of the Kankakee (Kolata et al.