New Insights on Upper Mississippi Valley Mineralization Based On
New Insights on Upper Mississippi Valley Mineralization Based on Solution Cavities in the Ordovician Galena Group at the Conco Mine, North Aurora, Illinois, USA Jared T. Freiburg1, Bruce W. Fouke2, and Zakaria Lasemi1 1Illinois State Geological Survey and 2Department of Geology, Institute for Genomic Biology, University of Illinois at Urbana-Champaign
! Conco Mine -2500 Elevation (ft)
0 100 mi Wisconsin Arch N 0 200 km
Michigan Basin -5000 0 ! er Arch 0 Kankak 0 ee Arch Forest City Basin Illinois Basin
Mississippi Riv
0
-2500 0
-2500 -5000 0
0 0
-2500 Ozark Uplift
-2500 Circular 581 2012 -2500 0 Mississippi Embayment Cincinnati Arch
ILLINOIS STATE GEOLOGICAL SURVEY Prairie Research Institute University of Illinois at Urbana-Champaign Front Cover: Major basins, arches, and domes in the north-central United States in relation to Conco Mine.
© 2012 University of Illinois Board of Trustees. All rights reserved. For permissions information, contact the Illinois State Geological Survey. New Insights on Upper Mississippi Valley Mineralization Based on Solution Cavities in the Ordovician Galena Group at the Conco Mine, North Aurora, Illinois, USA Jared T. Freiburg1, Bruce W. Fouke2, and Zakaria Lasemi1 1Illinois State Geological Survey and 2Department of Geology, Institute for Genomic Biology, University of Illinois at Urbana-Champaign
Circular 581 2012
ILLINOIS STATE GEOLOGICAL SURVEY Prairie Research Institute University of Illinois at Urbana-Champaign 615 E. Peabody Drive Champaign, Illinois 61820-6964 http://www.isgs.illinois.edu
CONTENTS
ABSTRACT 1 INTRODUCTION 1 GEOLOGIC SETTING AND BACKGROUND 5 Structural Geology 5 Stratigraphy 6 Dolomitization 6 METHODOLOGY 7 Sampling 7 Plane-Light Petrography and Cathodoluminescence 11 X-ray Diffraction 11 Environmental Scanning Electron Microscopy 11 Radiogenic Isotope Analyses 12 Stable Isotope Analysis 12 Elemental Analyses 12 RESULTS 13 Sedimentology and Stratigraphy 13 Structural Geology, Solution Cavities, and Mineralogy 13 Paragenetic Sequence 19 Geochemical Analyses 25 DISCUSSION 26 Depositional History of the Solution Cavity Host Rock 26 Diagenetic History 27 Early-Stage Diagenesis 27 Late-Stage Diagenesis 28 Fracturing and Dissolution The Sequence of Dissolution, Sulfide Precipitation, and Calcite Precipitation 28 Regional Paragenetic Comparisons 31 Upper Mississippi Valley Zinc-Lead District 32 Western Michigan Basin (Eastern Wisconsin) 34 Southeastern Michigan Basin (Michigan and Ontario) 35 CONCLUSION 36 ACKNOWLEDGMENTS 37 REFERENCES 37 APPENDIX 43 FIGURES 1 Major basins, arches, and domes in the north-central United States in relation to Conco Mine 2 2 Magnetic anomaly map and structures of northeastern Illinois 3 3 Stratigraphic column of northeastern Illinois 4 4 Mapped fractures and solution cavities in the Conco Mine overlying local magnetic anomalies 8 5 (a) The studied solution cavity showing its elliptical shape. (b) Giant euhedral calcite crystals line the cavity wall. 9 6 (a) A two-dimensional sketch of the mine room containing the solution cavity shown in Figure 5. (b) A three-dimensional sketch of the mine room of Figures 5 and 6a 10 7 Photographs of a representative euhedral calcite crystal lining the solution cavity wall 11 8 Paragenetic sequence of the solution cavity and the surrounding host rock 12 9 Plane-light petrography and cathodoluminescent photomicrographs of solution cavity host rock including paragenetic sequence events (A3, A7, A11) 15 10 Plane-light petrography and cathodoluminescent photomicrographs of rock including paragenetic sequence events (A3, A5, A6, A10, A11, A12, A27) 16 11 (a) Plane-light petrography photomicrograph of a fracture crosscutting wackestone and cemented with the first cavity cement. (C3) A12. (b) PL photomicrograph of the top edge of the cavity displaying clay bed (Dygerts bentonite), cavity calcite cement (C3) A12, and wackestone host rock. (c) Cathodoluminescent (CL) photomicrograph of a fracture crosscutting wackestone and cemented with the first cavity cement (C3) A12. (d) CL photomicrograph of correlating photomicrograph in part b. 17 12 Photographs of the top east corner of the studied solution cavity and another solution cavity within the mine 18 13 Scanned images of thin sections and correlating cathodoluminescent images reflecting underexaggerated lateral cross section from host rock-cavity dissolution boundary through the cavity cements 20 14 Plane-light photomicrographs of sulfide zones S1 through S4 within the cavity 21 15 Plane-light photomicrographs of sulfide zones S5 through S7′ within the cavity 22 16 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S1 through S3 23 17 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S4 and S5 24 18 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S6 and S7 25 19 Theδ18 O VPBD values of cavity calcite zones C3 through C7 compared with their δ13C VPBD values 27 20 Paragenetic sequence of the Upper Mississippi Valley Zinc-Lead District primary ore deposition 31 21 Paragenetic sequence of the general mineralization observed in the Sinnipee Group (Galena-Platteville Groups) in eastern Wisconsin on the western margin of the Michigan Basin 32 22 Paragenetic sequence of the Trenton Group (Galena Group) in southwestern Ontario on the southeastern margin of the Michigan Basin 32 23 Comparison of paragenetic events from the Upper Mississippi Valley Zinc-Lead District, eastern Wisconsin in the western Michigan Basin, and southwestern Ontario in the southeastern Michigan Basin, all relative to the study area at the Conco Mine 33
TABLES 1 Paragenetic table outlining and describing all diagenetic mineralization events at the Conco Mine 14 2 Elemental and isotopic abundances of the solution cavity cements 26 ABSTRACT A comparison of the paragenesis at the timing, composition, and source Conco Mine with that from other of the diagenetic fluids. Therefore, the Geologists have long recognized the regional MVT deposits indicates a present study was undertaken to deter- important role of continental- to basin- related succession of diagenetic events, mine whether massively mineralized scale fluid flow as a mechanism for dia- thus supporting the possible involve- solution cavities in the Ordovician lime- genesis, including the dissolution of rock ment of a similar diagenetic fluid history stone located in the Conco Mine, North and the deposition of Mississippi Valley- during dissolution and mineralization. Aurora, Illinois, are the diagenetic result type (MVT) minerals in sedimentary However, the paragenesis at Conco Mine of interaction with (1) MVT hydrother- basins. Tens-of-centimeter– scale calcite most resembles the paragenesis of the mal waters or (2) more common burial crystals, concentrically zoned with sul- Upper Mississippi Valley Zinc-Lead diagenetic waters. fide minerals and growing within meter- District deposits, which are thought to scale solution cavities, were recently be sourced from fluids expelled out of The MVT mineral deposits account for discovered on the flank of the Michigan the Illinois Basin. Conversely, the geo- some of the most important reserves Basin in the Conco Mine (North Aurora, graphic location, structural boundaries, and resources of lead and zinc in the Kane County, northeastern Illinois). and similar paragenesis of the Conco world. These epigenetic, strata-bound, These mineral deposits reflect the tem- Mine samples to MVT deposits in the carbonate-hosted sulfide bodies are poral changes in chemical composition, Michigan Basin cannot rule out a Michi- predominately composed of sphalerite, fluid flow pathways, and the source of gan or Appalachian Basin fluid source. galena, and numerous gangue minerals paleofluids migrating through the mid- such as pyrite, marcasite, calcite, and continent. Meter-scale solution cavities The results of this study have important dolomite (Paridas et al. 2007). The MVT were created by subsurface dissolution implications with respect to diagenesis deposits are thought to precipitate from along primarily northwest-southeast– of sedimentary rocks in midcontinent deep saline basinal hydrothermal fluids trending fractures in the Wise Lake North America, including MVT mineral that typically range from 70 to 200ºC Formation limestone of the Ordovician deposits and factors controlling cavern- (Leach and Sangste 1993). The genesis Galena (Trenton) Group. Plane-light ous porosity development. This study of MVT deposits has been thought to and cathodoluminescent petrography provides documentation and data that be related to the enhancement of sub- and scanning electron microscopy were help tie the diagenetic features observed surface reservoir rock porosity and used to identify a series of repeating dia- in the Galena Group carbonates with permeability, hydrocarbon generation, genetic events within the cavities: disso- potential diagenetic processes, which and subsequent migration (Jackson and lution followed by marcasite, pyrite, and should help in the development of pre- Beales 1967, Anderson 1991, Gregg 2004). calcite precipitation. dictive regional subsurface paleofluid The economic importance of these flow models. Enhanced diagenetic and enhancements has motivated many pre- The repetition of these diagenetic events basin flow modeling should improve the vious studies on the nature and genesis and the paragenetic ordering of miner- accuracy of mineral and hydrocarbon of MVT deposits. However, there is still alization, in conjunction with changes exploration and exploitation models. little consensus on interpretation of the 87 86 13 18 in isotopic ( Sr/ Sr, δ C, and δ O) and genesis, duration, and fluid migration elemental concentrations (Ca, Na, Sr, Fe, pathways of diagenetic events respon- Mg, and Mn) in calcite growth zones, INTRODUCTION sible for MVT mineralization. suggest cyclical changes in the fluid The process by which subsurface fluid chemistry. Furthermore, these repeated flow precipitated Mississippi Valley-type This study examines the paragenesis of changes reflect the source of the subsur- (MVT) mineral deposits in sedimentary spectacular mineralization in large solu- face diagenetic waters that were respon- basins throughout the midcontinent of tion cavities that have formed within sible for dissolution and mineralization. North America has long been a topic of the Ordovician Galena Group in north- The chemical composition of the older, debate (e.g., Noble 1963, Jackson and eastern Illinois. Field relationships have innermost calcite cement zones within Beales 1967, Garven and Freeze 1984, been integrated with high-resolution 87 86 the cavity ( Sr/ Sr, 0.71015 ± 0.00002 Bethke 1985, Sverjensky 1986, Garven petrographic and geochemical analyses standard SRM976; δ13C , –1.62 to to reconstruct the timing, composi- VPDB and Raffensperger 1997). Decades of –4.88; δ18 ,–6.99 to –3.37; high Mg; tion, and source of the diagenetic fluids OVPDB geochemical, geophysical, and hydro- and high Fe) is substantially different logical modeling analyses indicate that from which the minerals precipitated. from the chemical composition of later these mineral deposits precipitated Furthermore, the paragenetic sequence 87 86 and younger calcite cements ( Sr/ Sr, from secondary (diagenetic) formation is compared with other regional MVT 0.70980 to 0.70898 ± 0.00002; δ13C , deposits as a means of predicting VPDB water during the deep burial water-rock –3.27 to –10.58; δ18O , –8.86 to –6.18; regional subsurface paleofluid flow in VPDB alteration of marine limestone (Hitchon low Mg; and low Fe). These differences 1984, Oliver 1986, Bethke and Marshak order to be able to develop more accu- indicate that there were at least two 1990, Garven et al. 1993). However, sig- rate models for economic mineral and major events of diagenetic fluids migrat- nificant uncertainty remains regarding hydrocarbon exploration. ing through the country rock.
Illinois State Geological Survey Circular 581 1 ! Conco Mine -2500 Elevation (ft)
0 100 mi Wisconsin Arch N 0 200 km
Michigan Basin -5000 0 ! er Arch 0 Kankak 0 ee Arch Forest City Basin Illinois Basin
Mississippi Riv
0
-2500 0
-2500 -5000 0
0 0
-2500 Ozark Uplift
-2500 -2500 0
Mississippi Embayment Cincinnati Arch
Figure 1 Major basins, arches, and domes in the north-central United States (Cohee et al. 1962, Collinson et al. 1988) in relation to Conco Mine.
Meter-scale solution cavities have been cipitated within the solution cavities lization sequence, therefore, create a discovered on the southwest flank of that are both obliquely and concentri- uniquely well-suited natural laboratory the Michigan Basin just off the Kanka- cally zoned with cyclical micron- to in which to study subsurface regional kee and Wisconsin Arches (Figure 1) in centimeter-scale pyrite and marcasite fluid transport, extensive diagenetic the underground Conco Mine in Kane overgrowths. These successive changes alteration, and inference to mechanisms County, northeastern Illinois (Figure 2). in mineral growth zonations are indica- of MVT mineralization. The cavities occur within the late Ordo- tive of the changes in bulk subsurface vician carbonates of the Galena Group diagenetic fluid properties. The min- Late Paleozoic episodes of brine migra- Wise Lake Formation (Figure 3), which is eralization record within the solution tion in the Illinois, Michigan, and Appa- laterally equivalent to the Trenton Lime- cavities, therefore, directly records the lachian Basins originated in the fore- stone in Michigan, Indiana, Ohio, New timing and composition of the parent lands of North American tectonic belts. York, and Ontario, and the Sinnipee fluids, which in turn provide insight Migratory pulses of fluid were driven by Group in Wisconsin. Calcite cements to the source of the diagenetic fluids. global-scale tectonic processes (Leach at tens-of-centimeter scale have pre- These solution cavities and their crystal- et al. 2001) that resulted in belt deforma-
2 Circular 581 Illinois State Geological Survey BOONE MCHENRY LAKE
COOK
KANE
DE KALB
DUPAGE
Conco Mine Sandwich F !
358 ault Zone 283 Aurora Syncline 234 195 er iv 162 R s 133 e n 105 i a 81 l P
58 KENDALL s e
35 D 13 Ϫ9 Ϫ30 Ϫ52 Ϫ74 WILL Ϫ97 Sandwich F Ϫ120 LA SALLE Ϫ145 ault Zone Ϫ172 Ϫ201 Ϫ234 Ϫ273 GRUNDY Ϫ322 0 10 20 mi Ϫ397 N 0 10 20 30 km nanoteslas KANKAKEE
Figure 2 Magnetic anomaly map and structures of northeastern Illinois (Nelson 1995, Daniels et al. 2008). Reds indicate high magnetic pull; blues indicate low magnetic pull.
tion (Bethke and Marshak 1990). This sedimentary basins before depositing and zinc minerals in MVT deposits (e.g., interpretation has been based largely lead- and zinc-rich minerals. Despite Sverjensky 1986, Garven et al. 1993) due upon early studies of MVT deposits and this general understanding of the hydro- largely to the economic significance the relationship of regional or subconti- logic processes and recent advances of these minerals. However, minerals nental-scale hydrologic processes (Jack- made in dating MVT deposits (Leach et of little economic importance— com- son and Beales 1967, Sharp 1978, Leach al. 2001), there is still substantial uncer- monly referred to as “gangue” minerals 1979, Anderson and Macqueen 1982, tainty about models for ore genesis and such as pyrite, marcasite, and calcite— Cathles and Smith 1983, Garven and the duration, migration pathways, and have rarely been the focus of any study Freeze 1984, Bethke 1985, Garven 1985, compositional changes of the depo- (Brannon et al. 1996, Corbella et al. Oliver 1986, Bethke and Marshak 1990, sitional fluids at particular deposits 2004). The nature of gangue mineraliza- Garven et al. 1993, Garven and Raffens- (Rowan and Goldhaber 1995). tion is significant to tracking the overall perger 1997, Morrow 1998). The general depositional fluid evolution and the life consensus is that deep basinal brines Many previous studies have given con- span of an MVT deposit. Iron sulfide migrated hundreds of kilometers out of siderable attention to the genesis of lead and calcite mineralization is the main
Illinois State Geological Survey Circular 581 3 GRAPHIC FORMATION COLUMN DESCRIPTION THICKNESS (feet) (not to scale) GROU P SERIE S SYSTEM HOLO- Cahokia Fm (0–15) Alluvium, sand, silt, and clay CENE Grayslake Peat (0–5) Peat and muck Equality Fm (0–35) Silt loam, massive Henry Fm (0–70) Sand, silt and clay, laminated Sand and gravel, stratified Peddicord Tongue Till, sand and gravel, laminated sand, silt and clay (0–35) Sand, silt and clay, laminated Robein Silt Fm (0–28) Organic-rich silty clay
QUATERNARY Glasford-Banner Till, sand and gravel, laminated sand, silt and clay PLEISTOCEN E Fm (0–375)
Wedron Fm (0–250) Joliet-Kankakee Dol (0–50) Dolomite, fine grained Elmwood Dol Dolomite, fine grained, cherty (0–50) ALEXAN - DRIAN SILURIAN Wilhelmi Fm (0–20) Dolomite; fine grained, argillaceous; shale, dolomitic a Shale, dolomitic; dolomite, fine to coarse grained, argillaceous
Maquoket (undiff.) (0–210) Wise Lake Fm CINCINNATIA N (120–150) Dolomitic limestone, fine to medium grained Dygerts K-bentonite Dolomite; some limestone, fine to medium grained,
Galena (“Trenton” ) Dunleith Fm- cherty, interbedded bentonites Guttenberg Dol (35–55) Dolomite, fine to medium grained, with red brown Quimby’s Mill- shaly laminae Nachusa Fm (50) Dolomite, fine to medium grained, slightly cherty Grand Detour- Dolomite, fine to medium grained, cherty
ORDOVICIAN Mifflin Fm (43)
Platteville Pecatonica Fm (38) Dolomite, fine to medium grained, cherty, sandy at base
CHAMPLAINIAN Glenwood Fm Sandstone, poorly sorted; silty dolomite and green shale St. Peter Ss. Sandstone, white, fine to medium grained, well sorted
Ancell (60–400) Shakopee Dol Dolomite, fine grained, New Richmond Ss Sandstone, fine to medium grained Oneota Dol Dolomite, fine to coarse grained Prairie du Chien (undiff.)
CANAD- IA N (0–400) Eminence Dol Dolomite, fine to medium grained, trace sand (20–150) and glauconite Potosi Dol (90–225) Dolomite, fine grained, trace sand and glauconite
Franconia Fm Sandstone, fine grained, glauconitic; green and red shale (75–150)
Ironton-Galesville Ss (155–220) Sandstone, fine to medium grained, dolomitic CAMBRIAN ST. CROIXAN Eau Claire Fm Sandstone, fine grained, glauconitic; siltstone, shale, (350–450) and dolomite
Mt. Simon Ss Sandstone, white, coarse grained, poorly sorted (1,400–2,600)
PRECAMBRIAN Granite, red
Figure 3 Stratigraphic column of northeastern Illinois (Bauer et al. 1991, Graese 1991). Column is not to scale. Red box indicates rock units encountered at the Conco Mine. Devonian and younger strata were eroded in post-Pennsylvanian times.
4 Circular 581 Illinois State Geological Survey emphasis in this study for determining GEOLOGIC SETTING and fault zones throughout the mid- the bulk depositional fluid properties continent are recognized: east-west to and overall fluid evolution. At MVT AND BACKGROUND northwest-southeast and north-south deposits, iron abundance in the form of To understand the controls on subsur- to northeast-southwest. Regionally and pyrite and marcasite is more variable face fluid migration and subsequent locally, major fault, fracture, and joint than Pb/Zn ratios (Leach et al. 1995, rock diagenesis, it is necessary to first systems trend northwest-southeast and Marie et al. 2001, Paradis et al. 2007) and review the local and regional geologic northeast-southwest. The Sandwich can therefore be used to understand ore setting of the Ordovician limestone at Fault Zone coincides with the Kankakee genesis. Differences in the fluid property the Conco Mine. Subsurface fluid flow Arch and follows this dominant regional requirements for marcasite and pyrite throughout the region is controlled by northwest-southeast trend (Figure 2). precipitation create parameters for the both structural and sedimentologi- The Sandwich Fault Zone is approxi- fluid flowing through the host rock and cal properties of the regional bedrock mately 135 km long and 1 to 3 km wide fractures. Distinct cyclical zoning of (Bethke 1985, Garven et al. 1993, (Kolata and Graese 1983). Throughout calcite, marcasite, and pyrite cements Luczaj 2006). Furthermore, much of most of this zone, rock formations are within the cavities create a temporal the regional subsurface hydrology has upthrown to the south with a maximum sequence relating to the composition been associated with extensive dolo- cumulative displacement reaching and source of the depositional fluids. mitization of Ordovician limestone, approximately 245 m in southeastern However, comprehensive analyses of the possibly related to carbonate-hosted De Kalb County. However, at the eastern overall history of deposition and diagen- sulfide mineralization (e.g., Anderson end, in Will County (Figure 2), the fault esis (paragenetic sequence) of a particu- and Macqueen 1982, Gregg 2004, Luczaj zone is downthrown to the south with a lar deposit have rarely been completed 2006). Therefore, the dolomitization his- total displacement of approximately 45 (Heyl et al. 1959, Hagni 1986). tory of the Ordovician limestone is also m (Kolata et al. 1978). On the northeast reviewed as an important contextual side of the fault zone, Paleozoic strata In the present study, three-dimensional framework for the present study. dip eastward into the Michigan Basin. mapping of field relationships has been Conversely, on the southwestern side, combined with detailed plane-light the strata dip southward toward the Illi- petrography (PL) and cathodolumines- Structural Geology nois Basin. cence (CL) analyses, environmental The research site for this study is located scanning electron microscopy (ESEM), in an underground room-and-pillar The Conco Mine site is located approxi- x-ray diffraction (XRD) analysis, elemen- limestone mine in North Aurora, Illi- mately 35 km northeast of the Sandwich tal abundance, and isotope analyses of nois, along the Fox River, approximately Fault Zone just off the northern flank the cement stratigraphy within solution 65 km west of downtown Chicago of the Aurora Syncline (Figure 2). The cavities. Results have been synthesized (Figure 1). The mine is located on the Aurora Syncline trends eastward across to establish a high-resolution parage- continental interior platform known as Kane County and then southeastward netic sequence with which to interpret the midcontinent region of the United through Kendall County and into north- the depositional and diagenetic history States (Sloss 1988, Marshak and Paulsen western Will County. The syncline is at the Conco Mine in relation to the 1996). The mine lies immediately east approximately 3 to 10 km wide and has sequence of mineral deposition within of the intersection of the Kankakee and a maximum relief of approximately 30 the solution cavities. This paragenetic Wisconsin Arches (Pirtle 1932, Ekblaw m on both contouring surfaces (Nelson sequence was then compared with 1938, Graese 1991). The northwest- 1995). Based on seismic reflection data sequences established for MVT deposits trending Kankakee Arch, a southeastern of poor quality, McGinnis (1966) inferred and similar studies of MVT mineral- extension of the Wisconsin Arch and a large fault just north of the Aurora bearing host rock, including (1) the Canadian Shield, began development at Syncline trending northwest-southeast Upper Mississippi Valley Zinc-Lead Dis- the close of the Canadian (Early Ordovi- through central Kane County. Other trict (UMV); (2) Ordovician limestone cian) time period, which resulted in the studies (Collinson et al. 1988, Heigold deposited along the eastern Wisconsin separation of the Michigan Basin to the 1990, Vaiden et al. 1988) found evidence Arch in the western Michigan Basin, and northeast from the Illinois Basin to the of smaller faults, but nothing near the (3) Trenton (Galena Group equivalent) south (Collinson et al. 1988). magnitude proposed by McGinnis hydrocarbon reservoirs in the southern (1966). However, those later studies were and southeastern Michigan Basin. The In addition to epeirogenic structures mostly localized and may have been high-resolution diagenetic interpreta- such as basins and arches, the region areally insufficient to recognize larger, tions have been used to reconstruct the is host to local fold and fault zones that more widespread structural features. A timing, composition, and source of the include brittle faults that offset Phanero- recent magnetic anomaly map of Illinois diagenetic fluids responsible for mas- zoic sedimentary stratigraphic contacts completed by the U.S. Geological Survey sive dissolution and mineralization of and monoclinal folds that locally tilt the (Daniels et al. 2008) suggests a scissored the Ordovician limestone at the Conco bedding (Marshak and Paulsen 1996) pattern of magnetic pull trending north- Mine. (Figure 2). Two dominant trends of fold west-southeast. The Aurora Syncline
Illinois State Geological Survey Circular 581 5 sits in a magnetic low and is parallel to exposed. The mine is currently operat- Peter Sandstone, a significant regional a magnetic high to the northwest. This ing in the Wise Lake and Dunleith For- aquifer (Figure 3). The St. Peter repre- magnetic high is followed by a magnetic mations of the Ordovician Galena Group sents a probable pathway for regional low to the northwest, similar to the (Figure 3). The distribution of the Galena fluid flow and may have played an magnetic low of the Aurora Syncline Group in northern Illinois reflects the important role in the transport of dia- (Figure 2). These magnetic anomalies truncation of the southeast-projecting genetic fluids and the development of may reflect deep crustal rock petrol- Wisconsin Arch (Willman and Kolata solution structures and massive mineral ogy, basement structures such as horst 1978) and the uplift of the Kankakee deposits at the Conco Mine. and graben or flower structures, and/ Arch. The Galena Group comprises a or significant deposits of iron sulfides nearly continuous succession (Tippe- In northeastern Illinois, the Galena (Sumner 1976). canoe sequence) of carbonate rocks Group is overlain by the Maquoketa (dolomite and limestone) deposited Group, which is primarily composed of Regardless, there is evidence for deep- on a platform in northeastern Illinois. a thick shale sequence (Figure 3). Many seated structural slip, as suggested by Galena Group rocks in northern Illi- studies have proposed that the contact the recent earthquake on February nois are dominated by lime mudstone between the Maquoketa and the Galena 10, 2010, with an epicenter located just and wackestone interbedded with thin represents a subaerial exposure after northwest of the Conco Mine at an beds of grainstone and packstone and the deposition of the Galena (Rooney estimated depth of approximately 5 to primarily represent a warm, shallow 1966, Taylor and Sibley 1986, DeHaas 11 km. Within the mine, small vertical marine environment characterized by and Jones 1988). However, more recent faults with centimeter-scale offset are low depositional energy (Willman and studies have concluded that the contact evident, including minor gauge within Kolata 1978). Conversely, rocks of the is a submarine hardground (Keith and fractures and trace slickensides along Galena Group in southern Illinois are Wilson1985, Keith and Wickstrom 1993, some horizontal clay-rich bedding dominated by grainstones and pack- Yoo et al. 2000, Kolata et al. 2001). The planes. As previously mentioned, the stones that are rich in bryozoans, bra- lowermost formation of the Maquo- most prominent features are fractures chiopods, and echinoderms, suggesting keta Group is the Scales Shale. The with no apparent offset but rather host deposition in zones of mixing where Scales Shale is a mostly dark gray to to massive solution cavities. With little cool, nutrient-rich waters encountered dark brown shale that creates an excel- to no vertical offset apparent on these warmer shelf waters (Kolata et al. 2001). lent impermeable seal over the Galena fractures, yet with the development of It has been suggested that North Amer- Group. The Galena Group is laterally massive solution structures, strike-slip ica remained mainly in low latitudes equivalent to the Trenton Limestone faulting or trans-tensional movement during continental migration during the in Michigan, Indiana, Ohio, New York, might be suspected (W. John Nelson, Paleozoic, thus favoring the deposition and Ontario and the Sinnipee Group in personal communication 2011). of the vast carbonate platforms (Leach et Wisconsin. al. 2001) found throughout the midcon- Stratigraphy tinent rock record. Dolomitization The midcontinent region of the United During the deposition of the Ordovi- Dolomitization of the Galena Group States consists of a relatively thin cian carbonates, a convergent plate (Trenton Limestone) has been widely veneer of Phanerozoic strata overlying boundary developed along the eastern studied and commonly related to MVT crystalline basement rock (Sloss 1988, coast of proto-North America, forming mineralization in the midcontinent Marshak and Paulsen 1996) (Figure continent-wide volcanoes situated in region (e.g., Budai and Wilson 1986, 3). The stratigraphy northeast of the an island arc system (Kolata et al. 1986). 1991; Haefner et al. 1988; Gregg 2004; Sandwich Fault Zone, on the southwest The volcanoes deposited widespread ash Luczaj 2006; Davies and Smith 2006). margin of the Michigan Basin, consists beds throughout the midcontinent and Regionally, the Galena Group is exten- of Precambrian igneous basement rock intermittently throughout deposition of sively to completely dolomitized; how- unconformably overlain by a sequence the Galena Group. Over time, diagenetic ever, within the Aurora Syncline, lime- of Cambrian to Silurian sedimentary alteration of the ash beds formed k-ben- stone facies have remained only par- deposits composed of sandstone, lime- tonite beds (Kolata et al. 1996) or illite- tially dolomitized (Graese 1991). Graese stone, dolomite, and shale. Devonian smectite clay-rich beds. Locally, benton- (1991) reported that the lack of dolomi- and younger strata covered the area but ite is encountered in the Wise Lake and tization in the Aurora Syncline implies were eroded post-Pennsylvanian times Dunleith Formations (Figure 3). that the syncline was actively down- (Kolata et al. 1978). Quaternary age gla- warped during deposition of the Galena, cial drift directly overlies the Silurian The Galena Group unconformably over- thus preventing dolomitization through bedrock (Figure 3) and generally ranges lies the Platteville Group (Kolata et al. a mixing-zone model. The Dorag model from 15 to 90 m thick (Kempton et al. 1998), which comprises mostly dolomite or mixing-zone hypothesis was first 1985); at the mine, however, drift thick- and cherty dolomite. The Galena and proposed as the cause of regional dolo- ness is less than 15 m. This thickness Platteville Groups unconformably overly mitization along the Wisconsin Arch by difference is occasionally the case in the Ordovician Glenwood Formation, a Badiozamani (1973). He indicated that river valleys where bedrock is typically shaly dolomitic sandstone, and the St. (1) all limestone to the east of the Wis-
6 Circular 581 Illinois State Geological Survey consin Arch is not dolomitized based on evolved Middle and Late Ordovician dolomitized but becomes less dolomi- one data point as a constraint and that seawater during the deposition of the tized or closer to a limestone-dolomite (2) Dorag dolomitization was focused Maquoketa Group shale. It was further interface west of the Wisconsin Arch exclusively along the arch. Badiozamani hypothesized that— during compaction (Heyl et al. 1959). Heyl et al. (1959) also (1973) concluded that Dorag-style dolo- of the shale— magnesium- and iron-rich notes a regional dolomitization event mitization resulted from the mixing fluid derived from illitization of smec- overprinted by a later-stage ore-related of seawater and groundwater in the tite in the shale (McHarque and Price dolomite cement lining fractures, vugs, phreatic zone positioned along the arch. 1982) was expelled moving laterally and cavities. Badiozamani (1973) further suggested along the top of the Galena, forming a that multiple generations of dolomite ferroan dolomite cap (Taylor and Sibley Studies along the margins of the Michi- precipitated during multiple episodes 1986, Yoo et al. 2000). Throughout most gan Basin (Shaw 1975, Budai and Wilson of marine transgression and regression of the Michigan Basin, the Trenton is 1986, Hurley and Budros 1991, Luczaj during the Middle to Late Ordovician limestone with a pronounced dolomite 2006) and along the northwestern time. cap (Taylor and Sibley 1986, Fisher et al. margin of the Illinois Basin at the UMV 1988), further supporting this hypoth- (Heyl et al. 1959) suggest a paragenetic Several studies cited problems with the esis. Graese (1991) suggested that the relationship between a late stage dolo- Dorag model (e.g., Hardie 1987, Land Galena cap dolomite does not occur mitization event and MVT mineral pre- 1998, Melim et al. 2004). Luczaj (2006) in Kane County except in the Aurora cipitation events. Furthermore, petro- noted many problems when comparing Syncline where it overlies a mostly lime- graphic, fluid inclusion, and geochemi- attributes of the hypothetical regional stone facies. Petrographic, fluid inclu- cal evidence supports a hydrothermal distribution of the mixing-zone model sion, and geochemical studies suggest fluid origin of the late-stage dolomite with extensive observations made of the that the third stage of dolomitization (Colquhoun 1991, Yoo et al. 2000, Smith actual regional distribution of dolomite was initiated along the margins of the 2006) and the MVT minerals (Haefner et in the Sinnippe Group (Galena equiva- Michigan Basin when hydrothermal, al. 1988, Luczaj 2006). lent) in eastern Wisconsin and north- saline fluids emerged from the deeper eastern Illinois. Luczaj (2006) observed part of the Michigan Basin (Coniglio et METHODOLOGY that regionally Ordovician limestone is al. 1994, Yoo et al. 2000, Luczaj 2006) or (1) completely dolomitized to the east of the Appalachian Basin (Haefner et al. Sampling the arch, regardless of structural posi- 1988). tion; (2) partially to completely dolomi- This project was undertaken to better tized along the arch; and (3) sporadically Deeper in the Michigan Basin, along understand the hydrologic and diage- dolomitized west of the arch. Luczaj the southern margin, hydrothermal netic controls resulting in the formation (2006) also found that dolomitized lime- dolomitization is concentrated along of massive solution cavities and subse- stone extends eastward well into the synclinal sags that are famous for quent mineral precipitation in the upper Michigan Basin. Luczaj (2006) concludes cavernous porosity such as the Albion- Ordovician Galena Group in northeast- that if there had been an early regional Scipio and Stoney Point oil fields (Figure ern Illinois. This investigation included dolomitization event, it has been over- 1) (Shaw 1975, Taylor and Sibley 1986, a structural, sedimentological, and printed by a later dolomite preserving Budai and Wilson 1986, Hurley and stratigraphic study with high-resolution a hydrothermal signature. Based on Cumella 1987, Hurley and Budros 1991, petrographic microscopy and geochemi- petrographic, fluid inclusion, and iso- Davies and Smith 2006). Albion-Scipio cal analyses to ultimately compile a topic evidence, Luczaj (2006) suggests and Stoney Point fields are character- detailed paragenetic sequence for the that a hydrothermal dolomitizing fluid ized by an en echelon series of faulted mineralization events that have filled sourced from the Michigan Basin is and fractured graben structures that solution cavities within the Conco Mine. genetically related to MVT minerals trend N30° W; the overall field trends The Conco Mine began as a surface on the western margin of the Michigan N15° W. Vugs, fractures, and caverns operation extracting Silurian age dolo- Basin. are typically lined with saddle dolomite mite (Figure 3). Due to surface permit- (Radke and Mathis 1980). Taylor (1982) ting constraints and depletion of near- Other detailed studies of the Trenton describes vertical fracture zones at the surface reserves, a decision was made to Limestone (Galena) throughout the Albion-Scipio Trend as dolomite dikes begin subsurface mining operations of Michigan Basin have revealed at least (Haefner et al. 1988). Fracture fill dolo- the Galena Group dolomitic limestone. three stages of dolomitization (Gregg mites are commonly associated with The thick overlying Maquoketa Group and Sibley 1983; Budai and Wilson 1986, anhydrite, calcite, pyrite, marcasite and shale (Figure 3) makes a structurally 1991; Taylor and Sibley 1986; Fara and trace amounts of later stage fluorite, sound roof and made tunneling or Keith 1988; Yoo et al. 2000); the first two sphalerite, and barite (Shaw 1975, Budai ramping down to the Galena feasible. stages are strongly to completely over- and Wilson 1986, Hurley and Budros The mine is divided into two locations printed by the third stage throughout 1991), suggesting a genetic relationship. based on property ownership: the North western Michigan Basin (Luczaj 2006). It should also be noted that the Galena Mine (owned by Lafarge) and the South Yoo et al. (2000) proposed that the first Group host rock in the UMV district Mine (owned by the City of Aurora). The two stages of dolomitization formed by in northwestern Illinois is primarily dimension of the North Mine is approxi-
Illinois State Geological Survey Circular 581 7 3 4
358 283 234 195 162 133 105 81 58 35 13 ‒9 ‒30 ‒52 ‒74 ‒97 ‒120 10 ‒145 9 ‒172 ‒201 ‒234 ‒273 Fractures ‒322 ‒397 Solution structures 0 100 200 300 ft N Studied solutions cavity 050 100 m nanoteslas
Figure 4 Mapped fractures and solution cavities in the Conco Mine overlying local magnetic anomalies. Subsurface fractures outlined in blue were mapped in the Wise Lake Formation in level 1 of the mine. Solution structures in red primarily coincide with mapped fractures. Note that mapped cavities do not include all cavities as mine personnel have only recently begun to map them. The cavity described in this study occurs on mine level 1. The roof of mine level 1 is approximately 80 m below the surface. mately 595 m long and 623 m wide, and 15 m wide and 15 m high with equally recently begun to be mapped (Figure the South Mine is approximately 736 measured support pillars. Due to limited 4). Many of the unmapped cavities were m long and 427 m wide. Both mines access and safety constraints within the blasted out during mining operations. are mined using the room-and-pillar mine, only one room in the North Mine Other cavities that are mapped were method and have two levels. Each level was accessible for the current research. not accessible due to mining activities contains numerous rooms and pillars; and/or the location of the cavity (above each excavated room is approximately It should be noted that many cavities bench levels or near the roof). However, exist throughout the mine and have only
8 Circular 581 Illinois State Geological Survey a Blocky calcite Host rock bedding plane
Cavity
b
Giant euhedral calcite crystals
50 cm
Figure 5 (a) The studied solution cavity showing its elliptical shape (with mine foreman for scale). (b) Giant euhedral calcite crystals line the cavity wall. during reconnaissance for an accessible of the room was chosen for the research to create a relative temporal frame- and representative cavity to complete due to its accessibility (Figure 5). work from which the depositional this study, observations were made of Another cavity exposed on the adjacent and diagenetic history of the rock are other cavities visited throughout the pillar is described, but the location near reconstructed (Fouke et al. 2005). The mine and are briefly reviewed in the the roof of the room made it inaccessible paragenetic sequence also provides a results for reference to the overall para- for sampling (Figure 6). framework to help constrain interpreta- genesis of the solution cavity formation tion of the geochemical data. In order and mineralization. One cavity exposed The paragenetic sequence is derived to construct a comprehensive para- within the east pillar on the floor level from the synthesis of observations genetic sequence, it was necessary to
Illinois State Geological Survey Circular 581 9 a Figure 6 (a) A two-dimensional sketch of the mine room containing the solu- tion cavity (lower left) shown in Figure 5. Samples were taken across verti- cal and horizontal transects based on Mine Roof field observations. Directly adjacent to the studied cavity is a collapse brec- cia. Above the breccia is a smaller solution cavity lined with calcite sca- lenohedrons. Both cavities and the 7.6 m collapse breccia are located along the same northwest-trending fracture. (b) Mine Room A three-dimensional sketch of the mine room of Figures 5 and 6a. The walls or pillars of the room are composed of the Wise Lake Formation, a dolomitic limestone. The room is approximately 15 m by 15 m. A linear vertical fracture Mine Floor penetrates the roof and down the pillars of the room intersecting the solution Wise Lake Collapse Calcite structures. These structures include the Breccia Formation studied solution cavity located directly below the Dygerts bentonite and an Dygerts Sulfide-Zoned Euhedral Sample adjacent smaller cavity located closer Bentonite Calcite Crystals Transects to the roof. Below the smaller cavity is a collapse breccia consisting of large blocks of calcite-spar in a marcasite- b and pyrite-rich sucrosic dolomite matrix.
systematically sample the cavity host rock and each of the mineral cements precipitated within the solution cavity. Thus, cements were first mapped within the cavity using physical observations of properties such as color, mineralogy, and cement geometries to differentiate temporal boundaries. Multiple cement generations were recognized based on these observations. Each individual cement generation was sampled accord- ingly (Figure 7). Ten samples were taken across two linear horizontal and verti- cal transects (Figure 5). These sampling transects accounted for each cement generation that grew horizontally and vertically within the cavity and also included samples of the host rock above the cavity and horizontally away from
7.6 m the cavity. Each sample was labeled and photographed in place before it was carefully extracted with a hammer and chisel. Each side of the extracted sam- ples was then photographed. Samples 7.65 m were then described before sawing rep- resentative samples approximately 25 Fracture Collapse Breccia Calcite mm wide, 40 mm long, and 10 mm thick Dygerts Sulfide-Zoned Euhedral for thin section preparation. Samples Bentonite Calcite Crystals were cut using an Edco TMS 10 tile saw. One to two standard, uncovered and
10 Circular 581 Illinois State Geological Survey a 2 cm x-ray diffraction (XRD), and environ- mental scanning electron microscope (ESEM), mass spectrometer, and elec- tron microprobe analysis.
X-ray Diffraction Following petrographic description, XRD was used to identify the mineral- ogy of each sulfide zone. Analysis was completed at the 3M Materials Chem- istry Laboratory at the University of Illinois (http://chemistry.illinois.edu/ about/facilities/x-ray/introduction_to_ service_facility.html). A 100-µm drill bit was used to sample individual sulfide zones embedded in calcite crystals. b 1 cm The XRD data were then collected on a Bruker General Area Detector Diffrac- tion System (GADDS) equipped with a P4 four-circle diffractometer and HiStar multiwire area detector.
Environmental Scanning Electron Microscopy Based on the XRD results, individual sulfide zones were imaged to determine paragenetic ordering of the minerals and the mineral morphology within each sulfide zone. Back-scattered imag- ing was completed on an ESEM at the Microscopy Suite at the Beckman Insti- tute, University of Illinois at Urbana- Champaign (http://itg.beckman.illi- nois.edu/). Samples were prepared by breaking calcite crystals along sulfide Figure 7 Photographs of a representative euhedral calcite crystal lining the zones. A few pieces of calcite with each solution cavity wall (Figure 5b). (a) Large calcite crystal from studied cavity. individual and specified sulfide zone The prominent crystal face is calcite zone C-7 in the paragenetic sequence, were then bathed in a 5.0 pH acetic acid and the sulfide zone exposed on top is sulfide zone S-7. (b) Cross section of solution buffered with sodium acetate. the same crystal, displaying high-frequency calcite-sulfide zones. Samples were monitored every 12 hours in order not to dissolve all calcite and to only partially expose sulfide crystals. polished thin sections, approximately 30 CITL cold cathode luminescence 8200 Samples were bathed for approximately µm thick, were prepared from each hand MK3 stage (operating at 12 to 15 kV and 60 hours before removal. Samples sample. Thin sections were impregnated 550 µA) at the University of Illinois at were then washed with Millipore puri- with a clear epoxy. All thin sections were Urbana-Champaign. An Optronics DEI- fied water, dried, and mounted with prepared by Wagner Petrographic in 750 three-chip HCCD thermoelectroni- carbon tape to an ESEM stage plate. Lindon, Utah. cally cooled camera was used to capture Samples were sputter-coated with Au/ and manipulate the low-light CL images Pd for approximately 70 seconds. Back- Plane-Light Petrography and directly (Fouke and Rakovan 2001). scattered images of sulfide crystals were Solution cavity limestone host rock Cathodoluminescence taken on a Phillips XL30 Field-Emission was described and classified using the Environmental Scanning Electron The PL and CL analyses were completed Dunham (1962) classification scheme. Microscope. Images were compared on each thin section to determine the High-frequency sulfide and calcite with petrographic observations and paragenesis of each sample. Polished cementation events were paragentically XRD results to determine the mineral- thin sections were examined using a identified and further described using ogy and paragenesis of each sulfide zone. petrographic, cathodoluminescence,
Illinois State Geological Survey Circular 581 11 Radiogenic Isotope Relative Timing Paragenetic Event Early Late Analyses A. Mixed Fossil Wackestone-Grainstone 87 86 Strontium ratios ( Sr/ Sr) were used 1. Deposition 2. Dissolution and Replacement ? to constrain the source of the diage- 3. Calcite Cement (C1) ? 4. Burrowing ? netic fluids and subsequent water-rock 5. Matrix Dolomitization ? 87 86 6. Burrow Dolomitization ? interaction. Analyses of Sr/ Sr ratios 7. Calcite cement (C2) ? were completed on the cavity cements 8. Fracturing 9. Dissolution (Cavity Formation) and compared with regional formation 10. Dolomitization water to further constrain the source of 11. Sulfide Cement (S1) Marcasite-Pyrite 12. Calcite Cement (C3) the diagenetic fluids. 13. Dissolution 14. Sulfide Cement (S2) Marcasite-Pyrite 15. Calcite Cement (C4 ) These analyses were completed prior 16. Sulfide Cement (S3) Marcasite-Pyrite to the beginning of this study and, * Sphalerite ? ? 17. Calcite Cement (C5) therefore, are limited to three calcite 18. Sulfide Cement (S4) Marcasite-Pyrite zones that were sampled prior to the 19. Sulfide Cement (S5) Marcasite-Pyrite 20. Calcite Cement (C6) development of the detailed parage- 21. Sulfide Cement (S6) Marcasite-Pyrite 22. Dissolution netic sequence. Samples were prepared 23. Sulfide Cement (S7) Marcasite-Pyrite and run at the University of Illinois at 24. Siderite 25. Calcite Cement (C7) Urbana-Champaign Geology Depart- 26. Dissolution 27. Hydrocarbon Migration ment, using an NU Plasma HR multi- ? ? collector inductively coupled plasma B. Bentonite? 1. Deposition mass spectrometer (MC-ICP-MS; 2. Illitization ? http://www.geology.uiuc.edu/about/ ICPMS/index.html). Three calcite zones were microdrilled using a 100-µm Figure 8 Paragenetic sequence of the solution cavity and the surrounding host drill bit, and microsampled powders rock. The paragenetic events forming the solution cavity and mineralization within were collected. Approximately 20 mg are emphasized by larger fonts. of powered sampled was weighed and added to 500 µl of 3 N nitric acid ±2E-05. Finally, a blank sample was Samples were reacted with 100% phos- using a clean pipette. Upon dissolu- screened to check for contamination phoric acid at 70°C. Reproducibility for tion, the samples were centrifuged for 5 during preparation, and Sr, Rb, Ca, Ba, δ13C and δ18O is typically less than ±0.1‰ minutes at 300 rpm to separate undis- and Kr concentrations were commensu- and ±0.15‰, respectively. Six standards solved, suspended particles from the rate with background concentrations. were run with the sample set, and dupli- solution such as sulfide minerals. Then cates were run on one sample. 300-mL samples were loaded into col- umns containing preconditioned 0.25 Stable Isotope Analysis strontium-specific resin. One microliter 13 18 Elemental Analyses Theδ CVPBD and δ OVPBD isotopes are of 3 M nitric acid was passed through used to further constrain the composi- Electron microprobe analysis (EMPA) the column followed by 2 µl of 3 M nitric tion and source of the diagenetic fluid. was completed at the Electron Micro- acid added three times. Strontium was Theδ 13C and δ18O values are reported probe Laboratory, Department of then collected by passing 2 mL of 0.05 M relative to the Vienna Peedee belemnite Geology and Geophysics, University nitric acid through the columns twice. (VPDB) reference standard as calibrated of Minnesota,Twin Cities. Elemental The solution was then evaporated until through analysis of NBS-19 with values abundances were obtained for Ca, Na, it was dry. The dried samples were then of 1.95‰ and –2.2‰, respectively Sr, Fe, Mg, and Mn for the cavity calcite dissolved in 2 mL of 2% nitric acid for 13 (Coplen et al. 1983). The δ CVPBD and cements by EMPA operating at 15 kV and MC-ICP-MS analysis. Samples were 18 δ OVPBD analyses were completed on five 5 nA with a beam diameter at 40 µm and screened for concentration and intro- calcite cement zones within the solution obtained for Fe, S, Pb, Zn, and Cu for the duced MC-ICP-MS using a desolvating cavity using three samples from each cavity sulfide cements by EMPA oper- nebulizer in auto-sampling mode using zone except one zone where four were ating at 15 kV and 20 nA with a beam Ar as the sweep gas. For quality control, used. Sample powders were acquired diameter at 5 µm. Estimated precision is duplicates were run on each sample, by microdrilling the hand samples expressed as 95% confidence limits and yielding a precision of approximately described in the preceding sections. is typically less than 2 wt% for Ca, Mg, ±2E-05. The precision of an unprocessed Sample powders were analyzed at the 3 wt% for Fe and S, and 4 wt% for Na, Sr, (SRM 979) standard was approximately Illinois State Geological Survey by the Mn, Pb, Zn, and Cu. Analyses were com- ±2E-05. Also, a processed coral standard Isotope Geochemistry Section using pleted on the same thin sections pre- was run and was 2.3E-06 lighter than a Finnigan Mat 252 isotope ratio mass pared for petrographic analyses. Analy- 87 86 expected ( Sr/ Srcoral = 0.70919). Preci- spectrometer with an attached Kiel III ses were completed on linear transects sion of reported results is approximately Individual Acid Bath Carbonate Device. every 100 µm across the calcite zones
12 Circular 581 Illinois State Geological Survey identified by PL and CL petrographic syntaxial calcite (Figures 9c, d). Trace Structural Geology, Solution analyses. Three points were analyzed amounts of fine, rounded quartz grains Cavities, and Mineralogy for each sulfide zone. Sulfide zone (S2) are also present. Dolomite is found in A14 was not available for analyses due to the host rock laterally surrounding the Fractures with consistent northwest- loss of sample. Elemental abundances cavity and above the cavity. Dolomite in southeast and northeast-southwest ori- were averaged for each mineral zone. the host rock is primarily a microcrys- entations were mapped in detail within All results were compiled to reconstruct talline dolomite that has replaced the the Conco Mine by the mine engineer the paragenetic sequence (Figure 8), mud matrix (Figure 10a). In the host rock and foreman (Figure 4). Solution struc- describe diagenetic cementation events above the cavity, nonplanar dolomites tures are exclusively located along (Table 1), and interpret the results in (Figure 10b) are confined exclusively fractures; the majority of meter-scale the context of multiple diagenetic fluids to slightly branching trace fossil Chon- solution cavities are found along north- migrating through the host Ordovician drites or Thalassinoides burrows (Figu- west-southeast trending fractures. The limestone. re10c, d). Host rock both above and lat- first occurrence of meter-scale solution erally surrounding the cavity contains structures are found in the first level of trace amounts of planar to subplanar the mine, which has a breast back (roof) RESULTS dolomites that are primarily confined to elevation of approximately 131 m and a bench level (floor) elevation of approxi- Sedimentology and areas of dissolution (Figure 10e). These dolomites are commonly partially dis- mately 116 m. Solution structures have Stratigraphy solved, slightly replaced with sulfide been noted in the roof and all the way The Galena Group has regionally been cements, and completely surrounded by down to the floor along fractures. Solu- subdivided into three formations, which calcite cement (Figure 10e). Microfrac- tion structures have also been found include (1) the Guttenberg Formation tures cemented with calcite are common in the second level of the mine but are at the base, (2) the Dunleith Formation throughout the host rock surrounding inaccessible due to mining operations. in the middle, and (3) the Wise Lake the cavity (Figure 11a, c). Two general types of solution structures Formation at the top (Figure 3). These have been identified throughout the formations are relatively continuous Directly above the solution cavity, mine: (1) open-spaced cavities and (2) and uniform in composition throughout separating cavity cements from the host closed-spaced solution structures. This northern Illinois (Kolata et al. 2001). rock above, is a clay bed (Figure 11b, study focuses on the open-spaced cavi- Currently, the mining operations at the d). The clay bed lies directly beneath a ties and mineralization within those Conco Mine occur in the Wise Lake prominent bedding plane at the base of cavities; extensive analyses were com- Formation. The solution cavity studied the host rock above the cavity (Figure pleted on a single cavity that was easily is located on the south end of the North 12a, b). Laterally, away from the cavity, accessible. Brief descriptions of other Mine, approximately 93 m below ground the clay bed is difficult to trace but solution cavities and closed-space solu- surface and 28 m below the top of the appears to be a consistent bed less than tion structures throughout the mine are Wise Lake Formation (Figure 3). Host 5 mm thick. Other cavities are observed given for diagenetic context to the over- rock laterally surrounding the cavity is throughout the mine at the same depth all paragenesis at Conco. beneath this clay bed (12c, d). The solu- primarily a medium- to course-grained The mined-out room chosen for this wackestone (Figure 9a) with localized tion cavity and the cavity cements terminate at the base of this clay bed study contains a prominent northwest- thin beds of course grainstone (Figure southeast–trending fracture (Figure 4). 9b). The grains constituting the wacke- (Figures 11b, d and 13b), and centimeter- scale Thalassinoides burrows in the Along this fracture, in the north pillar stone are primarily echinoderm frag- of the room, is a highly brecciated lime- ments with rare to moderately common host rock above the cavity terminate at the top of the clay bed (Figure 12b). stone (Figure 6), a closed-space solution skeletal fragments of brachiopods, dasy- structure. The breccia is composed of cladacean green algae, pelecypods, and The clay along this boundary is slightly expanded, especially in the top corner tens of centimeter to centimeter-scale bryozoans (Figure 9a). The matrix also clasts of calcite; centimeter-scale clasts contains trace amounts of fine-grained boundaries of the cavity, and contains micrometer- to centimeter-scale pieces of dolomitized host rock; and marca- rounded quartz grains. The grainstone site, pyrite, and calcite cements. Miners is composed primarily of echino- of dolomitized host rock (Figures 11b and 12c). The clay is generally a light commonly refer to the breccia as “leop- derm fragments and is typically well ard rock” due to the pattern appear- cemented with blocky calcite cements gray to light blue-green color and exhibits a distinct blue-colored cath- ance of iron sulfide precipitated around (Figure 9b). The host rock just above angular dolomite clasts. The breccia is the cavity is a thin grainstone bed com- odoluminescence (Figure11b). Fractures where cavities did not develop but that approximately 2 m thick and appears as posed primarily of medium- to course- a pipe focused along the vertical frac- grained echinoderm fragments with penetrate this clay bed are observed throughout the mine and are commonly ture. Approximately 5 m up along the trace amounts of dasycladacean green breccia pipe and near the room roof is a algae, pelecypod, brachiopod, and bryo- filled with an expanded light gray to light blue-green clay. solution cavity lined with euhedral cal- zoan fragments cemented by blocky and cite crystals. The cavity is approximately
Illinois State Geological Survey Circular 581 13 tabular, and rhombohedral Equant blocky calcite hairline zoning Marcasite: distorted tabular rhombohedrons; pyrite: distorted stacked octahedrons Marcasite: tabular and acicular; pyrite: octahedral and pyritohedral cuboctahedral and pyritohedral Marcasite: tabular and bladed; pyrite: cuboctahedral, pyritohedral, and cubic zoning dark to light Marcasite: radiating blades; pyrite: cuboctahedral Highly complex, distorted, and commonly twinned Rhombohedral (in studied cavity), scalenohedral in other cavities Nonplanar, curved faces, rhombohedral Marcasite: irregular, tabular and prismatic, commonly twinned; pyrite: octahedral and pyritahedral Equant blocky calcite Marcasite: Interconnected spear-shaped coating, NA NA gradual zoning dark to light NA lamental, acicular; pyrite: Marcasite: fi Light purple to yellow- Microcrystalline Light purple, Purple to dark orange, Equant blocky and syntaxial calcite Dark purple, nonzoned Nonplanar, equant orange, nonzoned Dark to light purple, Microcrystalline NA nonzoned
CL color/ character Crystal morphology and gross crystal form Clear-milky gray Dark purple to dark red, Equant blocky calcite Brassy gold NA NA NA NA Gold-brown NA NA cloudy white Irridescent red- Translucent dark NA NA Clear NA NA
Yellow-orange Clear NA
Gold-dark brown NA to cloudy white
thickness or crystal thickness (cm) Color
0.500 0.001
cement blocky replacement replacement cement
pyrite to crystalline cement pyrite cement pyrite pyrite to crystalline cement Sphalerite Crystalline
A20 (C6) pyrite cement Calcite Blocky cement 0.400 Paragenetic sequence Mineralogy Component maximum Table 1 Paragenetic table outlining and describing all diagenetic mineralization events at the Conco Mine. Zone A14 (S2) Marcasite- Replacement bladed 0.035 A18 (S4) Marcasite- Bladed to crystalline A19 (S5) 0.050 Marcasite- Bladed to crystalline 0.045 A21 (S6) Marcasite- A23 (S7) Bladed to crystalline Marcasite- 0.040 Replacement bladed 0.050 A15 (C4) Calcite Blocky cement 10–35 Dark yellow to Purple to red, gradual Equant blocky calcite A3 (C1) Calcite Mycrocrystalline- NA A16 (S3) Marcasite- Bladed to crystalline pyrite cement 0.040 A17 (C5) Calcite Blocky cement brown-purple-red purple-blue-green 0.050
A7 (C2) Calcite Blocky-syntaxial NA A5 A6 replacement Dolomite nonzoned Mycrocrystalline Dolomite Nonplanar cement <0.001 0.030 NA A24 Siderite Crystalline A25 (C7) Calcite Blocky cement 0.5–2
A11 (S1) Marcasite- A12 (C3) Replacement bladed pyrite cement Calcite 0.030 Blocky cement 38–102 Dark yellow Deep purple
14 Circular 581 Illinois State Geological Survey a c
A7
A3 A3 500 m 200 m
b d A11
A3 Quartz A7
A3
A7 200 m 200 m
Figure 9 Plane-light petrography (PL) and cathodoluminescent (CL) photomicrographs of solution cavity host rock including paragenetic sequence events (A3, A7, A11). (a) PL photomicrograph of wackestone composed of microcrystalline matrix and medium to coarse grains of echinoderm, green algae, pelecypods, bryozoan, and brachiopod fragments. (b) PL photomicro- graph of grainstone composed primarily of echinoderm fragments cemented by blocky and syntaxial cement. (c) PL photomi- crograph of grainstone with sulfide cement over earlier blocky and syntaxial calcite cement. (d) CL photomicrograph of previ- ous photomicrograph in c displaying two generations of calcite cement overprinted by later sulfide cement. Trace amounts of rounded quartz grains are also common throughout the host rock.
1 m in width and height and contains estimated its depth at approximately 30 intergrown, rhombohedral calcite crys- primarily tens of centimeter-scale euhe- m. Only the top few meters of the cavity tals have grown directly over the blocky dral calcite scalenohedron crystals with were accessible for this research. The calcite cement. A closer examination of micrometer- to millimeter-scale zones of apex of the solution cavity is approxi- all calcite cements precipitated within marcasite and pyrite. This cavity was not mately 1 m above the level 1 floor and the cavity revealed calcite growth zones researched due to its location near the opens up in the south pillar or wall of separated by sulfide mineral zones mine roof. the room (Figure 6a). The top of the (Figure 7). cavity has an elliptical shape measur- Adjacent to the breccia pipe and smaller ing approximately 3 m × 2 m (Figure 5); To build a contextual framework of the cavity and trending along the same however, the original width before exca- diagenetic events at the Conco Mine, it fracture is a large solution cavity that is vation was probably more. The cavity is necessary to briefly describe the other the main focus of this study (Figures 5 walls are completely lined with calcite structures noted throughout the mine. and 6a). The cavity descends vertically and sulfide cements (Figures 5 and 7). Also, because this is the first extensive to an unknown depth. Prior to filling The calcite crystals are thick, blocky, study of the solution structures and in the cavity for safety reasons, miners and often fractured (Figure 5). Euhedral, mineralization at Conco, it is necessary
Illinois State Geological Survey Circular 581 15 a d A10
A12
A5
A11
200 m 200 m
B e
A11 A3 A6
A6
A27
200 m 200 m
C f
A11 A7
A12
A4 Host Rock 200 m 1,000 m
Figure 10 Plane-light petrography (PL) and cathodoluminescent (CL) photomicrographs of rock including paragenetic sequence events (A3, A5, A6, A10, A11, A12, A27). (a) PL photomicrograph of nonplanar microcrystalline dolomite matrix. (b) PL photomicrographs of non-planar dolomites with trace bitumen inclusions infilling burrows (c) in wackestone. (d) CL photomicrograph of burrow dolomites. (e) PL photomicrograph of planar dolomites and sulfide crystals concentrated along dissolution boundary and cemented by blocky calcite. Dolomites are partially dissolved and slightly replaced by a later sulfide cement. (f) “Floating” host rock suspended in the cavity calcite cement C3 and coated with sulfide cement S1.
16 Circular 581 Illinois State Geological Survey a c
A12
Fracture
200 m 200 m
b d
B1, B2
Bentonite
A12
Cavity
A7 A3
A3 Host Rock A7
200 m 200 m
Figure 11 (a) Plane-light petrography (PL) photomicrograph of a fracture crosscutting wackestone and cemented with the first cavity cement (C3) A12. (b) PL photomicrograph of the top edge of the cavity displaying clay bed (Dygerts bentonite), cavity calcite cement (C3) A12, and wackestone host rock. (c) Cathodoluminescent (CL) photomicrograph of a fracture cross- cutting wackestone and cemented with the first cavity cement (C3) A12. (d) CL photomicrograph of correlating photomicro- graph in part b. The clay exhibits distinct blue CL and includes clasts of host rock near its base. The host rock exhibits two distinct calcite generations. Cavity is cemented by first cavity calcite cement (C3) A12.
Illinois State Geological Survey Circular 581 17 a c Fracture
Bentonite 25cm
b d
Bentonite Clay Bentonite 1cm
Figure 12 Photographs of the top east corner of the studied solution cavity (a, b) and another solution cavity within the mine (c, d). (a) Zak Lasemi standing in front of the solution cavity; its top terminates at a prominent bedding plane. (b) Close-up photograph of the same bedding plane shows a slightly swelled Dygerts bentonite with clasts of limestone between the bed- ding plane and the cavity cements. (c) A different cavity in the north mine shows the cavity (bottom right corner) is focused along a vertical fracture and top of the cavity terminating at the Dygerts bentonite bed. (d) Millimeter-scale Dygerts bentonite bed above cavity. to briefly describe observed structures However, most scalenohedrons include cipitated is typically highly dolomitized. and subsequent mineralization for their co-genetic and zoned sulfide minerals Dolomitic host rock is sometimes highly scientific significance. Brief visits to (pyrite and marcasite) that sometimes cemented with calcite and other times is other open-spaced cavities throughout exhibit slight iridescent coloring. Sca- poorly cemented to completely uncon- the mine suggest that the extent of min- lenohedrons are commonly zoned solidated and appears as a sucrosic eralization and crystal growth habits with sulfide mineralization exhibiting dolomitic sand. Miners commonly refer vary within each cavity. The solution each calcite generation accentuated by to it as “sugar” when encountered. Old- cavities that are more laterally extensive sulfide overgrowth. Early generations time drillers drilling oil and gas wells in than vertically extensive typically are of calcite growth within the scalenohe- the late 1800s in the Giant Lima Trenton dominated by euhedral calcite crystals dron commonly exhibit rhombohedral Field in eastern Indiana and western growing in a scalenohedral habit (dog- growth habits overlain by a later scale- Ohio referred to this type of dolomite tooth). Single calcite scalenohedrons nohedral calcite growth habit. Calcite as “sugar sand”. It was considered one have been found that are up to 75 cm tall and sulfide mineralization is sometimes of the best reservoir facies in the Lima and 25 cm wide. Some scalenohedrons concentrated around preferentially Trenton Field (Dennis Prezbindowski, appear to be composed of one single dolomitized cylindrical burrows. This personal communication 2011). Dolo- calcite generation varying in character- phenomenon will be documented and mitization is concentrated along the istics from an extremely high luster cal- discussed in a future publication. The vertical fractures leading to the solution cite to a dull and typically etched calcite. host rock on which the crystals are pre- cavities, and dolomitization appears to
18 Circular 581 Illinois State Geological Survey be less above the cavity. The majority Sphalerite crystals are highly complex, (Figure 9a), including deposition of of fractures, if not all, especially in the distorted, and commonly twinned. localized thin beds of fossiliferous grain- south mine, have similar dolomitiza- Sphalerite crystals have been noted up stone (Figure 9b). A clay zone described tion concentrated along fractures, even to 7 mm in size. as altered bentonite was deposited where solution cavities did not develop. following deposition of the host rock Other closed-space solution structures laterally surrounding the cavity, with The cavities that are more vertically found throughout the mine include subsequent grainstone-wackestone extensive contain more massive blocky dolomitized fractures or pipes. These deposition. All wackestone comprises a calcite with intergrown rhombohedrons are solution-enhanced fractures where mud matrix containing echinoderms, and modified rhombohedron-scaleno- preferential dolomitization along the green algae, bryozoans, brachiopods, hedron combination euhedral crystals. fracture occurs up to 2 m laterally away and pelecypods (Figure 9a). Following Euhedral calcite rhombohedrons and from the fracture, giving the appearance deposition, dasycladacean green algae modified rhombohedron-scalenohedron of a pipe. Dolomite is a fine- to medium- and pelecypods internal structures combinations are typically deposited grained crystalline sucrosic dolomite were partially dissolved (A2) and later over blocky calcite cement up to 1 m that is typically poorly cemented; cemented by calcite (C1) A3 (Figure 9a). thick and less commonly growing however, some areas appear to be well The wackestone mud matrix was also directly on the host rock, unlike the sca- cemented by calcite and sulfide miner- partially cemented by microcrystalline lenohedron dominant cavities. Single als. These dolomite “pipes” are com- calcite (C1) A3 undifferentiated from rhombohedron and combined rhom- monly highly oxidized, and miners have calcite internally cementing fossils bohedron-scalenohedron crystals have reported that they occasionally leak (Figure 9c, d ). Most of the echinoderm, been found up to 90 cm in length. Mas- fluids. Occasional open-spaced meter- brachiopod, and bryozoan fragments sive sulfide deposits are also found in scale cavities occur at top of these pipes, retain some original skeletal structures solution cavities. These include tens-of- much like the breccia pipe and the small that are also altered to microcrystalline centimeter–scale marcasite and pyrite solution cavity adjacent to the studied neomorphic calcite (C1) A3 as suggested clusters and are typically deposited solution cavity (Figure 6). In the South by CL (Figures 9c, d ). Fossils are later directly on the cavities’ host rock. Most Mine, fractures with preferential dolo- cemented with blocky and syntaxial notable are iridescent marcasite and mitization are common with trace to no calcite cement (C2) A7 (Figures 9b, c, d). pyrite stalactites growing in large finger- calcite or sulfide cements. A7 (C2) exhibits brighter CL than A3 (C1) like clusters. Single stalactites have been and is typically overprinted with sulfide found that are up to 12 cm long and 4 Paragenetic Sequence cement (S1) A12 (Figure 9d). cm wide. Rarely, a few stalactites have a small inner core. Most stalactites have A high-resolution paragenetic sequence Some intervals of the host rock wacke- a structure radiating from the center of the solution cavity and surrounding stone mud matrix are partially to com- of the stalactite, which is a common host rock was reconstructed (Figure pletely replaced by microcrystalline growth habit of marcasite. Green, teal, 8). The paragenetic sequence reflects a dolomite (A5, Figure 10a). Burrowing purple, blue, and silver-black marcasite temporal framework of the diagenetic of host rock (A4) above the clay bed and is found in millimeter-scale bars and history of the solution cavity studied. likely below occurred following the centimeter- to tens-of-centimeter–scale Twenty-nine paragenetic events, includ- deposition of the host rock. Non-planar colloform masses precipitated on ing deposition and subsequent diagen- dolomites (A6, Figure 10b) occur exclu- dolomite matrix. Pyrite is rarer and is esis, have been identified. One mineral sively in burrows (Figure 10c). Burrows sometimes found in centimeter-scale event is pending. Sphalerite has yet to do not appear to crosscut fossils or clusters of distorted cubes and other be identified in place and has not been calcite cement A3 (C1) and A7 (C3) but times as millimeter-scale octahedrons found in the studied cavity. However, exhibit a darker, less intense CL emis- with a thin copper-red patina coating sphalerite has been found in other cavi- sion relative to the surrounding calcite that appears to be a result of oxidation. ties, and its paragenetic placement in cements (A3 and A7, Figure 10d). Hydro- However, this coating has not been ana- context to the studied cavity is inferred carbon (A27) inclusions are found exclu- lyzed. One previous cavity encountered by multiple calcite and sulfide zones sively in burrow dolomites (A6, Figure by the miners was nearly barren of min- found in situ with the sphalerite. The 10b). However, hydrocarbon coating of eralization except for a few specimens solution cavity mineralization events mineralized vugs is noted in the top 1.5 of massive colloform iron-sulfides, a constituting the bulk of the paragenetic m of the Wise Lake Formation in a drill thin coating of millimeter-scale “ruby sequence are outlined and described in core examined from the South Mine. jack” sphalerite crystals, and partially Table 1. The stratigraphic distribution of Fractures (A8, Figure 12a) crosscut host dissolved or “skeletal” calcite scaleno- each paragenetic event with respect to rock, including calcite cement (C1 and hedrons zoned with sulfides and up to 6 the solution cavity is used to establish C2) A3. Dissolution (A9) is primarily cm in length. A specimen was recently the relative timing of diagenesis. focused along fractures (Figure 11a), but obtained from this cavity confirming the also occurs within some areas of unfrac- The host rock surrounding the cavity mineralogy. One other specimen owned tured host rock (Figure 10e). Planar and was deposited primarily as a wackestone by a miner was identified as sphalerite. subplanar dolomites A10 are typically
Illinois State Geological Survey Circular 581 19 500 m 200 m 200 m
A9, A11 (S1) A13, A14 (S2) A16 A18 A19 A21 A22, A23 (S3) (S4) (S5) (S6) (S7) Cavity Host Rock Pore Space
A12 (C3) A15 (C4) A17 (C5) A20 (C6) A25 (C7)
Figure 13 Scanned images of thin sections and correlating cathodoluminescent images reflecting underexaggerated lateral cross section from host rock-cavity dissolution boundary through the cavity cements. All cavity sulfide and calcite zones are represented in cross section. associated with these dissolution zones by the precipitation of the calcite zone Calcite zone (C3) A12 follows precipita- (Figure 10e). Planar and subplanar dolo- (C3) A12 (Figures 10f and 13). “Float- tion of the sulfide zone (S-1) A11 and is mites along dissolution zones are later ing” clasts of host rock are commonly the thickest cement precipitated within partially replaced by sulfide cement found along the dissolution boundary the cavity, ranging in thickness from (S1) A11 (Figure 10e), followed by calcite of the solution cavity and are partially 38 to 102 cm. The calcite zone is very cementation (C3) A12 (Figure 10e). All cemented with sulfide cement (S1) A11 blocky, is highly fractured, and has a fractures (Figure 12a) and dissolution and completely cemented by calcite yellow to cloudy white color with limited zones (Figure 10e), including the solu- cement A11 (Figure 10f). luster and clarity. A12 (C3) first precipi- tion cavity, are partially cemented with tates as a yellow color and grades into a sulfide cement (S1) A11 (Figures 10e, Sulfide zone (S1) A11 (S-1, Figure 14) is more cloudy white color near the end. f) and completely cemented by calcite composed of marcasite (S-1, Figure 16), Overall, A12 (C3) maintains a low dark cement (C3) A12 (Figure 10e, f; Figure which is the first sulfide to precipitate purple CL intensity (Figures 11d and 13). 12a, b and Figure 13) postdissolution. followed by epitaxial pyrite (S-1′, Figure 16). Marcasite crystals are tabular to A12 (C3) is terminated by dissolution The solution cavity is host to multiple prismatic, typically are twinned, and surface A13 and deposition of sulfide generations of diagenetic events (Figure commonly have irregular or distorted zone (S2) A14 (S-2, Figures 13 and 14). 13) consisting of distinctive sulfide habits. The marcasite forms a partial A14 (S2) begins with marcasite precipi- cements (Figures 13 through 18), cal- coating over the host rock up to 300 µm tation on the dissolution surface of A12 cite cements (Figure 13), and dissolu- thick but is not continuous along all followed by epitaxial pyrite (S-2, Figure tion (Figure 13). Post dissolution (A9), limestone surfaces of the cavity. Epi- 16). A14 forms a continuous coating of after solution cavity formation, the first taxial pyrite habits include octahedrons 350-µm-thick marcasite over A12 (C3). cement precipitate is the sulfide zone and pyritohedrons. The coating consists primarily of irregu- (S1) A11 (Figures 10f and 14), followed lar, twinned, flattened rhombic prisms
20 Circular 581 Illinois State Geological Survey S-1 S-3
500 m 200 m
S-2 S-4
200 m 200 m
Figure 14 Plane-light photomicrographs of sulfide zones S1 through S4 within the cavity (Figure 13). S-1, the first and earli- est sulfide cement (S1) A11 partially coating host rock-cavity dissolution boundary; S-2, sulfide cement (S2) A14 precipitated on A13 dissolution surface of calcite cement (C3) A12; S-3, sulfide crystals (S3) A16 precipitated along symmetric crystal growth plane of calcite zone (C4) A15; S-4, multiple zones of sulfide crystals precipitated along symmetric growth planes of calcite zones (C4 and C5) A15 and A17. of marcasite growing laterally over A12 cloudy white color near the end. Relative calcite growth plane (S-3, Figures 13 (C3). Some marcasite clusters are exam- to A12 (C3), A15 (C4) is not terminated and 14). A16 (S3) marks the beginning of ined growing over the coating; these by a dissolution surface, but instead the euhedral calcite crystal growth and consist of large clusters of irregular, is divided by multiple zones of sulfide is the first sulfide to precipitate within tabular rhombic prisms (S-2, Figure 16). precipitates (S3 and S4) A16 and A18 the crystals (Figure 7b). A16 (S3) is com- Epitaxial pyrite exhibits pyritohedrons (S-3, Figures 13 and 14) and CL variation posed primarily of marcasite, exhibiting and octahedron habits (S-2′, Figure 16). with the overlying calcite cement (C5) well-formed blades to irregular cocks- All pyrite crystals are approximately 50 A17 (Figure 13). A15 maintains a steady comb blades (S-3, Figure 16). Epitaxial µm long. moderate to low purple-red CL intensity pyrite cuboctahedrons are common on until precipitation of A16 where it gradu- well-formed marcasite blades and bars A14 (S2) is followed by the precipita- ally increases in intensity before the pre- (S-1′, Figure 16). All cuboctahedrons are tion of calcite zone (C4) A15 (Figure 13), cipitation of sulfide zone (S4) A17 where approximately 20 µm in length. A16 (S3) which ranges in thickness from 10 to intensity decreases slightly (Figure 13). exhibits iridescence under close exami- 35 cm. A15 (C4) is very blocky and frac- nation of the hand-sample specimens tured; it is more yellow at the beginning Sulfide zone (S3) A16 is the first sulfide (Figure 7). A16 is approximately 400 µm of the zone and ends up with a more that is concentrically zoned along the thick.
Illinois State Geological Survey Circular 581 21 S-5 S-6
100 m 200 m
S-7 S-7Ј
50 m 50 m
Figure 15 Plane-light photomicrographs of sulfide zones S5 through S7′ within the cavity (Figure 13). S-5, sulfide zone (S5) A19 is precipitated along a symmetric growth planes of calcite zones (C5 and C6) A17 and A20 and consists of blocky and elongated irregular sulfide crystals; S-6, sulfide zone (S6) A21is precipitated along symmetric growth planes of calcite zone (C6) A20 and consists primarily of acicular to filament form sulfide crystals; S-7 and S-7′, the last sulfide zone (S7) A23 is precipitated along symmetric growth planes of calcite zone (C6) A20 and on a dissolution boundary at the top of A20. A23 starts as irregular, elongated crystals and grades to mostly blocky crystals. Slightly curved, orange siderite crystals are pre- cipitated over blocky sulfide crystals.
Sulfide zone (S4) A18 marks the end of 25 µm wide. Many of these octahedron tal precipitates in sulfide zone (S4) A18 calcite zone (C4) A15 and the precipita- stacks are skewed and appear stretched (S-4, Figure 14) at the base of A17 and tion of calcite zone (C5) A17. A17 (S4) is (S-4′, Figure 17). Trace tabular and irreg- large sulfide crystal precipitates at the composed of multiple sulfide zones less ular marcasite rhombic prisms are also top of sulfide zone (S5) A19 (S-5, Figure than 100 µm long and grouped together, found throughout S-4. Calcite zone (C5) 15) at the top of A17 (Figure 13). forming a zone of sulfides approximately A17 is approximately 600 µm thick and 500 µm thick (S-4, Figure 14). A17 (S4) is is characterized by a relatively high CL Sulfide zone (S5) A19 contains primarily characterized primarily by stacked and intensity; however, at approximately 150 pyrite crystals and is characterized by skewed pyrite octahedron verticals (S-4′, µm from the top of the A17 (C5), there larger pyrite crystals, all approximately Figure 17). These stacked octahedrons or is a noticeable lower CL intensity zone 150 µm thick at the base of A19 (S-5, Fig- chains of octahedrons range from 50 to (Figure 13). Calcite zone (C5) A17 is clear ures 13 and 15). A19 grades upward into 300 µm in length and are typically about and has very high luster. A17 is a transi- clusters of pyrite pyritohedrons, octahe- tion zone between smaller sulfide crys- drons, and pyrite bars (S-5, S-5′, Figure
22 Circular 581 Illinois State Geological Survey ′ S-1 Sulfide Coating S-1
Pyrite
Marcasite Limestone 200 m 200 m
S-2 S-2′ Marcasite
Pyrite
Marcasite
200 m 100 m
S-3 Calcite S-3′ Marcasite
Pyrite
Marcasite
200 m 20 m
Figure 16 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S1 through S3. Brighter emission surface represents sulfides and darker emission surface represents calcite. S-1 and S-1′, sulfide cement (S1) A11 partially coating host rock; marcasite grows as a mostly irregular coating with later epitaxial pyrite octahedrons; S-2 and S-2′, sulfide cement (S2) A14 consists of a heavy coating of irregular, tabular, and rhombic prisms with later epitaxial pyrite pyritohedrons and octahedrons; S-3 and S-3′, sulfide cement (S3) A16 consists of spaced out bladed marcasite with later epitaxial pyrite cuboctahedrons.
Illinois State Geological Survey Circular 581 23 S-4 S-4′
Pyrite
Pyrite
100 m 100 m
S-5 S-5′ Pyrite
Pyrite
20 m 20 m
Figure 17 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S4 and S5. S-4 and S-4′, sulfide zone (S4) A18 exhibits stacked and stretched pyrite octahedron verticals and trace irregular marcasite rhombic prisms. S-5 and S-5′, sulfide zone (S5) A19 consists primarily of pyrite octahedrons and pyritohedrons and trace marcasite.
17). As A19 (S5) crystals fine upwards, CL sulfide needles. Sulfide zone (S6) A21 is sity just before precipitation of sulfide intensity decreases in calcite zone (C6) approximately 400 µm thick. zone (S7) A23 but decreases in inten- A20 (Figure 13). Sulfide zone (S5) A19 is sity during precipitation of A23 (S7), Calcite zone (C6) A20 is the last zone approximately 450 µm thick. Another increases again, and then decreases at of calcite that includes sulfides (Figure sulfide zone (S6) A21 (S-5, Figure 15) at the very end of A23 (S7) precipitation 13). A20 begins at the top of sulfide zone the base of A20 (Figure 13) is identified (Figure 13). A20 is approximately 4 mm A19 and continues through A20 until based on morphological differences thick. termination by a slight dissolution zone compared with A19 (S-6, Figure 15). at the base near the top of sulfide zone The base of A23 (S7) is dominated by Sulfide zone (S6) A21 is characterized (S7) A21 (S-7′, Figure 15). Throughout irregular blades and tabular rhom- by needle-like sulfide crystals in the the middle zone of A20, numerous zones bohedrons of marcasite (S-7, Figure hand sample (Figure 7) and thin sec- of <10-µm-thick pyrite are included 18). The largest marcasite crystals are tion (S-6, Figure 15). The needle-like or but were undifferentiated (Figure 13). approximately 380 µm, but most aver- filamentous form crystals in A21 have Also, numerous zones of <50-µm-thick age around 200 µm. All marcasite little to no symmetry (S-6, S-6′, Figure marcasite and pyrite are included near crystals are suspended in calcite zone 18). The XRD analysis suggests that the the top 600 µm of A20 and are grouped (C6) A20. Epitaxial pyrite octahedrons needles are marcasite (S-6, Appendix). together as sulfide zone (S7) A7. A20 is are common on irregular marcasite Epitaxial pyrite pyritohedrons and characterized by mostly low intensity crystals. A23 (S7) grades upward from cuboctahedrons are common on these CL, especially near the base and end marcasite to pyrite crystals. Pyrites are (Figure 13). The CL increases in inten-
24 Circular 581 Illinois State Geological Survey S-6 S-6′
Pyrite
200 m 50 m
S-7 S-7′
Pyrite
Marcasite
50 m 50 m
Figure 18 Environmental scanning electron microscopy back-scattered photomicrographs displaying cavity sulfide zones S6 and S7. S-6 and S-6′, sulfide cement (S6) A21 consists of elongated filamental crystals with epitaxial pyrite pyritohedrons; it is unclear if filaments are pyrite or marcasite; S-7 and S-7′, sulfide cement (S7) A23 exhibits irregular marcasite blades that grade up into pyrite pyritohedrons, octahedrons, and cuboctahedrons. precipitated at the top of A20 on a slight top of A23 along the dissolution bound- Geochemical Analyses dissolution surface A22 (S-7′, Figure 16). ary A22. It is the outermost cement over The isotopic and trace element composi- Pyrites could be grouped into their own every euhedral crystal lining the cavity tion of the calcite and sulfide solution paragenetic event; however, they are (Figure 7). In the solution cavity, A25 cavity cements that constitute the para- epitaxial with and closely related to the (C7) is thickest on the lateral and base genetic sequence following the forma- marcasite suspended in calcite A20 (C6) sides of the crystal face, inward toward tion of the solution cavity is presented and therefore were grouped in A23 (S7). the center of the solution cavity. A25 (C7) in Table 2. Based on the elemental Pyrites exhibit octahedral, pyritohedral, is not found on the top of the protruding abundance results for calcite cements, and cubic habits (S-7′, Figure 18). Trace euhedral crystals (Figure 7). A25 (C7) A15 (C4) was reported as two distinct oxidized siderite rhombohedrons with forms sharp crystal faces and symmetric cements; C4 and C4′. The most distinct curved faces are common precipitates and asymmetric geometries. Some crys- geochemical characteristics of the dia- around pyrite in sulfide zone (S7) A23 tal faces appear stretched with sharp genetic cementation events are (1) high (S-7, S-7′, Figure 15). Pyrite and siderite but curved edges. A25 (C7) is extremely Mg concentrations (1,113 to 1,615 ppm) are cemented with calcite cement (C7) lustrous, although some crystal faces in earlier calcite cements (C3 and C4) A25. are slightly etched, dulling the luster A12 and A13 relative to lower concentra- and exhibiting a frosty white color. Pit- tions (395 to 526 ppm) for later calcite Calcite zone (C7) A25 is a 0.5- to 2-cm- ting caused by dissolution event A26 is cements (C4′ to C7); (2) more radiogenic thick calcite layer that forms a partial common on the lustrous crystal faces of 87Sr/86Sr ratios for earlier calcite cement coating over A23 (S7) and cements the A25 (C7) cement and exhibits internal crystal geometries.
Illinois State Geological Survey Circular 581 25 Table 2 Elemental and isotopic abundances of the solution cavity cements. 87Sr/86Sr Paragenetic Ca Na Sr Fe Mg Mn (standard δ13C δ18O sequence (mol%) (ppm) (ppm) (ppm) (ppm) (ppm) SRM976) (VPDB) (VPBD) Cavity calcite A12 (C3) 40.7 47 164 355 1,113 238 NA –1.95 –7.10 –1.89 –6.99 –1.62 –7.35 A15 (C4) 40.0 52 150 77 1,615 566 0.71015 –2.20 –5.05 –2.13 –4.22 A15 (C4′) 40.5 44 196 242 395 372 NA –4.24 –6.13 –3.60 –4.62 A17 (C5) 40.6 64 23 bd 490 451 NA –4.63 –3.47 –4.88 –3.37 –4.73 –3.55 A20 (C6) 40.4 43 155 146 375 203 0.70980 –3.53 –8.86 –3.27 –9.27 –3.44 –9.24 A25 (C7) 41.0 147 149 bd 526 158 0.70898 –8.57 –6.31 –10.58 –8.44 –8.57 –6.18 Fe S Cu Pb Zn (mol%) (mol%) (ppm) (ppm) (ppm) Cavity sulfide A11 (S1) 46.3 50.7 376 bd bd A14 (S2) NA NA NA NA NA A16 (S3) 45.4 51.1 87 2,172 423 A18 (S4) 46.2 51.1 44 889 0 A19 (S5) 45.4 51.3 144 6,013 767 A21 (S6) 45.2 51.4 29 4,203 0 A23 (S7) 46.4 51.5 694 0 110
1Abbreviations: VPBD, Vienna Pee Dee Belemnite laboratory standard; bd, below detection level; NA, not available at time of analysis.
(C4) A15 relative to 87Sr/86Sr ratios high-resolution examination using an detail in northern Illinois (Willman and (0.70980 to 0.70898) of later cements (C6 ESEM confirms that pyrite exists in all of Kolata 1978) and likely maintain unifor- and C7) A20 and A25; (3) two distinct the sulfide zones. mity and continuity in sedimentological trends of δ13C VPBD and δ18O VPBD cor- composition over hundreds of kilome- relating to the paragenetic events and ters, further indicating deposition in a following a similar pattern of δ13C VPBD DISCUSSION shallow marine ramp environment with 18 getting lighter and δ O VPBD getting Depositional History of the low sedimentation rates (Kolata et al. heavier with respect to time (Figure 19); 2001). Furthermore, Kolata et al. (2001) (4) high concentration of Na (147 ppm) Solution Cavity Host Rock suggest that the lack of micrite in south- and Sr (249 ppm) in the latest calcite The solution cavity at the Conco Mine ern Illinois and its abundance in north- cement (C7) A25; and (5) high Pb con- is located in the upper Ordovician Wise ern Illinois are evidence indicating that centrations (889 to 6,013 ppm) in sulfide Lake Formation of the Galena Group. the Galena Group was deposited more cements (S3 through S6) A16 through The Wise Lake is described as fossilifer- shoreward in warmer waters in northern A23 and relative Zn concentrations (110 ous wackestones with thin beds of grain- Illinois than in southern Illinois. to 767 ppm) in sulfide cements (S3, S5, stones deposited in a low-energy, sub- and S7) A16, A19, and A23. The XRD tidal environment as suggested by Will- The clay bed noted in the Wise Lake analyses of sulfides indicate that they man and Kolata (1978). The grainstone Formation host rock, directly above the are all composed primarily of marcasite beds have been attributed to deposition present-day solution cavity, is probably (Appendix). Pyrite was indicated by from periodic storm events. The Galena equivalent to the Dygerts K-bentonite XRD in zones S1, S5, S6, and S7; however, Group rock units have been described in bed described by Willman and Kolata
26 Circular 581 Illinois State Geological Survey Relative Timing (per trend line) Furthermore, fracture-focused fluids Older Younger 0.00 diagenetically altered surrounding host C3 C4 rock. The process of host rock dissolu- tion, sulfide cement precipitation, and –2.00 C6 calcite cement precipitation is repeated C5 –4.00 several times within the solution cavity. Geochemical and high-resolution microscopic analysis of these mineral –6.00 precipitation events suggest that they
C (‰ VPDB) C3, C4, and C5 were caused by changes in the geochem- 13 –8.00 δ C6 and C7 ical composition and extent of reaction C7 within the diagenetic system. –10.00 Trend (C6 and C7) Trend (C3, C4, and C5) –12.00 Early-Stage Diagenesis –10.00 –9.00 –8.00 –7.00 –6.00 –5.00 –4.00 –3.00 –2.00 Early diagenetic alteration consists of δ18O (‰ VPDB) paragenetic events A2 to A7 (Figure 8) (Table 1). Depending on the original sed- imentological composition of the host 18 Figure 19 The δ O VPBD values of cavity calcite zones C3 through C7 com- limestone, wackestone and grainstone 13 pared with their C VPBD values. Two distinct trend lines are indicated for fossils either (1) are dissolved (A2) and zones C3 through C5 and C6 through C7. Because calcite zones are labeled later cemented by calcite cement (C1) A3 by the their paragenetic event ordering, isotopic values of calcites on each (Figure 9a) or (2) partially retain their 13 18 trend line are getting lighter C VPBD and heavier δ O VPBD with respect original structure due to conversion to to time. This isotopic comparison suggests at least two different fluids were neomorphic calcite (C1) A3 (Figures 9a, responsible for precipitation of calcite cements within the solution cavity. c) as exhibited by dull, low intensity CL (Figure 9c). A similar CL color of inter- (1978). A previous study near the Conco barrier for downward burrowing organ- nal fossil structure and blocky calcite Mine (Graese 1991) notes that the isms and later as an impermeable aqui- cement in dissolved fossil interiors Dygerts K-bentonite bed lies approxi- tard for upward migrating fluids. imply that calcite’s (C1) A3 are related mately 24 to 30 m below the top of the and formed diagenetically early rather than being primary (Figure 9c, d). Bur- Wise Lake Formation, which is con- Diagenetic History sistent with the depth of the clay bed rowing (A4) of grainstone (Fig 10c) likely observed above the solution cavity and The combination of petrographic, ESEM, occurred before or during calcite altera- approximately 28 m below the top of and geochemical evidence suggests that tion and cementation (C1) A3; however, the Wise Lake Formation. The Dygerts diagenetic alteration of host rock and these events are paragenetically undif- K-bentonite bed (originally volcanic ash) the formation of the solution cavity and ferentiated. Burrows are cemented by is thought to be the product of volcanoes mineral cements occurred in two major nonplanar dolomites A6 (Figure 10b). A3 situated in an island arc system in the events: (1) early-stage diagenesis and (2) calcite and A6 dolomites are also para- Appalachian mobile belt (Kolata et al. late-stage diagenesis. This study focuses genetically undifferentiated. It might be 1986). Burrowing is not apparent in host primarily on late-stage diagenesis, expected that the early calcite cement rock below the clay bed at the studied which includes solution cavity forma- (C1) A3 would tightly cement the burrow solution cavity; however, mineralized tion and subsequent cementation and if A3 precipitated. However, burrowing burrows inside other cavities below the alteration events. Late-stage diagenesis likely resulted in a higher permeability bentonite zone indicate that burrows are is discussed in two contextual groups: and may have therefore acted as a sink not confined to beds above it. Similarly, (1) fracturing and dissolution and (2) for organic matter and clay that later it is well noted that the Galena-Platte- sequences of dissolution, sulfide pre- enhanced dolomite precipitation (Fouke ville throughout the region is highly cipitation, and calcite precipitation. The at al. 1996, Gingras et al. 2004). In this burrowed and bioturbated and is com- fracturing of the host rock is directly fol- case, burrow dolomites precipitate monly described as mottled due to the lowed by fracture-focused dissolution, after A3. A microcrystalline dolomite color appearance of preferentially dolo- indicating that the fractures at Conco (A5) that partially replaces wackestone mitized burrows (Kendall 1977, Willman were the significant conduits for fluid matrix (Figure 10a) is indistinguishable and Kolata 1978). Burrows in host rock transport. However, the sedimento- in PL and CL from cement A6. Calcite above bentonite terminate at the top of logical and stratigraphic context of the cement (C2) A7 is described primarily as it. This volcanic ash bed, composed of cavity (i.e., overlying clay bentonite bed a syntaxial cement over calcite cement fine-grained glass, may have deterred and mineralized burrows in other cavi- (C1) A3 and appears to surround the burrowing by marine organisms. Thus, ties) suggests that stratigraphic facies burrow without a crosscutting relation- the early bentonite may have acted as a strongly influenced fluid flow dynamics ship, suggesting A7 (C2) precipitated and resulting water-rock interactions. after burrow dolomites A6 (Figure 10c).
Illinois State Geological Survey Circular 581 27 Late-Stage Diagenesis creating conduits for fluid transport and The Sequence of Dissolution, subsequent deposition of MVT minerals Sulfide Precipitation, and Calcite Fracturing and Dissolution (Garven et al. 1993). A similar process Precipitation Microfractures (A8) crosscut the host was likely possible at Conco as indicated The diagenetic fluids that created rock wackestone matrix as well as the by the fracture-focused meter-scale cav- the solution cavity presumably were fossil fragments replaced and cemented ities and mineral precipitates through- strongly buffered by the host rock and by calcite (C1 and C2) A3 and A7 (Figure out the Conco Mine (Figures 4 and 5). later chemically evolved to precipitate 11a), which indicates that fracturing of Furthermore, the vertical geometry of sulfide cement (S1) A11 and later cal- the host rock postdates precipitation the cavity and the lenticular geometry at cite cement (C3) A12 (Figures10f and of calcite cements (C1 and C2) A3 and the top of the cavity (Figure 5) where it 13), which indicates a repeating his- A7. The CL of calcite cements in host terminates at the B1 clay bed (Figure 12) tory of fluid chemistry oscillation from rock microfractures (Figure 11c) are suggests that fluid ascended up vertical calcite undersaturation to sulfide and comparable to CL of early cavity calcite open fractures where it was impeded calcite oversaturation (repeated from cements (C3) A12 (Figure 11d), suggest- at the clay bed acting as an aquitard, the beginning of the A9 solution cavity ing that microfractures are cemented forcing lateral fluid flow and associated formation through last A26). Previous by calcite (C3) A12. A12 cementation dissolution. studies have interpreted this evidence around grains relative to the fractures to represent distinct pulses of upwell- (Figures 11a, c) suggests slight dissolu- Millimeter- to micrometer-scale dissolu- ing subsurface waters (Cathles and tion along microfractures; however, tion (A9) of the slightly fractured host Smith 1983, Deloule and Turcotte 1989) most dissolution (A9) occurs along rock predates later planar to subplanar transported through fractures and com- larger fracture networks generally dolomite A10 (Figure 10 D). The distribu- positionally evolving over time. Further- trending northwest-southeast (Figure 4), tion of A10 dolomites along these mil- more, the mixing of multiple fluid types resulting in the formation of meter-scale limeter- to micrometer-scale dissolution (the “mixing hypothesis”) (Beales and solution cavities (Figure 5). Preferential zones constrains the timing of this late- Jackson 1966, Anderson 1983, Sverjen- dissolution along northwest-southeast stage dolomitization event. A10 dolo- sky 1986) may be another controlling fractures rather than other promi- mites are always slightly replaced by factor in this sequence of diagenetic nent northeast-southwest fractures sulfide cement (S1) A11 and completely events. The mixing model proposes that at Conco Mine may be indicative of cemented and occasionally “‘floating’” MVT sulfide precipitation is the result continental compressional stress at the in calcite cement (C3) A12 (Figure 10d). of a metal-bearing fluid such as hydro- time of cavity formation. Daniels et al. Clasts of host rock are also found “float- thermal brine migrating from deep to (1990) suggested that mineralogical ing” in calcite cement (C3) A12 and shallow. These basinal waters mix with and chemical differences in authigenic typically replaced by sulfide cement (S1) formation waters rich in H S, resulting clays found in orthogonal joint sets A11 (Figure 10), which may imply that 2 in the precipitation of metal sulfides. formed in the eastern Appalachian the diagenetic fluids that caused vary- However, the mixing model may not Basin during Alleghenian orogenic ing degrees of dissolution are geneti- account for the dissolution of the host deformation (Permian) were related to cally related to A10 dolomitization, A11 rock. Anderson (1983) suggested that the permeability variations controlled by sulfide precipitation, and A12 calcite mixing of a reduced sulfur-bearing solu- the orientation of the Alleghenian stress precipitation. There is no evidence that tion and metal-bearing solution theo- fields. They suggested that joint sets that floating or cemented dolomite grains retically reduces the saturation state and were oriented roughly perpendicular and host rock clasts are a product of dissolves carbonate minerals. However, to the maximum paleostress direction brecciation, but instead may have a co- the opposite is observed at Conco, where (northeast-southwest) were compressed genetic relationship of the dolomitizing, dissolution is followed by sulfide and during the Allegheny Orogeny, while dissolving, and calcite precipitating calcite precipitation, as indicated by the joint sets oriented roughly parallel to the fluids. Furthermore, Mg concentration paragenetic sequence (Figure 8). maximum stress direction (northwest- (1,113 ppm) in A12 (C3) is substantially southeast) were opened. The opening of higher than most late-stage calcites (C4′ The mixing of two solutions with dif- these joint sets increased permeability, to C7; Table 2), suggesting that the early ferent compositions, each of which is promoting fluid flow through the joints diagenetic fluids responsible for dissolu- in equilibrium with respect to calcite, and encouraging diagenesis along frac- tion were evolving, eventually reaching has been shown to result in calcite dis- tures. This process would explain the the Mg supply needed to precipitate solution and precipitation (Wigley and majority of the solution structures found dolomite (Land 1985) and subsequently Plummer 1976, Rimstidt 1997, Anderson at Conco Mine that are roughly oriented precipitate A11 iron sulfides and A12 2008). Corbella et al. (2004) conducted along northwest-trending fractures or calcite. However, only trace amounts of reactive transport modeling and cal- joint sets, further suggesting that fluid dolomite are found within these micro- culations for the mixing of two fluids to flow initiated during or after the Allegh- dissolution zones of host rock and are account for the dissolution of host rock eny Orgogeny (Permian). It has been virtually nonexistent along the dissolu- and the precipitation of sulfides and noted that regional-scale fracturing was tion surface of the solution cavity-host calcite. In these experiments, reduced likely the most important process for rock dissolution boundary. sulfur-rich groundwater and reduced metal-rich brine (both saturated with
28 Circular 581 Illinois State Geological Survey respect to calcite) resulted in both dis- of marcasite followed sequentially ter 87Sr/86Sr decreased from 0.70900 to solution and precipitation of sulfide and by pyrite precipitation; however, the 0.70780 during Ordovician time (Shields calcite cement. Corbella et al. (2004) amount of precipitated sulfides is com- et al. 2003) suggesting that calcite also found that when the host rock pore parable. Although, based on the amount cements A15 and A20 were precipitated water contains H2S, and the brine con- of dissolution and calcite precipitation, from older seawater or diagenetic water tains dissolved zinc (or iron), sulfide the second sequence (A13, A14, and with increased radiogenic Sr. The sea- cement is precipitated, and dissolution A15), underwent less reactive fluidiza- water from which calcite cement A25 occurs due to sulfide-induced lower- tion or reached saturation faster than precipitated would be of Lower Ordovi- ing of pH (Anderson and Thom 2008). the first sequence (A9, A11, and A12). cian according to this secular variation As previously mentioned, this does not curve (Shields et al. 2003). Winter et al. appear to be the case at Conco Mine. The last sequence of diagenetic events (1995) analyzed the 87Sr/86Sr of dolomite in the cavity is similar to the first two, cement in the Lower Ordovician St. An interesting observation is the pre- except, importantly, it is in reverse order. Peter Sandstone of the Michigan Basin cipitation of marcasite followed by Near the end of calcite cement (C6) A20 and suggested that four different fluids epitaxial pyrite, which may suggest precipitation, marcasite begins to co- were involved in dolomitization: Fluid that the fluid responsible for dissolu- precipitate within the sulfide cementa- 1, older seawater (0.7093 ± 1); Fluid 2, tion, sulfide precipitation, and calcite tion event (S7) A23. The size of marcasite Ordovician seawater (0.7084 ± 1); Fluid 3, precipitation contained a bulk solution cements then increase (Figure 13) until fluid derived from water-rock interaction pH that was increasing as a function of a dissolution event (A22) occurs. Sulfide with potassium feldspar crystals (igne- time. This explanation is supported by cementation continues with a change ous and/or sedimentary) at the Precam- experiments that suggest that the pH to the epitaxial precipitation of pyrite brian-Cambrian contact (0.710 to 0.711); of precipitant fluid differs substantially cement over marcasite cement as well as and Fluid 4, derived from water-rock for marcasite and pyrite. Marcasite pre- the dissolution surface (S-7, S-7′, Figure interaction with Silurian gypsum (0.7085 cipitation is strongly favored in a more 15). Calcite (C7) A25 and A24 siderite ± 1). The present study’s 87Sr/86Sr analy- acidic (pH approximately 4 to 5) solu- cements precipitate co-genetically with ses of calcite cement suggest that the tion, and pyrite is favored in less acidic the A23 pyrite crystals. This paragenetic diagenetic fluids responsible for the first solutions (Murowchick and Barnes 1986; sequence (A20 calcite cement, A23 sul- two sequences of dissolution and min- Schoonen and Barnes 1991a, 1991b, fide cement [marcasite], A22 dissolution, eral precipitation in the Conco solution 1991c). Thus, compositional shifts in A23 sulfide cement [pyrite], A24 siderite, cavity may have originated from water- the pH of the diagenetic waters are sug- and A25 calcite cement) is suggestive rock interaction with arkosic sandstones gested by sequential mineralization of fluid pH and saturation state with in the basal Cambrian Mt. Simon Sand- events in the solution cavity, including respect to the sulfides and calcites. A stone and/or Precambrian granitic base- sulfide cementation event (S1) A11 and possible mechanism would be the slow ment rock (Figure 3) (Winter et al. 1995). all the other sulfide cementation events influx of slightly acidic brine mixing Later cement precipitation events, such (S2–S7; Figure 8) consisting of marcasite with oxidizing formation waters. as A25, may have been derived from and epitaxial pyrite followed by calcite water-rock interaction with Ordovician co-precipitation. These changes may These three sequences of diagenetic events (Table 1) also exhibit distinct or Silurian carbonates and connate be explained by fluidization events burial fluids, as implied by the 87Sr/86Sr consisting of acidic and possibly under- geochemical compositional differences that can be used to interpret the source composition of A25 (0.70898). Less acidic saturated brines that are composition- brine or less extensive water-rock inter- ally changing as a result of water-rock of the diagenetic fluids. As an example, calcite cement (C4) A15 is physically action events of acidic brine and/or the interaction (dissolution) or mixing with influence of shallower and less acidic formation waters. (Table 1) and chemically (Table 2) simi- lar to calcite cement A12 relative to later Ordovician or Silurian formation waters The first solution cavity diagenetic calcite cements (C4′ through C7). Spe- might explain the overall lack of dis- sequence of A9 dissolution, A11 sulfide cifically, the A15 calcite 87Sr/86Sr ratios solution after the precipitation of calcite precipitation, and A12 calcite precipita- are more radiogenic (0.71015) than ratios zone (C3). These shallower formation tion is followed by a second sequence for A20 (0.70980) and A25 (0.70898) cal- waters may explain the increased con- consisting of A13 dissolution, A14 sulfide cites. This difference suggests that the centrations of Na (147 ppm) and Sr (259 precipitation, and A15 calcite precipita- diagenetic water that precipitated each ppm) in the last calcite precipitation tion (Figures 8 and 13). Although the of these calcites was derived from dif- event (C7) A25 relative to earlier calcite extent of A13 dissolution is unknown, ferent sources, as illustrated in the fol- events (Table 2). Fluids with higher con- the volume of burial water involved in lowing examples. The 87Sr/86Sr ratios of centrations of Na and Sr may indicate the reaction must have been less than A20 and A25 are consistent with those that the fluid interacted with evaporite that of the A9 dissolution event that of Lower to Middle Ordovician seawater deposits, such as those in the overlying formed the cavity. Also, the amount of (Burke et al. 1982), including the Galena Silurian bedrock (Figure 3) (Fisher et al. calcite deposited in cementation event Group formation water in the Illinois 1988, Winter et al. 1995). Such interac- A15 is less than A12 (Table 1). Similar to Basin (Stueber et al. 1987). However, a tion would indicate that an element the first sequence sulfide cement (S1) newer secular variation in 87Sr/86Sr of of downward flow occurred, probably A11, sulfide cement (S2) A14 is composed Ordovician brachiopods reports seawa-
Illinois State Geological Survey Circular 581 29 farther and deeper in the basin, before of calcite zones by thin sulfide precipi- ture and degree of supersaturation is being transported up through the dis- tates. Lighter carbon values may also be reflected by pyrite morphology. Further- solved fractures at Conco. explained by meteoric diagenetic fluids more, the CL character implies that the where δ13C values reflect dissolved lime- diagenetic fluid was undersaturated or Stable isotope data also support at least stone and dissolved soil gas. Although at low supersaturation with respect to two different diagenetic fluids respon- meteoric calcites commonly have light iron, resulting in sulfide crystal precipi- sible for mineral precipitation at Conco δ13C values, δ18O values typically remain tation, which might explain the stacked 13 Mine. A crossplot (Figure 19) of the δ C constant (Lohmann 1988). The first of appearance of the octahedrons (Figure 18 VPBD and δ O VPBD values of calcite these two processes is more plausible 17 S-4) and suggest acicular growth in cements (C3) A12 through (C7) A25 to explain the light carbon. However, sulfide zone (S6) A21 (S-6, S-6′, Figure exhibits two distinct trends: (1) cement the increasing values of δ18O in the cal- 15). (C3) A12 through (C5) A17 increasing in cite cements remain ambiguous. The 18 δ O (–7.35‰ to –3.37‰) and decreas- δ18O possibly is a reflection of the fluid A18 is one of numerous small sulfide 13 ing in δ C (–1.62‰ to –4.88‰) and temperature, but fluid inclusion and cements (S3 through S6) A16, A18. A19, (2) cement (C6) A20 through (C7) A25 other studies are required to verify this and A21 precipitated along concentric 18 increasing in δ O (–9.27‰ to –6.18‰) suggestion. Further, each calcite zone growth planes of calcite cements (C4 13 and decreasing in δ C (–3.27‰ to likely reflects pulses of fluid that begin through C6) A15, A17, and A20. These –0.58‰). With respect to the paragenetic with sulfide precipitation as a result of sulfide cements (S3 through S6) may be ordering of each diagenetic cementation gas build up in the cavity and the mixing interpreted as very weak pulses of metal event within the cavity, each trend line of upwelling fluids. With regards to the (iron) carrying slightly acidic brine 18 or fluid increases inδ O and decreases two isotopic trends (Figure 19), calcite moving up the fractures and mixing 13 in δ C over to time. A study on Upper cements (C3 to C5) A12 to A17 would be with formation waters to precipitate the Ordovician brachiopods from 10 world- one similar sourced fluid, and (C6 to C7) sulfides. The lack of dissolution suggests 18 wide localities reports δ O values from A20 to A25 would be another fluid. that these pulses are very weak but suf- –3.5‰ to –6.9‰ and δ13C from 0.2‰ to ficient to change the pH slightly and 5.8‰ (Shields et al. 2003). These values As previously noted, the sulfide precipi- precipitate marcasite. A component of are reflective of δ18O and δ13C values for tation (marcasite and pyrite) sequence is mixing of two fluids is suggested based Upper Ordovician carbonates. Calcite an excellent indicator of compositional on the descending 87Sr/86Sr ratios cor- events (C3) A12 and (C6) A20 are the changes within the diagenetic fluid with responding to the calcite cements in the first cements of each trend line (Figure respect to pH. Further inference may cavity as a function of time. Further- 19), and both have δ18O values lighter also be made based on the morphol- more, the repetitive sequences of diage- than the range for worldwide Upper ogy of the sulfide crystal. Experiments netic events and the overall decrease of Ordovician carbonates. All of the calcite run by Murowchick and Barnes (1987) diagenesis as a function of time favors a cements (C3 to C7) A12 through A25 showed that the crystal morphology of weakening influx of deeper subsurface have much lighter δ13C values than the pyrite directly reflects the temperature fluids mixing with formation waters. worldwide Upper Ordovician carbon- and degree of supersaturation of the An alternative hypothesis of mixing, as ates, indicating a different source of solution from which the crystals pre- mentioned previously, is that a warm, carbon. The isotopically light carbon cipitate. They found that low degrees of acidic, sulfate- and metal-bearing solu- may have been derived from the oxidiza- supersaturation at temperatures below tion heated and dissolved the carbon- tion of organic matter such as petroleum 250°C produced acicular pyrite crys- ate host rocks, resulting in kerogen and/or kerogen (Zhu et al. 2011) during tals that presumably grew via a screw- maturation, generation of methane, and thermochemical sulfate reduction. As dislocation growth mechanism. Higher reduction of sulfate by thermochemi- suggested in the paragenetic sequence degrees of supersaturation resulted in cal sulfate reduction, H2S buildup, and (Figure 8), the association of hydro- the development of cubic, octahedral, the precipitation of sulfides (Anderson carbon to later mineralization is likely and pyritohedral forms. As a result, the 2008). one of the latest diagenetic events and sulfide zones have been used as another correlates with calcite cement (C7) A25, tool to delineate compositional and tem- In summary, the solution cavity and which represents the lightest carbon poral changes of burial diagenetic fluid subsequent mineralized cements calcite. Warm brines associated with flow. The most notable change in pyrite formed by pulses of at least two different MVT mineralization cause thermal morphology is sulfide zone (S4) A18. A18 fluids that decreased in intensity over maturation of organic matter in host (S4) is primarily composed of stacked time and likely mixed with other sub- rocks producing hydrocarbon gases and skewed or stretched pyrite octahe- surface formation waters or gas buildup within the cavity. Isotopic signatures of such as CH4, C2H6, and C3H8 (Anderson drons (S-4, Figure 17). A18 is located just 1991). If these gases are trapped, they below calcite zone (C5), which has a very early cavity cements reflect deep basin can provide a reductant for sulfate in high CL intensity (Figure 13), suggesting brines, whereas later cements reflect the the fluid such as dissolved anhydrite a relatively high Mn/Fe ratio or relatively influence of shallower formation water or gypsum and, furthermore, produce low abundance of quenching elements that may have been in contact with 2+ evaporite minerals, such as those in the H2S for metal sulfide precipitation. This such as Fe . Murowchick and Barnes process may explain the interruption (1987) hypothesized that fluid tempera- overlying Silurian carbonates (Fisher et al. 1988, Winter et al. 1995). It is likely
30 Circular 581 Illinois State Geological Survey that the progressively weaker pulses of principally in limestone and dolostone area and similarities relating paleofluid upwelling migrating fluid may directly that form a thin sedimentary cover over flow to other regional diagenetic events. reflect regional stress regimes and/or Precambrian igneous basement rock; Paragenetic sequences established subsidence dynamics imposed on the they are strata bound and epigenetic; for the UMV (Figure 19), Ordovician basin fluid dynamics and may further they consist of massive sulfide and cal- limestone deposited along the eastern be a product of major tectonic events cite deposits; there is no known associa- Wisconsin Arch in the western Michigan such as the Taconic, Acadian, and/or tion with igneous rocks; they exist in a Basin (Figure 20), and Trenton hydro- Alleghenian orogenies (Garven et al. region of mild deformation expressed carbon reservoirs in the southern and 1993, Coakley et al. 1994, Winter et al. by a gently sloping basin, slight faulting, southeastern Michigan Basin (Figure 1995). To further test this hypothesis, the and fractured bedrock; the minerals are 21) are reviewed and compared with study of paragenetic events of proposed present at shallow depths on the flank the paragenetic sequence established MVT deposits at Conco Mine should be of a basin; and along with massive solu- for Conco Mine. Common paragenetic compared with other paragenetic stud- tion cavities, there is other evidence for events between localities are reported ies of regional MVT deposits and MVT- dissolution expressed by collapse and (Figure 22) and discussed in the context bearing host rock. brecciation of the bedrock and etching of diagenetic fluids. of crystals. This new discovery of MVT Regional Paragenetic deposits at Conco Mine may provide Upper Mississippi Valley new clues to fluid flow mechanics and Comparisons the subsequent diagenesis of bedrock. Zinc-Lead District The results of this study suggest that Therefore, it is necessary to compare The UMV mineral deposits are hosted solution structures and mineral deposits paragenetic relationships of structures primarily by Ordovician carbonates on at Conco Mine are MVT deposits. Min- and mineralization at Conco with other the northwest flank of the Illinois Basin eral deposits at Conco generally follow regional MVT deposits and similar age (Figure 1) located in northwestern Illi- the general characteristics of MVT host rock in order to understand para- nois, southwestern Wisconsin, and east- deposits (Sverjensky 1986), occurring genetic differences unique to the study ern Iowa on a structural high defined by
EARLY LATE Development of
ents ore fractures xxxxx Development of Period of mineralization shear faults xxxxxxxxxx Development of minor Sequence of
tectonic ev fractures during mineralization xxxxx x xxxx xxxxxxx xxxx xxx x
Quartz......
Dolomite...... Solution and shalification of calcareous beds ??
Pyrite......
Marcasite...... ks
Sphalerite and Wurtzite (?)...... all roc
Cobaltite (?)-smaltite (?) ......
Galena ...... and solution of w
Sequence of mineral deposition Barite ...... Re-solution of early barite
Chalcopyrite ......
Millerite ......
Calcite ......
Figure 20 Paragenetic sequence of the Upper Mississippi Valley Zinc-Lead District primary ore deposition (Heyl et al. 1959). All dolomitization is paragenetically earlier than primary ore mineral deposits except for quartz, and pyrite is earlier than marcasite. Four stages of calcite are noted.
Illinois State Geological Survey Circular 581 31 the Mississippi River, Wisconsin Arch, and Kankakee Arch, which separate the Dolomite, types 1 and 2 Illinois Basin from the Forrest City Basin to the west and the Michigan Basin to Brecciation and fracturing the northeast (Figure 1). The seminal Quartz manuscript on the geology of the UMV by Heyl et al. (1959) is reviewed here to K-Feldspar compare the geology and paragenesis of Dolomite, type 3 those sites with that of Conco Mine. The total area of the district is approximately Iron sulfides 10,369 km2. The vast majority of pri- mary mineral deposits are found within Sphalerite and Galena dolomitic limestone. Island-like areas of unreplaced limestone have been Fluorite noted at UMV; however, most of the Galena Group is dolomitized. The UMV Barite is famous for zinc- and lead-based min- erals, especially sphalerite and galena. Calcite Iron sulfides are also primary miner- als at UMV and are paragenetically Early Late associated with lead and zinc sulfides, which is not always the case; it should Figure 21 Paragenetic sequence of the general mineralization be noted that iron sulfides are highly observed in the Sinnipee Group (Galena-Platteville Groups) in east- variable at MVT deposits (Marie et al. ern Wisconsin on the western margin of the Michigan Basin (Luczaj 2001). In general, iron sulfide deposits at 2006). All iron sulfides, calcite, and the final stage of dolomitization UMV, along with other continental MVT are precipitated post fracturing and brecciation. deposits known for significant amounts of iron sulfides, are paragenetically ear- lier than lead and zinc minerals (Marie Limestone Diagenesis et al. 2001) but are typically deposited (early diagenetic pyrite) throughout the duration of the lead and Dolomitization zinc mineralization. (some early silicification)
Heyl et al. (1959) describe the paragen- Xenotopic A Dolomite esis of UMV with a focus on tectonic events and the deposition of primary minerals (Figure 20). At UMV, fractures Idiotopics S Dolomite are the main control of the ore body and Idiotopic E Dolomite are proposed as being early deforma- tion events predating mineralization. Saddle Dolomite Hydrocarbon Emplacement Dissolution along fractures and the formation of solution structures is pro- Anhydrite Silicification posed to follow shortly thereafter. One Dedolomite/Calcite of the main differences at UMV relative to Conco Mine is the presence of a wide Equant Blocky Calcite range of MVT minerals, but specifi- Marcasite/Pyrite cally zinc and lead sulfides. However, a small amount of sphalerite was found in one of the cavities at Conco during Sphalerite this study, suggesting more may yet be Dogtooth Calcite discovered. At UMV, the first ore stage mineral to precipitate in the paragenetic sequence is quartz, which is dominant as a replacement in the host rock in Pyrite/Celestite the form of chert and is predominately found in close relation to fracture Figure 22 Paragenetic sequence of the Trenton Group (Galena zones. Dolomite is the second stage of Group) in southwestern Ontario on the southeastern margin of mineralization at UMV. Dolomite is the Michigan Basin (Colquhoun 1991). The vertical bar represents deposited within vugs and fractures that observed hydrocarbons (dark indicates 100% of the time; stippled crosscut an earlier, widespread regional indicates 25% of the time).
32 Circular 581 Illinois State Geological Survey Paragenetic Early Relative Timing Late Event Fracturing NA, UMV, EW, SWO Dissolution NA, UMV EW, SWO Quartz UMV, EW, SWO Dolomite NA, UMV, EW, SWO Pyrite NA, UMV, EW, SWO Marcasite NA, UMV, EW, SWO Sphalerite NA, UMV, EW, SWO Calcite NA, UMV, EW, SWO
Figure 23 Comparison of paragenetic events from the Upper Mississippi Valley Zinc-Lead District (UMV; comparison 1, Figure 1), eastern Wisconsin (EW; comparison 2, Figure 1) in the western Michigan Basin, and southwestern Ontario (SWO; comparison 3, Figure 1) in the southeastern Michigan Basin, all relative to the study area (Figure 1) at the Conco Mine (NA). Only paragenetic events from Conco are represented if one or more of the study locations include a similar event. However, a quartz cementation event is included because all of the study locations included this event except Conco Mine. Marcasite and pyrite are cementation events that begin relatively early and end late. matrix dolomitization. Dolomites are cipitate through most of the remaining differences and many interesting described as fine-grained and pink. ore deposition and is proposed to have similarities that may correlate (Figure They are restricted to fractures and ore increased in deposition after sphalerite 23). One major difference is the lack of zones and predate all sulfides. A second deposition, gradually decreased there- hydrocarbons at UMV. At Conco Mine, dissolution event is described that dis- after, and ended after all other sulfides hydrocarbons altered to bitumen have solves limestone and dolomites along were emplaced. Other primary miner- been noted as inclusions in dolomitized fractures, creating very porous media als to be deposited during marcasite burrows (A5) above the solution cavity for the transport of fluid and subsequent deposition include barite, sphalerite, and at the top of the Wise Lake Forma- sulfide precipitation. Heyl et al. (1959) galena, chalcopyrite, millerite, and tion, heavily staining fractures and vugs. notes that dissolving solutions did not suspected wurtzite, cobaltite, and saf- Briefly mentioned at UMV is oil staining alter the paragenetically earlier dolomi- florite. However, sphalerite is the most associated with the dark, organic-rich tized limestone as much as adjacent less abundant mineral found at UMV. Calcite Guttenberg Dolomite (lower Galena dolomitized limestone of the same beds. is the final stage of mineralization and Group, Figure 3); however, there is no The main period of dissolution is sug- includes four substages. Marcasite is mention of hydrocarbons throughout gested to be after dolomitization. Barite deposited in the first two stages of cal- the primary ore mineral paragenesis. and pyrite follow the main dissolution cite precipitation. Early calcite is cloudy Quartz cement and silicification is event. Barite is brief and not widespread. and colored; as deposition continued, another paragenetic difference. Quartz The pyrite is described as lining solution calcite became increasingly clear to col- cement along fractures and silicification cavities and cementing interstices in orless, with fewer impurities. Substages of the host rock are prevalent at UMV. host rock and partially dissolved dolo- of calcite deposition are recognized by The only occurrence of quartz in the mites. Pyrite mineralization is suggested three early scalenohedral habits fol- solution cavity at Conco noted in this to have occurred over a long period of lowed by a final rhombohedral habit. All study is primary depositional rounded time but to have ended before calcite substages can be further identified by quartz grains in the host rock; however, precipitation. Early pyrite likely formed episodes of dissolution or slight etching chert has been noted in the Wise Lake octahedral crystal habits, whereas of the crystal face and marcasite zones Formation in drill core examined from later pyrite formed more cubic habits. in the first two stages (Heyl et al. 1959). the South Mine. Marcasite precipitation is thought to have formed directly after pyrite and is Relative to the paragenesis of the Heyl et al. (1959) proposed a later-stage separated by a sharp contact between solution cavity researched at Conco dolomitization at UMV that postdates the pyrite. Marcasite continues to pre- (Figure 8), UMV has clear paragenetic quartz precipitation. This dolomitiza-
Illinois State Geological Survey Circular 581 33 tion event is likely related to the main and subsequent calcite precipitation. toward Conco during the late Paleozoic dissolution event. A similar stage dolo- This similarity is suggestive of similar would have been bound to the south by mite (A10) is proposed at Conco Mine late-stage diagenetic environments. pre-existing structural highs such as the that may be closely related to the main Interesting also are the crystal habits of Kankakee Arch and uplifted strata on cavity-forming dissolution event (A9). each calcite stage documented at UMV. the southern side of the Sandwich Fault. At both locations, dolomites are slightly The first three stages are noted as sca- The MVT trace minerals deposits are dissolved and replaced by the first sul- lenohedral habits and the last stage as found on the steeply dipping northeast fide cement. However, UMV later-stage rhombohedral. It was previously noted flank of the Illinois Basin in Indiana dolomite fits a description of saddle that smaller cavities in Conco typically (Shaffer 1981) but terminate quickly dolomite where dissolution-related dolo- contain only scalenohedron calcite before reaching the Kankakee Arch, fur- mites at Conco are described as more crystals, suggesting that these are para- ther suggesting structural constraints planar dolomites (Figure 10d). Pyrite is genetically early relative to UMV para- on fluid flow. However, on the north side proposed as the first sulfide to precipi- genesis. Other larger cavities included of the Kankakee Arch in Indiana there tate at UMV with marcasite following combined scalenohedral-rhombohedral are multiple occurrences of MVT min- shortly after. At Conco Mine, marcasite habits where both scalenohedrons with eralization, especially in northwestern is the first sulfide to precipitate with later stage rhombohedral growth and Indiana (Brock and Slater 1978, Shaffer pyrite co-genetically precipitating after- rhombohedral with later scalenhohedral 1981), indicating that fluids were being ward. Based on the results of pyrite and growth are exhibited. These findings transported along the northern flank marcasite precipitation studies (Murow- further suggest relatively similar diage- of the Kankakee Arch. Possible fluid chick and Barnes 1986; Schoonen and netic environments between UMV and sources include the Michigan Basin or Barnes 1991a, 1991b, 1991c), it makes Conco Mine, but further examination is the Eastern Appalachian Basin. more sense for marcasite precipitation needed of other cavities at Conco. followed by pyrite precipitation to follow Western Michigan Basin a dissolution event if both events are There were pre-existing structural genetically related to the same fluid. In boundaries between the two deposits (Eastern Wisconsin) this case, an acidic fluid evolving to less before mineral deposition. The Cam- Conco Mine is located on the far south- acidic fluid is expected. In the UMV, brian to Devonian age uplift of the west flank of the Michigan Basin. Previ- based on the paragenetic ordering, a Wisconsin Arch (Paull and Paull 1977, ous research has suggested a fluid flow fluid would be expected that is becom- Nelson 1995) predates a Permian age out of the Michigan Basin (Colquhoun ing increasingly more acidic. However, of ore emplacement at UMV, based on 1991, Coniglio et al. 1994, Yoo et al. 2000, with the lack of dissolution constraints Rb-Sr dating of sphalerite (Brannon Luczaj 2006) to explain late-stage dolo- during sulfide precipitation at UMV, it et al. 1992). Based on fluid inclusion mitization and MVT mineralization. is difficult to track the pH changes and studies of sphalerite from UMV (McLi- Luczaj (2006) suggested that Ordovician individual sulfide precipitation of the mans 1977) and calculated thermal carbonates are nearly to completely fluids. requirements (Bethke 1985, Rowan and dolomitized east of the Wisconsin Arch Goldhaber 1996), UMV ore-depositing in Wisconsin and provided evidence for A major similarity between the two fluids likely migrated from south to a strong hydrothermal dolomite signa- locations, in addition to co-genetic north and had a recharge zone in the ture, and tracing MVT mineralization precipitation of marcasite and pyrite, Appalachian-Ouachita fold belt (Bethke along the western flank of the Michigan includes multiple stages of calcite pre- and Marshak 1990). Furthermore, Perm- Basin. Thus, we compared the mineral cipitation. However, only marcasite is ian Ar-Ar age dates of hornblende and paragenesis in the Sinnipee Group described to be co-genetic with the first biotite in igneous dikes and breccias (Galena Group equivalent) in eastern two stages of calcite at UMV. At Conco, associated with the fluorspar district Wisconsin (Figure1) with that at Conco. marcasite and pyrite are parageneti- in southern Illinois (Snee and Hayes cally related to cavity calcite cements 1992) correlate with age dates at UMV, A paragenetic sequence of the general (C3 through C6). Calcite appears to be indicating that igneous activity in mineralization within the Sinnipee the last paragenetic event at both sites. southern Illinois may have been a heat Group in eastern Wisconsin (Figure 20) An interesting similarity between the source for northward migrating fluids. begins with two stages of dolomitiza- sites is the association of two dissolution Further evidence that fluid flowed from tion; the first is described as a planar events with marcasite and calcite pre- the south to north is provided by the dolomite composing most of the rock, cipitation. At UMV, the first and second abundance of MVT minerals along the and the latter is described as a finer calcite precipitation events or stages Mississippi Valley Arch, such as chal- dolomite that typically replaces cloudy end with a dissolution event. Marcasite copyrite, galena, barite, and sphalerite cores of the first dolomite. Breccia- precipitation continues through the first in eastern Iowa (Garvin 1995) and the tion and fracturing occur during the dissolution event and ends at the second decreasing concentrations of fluorine first and second dolomitization events. dissolution event, suggesting a parage- in drill core from the fluorspar district Undifferentiated iron sulfide precipita- netic relationship. At Conco Mine, the northwest up to UMV (Goldhaber et al. tion is associated with dolomitization first two dissolution events A9 and A13 1994). Hypothetically, fluid flowing from events 1 through 3. Quartz cementa- are related to marcasite precipitation deep in the Illinois Basin northeastward tion during the first two dolomitization
34 Circular 581 Illinois State Geological Survey events marks an increase in iron sulfide a barrier for westward migrating fluids. staining the crystal faces or along crystal precipitation. Dolomitization event 3 The Wisconsin Arch and Kankakee Arch boundaries. Idiotopic-E dolomites are occurs near the end of the major iron would have acted equally as barriers in also found enclosing xenotopic-A dolo- sulfide precipitation and at the begin- northeastern Illinois for southward and mites and are commonly stained with ning of trace sphalerite and galena westward migrating fluids. However, bitumen. Saddle dolomites are proposed precipitation. Iron sulfide precipitation these lines of evidence do not rule out an as the next dolomite to precipitate, fol- continues in trace amounts, increases Appalachian Basin fluid source. lowed by hydrocarbon emplacement. slightly during brief barite precipitation, Silicification is thought to occur after and decreases during calcite precipita- Southeastern Michigan idiotopic-S dolomites; however, parage- tion. Calcite is the last mineral proposed netic ordering of silicification in relation to precipitate in the Sinnipee Group Basin (Michigan and Ontario) to saddle dolomites is unclear. Saddle (Galena Group) in eastern Wisconsin. Diagenetic alteration of the Trenton dolomites are found exclusively lining No hydrocarbons were observed in this (Galena Group) in the southern and fractures and vugs. Saddle dolomites are study. southeastern Michigan Basin has been recognized as a factor in the creation described as zoned and contain dedo- Similarities occur in the mineral para- of hydrocarbon reservoirs. The para- lomite replacements zones of blocky genesis of the Sinnipee Group in eastern genetic sequence of hydrocarbon res- calcite, suggesting calcite precipita- Wisconsin and at Conco and may be ervoirs on the southeastern flank of the tion as the next event within fractures correlated (Figure 22). Both sites appear Michigan Basin in Ontario is reviewed and vugs. Anhydrite likely precipitates to have undergone three dolomitization (Figure 21) and compared with that at over saddle dolomites; however, the events, and at least one of those was inti- Conco (Figure 22). Also, the famous paragenetic relationship of dedolomite mately related to sulfide or MVT miner- Albion-Scipio Trend in the southern replacement calcite is unclear. Pyrite alization. Luczaj (2006) noted that mar- Michigan Basin (Figure 1) is reviewed and marcasite are described as the next casite either replaced or co-precipitated briefly for contextual diagenetic com- cementation events lining fractures and with dolomite 1, suggesting marcasite is parison with the cavities at Conco. Both cavities. Sphalerite is deposited over the first iron sulfide to precipitate simi- locations provide evidence for late-stage pyrite cement, placing it as a parageneti- lar to the occurrence at Conco. Also, iron paleofluid flow and MVT mineraliza- cally later event; however, blocky calcite sulfides and calcite are the last minerals tion. is described as fracture fill over pyrite to precipitate. Both paragenetic studies cement in some cases. Calcite scaleno- found that sulfide minerals are pref- A paragenetic sequence of hydrocar- hedrons (dogtooth spar) are suggested erentially precipitated along fractures bon reservoirs in the Trenton Group as always being found in association and joints in the bedrock, suggesting an (Galena Group) on the southeast flank with sphalerite precipitation and occur- element of structural control on fluid of the Michigan Basin (Figure 21) was ring paragenetically later. Celestite is transport. However, dissolution events compiled to compare and relate petro- proposed as the last cement; however, distinguishing compositional changes graphic textures to hydrocarbon genera- the paragenetic sequence suggests a in fluids and constraining the timing tion (Colquhoun 1991). Little evidence relationship with late-stage pyrite. There of events at Conco Mine are absent in of the original rock type exists due to is no description of this pyrite. the paragenetic sequence of eastern pervasive dolomitization, but overall the Colquhoun (1991) suggests that dolo- Wisconsin. Also differing from Conco rock is referred to as a wackestone with mitization of the Trenton in Ontario is the occurrence of quartz, k-feldspar, thin beds of interbedded grainstones occurred in a fluid-dominated environ- galena, fluorite, and barite, which are (Colquhoun 1991). An early diagenetic ment in which hydrothermal fluids were found throughout eastern Wisconsin. pyrite is proposed to predate dolomiti- emerging from deeper in the basin. This finding may be related to scale dif- zation. These hydrothermal fluids are proposed ferences between the two studies and, to be responsible for the generation therefore, the scale of the paragenetic Widespread dolomitization is the next of hydrocarbons. Relative to Conco, sequence. event described. All dolomite descrip- tions are based on the classification idiotopic-S dolomite in southwestern Luczaj (2006) suggests fluid flow out of scheme of Gregg and Sibley (1983). Ontario is most closely described as the Michigan Basin based on multiple Xenotopic-A (anhedral) dolomite is pro- burrow dolomitization A6. Idiotopic-E, lines of evidence, but does not rule out posed to be the paragenetically earliest xenotopic-A, or saddle dolomite may be the Precambrian basement as a fluid event of dolomitization. However, xeno- related to dolomitization events A5 or and heat source. Middle Paleozoic ages topic-A crystals are described as being A10 at Conco; however, this is unclear. yielded for k-silicate mineralization are the most pervasive dolomite adjacent to Relationships between the two para- consistent in eastern Wisconsin and into fractures, which are not included in the genetic sequences (Figure 22) include the Michigan Basin. This finding differs paragenetic sequence. Idiotopic-S (sub- calcite and sulfide cementation events from the later Paleozoic ages that have hedral) dolomite and idiotopic-E (euhe- within fractures with the exception of been suggested for ore deposition at dral) dolomite are suggested as later sulfate precipitates (anhydrite and celes- UMV. Luczaj (2006) also found that dolo- dolomitization events in the paragenetic tite). Blocky calcite is proposed as the mitization decreases west of the Wis- sequence. Bitumen is found most com- first later-stage cementation event, but consin Arch; the arch probably acted as monly around idiotopic-S dolomites the exact paragenetic timing between
Illinois State Geological Survey Circular 581 35 the calcite, marcasite, and pyrite is contribution from uranium or thorium, events comprising the cavity and min- unclear. In southwestern Ontario, as would be expected from bentonites eral cements, a repetitive sequence of blocky calcite precipitates first, and later (Fertl 1983). Gamma-ray emissions diagenetic events is recognized and more euhedral calcite crystals precipi- resulted solely from potassium, sug- interpreted as a reflection of the timing tate, much like the findings at Conco gesting that if they were from benton- and compositional change of the diage- Mine. Also, precipitation of sphalerite ites, the bentonites were originally netic waters, and, furthermore, multiple after pyrite at southwestern Ontario is potassium-rich (Hurley and Budros fluidization events or pulses of fluids not included in the paragenesis of the 1991) and severely altered to illite. Dolo- flowing through the fractures. This cavity studied at Conco Mine, but has mite zones commonly develop near or repetitive sequence more or less con- been noted in another nearby cavity at beneath shale, clay, or bentonite layers sists of carbonate mineral dissolution, Conco. Slight structural and paragenetic at Albion-Scipio (Hurley and Budros marcasite precipitation, epitaxial pyrite similarities occur between the reser- 1991), suggesting slight facies control on precipitation, and finally calcite precipi- voirs studied by Colquhoun (1991) in fluid migration. Among all of the inter- tation. The PL and CL petrographic evi- the southeastern Michigan Basin and esting characteristics at Albion-Scipio, dence suggests that these events within results for Conco. Other prolific hydro- the caverns are of greatest interest due a sequence are genetically related and carbon reservoirs in the southern Michi- to the enormous porosity enhancement that they precipitated from the same gan Basin, such as the Albion-Scipio of the reservoir and the ability of the fluid that was compositionally changing Trend, are famous for cavernous poros- caverns to host hydrocarbons. Caverns as a function of time. ity thought to be caused by fracture or have been recorded by bit drops when fault focused hydrothermal fluids. The drilling and have been recorded up to 19 To further characterize these sequen- diagenesis of the Albion-Scipio Trend m in Albion-Scipio (Smith 2006). Some ces, cavity calcite cementation events 87 86 13 18 is compared with diagenesis at Conco studies related cavern formation to karst were analyzed for Sr/ Sr, δ C, and δ O 87 86 Mine for regional and diagenetic con- processes (DeHaas and Jones 1988). (Table 2). The Sr/ Sr ratios for early text. However, Hurley and Budros (1991) out- cavity calcite cements record a fluid that lined evidence against karst processes, is radiogenic (0.71015 ± 0.00002) and The Albion-Scipio field is one of the most further supporting fracture-controlled interpreted as deep basin brine expelled 87 86 well-known fracture-controlled hydro- solution structure formation. Overall, to the basin margins. The Sr/ Sr ratios carbon reservoirs in the Michigan Basin. the structural, stratigraphic, and miner- become less radiogenic (0.70980 ± Unfortunately, a paragenetic sequence alogical parallels between Albion-Scipio 0.00002 and 0.70898 ± 0.00002) in later, of Albion-Scipio was not found to enable and Conco are fascinating and may be younger cavity cements and are inter- a paragenetic comparison with that of analogous in nature; however, a high- preted as an influx of formation water Conco. Instead, structural and miner- resolution paragenetic sequence needs as deep basin brine migration slows. 13 18 alogical similarities are noted herein. to be completed and compared before δ C and δ O values for calcites support Like Conco, Albion-Scipio is located an association can be made. this notion of two differently sources on a shallow synclinal structural sag fluids. Also, light carbon in the calcites with prevalent northwest-southeast– suggests that the fluids were warm and trending fractures. Porosity within the CONCLUSION capable of thermally maturing bedrock hydrocarbon trap is diagenetic in origin At Conco Mine, the discovery of meter- as they passed through, producing and is developed solely in areas of per- hydrocarbon gasses, leading to H S scale solution cavities lined with tens- 2 vasive dolomitization along and above of-centimeter-scale sulfide-zoned buildup and sulfide precipitation. The 18 fractures (Hurley and Budros 1991). calcite cements in the Ordovician age δ O values may reflect temperatures of Dolomitized zones are commonly vuggy Wise Lake Formation are indicative of the fluids and, if so, suggest pulses of and cavernous. Saddle dolomites line localized diagenetic events caused by fluids are cooling over time. Further- vugs and fractures (Radke and Mathis the transport of subsurface diagenetic more, the latest and youngest cavity 1980), and other minerals such as anhy- waters out of midcontinent sedimentary calcite cement records high concentra- drite, pyrite, calcite, and trace amounts basins. Structural, sedimentological, tions of Na and Sr relative to older calcite of fluorite, sphalerite, and barite have and geochemical analyses based on a cements (Table 2). Elevated Na and Sr 87 86 been noted as late-stage cementation paragenetic sequence of one of these concentrations coupled with the Sr/ Sr events (Shaw 1975, Budai and Wilson massive solution cavities and mineral- ratio (0.70898 ± 0.00002) are interpreted 1986, Hurley and Budros 1991). Collapse ization indicate that the solution cavi- as diagenetic fluids that interacted breccias are common and associated ties and minerals within them formed with evaporite deposits in the overly- with caverns (Hurley and Budros 1991) when deep basin diagenetic fluids ing Silurian bedrock (Figure 3) (Fisher or solution cavities. Thin shaly clay beds migrated through regional permeable et al. 1988, Winter et al. 1995). Overall, are also common within the reservoir beds and eventually through localized the late-stage diagenetic events form- rock and have previously been thought northwest-trending fractures, causing ing solution cavities and massive calcite to be of volcanic origin (Wilson et al. fracture- and facies-focused dissolution and sulfide deposits record multiple 1985, Harrison et al. 1986, Dehass and below impermeable boundaries. On the fluidization events and regional fluid Jones 1988). However, spectral gamma- basis of a high-resolution paragenetic transport through sedimentary basins. ray tests show no gamma-ray emission sequence (Figure 8) of the diagenetic These events probably reflect regional
36 Circular 581 Illinois State Geological Survey stress regimes or subsidence dynamics REFERENCES Journal of Geophysical Research, imposed by major tectonic events such v. 90, no. B8 (10 Jul.), p. 6817–6828. as the Taconic, Acadian, and Allegh- Anderson, G.M., 1983, Some geochemi- enian orogenies (e.g., Garven et al. 1993, cal aspects of sulfide precipitation in Bethke, C.M., and S. Marshak, 1990, Coakley et al. 1994, Winter et al. 1995). carbonate rocks: Contributions of the Brine migrations across North Amer- Economic Geology Research Unit, v. 8 ica: The plate tectonics of groundwa- The paragenetic sequence reconstructed (Nov.), p. 16. ter: Annual Review of Earth and Plan- for solution cavities at the Conco Mine etary Sciences, v. 18, p. 287–315. was compared with other regional MVT Anderson, G.M., 1991, Organic matura- deposits and MVT mineral-bearing host tion and ore precipitation in south- Brannon, J.C., S.C. Cole, F.A. Podosek, rock. 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Podosek, and R.K. compared, the paragenetic sequence sippi Valley-type ore deposits: Eco- McLimans, 1992, Alleghenian age of of the solution cavity and subsequent nomic Geology and the Bulletin of the the Upper Mississippi Valley zinc- mineralization at Conco resembles the Society of Economic Geologists, lead deposit determined by Rb-Sr paragenetic sequence of primary min- v. 103, no. 8 (Dec.), p. 1683–1690. dating of sphalerite: Nature (London) eralization events at the UMV. However, v. 356, no. 6369, (09 Apr.), p. 509–511. the geographic location and structural Anderson, G.M., and R.W. Macqueen, boundaries cannot rule out the Michi- 1982, Ore deposit models, 6, Missis- Brock, K.J., and L.D. Slater, 1978, Epitax- gan or Appalachian Basin as a fluid sippi Valley type lead-zinc deposits: ial marcasite on pyrite from Rensse- source. Gangue mineralization at the Geoscience Canada, v. 9, no. 2 (Jun.), laer, Indiana: American Mineralogist, UMV should be analyzed and compared p. 108–117. v. 63, no. 1–2 (Feb.), p. 210–212. with mineralization at the Conco Mine Anderson, G.M., and J. Thom, 2008, and other surrounding mines to inter- Budai, J.M., and J.L. Wilson, 1986, The role of thermochemical sulfate pret and re-examine the source of min- Depositional patterns and diagenetic reduction in the origin of Mississippi eralizing fluids at the UMV. The results history of Trenton-Black River Forma- Valley-type deposits. II. Carbonate- of this study imply that other such MVT tions in Michigan Basin: AAPG East- sulfide relationships: Geofluids 8, deposits and cavernous porosity could ern Section Meeting Abstracts: AAPG p. 27–34. exist throughout the midcontinent. Bulletin, v. 70, no. 8 (Aug.), p. 1063. The paragenesis at Conco Mine may Badiozamani, K., 1973, The Dorag dolo- Budai, J.M., and J.L. Wilson, 1991, Diage- be applied as a tool to predict regional mitization model; Application to the netic history of the Trenton and Black paleofluid flow and assist in future Middle Ordovician of Wisconsin: River Formations in the Michigan mineral and hydrocarbon exploration. Journal of Sedimentary Petrology, Basin: Early sedimentary evolution Exploration should focus on areas with v. 43, no. 4 (Dec.), p. 965–984. of the Michigan Basin: Boulder, Colo- similar structural geology (synclinal rado, Geological Society of America, features and fractured bedrock), sedi- Bauer, R.A., B.B. Curry, A.M. Graese, R.C. Special Paper, no. 256, p. 73–88. mentology, and stratigraphy (partially Vaiden, W.J. Su, and M.J. Hasek, 1991, dolomitized limestone, burrowing, Geotechnical properties of selected Burke, W.H., R.E. Denison, E.A. Hether- and interbedded bentonites). Relating Pleistocene, Silurian, and Ordovi- ington, R.B. Koepnick, H.F. Nelson, these geologic features to regional and cian deposits of northeastern Illinois: and J.B. Otto, 1982, Variation of sea- localized paragenetic studies such as at Illinois State Geological Survey, Envi- water 87Sr/86Sr throughout Phanero- Conco Mine may improve mineral and ronmental Geology 139, 92 p. zoic time: Geology, v. 10, no.10 (Oct.), hydrocarbon exploration efforts. p. 516–519. Beales, F.W., and S.A. Jackson, 1966, Pre- cipitation of lead-zinc ores in carbon- Cathles, L.M., and A.T. Smith, 1983, ACKNOWLEDGMENTS ate reservoirs as illustrated by Pine Thermal constraints on the forma- Point Ore Field, Canada, Institute of tion of Mississippi Valley-type lead- We gratefully thank Lafarge North Mining and Metallurgy Transactions, zinc deposits and their implications America and the personnel at the Conco Sec. B, Bull. 720, v. 75, B278–B285. for episodic basin dewatering and Mine in North Aurora, Illinois. We are deposit genesis: Economic Geology specifically indebted to Brian Wash- Bethke, C.M., 1985, A numerical model and the Bulletin of the Society of Eco- owiak for his patience, time, and great of compaction-driven groundwa- nomic Geologists, v. 78, no. 5 (Aug.), interest in the geology at the mine. We ter flow and heat transfer and its p. 983–1002. also thank Dennis Prezbindowski and application to the paleohydrology of W. John Nelson, Illinois State Geological intracratonic sedimentary basins: Survey, for their thoughtful reviews of this paper.
Illinois State Geological Survey Circular 581 37 Coakley, B.J., G.C. Nadon, and H.F. for mechanisms of anthracite forma- Fouke, B.W., A.-J.W. Everts, E.W. Zwart, Wang, 1994, Spatial variations in tion: Geology (Boulder), v. 18, no. 3 W. Schlager, P.C. Smalley, and H. tectonic subsidence during Tippe- (Mar.), p. 247–250. Weissert, 1996, Subaerial exposure canoe I in the Michigan Basin: unconformities on the Vercors car- Thematic set on cratonic basins: Davies, G.R., and L.B. Smith Jr., 2006, bonate platform, SE France, and their Basin Research, v. 6, no. 2–3 (Sep.), Structurally controlled hydrother- sequence stratigraphic significance: p. 131–140. mal dolomite reservoir facies: An High resolution sequence stratigra- overview; Structurally controlled phy; Innovations and applications: Cohee, G.V., and 15 others, 1962, Tec- hydrothermal alteration of carbonate Geological Society Special Publica- tonic map of the United States: Tulsa, reservoirs: AAPG Bulletin, v. 90, no. tions, v. 104, p. 295–320. Oklahoma, American Association of 11 (Nov.), p. 1641–1690. Petroleum Geologists, 1:2,500,000; 2 Fouke, B.W., and J. Rakovan, 2001, An sheets. DeHaas, R.J., and M.W. Jones, 1988, integrated cathodoluminescence Cave levels of the Trenton-Black video-capture microsampling Collinson, C., M.L. Sargent, and J.R. 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Turcotte, 1989, The Early Cretaceous Vercors carbonate Colquhoun, I.M., 1991, Paragenetic his- flow of hot brines in cracks and the platform, SE France: Sedimentology tory of the Ordovician Trenton Group formation of ore deposits: Economic in the 21st century; A tribute to Wolf- carbonates, southwestern Ontario: Geology and the Bulletin of the Soci- gang Schlager: Sedimentary Geology, M.S. thesis, Brock University, Saint ety of Economic Geologists, v. 84, no. v. 175, no.1–4 (15 Apr.), p. 297–314. Catharines, Ontario, Canada. p. 8 (Dec.), p. 2217–2225. 1–250. Garven, G., 1985, The role of regional Dunham, R.J., 1962, Classification of fluid flow in the genesis of the Pine Coniglio, M., R. Sherlock, A.E. Williams- carbonate rocks according to the Point deposit, western Canada sedi- Jones, K. Middleton, and S.K. Frape, depositional texture, in W.E. 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Keith, 1988, Depo- v. 302, no. 5905, p.236–238. sitional facies and diagenetic history Garven, G., S. Ge, M.A. Person, and D.A. of the Trenton Limestone in northern Sverjensky, 1993, Genesis of strat- Corbella, M., C. Ayora, and E. Cardel- Indiana: The Trenton Group (Upper abound ore deposits in the midconti- lach, 2004, Hydrothermal mixing, Ordovician Series) of eastern North nent basins of North America, 1, The carbonate dissolution and sulfide America: Deposition, diagenesis, and role of regional groundwater flow: precipitation in Mississippi Valley- petroleum: AAPG Studies in Geology, American Journal of Science, v. 293, type deposit: Mineralium Deposita, v. 29, p. 277–298. no. 6 (Jun.), p. 497–568. v. 39, no. 5 (May), p. 344–357. Fertl, W.H., 1983, Gamma ray spectral Garven, G., and J.P. Raffensperger, 1997, Daniels, D.L., R.P. Kucks, and P.L. 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38 Circular 581 Illinois State Geological Survey lore: Iowa City, Iowa, University of Hagni, R.D., 1986, Mineral paragenetic of diagenesis: Bulletin of Canadian Iowa Press, p. 260. sequence of the lead-zinc-copper- Petroleum Geology, v. 15, no. 4, p. cobalt-nickel ores of the Southeast 383–433. Gingras, M.K., S. George Pemberton, Missouri Lead District, U.S.A., in K. Muelenbachs, and H. Machel, J.R. Craig, R.D. Hagni, W. Kiesl, Keith, B.D., and L.H. Wickstrom, 1993, 2004, Conceptual models for burrow- I.M. Lange, N.V. Petrovskaya, T.N. Trenton Limestone: The karst that related, selective dolomitization with Shadlum, G. Udubasa, and S.S. wasn’t there, or was it?: Paleokarst textural and isotopic evidence from Augustithis, eds., Mineral paragen- related hydrocarbon reservoirs: SEPM the Tyndall stone, Canada: Geobiol- eses: Athens, Greece, Theophrastus Core Workshop 18, (25 Apr.), ogy, v. 2, no. 1 (Jan.), p. 21–30. Publ. S. A. p. 93–132. p. 167–179.
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Illinois State Geological Survey Circular 581 39 Kolata, D.R., W.D. Huff, and S.M. Berg- paleokarst, in N.P. James, P.W. Cho- Murowchick, J.B., and H.L. Barnes, 1987, strom, 1998, Nature and regional quette, ed., Paleokarst: New York, Effects of temperature and degree of significance of unconformities asso- New York, Springer-Verlag, supersaturation on pyrite morphol- ciated with the Middle Ordovician p. 55–80. ogy: American Mineralogist, v. 72, no. Hagan K-bentonite complex in the 1112 (Dec.): 1241–1250. North American midcontinent: Geo- Luczaj, J.A., 2006, Evidence against the logical Society of America Bulletin, v. Dorag (mixing-zone) model for dolo- Nelson, W.J., 1995, Structural features 110, no. 6 (Jun.), p. 723–739. mitization along the Wisconsin Arch: of Illinois: Illinois State Geological A case for hydrothermal diagenesis: Survey, Bulletin 100, 144 p. Kolata, D.R., W.D. Huff, and S.M. 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40 Circular 581 Illinois State Geological Survey Rowan, E.L., and M.B. Goldhaber, 1995, Cosmochimica Acta, v. 67, no. 11, p. istics of dolomite types and the origin Duration of mineralization and fluid- 2005–2025 of ferroan dolomite in the Trenton flow history of the Upper Mississippi Formation, Ordovician, Michigan Sloss, L.L., 1988, Tectonic evolution of Valley Zinc-Lead District: Geology, Basin, U.S.A.: Sedimentology, v. 33, the craton in Phanerozoic time, in v. 23, no. 7 (Jul.), p. 609–612. no. 1 (Feb.), p. 61–86. L.L. Sloss, ed., Sedimentary cover: Rowan, E.L., and M.B. Goldhaber, 1996, North American Craton: U.S., v. D-2, Vaiden, R.C., M.J. Hasek, C.R. Gendron, Fluid inclusions and biomarkers in United States: Denver, Colorado, Geo- B.B. Curry, A.M. Graese, and R.A. the Upper Mississippi Valley Zinc- logical Society of America, p. 25–51. Bauer, 1988, Geological-geotechnical Lead District—Implications for the studies for siting the superconducting Smith, L.B., Jr., 2006, Origin and reser- fluid-flow and thermal history of the super collider in Illinois: Results of voir characteristics of Upper Ordovi- Illinois Basin: Reston, Virginia, U.S. drilling large-diameter test holes in cian Trenton-Black River hydrother- Geological Survey Bulletin, 34 p. 1986: Illinois State Geological Survey, mal dolomite reservoirs in New York: Environmental Geology 124, 58 p. Schoonen, M.A.A., and H.L. Barnes, Structurally controlled hydrothermal 1991a, Reactions forming pyrite and alteration of carbonate reservoirs: Wigley, T.M.L., and L.N. Plummer, 1976, marcasite from solution, I, Nucleation AAPG Bulletin, v. 90, no. 11 (Nov.), Mixing of carbonate waters: Geochi-
of FeS2 below 100°C: Geochimica p. 1691–1718. mica et Cosmochimica Acta, v. 40, no. et Cosmochimica Acta, v. 55, no. 6 9 (Sep.), p. 989–995. (Jun.), p. 1495–1504. Snee, L.W., and T.S. Hayes, 1992, 40Ar/39Ar geochronology of intrusive Willman, H. B., and D. R. Kolata, 1978, Schoonen, M.A.A., and H.L. Barnes, rocks and Mississippi-Valley-type The Platteville and Galena Groups in 1991b, Reactions forming pyrite and mineralization and alteration from northern Illinois: Illinois State Geo- marcasite from solution, II, Via FeS the Illinois-Kentucky Fluorspar Dis- logical Survey, Circular 502, 75 p. precursors below 100°C: Geochimica trict: Mineral resources of the Illinois et Cosmochimica Acta, v. 55, no. 6 Basin in the context of basin evolu- Wilson, J.L., A. Sengupta, and J.L. (Jun.), p. 1505–1514. tion: Reston, Virginia, U.S. Geological Wilson, 1985, The Trenton Formation Survey, OF 92-0001. in the Michigan Basin and environs: Schoonen, M.A. A., and H.L. Barnes, Pertinent questions about its stratig- 1991c, Mechanisms of pyrite and Stueber, A.M., P. Pushkar, E.A. Hether- raphy and diagenesis: Ordovician and marcasite formation from solution, ington, J.S. Hanor, Y. K. Kharaka, and Silurian rocks of the Michigan Basin III, Hydrothermal processes: Geochi- L.S. Land, 1987, A strontium isotopic and its margins: Michigan Basin mica et Cosmochimica Acta, v. 55, no. study of formation waters from the Geological Society, Special Paper 4, 12, (Dec.), p. 3491–3504. Illinois Basin, U.S.A: Geochemistry of p. 1–13. waters in deep sedimentary basins: Shaffer, N.R., 1981, Possibility of Missis- Selected contributions from the Ben- Winter, B.L., C.M. Johnson, J. A. Simo, sippi Valley-type mineral deposits in rose Conference: Applied Geochemis- and J.W. Valley, 1995, Paleozoic fluid Indiana: Bloomington, Indiana, Indi- try, v. 2, no. 5–6 (Sep.), p. 477–494. history of the Michigan Basin: Evi- ana Geological Survey, Special Report dence from dolomite geochemistry 21, 49 p. Sumner, J.S., 1976, Principles of induced in the Middle Ordovician St. Peter polarization for geophysical explora- Sandstone: Journal of Sedimentary Sharp, J.M. Jr., 1978, Energy and momen- tion: Developments in economic geol- Research, Section A: Sedimentary tum transport model of the Ouachita ogy, v. 5: Amsterdam, Netherlands: Petrology and Processes, v. 65, no. 2 Basin and its possible impact on for- Elsevier Scientific Publishing Co., (03 Apr.), p. 306–320. mation of economic mineral deposits: 277 p. Economic Geology and the Bulletin of Yoo, C.M., J.M. Gregg, and K.L. Shelton, the Society of Economic Geologists, v. Sverjensky, D.A., 1986, Genesis of Mis- 2000, Dolomitization and dolomite 73, no. 6 (Oct.), p. 1057–1068. sissippi Valley-type lead-zinc depos- neomorphism: Trenton and Black its: Annual Review of Earth and Plan- River Limestones (Middle Ordovi- Shaw, B.R., 1975, Geology of the Albion- etary Sciences, v. 14, p. 177–199. cian), northern Indiana, U.S.A.: Jour- Scipio Trend, southern Michigan: nal of Sedimentary Research, v. 70, M.S. thesis, University of Michigan, Taylor, T., 1982, Petrographic and chem- no.1 (Jan.), p. 265–274. Ann Arbor, Michigan, 138 p. ical characteristics of dolomite types and the origin of ferroan dolomite Zhu, G., S. Zhang, H. Huang, Y. Liang, Shields, G.A., G.A. Carden, J. Veizer, T. in the Trenton Formation, Michigan S. Meng, Y. Li, 2011, Gas genetic type Meidla, J.Y. Rong, and R.Y. Li, 2003, basin: PhD. dissertation, East Lan- and origin of hydrogen sulfide in the Sr, C, and O isotope geochemistry of sing, Michigan, Michigan State Uni- Zhongba gas field of the western Sich- Ordovician brachiopods: A major iso- versity, 75 p. uan Basin, China: Applied Geochem- topic event around the Middle-Late istry 26, p. 1261–1273. Ordovician transition: Geochimica et Taylor, T.R., and D.F. Sibley, 1986, Petro- graphic and geochemical character-
Illinois State Geological Survey Circular 581 41 APPENDIX X-ray diffraction analyses from sulfide zone S1 (A11) through S7 (A23).
47-1743Ͼ Calcite - CaCO3
42-1340Ͼ Pyrite - FeS2
36-0426Ͼ Dolomite - CaMg)CO3)2
37-0475Ͼ Marcasite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
05-0586Ͼ Calcite, syn - CaCO3
37-4075Ͼ Marcasite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
42 Circular 581 Illinois State Geological Survey 47-1743Ͼ Calcite - CaCO3
37-0475Ͼ Marcasite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
47-1743Ͼ Calcite - CaCO3
37-0475Ͼ Marcasite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
Illinois State Geological Survey Circular 581 43 05-0586Ͼ Calcite, syn - CaCO3
37-0475Ͼ Marcasite - FeS2
42- 1340Ͼ Pyrite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
05-0586Ͼ Calcite, syn - CaCO3
37-0475Ͼ Marcasite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
44 Circular 581 Illinois State Geological Survey 37-0475Ͼ Marcasite - FeS2
42- 1340Ͼ Pyrite - FeS2 Intensity (Counts)
10 20 30 40 50 60 70 80 Theta (Њ)
Illinois State Geological Survey Circular 581 45