CONTROLS ON ORE DEPOSffiON IN THE LAMOITE SANDSTONE, GOOSE
CREEK MINE, INDIAN CREEK SUBDISTRICT, SOUTHEAST MISSOURJ TIIlS THESIS IS DEDICATED TO MY PARENTS,
ERNEST EARL AND RUTH W. TAYLOR,
ANDOSKAR CONTROLS ON ORE DEPOSITION IN THE LAMOTTE SANDSTONE, GOOSE CREEK MINE, INDIAN CREEK SUBDISTRICT, SOUTHEAST MISSOURI
BY
GAY NELL GUTIERREZ, B.S.
THESIS Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of
MASTER OF ARTS
THE UNlVERSITY OF TEXAS AT AUSTIN AUGUST 1987 ACKNOWLEDGMENTS
I would like to thank my supervisor, Dr. J. Richard Kyle for suggesting this thesis and making arrangements with St. Joe Minerals Corporation for access to the Indian Creek mines and drill cores. I am also indebted to him for taking time to visit Goose Creek mine while I was doing field work. Many of his suggestions during this visit proved to be invaluable since the mine was closed and allowed to flood shortly after I finished my field work. Most commendable however, he has remained my supervisor for this extended period of study, when the normal supervisor would have looked for a more energetic student.
Ors. Lynton S. Land and Earle F. McBride served as committee members. They read the thesis and made many valuable suggestions. Dr. Harry H.
Posey of the Bureau of Economic Geology served as an informal committee member, "cheerleader", and "coach". He read the first draft of the thesis, and his knowledge of the Southeast Missouri lead-zinc district made his corrections and comments extremely useful.
I am indebted to St. Joe Minerals Corporation, especially Paul
Gerdemann, for permission to study the Goose Creek deposit and for providing access to the Indian Creek mines, drill cores and mine maps. James Pettus, former
Indian Creek mine geologist, showed me around the Indian Creek mines and generously shared his knowledge of the local geology. Gary Miller served as my guide while underground. He also helped me carry samples, and kept my camera dry.
IV Richard Moralas made many of my polished thin-sections and provided assistance and constructive criticism for those that I made. Oskar Gutierrez and Rick
Ozment assisted in computer mapping, and Pat Bobeck and Kitty Milliken helped me use the microprobe. For those illustrations that "look professional", the drafting was done by Kerza Prewitt and Joel Lardon. Partial support for field work was provided by the Geology Foundation.
Lastly, I would like to thank Drs. Rob Finley, Steve Fisher, Shirley
Dutton, Jon Price, and Ronit Nativ of the Bureau of Economic Geology for providing me with employment and the opportunity to work in different fields of geology while pursuing this degree.
This thesis was submitted to the Committee in July 1987.
v ABSTRACT
The Indian Creek subdistrict is the northernmost mineralized area in the Southeast Missouri district and is unique because ore-grade concentrations of sulfides occur within the Lamotte Sandstone. The Lamotte Sandstone-hosted Goose Creek mine is located on the northern end and the Bonneterre Dolomite-hosted Indian Creek mine on the northwestern side of a N30°E-trending, Precambrian rhyolite ridge. A saddle on the northern end of the ridge separates the Indian Creek subdistrict from another probable high along the same trend to the north. Lamotte deposition was influenced by pre-Lamotte basement topography, and local thickness ranges from 0 where it pinches out again st the ridge to over 100 ft toward the basin. It is comprised of a thin, discontinuous basal cobble conglomerate overlain by a medium-grained, moderately to poorly soned, well-rounded quanzarenite. Fourteen authigenic minerals, plus hydrocarbons cement the Lamotte Sandstone at Goose Creek in the following paragenetic sequence: dolomite - framboidal pyrite - marcasite - cuboctahedral pyrite - bravoite - bladed marcasite - pyrite - quartz dissolution - brecciation - siegenite - marcasite - dolomite - brecciation - chalcopyrite - quartz dissolution - sphalerite - galena (cuboctahedral) - quartz - galena (cubic) - dolomite - gypsum - hydrocarbon - kaolinite - illite - calcite - • hydrocarbon. Primary and secondary porosity in the Lamotte vary between 1 and 20 volume percent and authigenic cements account for up to 35 volume percent of the sandstone. Quartz overgrowths are the most common cement in the Lamotte Sandstone at Goose Creek, comprising from 1 to 11 volume percent of the rock. Galena is the most abundant sulfide and commonly occurs in 1 to 3 mm blebs,
V1 averaging 3-4 volume percent. Chalcopyrite averages 0.5 volume percent, but high grade concentrations reach 8-10 volume percent locally. Sulfides in the Lamotte
Sandstone in the Indian Creek subdistrict commonly occur within 40 ft of the
Bonneterre-Lamotte contact, with the highest concentrations within 20 ft or less of the contact
Structure maps of the lead- and copper- bearing-zones mimic the basement topography, suggesting that the Precambrian basement was the major controlling factor on ore deposition in the Indian Creek subdistrict. Vertical tubes of
sulfides, which cross-cut bedding near the Lamotte pinchout in the Goose Creek mine, suggest that the ore-bearing fluids moved through the sandstone aquifer until
the pinchout forced them into the overlying Bonneterre. There the fluids were channeled through the grainstone-algal reef complex along the N30°E-trending
Precambrian ridge. Limited fluid inclusion data for Bonneterre-hosted sphalerite indicate that the mineralizing fluid was a Na-Ca-0 brine with temperatures between
105 and 120° C.
Vll TABLE OF CONTENTS
~ INTRODUCTION 1
General Information 1 Location of Study Area 3 History of Southeast Missouri and Indian Creek 3 Purpose of This Study 5 Methods of Study 6 Previous Work 7 REGIONAL GEOLOGY 9 General Information 9 Precambrian Basement Rocks 9 Upper Cambrian Stratigraphy 12 Introduction 12 Lamotte Sandstone 12 Bonneterre Formation 18 Davis Formation 20 Structural Geology 20
INDIAN CREEK STRUCTURE 23
LAMOTTE COMPOSmON AND DEPOSmO~AL HISTORY 28 General Information 28 Basal Cobble Conglomerate 28 Sandstone 32 Detrital composition 32 Introduction 32 Quartz 34 Sedimentary rock fragments 35 Volcanic rock fragments 37 Fossils 39 Grain size, sorting and roundness 39 Lamotte-Bonneterre Transition 40 Structural Influence On Deposition 41 Sedimentary Structures and Depositional Environment 41 Cross-bedding 41 Interbedded shales and carbonates 41 Ripup clasts 42 Soft-sediment deformation 44 Depositional environment 44
Ylll Page DIAGENESIS 46
Sediment Compaction And Pressure Solution 46 Interlocking quartz grains 46 Stylolites 47 Pressure solution associated with phosphorite 49 Authigenic Cements 49 Sulfides 49 Pyrite and marcasite 49 Grue~ 52 Chrucopyrite 54 Siegenite 57 Bravoite 60 Sphruerite 60 Non-sulfides 64 Quartz 64 Dolomite 64 Kaolinite 65 Feldspar 67 Gypsum 67 Illite 68 Calcite 68 Hydrocarbons 68 Quartz Dissolution and Brec.tjanwi 68 Paragenetic Sequence 72 Comparison with other studies in Southeast Missouri 74 Goose Creek mine 7 4 Lamotte Sandstone 76 Southeast Missouri disrrict 78 Comparison with other sandstone -hosted deposits 80 Cement Textures 82 Between cements and sedimentary structures 82 Between cements 82
MINERALIZATION 85
Ore Disrribution 85 Minerruization Controls 86 Introduction 86 Sedimentary structures and locru sulfur sources 86 Lamotte pinchout and Bonneterre permeability 88 Precambrian basement topography 92 Nature of Ore-forming Solutions 100
ix Sources of Metals and Sulfur 102 Lead and sulfur 102 Copper, cobalt, and nickel 104 Transporting Mechanism 105 Timing of Ore Deposition 106 Other Sandstone-hosted Base Metal Deposits 107
CONCLUSIONS 110 REFERENCES 112 VITA 119
x LIST OF TABLES Table Page 1. Scanning electron microscope analyses of phosphorites and calcium phosphate fossil fragments. 38
2. Microprobe analyses of siegenite. 59
3. Microprobe analyses of bravoite. 63
4. Fluid inclusion analyses of sphalerite. 101 5. Characteristics of sandstone-hosted Pb-Zn-Cu and carbonate-hosted Pb-Zn deposits, and the Goose Creek deposit. 108
xi LIST OF FIGURES Figure
1. Generalized geology of the Southeast Missouri district. 2 2. Precambrian basement structure map of the Indian Creek subdistrict with an outline of Indian Creek and Goose Creek mines. 4
3. Basement-rock types in Missouri. 11 4. Generalized stratigraphic column for Upper Cambrian strata in the Southeast Missouri lead district showing schematic position of the ore zone in the Indian Creek subdistrict. 13 5. Location of Precambrian and Lamotte outcrops in the St. Francois Mountains. 14
6. Lamotte isopach map for Missouri and Illinois. 15
7. Precambrian basement structure map for Missouri. 21 8. Precambrian basement structure map for the Indian Creek subdistrict. 24 9. Basement structure on the northern flank of the St. Francois Mountains based on aeromagnetic data. 25 10. Schematic evolution of the Precambrian basement structure and topography in Indian Creek subdistrict 27 I la. lsopach map of the Lamotte Sandstone at Indian Creek. 29 llb. Geologic cross-section A-A' of the Indian Creek ridge showing the relationship between basement structure, Lamotte Formation, and lead mineralization. 30 I le. Geologic cross-section B-B' of the Indian Creek ridge showing the relationship between basement structure, Lamotte Formation, and lead mineralization. 31 12. Plot of quartz, feldspar, and rock fragments in the Lamotte Sandstone at Goose Creek. 33
XU 13. Photomicrograph of a phosphorite and quartz clast in the ~ Lamotte quartzarenite.
14. Lamotte Sandstone with ripup clasts of the underlying 36 unit.
15. Lamotte Sandstone showing soft sediment deformation. 43
16. Photomicrograph of quartz dissolution associated with 45 organic-rich clays seams.
17. Medium-grained quartzarenite with sulfides outlining 48 crossbedding.
18. Lamotte Sandstone with a higher concentration of galena 50 at the pinchout of the truncated cross-bed.
19. Z.Ones of high lead and copper content with respect to 51 Indian and Goose Creek mines and the Lamotte pinchout.
20. Quartzarenite with disseminated patches of chalcopyrite. 53
21. Reflected light photomicrograph of a rhomb-shaped 55 chalcopyrite crystal that indicates the replacement of dolomite.
22. Plot of Fe-Co-Ni molar percentage for 56 siegenite.
23. Plot of Fe-Ni-Co molar percentage for 61 bravoite.
24. Cathodoluminescent photomicrograph of rhombic 62 dolomite showing the five zones found at Goose Creek.
25. Reflected light photomicrograph of galena in secondary 66 porosity created by the dissolution of quartz grains.
26. Reflected light photomicrograph of post-pyrite quanz 70 dissolution which resulted in brecciation.
27. Paragenetic sequence for the Lamotte cements in the 71 Goose Creek mine.
28. Paragenetic sequence for the Goose Creek mine. 73
75
XJll 31. Paragenetic sequence for the Laisvall sandstone-hosted deposit, Sweden. 81
32. Congruent and non-congruent cement textures. 84
33. Lead and copper values for assayed intervals in drill hole 64W32. 87
34. Generalii.ed lithologic logs of the Lamotte Sandstone from Goose Creek. 89
35. Vertical tubes of sulfides cross-cut bedding near the Lamotte pinchout in the Goose Creek mine. 91
36. Location of drill holes with assayed amounts of lead in the Indian Creek subdistrict 93
37. Location of drill holes with assayed amounts of copper in the Indian Creek subdistrict. 94
38. Computer-generated isometric plot of the Precambrian basement topography in the Indian Creek subdistrict showing a schematic flow pattern for the metal-bearing fluids. 95
39. Structure map for (a) the base of the copper-bearing zone and (b) the top of the copper-bearing zone. 96
40. Structure map for (a) the base of the lead-bearing zone and (b) the top of the lead-bearing zone. 98
41. Ternary diagram for fluid inclusions with Na-Ca-Cl fluids. 103
1. Map of sample collecting sites and drill holes of logged cores for the Goose Creek mine. pocket
xiv IN1RODUCTION
General Information
Southeast Missouri is the world's largest lead-producing district, having produced over 9 million tons of lead as of 1968, mainly from the Old Lead Belt
(Snyder and Gerdemann, 1968). Futhermore, the estimated mineable reserves of 7 percent combined lead and zinc for the more recently discovered Viburnum Trend are approximately 800 million tons (Kisvarsanyi and others, 1983). Southeast
Missouri deposits are classified as Mississippi Valley-type, having the following major characteristics: 1) epigenetic stratabound origin, which is shown by mineralization of pre-existing strucrures and generally open space precipitation; 2) lack of igneous activity to provide a suitable source for ore-forming components; 3) ore-forming fluids were low temperature (< 200 °C) sodium-chloride brines as indicated by fluid inclusions; 4) commonly dolomitized carbonate host; exceptions include sandstone hosts in Southeast Missouri and elsewhere; 5) normally located in structually positive areas near the margins of intracratonic basins; and 6) only contain a few major minerals with simple chemical compositions.
In Southeast Missouri, the ore deposits are mainly concentrated around the flanks of the St Francois Mountains, an erosional remnant of an uplifted block of Precambrian basement, on the northeastern flank of the Ozark dome (Fig. 1).
The Bonneterre Dolomite is the major ore-hosting formation in Southeast Missouri.
However, the underlying Lamotte Formation hosts ore-grade deposits in two of the four major subdistricts. In the Old Lead Belt subdistrict, ore is concentrated throughout the entire Bonneterre and extends 100 ft into the Lamotte (Snyder and
1 2
INDIAN CREEK Q Missouri
,', ... -...... __ _
I I I Th I I I OLD / LEAD BELT
N ' ' ,() ' '{) iJJ,~ .~ ', VIBURNUM TREND ~ ./>- - • · , • ..: I ~ I • I '4 ~J ; ...; ' ANNAPOLIS . .:
r.-..,. Bonneterre ~ Minerohzeo Areas - Precombn on Outcrop , - ',Edge of Bock· Reef • ,• ' Focies 0 0 16km
Fig. 1 Generalized geology of the Southeast Missouri district
The subdistricts are located around the flanks of the Precambrian St. Francois
Mountains. Indian Creek is the northernmost subdistrict. Modified from Wharton
(1975) 3
Gerdemann, 1968). The Goose Creek mine, in the Indian Creek subdistrict, is almo~t entirely in the Lamotte Sandstone and Lamotte-Bonneterre transition.
Location of Study Area
Indian Creek subdistrict, which is located about 160 km south of St.
Louis, Missouri, is the northernmost mined area of the Southeast Missouri district
(Fig. 1). The subdistrict is situated along a N30°E trending Precambrian ridge and is divided into two mines; the Bonneterre-hosted Indian Creek mine on the northwestern side of the ridge and the mainly Lamotte-hosted Goose Creek mine on the northern end of the ridge (Fig. 2).
History of Southeast Missouri and Indian Creek
Snyder and Gerdemann (1968, p. 328) provided the following picturesque description of the discovery of ore and the early history of the Southeast
Missouri district: Philip Fancis Renault lead a party of French miners and slaves up the Mississippi River from New Orleans in the year 1720 and established himself near Kaskaskia, Illinois. From that point, parties were sent out in search of precious metals. One of these led by M. LaMotte discovered the deposits that bears his name in Madison County, Missouri, near Fredericktown. Gradually other mines were opened in Washington and St. Francois Counties, and mining has been continuous nearly to the present time. This early mining was entirely for chunk galena found in the residuum, which required no milling. The history of modem mining, which led to the opening of the Lead Belt in St. Francois County, began on a 946-acre tract of land, located at the present site of Bonne Terre. It was known as La Grave Mines. On May 2, 1864, the St. Joseph Lead Company purchased this tract of land, and mining began in open pits. These gradually 4
0 ('\I _, • c I z ... 0 a::,_~ c -za...... c ... c ...... z.., • ID ll: C ~~ E..,u -... I a: ID 0 U a: L ICo ... ID ID i:• 0 :: ...... z • ID u 0 O'> (T')
,....0 (T')
0 L/) (T')
0
•(T')
0
(T')
0 0 (T')
0 O'> ('\I
0 ~~~~~~~~~~~~-.-~~~~~~~~~--.~~-r-~--1~ CD ('\I OL21 0921 0521 ant OE21 0221 0121 0021 0611 OQl l OLl l 0911
Fig. 2 Precambrian basement structure map of the Indian Creek subdistrict
with an outline of Indian Creek and Goose Creek mines.
The area in black outlines the mines, and the No. 32 and No. 24 shafts are white circles. The scale along the left and bottom margins is Universal Transverse
Mercator (UTM) coordinates X 10 3. 5
sloped downward until the operation developed into an underground mine; shafts were later sunk for better access to the ore. In 1869, the first diamond core drill was brought to Bonne Terre. Through its use, large bodies of ore were discovered that assured the development of the area as a great mining district Since that time, more than 100,000 diamond drill holes have been completed; from these were obtained 50,000,000 feet of core. As many as 15 companies were in production during the early years from the 1860's to the early 1900's. The St. Joseph Lead Company gradually acquired the holdings of these companies and became the only operator in the district in 1933. The mine at Bonne Terre, which began production in 1864, was closed July 3, 1961, after 97 years of continuous mining.
The discovery of Indian Creek and the Viburnum Trend was less romantic but of great economic importance. Indian Creek subdistrict was discovered in 1948 (Wharton, 1975), and mining began in 1953. Later exploration resulted in the discovery of the Goose Creek orebody in the Lamotte Sandstone. The combined production from both mines is over 14 million tons of ore containing over 350 thousand tons of lead. Both the Indian Creek and Goose Creek mines ceased production in 1982. The discovery of the first Viburnum Trend ore body occurred in
1955 (Wharton, 1975), and the Trend is still in production.
Purpose of This Study
Indian Creek subdistrict is unique because of the Lamotte-hosted Goose
Creek mine. It also has been considered by some (Brown, 1967) to link the Old
Lead Belt with the Viburnum Trend. Additionally, Indian Creek is about 16 km north of the main back reef facies, which has been proposed as a critical source of sulfate and organic material for bacteria production of reduced sulfur (Gerdemann and Myers, 1972). This study was undertaken because previous studies of the 6
Southeast Missouri district have not addressed the question of why ore concentrations of sulfides occur in the Lamotte Sandstone at Indian Creek, or in the
Old Lead Belt
Methods of Study
All accessible exposures in the Goose Creek mine were examined, just
prior to the closing of the mine during a two week period in the summer of 1982, for
depositional and diagenetic indicators (i.e bedding features and mineralization
patterns). Sketches and photographs were made of exposed faces, and one complete
section from the Precambrian basement, through the Lamotte Formation to the
Bonneterre Dolomite was mapped at one inch to one foot. Sixty samples, mainly of
Lamotte Sandstone, but also of Lamotte-Bonneterre transition and Lamotte cobble
conglomerate were collected for laboratory examination. Additionally, samples of
Precambrian basement rocks, basement fault gouge, and Bonneterre Dolomite were
collected where available. Five diamond drill cores from an unmineralized area west
of the Goose Creek mine were also logged and samples taken for microscopic study.
Plate 1 shows the collecting sites within the mine and the location of the core holes.
Approximately 125 polished thin-sections were made from the samples
collected in the mine and from the urunineralized drill core. These were examined
using a standard reflected light and transmitted light petrographic microscope.
Several thin-sections were stained for potassium feldspars, calcite, iron-rich calcite,
dolomite, and iron-rich dolomite. Cathodoluminescence microscopy was used to
distinguish quartz grains with authigenic overgrowths from grains with pressure
solution, and to examine the microstratigraphy of authigenic dolomite.
Many thin-sections and rock fragments were examined with an
International Scientific Instruments scanning electron microscope using an energy 7 dispersive spectrometer. Also, two samples with siegenite and bravoite were anal)'.zed by an Applied Research Laboratory microprobe using wavelength dispersive spectrometers to determine the exact elemental composition. Fluid inclusions from several doubly polished sections of sphalerite were analyzed using a
Fluid Inc. modified U. S. G. S. gas-flow heating/freezing stage to determine the nature of the ore bearing fluids.
Data supplied by St. Joe Minerals Corporation from approximately 400 drill holes were used to generate a structure map of the Precambrian basement, and structure and isopach maps of the Lamotte Sandstone, orebodies, and metal concentrations in the Indian Creek subdistrict. The maps were drawn using CPS-1 software (Radian Corp.) on the University of Texas' Dual Cyber 170n50. These data were also used to construct subdistrict cross-sections.
Previous Work
Numerous publications, theses, and dissertations on the Southeast
Missouri lead district are available. Economic Geology devoted an entire volume
(Vol. 72, No. 3, 1977) to the Viburnum Trend, Southeast Missouri. Futhermore, a symposium on Mississippi Valley-type deposits in 1982 resulted in a proceedings volume (Kisvarsanyi and others, 1983) containing numerous articles about or relating to Southeast Missouri.
However, there is very little published information about the Indian
Creek subdistrict. Brown ( 1967) and Snyder and Gerdemann (1968) reported isotope ratios for Indian Creek subdistrict, Snyder and Gerdemann (1968) and Ohle
(1985) briefly described the Indian Creek mine orebody, Horrall and others (1983) included a paragenetic study of the Goose Creek mine, Jessey (1983) gave a nickel-cobalt ratio for Indian Creek, and Pignolet-Brandom and Hagni (1985) 8 reported microprobe analyses of Indian Creek siegenites. Preliminary results of aspects of this research project were presented at the Seventh Quadrennial symposium of the International Association on the Genesis of Ore Deposits (Kyle and Gutierrez, 1986; in press)
More information is available on the regional character of the Lamotte
Sandstone. Studies by Ojakangas (1960; 1963), Houseknecht (1975), Houseknecht
and Ethridge (1978), and Yesberger (1982) cover the depositional history of the
Lamotte. The diagenetic history of the Lamotte Sandstone is covered by Abraham
(1978), Huggins (1981), and Rothbard (1982; 1983). REGIONAL GEOLOGY
General Information
The Southeast Missouri lead district surrounds the St. Francois
Mountains which lie on the southern edge of the Central Stable Platform and the northeastern side of the Late Precambrian-Early Cambrian (Thacker and Anderson,
1977) Ozark uplift. The Precambrian basement rocks were subjected to regional doming, block faulting, tilting, and an extended period of erosion which resulted in a rugged basement topography (Kisvarsanyi, 1977).
During the Paleozoic, a basal conglomerate and sandstone sequence, followed by a thick series of carbonate units were deposited over the rugged
basement. Each sucessive unit has an updip depositional pinchout against the remnant highs. By late Ordovician only the highest Precambrian peaks remained emergent (Thacker and Anderson, 1977). Periods of deposition and erosion followed until the end of the Pennsylvanian, after which the Ozark uplift and
surrounding region have remained positive (Thacker and Anderson, 1977).
Post-Paleozoic uplift and erosion exposed a 10,000 km2 area of basement rock, of
which the St. Francois Mountains are the largest continuous outcrop (Kisvarsanyi,
1974).
Precambrian Basement Rocks
The Precambrian in Missouri has been divided into two groups; the
"older rocks" of 1,500 Ma or older and the 1,200 to 1,350 Ma St. Francois igneous
rocks (Muehlberger and others, 1966). The older rocks consist of metamorphosed
sediments, gneissic granites (or granitic gneisses of probable igneous origin), and 9 10
basic plutonic rocks (troctolite, olivine gabbro, norite, and pyroxene and hornblende gabbro) in layered intrusions (Kisvarsanyi, 1974). Metamorphic rocks occur in the
subsurface in central and western Missouri and basic plutonic rocks are found in
southeastern, central and western Missouri (Fig. 3) and are associated with large
magnetic anomalies (Kisvarsanyi, 1974).
The younger St. Francois igneous rocks, unlike older Precambrian
rocks, have not been regionally metamorphosed and were both emplaced at a
shallow depth and extruded onto the Precambrian surface (Kisvarsanyi, 1974). The
extrusive volcanics, mainly rhyolites with minor trachytes and trachyandesites,
consist of flows, ash-flow tuffs, coarse-grained pyroclastics, and fine-grained tuffs
(Kisvarsanyi, 1974 ). Based on field evidence, Muehlberger and others (1966)
suggested that two sequences of rhyolite eruptions were followed by two sequences
of granite intrusions. Other than granite, the silicic intrusions include adamellite
porphyries, alkali granite, syenite, and adamellite (Kisvarsanyi, 1974). Dikes of
intermediate composition are also found in southeast Missouri and postdate the
rhyolites but predate the most recent granitic event (Kisvarsanyi, 1973).
The St Francois rhyolites and granites are found in both southeast and
southwest Missouri (Fig. 3). They are probably part of a large igneous complex that extends from western Ohio and eastern Indiana through nonheastern Oklahoma into
western Texas and eastern New Mexico (Muehlberger and others, 1966; Snyder,
1968; Kisvarsanyi, 1974). The igneous belt becomes progressively younger
southwestward (Muehlberger, 1966) and formed a continental divide from
Precambrian through mid-Cambrian. Sediments were deposited in the Keweenawan
basin to the northwest and the Appalachian basin to the southeast (Snyder, 1968). 11
- ,,......
G.!.i.!I~ ...· -...... - - _...... ,...... °"""' ...... QA.ll,M" 0 ...... ,~ --" " ro-c i.oca o: ·····...... · : - -
Fig. 3 Basement-rock types in Missouri.
From Kisvarsanyi (1974). 12
Upper Cambrian Stratigraphy
Introduction
In the Late Cambrian, the Precambrian basement was mostly covered
by a time-trangressive, epicontinental sea. The Upper Cambrian formations (Fig. 4)
in the St. Francois Mountains region represent both trangressive and regressive
deposition, and are mainly carbonates with the exception of the Lamotte Sandstone
(Howe and others, 1972).
Lamone Sandstone
The Lamotte Sandstone unconfonnably overlies the Precambrian
basement. It occurs in the subsurface throughout most of Missouri and crops out in
the St. Francois Mountains (Fig. 5). Howell and others (1944) claimed that the St.
Simon Sandstone in Minnesota and Wisconsin and the Reagan Sandstone in
Oklahoma are time-transgressive Lamotte equivalents. Kurtz and others (1975),
correlated the Reagan Sandstone in eastern Oklahoma, northern Arkansas, and
southwestern Missouri with nearshore facies of the Bonneterre and Davis
Formations. The Larnone Sandstone pinches out in southwest Missouri against the
eastward slope of the Precambrian terrane (Kurtz and others, 1975) and thickens
eastward from 100 ft in western Missouri to 400-500 ft in eastern Missouri
(Ojakangus, 1960; 1963) (Fig. 6). This trend of easr.i.·ard thickening continues with
the St. Simon Sandstone in the Illinois Basin. In the St. Francois Mountain region,
however, 2000 ft of erosional topography (Howe, 1968) produced local variations
in Lamotte thickness and pinch-outs against topographic highs.
The white to gray, mineralogically and texturally mature quartzarenite is
the dominant Lamotte lithology in Missouri (Ojakangas, 1960; 1963). Usually the
medium (0.25-0.5 mm) size fraction is dominant and well rounded, whereas the 13
Em inence Dolomite ( 150' - 300°)
Potosi Dolomite (250' - 300')
z Derby - Doerun Dolomite 4 (100' - 200') a: Davis Formot1on CD (125' - 225') ~ UJz 0 4 N u UJ 0:: 0 a: Bonnete rre For motion (200' -450') w ~
~ ::>
Lo motte Sandstone t (0'-500')
PROTEROZOIC
Fig. 4 Generalized stratigraphic column for Upper Cambrian strata in the
Southeast Missouri lead district showing schematic position of the ore
zone in the Indian Creek subdistrict
From Kyle and Gutierrez (in press) 14
c . 2 I = 3 • ' 6 ? a o l O I ~ I L--J
WAS> ' - / ' r. " .. · ~ - Fig. 5 Location of Precambrian and Lamotte outcrops in the St. Francois Mountains. Modified from Ojakangas ( 1960) 15 • WELL LOCATIONS II - LAMOTTE OUTCROPS -...r ISOF't.CHS l COHTOUR IHTER-...L. • 100 FEET Fig. 6 Larnone isopach map for Missouri and Illinois. Modified from Ojakangas (1960) 16 smaller size fractions are subangular to angular (Ojakangas, 1960; 1963). Sorting varies from fair to very good (Ojakangas, 1960: 1963). The upper Lamotte contains wavy shaley partings and the lower Lamotte locally has thin, hematitic shale beds (Thacker and Anderson, 1977). The lowermost Lamotte near the flanks of igneous knobs is arkosic or conglomeratic (Ojakangas, 1960; 1963; Snyder, 1968; Thacker and Anderson, 1977). The arkosic sandstones are texturally and mineralogically immature (Ojakangas, 1960; 1963). According to Howe and others (1972), the age of the Lamotte is unknown and could represent deposition during any time in the Cambrian. The middle zone of the Cedaria Zones which are used in dating the overlying Bonneterre Formation, is absent in the Lamotte, and it is at least probable that some of the sandstone is pre-Late Cambrian (Howe and others, 1972). In southeast Missouri the base of the Lamotte-Bonneterre transition is placed where the slightly dolomitic sandstone becomes sandy dolomite (Thacker and Anderson, 1977). This transition appears to be conformable; however, in western and central Missouri the transition zone contains reworked Lamotte and possibly represents a regional unconformity (Howe and others, 1972). Outcrops of the Lamotte Sandstone occur only in the area around the St Francois Mountains where the depositional environment and source were influenced by the Precambrian uplift. Thus the depositional environments found in this area cannot possibly be applied to the entire region of Lamotte deposition. Ojakangas ( 1960; 1963) described the Lamotte in the St Francois area as a time-transgressive marine sandstone but suggested that the cross-bedded arkoses might have a fluvial origin. However, he concluded that arkosic sediments which are mixed with well-rounded quartz sand were reworked. In more recent studies however, 17 Houseknecht (1975), Houseknecht and Ethridge (1978), and Yesberger (1982) interpreted the Lamotte in the St. Francois area as braided fluvial and alluvial fan deposits, except nearshore and marginal marine deposits in the upper Lamotte. Yesberger (1982) also reported eolian dune and interdune complexes associated with braided steam deposits in the area northwest of the St. Francois Mountains. Although the St. Francois Mountains were the source of arkose, and some angular quartz and heavy minerals, the area is too small to have provided enough quartz for the entire Lamotte Formation (Ojakangas, 1963). Also the heavy mineral assemblage of the quartzarenite sandstone consists of zircon and tourmaline which are not found in the St. Francois region (Ojakangas, 1960; 1963). A reworked sedimentary source is indicated for the Lamotte by well-rounded tourmaline and quartz grains with abraded overgrowths (Ojakangas, 1960; 1963). Tourmaline-bearing Precambrian sandstones in the Lake Superior region are mineralogically similar to the Lamotte and the paleogeography of the region and southward dipping paleoslope makes them the logical source of Lamotte sediment (Ojakangas, 1960; 1963). Paleocurrent directions also indicate a northwestern source. However, Houseknecht (1975), Houseknecht and Ethridge (1978), and Yesberger (1982) believed the source was a pre-Lamotte sedimentary cover to the northwest of the St Francois Mountains which was related to the Lake Superior sandstones. This belief is based on sets of crossed-bedded fluvial deposits which they conclude would unlikely have occurred in a fluvial system transporting sediment all the way from the Great Lakes region. Ojakangas (1960; 1963) also considered the peneplaned Precambrian basement in Missouri and adjacent areas to the west and northwest as a source, but only a minor one and not related to the Lake Superior sandstones. 18 Evidence for climatic conditions during Lamotte deposition are not completely clear. Ojakangas (1960) suggested a temperate climate with moderate rainfall based on what he identified as detrital kaolinite. Paleomagnetic data however, indicate that during the Late Cambrian the equator probably extended north-south across North America (Dott and Batten, 1976; Dott 1974) and the approximate position of southeast Missouri was near latitude 10° S. Violent tropical stonns would be expected in a low latitude, even during the Cambrian (Dott, 1974), and a climate with short periods of intense rainfall is suggested by the alluvial fan deposits that occur within the Lamotte (Houseknecht, 1975). Also according to Houseknecht (1975), a wann and humid climate conducive to chemical weathering is indicated by badly altered feldspar grains and the scarcity of feldspar in subarkosic sandstones derived from granite in southeast Missouri. However, alteration and dissolution of feldspar also occur during diagenesis. Bonneterre Fonnation The Bonneterre has been divided into lower, middle, and upper units. The lower Bonneterre is the Lamotte-Bonneterre transition (Kurtz and others, 1975) that is comprised of dolomite and quartz sand, and commonly shale, glauconite, and igneous rock fragments and cobbles near the Precambrian highs. Dolomite content usually increases upwards and the transition contains more large, oboloid brachiopods than the upper Lamone (Kunz and others, 1975). The middle Bonneterre is mainly composed of oolite facies intertonguing with contemporaneous micrite and shale facies (Kurtz and others, 1975). Gerdemann and Myers (1972) labeled the fine-grained carbonates and elastics as the offshore facies. The oolites also formed broad, shallow-water platfonns in offshore areas (Kunz and others, 1975). In the St. Francois region, 19 reef facies of algal boundstone and well developed digitate stromatolites formed barriers near the Precambrian highlands (Gerdemann and Myer, 1972). A dolomitized baclaeef facies of algal laminated mudstones, burrowed mudstones, rubble and pebble conglomerates, and patchy green shales formed landward of the barrier reefs (Gerdemann and Myers, 1972). Locally this zone is referred to as "white rock" and probably represents tidal flat deposition (Gerdemann and Myers, 1972). Thacker and Anderson ( 1977) interpreted the reef and back:reef facies as being contemporaneous with the oolitic facies. The Sullivan Siltstone Member and the overlying inhomogeneous Whetstone Creek Member make up the upper Bonneterre. The Sullivan Siltstone has a sharp, unconformable contact with the middle Bonneterre and is a micritic, laminated siltstone with calcarenite and mudchip conglomerate layers locally (Kunz and others, 1975). There is a gradational contact between the siltstone and Whetstone Creek Member (Larsen, 1977). The Whetstone Creek is very inhomogeneous and shale, silt, sandstone, or carbonate may be the dominant lithology anywhere in the unit (Kunz and others, 1975). Most of the Bonneterre Formation has been dolomitized in the St. Francois region but is still predominantly limestone elsewhere (Gerdemann and Myers, 1972; Snyder and Gerdemann, 1968). It thickens southeastward in Missouri and is 350-400 ft thick in the St. Francois Mountains, except where it pinches out against Precambrian highs (Larsen, 1977; Howe and others, 1972). The Bonneterre can be correlated with the Eau Claire Formation in Illinois (Howe and others, 1972) and the Reagan Sandstone in eastern Oklahoma (Kurtz and others, 1975). 20 Davis Formation The Davis is the lower formation of the Elvins Group and rests unconformably on the Bonneterre Formation in southeast Missouri (Thacker and Anderson, 1977). In areas where the Whetstone Creek Member is indistinguishable from the Davis, the contact has a conformable appearance (Thacker and Anderson, 1977). The Davis consists of interbedded burrowed silty to sandy green shale, silty to sandy limestone or dolostone, calcareous siltstone, and irregular zones of mud-chip conglomerate (Thacker and Anderson, 1977; Howe and others, 1972). The lower part of the Davis Formation is mainly green shale and glauconite pellets, and there is a decrease in elastic material upwards (Thacker and Anderson, 1977; Kurtz and others, 1975). The average Davis thickness is 170 ft (Snyder and Gerdemann, 1968). Structural Geolo~ One of the major structural features in Missouri is the basement topography. A structural map (Fig. 7) constructed by Kisvarsanyi (1974) shows the exposed Southeast Missouri high and three subsurface basement highs (Southwest, Central, and Northeast Missouri high) that lie along a southwest-northeast trend. The basement dips steeply from the Central and Southeast highs to the northeast toward the Illinois Basin, and from the Southeast high toward the Mississippi Embayment. A more gently dipping slope exists from the Central and Northeast highs to the northwest. Another major structural feature in Missouri is faulting. In the St. Francois region, the basement and overlying sedimentary units are highly faulted. Most faults fall into one of three major trends: N30°W (Saint Genevieve, Simms Mountain, Black, Ellington), N50°E (Greenville, Conway, Big River), and E-W 21 Fig. 7 Precambrian basement structure map for Missouri. Areas above sea level are shaded and faults are shown by a heavy line. Contour interval 200 ft. From Kisvarsanyi (1974). 22 (Palmer, Saint Genevieve, Sweetwater) (Pratt, 1979). Fault movement during Lamotte deposition is shown by the thickening of the downthrown side of a fault on the northern side of the St. Francois high (Thacker and others, 1979). INDIAN CREEK STRUCTURE The Indian Creek subdistrict is located along a N30°E-trending, Precambrian rhyolite ridge (Fig.8). The ridge slopes steeply toward the basin on the northwest and southeast sides. Along this high are two prominent peaks (labeled P 1 ) and P2 separated by an east-trending valley. A spur on the southwest part of P1 runs parallel to the valley. Another prominent feature is a saddle on the north-northeastern tip of the Indian Creek ridge. A structure map of the Rolla quadrangle (Kisvarsanyi, 1979) shows a large topographic high in which the Indian Creek subdistrict occupies approximately the southern half (Fig. 9). The saddle appears to separate the Indian Creek ridge from another ridge to north along the same trend. A N70°E-trending fault intersects the northern end of the Indian Creek ridge (Fig. 8). The fault was observed cutting the Precambrian basement in the Goose Creek mine, but the direction of movement on the fault could not be determined. The general basement structure, however, suggests the raised block is to the south. Many Precambrian faults in southeast Missouri were reactivated during or after Cambrian deposition (Kisvarsanyi, 1977). This fault was not found elsewhere in the Goose Creek mine, in either Lamotte or Bonneterre, even though extensive faulting is suggested by the six ft of fault gouge found along the fault. Therefore, the Indian Creek fault appears to have been inactive at least since the · onset of Cambrian deposition. Structural offsets along the Indian Creek ridge (Fig. 8) suggest that other 23 24 Fig. 8 Precambrian basement structure map for the Indian Creek sulxlisoict. Important features referred to in the text are labeled; fault is shown by a bold line. Plus marks show locations of drill holes used to construct map. 25 ' .. 00 0 4 rfli Contowt il'ltt r • I I 200 ft Fig. 9 Basement sturcture on the northern flank of the St. Francois Mountains. The 200 and 400 ft Precambrian structure contour lines form the Indian Creek structure map (Fig. 8) have been added. A dashed 200 ft contour line to the north of Indian Creek is the proposed secound ridge along the same trend. Indian Creek subdistrict is outlined by a stippled pattern and the black areas are exposed Precambrian basement. Contour interval 200 ft. Modified from Kisvarsanyi (1979). 26 east-trending faults may exist Also, the ridge itself is probably bounded on both sides by northeast-trending faults. Prior to east-west faulting, the linear ridge developed a shallow-sloping northwest flank and a steeply sloping southeast flank (Fig. lOa). Subsequently, the ridge was faulted and then exposed to a long period of erosion which resulted in the pre-late Cambrian topography (Fig. lOb). Both the northeast bounding faults and the east-trending faults correspond with regional faulting patterns shown by Pratt (1972). However, the northwest faults (Saint Genevieve, Simms Mountain, Black, Ellington) that are present elsewhere in southeast Missouri are not suggested by the Indian Creek structure, even though the northwest-trending Aprus fault lies to the east of Indian Creek. 27 (a) Fig. 10 Schematic evolution of the Precambrian basement structure and topography in the Indian Creek subdistrict. (a) Linear ridge has a shallow sloping northwestern flank and a steeply sloping southeastern flank; (b) East-trending faults cut the ridge and extensive erosion produced the rugged topography. Bold lines are faults. LAMOITE COMPOSffiON AND DEPOSITIONAL HISTORY General Infonnation In the Indian Creek subdistrict, the measured Lamotte thickness varies from 0 ft where it pinches out against the Precambrian ridges to greater than 100 ft toward the basin (Fig 11 a). The Lamotte is comprised of a thin, discontinuous basal cobble conglomerate overlain by quartzarenite. A thin sandy dolomite transition zone lies between the Lamotte quartzarenite and the overlying Bonneterre dolomite. This transition zone is considered to be the basal unit of the Bonneterre. Basal Cobble Conglomerate A basal cobble conglomerate discontinuously underlies the Lamotte Sandstone(Fig. 11 b). A discontinuous cobble conglomerate also underlies the Bonneterre Dolomite where the dolomite directly overlies the Precambrian basement (fig. llb). These cobble conglomerates probably represent reworked regolith and colluvium from the Precambrian basement However, another cobble conglomerate also occurs locally between the Lamotte and Bonneterre near the Lamotte pinchout (Fig 1 lc). Conglomerates were considered by Ojakangas (1960) to be a function of local relief and best developed next to high Precambrian knobs, and interformational conglomerates developed as the seas transgressed, reaching new supplies of detritus. However, interformational conglomerates could be related to syndepositional tectonic activity (Posey, 1987, pers. comm.). Tectonic activity would explain why the conglomerates are restricted to thin intervals. However a storm deposit might also produce similar results. 28 29 ....,0 ...., 0 ('\j (T) 0 ...., 0 0 (T) 0 0) ('\j 0 t--~-.~~r-~-r~~-r-~~~~-r--~-.~~-.--~--.~~--r-~---1~~ Ol2t 0921 0521 ant 0£21 0221 0121 0021 0611 0811 OLll 0911 Fig I la. Isopach map of the Lamotte Sandstone at Indian Creek. Zero contour is the pinchout of the Lamotte against the Indian Creek ridge. A - A' and B - B' are cross-section lines shown as Figs. 11 b and 11 c. Small circles are locations of drill holes used to construct the map. 30 • ~ • ~ 0 c .e e ... ~ E 0 0= ~ ~ 0 0 c c 0 0 ~ .2 r0 CL li c u t E • "'~ . ~ o E 0 : c ¥ : E ;; •c ,_; ~ ~ 0 0 0 0 "' CL~ J Cl) m t_J dm . rn D' ~ D ~ ~ 8 8 ' N' "'' Fig. 1 lb Geologic cross-section A - A' of the Indian Creek ridge showing the relationship between the basement structure, Lamotte Formation, and lead mineralization. Datum is the top of the Bonneterre Formation. 31 ·:._,· .··.· I f I / . I ,--I · 1. · ( . Fig. I le Geologic cross-section B - B' of the Indian Creek ridge showing the relationship between the basement structure, Lamotte Formation, and lead mineralization. Datum is the top of the Bonneterre Formation. Modified from Kyle and Gutierrez (in press) 32 The Lamotte conglomerate was not commonly encountered by the numerous wells drilled at Indian Creek, and the greatest thickness penetrated was 8 ft Subsurface exposures of the Lamotte basal conglomerate occur in four locations in the Goose Creek mine, but only two exposures were accessible for detailed examination (Plate 1, Sites 2 and 12). The conglomerate consists of rounded to sub-angular, equant to elongate, 2 to 30 cm cobbles of weathered porphyritic rhyolite. Cobbles are generally rounded, slightly elongate, and 15 to 20 cm in diameter. The space between cobbles is filled with medium to coarse grained sublitharenite of volcanic rock fragments and probable illitic rock fragments. Distinct weathering rinds as much as 2 cm deep were observed on the rounded surfaces of the cobbles, but even the interior of the cobbles are not pristine. Angular or broken surfaces of the cobbles do not have alteration, which indicates that the cobbles were rounded and rinds formed before deposition and not as a product of hydrothermal alteration as suggested by Sverjensky (1986) for such features in the Viburnum Trend. Sandstone Detrital composition Introduction The major elastic constituents are quartz, sedimentary rock fragments, volcanic rock fragments, and fossils. Quartz is the most abundant grain type, commonly comprising nearly 100 percent of the Lamotte (Fig. 12). Sedimentary rock fragments, which are locally derived ripup clasts, occur in concentrations of up to 20 percent in some beds. Volcanic rock fragments are more common than sedimentary rock fragments, but rarely occur in concentrations greater than 1 to 2 percent Calcium phosphate fossil fragments are usually found in concentrations of 33 a • 6 samples a 2 samples O 1 sample F R Fig. 12 Plot of quartz (Q), feldspar (F), and rock fragments (R) in the Lamotte at Goose Creek. The rock fragments are locally derived ripup clasts and volcanic sand. 34 less than 1 percent and are part of many sedimentary rock fragments. Except for beds with ripup clasts and local beds of silty carbonate, the Lamotte Sandstone in the Indian Creek subdistrict is quartzarenite (all sedimentary classifications are based on Folk, 1974). Minor constituents include clay and organics, tourmaline, feldspar, zircon, muscovite, and rutile. Clay and organics are mostly concentrated in thin beds and wispy partings. Other minor constituents are commonly found in the silt size-fraction, with tourmaline being the most common. The scarcity of feldspar in the Lamotte Sandstone in the Indian Creek subdistrict indicates either the absence of feldspar during deposition, or the diagenetic loss of feldspar after the Lamotte was buried. No evidence exist to indicate that the latter occurred. In fact, the presence of feldspar in the silt-size fraction in some samples suggests that significant loss of feldspar did not occur. A study by Rothbard (1982; 1983) of the Lamotte Sandstone from the St. Francois Mountains area revealed two types of clay seams. These include a dark gray organic-bearing type and a lighter gray and almost pure illite type. A core from Washington Co. contained many dark gray stylolites of brown illite, orange-amber organics, and pyrite microcrystals (Rothbard, 1982; 1983). This orange-amber gel is common in the thin clay-rich beds and stylolites at Goose Creek. The gel was identified as a partially soluble pyro-bitumen (Rothbard, 1982; 1983). Quartz Almost all of the detrital quartz at Indian Creek is common quartz (after Folk, 1974 ); poly-quartz is very rare. Most grains have straight to slightly undulose extinction and microlites are fairly common. The microlites inc'lude (in the order of abundance) tourmaline, rutile, muscovite, zircon, biotite, and magnetite and 35 hematite. Tourmaline occurs in many colors: green, olive, yellow-green, amber, brown, pink, and colorless. Many of the minor grain types, normally found only in the fine grain-size fraction, are the same as the mineral inclusions in the quartz. Sedimentary rock fragments There are six types of sedimentary rock fragments in the Lamotte at Goose Creek: silty carbonate clasts, sandstone and siltstone clasts, silty claystone clasts, carbonate clasts, sandy phosphorite clasts, and phosphorite clasts. Silty carbonate clasts are slightly more abundant than sandstone and siltstone clasts, and the silty claystone clasts, which occur in equal abundance. The carbonate to silt ratio varies from 1:1 to1:15. Silt grains are mostly quartz with some feldspar and minor tourmaline. The sand grains are all quartz except for minor calcium phosphate fossil fragments. The silty claystone clasts appear to be illitic, but organic and iron-staining, and the generally squashed appearance make positive identification difficult. Carbonate clasts that are free of silt are uncommon. Both the carbonate and the silty carbonate clasts have been dolomitized and the rare ghost of cystoid colurnnals are all that remains of the original texture. The most unusual rock fragments are the phosphorite and sandy phosphorite clasts. In the sandy clasts, sand and silt grains are supported by a phosphorite matrix (Fig. 13). Phosphorites also occur as wispy partings that probably represent in situ precipitation at Goose Creek. Although not very abundant, the phosphorites are unique and have not been previously reported for the Lamotte Formation, or in the Southeast Missouri district. 36 I ' .___.. J ,.. .._ 7 - - - Fig. 13 Photomicrograph of a phosphorite (brown matrix) and quartz clast in the Lamotte quartzarenite. Quartz grains in contact with phosphorite have been subjected to intense pressure solution. Sample L-56B. Maximum dimension 7.2 mm. 37 Chemical analyses (Table 1) of the phosphorites and calcium phosphate fossil fragments show that they are very similar in composition. The phosphorites, however, do not appear to have developed from the accumulation of hard pans of fossils. Also, the lack of an internal texture suggests that the phosphorites were deposited as chemical precipitates. This implies that the similarity in composition between the fossils and the phosphorites is a characteristic of the sea water from which both were derived. Most phosphorite deposits fonn at low latitudes, although some Cambrian deposits possibly fonned at intennediate (30 - 50°) latitudes (Cook and McElhinny, 1978). Phosphatic ooids, in addition to phosphorites associated with algal growths, srrongly suggest a shallow water origin (Bushinski, 1964 ). Phosphorites are also associated with large-scale coastal upwelling zones (Ziegler and others, 1978). Deep cold oceanic water contains about 0.3 ppm P04, whereas warm surface water has only 0.01 ppm or less (McKelvey, 1967). Upwelling occurs when wind and currents move warm surface water offshore, and deep cold phosphate-rich water moves up to replace it (McKelvey, 1967). As temperature and pH increase, phosphate is precipitated (Krauskopf, 1979). Although the phosphorites mostly occur as ripup clasts in the Goose Creek mine, they probably exist elsewhere in the Lamotte as thin beds. Volcanic rock fragments Volcanic rock fragments are subangular to well-rounded and are in various stages of weathering and silicification. This feature indicates that some of the weathering, silicification, and minor glauconitization may have occurred prior to deposition. Volcanic rock fragments contain quartz and feldspar phenocrysts. 38 TABLE 1 ANALYSES OF PHOSPHORITES AND FOSSIL FRAGMENTS p p F F F eao 53.4 66.8 65.8 64.6 65.7 P205 44.9 31.3 34.0 34.9 34.3 MgO 1.1 1.8 0.0 0.3 0.0 Cl 0.6 0.0 0.2 0.2 0.0 P= Phosphorite (L-56B); F= Calcium phosphate fossil fragment (L-42) Analyzed with an International Scientific Instruments Scanning Electron Microscope, Electron Dispersive Spectrometer (EDS); 30 second analysis rime. Weight percent normabzed to 100 percent 39 Some of the large feldspar laths have Carlsbad twinning. Both typical volcanic quartz and spherical quartz crystals occur as phenocrysts. Resorption of comers of the equant quartz probably produced the sphericity. Feldspar, quartz, apatite, and volcanic glass make up the groundmass. Fossils Calcium phosphate fossil fragments occur throughout the Lamotte Sandstone within the Goose Creek mine. Almost all the fragments display an internal shell structure. However, broken and rounded surfaces, plus the calcium phosphate composition, made identification of the fossils impossible. Brachiopods were the most common calcium phosphate fossil during the Late Cambrian (Shimer and Shrock, 1980) and may be the source of the fossil hash. Ojakangas (1960) reponed phosphatic fossil fragments in the Pea Ridge area with some occurring as much as 115 ft below the top of the Lamotte. He also found phosphatic fossil fragments at Flat River and Pilot Knob, but only in the upper Lamotte. Grain size, sorting and roundness The average quartzarenite grain size is medium (250 - 500 m). However, size ranges from very fine to very coarse resulting in a moderately to poorly sorted sandstone. Most of the grains are rounded to well-rounded; finer fractions range from subangular to well-rounded. Except for sorting, the monomineralogy and well-rounded grains indicate a mature sandstone. Folk (1974) refered to this situation as a textural inversion, which indicates mixing of depositional enviroments or erosion and redeposition of older sandstone. This is in agreement with both Ojakangas' (1960; 1963) interpretation of the source area and Houseknecht's (1975) and Houseknecht and Ethridge's (1978) opinion on the mixing of sediments from both marine and fluvial environments. 40 Sedimentary rock fragments and volcanic rock fragments range fro m coarse sand to pebbles and are subangular to well-rounded. Tabular sedimentary rock fragments can be very long (up to 1.5 m), but they typically are only 2-3 times as long as wide. Lamotte-Bonneterre Transition The Larnotte-Bonneterre transition is considered to be the lowermost unit of the Bonneterre Formation and is conformable with the Lamotte. In the Goose Creek mine, the contact between Lamotte Sandstone and the transition zone is abrupt. At one location (Plate 1, Site 8 through Tl) a 7.5 ft bed of quartzarenite in the Lamotte is overlain by two, 3 ft beds of lower Bonneterre. The lower of the two beds contains approximately 12 percent common quartz, 12 percent volcanic rock fragments, 5 percent glauconite, plus traces of echinoderm (cystoid columnals) fossils. The quartz grains are similar in genetic classification (i.e. common), size, soning, and roundness to those in the underlying Lamotte. The volcanic rock fragments are porphyritic , weathered, silicified, and glauconitized. The upper bed however, is mostly dolomite with only traces of poly-quartz, volcanic rock fragments, feldspar, gl auconite, and echinoderm fossils. Common quartz is extremely rare. The difference between detrital components of the two transition beds indicates a change in the source. Similarities between the Lamotte quartz sand and that in the lower bed suggest either th at Lamotte was reworked during lower Bonneterre deposition or that input of common quartz dwindled. Poly-quartz in the upper bed is similar to quartz in St. Francois Precambrian rocks and was probably derived by weathering. 41 Structural Influence On Deposition The Lamotte Sandstone pinches out against the Precambrian high at approximately 80 ft MSL in the Indian Creek subdistrict. An isopach map of the Lamotte Sandstone and two cross-sections (Fig. 1 la,b,c) show the influence of the pre-Lamotte structure on Lamotte deposition. Variations in sandstone thickness correspond with many prominent features of basement topography (Fig. 8). The Lamotte thins across the saddle in the nonheast, thickens toward the basin on both sides of the nonheast-trending high, and thickens above the east-west trending valley (Fig. 1 la). Sedimentary Structures And Depositional Environment Cross-bedding Only the upper 40 ft of the Lamotte Sandstone is exposed in the Goose Creek mine. The sandstone was mostly deposited in horizontally laminated and planar cross-bedded units, ranging from 0.5 to 8 ft thick. On the western side of the saddle, cross-bedding has a N25°W strike and apparent dips of 30° SW to 25° NE. However, average apparent dip is less than 10° NE. Trough cross-bedding occurs eastward over the saddle. Apparent dips on these trough cross-beds range from nearly horizontal to 25° SE. The apparent dip is always less than the true dip unless the surface from which the measurement was taken was perpendicular to strike. Therefore the true dips could actually be much steeper than the dip measurements listed above . Interbedded shales and carbonates Some sandstone beds contain wispy shale partings and are locally interbedded with thin, silty shale. Sandstone beds above the saddle are thinner and 42 interbedded with thin, silty, dark gray carbonate. These thin carbonate beds are 10-15 ft below the Larnotte-Bonneterre transition. These beds vary from slightly silty carbonates to a siltstone with a carbonate matrix. In contrast, the transition is mostly carbonate with minor glauconite, volcanic rock fragments, and up to 12 percent quanz sand. Also, the transition carbonate has ghosts of cystoid columnals, whereas the interbedded silty carbonate in the Lamotte contains calcium phosphate fossil hash. Ripup clasts Sandstone beds containing ripup clasts occur throughout the Goose Creek mine. They vary from about 0.5 to 4 ft thick. The clasts are usually pebble size or smaller and are rounded or angular. Larger, tabular ripup clasts are also common. A ripup clast approximately 1.5 m long was observed on the western side of the mine (Plate 1, Site 13). The ripup clasts occur both randomly and aligned with cross-bedding. Sandstone, siltstone, mudstone, silty carbonate, and carbonate clasts are found in varying combinations throughout the mine. Sandy, calcium phosphate clasts were only found in the central and eastern pans of the Goose Creek mine and were not very abundant (Plate 1, Sites 17 and 19). Many of the ripup clasts resemble the immediately underlying bed, suggesting local erosion and nearby redeposition (Fig. 14). Also the fragility of sand and silt clasts, and the size of some of the clasts suggesting a very short transport distance. 43 Fig. 14 Lamone Sandstone with ripup clasts of the underlying unit Gray spots are disseminated galena. Sample L-22. 44 Soft-sediment deformation A few examples of soft-sediment deformation occur in the Lamotte. Slump folding (Fig. 15) is the most prominent, with some of the folds completely recumbent Contorted bedding also occurs. Depositional Environment The Lamotte Sandstone at Indian Creek appears to have been deposited in a marginal marine environment. Wispy stringers of phosphorite, carbonate and phosphorite ripup clasts, calcium phosphate fossil fragments, and interbedded carbonate beds are indicative of a marine setting. Yesberger (1982) reported intertidal channel, tidal flat, and nearshore marine deposits in Washington Co., where the Indian Creek subdistrict is located. A channel, which is probably a shallow tidal inlet (interpretation based on descriptions in Reinson, 1980), cuts the planar cross-beds on the western side of Goose Creek mine. The cross-bedding observed in the mine supports, but does not confirm, marginal marine conditions, and possibly represents the upper shoreface-foreshore facies (interpretation based on descriptions in Elliott, 1978, and Reinson, 1980). 45 Fig. 15 Lamotte Sandstone showing soft sediment defonnation. Sample L-49. DIAGENESIS Sediment Compaction And Pressure Solution According to Rothbard (1982, 1983), the Lamotte was never buried deeper than 0.5 km. Present day burial depths around the Goose Creek mine range from 275 to 380 m. Evidence of compaction is mostly obscured by primary and secondary porosity (up to 20 percent) and authigenic cements (up to 35 percent). Nevertheless, compaction with pressure solution is displayed by interlocking quartz grains, and stylolites found throughout the Lamotte. Interlocking quartz grains Interlocking occurs when the stress on a grain-to-grain contact results in dissolution. Normally only one grain dissolves, or dissolves faster than the other, thus allowing the more stable grain to slowly penetrate, interlocking the two grains. If both grains are homogeneous, crystallographic orientation, origin (i.e. plutonic, volcanic, metamorphic), fluid inclusion content, detrital history, or size may determine the grain stability. Authigenic quartz overgrowths that completely fill the pore space can resemble interlocking grains unless a "dust ring" or other feature makes it possible to differentiate between the two. A few quartz grains in the Lamotte at Goose Creek have "dust rings". Other grains contain numerous tiny fluid inclusions that outline the grain from the overgrowth. However, most of the anhedral quartz overgrowths are only discernible under cathodoluminescent ligh t. Detrital quartz luminesces blue to bluish red and the authigenic quartz luminesces a dull red. Some detrital quartz 46 47 grains also exhibit a dull red luminescence and are not distinguishable from their authigenic overgrowth. Some interlocking quartz grains occur as both tightly stacked columns and clusters at Goose Creek. Columns of quartz grains are also commonly found elsewhere in the Lamotte in southeast Missouri (Rothbard, 1982). Columns consist of vertically stacked quartz grains with sutured boundaries along horizontal contacts and unsutured boundaries elsewhere. The columns are commonly only one grain in diameter. However, grains in interlocking clusters have sutured boundaries at all contact. The columns of grains indicate vertical pressure only and probably formed early, thus supporting the overburden and preventing futher compaction with more efficient spacing. Where futher compaction with more efficient spacing occurred, overcrowding produced a horizontal pressure that resulted in the pressure solution observed in the clusters. It should be noted however, that this occurs on a microscopic scale and both types of pressure solution are found within the same thin-section. Stylolites Stylolites occur throughout the Lamotte and quartz grains abutting them are partially dissolved (Fig. 16). The stylolites contain illite, pyro-bitumen (organics identified by Rothbard, 1982, 1983), and minor amounts of silt. They also contain minor amounts of sulfides, mostly frambodial pyrite. The intense dissolution of quartz in the presence of organic matter suggests that the solubility of silica is enhanced by organic acids. Blatt and others (1980) suggested a similar chemical response based on an increase in stylolitization near coal beds. Although organic complexes were shown to increase the solubility of aluminosilicates (Surdam, and others, 1984), these tests were not applied to quartz. 48 Fig. 16 Photomicrograph of quartz dissolution associated with organic-rich clay seams Sample L-40. Maximum dimension 7.2 mm. 49 Another explanation for the association of clay- and organic-rich zones with with intense pressure solution is the concentration of an insoluble residue of .clays and organics along the dissolution front. However, where only minor pressure solution occurs, the contact between grains does not contain even a small amount of clay or organics. Pressure solution associated with phosphorite Quartz grains in a phosphorite matrix in the Lamotte commonly exhibit more intense pressure solution than the nearby clean sandstone (Fig. 13). Quartz with phosphorite is found in ripup clasts and wispy zones similar to the wispy shale partings observed throughout the Lamotte at Goose Creek. As with the organic matter, pressure solution seems to have been enhanced by the presence of phosphorite. Authi genic Cements Sulfides Seven sulfide cements occur in varying concentrations in the Lamotte Sandstone at Goose Creek. These include pyrite, marcasite, galena, chalcopyrite, siegenite, bravoite, and sphalerite. Most sulfides occur as randomly disseminated blebs within the sandstone (Fig. 14) and others occur along organic-rich clay seams and stylolites, along cross-bedding, and in cross-bedding pinchouts (Figs. 17 and 18). Pyrite and marcasite Iron sulfides are found throughout the mineralized Lamotte Sandstone in the Goose Creek mine. Pyrite and marcasite concentrations range from a trace to approximately 15 volume percent. These two iron sulfides are commonly found together, either mixed or in alternating paragenetic positions. Locally, the ratio of 50 Fig. 17 Medium-grained quartzarenite with sulfides outlining crossbedding. A wispy shale stringer occurs in the left, lower quarter of the photo. Height of exposure is 2 ft. Site 10 (Plate 1). 51 Fig. 18 Lamotte Sandtone with a higher concentration of galena at the pinchout of the truncated cross-bed. The crossbedded unit is bounded by thinly bedded sandstone and silty shale. Arrow points to pinchout. Height of exposure is 5 ft. Site 24 (Plate I). 52 the two minerals is extremely variable and either polymorph can be the dominant phase. Typically, the dominant iron sulfide is several times more abundant than the other polymorph. Pyrite is most often the most abundant of the two. Pyrite was precipitated as framboids, cubic and cuboctahedral crystals, and anhedral pore cement The framboids formed early and locally served as nuclei for later pyrite growth. Marcasite formed small elongate euhedral crystals, large (up to 1 mm) equant crystals, and twinned blades, either completely filling the pore space or as short radiating clusters. Iron sulfides along with other pre-galena sulfides are preferentially concentrated in and next to organic-rich stylolites, silty shale partings, elastic and carbonate ripup clasts, phosphorites, and volcanic rock fragments within the quartzarenite. These zones may have provided reduced sulfur or channeled fluid flow, resulting in localized precipitation. Galena is not normally concentrated in these zones which suggests that channeled flow was not the major controlling factor in pre-galena precipitation. Galena Galena is the most abundant sulfide at Goose Creek even though it is not ubiquitous like pyrite and marcasite. The average concentration of galena is 3-4 volume percent, with local concentrations of nearly 40 volume percent. The location of the high grade lead zones for the Indian Creek subdistrict are shown in figure 19. Maximum lead concentrations in the Lamotte at Goose Creek occur on the eastern and western ends of the mine. Galena, unlike the earlier sulfides, is not nonnally associated with the silt and organic-rich zones. Commonly galena occurs as 1-3 mm blebs, either aligned with crossbedding or randomly disseminated (Figs. 14 and 17). Galena crystals are 53 0 CD (T) ,....0 (T) 0 CD (T) 0 L/') (T) ....0 (T) 0 (T) (T) 0 C\) (T) 0 (T) 0 0 (T) 0 en C\) 0 1--~~~~~~~~~~~--.~~-.-~~.--~-.-~~r-~~~~+-~ OLCl 0921 0521 0•21 0£21 0221 01 21 0021 0611 OBl 1 OL 11 0911 Fig. 19 Zones of high lead and copper content with respect to Indian and Goose Creek mines and the Lamotte pinchout High copper (> 20 ft percent) is shown by a striped pattern and high lead (> 40 ft percent) by a stippled pattern. 54 usually anhedral, but cubic and cuboctahedral terminations are found in pores and fractures. Cubic galena surrounding several sand grains is rarely observed, and then only a vague cubic shape exists. Nevertheless, the equant galena blebs reflect the cubic habit. Elongate spots only occur where blebs have coalesced. Some of the cubic crystals in the transition zone have minor cuboctahedral modifications. Horrall and others (1983) reported spots of galena with crude cuboctahedral habits as being typical in the Lamotte Sandstone at Goose Creek. However, spots of cuboctahedral galena were not observed in the Lamotte in this study. Rare occurrences of cuboctahedral galena were observed, but only on a pore-size scale. Cuboctahedral galena was precipitated before authigenic quartz. Chalcopyrite About half of the samples collected at Goose Creek, which were distributed throughout the mine, contain chalcopyrite. Chalcopyrite averages 0.5 volume percent for the mine, however high grade concentrations approaching 8-10 volume percent are present locally (Fig. 20). A map of the copper content in the Indian Creek subdistrict shows high concentrations in the Lamotte on the northern end of the Indian Creek ridge, west of and over the saddle (Fig. 19). Like the iron sulfides, chalcopyrite occurs more frequently in organic-rich zones, silty partings, and rock fragments. However, large concentrations of chalcopyrite occur more frequently in the quartzarenite than do the iron sulfides. Chalcopyrite occurs in an anhedral mass, except for the rare bladed crystal or curved rhomb. Blades of chalcopyrite indicate the replacement of marcasite, and dolomite replacement is shown by the chalcopyrite rhombs (Fig. 21). 55 Fig. 20 Quartzarenite with disseminated patches of chalcopyrite. Sample L-45 is from a high-grade copper zone (Site 27, Plate 1). 56 Fig. 21 Reflected light photomicrograph of a rhomb-shaped chalcopyrite crystal that indicates the replacement of dolomite. Chalcopyrite (cp) is ye11ow; quartz (qz) is gray; porosity (p) is black. Sample L-43. Maximum dimension 1.1 mm. 57 Siegenite Siegenite [(Co,Ni)3S4] is almost as common as chalcopyrite at Goose Creek, but never occurs in concentrations greater than about 1 volume percent. It is distributed throughout the Goose Creek mine and also occurs preferentially next to organic-rich zones, silty parting, and rock fragments. Additionally, siegenite is a pore cement within the quartzarenite. Commonly, siegenite occurs in pyritohedral and cuboctahedral crystals. Pyritohedral crystals are generally smaller than the cuboctahedral. Anhedral masses of siegenite are much less common. Siegenite commonly contains chalcopyrite-filled fractures, even where there is no other chalcopyrite in the immediate area. However, siegenite is typically coated by chalcopyrite. Unlike chalcopyrite, siegenite occurs only rarely around or adjacent to iron sulfides, even though both are normally found in the same general location (thin-section size area). The lack of association between iron sulfides and siegenite might be indicative of the timing of precipitation. Alternating bands of pyrite and bravoite show that nickel and cobalt were present in the ore fluids at the time of pyrite precipitation. If the precipitation of siegenite closely followed that of marcasite and pyrite, then the reduced sulfur could have been locally exhausted, thus forcing the siegenite to form at a distance. Fractures in the siegenite indicate time break between siegenite and chalcopyrite deposition. During this time reduced sulfur could have been replaced, therefore allowing chalcopyrite to precipitate around both iron sulfides and siegenite. Siegenite crystals from two Lamotte samples (Plate 1, Sites 17 and 24) were analyzed by microprobe. The siegenite in both samples is veined by chakopyrite, and some crystals are surrounded by chalcopyrite. Clearly the 58 siegenite in both samples is the early, pre-chalcopyrite type; however a large variation in the nickel:cobalt ratio exists between the two samples and within indi victual crystals (Table 2). Four patterns of ratios are discemable between the two samples: type A (0.5 - 0.55); type B (0.7 - 0.85); type C (0.95 - 0.975); type D (1.1 - 1.2). Only type B is found in both samples. Siegenite from the central part of the Goose Creek mine (Plate 1, Site 17) has a nickel:cobalt ratio of 0.5 to 0.75, whereas the ratio for siegenite from the eastern part of the mine (Plate 1, Site 24) is 0.85 to 1.2. Other studies (Jessey, 1983; Pignolet and Hagni, 1983) of nickel-cobalt minerals in the Southeast Missouri district, have suggested an increase of nickel:cobalt with time, from north to south for the district, and in stratigrnphically higher deposits. If the nickel:cobalt ratio at Goose Creek increased with time, then the west to east increase suggests a west to east movement of metal-bearing fluids for the Indian Creek subdistrict The copper and iron content of the two Goose Creek samples also varies. The sample from the central part of the mine has an average of 2.5 weight percent copper and 2 weight percent iron, whereas the eastern sample has less than 0.5 weight percent copper and 2.5 weight percent iron. The copper and iron content in siegenite from these two samples differ considerably from the less than 2 weight percent iron and up to 8 weight percent copper that Pignolet-Brandom and Hagni (1985) reported for early siegenite. Microprobe analysis of siegenite from the Bonneterre transition zone in the Madison mine (Pignolet, 1983) has only slightly higher nickel:cobalt ratios than the Lamotte Sandstone on the eastern side of Goose Creek mine (1.2 -1.3 and 1.0 - 1.2, respectively). Cobalt-rich siegenite was previously believed to exist only in Mine La Motte and the Fredericktown (Madison mine) subdistrict and to have a 59 TABLE2 ANALYSES OF SIEGENITE FROM IBE GOOSE CREEK MINE Sample no. Oystal Type Ni Co Fe Cu s Total L-56B 1 A 19.1 35.3 1.3 2.4 43.0 101.2 L-56B 2 A 17.8 37.5 1.5 3.2 43.3 103.3 L-56B 3 A 17.7 37.8 1.4 3.5 42.8 103.3 L-56B 4 A 20.1 37.1 1.7 3.7 43.1 105.6 L-56B 5 B 21.4 31.4 2.8 2.4 42.8 100.9 L-56B 6 B 22.6 32.1 2.1 1.6 43.0 101.4 L-56B 7 B 23.3 30.8 3.0 1.6 41.9 100.6 L-40E 1 B 26.5 31.2 2.2 1.2 44.5 105.5 L-40E 1 c 27.8 28.5 2.6 0.0 44.1 103.0 L-40E 2 c 27.4 28.6 2.5 0.6 43.9 103.0 L-40E 1 D 30.3 27.3 3.3 0.0 44.6 105.5 L-40E 2 D 29.9 25.2 3.6 0.0 43.2 101.8 L-40E 2 D 30.2 26.0 2.7 0.0 43.1 101.9 L-40E 2 D 30.5 24.8 3.1 0.0 43.8 102.2 L-40E 3 D 30.2 28.6 1.4 1.2 44.6 105.9 L-40E 4 D 29.7 26.2 2.2 1.3 43.1 102.4 L-40E 5 D 29.2 24.2 3.2 0.6 42.6 99.8 Four catagories of Ni :Co ratios: type A (0.5 - 0.55); type B (0.7 - 0.85); type C (0.95 - 0.975); type D (1.1 - 1.2). Only type Bis found in both samples. Analyzed on an Applied Research Laboratory Microprobe using wavelength dispersive spectrometers (WDS) for cobalt, nickel, and copper; electron dispersive spectrometers (EDS) were used for iron and sulfur. Analyses were made with a beam current of 20 nanoamps, an excitation voltage of 20 Kv, and a 50 second analysis time. Microprobe standards: cobalt metal for Co, nickel metal for Ni, covellite for Cu, and pyrite for Fe and S. 60 nickel:cobalt ratio of approximately 0.85 (Jessey, 1983). However, early, cobalt-rich siegenite also occurs in the Buick and Fletcher mines and Indian Creek subdistrict (Pignolet-Brandom and Hagni, 1985). Pignolet-Brandom and Hagni (1985) and the current study demonstrates that the Indian Creek subdistrict has the most cobalt-rich siegenite yet recorded in the Southeast Missouri district (Fig. 22). Bravoite Bravoite [(Ni, Fe)S2] was found only in two locations in the Goose Creek mine; both on the northern end of the Indian Creek ridge, over the saddle (Plate 1, Sites 19 and 24). Bravoite is intimately associated with pyrite, occuning as alternating bands within the iron sulfide. Commonly, both bravoite and pyrite exhibit a cubic or cuboctahedral habit. Bravoite is not closely associated with marcasite, probably because of their different crystal systems. Microprobe analyses of bands of bravoite alternating with pyrite show a wide variation in the nickel:iron ratio (Fig. 23, Table 3). Darker bands of bravoite have a ratio of approximately 0.15, whereas the ratio of the lighter bands is 0.25. High iron content is probably responsible for the darker color. Sphalerite Sphalerite occurs in minute quanities throughout the Goose Creek mine. It rarely occurs with other sulfides and only one tetrahedral crystal of sphalerite was observed. Sphalerite is also uncommon in the Bonneterre Dolomite at the Indian Creek mine, but small dark brown crystals occur locally in vugs. 61 Fe Co Ni + Goose Creek mine siegenite • Indian Creek sub-district siegenite o Southeast Missouri district siegenite Fig. 22 Plot of Fe-Co-Ni molar percentage for siegenite. Goose Creek data are from this study; Indian Creek and Southeast Missouri data are from Pignolet-Brandom and Hagni (1985). 62 Fe Fig. 23 Plot of Fe-Co-Ni molar percentage for bravoite. Data fall into two groups; higher iron represents darker bands of bravoite. 63 TABLE3 ANALYSES OF BRA VOITE FROM THE GOOSE CREEK MINE Sample no. Crystal Fe Ni Co Cu s Total L-40E 1 38.7 6.1 0.4 0.9 54.2 100.2 L-40E 1 36.0 9.7 0.7 0.7 53.5 100.5 L-40E 1 40.0 5.7 0.0 0.8 55.1 101.6 L-40E 1 40.1 6.0 1.0 0.0 54.9 102.0 L-40E 1 35.3 11.8 1.0 0.0 55.1 103.2 L-40E 2 36.6 11.5 1.4 0.0 55.9 105.3 L-40E 2 35.5 10.8 1.2 0.0 55.2 102.7 L-40E 2 35.2 9.8 0.7 0.4 54.8 100.8 Analyzed on an Applied Research Laboratory Microprobe using wavelength dispersive spectrometers (WDS) for cobalt, nickel, and copper; electron dispersive spectrometers (EDS) were used for iron and sulfur. Analyses were made with a beam current of 20 nanoamps, an excitation voltage of 20 K v, and a 50 second analysis time. Microprobe standards: cobalt metal for Co, nickel metal for Ni, covellite for Cu, and pyrite for Fe and S. 64 Non-sulfides Quartz Authigenic quartz overgrowths are the most common cement in the Lamotte Sandstone at Indian Creek. Quartz overgrowths make up from 1 to 11 percent of the rock. Most of the overgrowths have euhedral terminations, but some are anhedral, especially between closely spaced grains. A few of the anhedral overgrowths can be identified by dust rings that separate the sand grains from the overgrowth. For others, the lack of fluid inclusions identifies the overgrowth from the inclusion-rich grain. For grains that lack both a dust ring and inclusions cathodoluminescent microscopy may be used to identify the anhedral authigenic quartz. Cathodoluminescence also shows that the authigenic quartz is unzoned except for a very thin luminescing band on the edge of a few of the overgrowths (JEOL Superprobe 733; scanning with 65 milliamps of current at 200X magnification). Dolomite Less than half of the samples collected in the Goose Creek mine contain authigenic dolomite, which is mainly restricted to the upper Lamotte. Authigenic dolomite occurs as slightly baroque rhombs in intergranular pore space. Five luminescent and non-luminescent zones can be distinguished with cathodoluminescence (Fig. 24). The bright zones are caused by impurities, especially Mn2+, and the luminescence in the dark zones has been dampened by iron (Plant, 1976). At Goose Creek the zones are: (1) early bright and dull mottled zone with a bright center, (2) dull zone with faint banding, (3) thin zone of very bright bands, ( 4) thick non-luminescent band, (5) thick slightly mottled non-luminescent band. Except for the fifth zone, the zones listed are very similar to those described 65 and numbered by Voss and Hagni (1985) for Viburnum Trend dolomites. 2'.ones 1 through 5 are not found in all the Goose Creek dolomites. Zones 2 and 5 are usually absent because of either non-deposition or dissolution. Euhedral crystals of dolomite suggest that Zone 5 was never deposited in most areas at Goose Creek. However, Zones 2 and 3, if present, commonly display a jagged contact of etched pits with the overlying zone (Fig. 24 ), which infers that Zone 2, when missing, was probably etched away. The etched contacts represent a period of disequilibrium between dolomite and the surrounding fluid. In one sample, only zones 1 and 3 were deposited and the outer surface of these rhombs are coated with a dark, probably organic substance. This coating probably prevented further growth, which is similar to the more commonly noted phenomenon of clay coatings preventing overgrowths on quartz grains . Bright luminescent bands in zone 3 were precipitated in a low iron environment that probably corresponds to periods of iron sulfide precipitation elsewhere in the system. In one sample, marcasite and galena were deposited directly over zone 3, and zone 4 post-dated the sulfides. Another area in the same sample contains marcasite within zone 3, either deposited in alternating bands or selectively replacing the dolomite. This feature suggests that the iron-poor environment, which produced the bright bands in zone 3, was a direct result of marcasite precipitation, at least locally. This must futher correspond to periods of sulfur availability. Kaolinite Sixty percent of the samples collected throughout the Goose Creek mine contain trace amounts of kaolinite. Kaolinite occurs as masses of stacked hexagonal plates, filling intergranular pore space. In a study of the ore zone in the Bonneterre 66 Fig. 24 Cathodoluminescent photomicrograph of rhombic dolomite showing the five zones found at Goose Creek. Zone 1 - bright and dull mottled zone with a bright center, Zone 2 - dull zone with faint banding, Zone 3 - thin zone of very bright bands, Zone 4 - thick non-luminescent band, Zone 5 - thick slightly mottled non-luminescent band. Dashed line separates the host rock from the cement. Zones labeled 1 -5, calcite (cc). Arrow points to bands with etched pits. Sample L-27. Maximum dimension 1.8 mm. 67 Formation at Fletcher and Brushy Creek mines, Huggins (1981) found both kaolinite and dickite. The polymorphs did not occur together and dickite crystals were larger and more hexagonal than kaolinite, and commonly exhibited a mottled texrure on the pinacoid and basal faces (Huggins, 1981 ). Paragenetically, kaolinite generally coated marcasite, dickite coated chalcopyrite and sphalerite, and both occurred with galena (Huggins, 1981). In this study, kaolinite was not analyzed to determine if dickite also occurs. However at Goose Creek, kaolinite is a late-stage mineral that is not associated with sulfide deposition, and typically does not occupy the same pore space as sulfides. Also, the crystals do not exhibit a mottled texture (SEM examination). Although this does not preclude dickite, it does suggests that the hydrous aluminum silicate is kaolinite. Feldspar Extremely small amounts of authigenic feldspar occur in about 25 percent of the samples collected in the Goose Creek mine. Authigenic feldspar occurs as skeletal overgrowths on feldspar sand and silt grains. Sand grains of feldspar are usually intensely weathered, but the overgrowths are pristine. Feldspar overgrowths occurred before authigenic quartz overgrowths, but their paragenetic position with respect to the early sufides could not be determined. Gypsum Trace amounts of fibrous gypsum occur in 21 percent of the Goose Creek samples, which are scattered throughout the mine. The gypsum formed elongate blades, broad swords, and anhedral crystals which grew into the intergranular pore space. Dolomite rhombs were also replaced by gypsum. Gypsum is a late stage cement and is not associated with sulfide deposition, and probably represents a 68 change from the saline environment of ore deposition to a less saline environment. Illite Approximately 12 percent of the Goose Creek samples contain illite, and most are located on the western side of the mine. Illite occurs in trace amounts as tiny round, randomly oriented plates that fill intergranular pore space. Calcite Calcite is extremely rare in the Lamotte Sandstone at Goose Creek. It only occurs on the western end of the mine, only slightly above the Precambrian basement. The only calcite actually cementing the sandstone consists of one large crystal surrounding several sand grains. The other two occurrences are fracture fill, one in a cobble from the cobble conglomerate overlying the basement, and the other in the sandstone. Hydrocarbons Fifteen percent of the Goose Creek samples contain late hydrocarbons. The amber-colored material coats pores and other authigenic minerals. Cracks in the hydrocarbons suggest the desiccation of a gel-like material. These hydrocarbons do not appear to be related to the pyro-bitumen reported by Rothbard (1982; 1983), which were remobilized during stylolite formation very early in the diagenetic history of the Lamotte Formation. Instead, the hydrocarbons are probably related to the solid, insoluble bitumen observed in the Magmont mine (Marikos and others, 1986). These black, brittle blebs were generated from kerogen in the Bonneterre carbonates during the late stages of mineralization (Marikos and others, 1986). Quartz Dissolution and Brecciation A few samples from the Goose Creek mine contain evidence of quartz dissolution which is not associated with pressure solution or stylolitization. In these 69 samples, quartz grains surrounded by early sulfides were partially or totally dissolved. Where only a few grains have been dissolved, the surrounding sulfides were strong enough to support the overburden and a complete outline of the dissolved quartz grain is preserved (Fig. 25). In most cases however, many grains were dissolved and the surrounding sulfides were brecciated (Fig. 26). The degree of brecciation depends on the extent of quartz dissolution. Even with extensive brecciation, partial outlines of the dissolved quartz grains can be identified (Fig. 26). The most extensive period of brecciation occurred after iron sulfide and bravoite precipitation. Another episode occurred after siegenite deposition. However, the second occurrence cannot be tied to quartz dissolution unless it is related to the dissolution that caused the first episode of brecciation. A second episode of quartz dissolution occurred after chalcopyrite precipitation but apparently without brecciation. Neither episode of quartz di ssolution appears to be related to precipitation of authigenic cements. However, most of the secondary porosity created by dissolution was later filled by galena. Secondary porosity created by quartz dissolution has been documented for the Rotliegend aeolian sandstone from the Permian Basin in the North Sea (Pye and Krinsley, 1985). In the early stage of quartz di ssolution, the grains contained small, irregularly shaped voids. Gradually these voids coalesced, leaving a highly porous aggregate (Pye and Krinsley, 1985). This type of dissolution is not observed at Goose Creek, where the undissolved portion of the grain is only irregular at dissolution contacts. Commonly, this ragged surface is covered by authigentic quartz overgrowths, partially filling the secondary porosity. 70 ...... ~ .. '( " # PY '.. t. .. 1' ga , Fig. 25. Reflected light photomicrograph of galena (ga) in secondary porosity created by the dissolution of quartz grains. The quartz was cemented by pyrite (py) prior to dissolution. One grain is only partly dissolved. Sample L-40E. Maximum dimension 1.1 mm. 71 Fig. 26 Reflected light photomicrograph of post-pyrite quanz dissolution which resulted in brecciation. Quartz grains (qz) were cemented by pyrite (py) prior to dissolution. Partial outlines of the quartz grains are still recognizable. Galena (ga) precipitated after brecciation. Sample L-40E. Maxjmum dimension 1.1 mm. 72 The cause of the quartz dissolution is difficult to determine. Pye and Krinsley (1985) attributed the dissolution of the Rotliegend quartz to a warm, reducing, alkaline pore fluid which also deposited late-stage ferroan carbonate and evaporite cements. Paragenetic Sequence Fourteen authigenic minerals, plus hydrocarbons, cement the Lamotte Sandstone at Indian Creek. Although no single sample provides a complete picture of the sequence of precipitation, the combination of sequences from many samples resulted in the following paragenetic sequence for the Lamotte at Goose Creek (Fig. 27): dolomite - fram boidal pyrite - marcasite - cuboctahedral pyrite - bravoite - bladed marcasite - pyrite - quartz dissolution - brecciation - siegenite - marcasite - dolomite - brecciation - chalcopyrite - quartz dissolution - sphalerite - galena (cuboctahedral) - quartz - galena (cubic) - dolomite - gypsum - hydrocarbons - kaolinite - illite - calcite - hydrocarbons. The paragenetic position of a few of the cements could not be precisely placed. Auth.igenic feldspar formed prior to quartz, but its paragenetic position with respect to the other authigenic minerals could not be determined. However, Rothbard (1982; 1983) documented the authigenic feldspar in the Lamotte as pre-ore. A similar problem exists with bravoite. Bravoite and cuboctahedral pyrite precipitated together but at least two pyrite phase are separated by marcasite. It is difficult to determine with which pyrite phase the bravoite is associated. The earlier pyrite phase was selected because bravoite is coated by bladed marcasite, which probably correlates with the bladed marcasite that separates the two pyrite phases. 73 framboidaJ pyrite • marcasite · • pyrite ---- bravoite . --- siegenite • chalcopyrite · sphalerite - galena - quartz •..·--· dolomite ~ ------~ ------~ gypsum · kaolinite • • illite calcite feldspar - - -? - - hydrocarbons quartz dissolution brecciation early ~~~~~~~~~~~~~-- late Fig. 27 Paragenetic sequence for the Lamotte cements in the Goose Creek mine. 74 Comparsion with other paragenetic studies in Southeast Missouri Goose Creek mine The paragenetic sequence proposed in this study differs in several aspects from one presented by Horrall and others (1983) for the Goose Creek mine (Fig. 28). Horrall and others (1983) based their interpretation on samples from the Lamotte Sandstone and the Lamotte-Bonneterre transition zone, whereas the current study considered only samples from the Lamotte. Horrall and others (1983) divided their sequence into four categories: host rock, disseminated, colloform, and crystalline vug fill. The current study does not include the host rock as part of the paragenetic sequence, and only the authigenic minerals deposited in thin silty shale beds, wispy shale partings, and carbonate rip-up clasts could be considered disseminated in the sense described by Horrall and others (1983). The fram boidal pyrite in the current study could definitely be considered disseminated. Although minor colloform growth occurs around carbonate rip-up clasts, the colloform minerals and their sequence of precipitation do not indicate that these minerals or the time of precipitation differ from the same non-colloform minerals in the Lamotte. The majority of authigenic minerals in the Lamotte were deposited as open space pore cement, which would probably correspond to Horrall and others (1983) crystalline vug fill category. Futhermore, the paragenetic position of some minerals within the Lamotte at Goose Creek are opposite of that reported by Horrall and others (1983). The major disagreement is the position of siegenite. Horrall and others (1983) reported siegenite as post-chalcopyrite, whereas most of the siegenite observed in the current study definitely precipitated before chalcopyrite, because chalcopyrite occurs in 75 r :H, ! (j t : . ' ·• 11I . . I . :j rl -- I ·t ' .; ·+ · 1 ' .~ ) i ·• ·•: I I I ' i ! I i I t i· : I i . ' ' I I 'r·l ~i i ~ I ri ;• ;: i Jl ! I 1I 1 '•' I I - I ! I I ~ .; i i I I ' ! I . I !l 4- - 4 - - - • ------:: - - -- - .. - - ~ I ! j ;! · • i I J : . .. , : 1, . i ! ii : ;J ' I I _ .~ ; i · ..., ---1: · · :·-~·- r -1-1-1· ·-- 1·-· : : r I ' l , ( ~ ! 1 r-r 1r,l i ;-'": ; 1 • ·1 I I I ! I I ! i I ' i ' i ; i I j i i j i 'j a ! l i • i .: Fig. 28 Parageneric sequence for the Goose Creek mine. Modified from Horrall and others (1 983). 76 fractures that cross-cut siegenite. Similar occurrences of post-siegenite chalcopyrite were reponed by Pignolet and Hagni (1983) from the Madison mine. Occurrences of siegenite without the fracture relationship are less cenain, however in most cases, siegenite is partly enveloped by chalcopyrite. Although it is possible that a second phase of siegenite deposition formed after chalcopyrite, it is a minor cement, at least in the Lamotte at Goose Creek. Another disagreement concerns the position of bravoite in the pre-galena iron-sulfide sequence. According to Horrall and others (1983), bravoite formed prior to pyrite and marcasite. However, the minor amount of bravoite found in the Lamotte occurs as bands within pyrite. Lamotte Sandstone A paragenetic sequence developed by Rothbard (1982, 1983) for the Lamotte Sandstone near the St. Francois Mountains (Fig. 29) was divided into four stages: pre-ore, ore, post-ore I (reducing), and post-ore II (oxidizing). Most of the events described by Rothbard (1982; 1983) in his pre-ore stage are observed at Goose Creek. However, glauconite occurs only as replacement of volcanic and sedimentary rock fragments, and glauconite pellets observed in the current study are only found in the Lamotte-Bonneterre transition. Also, the only quartz overgrowths observed in the pre-ore stage at Goose Creek are those that formed as the direct result of pressure solution. Rothbard's (1982; 1983) ore mineralization stage does not contain all the minerals or precipitation events found at Goose Creek, however except for kaolinite, his sequence of precipitation is similar to that observed in the current study. In the ore stage, Rothbard (1982; 1983) reported that abundant flakes of kaolinite occurred 77 I I I I I I I I I I iI I I • - -~• E -Cl ;,• E a. . ,.. : C) Fig. 29 Paragenetic sequence for the Lamotte Sandstone for the Southeast Missouri district Modified from Rothbard (1983). 78 within the galena and marcasite cements. However at Goose Creek, kaolinite was one of the last cements to fonn and is not associated with the sulfides. Most of the minerals included in Rothbard's (1982; 1983) post-ore I stage are part of the ore mineralization stage at Goose Creek. Kaolinite and illite occur in the post-ore stage at Goose Creek and are in agreement with Rothbard's (1982; 1983) sequence. However at Goose Creek, kaolinite is the most abundant clay mineral, unlike illite as reponed by Rothbard (1982; 1983). None of the minerals from Rothbard's (1982; 1983) post-ore II stage occur in that paragenetic position at Goose Creek. Gypsum is a late-stage mineral at Goose Creek; however it occurs prior to kaolinite and illite, and hematite was not observed. Southeast Missouri district The current paragenetic study compares more favorably with the paragenetic sequence developed by Hagni ( 1986) for the entire Southeast Missouri district than with Horrall and others (1983) and Rothbard (1982; 1983). Hagni (1986) divided the sequence into five categories: disseminated, bornite pods, veins, main Pb-Zn ores, and crystals in vugs (Fig. 30). By excluding the categories that are not present in the Lamotte at Goose Creek (bomite pods, veins, and the second part of the main Pb-Zn ores after the first galena occurrence), the two sequences are compatible with a few exceptions. In Hagni's (1986) paragenetic sequence, the early galena is predominantly cuboctahedral, and late-stage galena associated with authigenic quanz is cubic (Hagni, 1986). These two stages of galena appear to correspond with those found in sandstone at Goose Creek. However, the main chalcopyrite stage occurs after galena in Hagni's (1986) sequence, but in the current study it occurs after siegenite and before galena. Also, many of the minerals in z 31 c: 3 ~ O" VJ C1I 0 ;;\ Dolomltee I '~; 4 , Pyrite ~ - . • ~ , -:::> · a Bra volte ~ • ., 0. C1I c:;· :::> Marcaslte -:- ~ . 'l J I» (1) • ""I Fletcherile ~ - ~ NI arrol lte - 5- Chalcopyr1te . . (1) -~~,. , ; ,. ) Bornlte • • 'O s. I l - "O - ,. ~ . ,-: 1 (") ~ !pha1er1/Hersdor te - l - I» '§: enn~nt1te ~ narg te ~- -- r - :::> ~. (1) Vaes1te 0 c:r. I .l :::> (") Digenite - (1) - "' ~ .0 ..0 ~~iljtea cocite c: c:: - C1I (1) Djurleite - :::> :::> (") (") - (1) ~lau~l~lbender Covellite I r C1I ove 1te -,. O' O' iegenite ""'I ""'I I 49' •,...._.. • ' C1I Galena - - w I» 5- I ~ ~.-r. , (") (1) ~rrhot ite . :r <.n agnetlte .l. 0 - 3 c: garlte . :::> (1) 5- uartz (1) I l ""I I» (I) ~11/er i te E. ... o ydymlte -- - .,, 3:: Luzontte -• a Calcite "' ....I l 3 ~ Dlcklte :i:: 5. Bitumen - I» - (1Q a. '-' = Dissolution 2. ,...... "'9...... 0 DISSEM. BORNITE PODS VEINS MAIN PB-ZN ORES CRYSTALS IN VUGS \D r 00 ':-"°' -J \D 80 Hagni's (1986) crystals in vugs category do not occur in the Lamotte at Goose Creek. Nevertheless, the paragenetic positions of many minerals are similar, including galena-quartz-galena, late calcite, dickite (kaolinite), and bitumen (hydrocarbons). Comparison with other sandstone-hosted lead deposits Laisvall and Vassbo lead-zinc deposits occur in Precambrian and Cambrian quartzitic sandstones along the eastern border of the Caledonides in Sweden (Rickard and others, 1979; Christofferson and others, 1979). These sandstone-hosted deposits are similar to the Lamotte-hosted deposit in many ways, especially the texture of ore described by Rickard and others (1979) and Christofferson and others (1979). However, the suite of authigenic minerals is fairly simple in comparison with Southeast Missouri deposits. Rickard ( 1983) reported five major minerals (galena, sphalerite, calcite, barite, and fluorite) and three minor minerals (pyrite, bravoite, and sulfosalts), plus secondary quartz for the Laisvall and Vassbo. Except for galena, the major minerals are either extremely minor (sphalerite and calcite) or do not occur (barite and fluorite) at Goose Creek. Also, pyrite is a major cement in the Lamotte, and secondary quartz was deposited during the ore-forming stage. Rickard's (1983) paragenetic sequence (Fig. 31) for the Laisvall deposit only includes the five major minerals, of which, only three are found in the Lamotte at Goose Creek. Nevertheless, some comparisons can be made. Calcite occurred early at Laisvall, predating the sulfides in each repetitive sequence, whereas at Goose Creek, calcite was one of the last minerals to form. Additionally, galena precipitated with sphalerite in the Laisvall deposit, but was deposited after sphalerite at Goose Creek. The precipitation sequence observed in the Laisvall deposit is 81 220 lower sandstone ore __...,J._ upper sandstont ore I ..... o., , , '~.a£'- ' ' 0 a-·,, T °C / H • ~ ' ' ' \40 - ' () - ~ 0 • ! 'O ' ' .Cl - l.J ~- _ , 0 .... Cl .... 0 0 '9·, 12 ..., o,J ; - ' ·~- ·-.. 0 QI .t= 0 0. o,J 0 t im e-- .t= "' 0. "' Fig. 31 Paragenetic sequence for the Laisvall sandstone-hosted deposit, Sweden. From Rickard and others ( 1983). 82 attributed to warming of fluids saturated with calcium carbonate, the oxidation of hydrocarbons by brine sulfate which increased sulfide activity, and finally the cooling of the fluids before another cycle began (Rickard, 1983). Cement Textures Between cements and sedimentary structures Rickard and others (1979) divided ore textures that occur in the Laisvall deposit into those that were congruent with sedimentary structures and those which were not (Fig.32). Only two of the congruent structures, thin bands of sulfides parallel to bedding and sulfides outlining cross-bedding, occur in the Lamotte at Goose Creek. Three of the non-congruent structures are found; spot structures, network structures, and ore patches. Spots are the most common structure for galena, whereas ore patches are typical for the earlier sulfides. Between cements Boundaries between authigenic cements in the Goose Creek mine include straight, scalloped, and jagged contacts. Contacts between sulfides vary considerably, even within the same sample. Boundaries between euhedral sulfide crystals are commonly unaltered, or slightly etched. In contrast, anhedral sulfides generally exhibit corroded contacts with the later sulfides. The type of corrosion varies from shallow, wide scallops to deep, narrow pits. In some cases, anhedral sulfides may have originally been euhedral, but there are no relic crystal faces to suggest this. Contacts between sulfides and non-sulfides, and between non-sulfides are commonly unaltered. However, dissolution pits between zones in dolomite are common (Fig. 24). Also, the replacement of dolomite by gypsum is common 83 locally, and the replacement of dolomite by chalcopyrite occurs rarely. The replacement of detrital quanz by sulfides also occurs, but only rarely. 84 cor~::.itm.V M • 7 ~.x.;:-REE $A.NO COt-4CE : . 7~Tt YE "' l " ~AA .. $ ,.,.AY BE E•~1. v S : y( OR E · at:AO. •t'.Ct ST~ucrvAE AS AT ,... ..l' ... .c. ANO A)o.)l(fRSC t ET Fig. 32 Congruent and non-congruent cement textures. From Rickard and others (1979). MINERALIZATION Ore Distribution Sulfides in the Lamotte Sandstone in the Indian Creek subdistrict are commonly found within 40 ft of the Bonneterre-Lamotte contact. The deepest mineralized Lamotte penetrated by drilling was 80 ft below the contact. The highest concentration of sulfides in the Lamotte is 20 ft or less from the contact. In the Goose Creek mine, the mine floor varies between 40 and only a few ft below the Bonneterre-Lamotte contact. The mined interval is approximately 15 to 25 ft thick, indicating that most of the ore was taken from the Bonneterre-Lamotte contact or slightly below it Drill cores within the proximity of the Goose Creek mine show most of the lead ore to be in the Lamotte except on the western side (Fig. 19). There both the Lamotte and the Bonneterre are mineralized, and the ratio of Bonneterre ore to Lamotte ore varies from west to east, from approximately 8: 1 to 1 :7 (Fig. llc). Most of the copper occurs in the Lamone, even on the western side. Most Lamotte samples from the Goose Creek mine were collected near the Bonneterre-Lamone contact because only a few exposures were available more than 20 ft below the contact The deepest sample (Plate 1, Site 27), collected 40 ft below the contact, contains very high concentrations of chalcopyrite (8-10%; Fig. 20). Here the high copper zone ends abruptly about 5 ft above the mine floor and galena becomes the dominant mineral. The contact between the chalcopyrite- and galena-bearing zones does not appear to be controlled by any sedimentary structure. A similar occurrence is present near the Bonneterre-Lamotte contact in another loca1ity (Plate 1, Site 26), where approximately 2 volume percent chalcopyrite and 85 86 no galena abruptly changed verticaUy to 1-2 volume percent galena and only a trace of chalcopyrite. Again the change seems not to be controlled by sedimentary souctures. Individual drill cores also show this trend (Fig. 33). In the Goose Creek mine, the highest copper concentrations are in the Lamotte Sandstone and the lead content increases higher in the Bonneterre Dolomite (Fig. 33). However in the Indian Creek mine, where copper is not very extensive, the copper horizon occurs at nearly the same level as the lead and decreases toward the Precambrian basement. Mineralization Controls Introduction The Indian Creek subdistrict is unique among the Southeast Missouri mineralized areas because a significant amount of ore was taken from the Lamotte Sandstone. The major purpose of this study was to determine why ore quanities of sulfides were concentrated in the Lamotte in the Goose Creek mine. Sedimentary structures and local sulfur sources Early sulfides (pyrite, marcasite, bravoite, siegenite, and chalcopyrite) in the Lamotte Sandstone are commonly concentrated along silty organic-rich clay seams and stylolites, and around rock fragments. Later sulfides (chalcopyrite and galena) are more commonly disseminated within the quartzarenite. Chalcopyrite was predipitated during the transition between early and late sulfides and is equally common in both environments. The clay seams, stylolites, and rock fragments may have controlled local circulation of the metal-bearing fluids, however, if this resulted in the early mineral concentrations, then a change in flow occurred prior to galena precipitation. Also, the sedimentary souctures could have provided a minor amount of reduced sulfur. 87 64 W32 I 80 w a: a: 60 w w~ z I z 0 40 a:l I I '"' ~Coppt r D Lt od 0 r;i [ .. .. • ~ / < .. ', • 'l 20 40 0 2 3 4 5 6 WE IGHT .,-. Fig.33 Lead and copper values for assayed intervals in drill hole 64W32. Vertical axis shows distance in feet relative to the Bonneterre-Lamone contact From Kyle and Gutierrez (in press). 88 Galena occurs as blebs within the quartzarenite, either randomly disseminated (Fig. 14) or aligned with crossbedding (Fig. 17). Earlier sulfides rarely occur in disseminated blebs; however, the most spectacular display of sulfides aligned with crossbedding (Fig. 17) observed in the Goose Creek mine consists of galena, chalcopyrite, and marcasite. This unusual occurrence is located on the western side of the mine, approximately 15 ft below the Lamotte-Bonneterre contact (Plate 1, Site 10). High concentrations of chalcopyrite cementing quartzarenite are more likely to occur as patches (Fig. 20), rather than in blebs like galena. Thin sandstone beds that are separated from surrounding beds by thin silty clay beds or wispy partings may contain higher sulfide concentrations than nearby thicker beds with similar porosities and composition. Sulfides are also concentrated in cross-bedding pinchouts where the overlying sandstone is interbedded with thin silty clay beds (Fig. 18). The pinchouts were formed when the cross-bedded unit was truncated by the overlying bed. These structures would channel metal-bearing fluids through a smaller volume of sandstone, hence more metal ions per pore volume than surrounding units would result in higher concentrations of sulfides. Lamotte pinchout and Bonneterre permeability Sedimentary features provided controls for local ore concentrations, but comparisons between mineralized and non-mineralized sandstone did not reveal a difference that could account for the ore deposit. The comparison (Fig. 34) of a measured section in the Goose Creek orebody (Plate 1, Sites 12 to B3) with the log of a core (Plate 1, 81W18) with only trace amounts of iron sulfides, galena, and chalcopyrite shows only minor differences. The transition zone is thinner in the core than in the measured section (1 ft compared to 6 ft). The upper 30 ft of the Lamotte 89 MEASURED SECTION 8 1Wl 8 - - ..~ ;;. ~ I " g ; 1 ~ IS > I' ' .> " w ! « J I J ...• I I .,,. .. I .. Colle> •• ..C " I ' [ Cfut104tr"' ton ·' hoq~• " ' VOICOl'l1( ...ca htt ..... fl.' •llOlt Clttic~•• .... , ...... c.... ,~...... 0 o-.c ..... Fig. 34 Generali red lithologic logs of the Lamotte Sandstone from Goose Creek. The measured section is from the orebody (Sites 12 to B3, Plate 1) and the core (81W18, Plate 1) is from an unm ineralired area west of the mine. 90 Sandstone in the core is medium sand and the next 40 ft is medium to coarse; however the reverse is true for other cores of unmineralized Lamotte. An average grain size of medium to coarse is constant for the measured section. Both the core and the measured section have ripup clasts and wispy partings, which appear to be more abundant in the core, but this may reflect the different methods of logging. A variety of porosities were found in both mineralized and non-mineralized Lamotte, and the secondary porosity that developed during sulfide deposition apparently precludes pre-mineralization porosity as a controlling mechanism. Also, the Lamotte Sandstone within the Indian Creek subdistrict is consistently a mature quartzarenite and thus the type of sandstone did not control the sulfide zones. In one location (Plate 1, Site 8) within a few feet of the pinchout of the Lamotte Sandstone against the Precambrian basement ridge, marcasite and galena occur in vertical "tubes" perpendicular to the nearly horizontal bedding plane (Figs. 35a & 35b). The lack of these vertical sulfide concentrations elsewhere along strike indicates that the ore-bearing fluids moved through the Lamotte until the pinchout forced them to flow into the overlying Bonneterre Dolomite. Once in the Bonneterre, the fluids encountered the porous grainstone-algal reef complex which channeled fluids along the N30°E-trending Precambrian ridge. Mineralization in the Bonneterre occurs along the northwest edge of the reef, above the reef, and in reef debris and oolitic dolomite of the fore-reef sediments (Snyder and Gerdemann, 1968). According to Ohle (1985), the reef-rock talus contains some of the richest and most continuous ore in the Indian Creek mine. Except for the mineralized Lamotte Sandstone and Bonneterre Dolomite on the northern end of the Indian Creek ridge, all lead and copper concentrations 91 Fig. 35 Vertical tubes of sulfides cross-cut bedding near the Lamotte pinchout in the Goose Creek mine. (a). Spots of galena are aligned with cross-bedding. Site 8 (Plate 1). Height of exposure is 6 ft. (b). A planar view of the vertical tube. Sample L-15,30. 92 encountered in an extensive drilling program in Indian Creek subdistrict occur in the Bonneterre overlying the Precambrian basement (Fig. 36; Fig. 37). Unmineralized Lamotte and overlying Bonneterre along the northwest side of the Indian Creek ridge suggest that either the fluids were not channeled into the Bonneterre along this front, or something prevented the sulfides from precipitating in the Lamotte and overlying Bonneterre on this side. The lack of mineralization in the Lamotte and overlying Bonneterre along the northwest side of the ridge, and the presence of sulfides in the Lamotte and overlying Bonneterre on the northern end of the ridge, along with the vertical sulfide structures which infer cross formational flow, indicate that the fluids were channeled into the Bonneterre carbonates on the northern end of the ridge (Fig. 38). Futherrnore, structural and sedimentary features support this local fluid flow model. A saddle at the northern end of the ridge separates the Indian Creek ridge from another ridge along strike to the northeast (Figs. 8, 9). Although the Lamotte Sandstone thins and becomes more dolomitic over the saddle, this zone would still have a lower potentiometric gradient than the pinchout against the impermeable basement rhyolite and the overlying Bonneterre Dolomite. Consequently, the fluids could have flowed through this pathway, concentrating sulfides in the Lamotte, and upon entering the porous Bonneterre grainstone-agal reef complex, followed that permeable zone to the southwest, mineralizing the Bonneterre. Precambrian basement topography Computer-generated structure maps of the top and base of the lead and copper orebodies reveal features that mimic the basement topography (Fig. 8; Figs. 39a,b; Figs. 40a,b). Structure maps based on the occurrence of lead contain more control points and are more exact copies of the basement stucture. The close correspondence of the ore body structure and the basement topography leads to the 93 • • • z I • c ... % . • • • • c ...... • • • • Zic~ • • • ~ u- E • • • • ,_ .., &.I _, • • ~ ...... Jct • • • L> -' IC - 1-f • • c '"' .., • _,2 ! • • c i ; - - -.... • I • / .. ~ ... / I • •• • J • ...... ' + I • +• + + +' .. I + + • 't"\ + . .. : . ~ • • +/ • • • • • • "' • ,.- - - - I + •.... r-. _, • • . . . . •.• "..! ~. • •• ...... : .·. . . .. : ...... ·...... ::: ...~··· :·. .. . . ··.7+· . .. ·: . .. ·: ... "...... ; ' ...... •••• • ' . # - • - - • • • • • • •• • • • • • J . , • .. • • •• • ...... '• .... s: • • • • • + '. l • \ • • • • • /+ •• ) ' •• • • • • t. • r • . ' ...... • .s: ...... ' .. .. .- ..., .. . ,_ ...... • . / I + + + + • ...... t• • I • ' . l' • ••,. . '°, . . ~ • .. I • Fig. 36 Location of drill holes with assayed amounts of lead in the Indian Creek sulxlistrict. Only drill holes with lead in the Lamotte or in the Bonneterre overlying the Lamotte occur on the nonhem end of the subdistrict 94 ., I.. ... z: ~ • C> ...... z -C ~ ~ ~ • C> :a "" ,.... . • ;:.-,t::; E I CZ. _, C> CE: • u _, u c - 11 C> "" .... . ~2 ~ • E .,,,. -- - 0 r· • " ' I I ! I ' I ' ' ' I ' \ • -- - - ! • .; . •.., I - -' \ -. • . .... • • • .. ' .. . l • ' ~ ,...... • • I "' \ .. . ' -- - -, • . . . . - ,. ' • l / I l • • I l I ( ' •l .... ·~ \ • / ' ,_ .... I I + • ~ t I f ! I • I \ ~ I,. • ; 1• \ I I I ~ ! ..I k.11 -· -· .... -· ...... , ...... M 11 ... Fig. 37 Location of drill holes with assayed amounts of copper in the Indian Creek subdistrict. Only drill holes with copper in the Lamotte or in the Bonneterre overlying the Lamotte occur on the northern end of the subdistrict. 95 Fig. 38 Computer-generated isometric plot of the Precambrian basement topography in the Indian Creek subdistrict showing a schematic flow pattern for the metal-bearing fluids. Flow pattern is inferred only for the Indian Creek subdistrict. Vertical exaggeration on topography is 5X. 96 ! .J c I I > - c ~; la.. c ~ ..... C> ...... ' 1o1L - !t'. E ... L-' ct C> ct C o e W C g.,, - ...z ..z r' a: C> u • I I I ! I f1 I ! I ! ! ; I • ! ! .... 8UI ...,, ...... ''" -· -· -· . "" Fig. 39a. Structure map for the base of the copper-bearing zone. Prominent topographic feature s of the Precambrian basement (Fig.8) are mimicked by the strUcture of the copper-bearing zone. 97 ! ...J z Cl: I > ....~ c: ; ... c: Cl: ...... c ...... L- E L L ...J !:: c c a: .... u c: C:o - z ....S"' '"' z i c u • ' I • • (!) I ! ! ! ;; I ~ I ! ONI ONI ..,, OCll I UI 1 11 1 ...... M ii "'' -· " " Fig. 39b. Structure map for the top of the copper-bearing zone. Prominent topographic features of the Precambrian basement (Fig.8) are mimicked by the structure of the copper-bearing zone. 98 • ! . J. \./:\• .,....c::i •• • \;(.- I ! I I •• ! • 1-~~--.-~~~~-~~~~~~~~~--.,..-~~~~~~..-~~~~~~..-~~-.-~~~~! ...,, IUI ..,, k ll .. ,, 1111 ...... Fig. 40a. Structure map for the base of the lead-bearing zone. Prominent topographic features of the Precambrian basement (Fig.8) are mimicked by the structure of the lead-bearing zone. 99 ! • .., _, I • z ~ / • 0 > • ... IC- ... c ...... ~ ; . . 0 c.., ! 1.. (:) ~ ... -::::: I . + L IW - - 0 _, _, ICC ~ ::le:> !..1 --o Ji. . o- ..,IC ... z z 1· • 0 E u • r' 1 - ooc- ~I i ~~ I I 1: ! ! .:: .;; I 4J- ! • ---...., - - --,. -.---- ! ui• I N I ...,. .. ,, IJUI IU I I U:l ...... -· "" Fig. 40b. Structure map for the top of the lead-bearing zone. Prominent topographic features of the Precambrian basement (Fig.8) are mimicked by the structure of the lead-bearing zone. 100 conclusion that the Precambrian basement was the major controlling factor in ore deposition in the Indian Creek subdistrict. This control is independent of formation boundaries, thus resulting in ore concentrations in the Lamotte Sandstone at the Goose Creek mine and in the Bonneterre Dolomite at the Indian Creek mine. The most general explanation of this relationship is that the depositional and diagenetic facies, which were locally controlled by the presence of the basement ridge, had a major influence on local fluid flow. Nature of the Ore-fonnin~ Solutions Numerous difficulties were encountered in the attempt to do a fluid inclusion study on the Indian Creek deposits, including the paucity of sphalerite, or other suitable minerals in both the Bonneterre and Lamotte orebodies. Futhermore, the available sphalerite is very dark and unsuitable for fluid inclusion work, and fluid inclusions, especially primary ones, are not abundant. Limited data (4 analyses) for sphalerite from the Bonneterre-hosted orebodies indicate homogenization temperatures between 105 and 120 °C and freezing point depressions from -22.9 and -19.7 °C (fable 4). These data are within the range reported by Roedder ( 1977) for sphalerite from the Viburnum Trend, and Sverjensky (1981) for late quartz coprecipitated with chalcopyrite. Initial melting of fluid inclusions in sphalerite from Indian Creek occurs between -52 and -48 °C (Table 4), indicating a NaCl-CaCl2 brine (eutectic temperature of -52.0 °C [Crawford, 1981)). Sverjensky (1981) reported inital melting temperatures for fluid inclusions in late quartz between -48 to -65 °C. He attributed this wide range temperatures to either a variety of salinities or metastable superheated ice. 101 TABLE4 FLUID INCLUSION DATA FROM SPHALERITE Th oc TIm °C Tfrn oc 10 105 10 105-110 -52 -20.0 - -19.7 10 108-1()<) 10 118-120 -22.9 Z' 101 -50.3 -15.2 Z' 109 -48 - -49 -15.0 1° =primary fluid inclusion; 2° =secondary fluid inclusion Th = homogenization temperature; Tim= initial melt; T fin= final melt Sphalerite from the Bonneterre Dolomite in the Indian Creek mine. Analyzed using a Fluid Inc. modified U.S.G.S. gas-flow heating and freezing stage. 102 The -22.9 to -19.7 range of freezing point depressions measured in sphalerite at Indian Creek have an equivalent weight percent NaCl of 24.6 to 22.4 (methods from Potter and others, 1978) and a CaC12/(NaCl + CaC12) range of 0 to 0.40 (Fig. 41). This ratio was determined from a H20-NaCl-CaC12 phase relations diagram (Konnerup-Madsen, 1979) using methods developed by Haynes and Kesler (1987). Also using the low temperature phase relations of HzO-NaCl-CaClz, Sverjensky (1981) estimated that a solution with a freezing point depression of -22 °C would contain 0.64 molal of Ca02 and 2.8 molal NaO. Sources of Metals and Sulfur Lead and sulfur Lead and sulfur isotopic studies of the Southeast Missouri district show a correlation between nonradiogenic lead and isotopically heavy sulfur in cuboctahedral galena, and more radiogenic lead and isotopically light sulfur in cubic galena (Sverjensky, 1981 ). This correlation suggests that lead and sulfur were transported together in the same solution; however the wide range of lead and sulfur isotopic compositions indicates that transport occurred in many different solutions with different isotopic compostions (Sverjensky, 1981). Sverjensky (1981) estimated o34SH2S of the ore-forming solutions was 28-35 O/oo and suggested that Ordovician or Cambrian evaporites may have been the source of sulfur for the Southeast Missouri district. 834S values of sulfide minerals that are lower than sulfur isotopic compositions that could have been derived from 103 +L,V -10°c ...... ______-20"C NaCl 0 .IO .20 .30 .50 .60 .70 .80 .90 1.0 Fig. 41 Ternary diagram for fluid inclusions with Na-Ca-Cl fluids. Freezing point depressions of -22.9 to -19.7 result in a range of CaC12J(NaCl + CaCl2) of 0 to 0.40, shown by the bold line. Modified from Hayes and others (1987). 104 most evaporites may have been contributed by diagenetic pyrite, petroliferous materials, or crystalline basement rocks (Sverjensky, 1981). The correlation between sulfur and lead isotopic compositions in Southeast Missouri indicates that the Lamotte Sandstone was not the only source of lead, as had been previously proposed by Doe and Delevaux (1972) (Sverjensky, 1981). However, Sverjensky (1981) suggested that at least some of the early, nonradiogenic lead in the cuboctahedral galena was derived from the Lamotte Sandstone aquifer before fluids entered the Bonneterre Dolomite. This lead was probably leached from inclusions in quartz, uranium-rich carbonate cement, iron-rich grain coatings, and uranium-poor feldspars within the quartzarenitic Lamotte by heated chloride brines (Doe and Delevaux, 1972). The source of the more radiogenic lead in the cubic galena is not known (Sverjensky, 1981 ); however the Bonneterre Formation, the younger Cambrian formations, and the Precambrian basement are isotopically unsuitable as the source (Doe and Delevaux, 1972). Copper, cobalt, and nickel Copper, cobalt, and nickel do not appear to have been leached from the Lamotte Sandstone. Horrall and others (1983) speculated that northwest-trending faults and river valleys in southeast Missouri are possibily transform faults and may be related to the New Madrid rift zone. Left-lateral slip is shown between the older rocks and the Quaternary alluvium where many of the rivers flow across the Mississippi escarpment (Horrall and others, 1983). If real, the contact between these transform faults and the Mississippi embayment would be an ideal locus for ultramafic and mafic intrusions of all ages (Horrall and others, 1983). Basinal brines would move preferentially along the embayment and faults and would leach 105 copper, cobalt, and nickel from these ultramafic and ma.fie plutons. However, Jessey (1983) suggested that the copper, cobalt, and nickel were derived from the Precambrian basement rocks. The most cobalt-rich, and presumably the earliest deposit occurs in Precambrian rocks near the northern end of the Viburnum Trend, and cobalt, nickel, and copper decrease to the south away from this deposit (Jessey, 1983). Jessey (1983) also proposed that the nickel:cobalt ratio increased with time and distance from the source. The most cobalt-rich siegenite occurs on the western side of the Goose Creek mine, and if the nickel:cobalt ratio increases with time and distance, then this pattern would suppon the eastward movement of fluids in the Indian Creek district Transponin~ Mechanism Basinal brines are commonly cited as the source of the metal-bearing fluids for Mississippi Valley-type deposits. According to Cathles and Smith (1983), continuous expulsion of fluids through normal basinal compaction cannot produce flow rates high enough to maintain deep-basin temperatures in the near-surface sites of ore deposition. Only episodic expulsion as fluid pressure builds up to lithostatic levels would allow basinal compaction to work (Cathles and Smith, 1983). Sharp (1978) modeled the Late Pennsylvanian-Permian faulting of geopressured zones in the Ouachita basin. In this model, fluids would have been released in discrete pulses as faulting brought the geopressured zones in to contact with updip aquifers. Widespread traces of mineralization appear to link the major ore districts (North Arkansas, Tri-State, Southeast Missouri, Central Missouri) in the Ozarks (Leach and Rowan, 1986). Fluid inclusion temperatures of 80-170 °C for the districts display a slowly cooling thermal gradient moving nonhward from the 106 Arkoma (Ouachita) basin (Leach and Rowan, 1986). Leach and Rowan (1986) suggested a gravity flow system for the Ozarks deposits, which was driven by a hydraulic head created by topographic relief in the uplifted Ouachita foldbelt where recharge occurred. Iimin~ Of Ore Deposition The timing of mineralization in Southeast Missouri is uncertain. The radiogenic nature of the J-type lead prevents dating with lead isotopes. Other methods using paleomagnerics, K-Ar in illite, and Rb-Sr in glauconites have been applied in the attempt to date the deposits. Wu and Beales (1981) found the Bonneterre Dolomite far away from mineralization has a different paleopole position than Bonneterre Dolomite host-rock and the ore, which are similar. Assuming that the ore fluids reset the paleomagnetic clock in the host rock, then the apparent age of the mineralizing event can be estimated by matching paleomagnetic signatures in the ore and host rocks with the apparent polar wandering path for the North American craton (Wu and Beales, 1981). Using this method, Wu and Beales (1981) proposed an Early Permian to Late Pennsylvanian age of mineralization for the Southeast Missouri district. Similarly, paleomagneric studies of the Bonneterre Formation (McCabe and others, 1982; Wisniowiecki and others, 1983) also resulted in an Early Permian to Pennsylvanian age for mineralization. Potassium-argon dating of post-galena illite from the Lamotte Sandstone in Southeast Missouri provided a minimum age for lead mineralization of 241-275 Ma (Huggins, 1981; Rothbard, 1982; 1983). Illites with feldspar and possible glauconite contamination resulted in much older ages (300-453 Ma; Rothbard, 1982). 107 In contrast, rubidium-strontium dating of glauconite from the Bonneterre Formation and the Davis shale in the Magmont mine resulted in a 359 ± 22 Ma isochron age (Stein and Kish, 1985). Stein and Kish (1985) concluded this glauconite age records the arrival of hot ore-bearing solutions. Glauconites not associated with known mineralization from several locations in southern Missouri yield model ages of 354-403 Ma, suggesting that the glauconites were only partially reset as compared to those in the Viburnum Trend (Posey and others, 1983). Other Sandstone-Hosted Base Metal Deposits Sandstone-hosted lead deposits are found in Europe, Africa, and North America in quartzitic sandstones, which were deposited at low latitudes, overlying a sialic basement (Bjorlyk.ke and Sangster, 1981 ). Table 5 (modified from Bj~rlykke and Sangster, 1981) compares the Goose Creek deposit with other sandstone-hosted lead deposits, red-bed copper deposits, and carbonate-hosted deposits. Southeast Missouri carbonate-hosted deposits are closely related to the Larnone Sandstone and display metal ratios (i.e. Pb >> Zn) normally associated with sandstone-hosted deposits (Bj¢rlykke and Sangster, 1981). There are numerous similarities between the Goose Creek deposit and the Laisvall and Vassbo deposits in Sweden. The Swedish sandstone-hosted lead-zinc deposits are two of several that occur along the eastern border of the Caledonian mountain chain (Rickard and others, 1979). Sulfide mineralization occurs mainly in a Lower Cambrian quanzitic sandstone at Vassbo, and in a Eocarnbrian quanzitic sandstone at Laisvall (Christofferson and others, 1979; Rickard and others, 1979). Both districts are underlain by Precambrian basement of sialic composition (Christofferson and others, 1979; Rickard and others, 1979). A thin phosphorite layer overlies the ore-bearing sandstones in both districts (Christofferson and others, 108 TABLES COMP ARIS ION OF SANDSTONE- AND CARBON ATE-HOSTED MET AL DEPOSIT TYPES Deposit type Red-bed Cu Sandstone Pb Goose Creek Carbonate Pb-Zn Mel.al Cu»Zn+Pb Pb>Zn>Cu Pb>> Cu >» Zn Zn>Pb>Cu ranking Host rock Arkose, feldspar Quartz sandstone, Quartz sandstone Dolomite shale Depositional Continen!.al Continental-shallow Shallow marine Shallow marine environment marine Tectonic Rifting Stable Stable Stable environment Oirnat.e Arid to Semi- Semiarid, wann Tropics (1) Arid to semiarid, arid to cool? warm Cement Calcite-dolomite- Quartz Quanz, dolomite Dolomite quanz Ultimate metal Basement Basement Basement Basement source Immediate Oxide coatings Feldspar in arkose Lamotte Carbonate, metal source or basement Sandstone (2) evaporite, shale Sulfur isotope Light Light and heavy Light (3) Heavy compostion Transpon Ground water Ground water Na-Ca-Cl Na-Ca-Cl medium brines brines (I) from Houseknecht (1975) (2) from Doe and Dclevaux (1972) (3) from Brown (1967) Modified from Bj~rlykke and Sangster (1986) 109 1979; Rickard and others, 1979). Elongate phosphorite fragments occur in coarser grained sandstones and are associated with limestone lenses in finer grained sandstones in the Laisvall deposit (Rickard and others, 1979). Similar! y, phosphorite ripup clasts are found in the Lamotte at Goose Creek. The sandstone units at Laisvall represent lagoonal beach and tidal channel-sand bar deposits, and the phosphorite bed a lagoonal deposit (Rickard and others, 1979). The Lamotte Sandstone at Goose Creek is probably a nearshore, marginal marine deposit. As with Goose Creek, galena is the dominant sulfide mineral in both of the Swedish deposits, and where crossbedding is common, galena follows the bedding planes (Christofferson, and others, 1979; Rickard and others, 1979). In the Laisvall deposit, galena occurs in spots that are very similar to those in the Lamotte Sandstone at Goose Creek, which suggests similar environments during precipitation. Fluid inclusion data for Laisvall indicate that ore formation occurred between 130 and 180 °C from a solution containing 24 equiv. wt percent NaCl with a maximum of 9 wt percent CaCl2 (Lindblom, 1986). Although similar in composition, these fluids were much hotter than those determined for the Indian Creek subdistrict (105-120 °C). CONCLUSIONS The Indian Creek subdisoict represents a transitional ore deposit type between "sandstone-hosted deposits" and "carbonate-hosted Mississippi Valley-type deposits". Sulfide deposits are associated with the Indian Creek ridge, an uplifted block of Precambrian rhyolitic basement, which has a shallow sloping nonhwestern flank and a steeply sloping southeastern flank. The Indian Creek ridge is probably bounded by nonheast-trending faults and intersected by three or more east-trending faults. A graben on the nonhern end separates Indian Creek from another ridge along the same trend. This complex structural pattern resulted in an irregular basement topography resulting from erosionn prior to Late Cambrian marine transgression. Wispy soingers of phosphorite, carbonate and phosphorite ripup clasts, calcium phosphate fossil fragments, and local carbonate beds indicate that the Lamotte Sandstone was deposited in a marginal marine environment, and planar cross-bedding appears to represent upper shoreface deposition. The Bonneterre Dolomite and younger carbonate units represent shallow marine depositional environments. Lead- and copper-bearing zones closely follow the basement topography and are independent of lithology and formation boundaries. This relationship suggests that the Precambrian basement was the major controlling factor on ore deposition in the Indian Creek subdistrict and that deposi tional and diagenetic facies, which were locally controlled by the presence of the basement ridge, had a major influence on the local mineralizing fluid flow. Major lead and copper concentrations occur in the Bonneterre Dolomite overlying the Precambrian basement, except for the mineralized Lamone Sandstone and Bonneterre Dolomite on the nonhern end of 110 111 the Indian Creek ridge. Vertical sulfide concentrations near the Lamotte pinchout suggest that the ore-bearing fluids moved through the Lamotte aquifer until the pinchout forced them into the overlying Bonneterre; the fluids were channeled through the porous grainstone-algal reef complex along the N30°E-trending ridge and formed the carbonate-hosted sulfide concentrations. Maximum lead and copper concentrations in the Lamotte Sandstone occur on the nonhem end of the Indian Creek ridge, west of and over the saddle, and the highest sulfides concentrations are within 20 ft of the Larnotte-Bonneterre contact. Maximum lead concentrations in the Bonneterre Dolomite occur along the western flank of the Indian Creek ridge. Intense early dissolution of quartz grains is associated with organic-rich clay zones and suggests that silica solubility may have been enhanced by the presence of organic acids. Phosphorites are also associated with intense quartz dissolution. Later quartz dissolution created secondary porosity during sulfide precipitation and precludes porosity as a major controlling factor in ore deposition. Fourteen authigenic minerals, plus hydrocarbons, cement the Lamotte Sandst0ne at Indian Creek in the following paragenetic sequence: dolomite - framboidal pyrite - marcasite - cuboctahedral pyrite - bravoite - bladed marcasite - pyrite - quartz dissolution - brecciation - siegenite - marcasite - dolomite - brecciation - chalcopyrite - quartz dissolution - sphalerite - galena (cuboctahedral) - quartz - galena (cubic) - dolomite - gypsum - hydrocarbon - kaolinite - illite - calcite - hydrocarbon. 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