Cover. Leached ilmenite grain approximately 60 microns in diameter surrounded by 

quartz, overgrowth in Lower Pennsylvanian sandstone in Indiana. Quartz overgrowths 

occlude much of the pore space in these rocks. Sample collected by Paula Hansley, U.S. 

Geological Survey.
Lead Isotopes from the

Upper Mississippi Valley District 

A Regional Perspective

By Timothy M. Millen, Robert E. Zartman, and Alien V. Heyl

EVOLUTION OF SEDIMENTARY BASINS^-ILLINOIS BASIN 

Jennie L. Ridgley, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2094-B

A multidisicplinary approach to research studies of 

sedimentary rocks and their constituents and the 

evolution of sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Millen, Timothy M.

Lead isotopes from the Upper Mississippi Valley district; a regional perspective / By 

Timothy M. Millen, Robert E. Zartman, and Alien V. Heyl

p. cm. (Evolution of sedimentary basins Illinois Basin; B) (U.S. Geological 

Survey bulletin ; 2094)

Includes bibliographical references.

Supt. of Docs, no.: 119.3:20948

1. Galena Illinois Basin Analysis. 2. Lead Isotopes Illinois Basin. 

I. Zartman, Robert E. II. Heyl, Alien Van, 1918- III. Tide. IV. Series. 

V. Series: U.S. Geological Survey bulletin : 2094. 

QE75.B9 no. 2094-B 

[QE319.G3] 

557.3 s dc20

[553.4'4'0977] 95-7133

CIP
CONTENTS

Abstract........................................................................................................................... Bl

Introduction..................................................................................................................... 1

Sample Localities............................................................................................................ 2

Analytical Procedure...................................................................................................... 2

Discussion....................................................................................................................... 3

Summary and Conclusions............................................................................................. 8

References Cited............................................................................................................. 9

Appendix Sample Descriptions and Localities............................................................ 11

FIGURES

1, 2. Maps of Upper Mississippi Valley district and vicinity showing galena sample localities and distribution of:

1. ^Pb/^Pb......................................................................................................................................................... B4

2. ^Pb/^Pb..................................^ 5

3, 4. Diagrams of:

3. ^PbWb versus ^PbWb............................................................................................................................ 6

4. ^PbWb versus ^Pb/^Pb............................................................................................................................ 7

TABLES

1. Revised lead isotopic data for the Upper Mississippi Valley district from Heyl and others (1966)........................ B2

2. New lead isotopic data for the Upper Mississippi Valley district and vicinity........................................................ 3

3. Comparison of regression slope, source rock age, and time of mineralization, as determined by

different authors and calculation methods............................................................................................................... 8

ill

LEAD ISOTOPES FROM THE

UPPER MISSISSIPPI VALLEY DISTRICT 

A REGIONAL PERSPECTIVE

By Timothy M. Millen, 1 Robert E. Zartman,2 and Alien V. Heyl3

ABSTRACT

New lead isotopic data on galena from within and 

peripheral to the Upper Mississippi Valley lead-zinc district 

make it possible, by extending coverage to outlying locations, to trace the pathway traversed by the mineralizing fluids beyond the boundary of the main district. All but one of 

the samples exhibit elevated ratios of the radiogenic isotopes 

typical of the Upper Mississippi Valley ore deposits;

206pb/204pb ranges from 19 38 to 24.46? 207pb/204pb ranges

from 15.73 to 16.24, and 208Pb/204Pb ranges from 39.24 to 

43.69. Galena from the Pints quarry near Waterloo, Iowa, 

has distinctly lower values of these ratios and may not be 

related paragenetically to the other samples. Otherwise, the 

lowest ratios are for samples in the southern part of the 

region in north-central Illinois, and the highest ratios are for 

samples to the northeast of the main district in the vicinity of 

Madison, Wisconsin. Thus, an isotopic pattern rather similar 

to that observed originally by Heyl and others (1966) prevails regionally, although the predominant fluid flow is now 

believed to have emanated from the Illinois Basin rather than 

from the Forest City Basin. Metal-bearing brines being 

driven northward out of the Illinois Basin probably played 

the key role in mineralization of the Upper Mississippi Valley district.

Both the new and the previously reported lead ratios for 

the Upper Mississippi Valley district are plotted on

207pb/204pb and pb208/pb204pb yersus 206pb/204pb dia.

grams, which permit their comparison and the calculation of 

refined slopes for the expanded data set. A two-stage model 

age for the time of mineralization can be determined from 

the 207Pb/204Pb-Pb206/Pb204 slope, provided that the source 

age of the lead is known. With our limited knowledge of this 

source age, the time of mineralization cannot be tightly

1 Department of Geology, Northern Illinois University, DeKalb, Illinois 60115.

"U.S. Geological Survey, MS 963, Denver Federal Center, Denver, 

Colorado 80225.

3P.O. Box 1052, Evergreen, Colorado 80439.

constrained but is permissive of a Permian or younger lateral 

secretion event, as suggested by other geochronological 

results.

INTRODUCTION

Previously, Heyl and others (1966) reported the lead 

isotopic composition of 17 galenas from the Upper Mississippi Valley lead-zinc district of southwestern Wisconsin, 

northwestern Illinois, and northeastern Iowa. Contouring of 

the 206Pb/204Pb and 208Pb/204Pb ratios across the district 

revealed a pattern of isotopic zonation in which lower ratios 

are on the western margin of the district and higher ratios are 

concentrated toward the northeast. The data of Heyl and others (1966) were reported as raw ratios uncorrected for mass 

fractionation (absolute standards were unavailable at that 

time), and they were later converted to absolute values and 

reinterpreted by Yonk (1970) and Richards and others 

(1972). The revised absolute ratios were not published, however, until more recently (Sangster, 1985) (see table 1). 

Aside from these adjustments generally leading to a refinement in the linear regressions of the data and to a recalculation of secondary isochron ages no other high-precision 

lead isotopic analyses have been reported for the Upper Mississippi Valley district since Heyl and others (1966).

The present study adds a significant number (22) of 

new galena isotopic analyses for the Upper Mississippi Valley district and vicinity that, hopefully, will renew interest in 

the source and transport mechanism of lead in this important 

mining region. Although a variety of new geologic research 

has been conducted on the Mississippi Valley deposits subsequent to Heyl and others (1966), this paper does not 

broadly discuss ore genesis in the Upper Mississippi Valley. 

The interested reader is referred to Heyl (1983) and Sverjensky (1986) for a review of some recent studies of Mississippi 

Valley-type zinc-lead deposits. Instead, we take this opportunity to revisit the Upper Mississippi Valley zinc-lead district and surrounding areas of Illinois, Wisconsin, Iowa, and 

Minnesota in order to draw more extensive regional lead

Bl
B2 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Table 1. Revised lead isotopic data for the Upper Mississippi Valley district from Heyl and others (1966).

[Sample localities are shown by number in figures 1 and 2. Locality numbers are the same as reported in Heyl and others (1966). Sample

locations and descriptions are summarized in the appendix. Data are converted to absolute values; original, uncorrected values are in

parentheses]

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

Locality number and name

Waukon (Mineral Creek) mines

Captain Turner mine

Holmes mine

Skene mine

Amelia mine (average of 3 samples)

Rockville mine

Piquette mine

Bautsch mine

Nigger Jim mine

New Hoskins mine

Calumet mine (cubic crystal)

Calumet mine (octahedral crystal)

Ohlerking mine

Old Slack mine

Ivey mine

North Yellowstone mine

Demby-Weist mine

206pb/204pb

20.70

21.22

21.74

21.92

21.91

21.89

22.06

22.18

22.07

22.17

22.80

22.62

23.18

23.48

23.76

23.60

24.24

(20.83)

(21.35)

(21.88)

(22.07)

(22.06)

(22.05)

(22.20)

(22.33)

(22.22)

(22.31)

(22.95)

(22.77)

(23.32)

(23.62)

(23.91)

(23.76)

(24.44)

207pb/204pb

15.85

15.99

15.95

16.02

15.98

16.01

16.01

16.06

15.97

15.98

16.15

16.04

16.10

16.14

16.17

16.16

16.22

(15.96)

(16.10)

(16.06)

(16.13)

(16.09)

(16.12)

(16.12)

(16.17)

(16.08)

(16.09)

(16.25)

(16.15)

(16.20)

(16.25)

(16.28)

(16.27)

(16.33)

208pb/204pb

40.19

40.83

41.29

41.67

41.60

41.44

41.64

41.98

41.79

42.09

42.58

42.43

42.40

42.93

43.27

43.40

43.65

(40.45)

(41.10)

(41.57)

(41.96)

(41.89)

(41.73)

(41.92)

(42.27)

(42.07)

(42.37)

(42.86)

(42.72)

(42.68)

(43.22)

(43.55)

(43.71)

(43.95)

isotopic zonation maps and to calculate refined secondary 

isochron ages based on the expanded data set.

SAMPLE LOCALITIES

Minor occurrences of Mississippi Valley-type sulfide 

mineralization outside the main Upper Mississippi Valley 

district were located and described by Heyl and West (1982), 

who noted (p. 103) that "Epigenetic lead, zinc and copper 

sulfides have been found in old mines, prospects and other 

occurrences over a wide area on all sides of the Upper Mississippi district. These sulfides have the same crystal habits, 

trace elements, lead isotope patterns, and sequence of formation" within the main district and in the outlying areas surrounding the main productive district. Samples from several 

of these localities, together with those from other mineral 

occurrences previously unreported in the literature, were 

included in the present study. The new sites were chosen on 

the basis of availability of galena, Mississippi Valley-type 

paragenetic relationships, and geographic distribution 

peripheral to the main Upper Mississippi Valley district.

The location, host rock, and mode of occurrence for 

many of the samples included in this study are described in 

Heyl and others (1966) and Heyl and West (1982); otherwise, a brief description is given in the appendix. All but one 

of the nine previously unreported occurrences (sample 21) 

are quarries containing noneconomic concentrations of 

galena, sphalerite, and iron sulfides. Most, but not all, have

associated calcite, and at one location (sample 38) substantial quantities of associated euhedral, acicular barite are 

present. Although galena has been reported in rocks ranging 

in age from Cambrian to Pennsylvanian (Heyl and others, 

1966), the present study only includes galena in Cambrian to 

Devonian rocks. Because the bulk of the ore extracted from 

the Upper Mississippi Valley district was hosted in rocks of 

Ordovician age, effort was concentrated on obtaining samples from rocks of this age in order to expand the lateral isotopic pattern developed by Heyl and others (1966) for the 

main ore district. By restricting sampling mostly to this horizon, however, vertical isotopic variation was not investigated, and its contribution to the overall pattern remains 

indeterminate.

ANALYTICAL PROCEDURE

Lead isotopic analyses from localities 18, 21, 27, and 

29-32 were initially conducted at Northern Illinois University on a 6Lin.-radius, Nier-type solid-source mass spectrometer to determine approximate isotopic compositions of 

galenas from outlying locations and to establish the general 

nature of the isotopic anomaly pattern. The precision of these 

analyses (±1 percent) was insufficient, however, to compare 

them with previously reported data from the Upper Mississippi Valley district. These samples were then reanalyzed on 

a 12-in.-radius, solid-source mass spectrometer at the U.S. 

Geological Survey to obtain the desired precision. In 

addition, other samples that had been previously analyzed at
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE B3

Table 2. New lead isotopic data for the Upper Mississippi Valley district and vicinity.

[Sample localities are shown by number in figures 1 and 2 and are described in the appendix. Data are given as absolute values based

on intercomparison with the NBS 981 lead standard]

Locality number and name

18. Spring Grove quarry 

19. Pints quarry 

20. Anamosa diggings 

21. Fesslermine

22. Orion mine

23. Doylestown mines 

24. Speedway roadcut 

25. Blue Mounds mines

26. Exeter diggings 

27. B redhead quarry

28. Avon lead-zinc occurrence

29. Vinegar Hill mine 

30. Ten Strike mine

31. Morseville quarry 

32. Mt. Carroll mines

33. Stebbins mine

34. Brookville quarry 

35. Oregon East quarry 

36. Dixon Southwest quarry 

37. Butler quarry

38. Troy Grove quarry 

39. Elmhurst quarry

206pb/204pb

21.83 

18.29 

22.65 

22.06

22.77

24.46 

23.11 

23.16

23.32 

23.09

21.70

22.30 

21.79

22.75 

21.80

22.12

20.47 

19.38 

19.55 

19.64

19.57 

20.81

207pb/204pb

15.99 

15.62 

16.08 

15.99

16.06

16.24 

16.10 

16.12

16.16 

16.14

15.97

16.03 

15.97

16.07 

15.99

16.00

15.87 

15.73 

15.76 

15.78

15.75 

15.88

208Ph/204Pb

41.36 

38.24 

41.48 

41.91

42.24

43.25 

43.04 

43.15

43.69 

43.39

41.78

42.25 

41.44

42.58 

41.66

42.30

40.50 

39.25 

39.36 

39.39

39.24 

40.22

the U.S. Geological Survey over the past decade (samples 

20, 22-26, 28, and 33), but not reported, were incorporated 

into the data set.

Approximately 1^2 mg of galena powder or crystal 

fragments was converted to Pb(NO3)2 by dissolution in 30 

mL of nitric acid (10 mL of 70 percent distilled HNO3 and 

20 mL ultrapure H2O). From this 30-mL solution, a 10-|LiL 

aliquot was transferred to a 5-mL centrifuge tube to which 1 

drop of ultrapure H3PO4 was added. After heating the sample almost to dryness, a single drop of ultrapure silica gel 

was added, and the total solution loaded onto a rhenium filament for mass spectrometric analysis. Samples were run 

alternately with the NBS 981 lead standard, and all new data 

were normalized to absolute values. Replicate measurements 

were made on about three-fourths of the samples, confirming 

a precision of ±0.1 percent or better (95 percent confidence 

level) for all ratios. The results of the new analyses are 

shown in table 2.

DISCUSSION

The 206Pb/204Pb and 208Pb/204pb isotopic data in tables 

1 and 2 were plotted and contoured on a base map of the 

Upper Mississippi Valley region (figs. 1 and 2, respectively). 

In order to minimize the effect of local isotopic scatter

known to be present even in samples from a single mine (for 

example, compare samples 11 and 12 in table 1; also the 

Stillhouse 40 mine has a range in 206Pb/204Pb of 

21.45-22.10; T.M. Millen, unpublished data), some smoothing of the contours was allowed in the computer program 

used to construct the figures. Expectedly, comparison of 

these plots with those of Heyl and others (1966) shows the 

retention of a similar pattern, as well as the extension of the 

contours well outside the original study area.

Heyl and others (1966) suggested that this districtwide 

gradient reflects fluid flow paths of the ore solutions and 

discussed two possible mechanisms for generating the 

regional lead isotopic.

1. Ore fluids migrated updip from adjacent source 

basins, depositing lead as they traversed the district. The 

increasingly radiogenic character of the galena across the 

Upper Mississippi Valley district was produced by additions 

of radiogenic lead to these migrating fluids. Left in doubt is 

whether the radiogenic lead was leached from the lower 

Paleozoic sedimentary rocks, which served as primary aquifers for the solutions, from underlying basement rocks, 

which also became involved in the hydrologic system, or 

from some combination of the two possible sources.

2. Ore fluids initially entered and migrated through 

basement rock at some structural high, such as along the
B4 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Wisconsin arch, where they became enriched in radiogenic 

lead. Subsequently, the metal-bearing fluids rose into overlying sedimentary rocks and then moved laterally toward the 

southwest. In this case, a less radiogenic lead component 

derived from the host rocks is required to contaminate the 

fluid as it migrated away from the site of upward flow.

Because of its geologic plausibility, the first model, 

which presumes that the fluids were derived by lateral secretion from, or, at least, flowed through, an adjacent sedimentary basin, seems preferable. The original contour map of 

Heyl and others (1966) appears to implicate the Forest City 

Basin, to the southwest in central Iowa, as the main ore-fluid 

source. Cathles and Smith (1983) and Bethke (1986), in their 

respective studies of compaction-driven fluid flow and of 

gravity-driven fluid flow as an ore-fluid forming process in 

sedimentary basins, assumed that the Illinois Basin, more

than 12,000 ft thick in southern Illinois, was the source of the 

fluids for the Upper Mississippi Valley district. The Forest 

City Basin, which contains only about one-third the thickness of sediments, was deemed less favorable by Cathles and 

Smith (1983) and, by itself, unable to produce the ore-fluid 

temperatures observed in the Upper Mississippi Valley district. Cathles and Smith (1983, p. 998) and Bethke (1986) did 

not, however, preclude some involvement of the Forest City 

Basin as an ore-fluid source for the Upper Mississippi Valley 

district.

Our new isotopic contour maps, which extend sample 

coverage a considerable distance to the south and west (figs. 

1, 2), are permissive of, but may not require, fluid flow originating from, or passing through, both of these basins. In particular, one might interpret the relatively high sample density 

between the Illinois River and Freeport to be the "tracks" of

92° 91° 90°

43°

42°

V

MINNESOTA «18 IOWA~ "

\

Qlowa City

Davenport ^

r 

Misst,issiPPl

0 20 40 60

I.I I I KILOMETERS

^

Figure 1. Map showing galena sample localities (solid circles) and distribution of ^"Pb/ Pb, Upper Mississippi Valley district and 

vicinity. Contour interval 0.5; contours smoothing was allowed in computer program; dashed contours are controlled solely by Pints quarry 

sample (locality 19)). Arrows indicate suggested flow paths for mineralizing fluids.
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE B5

an isotopically evolving lead emerging from the Illinois 

Basin and flowing northward into southwestern Wisconsin. 

Much poorer control, anchored mainly by the low 

206Pb/204Pb sample 19 from Pints quarry near Waterloo, 

serves to define a second possible flow path from the Forest 

City Basin. Paragenetic relationships and Late Devonian age 

of the limestone at Pints quarry are, however, the most dissimilar to the Upper Mississippi Valley occurrences of any 

sample included in this study (M. Goldhaber, written commun., 1993). Accordingly, those contours in figures 1 and 2 

that are controlled solely by the Pints quarry sample are 

shown as dashed, and we interpret the Forest City Basin flow 

path with great caution. Evidence for an isotopic high separating the Illinois Basin and Forest City Basin flow regimes 

is provided almost entirely by sample 20 from the Anamosa 

diggings. Although an extensive field search was made for

additional samples from east-central Iowa, we have, as yet, 

been unsuccessful in better defining this weakly mineralized 

area. Indeed, the presence or absence of trace galena in limestone quarries may be, in itself, an important clue to fluid 

paths.

Whether or not formation of these outlying occurrences 

of lead and zinc sulfides is related to deposition of ores in the 

main Upper Mississippi Valley district may be questioned. 

In order for the lead data from this study to be of significance 

to the study of the Upper Mississippi Valley district, these 

outlying occurrences must have been a result of the same 

mineralizing process. Interpretations of paragenetic, fluid 

inclusion, and stable isotopic studies of outlying and main 

district occurrences (Jenkins, 1968; Garvin and others, 1987; 

Millen and Ludvigson, 1987; Ludvigson and Millen, 1988; 

Kutz and Spry, 1989) do not unequivocally indicate a genetic

92 \

__ MINNESOTA 

IOWA

43°

42°

Davenport ~ t'

0 20 40 60 I

lil I I KILOMETERS /

Figure 2. Map showing galena sample localities (solid circles) and distribution of 208Pb/204Pb, Upper Mississippi Valley district and vicinity. Contour interval 0.5; contours smoothing was allowed in computer program; dashed contours are controlled solely by Pints quarry 

sample (locality 19). Arrows indicate suggested flow paths for mineralizing fluids.
B6 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

relationship between the outlying occurrences of Upper Mississippi Valley-type mineralization and deposits of the main 

district. Although most geochemical and paragenetic studies 

on Upper Mississippi Valley mineralization within the district are on pitch-and-flat deposits, many studies of the outlying occurrences have been conducted on disseminated, 

gash-vein, karst-fill, replacement, and vug-fill deposits. 

Paragenetic similarities exist between many of these types of 

occurrences and the main ore district pitch-and-flat deposits 

(for example, Bradbury, 1961), but the full range of the 

paragenetic sequences in outlying mineralized zones is usually not present at a given location.

Sulfur isotopic studies show that most outlying occurrences are isotopically lighter than the deposits in the main 

district, although some overlap between pitch-and-flat 

deposits and outlying occurrences does exist (McLimans, 

1977; Garvin and others, 1987; Millen and Ludvigson, 1987; 

Ludvigson and Millen, 1988; Kutz and Spry, 1989). Sulfur 

isotopic geothermometry and fluid inclusion studies on

outlying occurrences (Ludvigson and Millen, 1988; Kutz 

and Spry, 1989) produce fluid temperatures similar to those 

that formed the Upper Mississippi Valley district; however, 

late-stage calcites from several localities exhibit higher 

salinities than is typical for the Upper Mississippi Valley district fluids (Kutz and Spry, 1989). The general conclusion 

from stable isotopic, paragenetic, and fluid inclusion studies 

is that the closer the outlying occurrence approximates values and relationships of the main Upper Mississippi Valley 

district deposits, the more likely it is genetically related. Of 

the new sites in this study, those within the historic mining 

district (samples 21, 29, 30, and 31) and those nearest the 

district (samples 22, 24-28, 32, and 33) are most likely 

genetically related to main district mineralization. Sites in 

this study least likely related to Upper Mississippi Valley 

mineralization are those farthest from the main district (samples 18, 19, and 36-39).

Lead isotopic studies seem, however, to support coprecipitation of most outlying occurrences and main district

16.2

16.0

15.8

15.6

EXPLANATION

Heyl and others (1966) 

M = 0.0976 R = 0.939

This study 

M = 0.1022

All data 

M= 0.1013

R = 0.998 

R = 0.984

18.0 20.0 22.0 

206pb/204pb

24.0 26.0

Figure 3. 207Pb/204Pb versus 2o6Pb/204Pb diagram for revised data of Heyl and others (1966) (solid circles) and data from 

this study (open squares). Slopes, M, and correlation coefficients, R, are based on least-squares regression.
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE B7

ores (Millen and Ludvigson, 1987; Ludvigson and Millen, 

1988). 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams plotting the combined revised Heyl and others (1966) 

data and the data from this study are shown in figures 3 and 

4. There is good agreement between the slopes of the new 

data from outlying occurrences and those from the main 

Upper Mississippi Valley district proper, suggestive of a 

genetic relationship. As previously discussed, of course, 

more paragenetic, fluid inclusion, and isotopic work needs to 

be completed before some of these outlying occurrences can 

be definitely related to the main Upper Mississippi Valley 

mineralizing event.

Assigning an age of mineralization in the Upper Mississippi Valley district has long been problematic. Stratigraphically, galenas have been reported in rocks of Cambrian to 

Pennsylvanian age. Because no Permian to pre-Pleistocene 

rocks are present in the Upper Mississippi Valley, a minimum age cannot be determined by stratigraphic means. On 

the basis of structural evidence, Heyl and others (1970) suggested that mineralization occurred between the late Paleozoic and the end of the Mesozoic. Zimmermann (1986) 

interpreted "young" apatite fission-track ages determined on 

Precambrian granitic rock from a northern Illinois drill core

and from exposed basement in Wisconsin and Michigan to 

indicate substantial post-Permian, possibly Cretaceous and 

early Tertiary, uplift of the region. He speculated that 

removal of several thousands of feet of cover rock at that 

time might have provided the hydrologic conditions for brine 

migration and lead-zinc mineralization. Doe and others 

(1983) and Peterman and others (1986) concurred, on the 

basis of disturbed whole-rock and zircon U-Pb systems, 

respectively, that middle to late Phanerozoic uplift had 

affected the rocks in the northern Illinois drill core a conclusion also reached by Afifi and others (1984) in a study of 

the whole-rock U-Pb system in Early Proterozoic metavolcanic rocks of northern Wisconsin. More recently, Brannon 

and others (1992a, b) obtained Rb/Sr ages of -270 Ma on 

sphalerite from main district ores. An episode of late Paleozoic brine migration through Cambrian carbonate rocks of 

the central and southern Appalachians, which may also have 

implications for contemporaneous Mississippi Valley-type 

mineralization in the continental interior, has been recorded 

by the growth of authigenic potassium feldspar (Hearn and 

others, 1987; however, also see Duffin and others, 1989, for 

evidence of both Devonian and Permian potassium 

diagenesis).

44.0

42.0 

EXPLANATION

Heyl and others (1966) 

M = 0.9810 R = 0.982

This study 

M = 0.9933

All data 

M = 0.9763

R = 0.974 

R = 0.975

20.0 22.0 

206pb/204pb

24.0 26.0

Figure 4. 208Pb/204Pb versus 206pb/204pb diagram for revised data of Heyl and others (1966) (solid circles) and data from 

this study (open squares). Slopes, M, and correlation coefficients, R, are based on least-squares regression.
B8 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

Table 3. Comparison of regression slope (M), source rock age fa), and time of mineralization fa), as 

determined by different authors and calculation methods. 

[Errors in slopes and ages are ±2d values]

Regression slope (M)

0.0882±0.0087U 

0.093810. 007 1 3 

0.0962±0.00704 

O.lOlSiO.00561 

0.101310.00561

0.1013±0.00561

0.1005±0.00344 

0.1005±0.00344

0.1005±0.00344

t\ (for f2=0 Ma)

l,410±180Ma 

l,530+130/-160Ma 

l,570±140Ma 

l,648+102/-108Ma

1,633+577-63 Ma

fe (for f i=l, 350 Ma)

100±250 Ma 

285+21 5/-240 Ma 

365+200/-240 Ma 

479+142/-144 Ma 

378+148/ 152 Ma5

255+1507 158 Ma6

456+8S/-92 Ma 

355+947 97 Ma5

230+977 99 Ma6

Reference

Heyl and others (1966). 

Richards and others (1972). 

Richards and others (1972). 

All data, this study. 

All data, this study.

All data, this study.

All data, this study. 

All data, this study.

All data, this study.

1 Slope determined by unweighted least-squares method. 

Calculated regression based on uncorrected lead values. 

3Slope assigns all errors to Y value (York, 1969). 

4Slope assigns errors to X and Y values (York, 1969). 

5Assumes ^=1,420 Ma. 

6Assumesfi=l,500Ma.

Attempts at dating the time of ore deposition using a 

two-stage lead isotope model have been made by Heyl and 

others (1966), Yonk (1970), and Richards and others (1972) 

for the Upper Mississippi Valley. Whereas Heyl and others 

(1966) used a simple least-squares method to determine the 

207pb/204pb_206Pb/204pb slope M of the regression line,

Yonk (1970) and Richards and others (1972) used variations 

of a more complex method, developed by York (1969), of 

determining best-fit lines. The first variant attributes all error 

in M, to the 7, or 207Pb/204Pb, value, whereas the second 

variant takes into account systematic and related errors in 

both the Y and X variables, as would be the case if error is due 

mainly to mass fractionation or imprecision in measurements of the lesser abundant 204Pb. Both the simple 

least-squares method and the second variant of the York 

(1969) method of obtaining a slope are utilized for all the 

data available in tables 1 and 2. A comparison of values for 

M and ti (the time of mineralization) of Heyl and others 

(1966), Richards and others (1972), and this study is shown 

in table 3. Results determined by least-squares regression 

(M=0.1013±0.0056) and those determined by using the York 

best-fit program (M=0.1005±0.0034) show little difference, 

and use of either slope value does not alter the conclusions 

of our study.

In addition to a precise determination of M, the ages of 

ore emplacement given in table 3 rely on the ability to define 

t\ (age of the source of the lead; usually taken to be the age 

of the local midcontinent basement, although it could be a 

hybrid age arising from multiple sources or even an apparent 

age transferred to overlying sedimentary rock that was 

derived from the basement). Heyl and others (1966), Yonk 

(1970), and Richards and others (1972) used ^=1,350 Ma, an 

average age of basement rocks in the Upper Mississippi Valley as determined by Kanasewich (1962), Tilton and others

(1962), Goldich and others (1966), Muehlberger and others 

(1966), and Lidiak and others (1966). Subsequent studies 

indicate that this choice of t\ might be too young. Doe and 

others (1983), in their whole-rock U-Pb study of granite 

from the northern Illinois deep drill core, determined an 

upper concordia intercept age of 1,416±20 Ma and a secondary isochron age of 1,451 ±19 Ma. Hoppe and others (1983) 

dated zircon by the U-Pb method from this same and adjacent cores and determined upper intercept ages of 

1,461-1,509 Ma for the granitic basement underlying the 

Upper Mississippi Valley area. The estimated age of ore formation from Doe and others (1983) is 260±35 Ma, as suggested by the lower intercept on their concordia plot. If these 

older ages for the basement source rocks are more appropriate, then mineralization ages calculated from the slope of the 

secondary isochrons will be about 100-200 m.y. younger 

than if the source rocks are 1,350 Ma, as previously assumed. 

Thus, our new results are reconcilable with other recent 

efforts to establish mineralization ages for the Upper Mississippi Valley deposits, but, unfortunately, geologic uncertainties together with inherent scatter in the secondary isochrons 

prevent us from further resolving this important question.

SUMMARY AND CONCLUSIONS

On the basis of this and previous studies, it is likely that 

mineralization in the Upper Mississippi Valley was the result 

of lateral secretion of metal-rich basinal brines from the Illinois and, possibly, the Forest City basins. Extension of the 

lead isotopic pattern of Heyl and others (1966) further 

defines fluid paths within the region of northern Illinois, 

southern Wisconsin, northeastern Iowa, and southeastern 

Minnesota. Although isotopic data exhibit significant
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE B9

variations in lead ratios within single mines and mining subdistricts (206Pb/204Pb=20.69-22.10 for the Mt. Carroll subdistrict as a whole; T.M. Millen, unpublished data), averaged 

values for these regions plot well with respect to contours in 

the original study of Heyl and others (1966). The regional 

contour pattern defined by this study helps to define specific 

flow paths for mineralizing fluids, as shown in figures 1 and 

2. On the basis of these plots, the Illinois Basin in particular 

and perhaps the Forest City Basin probably contributed to 

the development of the Upper Mississippi Valley district 

mineralization. Because the lead isotopic control in eastern 

Iowa is scant, the role of the Forest City Basin in Upper Mississippi Valley mineralization cannot be fully assessed.

Our data are nonetheless compatible with the following 

scenario. At the time of mineralization, dominantly northward flowing, metal-rich brines from the Illinois Basin 

combined with eastward flow, if any, from the Forest City 

Basin apparently were deflected to the northeast through 

the Upper Mississippi Valley district. If the flow path was 

mostly confined to a lower Paleozoic aquifer, such as the 

Cambrian Mount Simon or Ordovician St. Peter Sandstones, 

the lead isotope pattern is best explained by the regional mixing of a basinal brine component (206Pb/204Pb=19.4 and 

18.3 and 208Pb/204Pb=39.2 and 38.2 where sampled in northern Illinois and northeastern Iowa, respectively) with a local, 

radiogenic-isotope-enriched component leached from the 

indigenous sedimentary rock. The proportion of the second 

component increased to the northeast because either the basinal brine component was being progressively spent or physicochemical conditions favored the leaching of radiogenic 

lead in that direction.

Alternatively, the flow path may have included excursions into, or intersection with fluids derived from, the 

underlying Precambrian basement from whence the radiogenic-isotope-enriched component was acquired. Precipitation of the galena and other sulfides took place when the 

mineralizing brine escaped through fractures and dissolution 

channels from the aquifers into the overlying carbonate rock, 

a phenomenon capturing the local lead isotopic composition 

and structurally focused in the main Upper Mississippi Valley district. Unfortunately, the isotopic characteristics of the 

underlying Precambrian basement are insufficiently known 

to predict the regional pattern in 206Pb/204Pb and 

208Pb/204Pb that might emerge from such a local sourcing of 

the lead.

Two-stage model ages determined from these galena 

leads do not tightly constrain the time of mineralization but 

are permissive of a Permian or younger lateral secretion 

event. Such an age is in agreement with the time of mineralization suggested by Heyl and others (1966, 1970), Yonk 

(1970), Richards and others (1972), McLimans (1977), Doe 

and others (1983), and Bethke (1986). Recent efforts by 

Brannon and others (1992a, b) and Hearn and others (1987) 

to date directly minerals from the Mississippi Valley paragenetic sequence undoubtedly represent a more definitive

approach toward establishing the late Paleozoic as the time 

of mineralization.

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BIO EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

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the Upper Mississippi Valley base-metal district: University of 

Wisconsin Geological and Natural History Survey, Information Circular 16, 45 p.

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1966, Isotopic study of galenas from the Upper Mississippi 

Valley, the Illinois-Kentucky and some Appalachian Valley 

mineral districts: Economic Geology, v. 61, p. 933-961.

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related to the Upper Mississippi Valley mineral district, Wisconsin, Iowa, Illinois, and Minnesota: Economic Geology, v. 

77, p. 1803-1817.

Hoppe, W.J., Montgomery, C.W., and Van Schmus, W.R., 1983, 

Age and significance of Precambrian basement samples from 

northern Illinois and adjacent states: Journal of Geophysical 

Research, v. 88, p. 7276-7286.

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using anomalous lead isotopes: Geophysical Journal of the 

Royal Astronomical Society, v. 7, p. 158-168.

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Upper Mississippi Valley district lead-zinc mineralization and 

minor base metal mineralization in Iowa, Wisconsin, and Illinois: Economic Geology, v. 84, p. 2139-2154.

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geochemistry of Mississippi Valley-type lead-zinc mineralization at Mount Carroll, Illinois, in Ludvigson, G. A., and Bunker, B. J., New perspectives on the Paleozoic history of the 

Upper Mississippi Valley An examination of the Plum River

fault zone: Iowa Department of Natural Resources Geological 

Survey Bureau Guidebook 8, p. 129-144.

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study of lead sulfide mineralization from the Fessler Mine No. 

2, Mines of Spain State Area, Dubuque County, Iowa: Iowa 

Geological Survey, Contract Report to the State Preserves 

Advisory Board, Iowa Conservation Commission, Contract 

No. SPAB-3, 14 p.

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southwest Wisconsin: University Park, Pennsylvania State 

University, Ph.D. thesis, 175 p.

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and S isotopic ratios in galenas from two lead mining areas in 

the Upper Mississippi Valley (UMV) Zn-Pb district: Geological Society of America Abstracts with Programs, v. 19, p. 234.

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1966, Geochronology of the basement rocks of the midcontinent region, United States, part 3, southern area: Journal of 

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from the Wichita and Arbuckle Mountains, Oklahoma, and the 

St. Francois Mountains, Missouri: Journal of Geophysical 

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Groups in northern Illinois: Illinois State Geological Survey 

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43 p.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication January 27, 1995 

Edited by Judith Stoeser 

Photocomposition and graphics by Joan Nadeau 

Cover design by Art Isom
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE Bll

APPENDIX SAMPLE DESCRIPTIONS 

AND LOCALITIES

[Samples are shown by number in figures 1 and 2. Descriptions and locations of samples 1-17 are summarized from Heyl and others (1966)]

1. Waukon (Mineral Creek) mines. Galena, smithsonite, 

and zinc clays in brecciated Oneota Dolomite. Clem 

Byrones farm, north of Waukon, Iowa, sec. 13, T. 99 

N., R. 6 W., lat 43°23'40" N., long 91°30/30" W., 

Dorchester, Iowa, 71/2-minute quadrangle. Reference: Heyl and West (1982).

2. Capt. Turner (Lansing) mine. Fissure vein galena, calcite, smithsonite, hemimorphite, and cerussite from 

Oneota and Trempleau formations. Lansing, Iowa, 

NW»/4 sec. 18, T. 99 N., R. 4 W, lat 43°23'50" N., long 

91°22'00" W, New Albin, Iowa, 71/2-minute quadrangle. Reference: Heyl and West (1982).

3. Holmes mine. Disseminated galena and calcite in 

Prosser Member of Galena Dolomite. 2 mi west of 

Guttenberg, Iowa, SWi/4 sec. 7, T. 92 N., R. 2 W, lat 

42°47'35" N., long 91°07/ 15" W, Guttenberg, Iowa, 

71/2-minute quadrangle. Reference: Heyl and West 

(1982).

4. Skene mine. Galena from veins in brecciated Galena 

Dolomite. 1 mi southwest of Elizabeth, 111., 

SWV4SWV4NWtt sec. 25, T. 27 N., R. 2 E., lat 

42°18'20'/ N., long 90°14/20" W, Elizabeth, 111., 

71/2-minute quadrangle. Reference: Heyl and others 

(1959, 1966).

5. Amelia mine. Disseminated galena from Guttenberg 

Limestone Memberof Decorah Formation from lower 

part of pitch-and-flat ore body. 3 mi south of Galena, 

111., SWV4SWV4 sec. 34,T. 28 N., R. 1 E., lat 

42°22/ 15//N., long 90°23/45'/ W. Bellvue, Iowa, 

71/2-minute quadrangle. Reference: Heyl and others 

(1959).

6. Rockville mine. Gash-vein deposit in middle part of 

Galena Formation. East side of U.S. Route 61, 2 mi 

north of Rockville, Wis., SWJ/4 SWJ/4 sec. 1, T. 3 N., 

R. 3 W, lat 42°45/30// N., long 90°40/40" W. Lancaster, Wis., 71/2-minute quadrangle. Reference: Heyl 

and others (1959).

7. Piquette mine. Cubic galena from bedded deposit in 

Decorah and basal Galena formations. 1 mi east of 

Tennyson, Wis., in center NE1/^ sec. 36, T. 3 N., R. 3 

W, lat 42°41 / 15// N., long 90°40/00// W, Potosi, Wis., 

71/2-minute quadrangle. Reference: Heyl and others 

(1959).

8. Bautsch mine. Cubo-octachedral galena from Decorah 

Formation in southernmost bedded ore body of mine. 

South of Galena, 111., north-central sec. 10, T. 27 N., 

R. 1 E., lat 42°21 /20'/ N., long 90°23'10" W, Bellvue,

Iowa, 71/2-minute quadrangle. Reference: Heyl and 

others (1959).

9. Nigger Jim mine. Cubo-octahedral galena from bedded lead-barite deposit in lower part of Decorah Formation. 2.5 mi north of Leslie, Wis., NE^SW1^ sec. 

22, T. 4 N., R. 1 E., lat 42°48'05// N., long 90°21 /50// 

W, Mifflin, Wis., 71/2-minute quadrangle. Reference: 

Heyl and others (1959).

10. New Hoskins mine. Galena from main inclined fracture zone of bedded deposit in Decorah Formation. 1.5 

mil northeast of New Diggings, Wis., SEl/4NEl/4 sec. 

14, T. 1 N., R. 1 E., lat 42°33/45// N., long 90°20/05// 

W, Long New Diggings, Wis., 71/2-minute quadrangle. 

Reference: Heyl and others (1959).

11. Calumet mine. Cubic galena from Gensler ore body. 

2.5 mi south of Shullsburg, Wis., N^NW^ sec. 27,T. 

1 N., R. 2 E., lat 42°32/30// N., long 90°14/30// W, 

Shullsburg, Wis., 71/2-minute quadrangle. Reference: 

Heyl and others (1959).

12. Calumet mine. Octahedral galena oriented on surface 

of sample 11. Reference: Heyl and others (1966).

13. Ohlkering mine. Galena in silicified dolomite breccia 

in Shakopee Member of Prairie du Chien Group. 

Highland, Wis., NW^SE^ sec. 31, T. 7 N., R. 1 E., lat 

43°02/ 15// N., long 90°25/00'/ W, Highland West, 

Wis., 71/2-minute quadrangle. Reference: Heyl and 

others (1959).

14. Old Slack mine. Disseminated cubic galena in Quimbys Mill Member of Platteville Group. Mifflin, Wis., 

SEi4SWi/4 sec. 26, T. 5 N., R. 1 E., lat 42°52/30// N., 

long 90°22/35'/ W, Long Linden, Wis., 71/2-minute 

quadrangle. Reference: Heyl and others (1959).

15. Ivey mine. Galena from copper-bearing lead-zinc ore 

in Decorah Formation. Southeast edge of Mineral 

Point, Wis., sec. 6, T. 4 N., R. 3 E., lat 42°51W/ N., 

long 90°10/35// W, Mineral Point, Wis., 71/2-minute 

quadrangle. Reference: Heyl and others (1959).

16. North Yellowstone mine. Cubic galena in bedded ore 

body in Decorah Formation. Yellowstone, Wis., 

N^NE^ sec. 19, T. 4 N., R. 5 E., lat 42°48/30//N., 

long 90°56'20" W, YellowstoneLake, Wis. 71/2-minute 

quadrangle. Reference: Heyl and others (1959).

17. Demby-Weist mine. Gash-vein mines in Prairie du 

Chien Group strata. Northeast of Dodgeville, Wis., 

NWV4 sec. 28, T. 7 N., R. 4 E., lat 43°03/45'/ N., long 

90°01 /40// W, Pleasant Ridge, Wis., 71/2-minute quadrangle. Reference: Heyl and others (1959).

18. Spring Grove quarry (south pit). Near-vertical, 

5-mm-wide veinlet in Middle Ordovician Platteville 

Formation. Cubic to cube-octahedral galena, sphalerite, iron sulfides, and calcite noted on surface of veinlet. At former site of the CMSP&P railroad overpass 

on Minnesota State Highway 44, 3.2 mi west of 

Spring Grove, Minn., NWV4NEV4 sec. 20, T. 101 N.,
B12 EVOLUTION OF SEDIMENTARY BASINS ILLINOIS BASIN

R. 7 W., lat 43°30'05" N., long 91°42'00" W., Spring 

Grove, Minn., 71/2-minute quadrangle. New galena 

locality; sample collected by T.M. Millen.

19. Pints quarry. Galena in Middle and Upper Devonian 

Cedar Valley Group. Near Raymond, Iowa, SWV4 sec. 

36, T. 89 N., R. 12 W., lat 42°28'10" N., long 

92°12'40" W., Gilbertville, Iowa, 71/2-minute quadrangle. Sample provided by P. Garvin, Cornell College, 

Mt. Vernon, Iowa. Reference: Anderson and Garvin 

(1984)

20. Anamosa diggings. Gash-vein, cubic galena in Silurian host rocks at site of several old lead mines. Sees. 

18-20, 30, T. 84 N., R. 3 W., lat 42°04'30 N., long 

91°14'00" W., Morley, Iowa, 71/2-minute quadrangle. 

Sample collected by A.V. Heyl. Reference: Heyl and 

West (1982).

21. Fessler mine no. 2. Galena in Upper Ordovician Wise 

Lake Formation. Area lies within the main Upper Mississippi Valley district and was mapped and studied by 

Brown and Whitlow (1960). When examined by A.V. 

Heyl in 1946, sphalerite and smithsonite were found 

on the mine dumps, but galena was the only ore mineral noted during later visit by T.M. Millen. Along 

Catfish Creek in Mines of Spain Nature Preserve, 1.3 

mi south of Dubuque, Iowa, SW^NEVi sec. 6, T. 88 

N., R. 3 E., lat 42°30'20" N., long 90°39'10" W, 

Dubuque South, Iowa, 71/2-minute quadrangle. Samples provided by J.C. Wright, Dubuque College. Reference: Ludvigson and others (1986).

22. Orion mine. Galena in brecciated, silicified dolomite 

in Lower Ordovician Prairie du Chien Group. 

NW^NW1/* sec. 28, T. 9 N., R. 1 E., lat 43°14'10// N., 

long 90°23'15// W, Muscoda, Wis., 71/2-minute quadrangle. Sample collected by A.V. Heyl and WS. West. 

Reference: Heyl and West (1982).

23. Doylestown lead mines. Cubic galena, quartz, and calcite from breccias and crevices in Lower Ordovician 

Prairie du Chien Group. T. Rudd farm, NE*4 sec. 2, T. 

11 N., R. 11 E., lat 43°27/20// N., long 89°09W/ W, 

Rio, Wis., 71/2-minute quadrangle. Sample collected 

by A.V. Heyl and WS. West. Reference: Heyl and 

West (1982).

24. Speedway roadcut. Galena in Middle Ordovician Platteville Formation. Roadcut 6 mi west of center of 

Madison, Wis., on County Road M near Pine Bluff, 

Wis., sees. 19 and 20, T. 7 N., R. 8 E., lat 43°03'40" 

N., long 89°35W W, Middleton, Wis., 71/2-minute 

quadrangle. Sample collected by R.A. Jenkins. Reference: Jenkins (1968).

25. Blue Mounds lead mines. Cubic galena in upper strata 

of Ordovician Galena Group. 0.5 mi south of Blue 

Mounds, Wis. Sample collected by A.V. Heyl and 

W.S. West. Reference: Heyl and others (1959).

26. Exeter diggings. Cubic galena in Ordovician Galena 

Group. Major old lead mine near Exeter, Wis. Sample 

collected by A.V. Heyl and W.S. West. Reference: 

Heyl and others (1959).

27. Brodhead quarry. Galena in Ordovician Shakopee Formation. 2 mi north of Brodhead, Wis., on east side of 

City Road G near conjunction of Rock, Green, and 

Dane Counties, Wis., NW^NW^SW^ sec. 30, T. 30 

N., R. 10 E., lat 42°41'40" N., long 89°22'05" W, 

Orfordville, Wis., 71/2-minute quadrangle. New galena 

locality; sample collected by T.M. Millen.

28. Avon lead-zinc occurrence. Quarry and drillhole 

occurrence of small veinlets of galena, sphalerite, and 

marcasite in Middle Ordovician Decorah and Platteville Formations. 1.5 mi east of Avon, Wis., SWViSE1/^ 

sec. 15, T. 1 N., R. 10 E., lat 42°32'30" N., long 

89°17'30" W, Brodhead East, Wis., 71/2-minute quadrangle. Sample collected by R.A. Jenkins. Reference: 

Jenkins (1968).

29. Vinegar Hill mine. Cubic galena taken from once 

major mine in Upper Ordovician Dunleith Formation. 

6 mi north of Galena, III, SW^SE^NWU sec. 21, T. 

28 N., R. 1 E., lat 42°28'00" N., long 89°25'50" W, 

Galena, 111., 71/2-minute quadrangle. Sample collected 

by A.V. Heyl and W.S. West. Reference: Heyl and others (1959).

30. Ten Strike mine. Dolomite-hosted, gash-vein occurrence of cube-octahedral galena, sphalerite, calcite, 

and oxidized iron sulfide in Upper Ordovician Dunleith and lower strata of Wise Lake Formations. 1.3 mi 

west of Galena, 111., sec. 23, T. 28 N., R. 1 E., lat 

42°24/40// N., long 89°28W W, Galena, 111., 

71/2-minute quadrangle. Samples collected by T.M. 

Millen from mine dump. Reference: Heyl and others 

(1959).

31. Morseville quarry. Galena and oxidized iron sulfides 

collected from vugs in Upper Ordovician Wise Lake 

Formation. Major old lead mines are just east of 

Morseville. 0.5 mi south of Morseville (Plum River), 

111., SEV4SEV4SEV* sec. 25, T. 27 N., R. 4 E., lat 

42°17/50'/ N., long 89°13'45" W, Galena, 111., 

71/2-minute quadrangle. Sample collected by T.M. 

Millen.

32. Mt. Carroll mines. Gash-vein and vug occurrences of 

cubic and cube-octahedral galena, sphalerite, dolomite, marcasite, and cubic to cube-octahedral pyrite. 

Primary host rocks are Upper Ordovician Wise Lake 

Formation and Silurian Gower Formation Vug-fill barite noted in Middle Ordovician Dubuque Formation. 

Major lead mines, quarries, and diggings west and 

northwest of Mt. Carroll, 111., sees. 2, 3, and 10, T. 24 

N., R. 4 E., lat 42°06/00" N., long*90°01'00" W, 

Wacker, 111., 71/2-minute quadrangle. Reference: Heyl
LEAD ISOTOPES, UPPER MISSISSIPPI VALLEY DISTRICT A REGIONAL PERSPECTIVE B13

and West (1982); Ludvigson (1988); Ludvigson and 

Millen(1988).

33. Stebbins (Yellow Creek) mine. Gash-vein galena in 

middle strata of Upper Ordovician Dunleith 

Formation. Galena also noted as vug-fill in nearby 

quarries. 0.25 mi west of Springfield Road on old U.S. 

Route 20, 2.5 miles east of Freeport, 111., 

NWViSWViSWV* sec. 2, T. 26 N., R. 8 E., lat 

42°16'40" N., long 89°33/ 10// W., Freeport East, 111., 

71/2-minute quadrangle. Sample collected by A. V. Heyl 

and W.S.West. Reference: Heyl and West (1982).

34. Brookville quarry. Minor fracture-fill galena in Middle Ordovician Platteville Formation. Center sec. 21, 

T. 24 N., R. 7 E., lat 42°03/50// N., long 89°41'50" W, 

Brookville, 111., 71/2-minute quadrangle. Stratigraphy 

of quarry described by Willman and Kolata (1978). 

New galena locality; sample collected by T.M. Millen.

35. Oregon East quarry. Disseminated and vug-fill galena, 

sphalerite, and dolomite in lower strata of Middle 

Ordovician Platteville Formation. Center sec. 2, T. 23 

N., R. 10 E., lat 42°00'50" N., long 89°19'45" W, 

Oregon, 111., 7!/2-minute quadrangle. New galena 

locality.

36. Dixon Southwest quarry. Fracture-fill galena and 

vug-fill sphalerite in Upper Ordovician Dunleith Formation. 1.3 mi southwest of Dixon, 111., on Hoyle 

Road, NWANEV4 sec. 7, T. 21 N., R. 9 E., lat

41°49'40" N., long 89°30W W, Dixon West, 111., 

71/2-minute quadrangle. New galena locality; sample 

collected by T.M. Millen.

37. Butler quarry. Fracture- and vug-fill galena, sphalerite, 

and iron sulfides in Middle Ordovician Mifflin Formation. 1.5 mi southeast of Lee Center, 111., center W!/2 

sec. 9, T. 20 N., R. 11 E., lat 41°44'20" N., long 

89°14'40" W, Sublette, III, 71/2-minute quadrangle. 

New galena locality; sample collected by T.M. Millen.

38. Troy Grove quarry. Vug- and fracture-fill calcite, 

galena, sphalerite, and iron sulfides in Middle Ordovician Mifflin and Pecatonica Formations. East edge of 

Troy Grove, 111., NE^NE1^ sec. 35, T. 35 N., R. 1 E., 

lat 41°28/00// N., long 89°04'20// W, Troy Grove, 111., 

71/2-minute quadrangle. Sphalerite and calcite noted by 

Buschbach (1970), but new galena locality; sample 

collected by T.M. Millen.

39. Elmhurst quarry. Octahedral galena, sphalerite, calcite, marcasite, and acicular barite in collapse breccias, vertical veinlets, and as disseminations in Upper 

Ordovician Wise Lake Formation. Underground limestone quarry 0.5 mi south of junction of Illinois Highways 83 and 64, SWViNWVi sec. 2, T. 39 N., R. 11 E., 

lat 41°54/00// N., long 87°57'30// W, Elmhurst, 111., 

71/2-minute quadrangle. New galena locality; sample 

collected by T.M. Millen.

6 U.S. GOVERNMENT PRINTING OFFICE: 1995-673-046/86090