Evidence for Hot Mississippi Valley–Type 

Brines in the Reelfoot Rift Complex,

South-Central United States, in

Late Pennsylvanian–Early Permian

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Professional Paper 1577
M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE 

Evidence for Hot Mississippi Valley–Type Brines 

the Reelfoot Rift Complex, South-Central United

States, in Late Pennsylvanian–Early Permian

By D.L. Leach, L.E. Apodaca, J.E. Repetski, J.W. Powell, and E.L. Rowan

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1577

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
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

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Mark Schaefer, Acting Director

Evidence for hot Mississippi Valley–type brines in the Reelfoot rift complex, south-central 

United States, in Late Pennsylvanian–Early Permian / by D.L. Leach . . . [et al.].

p. cm. (U.S. Geological Survey professional paper ; 1577)

Includes bibliographical references (p. ).

Supt of Docs. No.: I 19.16:1755

1. Hydrothermal alteration—Missouri—New Madrid Region. 2. Geology,

Stratigraphic—Pennsylvanian. 3. Geology, Stratigraphic—Permian. 4. Geology—

Missouri—New Madrid Region.

I. Leach, D.L. II. Series.

QE390.5.E85 

551.230976—dc21 97-12553

cip
CONTENTS

Abstract ........................................................................................................................... 1

Introduction..................................................................................................................... 2

Acknowledgments........................................................................................................... 2

Geologic Setting.............................................................................................................. 2

Southeast Missouri Lead District............................................................................ 4

Sparry Dolomite in the Ozark Region..................................................................... 5

Age of Sparry Dolomite Cement ............................................................................ 5

Temperature, Salinity, and Composition of the MVT Fluids in the Ozark Region 7

Thermal Regime of the Reelfoot Rift ..................................................................... 7

Methods and Results ....................................................................................................... 7

Samples of Sparry Dolomite ................................................................................... 7

Cathodoluminescent Microstratigraphy.................................................................. 8

Fluid Inclusion Studies and the Limitation of Fluid Inclusion Data for Dolomite . 9

Necking of the Fluid Inclusions ...................................................................... 9

Corrections to Homogenization Temperatures for Pressure ........................... 10

Changes in the Density of Inclusion Fluids Due to Reequilibration

During Post-entrapment Burial or Heating Events ..................................... 10

Leakage Into or Out of the Fluid Inclusions During Post-entrapment

Thermal Decrepitation ................................................................................ 11

Estimates of Paleotemperatures from Thermal Alteration of Conodonts

in the ARK-DW Drill Hole................................................................................. 11

Reelfoot Rift Drill Holes......................................................................................... 12

Samples from the Ozark Dome............................................................................... 15

Nashville Dome....................................................................................................... 16

Final Ice-melting Temperatures .............................................................................. 16

Salinity-Temperature Relationships........................................................................ 16

Discussion ....................................................................................................................... 18

Implications for Migration of MVT Brines ............................................................ 18

Thermal Regime in the Reelfoot Rift...................................................................... 19

Source of the Brines................................................................................................ 19

Variation in Temperature and Salinity.................................................................... 19

Changes in Temperature and Salinity Related to Fluid Mixing...................... 20

Post-trapping Thermal Stretching or Reequilibration..................................... 20

Conclusions..................................................................................................................... 21

References Cited ............................................................................................................. 22

Appendix of Supplemental Data ..................................................................................... 27

FIGURES

1. Location map showing drill holes, mining districts, major tectonic features, sedimentary basins, and

likely ore fluid flow-paths in the Ozark region ...........................................................................................................

2. Photomicrographs of cathodoluminescent zones of dolomite from drill cores in Missouri, Arkansas,

and Tennessee .............................................................................................................................................................

3. Graph of homogenization temperature versus depth for all drill hole fluid inclusion data ........................................

4. Idealized stratigraphic section for NMTW, Missouri, and ARK-DW, Arkansas, showing drill hole

sample locations for fluid inclusions and conodonts .................................................................................................
IV CONTENTS

5. Histogram plot of homogenization temperature for ARK-DW drill hole ...........................................................

6. Histogram plots of homogenization temperature for all drill hole fluid inclusion data ......................................

7. Graph of final ice-melting temperature versus depth for all drill hole fluid inclusion data ................................

8. Histogram plots of final ice-melting temperature for all drill hole fluid inclusion data......................................

9. Graph of final ice-melting temperature plotted against homogenization temperature for fluid inclusion

data of zones 3 and 4 in dolomite........................................................................................................................
Evidence for Hot Mississippi Valley–Type Brines in the 

Reelfoot Rift Complex, South-Central United States, in

Late Pennsylvanian–Early Permian

By D.L. Leach, L.E. Apodaca, J.E. Repetski, J.W. Powell, and E.L. Rowan

ABSTRACT

Petrographic and fluid inclusion studies of sparry dolomite cement from Upper Cambrian to Lower Ordovician

rocks and conodont thermal-alteration indices provide evidence that hot Mississippi Valley–type brines were once

present in the Reelfoot rift complex. The cathodoluminescent microstratigraphy of sparry dolomite cement in the

Reelfoot rift resembles that of sparry dolomite cement associated with widespread Mississippi Valley–type mineral deposition in the Ozark region. If correlative

cathodoluminescent zones in the sparry dolomite from the

Ozark and Reelfoot rift regions indicate broadly contemporaneous dolomite deposition, then the results show that the

Ozark Mississippi Valley–type hydrothermal system extended into the Reelfoot region and onto the western flank of the

Nashville dome. Independent evidence supports the migration of Mississippi Valley–type brines into the Ozark region

from the Reelfoot rift complex in late Paleozoic time.

Values for conodont thermal-alteration indices found in

the Dow Chemical #1 Wilson drill hole in the Reelfoot rift

complex provide evidence for the amount of post-Early

Ordovician strata that eroded before the Cretaceous. For a

burial thermal gradient of 30°C/km, about 2.1–3.7 kilometers

of strata have been removed at the location of the Dow

Chemical #1 Wilson drill hole. The total thickness of

post-Lamotte Sandstone strata present before pre-Cretaceous

erosion could have been between 5.3 and 6.3 kilometers. The

addition of as much as 1.5 kilometers of Lamotte Sandstone

could raise that amount to 7.8 kilometers for pre-Cretaceous

strata.

Fluid inclusion homogenization temperatures provide

estimates of the minimum temperatures for sparry dolomite

precipitation. Pressure corrections for these data in the Ozark

and Nashville domes are probably no more than 15°–20°C,

whereas in the Reelfoot rift, corrections to the homogenization temperatures may be as high as 45°–60°C. Uncertainties

regarding actual fluid pressures limit interpretation of the

homogenization data. Nevertheless, the data provide i

tant constraints on the thermal regime in the Reelfo

complex in late Paleozoic time. 

Fluid inclusion homogenization temperatures in 

dolomite from the Reelfoot rift complex are at least 60

more than 100°C hotter than in cathodoluminescent co

tive sparry dolomite from the Ozark and Nashville d

Conodont thermal-alteration indices in Lower Ordo

rocks in the Reelfoot rift are consistent with a min

paleothermal burial gradient of about 30°C/km. Fluid 

sion homogenization temperatures from sparry dolom

the Reelfoot rift are about 40°C higher than the min

burial temperatures estimated from conodont thermal 

ation. Pressure corrections to the homogenization tem

tures could increase this difference by an add

30°–50°C. This difference may reflect the fact that con

thermal-alteration values are estimates of the minimum 

temperature achieved by the rocks. Fluid inclu

however, may record the passage of transient but 

temperature fluids or a short-lived thermal pulse 

igneous activity. 

One of the most significant findings in this study 

high fluid inclusion temperatures recorded in sparry 

mite from the Reelfoot rift. Homogenization values 

deepest part of the Dow Chemical #1 Wilson dril

(present depth of 3.66 km) are in the range of 150°–2

most of the data are in the range 220°–260°C. Appl

60°C pressure correction, the actual temperatures ach

in the deepest part of Dow Chemical #1 Wilson dril

could be in the range of 210°–340°C. This range wo

equivalent to burial geothermal gradients of approxim

35°–60°C/km. The high fluid inclusion temperatures 

either the temperatures of advecting hydrothermal bri

thermally reequilibrated fluid inclusions in response to 

er basement-derived, conductive geothermal gra

Widespread igneous activity in the Reelfoot rift comp

Late Pennsylvanian to Permian time may be responsib

the unusually high temperatures recorded by the 
2 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

inclusions. In either case, the fluid inclusion data point to a

thermal event in the central Reelfoot rift where temperatures

reached 300°C in the Bonneterre Formation.

In Dow Chemical #1 Wilson, high homogenization

temperatures and relatively low salinities of many fluid

inclusions from zone 3 dolomite from the Bonneterre Formation suggest the presence of a hotter and more dilute fluid

component in the Reelfoot rift. Fluid inclusion data from

other samples in the study area generally show significant but

independent variation between homogenization temperature

and salinity. These variations in temperature and salinity

may reflect transient variations in fluid flow or fluid mixing

within the Ouachita basin complex. However, the salinity-temperature relationship for samples from the New

Madrid test well shows significantly greater variation in

temperature relative to salinity, suggestive of post-trapping

stretching or thermal reequilibration of the fluid inclusions. 

INTRODUCTION

The Ozark region of the United States midcontinent is

host for the largest Mississippi Valley–type (MVT) lead-zinc

province in the world. This region, located north of the Ouachita foldbelt and covering more than 240,000 km2, includes

the world-class MVT districts of Old Lead Belt, Viburnum

Trend, and Tri-State, and the smaller Northern Arkansas,

Central Missouri, and Southeast Missouri barite districts (fig.

1). Leach and Rowan (1986) presented evidence that brine

migration from the Arkoma foredeep (fig. 1), in response to

Late Pennsylvanian–Early Permian orogenesis in the Ouachita foldbelt, was responsible for the formation of MVT deposits in the Ozark region. However, we are becoming

increasingly convinced that a major component of fluid flow

for the lead-rich deposits in the Viburnum Trend and Old

Lead Belt in southeast Missouri was from the Reelfoot rift

zone (Farr and Land, 1985; Farr, 1989a, 1989b; Erickson and

others, 1988; Diehl and others, 1991; Goldhaber and Mosier,

1989; Viets and Leach, 1988, 1990; Horrall and others,

1993). In addition, Shelton, Bauer, and Gregg (1992)

suggested the possibility of brine migration from the Illinois

basin. 

We report herein a reconnaissance fluid inclusion study

of epigenetic sparry dolomite from Upper Cambrian and

Lower Ordovician strata in two drill cores located in the

Reelfoot rift complex, drill cores along the southeastern

flank of the Ozark dome and the western flank of the Nashville dome, and samples from a rock quarry in northern

Arkansas. In addition, conodont thermal-alteration studies

were conducted where possible to provide additional thermal

constraints. The fluid inclusion data, combined with

conodont thermal-alteration studies, indicate th

highly saline brines were present in the Reelfoot ri

during the Paleozoic. Cathodoluminescent (CL) 

sparry dolomite from the Reelfoot rift is similar to 

sparry dolomite in the Ozark region that is closely 

with MVT sulfide deposition. If the sparry dolom

in this report is temporally related to sparry dolo

Ozark region, then these data provide insights into 

ble ore-fluid migration with respect to the tectonic 

the rift complex. Because many of our interpre

based on previous studies of fluid inclusions a

dolomite in the Ozark region, we summarize 

important aspects of these earlier reports. In ad

present a brief overview of the complex ge

tectonics of the Reelfoot rift to provide insight in

tectonic implications of our data. 

ACKNOWLEDGMENTS

We appreciate the many helpful discus

suggestions from John Viets, Jim Palmer, Anita H

Albert Hofstra. Many of the samples were obtaine

Missouri Geological Survey with the help of Ji

David Weary’s help was invaluable in proc

ARK-DW (Dow Chemical #1 Wilson) and 

samples. The manuscript was significantly impr

constructive reviews by Martin Goldhaber, Dav

knecht, and J.A. Allan.

GEOLOGIC SETTING

Development of the Arkoma and Black Wa

deeps and the Ouachita foldbelt (fig. 1) is generall

to be the result of closure of a Paleozoic ocean bas

dipping subduction of the North American plate 

magmatic arc or continental plate formed an ac

wedge and culminated in plate suturing (Viele, 19

and others, 1982). This accretionary wedge and 

Ouachita foldbelt overlie the thick sedimentary 

Late Proterozoic–early Paleozoic Reelfoot rift b

eastward extension of the Arkoma foredeep forms 

with the Black Warrior foredeep. This juncture, 

poorly understood, is buried beneath Cretaceous an

rocks and sediments of the Mississippi E

(Thomas, 1976). During late Paleozoic time, th

rift basin was part of a more extensive basin that in

southern part of the Illinois basin and the Arkoma 

Warrior foredeeps. This basin complex was boun

south by the frontal Ouachita fold belt and to the so
LA TX

OK

MO

AR

AL

Ouachita Mountains

REELFOOT RIFT Ozark Dome

6

4

3

5 1

2

P-1

GP-10

PBW-3 PB-7

IMBODEN

ARK-DW

DP-4

NMTW DP

MS

PASCOLA ARCH

Area of

fig. 1

96° 94° 92° 90° 88°

37°

34° Mississippi Embayment

IL

Illinois

Basin

Black

Warrior

Basin

Arkoma Basin

N

0 50 100 KILOMETERS

Figure 1. Location map showing drill holes, mining districts, major tectonic features, sedimentary basins, and likely ore fluid flow-path

districts: 1, Viburnum Trend subdistrict of southeast Missouri; 2, Old Lead Belt subdistrict of southeast Missouri; 3, Southeast Missouri 5, Tri-State; 6, Central Missouri. Sawteeth represent fold belts. NMTW, New Madrid test well; ARK-DW, Dow Chemical #1 Wilson hole.
4 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

the southern Appalachian fold belt. Mississippi Valley–type

fluids may have migrated from the eastward portion of the

Arkoma basin or from the tectonically related Black Warrior

foredeep northward through the Reelfoot rift complex into

the Southeast Missouri lead districts.

The Reelfoot rift (fig. 1), buried beneath approximately

1 km of Cretaceous to Holocene rocks and sediments of the

Mississippi Embayment, is generally thought to be a late

Precambrian to Early Cambrian failed continental rift that

formed during supercontinent rifting that led to the formation

of the Paleozoic Iapetus ocean (Ervin and McGinnis, 1975;

Kane and others, 1981; Kolata and Nelson, 1991; Nelson and

Zhang, 1991). Nelson and Zhang (1991) showed that the total sedimentary fill of the Mississippi Embayment is variable

in thickness and ranges from about 4 to 8 km in depth. Subsidence of the rift along steeply dipping bounding faults resulted in the accumulation of as much as 1.5 km of probable

syn-rift sediments of the Lamotte Sandstone (Late Cambrian

age) (Nelson and Zhang, 1991). Active rifting ceased, probably by Middle to Late Cambrian, and the Reelfoot region

evolved into a sag basin by Late Cambrian. Continued subsidence of the Reelfoot basin continued into the late Paleozoic, with interspersed episodes of mild deformation and uplift

along the margins of the North American craton during Paleozoic orogenies. Post-rifting subsidence extended beyond

the rift bounding faults and formed a broad sedimentary sag

basin that overlies the Reelfoot graben. Post-rift Cambrian

and Ordovician sedimentary rocks, described by Nelson and

Zhang (1991), include the stratigraphic units they assigned to

the Bonneterre Formation, Elvins Group, and Knox Group.

These sediments and rocks are thickest (2.7–3 km) within the

rift and thin to about 2.1 km on the southeast shoulder of the

rift (Nelson and Zhang, 1991). Little is known about the

original thickness of post-Knox sediments because of extensive erosion during Late Pennsylvanian–Permian uplift of

the northern Reelfoot rift. Schwalb (1982) estimated that at

least 2.8 km of post-Knox through Pennsylvanian sediments

accumulated in the northern Reelfoot zone. Thus, by Late

Pennsylvanian, about 7 km of Paleozoic rocks filled the central rift basin. As a consequence of crustal downwarping during the Ouachita orogeny, Pennsylvanian sediments are

likely to thicken southward to the juncture with the Ouachita

fold belt. Therefore, in the southern part of the Reelfoot rift

complex, the total thickness of Paleozoic rocks could exceed

7 km.

Late Paleozoic to Mesozoic uplift resulted in the formation and subsequent erosion of the Pascola arch (Kolata and

Nelson, 1991), which trends northwest-southeast and forms

the southern end of the Illinois basin (fig. 1). The late Paleozoic to Mesozoic uplift in the Reelfoot rift is reflected by the

fact that the pre-Mt. Simon/Lamotte Sandstone underlies the

Cretaceous unconformity at the crest of the Pascola arch

(Schwalb, 1982). Stearns and Marcher (1962) suggested that

between 2.5 and 4.5 km of Paleozoic rocks were eroded

during uplift of the Pascola arch. Another large antiformal

structure, called the Blytheville arch, parallels th

the Reelfoot rift and may connect with the Pa

(McKeown and others, 1990). The nature of these 

highs is of considerable interest because most earth

the New Madrid seismic zone are associated with 

of these structures (Hildenbrand and others, 1977)

of the arches may have influenced migration of MV

ids in late Paleozoic time. Howe and Thompso

Hildenbrand (1985), Stearns and Marcher (1

Hamilton and McKeown (1988) suggested that 

are tectonic features related to basement faulti

(1985) and Nelson and Zhang (1991) directly r

arches to compressional forces during a late Paleo

eny. Other proposed mechanisms include uplift a

quence of igneous activity (Crone and others, 

diapiric intrusion (McKeown and others, 1990). 

Formation of the modern Mississippi E

began with subsidence of the Gulf Coastal Pla

breakup of Pangea in the Late Cretaceous (Cap

Stearns and Marcher, 1962). West of Memph

Cretaceous sediment and rocks reach a thickness o

Post-rifting igneous activity in the Reelfoot ri

in Late Pennsylvanian to Permian alkalic dikes a

indicated in drill core in the vicinity of the Pascol

in Hicks dome in the southern Illinois fluorsp

(Zartman, 1977). Hildenbrand (1985) used geoph

to identify possible post-early Paleozoic intrusion

margins of the Reelfoot rift.

SOUTHEAST MISSOURI LEAD DIST

The Southeast Missouri lead district include

jor subdistricts of the Viburnum Trend and Old 

(fig. 1). The ore deposits are restricted to Upper 

rocks, which unconformably overlie the Precamb

talline rocks of the St. Francois Mountains at th

the Ozark dome. About 95 percent of the ore 

produced from dolostones in the Bonneterre 

(Upper Cambrian) and the remainder from 

Lamotte Sandstone (Upper Cambrian). The Lam

stone is variable in thickness, ranging from 0 

whereas the Bonneterre Formation averages ab

thick. The overlying Cambrian-Ordovician f

have an aggregate thickness of about 325 m (G

and Meyers, 1972). Cambrian and Ordovician ro

Southeast Missouri lead districts (about 500 m 

thickness) dramatically thicken southeastward 

Reelfoot rift zone, where these rocks may have a 

thickness of as much as 4.5 km. 

Detailed descriptions of the mines of the 

Trend are given in a series of papers in Economi

(v. 72, no. 3, 1977). Other important papers inclu

and Gerdemann (1968), Gerdemann and Meyers (

Sverjensky (1981).
GEOLOGIC SETTING 

SPARRY DOLOMITE IN THE OZARK REGION

Sulfide ore minerals in the Ozark MVT districts are

associated with coarse, saddle-shaped, sparry dolomite that

paragenetically spans deposition of ore minerals. This

sparry dolomite is present as fractures and vug linings within

secondary or secondarily enhanced porosity and as replacement of limestones. Distant from the ore districts, sparry

dolomite may occur without associated sulfides but, as

studied by Erickson and others (1981, 1988), commonly is

closely associated with trace sulfides in ore-barren rocks.

Palmer and Hayes (1989) used iron staining and a distinctive

dolomite color sequence in the sparry dolomite to correlate

specific paragenetic stages of sulfides in each ore district.

They demonstrated that many stages of sulfides in each

district and the widespread occurrences of sparry dolomite

and trace sulfides are correlatable throughout the Ozark

region, providing strong evidence for a large interconnected

hydrothermal system for the Ozark region.

The iron staining and dolomite color sequence in sparry

dolomite observed by Palmer and Hayes (1989) correlates

with previously established cathodoluminescent (CL)

microstratigraphy in sparry dolomite for the Ozarks (Voss

and Hagni, 1985; Frank and Lohmann, 1986). Voss and

Hagni (1985) established a CL microstratigraphy in sparry

dolomite from the Viburnum Trend over a distance of

approximately 30 km. Gregg (1985) identified the

microstratigraphy in dolomite cements in the basal

Bonneterre Formation as far as 80 km west of the Viburnum

Trend district. Rowan (1986) observed correlative CL

zonation a distance of 350 km to the southwest near the

Tri-State and Northern Arkansas districts. Farr and Land

(1985), Farr (1989b), and Shelton, Bauer, and Gregg (1992)

expanded the microstratigraphy into southeastern Missouri

and northern Arkansas, near the limit of the Mississippi

Embayment. 

The sparry dolomite microstratigraphy consists of four

zones or bands (fig. 2), each consisting of subbands (Voss

and Hagni, 1985; Voss and others, 1989). Zone 1 is typically

moderately luminescent and is characteristically mottled.

Zone 2, commonly not well developed and often difficult to

identify, consists of very narrow, micron (micrometer)-width, regular bands; Frank and Lohman (1986) did not

recognize zone 2 as a distinct band. Zone 3 consists of

numerous alternating bright and dark bands, and zone 4 is

characteristically dull to nonluminescent.

The oldest zone (1) and the youngest zone (4) bracket

unambiguously main ore-stage sulfides (octahedral-galena

stage) in the Viburnum Trend (Voss and others, 1989; Rowan and Leach, 1989). Zones 1 and 2 predate main-stage ore

deposition. Zone 3 dolomite formed intermittently with

episodes of main-stage sulfide deposition in the Viburnum

Trend (Rowan and Leach, 1989, p. 1953). However, Voss

and others (1989) considered zone 3 to be entirely post-main

stage galena (octahedral galena) but predate late-stage

galena (cubic galena). They considered zone 4 dolom

have formed after late-stage galena, whereas Rowan 

Leach (1989) observed zone 4 dolomite to both predat

postdate deposition of cubic galena. In other Ozark 

districts, we have observed that zone 3 dolomite, w

present, was deposited prior to main-stage sphalerit

galena, whereas zone 4 dolomite was deposited both 

mittently with, but mostly after deposition of main-stag

fides. It appears that zone 3 dolomite is temporally re

to deposition of main-stage ores in southeast Mis

whereas zone 4 dolomite corresponds to the time of de

tion of late-stage sulfides in southeast Missouri 

main-stage ores in the other Ozark MVT districts.

Where recognition of the CL zone that hosts a 

inclusion in sparry dolomite is possible, the CL microst

raphy provides a relative time frame that facilitates com

ison of fluid inclusion data from different sam

However, correlation of a particular band can be diff

The thickness of the zone can be highly variable becau

limited crystal growth and (or) orientation of sec

relative to the direction of crystal growth; zones may al

completely absent.

AGE OF SPARRY DOLOMITE CEMENT

In view of the paragenetic association of regional 

ry dolomite with the MVT deposits in the Ozark regio

age of MVT ore formation should also broadly refle

time of deposition of ore-stage sparry dolomite in the 

area. Recent attempts to date MVT mineralization i

Ozark region have been mainly by paleomagnetic me

using apparent polar wandering paths (Wisniowieck

others, 1983; Pan and others, 1990; Symons and San

1991, 1994). The paleomagnetic results yield a regio

consistent age of Late Pennsylvanian to Permian. Cons

with the paleomagnetic results, isotopic studies in the 

midcontinent region indicate an episode of regional h

thermal fluid migration and formation of MVT depos

late Paleozoic time (Hay and others, 1988; Desboroug

others, 1985; Bass and Ferrara, 1969; Shelton and o

1986; Aronson, reported in Rothbard, 1983; Brannon 

others, 1992; Chesley and others, 1994; Brannon and o

1995). In view of the late Paleozoic to Early Permian a

MVT ore deposition, we suggest that the ore-stage s

dolomite was deposited at this time. In contrast to the 

Pennsylvanian to Permian ages, other isotopic studies 

midcontinent indicate additional widespread fluid m

tions and precipitation of authigenetic minerals in Lat

vonian and Early Devonian time, which are po

associated with earlier North American orogenies. (Se

example, Hay and others, 1988; Posey and others, 

Stein and Kish, 1985, 1991; Duffin and others, 1989.)
6 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

A B

D E

C

1

2

3

4

2

3 4

1

3

3

4

4

3

4

1

3

1 3

4

3

4
METHODS AND RESULTS 

TEMPERATURE, SALINITY, AND

COMPOSITION OF THE MVT FLUIDS

IN THE OZARK REGION

Studies of MVT ore minerals and associated sparry dolomite in the Ozark region show that the mineralizing fluid

was a brine at temperatures predominantly between 80° and

140°C (see references in Leach and Rowan, 1986; Rowan

and Leach, 1989; and Shelton, Bauer, and Gregg, 1992).

Final ice-melting temperatures of fluid inclusions in samples

throughout the Ozark region (Leach and others, 1975; Leach,

1979; Leach and Rowan, 1986; Rowan and Leach, 1989)

range mainly from –10° to –27°C, with a well-defined mode

at –21°C, corresponding to 23 equivalent weight percent

NaCl. These studies show a range of final ice-melting temperatures with a small secondary mode at around –10° to

–15°C, corresponding to salinities of 14–18 equivalent

weight percent NaCl. Rowan and Leach (1989) proposed

that this secondary mode of final ice-melting temperatures is

evidence for another, more dilute brine present in the region

during mineralization, consistent with Bauer and Shelton

(1989) and Shelton, Bauer, and Gregg (1992). Recent

studies by Bauer and Shelton (1989) and Shelton, Bauer, and

Gregg (1992) on sparry dolomite from the Ozark region

indicated a larger range in homogenization and final

ice-melting temperatures than had been previously reported. 

THERMAL REGIME OF THE REELFOOT RIFT

In a recent study by Shelton, Haeussler, and Burstein

(1992), fluid inclusion homogenization temperatures of

180°–250°C were measured in sparry dolomite from the

Bonneterre Formation at 2–3 km depth within the Ree

rift. Tobin (1991) presented fluid inclusion homogeniz

temperatures from sparry dolomite and calcite cement 

Bonneterre Formation from the Amoco No. 1 Spence T

well in the Reelfoot rift. These samples were from pr

depths of about 2.3–2.4 km. Homogenization tempera

range from a mean of 235°C for primary inclusions in s

dolomite to a mean of 169°C for primary and secondary 

inclusions in calcite cement; these temperatures are co

tent with thermal alteration indices (TAI) of palynomo

Tobin (1991), assuming that these fluid inclusions 

trapped at shallow depths (and low temperature) and 

they had subsequently “reequilibrated” during buri

higher temperatures, used these data to derive a b

history of the Bonneterre Formation. Judging from To

petrographic descriptions, his samples appear to be si

to sparry dolomite studied in this report. 

METHODS AND RESULTS

SAMPLES OF SPARRY DOLOMITE

Samples of sparry dolomite were obtained from 

USGS New Madrid Test Well (NMTW) and the 

Chemical #1 Wilson (ARK-DW) drill core located i

Reelfoot rift (fig. 1). Other samples were obtained from 

core (PB-7, P-1, GP-10, and PBW-3) located on the s

eastern flank of the Ozark dome and from a drill hole (D

on the western flank of the Nashville dome. Samples 

also collected in northeastern Arkansas at the Black 

Quarry near Imboden, Ark. (JR-series). The sample se

sists of sparry dolomite cement from Upper Cambri

Lower Ordovician carbonate rocks (see appendix). 

samples of sparry dolomite in drill core, pairs of pol

thin sections were prepared from samples from sel

depths; of each pair, one was used for CL examinatio

one for fluid inclusion study. However, most of the sam

of sparry dolomite were obtained from insoluble residu

acetic acid digestions of the host rock obtained for stud

conodonts. Surprisingly, sparry dolomite crystals were 

moderately corroded by acid digestion of the host limes

and dolostones. Grain mounts of hand-picked s

dolomite were polished for CL and fluid inclusion stu

For each sample, a description of the CL zone(s) was 

mented. In separate polished plates, fluid inclusions 

studied by microthermometry and their locations with

crystals carefully documented with photomicrogr

After completion of the fluid inclusion microthermom

the CL zone that hosted the inclusions was determ

Identification of the CL zone hosting the fluid inclu

was made after fluid inclusion measurements, becaus

electron beam used in CL analysis produces more 

sufficient heat to stretch or decrepitate the inclusions. 

Figure 2 (previous page). Cathodoluminescent zones of dolomite from drill cores in Missouri, Arkansas, and Tennessee. Zone

1 is the earliest generation of dolomite, showing a moderate luminescence, a mottled texture, an absence of banding, and a gradational contact with the host rock. Zone 2 is not well defined, consisting

of narrow micron (micrometer)-width bands with a dull luminescence. Zone 3 contains numerous alternating bright and dark subbands. Zone 4, the outermost zone, is dull to nonluminescent. This

zoning is consistent with the dolomite cathodoluminescence microstratigraphy defined for the Viburnum Trend (Voss and Hagni,

1985; Voss and others, 1989; Rowan, 1986). A, Buick Mine, Missouri; typical of the Viburnum Trend cathodoluminescent stratigraphy; zone 2 is not well developed. Scale bar = 0.2 mm. B, GP-10,

southeast Missouri; stratigraphy similar to that seen in the Viburnum Trend. Scale bar = 0.4 mm. C, ARK-DW-12,000, Arkansas;

grain mount. Zone 3 observed most often, consisting of alternating

dull and bright bands. Scale bar = 0.08 mm. D, NMTW-2,055,

Missouri; grain mount, predominantly zone 3 banding with some

zone 1 (mottled). Scale bar = 0.12 mm. E, DP-4, Tennessee; shows

zonation similar to that seen in the Viburnum Trend. Symmetry of

banding is not as well developed. Scale bar = 0.4 mm.
8 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

CATHODOLUMINESCENT

MICROSTRATIGRAPHY

Recognition of a CL zone that hosts a fluid inclusion in

sparry dolomite aids in determining a relative time frame

that facilitates comparison of fluid inclusion data from

different samples. For the Ozark region, where the CL

microstratigraphy is well established, this comparison is

relatively straightforward but requires the assumption that

the CL zones are recording a fluid flow event that was

broadly contemporaneous. However, extrapolating the

Ozark CL microstratigraphy into areas adjacent to the Ozark

region is admittedly less certain. 

The samples of sparry dolomite used in this study

exhibited CL zones similar to those observed in the Ozark

region. The microstratigraphy was generally simpler, with

fewer subbands, and many of the CL zones were thinner than

those observed in the Viburnum Trend. For many samples,

only two or three zones could be identified with confidence.

As in the Ozark region (Rowan, 1986; Rowan and Leach,

1989), zone 2 dolomite was generally difficult to identify

with confidence. The dull to moderately luminescent zone 1

dolomite was commonly the thickest CL zone in samples

from the deepest part of the Reelfoot rift. In addition, the

highly banded CL zone identified as zone 3 in our samples

was generally thinner and contained fewer subbands relative

to typical zone 3 dolomite in the Viburnum Trend. The dull

to nonluminescent zone 4 dolomite likewise was thinner in

our samples than in samples typically collected in the

Ozarks. 

Although there are minor differences in the CL zones

(for example, thinner zones 3 and 4) in many of our samples

compared to samples from the Viburnum Trend, we suggest

that CL zones in our samples of sparry dolomite can be correlated with the CL microstratigraphy established for the

Ozark region. In samples from the southern Ozark dome,

this correlation is consistent with previous studies. Using

this correlation as a working hypothesis, we have placed

constraints on the relative time of fluid entrapment for the

fluid inclusion data presented in this study. In the absence of

independent age constraints on the sparry dolomite, this requires two assumptions: (1) that we have correctly identified

the CL zones, and (2) that a primary fluid inclusion hosted in

a particular CL zone in sparry dolomite from the Reelfoot

rift, for example, trapped a sample of fluid from the same

hydrological system that deposited the comparable zone in

sparry dolomite from the Ozark region. In view of the

evidence for the migration of MVT ore fluids out of the

Reelfoot rift into the Ozark region, we find the latter assumption to be reasonable.

Sparry dolomite from drill holes PB-7, P-1, GP-10, and

PBW-3 located on the southern and southeastern flank of the

Ozark dome (fig. 1) are from the Upper Cambrian

Bonneterre and Derby–Doe Run Formations (see appendix).

All four CL zones were observed in samples from these drill

holes (fig. 2B); however, zone 2 was commonly 

difficult to identify with confidence. Samples 

Black Rock Quarry at Imboden, Ark., contain p

dolomite characteristic of the Northern Arka

Tri-State districts. The CL zones were predomin

4, with occasional crystals showing zone 3. 

Samples studied from drill hole DP-4, loca

western flank of the Nashville dome (fig. 1), are 

Conasauga Group (Middle to Upper Cambrian). D

in these samples are zones 1, 3, and 4 (fig. 2E)

zones in sparry dolomite from DP-4 are distinct fr

microstratigraphy described by Kopp and others 

sparry dolomite from Lower Ordovician rocks in t

and East Tennessee districts. The CL zones in sam

DP-4 are also different from those observed in sp

mite in Lower Ordovician rocks in north-central T

described by Gorody (1980). We also examined 

in two drill cores from sites located approximat

east of DP-4. In these samples, we did not observe 

that we could correlate with the Ozark CL micr

phy. Rather, the CL bands are most similar to 

scribed in the Central Tennessee district (Kopp a

1986). We conclude that the Ozark hydrotherm

may have extended as far east as DP-4 on the 

dome (fig. 1).

Samples of sparry dolomite from the ARK

hole in the Reelfoot rift complex were obtained 

insoluble residues of the Upper Cambrian Bonne

mation (depths of approximately 3.58–3

(11,726–12,200 ft1)), the Upper Cambrian Elv

(depths of approximately 3,250–3,575 m (10,6

ft)), and the Upper Cambrian–Lower Ordovician 

Arbuckle Group. CL zones 1, 3, and 4 were read

fied (fig. 2C); zone 2 was either not present or d

identify with confidence. Although zone 3 dol

observed throughout most of the drill hole, the m

dant and best developed zone 3 dolomite was fo

Bonneterre Formation at depths between 3.62 an

(11,880 and 12,000 ft). Dolomites containing z

absent from the Bonneterre Formation (3.62–3.6

are best developed at shallower stratigraph

1.07–2.9 km (3,540–9,540 ft). These observa

gest that fluid migration at the ARK-DW site du

3 time was mainly in the lowermost Paleozo

(Bonneterre Formation and probably in the u

Lamotte Sandstone), whereas by zone 4 time, flu

tion was occurring at shallower levels and large

the Knox or Arbuckle Group aquifers. Alt

sparry dolomite was not precipitated in t

Bonneterre during migration of brines that depo

4 dolomite. It is possible that the brines were no

with respect to dolomite or that permeability was 

1Original depths were measured in feet; feet are thus gi

completeness, along with metric equivalents.
METHODS AND RESULTS 

in the lower part of the stratigraphic section during migration of brines in zone 4 time.

Samples of sparry dolomite from the NMTW (fig. 1)

are from the Upper Cambrian carbonates (Bonneterre Formation) located on the Pascola arch 25 km north of the

ARK-DW drill hole. These samples were obtained from

acid insoluble residues at drill hole depths of 626–701 m

(2,055–2,300 ft). Examination of the CL zones identified

only zones 1 and 3 with confidence; zone 2 is generally not

present or is difficult to identify (fig. 2D). Zone 4 dolomite

was not observed in the Bonneterre Formation in the

NMTW, which is consistent with the absence of zone 4

dolomite in the Bonneterre Formation in drill hole

ARK-DW. 

FLUID INCLUSION STUDIES AND THE 

LIMITATION OF FLUID INCLUSION DATA 

FOR DOLOMITE

Microthermometric studies were made on 303 primary

fluid inclusions in samples of sparry dolomite, mostly from

drill cores NMTW, ARK-DW, DP-4, and the JR series.

Measurements were made on a Fluid Inc. gas-flow fluid inclusion stage. Temperature measurements of homogenization (Th) and final ice-melting temperatures (Tm) have a

precision of ±1.0°C and ±0.2°C, respectively. Possible overheating of fluid inclusions and stretching of fluid inclusions

during homogenization studies were carefully avoided. The

data for each sample are given in the appendix. The number

of inclusion measurements is small because of the difficulty

in identifying primary inclusions in which the CL zone could

be identified with confidence, and that had not obviously

experienced changes in inclusion volume (for example,

necking). Measurements were made more difficult by the

small size of the inclusions, commonly less than 10 microns

(micrometers) in long dimension.

The sparry dolomite crystals typically display cloudy

cores produced by dense populations of fluid inclusions;

these cloudy cores typically are bounded by outer clear

growth zones that contain considerably fewer inclusions.

The primary nature of the inclusions was largely established

by their occurrence in three-dimensional arrays related to

crystal growth (Roedder, 1984, p. 43) or along distinct

growth bands.

Primary fluid inclusions provide estimates of the hydrothermal environment and composition of the fluid from

which the crystal precipitated. However, post-entrapment

changes in fluid inclusion density and composition can, in

some cases, seriously limit the determination of actual

trapping conditions and fluid composition. In addition,

homogenization temperatures provide estimates of the

minimum temperatures of fluid entrapment; pressure

corrections are necessary to obtain actual trapping

temperatures. 

The most serious sources of uncertainty in the flu

clusion studies presented include: (1) uncertainties re

ing the actual fluid pressures at time of entrapment o

fluid and therefore actual trapping temperatures; an

changes in fluid inclusion size and density related to flu

clusion necking, stretching, and decrepitation or le

into or out of the inclusion. 

NECKING OF THE FLUID INCLUSIONS

The necking of fluid inclusions was suggeste

Rowan and Leach (1989) and Leach and Rowan (1993) 

an inherent problem in sparry dolomites from the O

region. In this study, we observed commonly abu

evidence for the necking of fluid inclusions in sparry 

mite. Necking typically produces several smaller, 

equant-shaped inclusions from a larger, elongate o

inclusion of higher surface area, commonly progressi

the point where no visible evidence of the process rem

If a vapor phase nucleates in the fluid inclusion befo

during necking of the fluid inclusion, the or

liquid/vapor ratios (and fluid density) will be altered b

necking process, resulting in scatter in the Th data. Fre

point depression, however, is not measurably affected. 

Roedder, 1984, p. 59–66 for a complete discussion.)

Most of the samples of sparry dolomite have a c

core consisting of abundant fluid inclusions that are bel

to be primary because they occur in a three-dimens

array related to crystal growth (Roedder, 1984, p. 43). W

in these cloudy cores, we commonly observed inclu

whose shapes and distributions indicated various stag

necking. Some inclusions were single-phase, liquid o

and others had clearly variable gas/liquid ratios. How

we also observed groups of inclusions with cons

gas/liquid ratios that yielded Th ranges of several tens 

grees. Barely detectable differences in gas/liquid ratio

equate to significant differences in volume of the gas 

and in Th. In CL zones 3 and 4, inclusions are less abun

but necking appears to have been prevalent there as 

Although we carefully selected inclusions that were no

ibly necked, we rarely found groups of inclusions that y

ed Th ranges of ±10°C or less. From these observation

suggest that necking has affected many more inclusio

sparry dolomite than is visually evident, and that it acc

for significant variability in Th in sparry dolomite rep

here and in other studies from the Ozark region.

Whereas the effects of undetected necking cann

quantified, a reasonable approach to the problem is to 

less credibility to extreme values of Th and focus interp

tions on temperatures where most of the data occur. 

have arbitrarily chosen to make comparisons of the c

tendency of the Th populations (median or 50th perce

and well-defined modes in the Th distributions. Consid

the large range of the Th data (from about 90° to 26

these comparisons appear to be a reasonable but admi
10 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

imprecise approach to the problem of undetected necking in

the sparry dolomites.

CORRECTIONS TO HOMOGENIZATION 

TEMPERATURES FOR PRESSURE

If a fluid inclusion is trapped above a liquid-vapor

curve, a correction for pressure must be made to arrive at

actual trapping temperatures. In the Ozark dome, the maximum burial depth at the time of dolomite precipitation is estimated to have not exceeded 1.5–2 km (Leach and others,

1975; Leach, 1979). Assuming hydrostatic pressures and a

simple H2O-NaCl fluid, the resulting pressure correction

would not exceed approximately 15°–20°C. 

Estimation of the burial depth at the time of sparry

dolomite precipitation in the Reelfoot rift is much more

problematic. The deepest samples from ARK-DW are at a

drill hole depth of 3.6 km, which includes a kilometer of Cretaceous and younger coastal plain rocks and sediments above

the erosional surface in Paleozoic rocks. Based upon

conodont thermal alteration studies (discussed in more detail

later), as much as 2.1–3.7 km of additional Paleozoic rocks

could have been present at ARK-DW drill hole in the late

Paleozoic; therefore, the total thickness of Paleozoic rocks

overlying the deepest sample could have been as much as 6.3

km. At the location of the NMTW drill hole, situated on the

Pascola arch, Stearns and Marcher (1962) estimated that

between 2.4 and 4.6 km of Paleozoic rocks were removed by

erosion during uplift of the arch. Our deepest sample from

the NMTW is at 713 m and yields a maximum burial depth

of 5.3 km. For fluid inclusions trapped above a liquid-vapor

solvus, at estimated maximum burial depths of 5.3–6.3 km,

a hydrostatic pressure correction could be as large as

45°–60°C. Burial depths of 4 km during fluid inclusion trapping would require as much as 30°–35°C correction to the

observed homogenization temperatures. Therefore, the

reported homogenization temperatures must be considered

minimum estimates of the actual trapping temperatures. 

Studies of gases in fluid inclusions from sparry dolomite in the Ozark region (Hofstra and others, 1989) suggest

high amounts of CO2 in the fluid inclusions. The anomalous

concentration of CO2 in the fluid inclusion gases together

with mineralogical evidence for low pH between 4 and 5 of

the fluids (Goldhaber and Stanton, 1987; Diehl and others,

1991) was interpreted to indicate that the dolomite-forming

fluids may have been at or near saturation with respect to

CO2 and that effervescence was occurring during dolomite

precipitation. Leach and others (1991) conducted chemical

reaction path calculations for precipitation of sparry

dolomite in the Ozark region. They concluded that

near-isothermal effervescence of a CO2-dominate gas phase

during regional brine migrations to shallower burial depths

was an attractive mechanism to account for the widespread

occurrence of sparry dolomite in the Ozark region. If the

fluid inclusions were trapped along a brine-CO2 s

homogenization temperatures record actual trappin

atures and do not require a pressure correction

absence of independent data on the CO2 content o

inclusions in this study, we must assume that some 

amount of pressure may be required to obtain actu

temperatures. 

Considering the uncertainty surrounding 

pressures during fluid trapping, the observed fluid 

data, especially from the Reelfoot rift, can only be 

on a broad basis from one location to another. 

uncertainty in the actual depth of burial during flui

yields about 15°–45°C uncertainty in the 

correction to the observed homogenization tempe

CHANGES IN THE DENSITY OF INCLUSION 

DUE TO REEQUILIBRATION DURING PO

ENTRAPMENT BURIAL OR HEATING EVE

Several studies suggest that fluid inclusions 

can reequilibrate during post-entrapment burial 

events (Goldstein, 1986; Prezbindowski and Lar

Burruss, 1987; see discussion in Goldstein and 

1994), and presumably dolomite would respond i

manner. Such reequilibration can be useful in esti

maximum temperature achieved by the sample dur

quent burial to greater depths (Prezbindowski an

1987; Burruss, 1987). The fluid inclusions exami

study were trapped in Late Pennsylvanian to middl

time at or near their maximum burial dept

post-entrapment burial history for our fluid 

included initial uplift (about 2.4–4.6 km for NMTW 

and Marcher, 1962)—less for ARK-DW) followe

1 km of subsidence during sedimentation of the co

sediments. Thus, net overburden has decreased 

inclusion entrapment, so burial reequilibration o

inclusions is unlikely to have reset the homo

temperatures. 

A more likely source of the uncertainty in 

inclusion study is the possible reequilibration or str

the fluid inclusions caused by post-entrapment he

igneous activity in the Reelfoot rift. Igneous acti

Reelfoot rift includes emplacement of Late Pennsy

Permian age alkalic dikes and sills in the vicinity 

dome in the southern Illinois fluorspar district 

Pascola arch (Zartman, 1977). Hildenbrand (1

geophysical data to identify possible post-rift 

along the margins of the Reelfoot rift. One of the

intrusions is about 30 km east of the ARK-DW dri

addition, anomalous concentrations of Be, Nb, T

(unpublished data from R.L. Erickson and M.B. G

were recorded in portions of the NMTW drill c

suite of elements is consistent with possible alkal

activity (Goldhaber and others, 1992). Although r
METHODS AND RESULTS 

NMTW have been geochemically altered, no clear evidence

suggests that they have been thermally altered (M.B.

Goldhaber, oral commun., 1992). 

Magmatism, rather than local igneous intrusions, might

have been extensive in the Reelfoot rift during Late Pennsylvanian to Permian time, as suggested by Lewis (1987). Extensive intrusive activity would likely result in

above-average heat flow in the Reelfoot rift at a time broadly

coincident with inferred time of sparry dolomite

precipitation. As discussed in the section about estimating

paleotemperatures from thermal alteration of conodonts, an

above-average heat-flow in the Reelfoot rift could account

for the elevated homogenization temperatures reported

here—including the possibility for stretching and reequilibrating of earlier trapped fluid inclusions.

If the fluid inclusions were stretched by

post-entrapment heating, we might have observed a correlation of fluid inclusion size with Th or gas/liquid ratios;

stretching of fluid inclusions during post-entrapment heating is more likely to affect larger inclusions (Bodnar and

Bethke, 1984). We did not observe any correlation of Th

or gas/liquid ratios with inclusion sizes that range from 5 to

30 microns (micrometers), which would indicate

post-entrapment stretching. However, as discussed later,

certain evidence from the observed homogenization

temperatures from samples in the rift may reflect

post-entrapment stretching of the fluid inclusions. 

LEAKAGE INTO OR OUT OF THE FLUID INCLUSIONS 

DURING POST-ENTRAPMENT THERMAL 

DECREPITATION

Partial to complete recrystallization of the dolomite

can result in formation of new primary fluid inclusions in

the host crystal. This could produce a range of homogenization and ice-melting temperatures that reflects the conditions of fluids in which recrystallization occurred. In our

samples, we did not observe any textural evidence (such

as disrupted or altered CL bands) that suggests recrystallization of the host dolomite occurred. However, the decrepitation of fluid inclusions during post-entrapment

thermal events and the infilling by new fluids are a possible concern in the Reelfoot rift samples. If this process

occurred in our samples, we would expect to see a range

in homogenization and ice-melting temperatures as well as

fractures and fluid inclusion textures indicative of

decrepitation. Although we did not observe textural evidence for decrepitation, the fluid inclusion data from dolomite in the Reelfoot rift show a wide range in salinity and

filling temperatures. Therefore, we cannot exclude the

possibility that decrepitation of fluid inclusions and the infilling by new fluids did take place.

ESTIMATES OF PALEOTEMPERATURE

FROM THERMAL ALTERATION OF 

CONODONTS IN THE ARK-DW DRILL HO

Conodonts were not present in the NMTW becau

host rocks are Upper Cambrian Bonneterre Formation

low the common occurrence of euconodonts (or true 

odonts). Studies of conodont thermal-alteration in

(CAI) in post-Bonneterre rocks in the ARK-DW drill 

provide constraints on paleothermal gradients in the rif

Repetski, unpub. data, 1992). In the samples from 

ARK-DW drill hole, conodont CAI values are cons

with normally progressive downhole temperatures, an

face corrosion of the conodonts is not obviously greate

what would be expected by diagenetic dolomitizati

some of the collection horizons. For example, the con

elements do not show several CAI values in any one sa

nor do they exhibit abnormal appearance of whitened 

gins or patinas, as would be expected from significant 

sure to hydrothermal fluids. (See, for example, Rejebia

others, 1987.) At 914 m the CAI is 23, indicating a min

burial temperature of about 100°C, whereas at 2.59 k

CAI is 31

/2, equivalent to a minimum burial temperatu

about 150°C (assuming 25°C ambient surface tempera

These inferred temperatures indicate a burial thermal g

ent of about 30°C/km. Because the CAI is a time-depe

indicator, a short-lived thermal event of less than 1 m

the Reelfoot rift might be undetected in the conodont C

If the CAI values found in the ARK-DW drill hole 

reflect burial temperatures, then we can estimate the am

of post-Early Ordovician, pre-Cretaceous-age strata 

have been present in this part of the rift. The overb

needed to generate a CAI of 21

/2–3, as found at the top 

preserved Paleozoic succession, at 914 m in the ARK

drill hole, is about 3,049–4,573 m, given a burial th

gradient of about 30°C/km and given the existence o

amount of burial for at least 1 m.y. before subsequen

sion (Epstein and others, 1977; Harris, 1979). By thi

soning, at least 2,134–3,659 m of Middle Ordovician 

younger sediments were deposited in this part of the 

before the Cretaceous, and most likely before the m

Permian. The conodont CAI values may reflect a short

thermal event from advective heat transport by hydroth

fluids or conductive heating from igneous intrusions r

than simple burial thermal gradients. In this situatio

amount of burial estimated solely on the CAI values m

too large. However, the consistent downhole CAI tren

the general absence of within-sample CAI, bleachin

corrosion variations argue against significant effects 

these nonburial heating sources.

The minimum burial temperatures estimated 

conodont CAI values are at least 30°–50°C lower 

homogenization temperatures from sparry dolomi

approximately the same stratigraphic positions. 

observed homogenization temperatures are mini
12 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

240

200

160

120

7,500 10,000

DEPTH, IN METERS

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS

0 2,500 5,000

EXPLANATION

ARK-DW

DP-4

NMTW

P-1

PBW-3

IMBODEN

PB-7

estimates of the temperatures (1) at which primary fluid inclusions were trapped or (2) at which thermal reequilibration

may have occurred during a possible post-trapping thermal

event. Therefore, the addition of pressure corrections to the

homogenization temperatures could increase this difference

by as much as another 30°–60°C. The most obvious possibility is that the conodont CAI values record the minimum

burial temperatures of the rocks, whereas the fluid inclusions

record a transient and short-lived thermal event of less than

1 m.y.

REELFOOT RIFT DRILL HOLES

The homogenization temperatures for fluid inclusions

hosted in zones 3 and 4 dolomite from all drill holes are

shown in figure 3. In view of the uncertainties related to

unknown pressure corrections to the fluid inclusion

homogenization values and uncertainties related to possible

post-trapping changes in the density of the fluid inclusions,

only general comparisons can be reasonably made between

the different sample locations. Based on reasonable

difference in the stratigraphic cover, maximum pressure

corrections to the homogenization temperatures for data

from the Reelfoot rift probably exceed by about 10°–40°C

the maximum corrections to the data for samples from the

Ozark and Nashville domes. The uncorrected Th values for

the Reelfoot rift drill holes (NMTW and ARK-DW) are

consistently higher than those on the flanks of the Ozark and

Nashville domes. With the exception of fluid inclusions in

samples from the NMTW, the data show a gener

increasing homogenization with increasing depth 

samples were obtained. 

Figure 4 shows the location of samples stud

ARK-DW drill hole. The homogenization tem

(Th) in figure 5 show a clear bimodal distribu

population with a mode around 250°C and a

about 190°C. It is clear from figures 5 and 6 tha

er temperature mode consists of fluid inclusions 

CL zone 3 dolomites. As discussed in the cat

nescent microstratigraphy section, sparry dolom

CL zone 3 occur throughout the ARK-DW dril

are best developed in the Bonneterre Formatio

and 3.66 km. Dolomites containing CL zone 4 

from the Bonneterre Formation but are commo

lower stratigraphic levels, 1.07–2.9 km. Fluid 

in zone 4 dolomite have distinctly lower homo

temperatures with a mode of about 190°C and m

ue of about 180°C. The apparent difference in h

zation temperatures between zones 3 and 4 fluid 

could be about 50°C. If pressure corrections 

mogenization temperatures, due to the stratigraph

tion between zones 3 and 4 dolomite, are consi

difference in temperatures could be about 60°C

estimates are based on the assumption that the p

Ordovician stratigraphic overburden was appr

the same for fluids that formed zones 3 and 4 do

the difference in post-Lower Ordovician o

between dolomite-hosted zones 3 and 4 fluids we

Figure 3. Homogenization temperature (Th) versus depth for all drill hole fluid inclusion data.
METHODS AND RESULTS 



































































































TERTIARY

CRETACEOUS

CAMBRIAN

NMTW

NMTW

ARK-DW

Sands and clays

Sands, silts, and clays

Clay, silty clay, clayey silt, and sand

Glauconitic sand

Dolostone

Sands, silts, and clays

Dominantly limestone with some dolostone

Shale, claystone, and arkosic sandstone

Dolostone

Dominantly arkosic sandstone

Granitic and gneissic rocks

EXPLANATION

QUATERNARY

Knox and

Arbuckle Group

equivalents

Elvins Group

Bonneterre Formation

basal clastic

rocks

crystalline rocks

Quaternary to Cretaceous

sedimentary deposits

ARK-DW

1,

2,

3,

4,

Depth 

surface 

LOWER ORDOVICIAN MIDDLE (?) AND UPPER CAMBRIAN e p

Figure 4. Idealized stratigraphic section for NMTW, Missouri, and ARK-DW, Arkansas, showing drill hole sample locations fo

inclusions (oval with enclosed dot) and conodonts (toothed symbol). Wavy line, unconformity. Vertical solid and dashed line in

distribution of conodonts.
14 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX NUMBER OF SAMPLES

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS

EXPLANATION

1.08

2.34

2.82

2.87

2.91

3.62

3.66

Depth of sample,

in kilometers

80 100 120 140 160 180 200 220 240 260 280

8

7

6

5

4

3

2

1

0

3–6 km, the separation in observed homogenization temperatures could be due to pressure alone. In view of the

inferred Late Pennsylvanian to Early Permian age for

zones 3 and 4 dolomite, the difference in the amount of

post-Lower Ordovician overburden between zones 3 and 4

dolomite was probably insufficient to account for the differences in the homogenization temperatures. Therefore,

the separation between the homogenization temperatures

for zones 3 and 4 fluid inclusions most likely reflects a real

difference in the temperatures of the fluids that formed

zones 3 and 4 dolomite.

All samples from the NMTW are from Upper Cambrian

Bonneterre Formation, located on the Pascola arch 25 km

north of the ARK-DW drill hole (fig. 1). Comparison of the

distribution of Th values (range and median value) for zone

3-hosted fluid inclusions from both drill holes suggests temperatures in the NMTW could possibly have been slightly

cooler than in the ARK-DW. However, in view of the uncertainty in pressures during fluid trapping, these slight differences in Th values may simply reflect different trapping

pressures.

In addition, Th values from the NMTW do no

to the broad trend of higher Th temperatures w

drill hole depths (fig. 3) observed for the other lo

appears that the Th values from samples in the N

anomalously hot when compared to the Th-depth r

ARK-DW (fig. 3). Dolomite from drill hole 

626–701 m in the Bonneterre Formation from th

have Th values that correspond to values obta

samples at drill hole depths of about 2.86–3.

ARK-DW. All Th measurements in the NMT

ARK-DW at 2.86–3.66 km are from zone 3-ho

inclusions. 

In view of the uncertainties about pressure c

and possible post-trapping changes in the fluid 

densities, the reason for the difference in Th valu

ulative. One possibility is that the zone 3 dolom

NMTW formed from fluids at lower temperatures 

sures relative to the fluids in ARK-DW. In this ca

fluid migration would probably have occurred aft

the Pascola arch. The second possibility is that zo

mite in both drill holes formed at approximately 

Figure 5. Histogram plot of homogenization temperature (Th) for ARK-DW drill hole. Variations in

temperature with depth are also indicated.
METHODS AND RESULTS 

Zone 3

16

12

8

4

0

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS

NUMBER OF SAMPLES

NMTW

16

12

8

4

0

80 120 160 200 240 

Zone 4

ARK-DW

Zone 4

Zone 3

Zone 3

Zone 4

DP-4

Minimum 25 50 75 Maxim

IMBODEN

Percentile

EXPLANATION

EXPLANATION

A B

C

D E

PB-7

GP-10

P-1

PBW-3

80 120 160 200 240 280 80 120 160 200 240 280

temperatures and pressures (burial depths) and the NMTW

experienced a net uplift, relative to ARK-DW, of about

2.2–3 km. If the thickness of Paleozoic rocks deposited at

the NMTW location were comparable to that at ARK-DW,

as much as 4.3–6.7 km of Paleozoic rocks were eroded on the

Pascola arch before the Cretaceous. A third possibility is

that the fluid inclusions in zone 3 dolomite in the NMTW

drill hole were stretched or thermally reequilibrated during a

thermal event that could be related to the postulated igneous

intrusion that produced the anomalous concentrations of Be,

Nb, Th, and La in samples from the drill hole. 

SAMPLES FROM THE OZARK DOME

All Th data from sparry dolomite from the Lower

Ordovician Powell Dolomite in the Black Rock Quarry at

Imboden, Ark. (figs. 3 and 6) are from zone 4-hosted fluid

inclusions. Zones 1 and 3 were recognized, but we were

unable to obtain Th measurements from these zones. The Th

data from zone 4 dolomite at Imboden are essentially

identical to the Th data from sphalerite from the Northern

Arkansas district (Leach and others, 1975; Long and others,

1986). 

Conodonts from the upper Lower Ordovician rocks at

the Imboden Black Rock Quarry (J.E. Repetski, unpub.

data, 1992) have CAI values of 11

/2 , indicating that they

experienced minimum post-depositional heating in the

range of 50°–90°C. Thus, the contrast in CAI value

tween the Reelfoot rift and Imboden demonstrates a 

consistent with the fluid inclusion data (as discussed 

following paragraphs).

Also shown in figure 6 are Th data from s

dolomite from Upper Cambrian Bonneterre and Derby

Run Formations in drill cores PB-7, GP-10, P-1

PBW-3. The Th data from these samples include zone

and 4. The few measurements from these samples pr

recognition of possible differences between the Th o

different zones, and we have chosen to combine them 

single histogram.

The Th values from sparry dolomite in the Ozark 

are clearly lower than from comparable sparry dolom

the Reelfoot rift. For example, fluid inclusions in z

dolomite from Upper Cambrian rocks in the ARK-DW 

hole have a median value of 180°C, whereas fluid inclu

in zone 4 dolomite from Lower Ordovician rocks 5

northwest at Imboden, Ark., have a median value of 1

Thus, Th values differ by at least 60°C in fluid inclusio

samples of CL-equivalent sparry dolomite located 5

apart and 2.87 km vertically (drill hole depths). Becau

the clear difference in burial depths between the Ozark 

and Reelfoot samples, the pressure corrected Th v

would indicate even greater difference in temperature

fluid in the Reelfoot rift relative to the Ozark dome. 

Figure 6. Histogram plots of homogenization temperature (Th). A, B, Ozark dome; C, D, Reelfoot rift; E, Nashville dome.
16 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

-7.5

-15.0

-22.5

7,500 10,000

DEPTH, IN METERS

0 2,500 5,000

EXPLANATION

ARK-DW

DP-4

NMTW

P-1

PBW-3

IMBODEN

PB-7

FINAL ICE-MELTING TEMPERATURE, IN DEGREEES CELSIUS

NASHVILLE DOME

Samples of sparry dolomite containing CL zones 1, 3,

and 4 were obtained from drill core DP-4 (fig. 1), located

along the western flank of the Nashville dome. Only zones

3 and 4 yielded usable fluid inclusions for Th measurements

(fig. 6). The data are from samples of sparry dolomite hosted

by the Conasauga Group (Middle and Upper Cambrian).

The present burial depth for the samples is 1.73 km. The Th

values for zones 3 and 4 lie in the range of 100°–145°C and

are similar to Th values for the Ozark dome. These

observations are significant in that they show the Ozark

MVT–Reelfoot rift hydrothermal system extended onto the

western flank of the Nashville dome. 

FINAL ICE-MELTING TEMPERATURES

Measurements of the final ice-melting temperatures of

fluid inclusions for all samples range from –4.9° to –29.9°C

(figs. 7 and 8), corresponding from 7.7 to greater than 23

equivalent weight percent NaCl (Bodnar, 1992). Although

samples from ARK-DW show the largest spread in Tm, salinity does not differ significantly between locations, between present burial depths, or between fluid inclusions

hosted by zones 3 and 4 dolomite. First-melting temperatures are lower than –50°C, indicating the presence of appreciable divalent ions in solution. These data are similar to

data obtained in other studies of sparry dolomite (Rowan and

Leach, 1989; Shelton, Bauer, and Gregg, 1992

sphalerite from MVT occurrences in the Oza

(Roedder, 1967; Leach and others, 1975; Lea

Hagni, 1983; Long and others, 1986). 

SALINITY-TEMPERATURE RELATIO

No clearly defined relationship is apparent be

mogenization and final ice-melting temperature

from individual fluid inclusions (fig. 9). Howeve

ter plot for ARK-DW does suggest the presence of 

tion of higher temperature and lower salinity fluid 

hosted by zone 3 dolomite (Th between 240° and 

Tm between –18° and –5.4°C). Fluid inclusions 

pose this population form the apparent high-te

mode at 3.6 km from the Bonneterre Formation in 

(fig. 5). 

Of particular interest shown in figure 9 is th

linear array of Th plotted against Tm for samples 

dolomite-hosted fluid inclusions from samples 

NMTW. This array reflects a large variation in 

relative to less variable Tm values and is consi

fluid inclusions that have experienced pos

stretching or thermal reequilibration. Consid

evidence for igneous activity in the vicinity of the 

post-trapping thermal event could account for th

Th values observed in the NMTW and possib

ARK-DW drill holes.

Figure 7. Final ice-melting temperature (Tm) versus depth for all drill hole fluid inclusion data.
METHODS AND RESULTS 

EQUIVALENT WEIGHT PERCENT NaCl

EXPLANAT

PB-

GP-

P-1

PBW

?

Minimum 25 50 75 Maxi

Percentile

EXPLANATION

-40 -30 -20 -10 0 -40 -30 -20 -10 0

-40 -30 -20 -10 

22.38 18.63 13.94 

FINAL ICE-MELTING TEMPERATURE, IN DEGREES CELSIUS

16

12

8

4

0

NUMBER OF SAMPLES

16

12

8

4

0

Zone 3

Zone 4

ARK-DW

A

Zone 4

IMBODEN

B C

NMTW

Zone 3

Zone 3

Zone 4

DP-4

D E

Zone 1

22.38 18.63 13.94 7.86 22.38 18.63 13.94 7.86

80 100 120 140 160 180 200 220 240 260 280 80 100 120 140 160 180 200 220 240 260 280

-10

-15

-20

-25

-30

-35

-40

B C

D E

A

Zone IV

Zone III

-5

0

-10

-15

-20

-25

-30

-35

-40

-5

0

80 100 120 140 160 180 200 220 240 

F

Zone III

Zone IV

Zon

Zon

Zon

Zon

Zone III

Zone IV

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS

FINAL ICE-MELTING TEMPERATURE, IN DEGREES CELSIUS

Figure 8. Histogram plots of final ice-melting temperature (Tm). A, B, Ozark dome; C, D, Reelfoot rift; E, Nashville dome.

Figure 9. Final ice-melting temperature (Tm) plotted against homogenization temperature (Th) for fluid inclusion data of zone 3 a

4 in dolomite from the Ozark dome (A, B), Reelfoot rift (C, D), Nashville dome (E), and ALL DATA (F). A, PB-7, GP-10, P-1, 
18 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

DISCUSSION

The CL microstratigraphy developed for sparry

dolomite in southeast Missouri provides a means to correlate

sparry dolomite outside the ore districts. However, in the

absence of independent age constraints on the sparry

dolomite cement, this comparison requires two important

assumptions.

First, we assume that the CL zones in our samples are

the same zones present in sparry dolomite from the Ozark

region. Identification of a CL zone can be subjective, particularly if the entire microstratigraphy is not present or if

one or more zones are poorly developed. Even in the

Viburnum Trend, where the Ozark CL microstratigraphy

was first recognized, samples of sparry dolomite with poorly developed CL zones are common. In most of our samples from the Reelfoot rift complex, the observed CL

microstratigraphy was similar to that of the Viburnum

Trend but the zones were typically thinner and less complex than those in the Viburnum Trend. For many samples,

only two or three zones could be identified with confidence.

In addition, the CL zone identified as zone 3 in our samples

was generally thinner and contained fewer subbands relative to typical zone 3 dolomite in the Viburnum Trend. We

suspect that the less well developed CL zones in the

samples from the Reelfoot rift complex may reflect the

small size of the dolomite crystals obtained from acid

digestion of the carbonate rocks. Nevertheless, we believe

that the CL zones in the Reelfoot rift samples are

sufficiently similar to the Viburnum Trend CL microstratigraphy to propose that they were deposited from a common

brine. 

Secondly, we assume that the correlative CL zones,

whether in samples from the Reelfoot rift or the Ozark

region, reflect the broadly contemporaneous precipitation

of sparry dolomite. In view of the strong evidence for an

interconnected hydrothermal system for the MVT deposits

and associated sparry dolomite in the Ozark region, we believe this to be a valid assumption. In addition, many lines

of evidence indicate that a MVT fluid migration out of the

Reelfoot rift is consistent with our assumption. 

Based on the CL microstratigraphy, we suggest that

the fluid inclusions in sparry dolomite from the Reelfoot

rift complex provide samples of the MVT brines in a portion of what may have been the source of the hot brines that

affected much of the Ozark region. The source basin probably included the combined Arkoma and Black Warrior

foredeeps together with the thick Paleozoic sedimentary fill

in the Reelfoot rift complex, as first suggested by Farr

(1989a, 1989b). The results of this study are consistent

with the suggestion that the Reelfoot rift zone was an important pathway for the migration of MVT brines into the

Southeast Missouri lead districts (Brecke, 1979; Farr and

Land, 1985; Farr, 1989a, 1989b; Erickson and others, 1988;

Viets and Leach, 1988; Goldhaber and Mosier, 1989; Diehl

and others, 1991; Viets and Leach, 1990). How

lead isotope studies in the region (Goldhaber a

1995) show that the lead in the main-stage o

Southeast Missouri ore districts contain lead isoto

inconsistent with derivation from the Reelfoot ri

count for the lead isotope values in the main-sta

southeast Missouri, Goldhaber and others (1995) 

that lead was leached from source rocks in the 

gion. Our results also suggest that the hydrotherm

responsible for the Ozark MVT districts may have 

onto the western flank of the Nashville dome. T

ble extension of the Ozark hydrothermal system 

the potential for undiscovered MVT resources 

flanks of the Reelfoot rift complex and along th

flank of the Nashville dome.

IMPLICATIONS FOR MIGRATION OF 

BRINES

Cathodoluminescent microstratigraphy inve

of sparry dolomite from the ARK-DW and NM

holes in the Reelfoot rift show that zone 3 dolom

abundant in the Bonneterre Formation. Presu

Bonneterre Formation was hydrologically connec

underlying Lamotte Sandstone. Therefore, the 

Paleozoic rocks may have been the dominant pa

migration of brines that deposited zone 3 dolom

Reelfoot rift complex and main-stage ores in sout

souri. Zone 4 dolomite was not observed in sam

the Bonneterre Formation, either in ARK-DW o

drill holes. However, zone 4 dolomite was deposi

rocks overlying the Bonneterre Formation in the 

drill hole. We suggest two possibilities to accou

lack of zone 4 dolomite in samples from the Bonn

mation: (1) the brines were undersaturated with 

dolomite in the lower carbonate section during zo

therefore, no zone 4 dolomite was deposited in th

(2) at the time of precipitation of zone 4 dolomite

gration was mainly through aquifers at higher str

levels. Zone 4 dolomite is broadly time equ

late-stage ores in southeast Missouri and main-sta

other Ozark MVT districts. This broad time equi

consistent with that of Viets and Leach (1990), wh

ed that main-stage ore in the Viburnum Trend w

tated from a fluid component that interacted with a 

aquifer. Shifting fluid migration to higher str

position in zone 4 time would possibly limit intera

the basal sandstone in the rift, and is consisten

lower potassium content in late cubic-stage gale

the Viburnum Trend and other MVT districts (

Leach, 1990). If brine migration did shift 

stratigraphic levels during zone 4 time, the shift co

tectonic or structural controls on fluid migrat

example, reactivation and uplift of preexisting 
DISCUSSION 

faults may have forced fluid flow to higher stratigraphic

levels in zone 4 time. 

THERMAL REGIME IN THE REELFOOT RIFT

Conodont CAI data suggest a minimum paleothermal

gradient of about 30°C/km for conodonts between 0.91 and

2.6 km and a minimum temperature of about 150°C at 2.6

km in the ARK-DW drill hole. Our Th data (uncorrected for

pressure) for zone 4 fluids are about 135°–205°C at depths

between 1.08 and 2.9 km. If the conodonts and Th data for

fluid inclusions in zone 4 dolomite record the same thermal

regime, then the homogenization temperatures are about

40°–50°C hotter than indicated from CAI. Pressure corrections to the Th data could increase this difference by as much

as 30°–50°C.

Homogenization temperatures (uncorrected for pressure) for fluid inclusions in zone 3 dolomite in the ARK-DW

at drill hole depths of 3.62 and 3.66 km are between 150° and

280°C, with possible modes at about 190° and 255°C (fig. 5).

At estimated maximum burial depths of about 5.3–6.3 km

(see previous discussion) in the late Paleozoic, a 30°C/km

(25°C ambient surface temperature) geothermal gradient

yields minimum temperatures of 184°–214°C. Thus, it appears that zone 3 fluids were hotter than obtainable by a

30°C/km gradient, indicated from conodont CAI in the overlying rocks. 

The conodont CAI data and the Th data provide independent estimates for minimum temperatures experienced

by the samples from the Reelfoot rift. Homogenization temperatures are on the order of 30°–50°C higher than estimates

from the conodont data. If pressure corrections are

considered for the Th data, the separation in temperatures

could be several tens of degrees greater. The most reasonable interpretation of this difference is that the fluid inclusions are recording a transient but short-lived thermal event

on the order of 1 m.y. or less. However, it is possible that

some of the fluid inclusions may have decrepitated or

leaked; however, we did not observe petrographic evidence

of this in our samples. 

One of the most significant findings in this study is the

anomalously high Th values recorded in the Reelfoot rift.

For example, Th values in the deepest part of the ARK-DW

hole (present depth of 3.66 km) are in the range of

150°–280°C; most of the data are in the range 220°–260°C.

Based on estimates of the stratigraphic overburden in late

Paleozoic time (5–6 km), possible pressure corrections to

these data would yield corrected temperatures of about

50°–60°C higher. For a 60°C pressure correction, the actual

temperatures achieved in the deepest part of ARK-DW

would be in the range 210°–340°C. This range would be

equivalent to burial geothermal gradients of approximately

35°–60°C/km. Assuming that the fluid inclusions have not

leaked, the high temperatures reflect either temperatures of

advecting hydrothermal brines or thermally reequilibrated

fluid inclusions in response to higher basement-de

conductive geothermal gradient. In either case, the flu

clusion data point to a thermal event in the central Ree

rift where temperatures of about 300°C were achieved 

Bonneterre Formation.

SOURCE OF THE BRINES 

The source of the highly saline fluids that forme

sparry dolomite and the MVT deposits in the Ozark r

remains problematic. However, the general opinion i

the high salinity levels of most subsurface sedime

brines are probably related to dissolution of evaporite

(or) subaerially produced brines (bitterns) associated 

evaporation of seawater. (See Carpenter, 1978; Lan

Prezbindowski, 1981; Hanor, 1987, among ot

Although the few drill holes in the Reelfoot rift have 

encountered bedded evaporites, McKeown and o

(1990) reported as much as 5 percent intergranular barit

anhydrite in part of the ARK-DW drill hole; about 1 pe

gypsum is present in the ARK-DW drill hole. De

(1984) also reported anhydrite in the crystalline basem

the ARK-DW drill hole. Furthermore, sulfur isotope st

of Paleozoic rocks in the Pascola arch and vicinity are 

sistent with sulfur derived from reduction of evaporit

fate by organic matter (Goldhaber and Mosier, 1

Continental rift basins commonly contain evap

sequences (Ziegler, 1988; Hardie and others, 1978). T

fore, a reasonable hypothesis is that evaporites accumu

in the Reelfoot rift during late Proterozoic and 

Cambrian rifting could have provided a source for the h

saline fluids that deposited the sparry dolomite.

VARIATION IN TEMPERATURE AND 

SALINITY

The scatter plots for temperature and salinity fo

samples (fig. 9) are similar to those of other fluid incl

studies throughout the region that indicate independent 

ations in temperature and salinity (Leach and others, 

Leach, 1979; Roedder, 1977; Long and others, 1986; R

and Leach, 1989). For samples from the Reelfoot rif

range in fluid inclusion data is exceptionally large. 

variation in the Th data can be attributed to undetected 

ing of fluid inclusions (Leach and Rowan, 1993); how

the large range of Th values observed for samples fro

Reelfoot rift is unlikely attributable to necking alone. 

ability related to recrystallization of the sparry dolom

leakage of the fluid inclusions was not important bas

our petrographic observations. Changes in fluid pre

during crystal growth can account for some variation 

values. However, at any given site during growth o

dolomite crystal, the possible variations in pressur
20 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

expected to be too small to account for significant variability

in the Th values. The observed range in Th and Tm values

more likely reflects changes in the salinity and temperature

of the hydrothermal fluids, or in the case of the Reelfoot rift

samples, post-trapping thermal stretching or reequilibrium of

the fluid inclusions.

CHANGES IN TEMPERATURE AND SALINITY RELATED 

TO FLUID MIXING

It is tempting to invoke fluid mixing at each site of mineral deposition to explain the large variation observed in

temperature and salinity of the fluid inclusions. Many studies call upon fluid mixing to account for ore deposition in the

Viburnum Trend district (Rowan and Leach, 1989; Viets and

Leach, 1990; Burstein and others, 1991; Shelton and others,

1991; Shelton, Bauer, and Gregg, 1992; Plumlee and others,

1995) and in other MVT districts such as East Tennessee

(Taylor and others, 1983) and Sweetwater district (Zimmerman and Kesler, 1981). In addition, a variety of fluid mixing

models are proposed for sparry dolomite precipitation.

Rowan and Leach (1989) suggested that the wide variation in

salinity and temperature of sparry dolomite in the Viburnum

Trend reflects the presence of more than one fluid during ore

deposition and that fluid mixing probably occurred. In a

recent study of sparry dolomite from the Ozark region

(Shelton, Bauer, and Gregg, 1992), fluid mixing was invoked

to explain the variance in homogenization–final melting

temperatures. 

An alternative to local, site-specific, fluid mixing to

explain the variation in salinity is the migration of pulses of

brines whose temperature and salinity reflect large-scale

processes within the source basin of the brines. Sedimentary

basins typically contain fluids that differ significantly in

composition and salinity. (See Hanor, 1979, 1987; Carpenter, 1978, among others.) As Hanor (1987, p. 54) pointed

out, these variations probably represent blends of various

end member fluids which include bitterns, brines produced

through dissolution of evaporites, and influxes of meteoric

water. Thus, fluid mixing within the Ouachita basin

complex would have produced a range of fluid inclusion

salinities in sparry dolomite in the Reelfoot rift and throughout the Ozark dome and western flank of the Nashville dome.

The expulsion of fluids from the Ouachita basin complex,

containing a range of fluid salinities, would likely result in

transient changes in fluid salinity at any given location along

the brine’s flow path. Fluid-rock reactions as well as additional fluid mixing during brine migration would further add

to the variability. We believe that much of the variation in

salinity in sparry dolomite observed for the Ozark region can

be related to inherent variations in salinity within the Ouachita basin complex produced by fluid mixing within the basin.

In this scenario, fluid inclusions in sparry dolomite in the

Ozark region and western flank of the Nashville dome record

this variation in temperature and salinity throug

fluid migration from the source basin. 

The large range in fluid inclusion homo

temperatures, particularly in samples from the Re

may in part reflect real variations in trapping tem

The temperature of the mineralizing fluid at a 

sparry dolomite is precipitating must be related t

migration pathway, starting temperature of the f

source basin, rate of fluid migration, local thermal 

the depositional site, and amount of thermal insu

vided by the overlying rocks. Fluid migrating th

Reelfoot rift complex in late Paleozoic time would 

perience considerable transient temperature variat

ing the closing stages of Ouachita orogeny in 

Paleozoic, compression taking place in the Re

complex probably reactivated basement faults t

disturb and modify fluid migration pathways. 

compression also produced uplift in arches (such 

arch) and domes (such as Nashville and Ozark), w

also cause transient changes in the fluid pathway

rates. The Paleozoic sedimentary fill in the Reelfo

ies as much as 4 km (Nelson and Zhang, 1991), 

migrating through this variable sedimentary fill c

as much as 120°C for a 30°C/km basement-contr

thermal gradient. In addition, the drive for the re

drothermal system was likely topographic he

uplifted Ouachita foldbelt; therefore, changes in to

head in the Ouachita foldbelt would also influence 

rates and pathways. 

POST-TRAPPING THERMAL STRETCHIN

REEQUILIBRATION

The fluid inclusion data for sparry dolom

Ozark dome are similar to data for the MVT ore de

evidence exists for igneous activity or other therm

near the ore deposits of the Ozark dome after ore 

that could have disturbed the Th data. However, 

tion in the Reelfoot rift is quite different. In the Re

the fluid inclusion data show a large range in valu

trusive activity is known to have been coeval with 

date the inferred time of sparry dolomite precipitat

of this variability may be related to the presence of 

one fluid, especially for samples from the ARK

hole. However, the extreme range of Th relativ

more restricted range in Tm, especially for NMT

gestive of stretching or reequilibration of the fluid 

from a post-trapping thermal event.

Known igneous activity in the Reelfoot rift co

cludes emplacement of Late Pennsylvanian to Pe

alkalic dikes and sills in the vicinity of Hicks do

southern Illinois fluorspar district and in the vici

Pascola arch. In addition, an inferred igneous int

about 30 km east of the ARK-DW drill hole. 
CONCLUSIONS 

thermal conditions, related to the known igneous activity,

possibly were coeval with or followed formation of the

sparry dolomite. Previously trapped fluid inclusions would

likely have expanded or stretched to achieve new fluid

densities approaching values more consistent with the

thermal conditions imposed by the elevated thermal environment. If stretching of fluid inclusions did occur, the reported

Th values would more closely reflect the imposed thermal

regime, therefore still providing evidence of paleothermal

conditions in the Reelfoot rift.

CONCLUSIONS

The fluid inclusion, CL, and CAI investigations must be

considered as a reconnaissance study of the involvement of

the Reelfoot rift complex in the Ozark MVT hydrothermal

system. Our study was of only two drill holes in the Reelfoot

rift and one along the Nashville dome. Certainly more work

is needed to confirm our observations and support our

conclusions. Nevertheless, the following conclusions appear to be reasonable: 

1. The CL microstratigraphy of sparry dolomite cement

in the Reelfoot rift complex is similar to that observed in the

Ozark region. Although the CL microstratigraphy in the

Reelfoot rift complex is simpler and less well developed than

in the Ozark region, the two are sufficiently similar to suggest deposition of sparry dolomite from a common brine.

2. Fluid inclusion and CL studies suggest that the hot

brines responsible for widespread formation of MVT mineralization in the Ozark region were also present in the

Reelfoot rift complex in the late Paleozoic. This interpretation is consistent with other studies that suggest the Reelfoot rift complex as an important pathway for migration of

MVT brines into the Southeast Missouri lead districts.

3. The Ozark hydrothermal system may have extended into the western flank of the Nashville dome. This extension is suggested by the presence of sparry dolomite

cement in drill hole DP-4, in which CL microstratigraphy

and fluid inclusion homogenization and salinities are similar to those in the Ozark region. The CL zoning in samples

from the western Nashville dome are distinctly different

from those reported from sparry dolomite along the crest of

the Nashville dome near the Central Tennessee MVT

district.

4. Zone 3 dolomite, temporally related to main-stage

ores in southeast Missouri, was observed to be best developed in the Bonneterre Formation and in drill holes

ARK-DW and NMTW in the Reelfoot rift complex. Zone 4

dolomite is temporally related to deposition of late-stage

ores in southeast Missouri and main-stage ores in the other

Ozark MVT districts. Zone 4 dolomite was observed only in

post-Bonneterre Formation rocks in ARK-DW. Possible

reasons for the absence of zone 4 dolomite in the lower

Bonneterre Formation in the Reelfoot rift are: (1) Zone 4

brines did not precipitate dolomite in the lowermost section

because the fluids were at or below dolomite saturati

these rocks; or (2) fluid flow shifted to higher stratigr

levels from zones 3 to 4 time.

5. Conodont CAI data from Lower Ordovician ro

the Reelfoot rift from the ARK-DW drill hole are cons

with a minimum paleothermal gradient of about 30°C

Fluid inclusion homogenization temperatures (uncorr

for pressure) obtained for fluid inclusions hosted in s

dolomite in the ARK-DW drill hole are similar (but 

30°–50°C higher) relative to temperatures estimated 

conodont CAI. Pressure corrections to the fluid incl

homogenization temperatures could extend the differen

as much as an additional 30°–50°C. The difference in 

peratures from conodont CAI and fluid inclusions is p

bly due to the fact that the CAI values are a time-depe

indicator; a short-lived thermal event of less than 1 m

the rift might be undetected in the conodont CAI. The 

inclusions may record the passage of transient but h

temperature fluids or a short-lived thermal event rela

igneous activity. 

6. CAI values found in the ARK-DW drill hole pr

evidence of the amount of post-Lower Ordovician 

eroded before the Cretaceous. For a burial thermal gra

of 30°C/km for at least 1 m.y., about 2.1–3.7 km of Pale

strata have been removed. The total amount of post-La

Paleozoic strata present before the pre-Cretaceous er

could be between 5.3 and 6.3 km. However, if the 

values are reflecting local thermal effects from adve

hydrothermal fluids or local igneous activity, these esti

may be too large, although there appears to be no dow

perturbation of either the CAI trend or within-sample 

value or surface textural variations to indicate these sm

scale, nonburial heating effects.

7. The fluid inclusion homogenization data pr

estimates of the minimum temperatures of fluid trappin

(or) burial temperatures for the sparry dolomite. Pre

corrections can be estimated, assuming hydrostatic 

pressures and estimates of the maximum depth of buria

the Ozark and Nashville domes, the maximum pressur

rections to the homogenization data probably do not e

15°–20°C. In the Reelfoot rift, estimated pressure c

tions could be as high as 45°–60°C. Uncertainties rela

actual burial depths limit comparisons between locatio

broad conclusions. Nevertheless, the data do pr

important constraints on the thermal regime in the reg

late Paleozoic time. 

8. Fluid inclusion homogenization and salinity da

sparry dolomite in the Ozark and Nashville dome

similar to values reported for sphalerite and sparry dol

from the MVT districts in the Ozark region. Most of th

homogenization temperatures fall between 100° and 1

pressure corrections to these data probably do not e

15°–20°C. Final ice-melting temperatures are m

between –10° and –25°C, corresponding to about 

greater than 23 equivalent weight percent NaCl salinit
22 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

9. Fluid inclusion homogenization temperatures for

samples of sparry dolomite and CAI values in Ordovician

rocks in the Reelfoot rift are significantly higher than those

for sparry dolomite from the Ozark and Nashville domes.

For example, fluid inclusions in zone 4 dolomite from the

ARK-DW drill hole in the Reelfoot rift are about 60°C hotter

than those in zone 4 dolomite located 50 km distant at

Imboden, Ark. Fluid inclusions in zone 3 dolomite in the

Reelfoot rift may be more than 100°C hotter than those found

in zone 3 dolomite in the Ozark dome. Pressure corrections

to the homogenization temperatures could extend these

temperature differences by as much as 40°–60°C.

10. In the ARK-DW drill hole, fluid inclusions in

zone 4 dolomite have distinctly lower homogenization temperatures relative to fluid inclusions in zone 3 dolomite.

The apparent difference in homogenization temperatures

between zones 3 and 4 fluid inclusions could be about

50°C. If pressure corrections to the homogenization

temperatures, due to the stratigraphic separation between

zones 3 and 4 dolomite, are considered, the difference in

temperatures could be about 60°C. The difference between

the homogenization temperatures for zones 3 and 4 fluid inclusions most likely reflects a real difference in the temperatures of the fluids that formed zones 3 and 4 dolomite.

11. One of the most significant findings in this study

is the high fluid inclusion temperatures recorded in sparry

dolomite from the Reelfoot rift. Homogenization values in

the deepest part of the ARK-DW drill hole (present depth

of 3.66 km) are in the range of 150°–280°C; most of the

data are in the range 220°–260°C. Applying a 60°C pressure correction, the actual temperatures achieved in the

deepest part of ARK-DW could be in the range

210°–340°C. This range would be equivalent to burial

geothermal gradients of approximately 35°–60°C/km. The

high temperatures reflect either the temperatures of

advecting hydrothermal brines or thermally reequilibrated

fluid inclusions in response to higher basement-derived,

conductive geothermal gradient. In either case, the fluid

inclusion data point to a thermal event in the central Reelfoot rift where temperatures of around 300°C were

achieved in the Bonneterre Formation.

12. For samples of sparry dolomite at any given location or depth in the Reelfoot rift, the range in homogenization temperatures is rather large. For example, at 3.62 to

3.66 km in ARK-DW, the homogenization temperatures

range from 155°–280°C. This large range in homogenization temperatures may be explained by significant but transient changes in temperatures of advecting hydrothermal

fluids. Alternatively, the range could be the result of the

stretching or reequilibration of the fluid inclusions from a

post-trapping thermal event. The possible post-trapping

thermal event could be related to the Late Pennsylvanian to

Permian intrusive activity known in the Reelfoot rift

complex. If stretching of fluid inclusions did occur, the

reported Th values would more closely reflect the imposed

thermal regime; therefore, they would stil

constraints on the thermal conditions in the Reel

late Paleozoic time.

13. High homogenization temperatures and 

low salinities of many of the fluid inclusions fro

dolomite in ARK-DW from the Bonneterre 

suggest the presence of a hotter and more d

component in the Reelfoot rift. Fluid inclusion 

other samples in the study area generally show 

but independent variation between homogenizatio

ature and salinity. These variations in temper

salinity may reflect transient variations in flui

fluid mixing within the Ouachita basin complex. 

the salinity-temperature relationship for samples 

NMTW shows significantly greater variation in 

ture relative to salinity, suggestive of post-trappin

ing or thermal reequilibration of the fluid inclusio

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sition of zoned dolomite and its relationship to sulfide m

paragenesis in the Viburnum Trend, Southeast Missouri

ety of Economic Paleontologists and Mineralogists, C

ates and Evaporites, v. 4, no. 2, p 195–209.

Wisniowiecki, M.J., Van der Voo, R., McCabe, C., and 

W.C., 1983, A Pennsylvanian paleomagnetic pole from m

alized Late Cambrian Bonneterre Formation, southeas

souri: Journal of Geophysical Research, v. 88, p. 6540–

Zartman, R.E., 1977, Geochronology of some alkalic 

provinces in eastern and central United States: Annual R

of Earth and Planetary Science, v. 5, p. 257–286.
26 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

Ziegler, P.A., 1988, Evolution of the Arctic-North Atlantic and the

western Tethys: American Association of Petroleum Geologists Memoir 43, 198 p.

Zimmerman, R.K., and Kesler, S.E., 1981, Fluid inclusi

for solution mixing, Sweetwater (Mississippi V

district, Tennessee: Economic Geology, v. 76, p. 1
APPENDIX OF SUPPLEMENTAL DATA 

APPENDIX OF SUPPLEMENTAL DATA

Original depths were measured in feet; feet are thus given here for completeness and were not converted to m

(conversion factor to meters: feet (ft) × 0.3048=meters (m)). Leaders (---), no data.

Zone Th(v) Tm(ice) Depth

(ft)

Formation/

Group

Sec. T. R.

Sample ARK-DW; location Arkansas

--- --- --- 3,000 Knox/Arbuckle 14 12N 9E

--- --- --- 3,300 Knox/Arbuckle 14 12N 9E

IV 147.4 --- 3,540 Knox/Arbuckle 14 12N 9E

IV 210 -19.4 3,540 Knox/Arbuckle 14 12N 9E

IV --- -11.1 3,540 Knox/Arbuckle 14 12N 9E

IV 143 -11.2 3,540 Knox/Arbuckle 14 12N 9E

--- --- --- 3,660 Knox/Arbuckle 14 12N 9E

--- --- --- 3,960 Knox/Arbuckle 14 12N 9E

--- --- --- 4,140 Knox/Arbuckle 14 12N 9E

--- --- --- 4,820 Knox/Arbuckle 14 12N 9E

--- --- --- 5,040 Knox/Arbuckle 14 12N 9E

IV --- --- 5,220 Knox/Arbuckle 14 12N 9E

--- --- --- 5,400 Knox/Arbuckle 14 12N 9E

--- --- --- 5,460 Knox/Arbuckle 14 12N 9E

--- --- --- 5,520 Knox/Arbuckle 14 12N 9E

--- --- --- 5,760 Knox/Arbuckle 14 12N 9E

IV --- --- 6,180 Knox/Arbuckle 14 12N 9E

IV --- --- 6,480 Knox/Arbuckle 14 12N 9E

IV --- --- 6,600 Knox/Arbuckle 14 12N 9E

--- --- --- 6,840 Knox/Arbuckle 14 12N 9E

IV --- --- 7,020 Knox/Arbuckle 14 12N 9E

--- --- --- 7,140 Knox/Arbuckle 14 12N 9E

IV 187 -7.9 7,680 Knox/Arbuckle 14 12N 9E

IV --- -7.3 7,680 Knox/Arbuckle 14 12N 9E

--- --- --- 7,980 Knox/Arbuckle 14 12N 9E

--- --- --- 8,520 Knox/Arbuckle 14 12N 9E

--- --- --- 8,640 Knox/Arbuckle 14 12N 9E

--- --- --- 9,180 Knox/Arbuckle 14 12N 9E IV 137.9 -4.9 9,240 Knox/Arbuckle 14 12N 9E

IV 137.2 --- 9,240 Knox/Arbuckle 14 12N 9E

IV 178.8 --- 9,240 Knox/Arbuckle 14 12N 9E

IV 173.9 --- 9,240 Knox/Arbuckle 14 12N 9E

IV 176.7 --- 9,240 Knox/Arbuckle 14 12N 9E
28 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

IV 176.7 --- 9,240 Knox/Arbuckle 14 12N 9E

IV 180.8 --- 9,240 Knox/Arbuckle 14 12N 9E

IV 192 --- 9,240 Knox/Arbuckle 14 12N 9E

--- 174 --- 9,420 Knox/Arbuckle 14 12N 9E

--- 194.2 -22.8 9,420 Knox/Arbuckle 14 12N 9E

--- 174 --- 9,420 Knox/Arbuckle 14 12N 9E

--- 174 --- 9,420 Knox/Arbuckle 14 12N 9E

--- 180.5 --- 9,420 Knox/Arbuckle 14 12N 9E

IV 178 -21.7 9,540 Knox/Arbuckle 14 12N 9E

IV 165 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 201 -21.6 9,540 Knox/Arbuckle 14 12N 9E

IV --- -19.7 9,540 Knox/Arbuckle 14 12N 9E

IV 142 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 178 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 179 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 187.8 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 198.5 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 157.5 -21.2 9,540 Knox/Arbuckle 14 12N 9E

IV --- -20.6 9,540 Knox/Arbuckle 14 12N 9E

IV 157.5 --- 9,540 Knox/Arbuckle 14 12N 9E

IV 186 --- 9,540 Knox/Arbuckle 14 12N 9E

IV --- --- 9,600 Knox/Arbuckle 14 12N 9E

IV --- --- 9,780 Knox/Arbuckle 14 12N 9E

IV --- --- 10,140 Knox/Arbuckle 14 12N 9E

IV --- --- 10,380 Knox/Arbuckle 14 12N 9E

IV --- --- 10,560 Knox/Arbuckle 14 12N 9E

--- --- --- 10,680 Elvins Group 14 12N 9E

IV --- --- 11,040 Elvins Group 14 12N 9E

IV --- --- 11,340 Elvins Group 14 12N 9E

III 256 -18.5 11,880 Bonneterre 14 12N 9E

III 274.9 -17.9 11,880 Bonneterre 14 12N 9E

III 250.7 -17.7 11,880 Bonneterre 14 12N 9E

III 263.3 --- 11,880 Bonneterre 14 12N 9E

III --- -17 11,880 Bonneterre 14 12N 9E

III 169.7 --- 11,880 Bonneterre 14 12N 9E

III 193 --- 11,880 Bonneterre 14 12N 9E

III 193 --- 11,880 Bonneterre 14 12N 9E

III 206.5 --- 11,880 Bonneterre 14 12N 9E

III 221.8 --- 11,880 Bonneterre 14 12N 9E

III 227.5 --- 11,880 Bonneterre 14 12N 9E

III 230.2 --- 11,880 Bonneterre 14 12N 9E

III 238.2 --- 11,880 Bonneterre 14 12N 9E

III 243.4 --- 11,880 Bonneterre 14 12N 9E
APPENDIX OF SUPPLEMENTAL DATA 

III 247 --- 11,880 Bonneterre 14 12N 9E

III 247.4 --- 11,880 Bonneterre 14 12N 9E

III --- -22.1 11,880 Bonneterre 14 12N 9E

III 277 -15.6 11,880 Bonneterre 14 12N 9E

III 250 --- 11,880 Bonneterre 14 12N 9E

III 255.5 -15.4 11,880 Bonneterre 14 12N 9E

III 247.6 -11.4 11,880 Bonneterre 14 12N 9E

III --- -12.3 11,880 Bonneterre 14 12N 9E

III 197 -15.2 11,880 Bonneterre 14 12N 9E

III 152 --- 11,880 Bonneterre 14 12N 9E

III 194 --- 11,880 Bonneterre 14 12N 9E

III 250 -15.9 11,880 Bonneterre 14 12N 9E

III 255 --- 11,880 Bonneterre 14 12N 9E

III 194.5 -23 11,880 Bonneterre 14 12N 9E

III 220.7 --- 11,880 Bonneterre 14 12N 9E

III 188.8 --- 11,880 Bonneterre 14 12N 9E

III 184.4 --- 11,880 Bonneterre 14 12N 9E

III 185.3 --- 11,880 Bonneterre 14 12N 9E

III 187.2 --- 11,880 Bonneterre 14 12N 9E

III 189.8 --- 11,880 Bonneterre 14 12N 9E

III 165.4 -25.9 11,880 Bonneterre 14 12N 9E

III 168.7 -29.5 11,880 Bonneterre 14 12N 9E

III --- -28 11,880 Bonneterre 14 12N 9E

III 183.6 --- 11,880 Bonneterre 14 12N 9E

III 184 --- 11,880 Bonneterre 14 12N 9E

III 195.4 --- 11,880 Bonneterre 14 12N 9E

III 245 -16.1 12,000 Bonneterre 14 12N 9E

III 150 -17.7 12,000 Bonneterre 14 12N 9E

III --- -13.9 12,000 Bonneterre 14 12N 9E

III --- -13.9 12,000 Bonneterre 14 12N 9E

III --- -17.7 12,000 Bonneterre 14 12N 9E

III --- -16.1 12,000 Bonneterre 14 12N 9E

III 242.2 --- 12,000 Bonneterre 14 12N 9E

III 263 -14.9 12,000 Bonneterre 14 12N 9E

III 247.5 -15.5 12,000 Bonneterre 14 12N 9E

III 260 --- 12,000 Bonneterre 14 12N 9E

III --- -18.8 12,000 Bonneterre 14 12N 9E

III 229.7 --- 12,000 Bonneterre 14 12N 9E

III 263 --- 12,000 Bonneterre 14 12N 9E

III 259 --- 12,000 Bonneterre 14 12N 9E

III 260.5 -9.9 12,000 Bonneterre 14 12N 9E

III 259 --- 12,000 Bonneterre 14 12N 9E

III 258.4 -6 12,000 Bonneterre 14 12N 9E

III 259 --- 12,000 Bonneterre 14 12N 9E

III 258.4 -6 12,000 Bonneterre 14 12N 9E
30 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

III 229.6 --- 12,000 Bonneterre 14 12N 9

III 239.9 -10.1 12,000 Bonneterre 14 12N 9

IV --- --- 12,180 Bonneterre 14 12N 9

Sample NMTW; location Missouri

IV 129 -23 2,055 Bonneterre (?) 32 21N 14

IV 136 -20.8 2,055 Bonneterre (?) 32 21N 14

IV 138 -17.2 2,055 Bonneterre (?) 32 21N 14

IV 176 -18 2,055 Bonneterre (?) 32 21N 14

IV 207.5 --- 2,055 Bonneterre (?) 32 21N 14

IV 183.2 --- 2,055 Bonneterre (?) 32 21N 14

IV 213 --- 2,055 Bonneterre (?) 32 21N 14

IV 216.5 --- 2,055 Bonneterre (?) 32 21N 14

IV --- -15.3 2,055 Bonneterre (?) 32 21N 14

IV 206 --- 2,055 Bonneterre (?) 32 21N 14

IV 207 --- 2,055 Bonneterre (?) 32 21N 14

IV --- -18.9 2,055 Bonneterre (?) 32 21N 14

IV 147.1 -17.1 2,055 Bonneterre (?) 32 21N 14

IV --- -17.2 2,055 Bonneterre (?) 32 21N 14

IV 191 --- 2,055 Bonneterre (?) 32 21N 14

IV 218.5 --- 2,055 Bonneterre (?) 32 21N 14

IV 236.6 --- 2,055 Bonneterre (?) 32 21N 14

IV 232.2 --- 2,055 Bonneterre (?) 32 21N 14

IV 205.9 -15.1 2,055 Bonneterre (?) 32 21N 14

IV --- -18.4 2,055 Bonneterre (?) 32 21N 14

IV 227 -18.1 2,055 Bonneterre (?) 32 21N 14

IV --- -17.2 2,055 Bonneterre (?) 32 21N 14

IV --- -24.3 2,055 Bonneterre (?) 32 21N 14

IV 248.5 -15.9 2,055 Bonneterre (?) 32 21N 14

IV 220.5 -15.3 2,055 Bonneterre (?) 32 21N 14

IV 234.5 --- 2,055 Bonneterre (?) 32 21N 14

IV 227.5 --- 2,055 Bonneterre (?) 32 21N 14

IV 128.1 -25 2,055 Bonneterre (?) 32 21N 14

IV 159.6 --- 2,055 Bonneterre (?) 32 21N 14

IV 160 -24.7 2,055 Bonneterre (?) 32 21N 14

IV 172.5 --- 2,055 Bonneterre (?) 32 21N 14

IV 164 --- 2,300 Bonneterre (?) 32 21N 14

IV 151 --- 2,300 Bonneterre (?) 32 21N 14

IV 218 -20.4 2,300 Bonneterre (?) 32 21N 14

IV 175 --- 2,300 Bonneterre (?) 32 21N 14

IV 169.4 --- 2,300 Bonneterre (?) 32 21N 14

IV 163 --- 2,300 Bonneterre (?) 32 21N 14

IV 169 --- 2,300 Bonneterre (?) 32 21N 14
APPENDIX OF SUPPLEMENTAL DATA 

IV 200.7 -21.8 2,300 Bonneterre (?) 32 21N 14E

IV 197.2 --- 2,300 Bonneterre (?) 32 21N 14E

IV 219.3 --- 2,300 Bonneterre (?) 32 21N 14E

IV 224 --- 2,300 Bonneterre (?) 32 21N 14E

IV 225 --- 2,300 Bonneterre (?) 32 21N 14E

Sample JR4-18-89; location Imboden, Ark.

IV --- --- 100 Powell 2 17N 2W

IV 101.2 -16.6 100 Powell 2 17N 2W

IV 113.2 -18.9 100 Powell 2 17N 2W

IV 131.5 --- 100 Powell 2 17N 2W

IV 129.5 -21.2 100 Powell 2 17N 2W

IV 117 -21 100 Powell 2 17N 2W

IV 117 -18.6 100 Powell 2 17N 2W

IV 118 --- 100 Powell 2 17N 2W

IV 127.1 --- 100 Powell 2 17N 2W

IV 131.9 --- 100 Powell 2 17N 2W

IV --- -20.2 100 Powell 2 17N 2W

IV 126.6 --- 100 Powell 2 17N 2W

IV 124.1 --- 100 Powell 2 17N 2W

IV --- -21.6 100 Powell 2 17N 2W

IV 166.5 -22.3 100 Powell 2 17N 2W

IV 107.8 -19.3 100 Powell 2 17N 2W

IV 112.5 -15.5 100 Powell 2 17N 2W

IV 110 -15.1 100 Powell 2 17N 2W

IV 113.5 -22.9 100 Powell 2 17N 2W

IV 115.5 -16.6 100 Powell 2 17N 2W

IV 115.5 -22.4 100 Powell 2 17N 2W

IV 124 -22.3 100 Powell 2 17N 2W

IV 123.5 --- 100 Powell 2 17N 2W

IV 124 --- 100 Powell 2 17N 2W

IV --- --- 100 Powell 2 17N 2W

IV --- -17.1 100 Powell 2 17N 2W

--- --- -23.7 100 Powell 2 17N 2W

--- --- -23 100 Powell 2 17N 2W

--- --- -17.2 100 Powell 2 17N 2W

--- --- -19.2 100 Powell 2 17N 2W

--- 128.7 -21.8 100 Powell 2 17N 2W

--- 120 --- 100 Powell 2 17N 2W

--- 119.8 --- 100 Powell 2 17N 2W

--- 114.6 --- 100 Powell 2 17N 2W

--- --- -16.1 100 Powell 2 17N 2W

--- 133.6 -24 100 Powell 2 17N 2W
32 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

--- 121.4 -16.4 100 Powell 2 17N 2

--- 142 -23.4 100 Powell 2 17N 2

--- 112.5 --- 100 Powell 2 17N 2

--- 112.5 --- 100 Powell 2 17N 2

IV --- --- 100 Powell 2 17N 2

IV 185 -21.3 100 Powell 2 17N 2

IV --- -21.5 100 Powell 2 17N 2

IV 118.5 -21.2 100 Powell 2 17N 2

IV --- -20.5 100 Powell 2 17N 2

IV 112.4 -17.6 100 Powell 2 17N 2

IV 107.7 -20 100 Powell 2 17N 2

IV --- -20.3 100 Powell 2 17N 2

IV 110.5 -16.8 100 Powell 2 17N 2

IV 107.5 -21.2 100 Powell 2 17N 2

IV --- -24.4 100 Powell 2 17N 2

IV --- -21.3 100 Powell 2 17N 2

IV 130.7 -22.5 100 Powell 2 17N 2

IV 122.5 -22.3 100 Powell 2 17N 2

IV 120 -16 100 Powell 2 17N 2

IV 120 -19.9 100 Powell 2 17N 2

IV 113 -16 100 Powell 2 17N 2

IV 124 -18.1 100 Powell 2 17N 2

IV 144 -22.2 100 Powell 2 17N 2

IV 151 -22.1 100 Powell 2 17N 2

IV --- -19.9 100 Powell 2 17N 2

IV --- -17.1 100 Powell 2 17N 2

IV 121 --- 100 Powell 2 17N 2

IV --- --- 100 Powell 2 17N 2

IV 185 -21 100 Powell 2 17N 2

IV --- -21.3 100 Powell 2 17N 2

IV 114.4 --- 100 Powell 2 17N 2

IV 135.8 -21.1 100 Powell 2 17N 2

IV 120.5 -18.3 100 Powell 2 17N 2

IV 110.9 -21.6 100 Powell 2 17N 2

IV 134 -22 100 Powell 2 17N 2

IV 131.2 -22.7 100 Powell 2 17N 2

IV 114.6 --- 100 Powell 2 17N 2

IV 112.3 --- 100 Powell 2 17N 2

IV 113 -23 100 Powell 2 17N 2

Sample JR4-19-89; location Imboden, Ark.

IV --- --- 100 Powell 17N 2
APPENDIX OF SUPPLEMENTAL DATA 

Sample JR5-22-89; location Imboden, Ark.

IV --- --- 100 Powell 2 17N 2W

IV --- --- 100 Powell 2 17N 2W

IV 137.3 22.4 100 Powell 2 17N 2W

IV 126.8 21.5 100 Powell 2 17N 2W

IV --- --- 100 Powell 2 17N 2W

IV 135.5 -22.6 100 Powell 2 17N 2W

IV --- -22.6 100 Powell 2 17N 2W

IV 126.3 -22.1 100 Powell 2 17N 2W

IV 126.3 -21.9 100 Powell 2 17N 2W

IV 113.2 -20.4 100 Powell 2 17N 2W

Sample PB-7; location Missouri

IV 132.7 -23.1 1,137 Derby/Doe Run 3 24N 5E

III 143.4 -17.1 1,659 Derby/Doe Run 3 24N 5E

III --- -23.5 1,659 Derby/Doe Run 3 24N 5E

IV --- -24.2 1,659 Derby/Doe Run 3 24N 5E

III 103.4 -24.9 1,659 Derby/Doe Run 3 24N 5E

III --- -20.9 1,659 Derby/Doe Run 3 24N 5E

III 97.3 -24.2 1,659 Derby/Doe Run 3 24N 5E

III 104.2 -22.3 1,659 Derby/Doe Run 3 24N 5E

IV --- -19.8 1,659 Derby/Doe Run 3 24N 5E

--- 109.7 --- 1,659 Derby/Doe Run 3 24N 5E

IV 113.5 -18.3 1,678 Derby/Doe Run 3 24N 5E

III or IV -9.7 --- 1,678 Derby/Doe Run 3 24N 5E

I 137.7 -18.1 1,678 Derby/Doe Run 3 24N 5E

IV 148.8 --- 1,678 Derby/Doe Run 3 24N 5E

IV 139.2 --- 1,678 Derby/Doe Run 3 24N 5E

III -20.5 1,678 Derby/Doe Run 3 24N 5E

III or IV -11.3 --- 1,678 Derby/Doe Run 3 24N 5E

I 143.3 --- 1,678 Derby/Doe Run 3 24N 5E

--- --- --- 2,190 Bonneterre 3 24N 5E

IV --- -22.3 2,202 Bonneterre 3 24N 5E

IV 127.5 -23.5 2,202 Bonneterre 3 24N 5E

IV --- -13 2,202 Bonneterre 3 24N 5E

IV 125 -10.5 2,202 Bonneterre 3 24N 5E

IV 120.1 -10.2 2,202 Bonneterre 3 24N 5E

Sample P-1; location Missouri

--- 115.6 -20.1 1,691 Post-Bonneterre 30 31N 10E

IV 102.3 -13.5 1,729 Post-Bonneterre 30 31N 10E
34 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

IV --- -21.8 1,729 Post-Bonneterre 30 31N 10

--- 114 --- 1,729 Post-Bonneterre 30 31N 10

--- 96.3 --- 1,755 Post-Bonneterre 30 31N 10

--- --- -17.9 1,755 Post-Bonneterre 30 31N 10

--- 119.8 --- 1,755 Post-Bonneterre 30 31N 10

--- --- -16.5 1,755 Post-Bonneterre 30 31N 10

--- 86.5 -21.9 1,755 Post-Bonneterre 30 31N 10

--- 125.7 --- 1,755 Post-Bonneterre 30 31N 10

--- --- -15 1,755 Post-Bonneterre 30 31N 10

--- --- -14.4 1,755 Post-Bonneterre 30 31N 10

--- --- -17.5 1,755 Post-Bonneterre 30 31N 10

--- 97.1 -21.1 1,755 Post-Bonneterre 30 31N 10

--- 58 -20.4 1,755 Post-Bonneterre 30 31N 10

--- --- -12.7 1,755 Post-Bonneterre 30 31N 10

--- 71 -22.5 1,755 Post-Bonneterre 30 31N 10

--- 73.2 -22.5 1,755 Post-Bonneterre 30 31N 10

--- --- -21.8 1,755 Post-Bonneterre 30 31N 10

--- --- -18.2 1,755 Post-Bonneterre 30 31N 10

--- 100.7 -18.8 1,848 Post-Bonneterre 30 31N 10

--- --- -6.8 1,848 Post-Bonneterre 30 31N 10

--- 106.2 -22.6 1,848 Post-Bonneterre 30 31N 10

--- 99 -19.6 1,848 Post-Bonneterre 30 31N 10

--- 93.6 -22.2 1,848 Post-Bonneterre 30 31N 10

--- 92.6 --- 1,848 Post-Bonneterre 30 31N 10

III --- -20.8 2,074 Bonneterre 30 31N 10

III 123.8 -20.5 2,074 Bonneterre 30 31N 10

Sample GP-10; location Missouri

III --- -22.3 1,850 White Rock --- --- --

IV --- -15.3 1,850 White Rock --- --- --

IV or III --- -15.8 1,850 White Rock --- --- --

--- 126.2 -19.6 2,237 Basinal limestone --- --- --

--- --- -11.3 2,237 Basinal limestone --- --- --

--- --- -12.2 2,237 Basinal limestone --- --- --

--- --- --- 2,384 Basinal limestone --- --- --

Sample PBW-3; location Missouri

--- 115.5 -1.5 1,710 Derby(?) --- --- --

--- 114.3 -0.3 1,710 Derby(?) --- --- --

--- 99.8 --- 1,710 Derby(?) --- --- --

--- --- -12.3 1,710 Derby(?) --- --- --

--- 96.3 --- 1,710 Derby(?) --- --- --
APPENDIX OF SUPPLEMENTAL DATA 

--- --- --- 1,821 Davis(?) --- --- ---

--- --- --- 2,223 Davis(?) --- --- ---

--- --- --- 2,279 Davis(?) --- --- ---

--- 110 -22.2 2,284 Davis(?) --- --- ---

--- 110 -22.2 2,284 Davis(?) --- --- ---

--- 123 -22.6 2,284 Davis(?) --- --- ---

Sample DP-4; location Tennessee

IV --- -19.2 5,192 Knox 15 6S 19E

IV --- -18.4 5,192 Knox 15 6S 19E

IV --- -20.1 5,192 Knox 15 6S 19E

IV 109.3 --- 5,648 Conasauga 15 6S 19E

IV --- -21.5 5,648 Conasauga 15 6S 19E

IV 119.8 --- 5,648 Conasauga 15 6S 19E

III --- -19.2 5,648 Conasauga 15 6S 19E

IV 101.8 -14.5 5,648 Conasauga 15 6S 19E

III --- -19.6 5,648 Conasauga 15 6S 19E

IV 118.7 --- 5,648 Conasauga 15 6S 19E

III 114.4 --- 5,648 Conasauga 15 6S 19E

IV --- -15.1 5,661 Conasauga 15 6S 19E

IV --- -17.7 5,661 Conasauga 15 6S 19E

IV --- -15.6 5,661 Conasauga 15 6S 19E

IV 115.8 -16.2 5,661 Conasauga 15 6S 19E

IV 114.3 -22.9 5,661 Conasauga 15 6S 19E

IV --- -21 5,661 Conasauga 15 6S 19E

I --- -19.1 5,661 Conasauga 15 6S 19E

IV 106.6 --- 5,661 Conasauga 15 6S 19E

III 111.3 -18.5 5,661 Conasauga 15 6S 19E

III --- -20.1 5,661 Conasauga 15 6S 19E

III --- -21.6 5,661 Conasauga 15 6S 19E

IV --- -19 5,661 Conasauga 15 6S 19E

III 110.5 --- 5,661 Conasauga 15 6S 19E

IV 121.2 --- 5,661 Conasauga 15 6S 19E

IV 128.1 --- 5,661 Conasauga 15 6S 19E

III --- -18.9 5,661 Conasauga 15 6S 19E

III 106 -19.2 5,661 Conasauga 15 6S 19E

IV 136.7 --- 5,661 Conasauga 15 6S 19E

IV 135.9 --- 5,661 Conasauga 15 6S 19E

IV 143 -23.1 5,661 Conasauga 15 6S 19E

IV 111.6 --- 5,661 Conasauga 15 6S 19E

I 105.9 --- 5,661 Conasauga 15 6S 19E

IV 134.5 -22.4 5,663 Conasauga 15 6S 19E

IV --- -17.5 5,663 Conasauga 15 6S 19E
36 HOT MISSISSIPPI VALLEY–TYPE BRINES, REELFOOT RIFT COMPLEX

III --- -20 5,663 Conasauga 15 6S 19

III --- -16.9 5,663 Conasauga 15 6S 19

III --- -17.9 5,663 Conasauga 15 6S 19

IV --- -22.7 5,663 Conasauga 15 6S 19

IV 114.9 -22.3 5,663 Conasauga 15 6S 19

III or I --- -21.9 5,663 Conasauga 15 6S 19

III --- -21.5 5,663 Conasauga 15 6S 19

III --- -21.6 5,663 Conasauga 15 6S 19

III or IV --- -21.6 5,663 Conasauga 15 6S 19

III 129.8 --- 5,663 Conasauga 15 6S 19

III --- -18.5 5,663 Conasauga 15 6S 19

IV 119.2 -19.3 5,663 Conasauga 15 6S 19

IV 109.2 --- 5,663 Conasauga 15 6S 19

IV 129.6 --- 5,663 Conasauga 15 6S 19

IV 134.8 --- 5,663 Conasauga 15 6S 19

Sample DP-2; location Tennessee

--- --- --- 4,420.7 Knox 14 6S 19

--- --- --- 4,996 Knox 14 6S 19

--- --- --- 6,087 Conasauga 14 6S 19

--- --- --- 6,377 Conasauga 14 6S 19

--- --- --- 6,380 Conasauga 14 6S 19

Manuscript approved for publication March 3, 1997

Published in the Central Region, Denver, Colorado

Graphics and photocomposition by Gayle M. Dumonceaux

Edited by Lorna Carter

On-line version by Joan G. Nadeau