Feldspar Diagenesis in Cambrian Clastic Rocks of the 

Southern Ozark Mountains and Reelfoot Rift, 

Southeastern Missouri and Northeastern Arkansas 

Implications for Mississippi Valley-Type Ore Genesis

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Chapter F

Feldspar Diagenesis in Cambrian Clastic Rocks of the 

Southern Ozark Mountains and Reelfoot Rift, 

Southeastern Missouri and Northeastern Arkansas 

Implications for Mississippi Valley-Type Ore Genesis

By S.F. DIEHL and M.B. GOLDHABER

U.S. GEOLOGICAL SURVEY BULLETIN 1989

STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, 

UNITED STATES

WARREN C. DAY AND DIANE E. LANE, Editors
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

Published in the Central Region, Denver, Colo. 

Manuscript approved for publication March 11,1994. 

Photocomposition by Gayle M. Dumonceaux.

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.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1995

For sale by

U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Diehl, S. F.

Feldspar diagenesis in Cambrian clastic rocks of the southern Ozark Mountains 

and Reelfoot Rift, southeastern Missouri and northeastern Arkansas: Implications 

for Mississippi Valley-type ore genesis / by S.F. Diehl and M.B. Goldhaber.

p. cm. (U.S. Geological Survey bulletin: 1989) 

(Strategic and critical minerals in the midcontinent region, United States; ch. F)

Includes bibliographical references.

Supt. of Docs, no.: 119.3:1989F

1. Diagenesis Missouri. 2. Feldspar Missouri. 3. Geology, 

Stratigraphic Cambrian. 4. Lamotte Sandstone (Mo.) I. Goldhaber, Martin 

B. II. Title. III. Series. IV. Series: Strategic and critical minerals in the 

midcontinent region, United States: ch. F. 

QE75.B9 no. 1989-F 

[QE571]

557.3s dc20 94-12432 

[552'.03] CIP
CONTENTS

Abstract Fl

Introduction Fl

Regional structure F2

Description of the Lamotte Sandstone and time-equivalent rocks in the Reelfoot

Rift F3

Methods and materials F3

Petrographic descriptions of samples from core fence A-A' F4 

Reelfoot Rift F4

Dow Chemical No. 1 Wilson drill hole F5 

Dow Chemical No. 1 Garrigan drill hole F5 

Strake Petroleum No. 1 Russell drill hole F6 

Drill hole BT1 F7

Indian Creek Subdistrict Goose Creek Mine F7 

Petrographic descriptions of samples from core fence B-B' F9 

Drill hole 63W72 F9 

Drill hole 63W5 F9

Results of petrographic studies of core from other drill holes F10 

Discussion F10

Timing of feldspar precipitation relative to Mississippi Valley-type ore

genesis Fll 

Contrasts between Reelfoot Rift and platform settings hi feldspar

diagenesis F13 

Formation and dissolution of potassium hi the Lamotte Sandstone outside the

Reelfoot Rift F14 

Additional evidence for a link between the Reelfoot Rift and lead-belt

mineralization F15

The anomalous nature of the Strake well samples F15 

Summary F15 

References cited F16

FIGURES

Fl. Map showing location of drill holes, major structural terrains, and ore districts

in the study area F2 

F2-F6. Photomicrographs showing:

F2. Deuital microcline grain and partially dissolved overgrowth, Dow Chemical No. 1 Wilson drill hole F5

F3. Plagioclase grain and albite overgrowths, and authigenic albite enclosing 

remnants of carbonate minerals, Dow Chemical No. 1 Garrigan drill 

hole F6

F4. Inclusion-rich authigenic potassium feldspar overgrowths on deuital feldspar grains, Strake Petroleum No. 1 Russell drill hole F6

F5. Etched rhombic potassium feldspar overgrowths on a hematitic feldspar 

grain and an orthoclase grain and its overgrowth, drill hole BT1 F7

F6. Galena cement that partially encloses a detrital potassium feldspar grain 

and its overgrowth, and galena cement that invaded dissolution voids hi a 

partially dissolved potassium feldspar grain, Goose Creek Mine F8 

F7. Scanning-electron photomicrographs of samples from Goose Creek Mine, in

Contents III
Indian Geek subdistrict F8 

F8. Diagram showing mineral paragenetic sequence in Indian Creek

subdistrict F9 

F9. Photomicrograph showing two generations of potassium feldspar overgrowths

on a microcline grain F9 

F10. Photomicrograph of a detrital feldspar grain and two generations of potassium

feldspar overgrowths that have undergone dissolution F10 

Fl 1. Map showing abundance of potassium feldspar cement and the amount of

dissolution of the cement plotted at the drill hole locations Fl 1 

F12. Photomicrograph showing that different generations of potassium feldspar overgrowths may be differentially affected by dissolution F12 

Fl 3. Plot of point-count data for albite and potassium feldspar from shallowly buried

Lamotte Sandstone and deeply buried siliciclastic rocks versus depth F13 

F14. Potassium feldspar stability diagram at 150°C. F14

TABLES

Fl. Percent of bulk rock constituents in selected thin sections F4

F2. Estimation of composition of ore-forming fluid hi the study area F14

IV Contents
STRATEGIC AND CRITICAL MINERALS IN THE MIDCONTINENT REGION, UNITED STATES

Feldspar Diagenesis in Cambrian Clastic Rocks of the 

Southern Ozark Mountains and Reelfoot Rift, 

Southeastern Missouri and Northeastern Arkansas 

Implications for Mississippi Valley-Type Ore Genesis

ByS.F. Diehl and M.B. Goldhaber

Abstract

The Upper Cambrian Lamotte Sandstone is considered to have been a major aquifer for warm basinal brines 

that formed the Mississippi Valley-type (MVT) mineral 

deposits in Missouri. Petrographic studies of authigenic 

potassium feldspar in the Lamotte Sandstone constrain the 

geochemistry of the ore-forming fluids. Authigenic potassium feldspar is important because (1) it has been dated as 

late Paleozoic, which broadly coincides with the inferred 

time of ore genesis, (2) previous petrographic studies constrain the relative time authigenic feldspar dissolved with 

respect to deposition of the lead-zinc ore, and (3) MVT 

ore fluids are typically anomalously rich in K+.

Published petrographic studies of Lamotte-hosted 

lead-zinc mineralization in southeastern Missouri demonstrate that the first episode of authigenic potassium feldspar formation predated mineralization and that dissolution 

of the overgrowths accompanied ore formation. Our data 

show that this ore-stage dissolution event was regionally 

extensive and was most intense along a northwest-trending 

corridor that includes the lead belts and the St. Francois 

Mountains. Fluid inclusion data from southeastern Missouri show very high K+ concentrations during ore deposition. Potassium feldspar would be stable in a high K+ 

environment; however, our data suggest that the ore fluid 

had a low pH (acidic), which dissolved potassium feldspar 

overgrowths and carried metals and H2S in solution. The 

broad zone of dissolved potassium feldspar in southeastern 

Missouri suggests that this acidic fluid originated hi the 

Reelfoot Rift.

Although dissolution of potassium feldspar is regional 

throughout southeastern Missouri, alteration products of

potassium feldspar are significantly different in shallowly 

buried Lamotte Sandstone than in time-equivalent deeply 

buried sedimentary rocks in the Reelfoot Rift. Potassium 

feldspar grains in the Reelfoot Rift samples show increasingly greater alteration to albite with depth, and albitization 

is an important process that enriches pore fluids in K+.

INTRODUCTION

The Upper Cambrian Lamotte Sandstone is of interest as a major aquifer that transported ore-bearing fluids 

for the Mississippi Valley-type (MVT) deposits of the 

southern midcontinent of the United States. We undertook 

a regional study of the Lamotte Sandstone to characterize 

diagenetic alterations that could possibly be related to the 

passage of the ore-forming fluids. The study was conducted as part of the U.S. Geological Survey (USGS) 

Midcontinent Strategic and Critical Minerals Project. Of 

the diagenetic phases recognized hi the Lamotte Sandstone, potassium feldspar offered potentially critical information about the fluids and their age (Rothbard, 1983; 

Gutierrez, 1987). Authigenic potassium feldspar hi the 

study area (Hearn and others, 1986) yielded late Paleozoic 

K-Ar radiometric ages, which are similar to paleomagnetic 

ages yielded by magnetite (Wu and Beales, 1981; Wisniowiecki and others, 1983). These late Paleozoic ages are 

considered to represent the time of mineralization, and 

thus they suggest a general link in time between potassium 

feldspar precipitation and ore formation. Furthermore, 

brines that formed MVT deposits are known to be anomalously rich in K+ compared to typical upper crustal (that is 

oil-field) brines (Manor, 1979). Even among MVT brines, 

the fluids of the main-stage mineralization of the deposits

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F1
in the southeastern Missouri lead belts are anomalously 

rich in this ion (Viets and Leach, 1990). Therefore, the ore 

fluids are good candidates for the source of K+ that 

favored authigenic potassium feldspar precipitation.

We studied the Lamotte Sandstone in a regional 

sense, paying particular attention to alteration of the 

framework minerals and diagenetic potassium feldspar. 

Initial phases of the work indicated that the distribution 

of potassium feldspar overgrowths and their regional pattern of dissolution were important in recognizing the flow 

path of ore fluids. In this paper we discuss in detail 

results determined from cores from two core fences (fig. 

Fl), which illustrate some key characteristics of alteration 

in the Lamotte Sandstone and basal clastic rocks in the

Reelfoot rift, and we summarize data for additional cores 

in surrounding drill holes in terms of regional trends.

Acknowledgments. We wish to thank George Breit, 

Donley Collins, and Paula Hansley of the U.S. Geological 

Survey for critical reviews. Elwin Mosier collected the 

Lamotte Sandstone samples from drill holes on the carbonate platform and Ozark uplift.

REGIONAL STRUCTURE

The Reelfoot rift (fig. Fl) is a major crustal structure that underlies the central part of the Mississippi 

Embayment syncline, approximately 150 km southeast of

FLUORSPAR

MINERAL

DISTRICT

: STRAKE PETROLEUM iXyK! : : -3 ' : -

: : : : -NO. 1 RUSSELL j

Tertiary and 

DOW CHEMICAL Cretaceous rocks

NO. 1

DOW CHEMICA

NO. 1 WILSON :

36° 

35° -

Figure Fl. Map showing location of drill holes, major structural terrains, and ore districts in the study area, southeastern 

Missouri and northeastern Arkansas.

F2 Strategic and Critical Minerals, Midcontinent Region, U.S.
the Missouri lead belts. The rift formed in the Late Proterozoic or Early Cambrian (Ervin and McGinnis, 1975) as 

a fault-bounded basin that has undergone several subsequent periods of extension (Diehl and others, 1992) and 

subsidence. Deposition of a thick section of Cambrian 

rocks (as much as 30,000 ft) along the middle of the rift 

(Howe and Thompson, 1984) is evidence that the rift was 

active during the Cambrian. Structurally, the northern part 

of the rift joins the Rough Creek Graben, the Ste. Genevieve fault system, and the Illinois Basin (fig. Fl). Because 

of the differences in lithologies and lack of drill hole data 

for the rift, the Cambrian-age carbonate rocks, shale, and 

siliciclastic rocks that fill the central part of the rift are not 

readily correlated with the Cambrian units on the adjacent 

carbonate platform (Collins and others, 1992).

Precambrian granite and volcanic rocks of the St. 

Francois Mountains are exposed in the central part of the 

Ozark uplift (fig. Fl). Unconformities within Paleozoic 

rocks in and around the St. Francois Mountains demonstrate that the Ozarks have been repeatedly uplifted and 

eroded throughout the Paleozoic Era. Clendenin (1989) 

and Palmer (1989) speculated that the St. Francois Mountains are part of the rift-border fault system or an uplifted 

rift shoulder behind the rift-border fault system.

DESCRIPTION OF THE LAMOTTE 

SANDSTONE AND TIME-EQUIVALENT 

ROCKS IN THE REELFOOT RIFT

The Upper Cambrian Lamotte Sandstone is the first 

sedimentary unit overlying Precambrian basement rock in 

the study area. It ranges from less than 100 ft in thickness 

where it pinches out against Precambrian knobby terrain 

in the Ozark Mountains, to several thousands of feet in 

midcontinent basins, particularly the Reelfoot Rift. The 

Lamotte Sandstone has been correlated with the Mount 

Simon Sandstone (Upper Cambrian) in the Illinois basin 

and with parts of the Reagan Sandstone (Upper Cambrian) 

in Kansas, Oklahoma, and Nebraska (Houseknecht and 

Ethridge, 1978).

The Lamotte Sandstone varies considerably hi 

lithology. The base of the Lamotte Sandstone is commonly conglomeratic where it overlies Precambrian basement in pinchouts. The lower part of the Lamotte 

Sandstone is a continental red-bed deposit (or its bleached 

equivalent) in much of Missouri. Upsection, there is a 

transition from a terrestrial depositional environment, 

which is recorded by a red feldspathic arenite, to a transgressive marine unit, which is commonly a white quartz 

arenite. From the quartz arenite, the Lamotte grades conformably upward through a glauconitic transition zone into 

the overlying Bonneterre Formation (Upper Cambrian). In 

outcrop, the Lamotte Sandstone is commonly thinly 

layered, crossbedded, and well sorted. The depositional

history and composition of the Lamotte Sandstone was 

described by Ojakangas (1963), Abraham (1978), Houseknecht and Ethridge (1978), and Yesburger (1982). Three 

lithologic types were recognized: feldspathic, lithic, and 

quartzitic (Houseknecht and Ethridge, 1978). Feldspathic 

units contain abundant potassium feldspar and plutonic 

rock fragments, lithic units contain a greater percentage of 

volcanic rock fragments, and quartzitic units are predominantly composed of monocrystalline quartz grains (Houseknecht and Ethridge, 1978). These lithologic types are 

commonly interbedded and represent parts of braided fluvial deposits in and around the St. Francois Mountains. 

These various lithologies are thought to represent detritus 

from different source areas (Houseknecht and Ethridge, 

1978), but we conclude from our study that the 

quartz-arenite composition of some units hi the Lamotte 

Sandstone may be the result of extensive dissolution of 

potassium feldspar.

The basal Cambrian rocks hi the Reelfoot Rift that 

may be time-equivalents of the Lamotte (Diehl and others, 

1992) consist of feldspathic graywacke, feldspathic arenite, and quartz arenite. These rocks are commonly crossbedded at thin-section scale. The shale overlying the 

graywacke and arenite may be time equivalent to the 

Bonneterre Formation and Elvins Group (Collins and 

others, 1992). Rock fragments of graphic-granite texture 

are common to sedimentary rocks both in the rift and in 

the Lamotte on the adjacent carbonate platform.

METHODS AND MATERIALS

Drill hole locations are shown in figure Fl. Core 

samples of the Lamotte Sandstone were obtained from 

cores housed at the Missouri Division of Land Survey. 

Drill cuttings of the Garrigan and Wilson wells were 

obtained from the Arkansas Geologic Commission. Seventy thin sections of the Lamotte Sandstone from 25 holes 

and 1 mine in southeastern Missouri and northeastern 

Arkansas were examined for feldspar content and dissolution effects. Generally, several depth intervals were 

sampled in each core. The results of these petrographic 

studies of selected thin sections from drill holes in the 

core fences are listed in table Fl. Petrographic data were 

determined with standard-size thin sections and a petrographic microscope. One-half of each thin section was 

stained with a solution of sodium cobaltinitrate for identification of potassium feldspar grains. The samples were 

impregnated with blue epoxy to determine porosity. 

Selected samples were examined with a Cambridge 250 

Mark 2 scanning electron microscope (SEM) equipped 

with an energy-dispersive X-ray (EDX). Sedimentary 

rocks were classified on the basis of the system of Pettijohn and others (1973). Modal analyses on core samples to 

determine bulk composition were made by 300

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F3
Table F1. Percent of bulk rock constituents in selected thin sections (percentage based on 300 points per thin section) of samples 

from core fence A-A' and B-B'.

[ , none observed]

Drill hole 

(depth, in ft)

Wilson

(12,523)

Wilson

(12,690)

Wilson

(13,930-

13,940)

Garrigan

(11.980-

12.040)

StrakeRussell

(4,717)

BT1

(2,092-

2,114)

Goose

Creek 2

Goose

Creek 3

63W5

(1,214-

1,267)

63W72

(1,288-

1,344)

Quartz

88

38

49

62

38

78.7

48.3

58.8

71

70.5

Feldspar

Potassium Plagio- Volcanic 

feldspar clase

1 3

35 55

27 7

7 1

37b

1.3 3

_ _ j 3

.3e - 7.7f

4 7

16.6h .6

Rock fragments

Igneous Other Carbonate Clay Voids Opaque 

cement matrix minerals

7 ... ... <l

9 1 ... 7

13 -- -- 2 2

8a 22

2 23

11.3 5.3 .3

39.3° 4 7d

21.3 5.9 6

11 138 .3

1.9 10.3

^iderite and dolomite.

Total of 10 percent overgrowths. 

cDolomite.

Galena and marcasite.

eOvergrowths on volcanic rock fragments. 

f Volcanic rock fragments containing sanidine.

gTotal of 8 percent dissolution voids in potassium feldspar overgrowths. 

Total of 4.5 percent overgrowths.

point-counts per thin section (table Fl). When only drill 

cuttings were available for thin sections, 100 point-counts 

per thin section were made, using several large chips of 

the same lithology. To estimate the amount of dissolution 

of potassium feldspar overgrowths, a small-scale grid was 

superimposed on 3-10 single, randomly selected feldspar 

grains that had overgrowths. Although dissolution of detrital feldspar grains was apparent, the dissolution effects on 

the overgrowths was the main topic of interest. Therefore, 

only the percentages of dissolution of the overgrowths 

were used to construct a map of dissolution areas. A Cold 

Cathode Luminescence scope Model 8200 MK 11 was 

used to examine detrital feldspar grains and their overgrowths in the Stroke Russell No. 1 hole.

PETROGRAPHIC DESCRIPTIONS OF 

SAMPLES FROM CORE FENCE A-A'

The line A-A' traces the abundance of potassium 

feldspar overgrowths and the amount of dissolution in drill 

hole samples of Cambrian siliciclastic rocks from the 

Reelfoot Rift northwestward onto the adjacent carbonate 

platform and then into the Ozark uplift (fig. Fl).

Reelfoot Rift

Samples of feldspathic arenite from three drill holes 

in the Reelfoot Rift were examined: the Dow Chemical 

Co. No. 1 Wilson, Dow Chemical Co. No. 1 Garrigan, and

F4 Strategic and Critical Minerals, Midcontinent Region, U.S.
Strake Petroleum No. 1 Russell (hereafter called the Wilson, Garrigan, and Strake holes) (fig. Fl). The upper part 

of the Cambrian siliciclastic deposits in the Wilson and 

Garrigan holes in the Reelfoot Rift are considered to be a 

time-equivalent of the Lamotte Sandstone (Diehl and others, 1992). The Strake hole, in the northwestern part of the 

rift near the shelf margin, bottomed in a feldspathic arenite 

that was identified as the Lamotte Sandstone (Grohskopf, 

1955).

Dow Chemical No. 1 Wilson Drill Hole

The Wilson hole, the deepest drill hole in the Reelfoot Rift, penetrates the Precambrian basement (total depth 

14,868 ft, or 4,531.8 m). The siliciclastic rocks penetrated 

by this drill hole consist of a basal red feldspathic arenite 

(12,630-14,300 ft, or 3,850-4,358.6 m) overlain by a gray 

feldspathic to quartz arenite (12,300-12,630 ft, or 

3,749-3,850 m). The latter is considered to be a 

time-equivalent of the Lamotte Sandstone (Diehl and others, 1992). However, previous studies have considered all 

of the basal siliciclastic deposits in the rift to be older than 

the Lamotte and have separated these rocks into distinct 

formations (Weaverling, 1987; Houseknecht, 1989).

Potassium feldspar in the red feldspathic arenite 

(12,630-14,300 ft, or 3,850-4,358.6 m) contains hematite 

inclusions, which impart a red coloration to the rock. The 

hematitic feldspar grains are commonly partially replaced 

by siderite and (or) illitic clay. Potassium feldspar is 

present as discrete framework grains and as part of igneous rock fragments, which commonly have a 

graphic-granite texture. Perthitic rock fragments are common near the basement contact, but many of these may be

Figure F2. Photomicrograph (crossed polars) of detrital 

microcline grain (DK) and partially dissolved overgrowth (KO; 

arrows). Other diagenetic products in photo are quartz overgrowths (QO) on detrital quartz grains (Q). Dow Chemical 

No. 1 Wilson drill hole, depth 13,426 ft (4,092.3 m) (location 

shown in fig. F1).

partially albitized potassium feldspar grains (for example, 

see Saigal and others, 1988). Framework grains consisting 

of illite and chlorite(?) appear to be altered volcanic rock 

fragments because they have remnant textures suggestive 

of phenocrysts.

In the gray feldspathic and quartz arenite samples 

(12,300-12,630 ft, or 3,749-3,850 m), the abundance of 

detrital potassium feldspar and plagioclase grains indicate 

an igneous source rock. In layers where detrital outlines 

are visible owing to iron-oxide dust rims, framework 

grains are generally well rounded. Potassium feldspar 

(mostly microcline) grains constitute between 2 and 25 

percent of these rocks.

Fossil-rich, feldspar-poor quartz arenite is interbedded with the feldspathic arenite (12,782 ft, or 3,896 m). 

The lack of feldspar in these units appears to be due to 

diagenetic alteration: feldspar is largely replaced by carbonate cement and clay minerals. Minor pyrite and galena 

cement were detected in these units.

Potassium feldspar overgrowths in both the red and 

gray arenite appear to be an early authigenic cement 

because the overgrowths are in contact with the edges of 

rounded detrital quartz grains. This suggests that authigenic potassium feldspar was the first pore-filling cement. 

However, earlier authigenic phases may have existed prior 

to the formation of the authigenic feldspar in the late Paleozoic, such as calcite, which is not present because of the 

long time interval. The potassium feldspar overgrowths 

constitute only 1 percent or less of the rock, are commonly twinned, and usually show partial dissolution (fig. 

F2). The black arrows in figure F2 point to potassium 

feldspar overgrowths that have ragged, partially dissolved 

edges on a microcline grain.

Importantly, throughout the red and gray arenite 

samples from the Wilson hole, authigenic albite occurs as 

overgrowths around plagioclase grains and as a replacement mineral of detrital potassium feldspar. Irregular 

staining by sodium cobaltinitrate and SEM semi-quantitative analysis show that the potassium feldspar in the Wilson hole has been partially replaced by albite. Table Fl 

shows a minor increase in bulk plagioclase content downhole, and this increase may reflect the albitization process.

Dow Chemical No. 1 Garrigan Drill Hole

The Garrigan hole (total depth 12,038 ft, or 3,669 

m) (fig. Fl) bottoms in a quartz and feldspathic arenite 

that may be a time-equivalent of the Lamotte Sandstone 

(Diehl and others, 1992). Detrital outlines are not visible 

on most of the framework grains. Because the pore spaces 

have been totally filled with authigenic cements, the 

quartz and feldspar grains have a deceptively angular 

appearance (fig. F3A). Microcline is the predominant 

detrital potassium feldspar. Feldspar grains and their

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F5
Figure F3. Photomicrographs (crossed polars) of sample from Dow Chemical No. 1 Garrigan drill hole.A Plagioclase grain (A) 

and albite overgrowths (AO) that fill the surrounding pore space, fi, Authigenic albite (AO) enclosing remnants of carbonate minerals (D). Q, quartz. Depth 11,980-12,040 (3,651.5-3,669.8 m) (location shown in fig. F1).

overgrowths are commonly partially replaced by illitic 

clay or carbonate minerals.

As in the Wilson hole, euhedral albite overgrowths 

are also common in the Garrigan samples as a late diagenetic cement, forming optically continuous overgrowths on 

plagioclase. The authigenic albite has inclusions of remnant carbonate minerals and therefore appears to have partially replaced earlier diagenetic carbonate cement (fig. 

F3B). The euhedral outlines of the albite affirm its authigenic nature.

Strake Petroleum No. 1 Russell Drill Hole

The Strake hole, which is near the northwestern 

shoulder of the rift in the area of the Pascola Arch, bottomed in the Lamotte Sandstone (Grohskopf, 1955) at 

4,740 ft (1,444.8 m) (fig. Fl). Staining revealed only 

potassium feldspar detrital grains and overgrowths; there 

are no plagioclase grains or diagenetic albite.

In contrast to the wells discussed above, at least two 

generations of potassium feldspar overgrowths are present 

in the Lamotte Sandstone at this location. The first and 

quantitatively predominant generation of potassium feldspar overgrowth is turbid because of abundant fluid inclusions (fig. F4). This contrasts markedly with observations 

of authigenic potassium feldspar elsewhere in the region. 

The second generation is clear, which is typical of potassium feldspar overgrowths in other drill holes. The 

first-generation potassium feldspar overgrowths constitute 

approximately 10 percent of the total volume of the rock 

at 4,717 ft (1,438 m) (table Fl). The sodium cobaltinitrate 

used to stain the feldspars colored both detrital cores and 

overgrowths the same dark yellow. Authigenic potassium 

feldspar has been documented to stain a darker yellow 

than do detrital cores (Duffin and others, 1989), and we

Figure F4. Photomicrograph (crossed polars) of inclusion-rich authigenic potassium feldspar overgrowths (KO) on 

detrital feldspar grains (DK); Q, quartz. Strake Petroleum No. 

1 Russell drill hole, depth 4,717 ft (1,437.7 m) (location 

shown in fig. F1).

also observed this staining phenomenon in samples from 

other drill holes. Therefore, the potassium feldspar in the 

Strake samples is anomalous and may indicate potassium 

metasomatism of the detrital cores. Further study is 

needed to confirm this hypothesis.

Under cathodoluminescence (CL), detrital potassium 

feldspar luminesces blue. The potassium feldspar overgrowths show a faint zonation, which may be related to 

the two generations of feldspar cement observed under 

transmitted light. There are two phases: a dark, nonluminescent phase and a brown, luminescent phase. It should 

be noted that Kastner (1971) reported that authigenic feldspar is nonluminescent. However, these feldspar overgrowths do contain trace amounts of iron that may be 

acting as an activator.

R> Strategic and Critical Minerals, Midcontinent Region, U.S.
Figure F5. Photomicrographs (crossed polars) of sample 

from drill hole BT1. A, Etched rhombic potassium feldspar 

overgrowths (KO) on a hematitic feldspar grain (DK). B, an 

orthoclase grain (DK) and its overgrowth (KO, upper left) that 

have not undergone the dissolution effects shown by the sanidine grain (DS) and its overgrowth. Other diagenetic products 

are dolomite (D), quartz overgrowths (QO) on detrital quartz 

grains (Q), and illitic clay (I). Void spaces (V) are due to dissolution. Depth 2,092-2,114 ft (637.6-644.4 m) (location 

shown in fig. F1).

The Lamotte Sandstone is fractured at this locality, 

and detrital grains are commonly brecciated and rotated. 

This structural feature is apparent under CL. The nonluminescent authigenic feldspar partially replaces as well as 

heals fractures in the detrital grains.

Dissolution effects are difficult to determine in 

these samples because of extensive replacement of the 

rock by dolomite cement. However, second-generation 

overgrowths on feldspar grains do show minor dissolution 

embayments next to quartz overgrowths.

Drill Hole BT1

This drill hole, located between the Reelfoot Rift 

and the St. Francois Mountains, bottoms in the Lamotte

Sandstone at 2,114 ft (644.4 m). The Lamotte Sandstone 

is thin at this locality, and the sample was obtained from 

the transition zone with the overlying Bonneterre Formation (E.L. Mosier, oral commun., 1992).

Potassium feldspar overgrowths are a pore-filling 

cement that commonly formed before quartz overgrowths, and they are present as a series of rhombs on 

detrital feldspar grains (fig. F5A). Many of the igneous 

rock fragments contain feldspar that is turbid because of 

hematite flakes, but their overgrowths are clear. Many of 

the devitrified volcanic rock fragments, which have a 

chertlike appearance, also have clear potassium feldspar 

overgrowths. Dissolution voids in the detrital feldspar 

grains and authigenic overgrowths are prominent and are 

filled with blue-epoxy resin, indicating that this void 

space is part of the effective porosity of the rock (Heald 

and Larese, 1973). Dissolution has also occurred between 

detrital quartz grains and their overgrowths.

Orthoclase, which is plutonic in origin, and sanidine, which is volcanic in origin show different dissolution effects (fig. F5B). For example, the detrital sodic 

sanidine, volcanic rock-fragment grains, and overgrowths have partially dissolved. Feldspar from a volcanic, high-temperature, more sodium-rich source is 

known to dissolve more easily than feldspar from a 

lower temperature igneous-rock source (Heald and 

Larese, 1973), and this may be the cause of the 

uneven dissolution effects at this locality.

Indian Creek Subdistrict Goose Creek Mine

The Lamotte Sandstone is extensively mineralized 

in pinchouts against the Precambrian terrain in the Goose 

Creek Mine, in the Indian Creek subdistrict, which is 

north of the Viburnum Trend and the St. Francois Mountains (Gutierrez, 1987) (fig. Fl). The Goose Creek Mine 

is in the Lamotte Sandstone and is the northernmost 

lead-zinc deposit in the southeastern Missouri lead district. Mineralization in this locality has been attributed to 

the control of fluid flow by the underlying Precambrian 

topography (Gutierrez, 1987; Kyle and Gutierrez, 1988). 

The "clotted" or "spotted" ore textures at Goose Creek 

Mine are similar to ore textures in sandstone in the lead 

deposits of Laisvall, Sweden (Kyle and Gutierrez, 1988). 

In the study of clotted ore textures in Laisvall, Sweden, 

galena was determined to have precipitated preferentially 

in feldspar-rich layers (Boorder, 1986). In the Goose 

Creek Mine samples, clotted galena textures do occur 

with altered feldspar, but it does not appear to be a preferential association. Rather, the galena occurs in conjunction with coarse-grained sandstone layers containing 

volcanic rock fragments and silicified shell material as 

well as detrital feldspar. In these layers, galena occurs 

within dissolution voids in detrital potassium feldspar

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F7
grains, silicified shell fragments, volcanic rock fragments, 

and dolomite rhombs. Porosity has been enhanced and 

intergranular void space has been enlarged by dissolution 

of framework grains and overgrowths. Galena and rnarcasite commonly have geopetal textures within these dissolution voids.

The volcanic rock fragments in the Goose Creek 

Mine contain sanidine phenocrysts. Many of the feldspar 

framework grains appear to be sanidine phenocrysts that 

weathered out of the volcanic rock fragments or the original volcanic matrix.

The contact relations between galena and authigenic 

potassium feldspar cement constrain the relative ages of 

these minerals. Early galena cement postdates the precipiFigure F6. Photomicrographs of sample from Goose Creek 

Mine, in Indian Creek subdistrict (location shown in fig. F1). 

At Galena cement (Ga) that partially encloses a detrital potassium feldspar grain (DK) and its overgrowth (KO); 

plane-polarized light. B, Galena cement (Ga) that invaded 

dissolution voids (arrows) in a partially dissolved potassium 

feldspar grain (DK) and its overgrowth; reflected light. Other 

cements are quartz overgrowths (QO) on quartz detrital 

grains (Q) and dolomite (D). V, void space.

Figure F7. Scanning-electron photomicrographs of samples 

from Goose Creek Mine, in Indian Creek subdistrict (location 

shown in fig. F1). A, Backscatter image of dissolution pitting 

in a detrital feldspar grain (DK) with galena (Ga) cement in 

the dissolution void (DV). 6, Etched authigenic feldspar 

rhombs (KO). QO, quartz overgrowth; Q, quartz.

tation of potassium feldspar overgrowths and predates the 

dissolution of those overgrowths (fig. F6A). This early 

galena surrounds the rhombic outlines of potassium feldspar overgrowths. However, a later stage of galena 

cement invades the dissolution voids in feldspar along 

with minute cubes of pyrite (fig. F6£).

Under the SEM, galena is observed both as an intergranular cement between framework grains and as an 

intragranular cement in dissolution voids in detrital potassium feldspar grains (fig. F7A). This represents at least 

two different generations of galena precipitation. The 

potassium feldspar grains are highly altered; cores of the 

grains are commonly pitted and dissolved and replaced by 

tiny feldspar rhombs (probably adularia) that also show 

dissolution etching (fig. FIB).

F8 Strategic and Critical Minerals, Midcontinent Region, U.S.
Earlier TIME Later

STAGES

ILLITE

POTASSIUM 

FELDSPAR

QUARTZ 

DOLOMITE

KAOLINITE 

PYRITE 

MARCASITE 

CHALCOPYRITE 

SPHALERITE 

GALENA

PRE-ORE

 

ORE STAGE

 

POST-ORE

-

 

Figure F8. Mineral paragenetic sequence in Indian Creek 

subdistrict compiled from work of Rothbard (1983), Cutierrez 

(1987), Horrall and others (1983), and this study. Dashed lines 

indicate dissolution.

According to these observations and to data from 

the literature (Gutierrez, 1987), the general diagenetic 

sequence appears to be (1) precipitation of potassium 

feldspar overgrowths, (2) precipitation of quartz overgrowths, (3) precipitation of galena cement, (4) dissolution of potassium feldspar and quartz overgrowths and 

precipitation of a second generation of galena, and (5) 

precipitation of pyrite and quartz overgrowths. A more 

extensive paragenetic sequence of ore minerals is given in 

Horrall and others (1983) and Kyle and Gutierrez (1988) 

(fig. F8).

PETROGRAPHIC DESCRIPTIONS OF 

SAMPLES FROM CORE FENCE B-B'

Cores along B-B' (fig. Fl) show a pronounced 

change in the dissolution of feldspar overgrowths between 

nearby drill holes. Drill hole 63W72 (fig. Fl) has abundant feldspar cement that has largely been unaffected by 

dissolution. In contrast, feldspar overgrowths from drill 

hole 63W5 have been almost completely dissolved, creating significant secondary porosity.

Drill Hole 63W72

In drill hole 63W72, two generations of potassium 

feldspar overgrowths are present, neither of which has 

been affected by subsequent dissolution (fig. F9). The 

feldspar overgrowths are generally rhombic, rarely 

twinned, and completely enclose detrital feldspar cores. 

This encircling of the potassium feldspar core suggests 

that the overgrowth precipitated in a loosely packed, 

porous rock. Overgrowths are evident because they are 

not in optical continuity with the detrital feldspar grains. 

The two generations of authigenic feldspar cement are in 

turn evident because they are not in optical continuity

with each other. Authigenic potassium feldspar showing 

little or no dissolution effects is characteristic of samples 

from several of the drill holes in the northern extent of 

the Lamotte Sandstone (figs. Fl and F9).

Potassium feldspar overgrowths are an important 

volumetric pore-occluding cement in the upper part of the 

Lamotte Sandstone in porous feldspathic arenite: in feldspar-rich layers, overgrowths make up as much as 18 percent of the authigenic cement (1,394-1,441 ft, or 

424.9-439.2 m) (fig. F9). A later authigenic illitic clay 

coats the authigenic feldspar and detrital framework 

grains and appears to have inhibited the development of 

quartz overgrowths.

The sample from the interval 1,441-1,472 ft 

(439.2-448.7 m) in this drill hole is a quartz arenite; no 

feldspar is present Because the rock overlying the crystalline basement (1,527 ft, or 465.4 m) has abundant 

microcline grains, one must speculate whether the absence 

of feldspar in this zone is due to dissolution, or to the 

absence of feldspar in the original sediment. There is no 

evidence in thin sections of remnant feldspar. However, 

grain-size pore spaces (a result of dissolution of framework grains), clay drapes, and pendant quartz cements 

indicate diagenesis in a vadose environment, which is 

subject to influxes of acidic meteoric waters.

Drill Hole 63W5

There is a marked vertical distribution to the dissolution textures seen in core from drill hole 63W5. 

Microcline grains and their overgrowths in samples from 

1,130 to 1,164 ft (344.4 to 354.8 m) have minor dissolution pitting. In samples from 1,164 to 1,214 ft (354.8 to 

370.0 m), potassium feldspar grains are more abundant,

Figure F9. Photomicrograph (plane-polarized light) showing 

two generations of potassium feldspar overgrowths (KOj, 

KC>2) on a microcline grain (DK). Q, quartz grain; V, void. 

Drill hole 63W72, depth 1,394-1,441 ft (424.9-439.2 m) 

(location shown in fig. F1).

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F9
0K

f KOIHBw,-

Figure F10. Photomicrograph (plane-polarized light; therefore, twinning is not apparent) of detrital feldspar grain (DK) 

and two generations of potassium feldspar overgrowths 

(KOlr KO2) that have undergone dissolution. Skeletal outlines of overgrowths remain. Q, quartz; V, void. Drill hole 

63W5, depth 1,214-1,267 ft (370-386.2 m) (location 

shown in fig. F1).

kaolinite and illitic clay appear as common alteration 

products, and dissolution of feldspar overgrowths is 

apparent. From 1,214 to 1,267 ft (370.0 to 386.2 m), the 

illitic clay forms layers several millimeters thick, above 

and below fine-grained feldspar-rich layers where intense 

dissolution of potassium feldspar overgrowths has taken 

place. Two generations of potassium feldspar overgrowths are recognized in this interval (1,214-1,267 ft), 

and both have undergone partial to total dissolution. Skeletal outlines of the feldspar overgrowths are visible 

because of clay and (or) iron-oxide coatings (fig. F10).

RESULTS OF PETROGRAPHIC STUDIES OF 

CORE FROM OTHER DRILL HOLES

The abundance and degree of dissolution of potassium feldspar overgrowths seen in the entire suite of 

Lamotte thin sections are summarized semiquantitatively 

in figure Fll. Because of the vertical variability in both 

abundance and dissolution data, this plot focuses on data 

from the Lower Lamotte, where dissolution effects were 

most apparent. This plot shows no systematic trends in 

the abundance of potassium feldspar cement, but it does 

indicate strongly that dissolution of the potassium feldspar cement was regionally extensive and was most 

intense along a northwest-trending corridor that includes 

the Reelfoot Rift, the lead belts, and the St. Francois 

Mountains. The major exception to this trend is in samples from the Stroke well, which do not show extensive 

dissolution. We discuss the significance of these dissolution trends below.

F10 Strategic and Critical Minerals, Midcontinent Region, U.S.

Detrital feldspar grains and the two generations of 

authigenic feldspar commonly show varying degrees of 

dissolution effects, which are probably due to the chemistry of the overgrowths versus the chemistry of the detrital 

grain (Kastner and Siever, 1979) because overgrowths 

tend to be pure end members of the alkali feldspar series 

and are thus more stable in low-temperature diagenetic 

environments. Any single feldspar-rich layer may show 

partial dissolution of both detrital grains and their overgrowths, dissolution of only the detrital grains, or dissolution of only the overgrowths. Where there are two 

generations of feldspar overgrowths, only one generation 

may be affected by dissolution (fig. F12).

Additional observations of diagenetic minerals in 

the Lamotte Sandstone and Reelfoot Rift samples may be 

pertinent. Several differing trends in alteration minerals 

exist between the Reelfoot Rift and the Ozark uplift. 

Within the rift-filling sedimentary rocks, diagenetic clay 

minerals are predominantly illite and chlorite, whereas on 

the carbonate platform and in the Ozark mineral districts, 

the predominant diagenetic clay product is kaolinite 

accompanied by minor illite. Iron-rich carbonate minerals, 

such as siderite and ferroan dolomite, are common 

cements in the Reelfoot Rift samples, whereas dolomite 

and calcite are more common carbonate cements in samples from west and northwest of the rift (see table Fl). As 

previously discussed, albite is a common cement in the 

rift but was not detected elsewhere in the study area.

DISCUSSION

A major focus of this study was to evaluate the 

regional implications of feldspar diagenesis in the Lamotte Sandstone and to determine if feldspar diagenesis and 

dissolution events are related to the genesis of the MVT 

districts in the region. As stated in the introduction, there 

is reason for suspecting such a relationship because of the 

very potassic nature of the ore-forming brines for MVT 

deposits. Furthermore, a strong geochemical correlation 

exists between lead and potassium in that potassium feldspar is a significant residence for lead in igneous and 

feldspathic sedimentary rocks (Doe and Delevaux, 1972). 

Thus, the fact that the lead belts surrounding the St. Francois Mountains of southeastern Missouri, which constitute 

the largest known concentration of lead in the crust of the 

earth (Kisvarsanyi and others, 1983), overlie the authigenic potassium feldspar-bearing Lamotte Sandstone at 

least suggest a link between the ores and potassium-bearing fluids in the Lamotte. In addition, the available evidence indicates that the mineralization in southeastern 

Missouri was profoundly affected by the proximity of the 

Reelfoot Rift. This influence of this structure led to major 

differences between ores in southeastern Missouri
94° 92° 90°

38'

37°

MISSOURI 

ARKANSAS

0 64W86 

64W58r* CX"W49

^<&y///) \ , , , , ?

| 0 20 40

EXPLANATION

50 MILES 

I

I 

80 KILOMETERS Garriganr)

Wilson^

A1

k BT1 Drill hole Size of circle and proportion shaded 

shows abundance of potassium feldspar cement 

and proportion of void space in feldspar overgrowths 

due to dissolution; see diagram below

Potassium feldspar cement, in percent

o O O O

<1 5 10 20

Areal extent of two generations of 

potassium feldspar cement

Ore district

Igneous rocks

B - B' Section discussed in text

Increasing dissolution, in percent

O 3

50 >90

Figure F11. Map showing abundance of potassium feldspar cement (as percentage of rock) and the 

amount of dissolution of the cement plotted at the drill hole locations.

compared to other MVT ores of the region. Below we 

discuss these influences in more detail.

Timing of Feldspar Precipitation Relative to 

Mississippi Valley-Type Ore Genesis

The most direct evidence linking the formation of 

potassium feldspar and dissolution trends (documented 

above) with MVT ores comes from both absolute and 

relative age data. Hearn and others (1986) obtained a late

Paleozoic age of 300+60 Ma for authigenic potassium 

feldspar samples from the transition zone between the 

Lamotte and the overlying Upper Cambrian Bonneterre 

Formation in the study area. For comparison, independent 

evidence based largely on paleomagnetic data has likewise placed the age of MVT mineralization in the Ozark 

region as late Paleozoic (Wu and Beales, 1981; Wisniowiecki and others, 1983; Pan and others, 1990). Rothbard 

(1983) reported that illite from the Lamotte Sandstone, 

which postdated galena, has an age of 255±15 Ma (Permian). Collectively, these dates place the formation of

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F11
Figure F12. Photomicrograph (plane-polarized light) (drill 

hole 63W5; location shown in fig. F1) showing that different 

generations of potassium feldspar overgrowths may be differentially affected by dissolution. Here, the first generation 

(KOi) has undergone total dissolution, but the second generation (KC>2) has undergone partial dissolution. The detrital 

core (DK) also remains. Q, quartz.

authigenic potassium feldspar in the same general late 

Paleozoic time frame as ore deposition.

Additional evidence of timing that is pertinent to 

the relations between potassium feldspar and ore mineralization comes from petrographic studies on potassium 

feldspar and the ore phases. These relations between minerals hi the Indian Creek subdistrict are summarized in 

the paragenetic diagram in figure F8. The figure shows 

that authigenic potassium feldspar predated the main episode of ore genesis, and that ore deposition was associated with the dissolution of authigenic potassium feldspar 

(Rothbard, 1983; this study).

The paragenetic sequence for the Indian Creek subdistrict (fig. F8) is probably valid for a large part of 

southeastern Missouri. This conclusion is based on the 

work of Horrall and others (1983), who showed that the 

Goose Creek Mine was part of a district-wide paragenetically correlated sequence of ore deposition. In particular, 

galena was deposited as "spots" in the Lamotte Sandstone 

and has a crude cuboctahedral habit, which correlates texturally with the main phase of cuboctahedral and disseminated mineralization in the Viburnum Trend and contrasts 

with the morphology of later (minor) cubic galena. Other 

correlations between Goose Creek and the Viburnum 

Trend exist. Horrall and others (1983) observed that specific generations of quartz, dolomite, bravoite, pyrite/marcasite, chalcopyrite, siegenite, sphalerite, galena, dickite, 

and calcite could be correlated between the Goose Creek 

Mine and at least two mines in the Viburnum Trend. On 

a more regional scale, Hayes and others (1989) and 

Rowan (1986) used detailed regional correlations of dolomite microstratigraphy to show that the ores of the entire 

southern Ozark region formed from a single huge

regional hydrothermal system. In view of this regional 

correlation of paragenetic events, the relative timing of 

deposition and dissolution of potassium feldspar for the 

Indian Creek subdistrict (fig. F8) may also apply to the 

formation of ores in other parts of the region.

Lead-isotope studies reveal an additional geochemical link between authigenic potassium feldspar in the 

Lamotte Sandstone and main-stage ores of the Viburnum 

Trend. These studies, which were conducted on authigenic 

potassium feldspars in the Lamotte Sandstone of southeastern Missouri (Aleinikoff and others, 1993), show that 

the fluids that formed the authigenic feldspars carried lead 

whose isotopic composition rather closely matches the 

208/204Pb/206/204Pb ratio of one end member of the lead 

that formed main-stage (cuboctahedral) mineralization of 

the Viburnum Trend ores. This similarity in isotopic compositions is important because the 208/204pb/206/204Pb 

ratios of main-stage Viburnum Trend ores are unique 

compared to all other MVT ores of the southern midcontinent (J.N. Aleinikoff, written commun., 1992), and trace 

lead in other aquifers higher in the stratigraphic section 

does not match the isotopic signature of the Viburnum 

Trend ores (Goldhaber and others, 1989). This indicates 

that the same isotopic lead, which was carried in solution 

through the Lamotte aquifer, was trapped in potassium 

feldspar overgrowths as well as in the main-stage ores.

On the basis of the absolute and relative timing evidence and geochemical data enumerated above, several 

conclusions are possible. Even though potassium feldspar 

formed before Pb-Zn mineralization, it nevertheless 

formed in the same late Paleozoic (Carboniferous-Permian) periods as the ores. Thus, authigenic potassium 

feldspar formation, like MVT ore genesis, may have been 

associated with the regional flow of brines in the late 

Paleozoic (Leach and Rowan, 1986) that resulted from the 

Appalachian-Ouachita orogeny. The lead-isotope data 

imply that the earlier episode of potassium feldspar precipitation, although predating main-stage ore deposition, 

nonetheless involved fluids with lead that is isotopically 

the same as the source of lead for the main-stage ores.

Importantly, fluid-inclusion studies of early to 

main-stage mineralization in the Viburnum Trend (Crocetti and Holland, 1989; Viets and Leach, 1990) revealed 

exceptionally high K/Q ratios compared to other MVT 

ore-forming brines of the region (table F2) and even compared to paragenetically later parts of the Viburnum Trend 

mineralization. These high K/C1 values were attributed by 

Viets and Leach (1990) to a fluid migrating through the 

Lamotte Sandstone, a conclusion that is consistent with 

the isotopic data of Aleinikoff and others (1993).

Overall, the evidence suggests that fluids elevated 

in potassium (compared to brines that formed other ores 

of the region) and containing isotopically distinctive lead, 

formed the Pb-Zn deposits in the lead belts of southeastern Missouri. Some key characteristics, such as elevated

F1 2 Strategic and Critkal Minerals, Midcontinent Region, U.S.
K+ concentration and specific lead-isotope ratios, of the 

main-stage ore fluids may have spanned the time interval 

during which authigenic feldspar formed and dissolved. In 

the next section, we examine the hypothesis that the 

source of this potassium (and by inference a significant 

part of the lead in the same fluid) came from the Reelfoot 

Rift, and, subsequently, we examine the shift from conditions favoring precipitation as compared to dissolution of 

potassium feldspar.

Contrasts Between Reelfoot Rift and Platform 

Settings in Feldspar Diagenesis

The data from this study show a major change in 

both the type and distribution of feldspar and the nature 

of feldspar diagenesis in the Cambrian rocks in southeastern Missouri. A significant difference was detected 

between the Lamotte Sandstone and basal clastic equivalents in the Reelfoot Rift, in contrast to the same units in 

the carbonate platform northwest of the rift and in the 

adjacent Ozark uplift. Feldspar in siliciclastic rocks in the 

rift is predominantly microcline, plagioclase, or intergrown microcline and plagioclase that were derived from 

plutonic rock fragments. In the platform area, sanidine is 

the common feldspar and volcanic rock fragments take 

precedence over plutonic rock fragments. Another difference is that the potassium feldspar overgrowths in the 

Reelfoot Rift are not as well developed or abundant as 

those in the Lamotte section northwest of the rift 

(although the Strake well is an exception; see below). 

Furthermore, and perhaps most importantly, extensive 

replacement of potassium feldspar by albite may have 

severely changed the original potassium feldspar content 

of feldspathic arenite and graywacke in the Wilson and 

Garrigan holes in the rift. No evidence of similar albitization was detected in time (Cambrian) and lithologic-equivalent rocks in the platform area outside the 

rift.

Figure F13 shows representative point-count data 

for feldspar in drill hole samples (table Fl) of shallowly 

buried Lamotte Sandstone and deeply buried siliciclastic 

rocks in the Reelfoot Rift, plotted against depth. We interpret these differences in feldspar alteration between the 

rift and platform areas to be a function of depth of burial 

in sedimentary rocks within the rift Dissolution of feldspar with or without albitization is a well-recognized process that occurs during deep burial diagenesis in a range 

of settings (Walker, 1984; Milliken, 1988; Saigal and others, 1988). Potassium feldspar is particularly susceptible 

to destruction by both dissolution and albite replacement, 

resulting in marked decreases in the ratio of potassium 

feldspar to total feldspar and increases in the percentage 

of albitized feldspar with deep burial diagenesis (Walker, 

1984; Saigal and others, 1988; Milliken and others, 1989)

EXPLANATION

A 63W5 drill hole 

T BT 1 drill hole

 Garrigan drill hole

 Wilson drill hole

18,000

0.05 0.10 0.15 0.20 

ALBITE/ALBITE + POTASSIUM FELDSPAR

Figure F13. Plot of point-count data for albite and potassium 

feldspar from shallowly buried Lamotte Sandstone and deeply 

buried siliciclastic rocks versus depth.

(fig. F13). (The correlation coefficient (r) of the linear 

regression line is 0.882917; the trend relating depth to 

Ab/(Ab + Kspar) is statistically significant. However, the 

data may also represent two distinct populations.)

The, contrast in the major detrital feldspar type 

between sanidine in the platform area and microcline in 

the rift may simply reflect a greater solubility of sanidine 

in the deeply buried rift section rather than a different 

provenance. Heald and Larese (1973) showed that in the 

Mount Simon Sandstone (a Lamotte equivalent) in Ohio, 

sanidine was partially dissolved whereas microcline was 

stable. Thus, partial destruction of feldspar can lead to an 

apparent change in provenance, whereas the actual difference may be due to diagenetic processes (Milliken, 1988).

The destruction of large amounts of feldspar, particularly potassium feldspar, in the deeply buried Reelfoot 

Rift sedimentary rocks would have resulted in the addition of large amounts of K+ to the pore fluids. We suspect that it is this potassium that led to elevated K/C1 

values in main-stage mineralization of the Viburnum 

Trend. Clearly, other deep sedimentary basins, particularly the Arkoma basin south of the Ozark region, could 

have contributed K+ by similar processes of burial 

diagenesis, but the lower K/C1 values of fluids from ores 

closer to the Arkoma basin (Viets and Leach, 1990) argue 

against such a source. The proximity of the Viburnum 

Trend and the other lead ores surrounding the St. Francois 

Mountains to the Reelfoot Rift thus suggests a fluid-flow 

path linking the Reelfoot Rift to the lead belt area; this 

fluid-flow path uniquely contributed extra K+ as well as 

other ore constituents to the lead belt area.

The inference that ore-forming fluids originated or 

passed through the rift section is strengthened by the trace 

amounts of lead and zinc minerals in the siliciclastic 

rocks of the Garrigan and Wilson drill holes (Diehl and 

others, 1992). Epigenetic galena occurs within pyrite in a 

matrix of siderite cement in samples from 12,782 ft 

(3,896 m) in the Wilson hole. In the Garrigan hole, 

epigenetic "mineral fronts," which are areas of sulfide

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F13
alteration in siliciclastic rocks from 7,993 to 8,002 ft 

(2,436.3 to 2,439 m), are enriched in lead and cobalt. Epigenetic pyrite having the same style of occurrence is 

present in siliciclastic rocks at 11,998-12,040 ft 

(3,657-3,670 m) in the same drill hole.

Formation and Dissolution of Potassium in the 

Lamotte Sandstone Outside the Reelfoot Rift

The precipitation and dissolution of potassium feldspar are related to sample locality and depth (fig. Fll; 

table Fl). Authigenic potassium feldspar overgrowths 

from lower in the Lamotte Sandstone northwest and 

southwest of the St Francois Mountains are partially to 

totally dissolved. However, higher in the Lamotte Sandstone and near the transition zone with the overlying Bonneterre Formation, potassium feldspar overgrowths are 

less affected by dissolution. Two episodes of potassium 

feldspar precipitation are commonly indicated because 

early overgrowths are partly dissolved and are surrounded 

by later, well-formed rhombic overgrowths. However, 

only one generation of authigenic potassium feldspar is 

evident southeast of the St. Francois Mountains toward 

the Reelfoot Rift (fig. F9).

T=150°C

Table F2. Estimation of composition of ore-forming 

fluid in the study area, southwestern Missouri and northeastern Arkansas.

-6.0 -4.0 

LOG(aSi09 (Aq)>

-2.0

Figure F14. Potassium feldspar stability diagram at 150°C 

(modified from Carrels and Christ, 1965, p. 359-370). Upper 

shaded rectangle shows calculated pH range over which 

potassium feldspar and muscovite would coexist. Lower 

shaded rectangle shows pH range over which dissolution of 

potassium feldspar would lead to formation of kaolinite. a, 

activity.

Weight ratio

Na/Cl

K/Na

Ca/Mg

Ca/Na

Octahedral 

galena1

0.463

.129

2.531

.229

Cubic 

galena1

0.490

.040

10.2

.188

Calculated 

ore fluid2

0.504

.091

5.72

.206

Values from Crocetti (1985).

2Molalities for ions in calculated ore-forming fluid: Cl~, 3.60; HCO3~, 0.50; 

HS-, 0.01; CA++, 0.35; Mg^, 0.10; K+, 0.15; Na+, 2.80.

The dissolution of potassium feldspar is of particular interest because if the results from ore samples can be 

extrapolated to areas away from the mineralization, then 

this dissolution event coincided in time with main-stage 

ore deposition in the lead belts (figs. Fl and F8). Figure 

Fll shows that feldspar dissolution is most intense in the 

area south of the St. Francois Mountains between the 

Reelfoot Rift and the lead belts and continues to the west 

and northwest of the Ozark Mountains. The proximity of 

this "dissolution corridor" to the Reelfoot Rift suggests 

that the source of fluids capable of dissolving feldspar 

was the rift area. This dissolution coupled with the anomalously high K/C1 values of main-stage ore fluids that 

formed deposits adjacent to the rift indicate that both the 

source of ore constituents and destruction of potassium 

feldspar was influenced by Reelfoot Rift fluids.

Geochemical modeling provides insight into the 

question of what caused the switch from feldspar precipitation to later dissolution. The most likely causal factor is 

a change in pH. The pH changes implied by the potassium feldspar precipitation-dissolution events were calculated by first modeling an MVT "ore fluid," and then 

comparing the activity of K+ in this fluid with the feldspar-stability diagram of Bowers and others (1984) (fig. 

F14; table F2). The chemistry of the ore fluid was constrained by the fluid-inclusion data of Crocetti (1985), 

who measured the chemistry of fluid-inclusion leachates 

from Viburnum Trend galena. As shown in figure F14, 

the fluid modeled here conforms to the weight ratios of 

Crocetti (1985) and, in addition, was constrained to be at 

equilibrium with dolomite and quartz. Ionic activities 

were calculated using the computer code Solveq (Reed, 

1982). The resulting value of aK+ was then used to calculate the pH shown along the right vertical axis of figure 

F14. The upper shaded rectangle in figure F14 shows that 

potassium feldspar dissolution requires a pH of about 5.0. 

If kaolinite formed as potassium feldspar dissolved, as 

indicated by the paragenetic diagram of ore formation in 

the Lamotte Sandstone (fig. F8), then the pH would have 

had to be at least 4.0 (lower shaded rectangle). This calculated pH for feldspar dissolution (and therefore ore

F14 Strategic and Critical Minerals, Midcontinent Region, U.S.
formation) corresponds with the value calculated by an 

independent technique using fluid-inclusion gas compositions by Leach and others (1991).

We do not have direct evidence for what caused the 

pH of the fluid to change from a brine of relatively higher 

pH that was capable of depositing potassium feldspar to a 

brine of lower pH, which caused the dissolution trend of 

figure Fll. It is likely that this shift was related to an 

increase in PCOJ of the fluid, which caused the low pH values of the MVT ore-forming fluids (Leach and others, 

1991). It thus appears that the fluid evolved from lower to 

higher values of this parameter. We can speculate that 

this pH change involved the thermal catabolic destruction 

of organic matter deep in the sedimentary rocks of the 

Reelfoot Rift. Early stages of sedimentation in the rift 

included deposition of large amounts of sedimentary 

organic matter. The release of CO2 is known to accompany deep burial and heating of organic matter (Capuano, 

1990). We can further speculate that this increase in PCOJ 

was due to a process that was slower than the one that 

released excess K+.

Additional Evidence for a Link Between the 

Reelfoot Rift and Lead-Belt Mineralization

In addition to the zone of dissolution of potassium 

feldspar that is adjacent to the Reelfoot Rift, other 

researchers have presented evidence for fluid migration 

from the rift towards the St. Francois Mountains. Erickson 

and others (1988) and M.B. Goldhaber (unpub. data, 

1990) documented a zone of lead enrichment in Cambrian 

carbonate rocks adjacent to the Reelfoot Rift, which suggests that ore-forming fluids originated in, or passed 

through, the Reelfoot Rift. This area of anomalous lead 

enrichment overlaps the zone of dissolution of the potassium feldspar adjacent to the rift.

Farr (1986) reported an increase in the ratios 

of strontium isotopes in epigenetic dolomite from 

less radiogenic (87Sr/86Sr=.7090) to more radiogenic 

(87Sr/86Sr=.7140) with increasing proximity to the 

rift. Radiogenic strontium is largely derived from feldspar dissolution in much the same way as we proposed above for K+, and thus Fair's work 

complements ours.

The Anomalous Nature of the Strake 

Well Samples

Samples from the Strake well have exceedingly high 

concentrations of authigenic potassium feldspar (as much 

as 10 percent of the whole rock), and this feldspar is visually different from feldspar overgrowths elsewhere in the 

area. Furthermore, two generations of overgrowths are

present in these samples in contrast to those from the rest 

of the southern part of the area in figure Fll. The 

anomalous abundance and appearance of the feldspar in 

this well may be related to the uplift of the Pascola arch, 

with which it is geographically associated. We attribute 

the elevated potassium feldspar content in the Strake well 

to fluids associated with alkaline igneous activity near the 

Pascola arch. This explanation is based on data from the 

USGS New Madrid test well (Crone, 1981), which is only 

12 mi northeast of the Strake drill hole. Samples from the 

test well contain exceedingly elevated levels of the 

rare-earth elements Nb, Be, La, and Th (Erickson and 

Goldhaber, unpub. data, 1990). This suite of elements is 

diagnostic for carbonatite magmatism (Mariano, 1989). 

The upper levels of carbonatite complexes are invariably 

strongly potassium metasomatized (for example, potassium fenitized, La Bas, 1984), and such a process probably has operated in the area of the Pascola arch. If this 

igneous activity and associated alteration was associated 

with the uplift of the Pascola arch, then it postdated the 

time of MVT ore formation in the Ozark area (Kolata and 

Nelson, 1990). Therefore, the anomalous potassium feldspar overgrowths in samples from the Strake well may be 

unrelated to fluids that moved from the Reelfoot Rift.

SUMMARY

We list four principal conclusions:

1. The variety of types of feldspar and the amount 

of dissolution in the potassium feldspar and its overgrowths in the Lamotte Sandstone vary regionally over 

southeastern Missouri. Microcline and plutonic igneous 

rock fragments that contain potassium feldspar were common in basal clastic rocks that were originally deposited in 

the Reelfoot Rift. Towards the St. Francois Mountains and 

to the west, volcanic rock fragments are more abundant 

than plutonic rock fragments, and sanidine was a common 

detrital feldspar in addition to microcline.

2. Although dissolution of feldspar occurred on a 

regional basis, dissolution is locally controlled by the type 

and abundance of feldspar and burial depth. Detrital 

microcline and orthoclase are less affected by dissolution 

than are sanidine grains. For example, at the Goose Creek 

Mine, where sanidine was the predominant feldspar in the 

Lamotte Sandstone, secondary porosity produced by feldspar dissolution is high. In the Reelfoot Rift, where the 

sedimentary rocks have undergone a long period of deep 

burial, albitization as well as dissolution have been important diagenetic processes that altered feldspar and added 

K+ to pore fluids.

3. Multiple episodes of precipitation and dissolution of potassium feldspar cement have occurred in the 

northern limits of the Lamotte Sandstone. These potassium

Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F15
feldspar cements are the result of the migration of potassium-rich brines through the Lamotte Sandstone in late 

Paleozoic time.

4. The potassium feldspar grains and their overgrowths dissolved by reaction with an acidic solution of 

pH 4. This acidic solution may have originated in, or at 

least passed through, the Reelfoot Rift. The diagenetic 

changes in the sedimentary rocks in the rift probably provided a large source of K+ for ore-forming solutions. The 

Reelfoot Rift had a major influence both on diagenesis of 

the Lamotte Sandstone and genesis of MVT ores.

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Feldspar Diagenesis in Cambrian Clastic Rocks, Southeastern Missouri and Northeastern Arkansas F17

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