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This is to certify that the

thesis entitled

Sulfur in Dolomite

presented by

Keith Charles Hill

has been accepted towards fulfillment
of the requirements for

Masters degree in Geology

 

 

 

vv

Major professor

&/ 3l/’3 5/
Date ‘ /é‘ €75" “
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0-7639 MS U i: an Aflirmau’ve Action/Equal Opportunity Institution

 

 

 

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RETURNING MATERIALS:
Place in book drop to
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SULFUR IN DOLOMITE

BY

Keith Charles Hill

A THESIS
Submitted to
Michigan State University
in partial fulfillment
for the degree of
MASTER OF SCIENCE

1985

ABSTRACT

SULFUR IN DOLOMITE

BY
Keith Charles Hill

The purpose of this study is to differentiate dolomites
formed in different natural waters by examining their
sulfur content. Dolomites and limestones from thirteen
formations ranging from Proterozoic to Holocene in age were
analyzed for sulfur abundance and oxidation state. These
values were compared to the sulfur content predicted by the
sulfur chemistry of the various waters in natural
environments of dolomitization.

The results of this study are that sulfur is of little
or no use as a trace element in differentiating dolomites
formed in different environments. The sulfur concentration
fields defined by dolomites presumed to be formed by
different environments showed no discernable separation.
There was also a wide scatter of sulfur concentrations
within individual formations. Dolomites and limestones
showed no appreciable difference in sulfur values. This
evidence contradicts the occult gypsum method used by
Beales and Hardy (1980) to assign a hypersaline origin to

most dolomites.

ACKNOWLEDGEMENTS

 

I would first like to thank Dr. Duncan F. Sibley for
his help and guidance throughout this study. I also wish
to thank Dr. Karl F. Bruder for sparking my interest in
geology. Dr. James H. Fisher, Dr. John T. Wildband and Dr.
Thomas A. Vogel were instrumental in teaching me the
methods and value of science during my studies at MSU.

I would like to thank the following people for
providing samples or assistance in the field collecting
samples used in this study: Dr. Chris von der Borch, Dr.
Ron Patterson, Dr. John Hudson, Dr. Eugene Shinn, Dr.
Steven Sears, Dr. Lynton S. Land, Dr. Thomas R. Taylor and
Shell Oil Company, Dr. Dave Eby and the 1984 AAPG Student
Chapter fieldtrip staff, Dr. Jay M. Gregg, Dr. Frank W.
Beales, Micheal Miller and the DSDP sample distribution
center at La Jolla.

I would like to thank Barringer-Magenta for the total
sulfur analyses and Bondar-Klegg for sulfate sulfur
analyses.

I wish to thank all of the friends and drinking buddies
for their emotional support throughout these years. I
shall especially miss John "Wad" Nelson, Tim "T.J."
Bartlett, Jimmy Gell, Steve "4-putt" Rohr, Mike "Serachoad"

Serafini, Tim "gosh I'm lucky" Flood, James "Rico" Carty,

ii

Bob "Jaws" Dedoes, Mike "Reb" Miller, Steve "Mongo" Mattson
and Gary "must-have" Lunsky.

And last, but certainly not least, I wish to pay homage
to my incredible wife Margo Ann Hill for her undying
support and help throughout my academic career. Her
tolerance of me under numerous circumstances was most

inspirational.

iii

TABLE OF CONTENTS

LIST OF TABLES . . . . . . . .
LIST OF FIGURES . . . . . . . .

INTRODUCTION . . . . . . . . .
Models of Dolomitization . . .
Water Chemistry . . . . . .

SAMPLES USED IN THIS STUDY . . . .

METHODS. . . . . . .
Occult Gypsu Method
Sample Preparation .
Total Sulfur Method .
Oxidation State . .

EXPERIMENTAL RESULTS . . . . . .
DISCUSSION . . . . . . . . .
CONCLUSIONS . . . . . . . . .
APPENDIX A
Formation Descriptions of Samples
Used in this Study . . . . .
APPENDIX B
Solubility Calculations in Occult
Gypsum Solution . . . . . .

BIBLIOGRAPHY . . . . . . . . .

iv

24
27
27
29
30
30
33
44

49

SO

78
80

Table 1.

Table 2.

Table 3.

LIST OF TABLES

page
Estimated dissolved sulfur content
in waters from the various
models of dolomitization. . . . . . . 14

Samples used in this study grouped
by proposed model of dolomitization. . . . 26

Sulfur analyses of all samples
used in this study. . . . . . . . . 35

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 1A.

Figure 2A.

Figure 3A.

LIST OF FIGURES

Changes in carbonate alkalinity
and sulfate abundances with depth. . . . 5

Sedimentary column showing processes
and dissolved species in an organic
influence environment. . . . . . . . . 8

Changes with depth of the concentration

of 8042’ and carbonate alkalinity

and carbon isotopes of the dissolved

C02 in the interstitial water of

South Guyamas Basin sediments, Gulf of
California. . . . . . . . . . . . 10

Estimates of sulfur hydrochemistry for the
various dolomitizing environments. . . . 16

Changes in carbonate alkalinity with depth
versus dissolved sulfides. . . . . . . 19

Changes in dissolved sulfur species
with depth in three organic-rich,
anoxic basins. . . . . . . . . . . 22

Sulfur chemistry of all dolomite
samples used in this study. . . . . . . 38

Sulfur chemistry of limestones used
in this study. . . . . . . . . . . 40

Sulfur chemistry of dolomites grouped
by proposed model of dolomitization. . . . 42

Fields of sulfur chemistry for
dolomites proposed to be formed

by the various models. . . . . . . . 46
Facies distribution in the Bonneterre

formation. . . . . . . . . . . . 53
Isotope data from the Mifflin member

in relation to carbonates precipitated
from other environments. . . . . . . . 57

Niagaran reef growth associated with

basin margins and shelf areas during
Silurian time. . . . . . . . . . . 60

.vi

LIST OF FIGURES (cont.)

page
Figure 4A. Dolomitization patterns from the A-l
carbonate and the Niagaran formation
in the pinnacle reef trend,
Northern Michigan. . . . . . . . . . 64

Figure 5A. Distribution of facies in the Edwards
formation. 0 O O O O O O O O O O 68

Figure 6A. Dolomitization of the Edwards formation

with respect to the Kirschberg
evaporite lagoon. . . . . . . . . . 7O

vii

I. INTRODUCTION

 

The purpose of this study is to differentiate
dolomites formed in different natural waters based on their
sulfur abundances and oxidation states. The amount and
oxidation state of sulfur incorporated into the dolomite
crystal will be proportional to the amount and oxidation
state of the sulfur in the dolomitizing solution. Because
the primary models of dolomitization differ widely in the
distribution of sulfur in the dolomitizing fluid, this
study proposes to test the hypothesis that this sulfur
signature can be used to distinguish between dolomite
types.

The structure of this paper will be; 1) to define the
models of dolomitization considered and delineate the
sulfur chemistry of the pore waters in these environments;
2) to briefly describe the samples analyzed (extensive
formations descriptions are included in Appendix A); 3)
describe the methods used for sulfur analysis; and 4) to
report the results of the analyses and interpret the sulfur

distribution.

Models of Dolomitization

 

Four models of dolomitization are be considered in

this study; 1) the hypersaline brine model; 2) the

2
mixed-water or Dorag model; 3) the organic influence model;
and 4) normal seawater dolomitization .

The hypersaline brine model (Adams and Rhodes, 1960)
consists of a dolomitizing fluid composed of seawater that
has evaporated to the point of gypsum precipitation. The
removal of calcium ions due to this precipitation raises the
magnesium—calcium ratio in the solution and thus is
thermodynamically favored for dolomitization. One strong
evidence for the plausibility of this model is that dolomite
is forming today in modern sabkhas which is believed to be a
result of this model (Illing et. a1., 1965; Patterson and
Kinsman, 1982; Butler, 1969). The periodic dilution of
these brines by seawater and/or freshwater to produce a
schizohaline environment may also be important (Folk and
Land, 1975).

The mixed-water or Dorag model (Land, 1973a;
Badiozamani, 1973; Back and Hanshaw, 1970) involves a
dolomitizing fluid that is a mixture of seawater and
freshwater. This fluid is supposed to be undersaturated
with respect to calcite and supersaturated with respect to
dolomite thus favoring replacement. The solution is also
dilute which may lessen the negative effect of interferring
ions on the replacement reaction (Folk and Land, 1975). Few
examples of dolomite forming in modern mixing zone
environments have been documented (Land , 1973b; Gebelein
et. a1., 1980)

The organic influence model consists of two organically

3

controlled processes which may favor dolomitization. The
first of these is sulfate reduction. Sulfate reduction
occurs in anoxic waters where bacteria use the oxygen in
8042' to convert organic material to C02 (Drever,
1982). Baker and Kastner (1981) found that the presence of
sulfate ions inhibited dolomitization in hydrothermal bomb
experiments. They believe that the large sulfate ions
"poison" the lattice sites on the growing dolomite crystal.
Therefore, a solution which has the sulfate reduced by
bacterial processes would be more favorable for
dolomitization.

Another important effect of sulfate reduction is the
increase in carbonate alkalinity (Lippman, 1973; Baker and
Kastner, 1981). The sulfate reduction reaction can be

written as follows (Berner, 1984):

zcnzo + $042- ==> H28 + 2HCO3‘ ( eq.1)

The increase in carbonate alkalinity caused by this
reaction favors dolomitization by increasing the activity
of the carbonate ion (Lippman, 1973). Figure 1 shows the
relationship between 5042‘ decrease and alkalinity
increase for certain environments.

The second biologically controlled process which may
favor dolomitization is methanogenesis. Methane producing
bacteria may not be able to grow in areas where dissolved

sulfate is present (Claypool and Kaplan, 1974). This

Figure 1.

Changes in carbonate alkalinity and sulfate
abundances with depth; a) Abu Dhabi Sabkha (data
from Patterson and Kinsman, 1982); b) Bahama
mixing zone hammocks, depth of 1 meter estimated
(data from Gebelein et. al., 1980); c) Gulf of

California (data from Goldhaber and Kaplan,
1980).

 

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6
causes a natural segregation of sulfate reduction and
methanogenesis environments (Fig. 2). As sediments are
buried and pass into the zone of methanogenesis, methane
production will proceed according to this equation

(Claypool and Kaplan, 1974):

HCO3- + 8H ==> CH4 + 2H20 + OH- (eq. 2)

The high HCO3' content of the pore fluids due to the
overlying sulfate reduction zone (Fig. 3) provides the
necessary HCO3‘. The bacterially controlled,
non-equilibrium reaction for methane production (eq. 2),
and the resulting increase in OH’ raises the pH of the

solution and causes this reaction to occur:

HCO3’ + OH‘ <==> c032' + H20 (eq. 3)

This will favor the precipitation of carbonate minerals:

Me2+ + c032‘ <==> MeCO3 (eq. 4)

Me2+ can be Ca2+ and/or Mg2+ so dolomite may be
precipitating. These two processes will deplete the
HCO3' content of the pore fluids (Fig. 3) but other
factors such as biogenic decarboxylation may act as a
source of carbonate (Irwin, 1980).

The fourth model of dolomitization considered here is

Figure 2. Sedimentary column showing processes and
dissolved species in an organic influence
environment (from Claypool and Kaplan, 1974).

water —>I air

‘l‘

v'V

 

sediment

 

 

 

 

 

 

 

 

 

 

 

water-rudimentary
column '93
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2 1 z : .' .' :reducinry _ 21"
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Figure 2

Figure 3.

Changes with depth of the concentration of

3042’ and carbonate alkalinity and carbon
isotopes of the dissolved C02 in the
interstitial water of South Guyamas Basin
sediments, Gulf of California (from Claypool and
Kaplan, 1974 using data from Goldhaber, 1974).
Note the changes in carbon isotopes and
titration alkalinity at the depth where

8042‘ is removed and methanogenesis

commences.

Depth (cm)

 

10

so" as.)
-20 45 -IO -5 0 +5

Concentration (mM)

 

 

" 3-9;. t- ~.__.'l'itrotion Alkalinity

 

Figure 3

11
dolomitization by normal seawater. Until recently this
model had received little consideration due mainly to the
fact that there are a large number of carbonate sediments
in contact with seawater which remain undolomitized (Land,
1980 and others). Two recent papers have demonstrated the
possibility that relatively unaltered seawater may be
responsible for dolomitization.

Saller (1984) studied dolomite found in cores on
Enewetak Atoll and concluded that cold normal seawater was
the dolomitizing solution. By examining the petrography
and strontium isotopes of the dolomite, he showed that the
dolomitization occured no later than middle to late
Miocene. This would place the sediments at a depth of more
than 900m, far below the expected depth of a mixing zone or
hypersaline brine. The temperature profile of the well and
the observations of tidal fluctuations within the well
suggests it is in communication with the surrounding
seawater. The oxygen isotopes of the dolomite are
consistant with dolomitization by cold normal seawater.
These factors taken together present good evidence for
Saller's conclusion.

Carbello and Land (1984) reported dolomite forming
crusts in Sugarloaf Key, Florida which they believe to be
the result of tidal pumping of normal seawater. The
highest concentrations of dolomite were found in the areas
of most active tidal pumping. Water analyses showed the

surface and subsurface waters were essentially normal

12

seawater but evidence of sulfate reduction is present.

Water Chemistry

 

With the exception of normal seawater, pore fluid
sulfur in the various dolomitizing environments is
difficult to characterize due to the effects of sulfate
reduction. Estimates of the sulfur hydrochemistry of the
various models are given in Table 1 and shown in Figure 4.

Hypersaline brine waters are enriched in sulfate due to
evaporative concentration. Sulfate values in the pore
waters of the upper 20cm range from 60 to 140 mmol/kg and
are reduced down to 35 to 109 mmol/kg at a depth of 70 cm
in areas of dolomite formation (Patterson and Kinsman,
1982; Butler, 1969).

Dissolved sulfide in hypersaline pore waters is much
more difficult to characterize. In oxic environments
dissolved sulfide is extremely low to non-existant (Berner,
1972; Goldhaber and Kaplan, 1974; Horne, 1969). So
essentially all dissolved H28 is the result of processes
occuring in anoxic environments, namely sulfate reduction.
As H25 is produced during sulfate reduction it will
immediatly react with iron minerals or dissolved iron to
form iron sulfides, the most abundant of which is pyrite
(Goldhaber and Kaplan, 1974; Drever, 1982). Because
seawater is extremely low in dissolved iron (.036 mmol/kg;

Drever, 1982) and there are few reports of iron minerals in

13

Table 1. Estimated dissolved sulfur content in waters
from the various environments of dolomitization.

14

 

 

 

TABLE 1
Environment 3042' H28
(mmol/kg) (mmol/kg)

Seawater 281 0
Freshwater < 1% of SW1 variable
Hypersaline Brine

Surface 60-1402 0

Pore Water 35-1092 2.5-256
Mixing Zone 2.674

1.19-2.155 .57-1.276

Organic Influence

Sulfate Reduction 5-257 0-.057

Methanogenesis 07 07

1Berner (1984)

2Patterson and Kinsman (1982); Butler (1969): Persian

Gulf sabkha, Kuwait

4Back and Hanshaw (1970) Isla Mujeres, Yucatan

Peninsula

SGebelein et. a1. (1980) North Hawk Creek, Andros

Island, Bahamas

6Calculated from alkalinity increase and sulfate

decrease (see text)

7Goldhaber and Kaplan (1980): Gulf of California

*note- the values calculated for H28 assume no
reaction with iron mineral or loss to surface
and thus represent a maximum that is probably
much too large (see text)

15

Figure 4. Estimates of sulfur hydrochemistry for the
various dolomitizing environments (data from
Table 1).

l6

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cupczcmw E

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.17
recent sabkha environments, dissolved sulfide cannot be
easily removed by precipitation of iron sulfides.

It is possible to make an estimation of the amount of
dissolved sulfide in hypersaline pore waters by setting a
minimum and maximum value for them. The maximum amount of
H28 produced is the amount of sulfate sulfur lost with
depth. This value must be used with caution however,
because gypsum precipitation may account for a large part
of it (Butler, 1969). Using data from Patterson and
Kinsman (1982) the maximum amount of H28 produced in
three sabkha wells would be 25mmols/kg.

‘In areas containing few iron minerals, the minimum
value can be obtained by using the increase in alkalinity
and assuming that the alkalinity increase is proportional
to the dissolved H25 at a ratio of 2:1 (fig.5, eq.1)
(Goldhaber and Kaplan, 1974). This value may be low due to
loss of CO32' via the precipitation of carbonate
minerals. Again using data from the same wells of
Patterson and Kinsman (1982) this yields a value of 2.5
mmols/kg for dissolved H28.

The maximum and minimum values obtained here may both
be too large due to the fact that H25 may simply diffuse
upward and become oxidized upon contact with seawater
(Aharon et. al., 1977; Jorgensen, 1979).

The values obtained here may not be applicable to all
hypersaline environments but they at least put a constraint

of one order of magnitude on one of the most studied ones.

Figure 5.

18

Changes in carbonate alkalinity with depth (left
to right) versus dissolved sulfides. The values
must be corrected for NH4+ due to the fact

that there is some nitrogen in organic materials
that will be converted to NH3. This will

react according to this equation:

NH3 + C02 ==> NH4+ + HCO3'

which gives anamalously high values (Goldhaber
and Kaplan, 1974) The data points are from a
carbonate mud in Devil's Hole, Bermuda and the
line drawn is the predicted relationship
assuming a closed system and eq. 1. (from
Goldhaber and Kaplan, 1974 using data from
Thorstenson and MacKenzie, 1971).

Total dissolved sulfide, millimoles per liter

 

l9

 

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Alkalinity, milliequivalent per liter- ammonia

millimoles per liter

Figure 5

20

Mixing zone pore fluids have a much lower concentration
of sulfate than other models due to the fact that
freshwater contains less than 1% the amount of sulfate that
seawater contains (Berner, 1984). The dissolved sulfide
concentration is also poorly understood but using the
amount of sulfate lost reported in Gebelein et. a1. (1980)
this yields a value of 0.57 to 1.27 mmol/kg mmol/kg for the
Bahama mixing zone.

Organic influence waters go through a wide range of
dissolved sulfur contents with depth. Examples of sulfur
contents with depth are given in Figure 6. In the upper
zone, sulfate is gradually depleted with depth from it's
initial seawater concentration by sulfate reduction. This
may or may not be accompanied by an increase in dissolved
H25 depending on the availibility of iron minerals for
reaction (Berner, 1981; Golhaber and Kaplan, 1974). In
some instances there is a buildup of dissolved H28 where
the production by bacterial reduction is greater than the
removal rate by reaction with iron minerals (Berner, 1972)
(see figure 6) . Below this zone the sulfate is completely
removed and there are essentially no dissolved sulfur
species in the pore water (Gieskes et. al., 1981).
Therefore a dolomite forming in the sulfate reduction zone
may have a wide range of sulfur signatures and one forming
in the zone of methanogenesis should have essentially no
sulfur.

The chemistry of surface seawater is fairly constant

21

Figure 6. Changes in dissolved sulfur species with depth
in three organic-rich, anoxic basins; a) and b)
Gulf of California, c) Santa Barbara Basin (from
Goldhaber and Kaplan, 1974).

Depth, centimeters

3.--. l

Dissolved Sulfide. micromoles per liter
e 2e so 40 , so

 
 
 

22

A
W s

b
0.
. NT.

Depth, centimeters
8
V

 

wL
5 ;
I i
' =oc‘i-
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i ’ i
/ l
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I i 1 l
25°33 5 io l5 20 25 so

Dissolved sulfate, millimoles per

Figure 6a

     

4O

8

O
O
I

Depth, centimeters
a
\

 

liter

Figure 6b

Dissolved sulfide or dissolved sulfate.
millimoles per liter

l 2 3 4 5 6

I T

Figure 6c

'5

       

 

29

Concentration, mullimoles per liter
'0

T

25

23
throughout the world (Holland, 1978). Since it is an oxic

environment there is little or no dissolved sulfide.

IV. SAMPLES USED IN THIS STUDY

Sixty-seven samples from 13 formations were analyzed in
this study. They are listed by proposed dolomite type in
Table 3. Most samples are >90% dolomite although some
limestones were analyzed to detect any sulfur content
differences. The samples range from Middle Proterozoic to
Holocene in age. Formation descriptions of the
depositional environment and evidences for the proposed

model dolomitization are given in Appendix A.

24

25

Table 2. Samples used in this study grouped by proposed
model of dolomitization (for evidences see
individual formation descriptions in Appendix
A).

 

Dolomitization
Model Formation

Hypersaline

Brine
Yalco
Muck
Edwards
Bahama Crust
Persian Gulf

Mixing Zone
Hope Gate
Seroe Domi

Organic

Influence

' Monterey
Gulf of Calif.

Uncertain
Bonneterre
Galena
Niagara
Plomo

26

TABLE 2

£95:

Proterozoic
Jurassic
Cretaceous
Holocene
Holocene

Pleistocene
Pliocene

Miocene
Quaternary

Cambrian
Ordovician
Silurian
Miocene

#
Location Dol
Australia 4
Scotland 1
Texas 4
Bahamas 2
Abu Dhabi 1
Jamaica 6
Bonaire 6
California 3
DSDP #478 2
Missouri 3
Iowa 4

Mich. Basin 11
Spain 3

_|b=w
OCDP‘OCD m

bob

ALHF‘H

V. METHODS

 

The samples in this study were originally intended to
be analyzed using the occult gypsum method (Beales and
Hardy, 1977; 1980). When this method proved inadequate,
quantitative methods of analyses for sulfur abundance and

oxidation state were employed.

Occult Gypsum Method

 

The first method attempted in this study was the occult
gypsum method (Beales and Hardy, 1977; 1980). Occult
gypsums are inclusions of gypsum within the dolomite which
are too Small to be detected by conventional means but
which can be "cultivated" to form larger detectable
crystals. ‘These inclusions are thought to indicate a
hypersaline origin for the dolomite.

The method proposed by Beales and Hardy (1977) was to
first dissolve the dolomite in dilute hydrochloric acid.
Then the insoluble residue is washed with distilled water
to remove the acid and placed in a watchglass filled with
distilled water. As the solution evaporates, small gypsum
crystallites should form around the edges.

This method was attempted on a number of samples from
the Bonneterre, Galena and Seroe Domi formations. Small
crystallites which had the form of gypsum were observed in

27

28
some samples from all formations. These were difficult to
identify because they were so delicate that when extraction
on a needle point was attempted for microprobe examination
the crystallites shattered on contact. The solution was
filtered and the cultured insoluble residue was analyzed by
x-ray diffraction but no gypsum peaks were found.

To determine whether or not the the "occult gypsum”
crystals observed in the above experiments were actually
from the dolomite, a stoichiometric mixture of reagent
grade calcium carbonate and magnesium carbonate was run
through the procedure as a blank. Small crystallites with
the occult gypsum form were observed. This prompted a more
detailed look into the mechanism of occult gypsum
formation.

There are several problems with the ”occult gypsum"
method. The main problem concerns the solubility of gypsum
in the solution used to dissolve the rock sample. It was
found that 250 ml of 5% HCl was needed to completely
dissolve 2 grams of dolomite. Solubility calculations (see
Appendix B), which were supported by laboratory
experiments, show that .718 grams of gypsum can be added to
the dissolution solution before it becomes saturated.
Therefore, in dolomites containing less than .718 grams (or
35.9 weight percent) of gypsum, the gypsum will all go into
solution and will be lost during the washing stage.
Dolomites with gypsum concentrations higher than 35.9%

would be easily discernable in thin section.

29

In an attempt to overcome this problem of losing gypsum
in solution, experiments were run in which the dissolving
solution was retained. This resulted in many crystals
having roughly the form of occult gypsums which were shown
to be CaClz by microprobe analysis. The large number of
these crystals (both large and small) prohibited the
identification of gypsum crystallites if present.

Another problem discovered with this method is the
reaction of other sulfur bearing minerals to the
dissolution process. Some metal sulfides, such as galena
and sphalerite, are soluble in HCl and will become oxidized
to 5042' in the dissolution fluid. Pyrite is exempt
from this problem as it is virtually insoluble in HCl.

This sulfate from oxidation of sulfide minerals could
combine with the abundant Ca2+ ions from the dissolved
dolomite to form gypsum. Although galena and spalerite are
not widespread minerals in carbonates and thus would not be
a problem in most dolomite analysis, they are a common
component of Mississippi valley-type ore host dolomites
which were emphasized in Beales and Hardy (1980).

There also may be contributions to the sulfate content
of the solution by other sulfate minerals such as barite or
celestite but these are probably not volumetricly
important.

The main difficulty with this method is that even if
the aforementioned problems are insignificant, it is at

best a semi-quantitative method for sulfate analyses.

30
Therefore, it was concluded that the use of quantitative

sulfur analyses would be more useful in this study.

Sulfur Analyses

 

Carbonate samples were first crushed in a chipmunk jaw
crusher and then ground using a rotary disc mill
pulverizer. For sulfate analysis, which required finer
grain sizes, the samples were powdered using a ball mill.

The samples were originally intended to be leached with
EDTA to remove calcite from the dolomite (Videtich, 1981).
This process was foregone due to the fact that sulfur
inclusions in the limestone, especially pyrite, would be
concentrated in the dolomite residual giving anomalous
values. It was decided to use pure end members of dolomite

and limestone.

Total Sulfur Method

 

Total sulfur was analyzed using a Leco induction
furnace. A weighed quantity of sample is mixed with a
conducting flux of pure iron, tin and copper. This mixture
is then placed within an induction coil. Oxygen is passed
over the mixture and power is applied to the coil. The
flux melts under the strong electromagnetic field
decomposing the sample. All forms of sulfur are gassed off

as sulfur dioxide. This gas is dissolved in a weakly acid

31
solution and is measured by titration with a standardized
solution of potassiun iodate in the presence of potassium
iodide and starch. The analytical error of this method for

replicant samples was .004 weight percent.

Oxidation State

 

The method used to quantify the oxidation state of the
sulfur is to analyze the amount of sulfur in a particular
oxidation state (either oxidized or reduced) and subtract
this value from the total sulfur value to obtain the amount
of sulfur in the other oxidation state (Ricke, 1960).
Methods for obtaining the percentage of sulfur in a
particular oxidation state were attempted by myself and two
commercial chemical laboratories.

The method I used was that of Murthy et. al. (1956) in
which powdered rock sample is decomposed in hydriodic acid
in a nitrogen atmosphere. Metal sulfides will react to
form H28. This gas is then washed through two gas
washing bottles filled with cadmium hydroxide. The gas
reacts to form cadmium sulfide and water. The amount of
sulfide sulfur can then be determined iodiometricly. This
process was standardized using analytical reagent grade
magnesium carbonate and calcium carbonate and analytical
reagent grade zinc sulfide as the spiking agent. The
results of these experiments were in agreement with the

predicted values with a standard deviation of .002 weight

32
percent. However, when actual dolomite samples were tested
(some with pyrite or sphalerite spikes) or pyrite was used
in the reagent matrix, the values were significantly lower
than predicted. Insoluble residues from pyrite spiked
samples contained pyrite which demonstrates incomplete
dissolution even though the recommended mercury catalyst
for pyrite decomposition was used (Murthy and Sharada,
1960).

A similar process was attempted by a commercial
laboratory using a slightly different acid mixture and more
vigorous heating. They too had problems with poor recovery
of sulfide standards and were unable to acheive
satisfactory results after 2 1/2 months despite numerous
adaptations on the method. They noted the appearance of a
condensate on the apparatus which they believed was
mercuric sulfide. This could explain the poor recovery.

An alternative method was attempted by another
laboratory which analyzed for sulfate sulfur by a leaching
process with sodium carbonate. A weighed quantity of
sample was leached for one hour with a boiling solution of
2% sodium carbonate solution. The filtrate was acidified
with HCl and the sulfate separated by precipitation as
barium sulfate. This precipitate was filtered off, ignited
and weighed to determine sulfate. These results were much
more satisfactory with an analytical error of .004 weight

percent for replicant samples.

V. EXPERIMENTAL RESULTS

 

The results of the sulfur analyses are given in Table 3
and shown in figures 7-9. Figure 7 shows all dolomites,
Figure 8 shows all limestones analyzed in this study.
Figure 9 shows dolomites grouped by proposed dolomite type.

There are a few interesting features to note in the
distribution of sulfur contents of the various samples. In
Figure 7 it can be seen that there is a wide scatter of
sulfur signatures but there is a concentration of dolomites
with no appreciable sulfate which plot along the y-axis.
This may be due to leaching of evaporitive minerals by
groundwater. There are a few dolomites with extremely high
contents of both oxidation states (Gulf of California,
Persian Gulf, Monterey).

The limestones (Figure 8) show approximately the same
distribution but with less points exhibiting extremely high
values, especially in sulfate contents. A t-test showed no
differenCe in the dolomite and limestone populations with
respect to both forms of sulfur. In fact, the percentage
of limestones exhibiting appreciable sulfate is essentially
the same as the dolomites (S of 18 for limestones as
opposed to 16 of 42 for dolomites) although not as high.

In Figure 9 it can be seen that there is a wide
distribution of sulfur contents not only within dolomites

33

34

Table 3. Sulfur analyses for all samples used in this
study.

Dolomites

 

Formation

 

Yalco

Bonneterre

Galena

Niagara

Muck

Edwards

Plomo

Monterey

Seroe Domi

Hope Gate

Sample #

ibUUNH

35

Table 3

%Total Sulfur

 

.007
.006
.025
.006

.009
.002
.006

.005
.005
.003
.002

.012
.017
.002
.017
.063

.016

.067
.084
.010
.007

.092
.092
.141

.002
.047
.169

.019
.022
.017
.020
.018

.006
.015
.022
.017
.040
.018

%Sulfate

.003
(.001
(.001

.003

(.001
(.001
(.001

(.001
(.001
(.001
(.001

.002
.002
(.001
(.001
(.001

(.001

(.001

.005
(.001
(.001

.003
.009
.004

(.001
.044
.031

(.001
(.001
(.001
(.001
(.001

(.001

(.001

(.001
(.001
.029
.006

%Sulfide

.004
.006
.025
.003

.009
.002
.006

.005
.005
.003
.002

.010
.015
.002
.063
.063

.016

.067
.079
.010
.007

.089
.083
.137

.002
.003
.138

.019
.022
.017
.020
.018

.006
.015
.022
.017
.011
.012

36

Table 3 (cont.)

 

Dolomites(cont.)

 

 

 

 

 

Formation Sample # %Total Sulfur %Sulfate %Sulfide
Gulf of Calif. 50 .152 .026 .074
51 1.40 .800 .600
Bahama Crust 6 .065 .028 .037
Persian Gulf 72 .460 .322 .138
Limestones
Bonneterre 28 .003 (.001 .003
Galena 23 .051 (.001 .051
Niagara 58 .007 (.001 .007
S9 .001 <.001 .001
60 .008 <.001 .008
Edwards 47 .006 (.001 .006
Plomo 83 .072 (.001 .072
84 .046 .011 .035
85 .020 .003 .017
86 .032 .003 .029
Seroe Domi 37 .013 (.001 .013
62 .005 .002 .003
63 .006 (.001 .006
64 .022 <.001 .022
Hope Gate 16 .046 .039 .007
80 .012 <.001 .012
81 .038 <.001 .038
82 .012 <.001 .012

37

Figure 7. Sulfur chemistry of all dolomite samples used in
this study.

38

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hzmommm Fromm: zu mnmgzm uh¢m43m
a. a. a.

0 on . .- TOW.
bbbb bL lb L .bLb D b (D l. bbbbbL b b b LI)
1 l

o

 

onAm
cmmacaz
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mmmmwwzzom
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>umm~zoz
upco mmo:
“zoo momma
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hwazo czcrcm
womczom

x03:

o04¢>

+
0'"
1N33838 IHOIBH NI 805103 3013109

XIIIIW. X N

' '

 

EJEJG +UX‘0‘6IKlfllbltakbd
n;
N
4
N

 

 

4n

39

Figure 8. Sulfur chemistry of limestones used in this
study.

40

w ouswfim

hzuommm tau—um: zu «Dayan mhcmqnwo

 

 

 

 

on: o.- T? e. an
.illl.ilL, it --.LL .It i. plii it; t on:
nu
HI
.3
e m.
a:
+ .n.
1
v.4.
N m
N m ..N.
Mu
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38 was: I. am
:8 momma e .m.
28.: N . a
85138. 4 , m.
5.552 x H a
5.38 x m .N.
535228 > .m.

41

Figure 9. Sulfur chemistry of dolomites grouped by
proposed model of dolomitization; a)hypersaline
brine; b)mixing zone; and c) organic influence.

42

 

A A A

 
 
 
    
 
  

mfl

A AAA-

 

A“.

d

III YRLCG

0 HUCK

A EDHRRDS

+ BRNRHR CRUST
X PERSIRN GULF

 

SULFIOE SULFUR IN HEIGHT PERCENT

 

v vvv vvvvvv' v

g

'63, -., id‘ - - to - - -.-1°.
SULFRTE SULFUR IN HEIGHT PERCENT

A AA_AA
e

Figure 9a

----4m‘

 

 

  
   
  

‘3 x HONTEREY

Z GULF OF CRLIF. 9 SERGE 00H!

4 HOPE GflTE

SULFIDE SULFUR IN HEIGHT PERCENT

A A

 

Q

fffvrvt W v vv—v'

To‘gfili ‘

to 10* ' Vivvio
SULFRTE SULFUR IN HEIGHT PERCENT

f‘

“JAI‘

Figure 9b

t‘ -

AAA

SULFIDE SULFUR IN HEIGHT PERCENT

 

 

 

TOT v----.‘6‘ - frnntt'l" - rf'fito'
SULFATE SULFUR IN HEIGHT PERCENT

Figure 9c

43
formed by the same model but also within dolomites from the
same formation. The Monterey formation shows probably the
largest variation with a point near the origin and a point
high in both forms of sulfur. This variation causes the
fields delineated by the different models to be large and
poorly defined.

The mixing zone dolomites have a fairly tight
distribution but there are a few anomalous points. They
are dominantly sulfate free as might be expected due to the
fresh water influence (freshwater contains less than 1% the
amount of dissolved sulfate found in seawater).

The high sulfate content in the organic influence
dolomites is rather hard to explain considering the highly
reducing conditions that are believed to exist in these
environments. There are reports of anhydrite beds in a
neighboring drill hole of the Gulf of California dolomites
which are believed to be the result of evaporation of
seawater during intrusion of dolerite dikes (Shanks and

Niemitz, 1981).

VI. DISCUSSION

 

From a consideration of the sulfur distribution of the
various dolomite types as shown in Figure 9, it appears
that sulfur is of little use as a trace element in
separating dolomites formed by the models considered in
this study. There is no separable variation between the
fields defined for the dolomites from the three different
models (Figure 10).

The reason for the non-conformity of the data to the
proposed distribution based on the sulfur hydrochemistry of
the pore fluids (Figure 4) is probably due to one or more
of the following factors; 1) Inhibition of sulfur
incorporation into the dolomite crystal; 2)
post-dolomitization leaching or precipitation of sulfur
compounds by pore fluids; and 3) recrystallization of the
dolomite at depth.

Sulfate ions will not be easily incorporated into the
dolomite lattice because they are so much larger than the
carbonate ions for which they might substitute. Sulfate
will probably be present only as cryptocrystalline or fluid
inclusions which may be more susceptable to alteration than
a lattice substituting trace element such as stontium.
Sulfide ions are small enough to fit in the carbonate
sites, but they may be so small that they are not tightly

44

45

Figure 10. Fields of sulfur chemistry for dolomites
proposed to be formed by the various models
(based on Figure 9).

46

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o3 muamam

q.r

v

hzmommm proumz za mauqam whcmgzm
a. we. re“ a

 

 

1T“

 

 

 

mupm: aux”: Wmmm

mzuxm mzu4cwmum>z = =

 

 

 

 

 

 

1N33833 lHflIBM NI 803109 3013109

 

47
bound.

Sulfate leaching by porewaters is believed to be quite
extensive (Beales and Hardy, 1980). This could account for
the high amount of samples in Figures 7 & 8 which plot on
or near the y-axis. Using a small amount of imagination
and Figure 7, it is possible to distinguish a trend in
sulfur contents from the different formations. If we
consider the Persian Gulf dolomites as a good example of a
recent hypersaline brine dolomite we can trace the
concentration path during sulfate leaching (arrow on Figure
7). Since sulfide minerals (dominantly pyrite in carbonate
rocks) are relatively stable above a pH of 3 (which
includes most natural waters), the concentration path will
be driven towards the y-axis. There are two hypersaline
dolomites that would lie along that pathway, the Plomo
Formation and the Edwards Formation. The three dolomites
decrease not only in sulfate content but also in age from
Recent to Miocene to Cretaceous. This may suggest that
there is gradual sulfate depletion through time. Obviously
a much larger data set including shorter time intervals
than used in this study would be necessary to conclusively
prove this hypothesis.

Another interesting feature in Figure 7 is the location
of the points for the Bonneterre and Galena formations.
They exhibit the lowest average sulfur values of all
formations even though the Bonneterre dolomite is suspected

of originally being hypersaline in origin. One feature

48
these two formations have in common (besides old age), is
that both are believed to have undergone some growth and/or
recrystallization by hydrothermal solutions (see Appendix
A). Recrystallization at this elevated temperature could
account for this lack of sulfur inclusions.

There may also be post-dolomitization precipitation of
sulfur minerals as pore-lining cements. This would mask
the original sulfur signature imparted to the rocks during
dolomitization. These cements were not observed in thin
section for any of the rocks used in this study, but they

may have been too small to be distinguishable.

CONCLUSIONS

 

Based on the distribution of sulfur abundances and
oxidation states in the dolomites studied, it appears that
sulfur is of little or no use as a dolomitization
environment indicator. There was no significant difference
between the sulfur concentration fields defined by
dolomites that were presumed to be of different origins.
Nor is there a significant difference between limestones
and dolomites. There was also a large variation of sulfur
contents within dolomite fields considered to be of the
same origin as well as dolomites from the same formation.

In view of the wide scatter of the sulfur data and the
aforementioned problems with the occult gypsum method, the
conclusion reached by Beales and Hardy (1980) that dolomite
is formed predominantly by hypersaline brines appears to be
invalid.

The reason for the non-conformity of the sulfur data to
the proposed model based on the sulfur hydrochemistry of
the pore waters is believed to be due to; 1) difficulty in
sulfur incorporation into the growing dolomite crystal;
and/or 2) leaching of evaporite minerals by groundwater;
and/or 3) loss during recrystallization at elevated

temperatures.

49

APPEND I X A

 

Formation Descriptions of Samples Used in this Study

50

YALCO FORMATION

 

The Yalco Formation (Middle Proterozoic) is found in
Northern Australia. It is overlain by the cross-bedded
quartz-rich Stretton sandstone and underlain by the
evaporitic and dolomitic Lynott Formation. Muir et. al.
(1980) proposed these rocks as an ancient analog to the
modern Coorong lagoon. The Coorong Holocene dolomites show
tidal flat characteristics alternating with estuarine
conditions. There are numerous stromatolites and algal
laminations along with dessication features such as
polygonal mudcracks (Muir et. al., 1980). The Yalco
Formation is remarkably similar to the Coorong dolomite
showing all these tidal flat features as well as a lack of
evaporites.

The Coorong lagoon is believed to have been dolomitized
by a form of the hypersaline brine model in which seawater
and freshwater is collected in coastal lakes during the
humid summer months and then evaporates during the dry
winter months. This forms a concentrated brine believed
responsible for the dolomitization (von der Borch, 1976).
One effect of the freshwater influence is to add
organically derived C02 thus raising the bicarbonate
content (Botz and von der Borch, 1984). A second effect

may be to create a mixing zone environment during the rainy

51
season. Few if any evaporites are found in the sediment.
This could be attributed to dissolution during the wet
season or the low concentration of sulfate in the fresh
water.

There were four core samples analyzed in this study
which came from the Amoco DDH 82-6 well. They consist
mainly of dolomitized algal breccia with silica cements of
varying amounts as well as a small amount of detrital

silica.

BONNETERRE FORMATION

 

The Bonneterre Formation (Upper Cambrian) contains the
Mississippi Valley-type ores in the Viburnam lead-zinc
province. There are four main facies in this district
which represent a transgressing sea onto the cratonic
shelf. The St. Francios mountains which formed a
Precambrian high served as a minor shelf area on which the
Bonneterre was deposited as a intertidal to supratidal
facies. Around this high is a stromatolitic reef with
associated oolite facies. The remainder of the Bonneterre
is a deep water micrite and shale facies (Larsen, 1977).
The facies distribution is given in Figure 1A.

The method of dolomitization is under debate but may be
due to hypersaline brines associated with the supratidal
facies (Beales and Hardy, 1980). There is a great deal of

mineralization which was associated with hydrothermal

52

Figure 1A. Facies distribution in the Bonneterre formation
(from Larsen, 1977).

53

o ' 4'15"

"FEET-film ’iTKaGiN
IGITATE STROMATOLITE
| FAClES lL /

stops TO BASIN ‘
lOOLlTE FACIES rim": ---- "§E\
41ml BASIN
i l""‘"""""""

.
MICRITE a SWC:

"91:5“!‘1‘1 FACIES as;

 

«In
300-“

‘50‘__~__‘

   
  

  
  
  

 

 

 

 

  

 

 

 

 

 

mssoum i
d e e

"'""““ LEGEND

- NEWMAN MW
-50- CONTOUR INTERVAL 50 Feet

 

 

 

 

Figure 1A

54
solutions and it is possible that this solution was
responsible for some of the dolomitization. Baroque or
saddle dolomite is quite common and may indicate formation
at elevated temperatures (Gregg and Sibley, 1984; Radke and
Mathis, 1980).

The rocks in this study consist of four samples from
the platform or "white rock" facies. There are three
dolomites, one of which has baroque dolomite and one
peloidal limestone. The dolomites appear to have been
dominantly algal material and mud but much of the original

texture has been obliterated by dolomitization.

GALENA FORMATION

The Galena Group (Ordovician) is found in the Upper
Mississippi Valley. It is a series of limestones and
dolomites which represent deposition on a shallow broad
shelf area (Delgado, 1983). The rocks are 95% bioturbate
mudstones and wackestones or their dolomitized
equivalents. The other 5% are grainstones which are
thought to be the result of storm deposits. The lack of
sedimentary structures such as ripples or rip up clasts
combined with the faunal assemblage and lack of exposure
features indicates that these deposits formed below wave
base (Delgado, 1983).

The mineralogy of the section at Guttenburg Iowa ranges

from 100% porous, vuggy dolomite at the top of the section

55
to mottled partially dolomitized areas in the middle of the
section to only slightly dolomitized limestones at the base
of the section. There are numerous chert nodules in the
middle of the section which are concentrated along bedding
planes.

The method of dolomitization was originally believed to
be the result of a mixing zone model (Badiozamani, 1973).
This was based on the low sodium (190 ppm) and strontium
(37 ppm) contents as well as carbon and oxygen isotopes.
Recent evidence implies that the dolomite has undergone
neomorphism by hot waters associated with lead-zinc
mineralization (Gregg and Sibley, 1984). In fact, Figure
2A from Badiozamani (1973) shows that the carbon and oxygen
isotopes of the Mifflin member fall in the range of
hydrothermally alteration, not in between freshwater and
seawater as you would expect from a mixing zone model.

This neomorphism and other recrystallizations with depth of
the original dolomite could account for the low sodium and
strontium values. Therefore, the origin of the original
regional dolomitization in the Galena Formation is still
uncertain.

There were five samples from the Galena Group analyzed
in this study. They consist of four 100% dolomite samples
from the upper porous unit and one fossiliferous limestone
wackestone from the base of the unit. The dolomite samples
consisted entirely of cloudy dolomite rhombs with good

porosity and the limestone sample consisted of large

56

Figure 2A. Isotope data from the Mifflin member in
relation to carbonates precipitated from other
environments (from Badiozamani, 1973).

57

 

30-

 

 

l'_"

 

 

 

 

13m.

 

Hydro-

Ireeh Weter "'"""7

Alt. ht.

 

 

 

 

 

1

 

-20

Ot-

-IO 10

5 (ll

Figure 2A

(root

60

58
gastropods, brachiopods and trilobites in a lime mud

matrix.

NIAGARAN FORMATION

 

The Niagara Formation (Middle Silurian) consists of
carbonates and evaporites deposited in a epicontinental sea
(Sears and Lucia, 1980). There was prolific reef growth
associated with the margins and shelf areas of the
developing intracratonic basins (Fig. 3A). In the Michigan
Basin a number of pinnacle reefs developed basinward of the
main reef trend. In Indiana, the reefs were characterized
by broad platform reefs and scattered patch reefs (Shaver
and Sunderman, 1982). This reef growth was inhibited and
eventually stopped by restriction and a sea level drop
which resulted in the A-1 and A-2 Evaporite deposition
which overlies the reefs (Briggs, 1980). The A-l Carbonate
was deposited in between these two evaporite sequences
during a sea-level rise (Sears and Lucia, 1980).

The reefs are composed of stromatoporoid and coral
framework infilled with interreef sediments and mud. They
are surrounded by a reef flank facies consisting of
crinoidal debris in the pinnacle reef zones (Sears and
Lucia, 1980) and echinoderm, brachiopod and gastropod
debris in the Indiana reefs (Shaver and Sunderman, 1982).
There are deeper-water mudstone facies away from the reefs.

The samples from the Niagaran used in this study come

59

Figure 3A. Niagaran reef growth associated with basin
margins and shelf areas during Silurian time;
dots are discrete reefs, stipples are carbonate
banks or barrier reefs (from Shaver et. al.,
1978).

6O

 

 

 

 

 

    

(EV . KY.
,. \- TERRE HAUTE
BANK

 

 

 

7

 

Figure 3A

 

61
from a Northern Michigan pinnacle reef and two quarrries in
North Central Indiana. The pinnacle reef samples are from
the Shell State Union 1-8 well in Grand Traverse County,
Michigan. This reef is approximately 130 meters thick and
is overlain by the A-l Carbonate (Cercone, 1984). There
is an abrupt contact between the top of the reef and the
overlying A-l Carbonate (Cercone, 1984). The A-l Carbonate
is a dark algal mudstone which shows evidence of subaerial
exposure (Sears and Lucia, 1980).

There are two petrographically distinct dolomite types
found in these pinnacle reefs (Sears and Lucia, 1980). One
type consists of clear euhedral rhombs which usually
comprise no more than 30% of the whole rock, the remainder
being calcite. The other consists of a brownish cloudy
anhedral dolomite which usually comprises 90-100% of the
rock. The clear dolomite appears to have predated
extensive freshwater neomorphism whereas the cloudy
dolomite appears to postdate it (Sears and Lucia, 1980).

Sears and Lucia (1980) believe the clear dolomite is of
mixed water origin mostly due to the lack of inclusions
which Folk and Land (1975) attributed to a dilute
solution. They believe the cloudy dolomite is the result
of hypersaline brines. Their evidence for this is that
there are evaporites overlying the dolomitized A-l
Carbonate which is petrographically similar to the cloudy
Niagara dolomite. The dolomitization pattern of the A-l

Carbonate and the Niagaran Formation coincides (see Figure

62

Figure 4A. Dolomitization patterns from the A-l Carbonate
and the Niagaran Formation in the pinnacle reef
trend, Northern Michigan (from Sears and Lucia,
1980).

63

KALKASKA AREA
Linesroummomre oisrmauriou
mun-nest A-I cannon“:

 

 

 

 

  
 

 

 

 

 

 

GOLOMITE

Limestone r.
:2: oatomrs

a: LIMESTONE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

KALKASKA AREA
unesrouuoommre DISTRIBUTION
NinoAnAN nears

 

 

 

o e
-s»-“’"
If an.
7' a
’I 9
gang; ooLomre
LlMESTONE e
:3 Domain
:3 LIMESTONE
ew '

 

 

Figure 4A

64
4A). Carbon isotopes of the cloudy dolomite are slightly
heavy (+3.7 PDB) which may suggest a methanogenesis
influence (Sears and Lucia, 1980). Strontium
concentrations are quite low for a hypersaline dolomite (74
ppm) compared to the predicted value of 600-700 ppm
(Behrens and Land, 1973). They believe this is due to
recrysallization of the dolomite and/or precursor
limestone.

Six samples were analyzed from this core. Three
samples (#10, 73, 79) are of the completely dolomitized
cloudy type (#10 is A-l carbonate) and three are of the
clear partially dolomitized type (#74, 75, 77). With the
exception of #10, all are from the pinnacle reef facies and
consist of coral-stromatoporoid wackstones and packstones.

The second suite of samples from the Niagara Formation
is from two quarries in Indiana, the Pipe Creek Jr. and
Delphi quarries. The two quarries expose two reefs which
have had their tops truncated by erosion. They are also
coral-stromatoporoid reefs and are flanked by echinoderm,
brachiopod and mollusc debris (Shaver and Sunderman, 1982).

An interesting feature about these two localities which
are only abount 40 miles apart is that the Delphi reef is
almost completely dolomitized and the Pipe Creek Jr. reef
is almost all limestone (Shaver and Sunderman, 1982). The
reason for this difference is believed to be early
cementation of the Pipe Creek Jr. reef which decreased

permeability and hence inhibited dolomitization (Lehmann,

65
1978). The method of dolomitization is unclear but
evaporites are found directly above the reefs (Shaver and
Sunderman, 1982) which may indicate hypersaline brine

dolomitization.

MUCK DOLOMITE

 

The Muck Dolomite (Middle Jurassic) is found in
North-West Scotland on the island of Muck. It is a part of
the Ostracod Limestone which is a member of the Great
Estuarine Series. As the name implies, these deposits were
formed in marginal marine to brackish water coastal lagoons
(Tan and Hudson, 1974). The water depth was quite shallow
and periods of evaporation are marked by mudcracks and
brecciation (Tan and Hudson, 1971). The faunal assemblage
is non-marine and shows conditions of variable salinity
(Tan and Hudson, 1971). The rocks themselves are
dominantly mudstones and mostly pure dolomite.

The dolomitization is believed by Tan and Hudson (1971)
to have taken place early in diagenesis and to be the
result of evaporation and concentration of the lagoon.
Their main evidence is the enrichment in isotope values
(-0.4 ppm PDB as compared to -2.4 PDB of the limestones and
-3.0 ppm PDB of other dolomites in the area) as well as
mudcracks and brecciation indicating evaporation. The
carbon isotopes in this locality show little organic

influence (-2.4 PDB) although other members of the series

66
have very light carbon isotopes (-14.1 PDB).
There is only one sample of the Muck Dolomite analyzed

in this study. It consists of a dolomitized mudstone.

EDWARDS FORMATION

 

The Edwards Formation (Lower Cretaceous) covers a large
part of Central Texas. It was deposited on a large
platform in shallow water which periodically experienced
conditions of restriction during which time evaporites were
deposited (Fisher and Rodda, 1969). Figure 5A shows the
distribution of the Edwards facies which were primarily
rudist reefs, carbonate grainstone interreef facies and
lagoonal deposits on the platform.

Dolomitization is associated with the evaporite lagoons
(Fig. 6A). Based on it's stratigraphic distribution and
the presence of collapse breccias, the dolomite is believed
to be a classic example of the hypersaline brine model of
dolomitization (Fisher and Rodda, 1969).

There were five samples analyzed from the Edwards
Formation. They consist of four dolomites and one
limestone. There are evaporites or solution breccias

associated with the dolomite samples.

PLOMO FORMATION

 

The Plomo Formation (Late Miocene) forms a series of

67

Figure 5A. Distribution of facies in the Edwards Formation
(from Fisher and Rodda, 1969).

68

.._._.—

OKLAHOMA

 

Figure 5A

69

Figure 6A. Dolomitization of the Edwards Formation with
respect to the Kirschberg evaporite lagoon
(from Fisher and Rodda, 1969).

70

   

 
 

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71
reefs with interreef facies on the southern coast of
Spain. These reefs were deposited in extremely shallow
water on the margin of the former straits which connected
the Atlantic Ocean and Medditerranean Sea (Addicott et.
al., 1978).

The rocks of the Plomo Formation consists of reefs
built with predominatly porites framework corals and
forereef debris. The porites framework is infilled with
halimeda, molluscs, coralline algea and lime mud (Addicott
et. al., 1978).

The diagenesis of this unit began with submarine
cementation and micritization of fossil fragments. This
was followed by extensive dolomitization (Armstrong et.
al., 1980). The mechanism of dolomitization is unclear but
there is a massive gypsum unit in the overlying beds that
represents the Messina salinity crisis (Estaban et. a1.
1977). This crisis would have generated a large volume of
hypersaline fluid which could be responsible for
dolomitization.

There are seven samples from the Plomo Formation
analyzed in this study. They consist of three dolomites

and four limestones.

MONTEREY FORMATION

 

The Monterey Formation (Miocene, California) is an

organic rich marine shale that was deposited in an

72
extensive anoxic ocean basin (Friedman and Murata, 1979).
Dolomite and limestone occur as beds, lenses and
concretions within the shale. The dolomite appears to be
concentrated along zones of denser carbonate fossils, but
is believed to be primarily a cement (Murata et. al.,
1969).

The dolomitization appears to be controlled by organic
processes. The carbon isotopes show extreme varibility
from highly positive (+21 ppt PDB) to highly negative (-30
ppt PDB) (Pisciotto and Mahoney, 1981). The light carbon
values are thought to reflect dolomite formed in the
sulfate reduction zone (Pisciotto and Mahoney, 1981). The
heavy carbon dolomites are probably the result of
methanogenesis (Pisciotto and Mahoney, 1981). During
methane production the light carbon is preferentially
removed leaving heavy carbon a residual (Irwin, 1980).

The samples analyzed in this study consists of three
outcrop samples from southern California. All occured as
concretions. They consist of dolomite which appears to be
mostly a cement and contained siliceous mud. These rocks
did have carbon isotopic analyses which yeilded two very
positive samples (#42 and 44) and one which was variable
from highly positive to negative (#43) (see petrographic

reports).

SEROE DOMI FORMATION

 

73

The Seroe Domi Formation (Pliocene to Pliestocene)
outcrops on the western coast of Bonaire, Netherland
Antilles. The rocks represent carbonates deposited on a
Cretaceous volcanic island during a sea level rise
(Bandoian and Murray, 1974). The rocks represent a
build-up of fore-reef carbonates up to a shallow-water
platform at the top (Bandoian and Murray, 1974).

Dolomitization of the platform carbonates is quite
extensive and was first thought to be the result of
hypersaline brines (Deffeyes et. al., 1965). This
hypothesis was developed mainly on the presence of
hypersaline lakes on the southern end of the island which
contained dolomite in the bottom sediments. Deffeyes et.
a1. (1965) believed that these lakes were a recent analog
to the environment which dolomitized the Seroe Domi.
Recent evidence has cast doubt on this model for the
Bonaire dolomites. There are few or no evaporites found
outside of the hypersaline lakes (Sibley, 1980) and the
hydrology of the lakes does not appear consistant with the
extensive refluxing conditions necessary for large scale
hypersaline dolomitization (Murray, 1969). The sodium
content of the dolomite is quite low which suggests fresh
water influence and there is freshwater cement included
within the dolomite (Sibley, 1980). Oxygen isotopes are
high (+2.1 to +4.1 PDB) but could be explained by
evaporation of freshwater (Sibley, 1980). These evidences

favor a mixed-water model for dolomitization of the Seroe

74
Domi Formation.
Ten samples from the Seroe Domi Formation were analyzed
in this study. They were all collected in the Santa
Barbara Hill area. They consist of six dolomites and four

limestones.

HOPE GATE FORMAT ION

 

The Hope Gate Formation (Middle Pliestocene, North
Jamaica) consists of a large reef system deposited on
Miocene pelagic chalks during a period of high sea level
(Land, 1973a). The rocks consist of reef debris such as
corals, red algea, foraminifera, molluscs and echinoderms
in a micrite supported matrix.

Dolomitization of these rocks is believed to be a
result of mixing-zone fluids by Land (1973a). His main
evidences are; 1) lack of associated supratidal sediments
or evaporites and 2) sodium (400 ppm), strontium (220 ppm)
and carbon-oxygen isotopes (+1.2 and +2.2 PDB respectivly)
which point to a freshwater influence on the dolomitizing
fluid. These rocks are similar to the Seroe Domi Formation
and Bahaman dolomites both depositionally and
diagenetically (Sibley, 1980).

Ten samples were analyzed in this study from the Hope
Gate Formation. They were collected on the north coast of
Jamaica. They consist of six dolomites and four

limestones. All samples contain some sparry calcite cement

75
(less than 10%) and the limestones appear to have been

totally replaced by low magnesian calcite.

GULF OF CALIFORNIA DOLOMITES

 

Dolomite occurs in Gulf of California sediments which
are Quaternary in age and consist of organic-carbon rich,
hemipelagic diatomaceous oozes. They were deposited in
deep quiet water anoxic basins similar to the Monterrey
formation environments (Kelts and McKenzie, 1982).

The dolomite occurs as thin beds within the mud and is
believed to form primarily as a cement with some
replacement of the small amount of calcium carbonate
fossils (Kelts and McKenzie, 1982). Although originally
interpreted as a product of sulfate reduction by Baker and
Kastner (1981), the dominance of heavy carbon isotopes
suggests methanogenesis is important in the dolomite
formation (Kelts and McKenzie, 1982).

Two samples consisting of mostly dolomite with some
silica were analyzed from the Gulf of California. The
samples are from site 478 of Leg 64 of the Deep Sea
Drilling Project. The dolomite is fine grained with no
discernable fossils or sedimentary structures. Sample #51

contained a small amount of pyrite visible in hand sample.

BAHAMA CRUST DOLOMITES

 

76

Holocene dolomite crusts are found on Andros Island in
the Bahamas. The crusts are composed mostly of intertidal
sediments which are washed up onto the supratridal zone by
high tides and storms. The sediments consist of mainly
peloidal carbonate mud with algal mats, roots, land snails
and a few marine foraminifera and gastropods. Subaerial
features such as polygonal desication cracks and karst
features are widespread (Shinn et. al., 1965).

The mineralogy consists of poorly ordered dolomite with
aragonite and calcite cements and grains. The
dolomitization is believed to be the result of evaporation
concentrated brines which are brought up to the surface by
capillary action (Shinn et. al., 1965). Shinn et. al.
(1965) found that the magnesium-calcium ratio of this fluid
was in excess of 40 to l with salinities 5 to 6 times that
of seawater thus making it an excellent hypersaline brine
for dolomitization.

Two samples of the Bahama crust dolomites were analyzed
in this study. They are from Andros Island and contain
poorly ordered dolomite and calcite. The textures are
poorly preserved but peloids and mollusc fragments are
discernable. The calcite is found primarily as a cement

but is also found in original grains.

Persian Gulf

 

Recent dolomite has been reported forming in the

77
Persian Gulf (Butler, 1969; Patterson and Kinsman, 1982).
It is supratidal and is found in sabkhas which is strong
evidence for a hypersaline origin.
One sample of this dolomite was analyzed in this study

and comes from the Abu Dhabi sabkha.

APPEND I X B

 

Solubility Calculations in Occult Gypsum Solution

II.

78

APPENDIX B
Solubility Calculations in Occult Gypsum Solution

 

Solubility product of gypsum at 25°C =10"4-61
(Drever, 1982)

Activity coefficient calculations
Ionic strength (I) = 1/2 mizi2

If no gypsum were present, the only input of ions
would be from the dissolving dolomite and the 5% HCl
solution.

Input from dolomite = 2g dol x 1 mol/184.3g
= 1.09x10'2 moles

so, this would contribute 1.09x10'2 moles of Ca2+
and Mg2+ ions and 2.18x10'2 moles of CO32'

(some C032“ will be lost due to shifting of the
carbonate equilibrium and C02 evolution)

Input from HCl = 38% HCl x 5% sol. x 1.1Bg/ml x 250ml
5.619 HCl x 1 mol/36.Sg
1.54x10'1 moles

so this would contribute 1.54x10’2 moles of H+ and
Cl' ions

multiplying the number of moles by 4 (to get
molarities for a 250 ml solution) this yeilds for I:

1/2([4.36x10'2 x 4] + [4.36x10'2 x 4] +
[8.72x10‘2 x 4] + [6.14x10’1 x l] +
[6.14x10‘1 x 1])

9.97x10‘1

I

Using the modified Debye-Huckel equation for activity
coefficients (log aC' = -Azi2 I / l + Bao I):

A =.5085; B =.3281x108 at 250C

ac(Ca) = 6x10'8; ac(SO4) = 4x10"8

(Drever, 1982)

this yeilds: ac(SO4) = .134
ac(Ca) = .209

79

III.Saturation state calculations

The sulfate in solution is contributed by the acid:

mSO4 = .00002% $04 in acid x 5% solution x

1.18g/m1 x 250ml x 1 mol SO4/96g
3.07x108' moles x 4(250ml solution)
1.23x10‘7 moles/l

The IAP for gypsum in this solution is:

(4.36x10’2)(.209)(l.23x10'7)(.l34)

IAP
gyp 1.47x10’10

to reach equilibrium IAP = KSp = 10'4-51

so, to reach equilibrium, the amount of gypsum that
must be added is:

(4.36x10'2 + mCa)é.209)(1.23x10’7 + mso4)
(.134) = 2.45x10'

mCa = mso4 = X

multiplying this out yeilds:

2.80x10’2x2 + 1.22x10‘3x - 2.4Sx10’5 = o
solving by the quadratic equation yeilds:
x = 1.50x10'2 m504/1

Converting to grams:

(1.50x10'2 mso4/1)(.251)(1729/mol)

wt. of gyp
6.43x10'lg CaSO4.2H20

this corresponds to 32.1 wt. % in the analyzed sample

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