CHEMICAL
GEOLOGY
fNCWDlNG
ISOTOPE
ELSEVIER
GEOSCIENCE
Chemical Geology 143 (1997) 121-125
Sources of basinal and Mississippi Valley-type mineralizing
brines: mixing of evaporated seawater and halite-dissolution
brine
Guoxiang Chi
Geological Survey of Canada-Quebec
*,
1
Martine M. Savard
Geoscience Centre. 2535 boul. Laurier. Ste-Foy. QC GIV 4C7. Canada
Abstract
Origins of basinal brines and Mississippi Valley-type (MVT) mineralizing fluids have been separately attributed to
evaporation of seawater or dissolution of halite, although brines originating from the two processes are not mutually
exclusive in basins. This study shows that the NajBr-CljBr diagram cannot distinguish between evaporated seawater and
halite-dissolution fluid. Using the Nadeficit-Caexcess
diagram which was previously proposed to characterize fluid-rock
interactions of basinal brines, it is shown that most basinal brines including MVT mineralizing fluids of the Viburnum Trend
deposits were probably initially a mixture of halite-dissolution fluid and evaporated seawater. Using the same diagram, we
suggest that the mineralizing brines of the Gays River MVT deposit were derived from an aquifer of clastics underlying a
thick succession of evaporites, where halite-dissolution fluid and evaporated seawater could have mixed. @ 1997 Elsevier
Science B.V.
Keywords:
Basinal brines; Mixing; MVT deposits; Evaporated
seawater; Halite dissolution
1. Introduction
Origins of basinal brines and Mississippi Valleytype (MVT) mineralizing fluids have been the subject of decades of research because of their importance in understanding the evolution of sedimentary
basins, large-scale fluid migration, and genesis of
mineral deposits. The high salinities of brines were
in most cases attributed to evaporation of seawater or
dissolution of halite (Hanor, 1994), and a number of
. Corresponding
author. E-mail: [email protected]
I Geological Survey of Canada Contribution Number 1997060.
0009-2541/97/$17.00
@ 1997 Elsevier Science BY
Pll SOO09-2541
(97)00096-X
previous studies have aimed to distinguish between
these two origins for basinal brines (e.g., Land and
Prezbindowski, 1981; Stoessell and Moore, 1983)
and for MVT mineralizing fluids (e.g., Kesler et aI.,
1995, 1996). These studies sometimes led to opposite conclusions for a given brine, i.e., seawater
evaporation vs. halite-dissolution origins.
We propose that brines derived from seawater
evaporation and halite dissolution are not mutually
exclusive. Their mixing is expected in sedimentary
basins, and can better explain the geochemical characteristics of brines. In particular, we evaluate the
applicability of the Na/Br-CljBr
(Walter et aI.,
All rights reserved.
122
G. Chi, M.M. Savard j Chemical Geology 143 (1997) 121 -125
4000
1990) and Nadeficit-Caexcessdiagrams (Davisson and
Criss, 1996) as to recognition of brine sources.
(mg/l) CI
Br Na
FI
245000 5860 8710
F2
200500 117 128063
'k3000
~
'0
2. The NajBr-CljBr
interpretation
diagram: an alternative
The NajBr-CljBr
diagram was proposed by .
Walter et ai. (1990) to characterize brines in the
Illinois Basin. According to the principle of the
diagram, brines derived from evaporation of seawater past halite saturation have NajBr and CljBr
ratios lower than seawater, whereas brines produced
by dissolution of halite likely have NajBr and CljBr
ratios higher than seawater. The NajBr-CljBr diagram is very suitable for fluid inclusion studies
where absolute concentrations of elements are difficult to obtain because it only requires measurements
of element ratios. The diagram has been applied to
studies of MVT deposits (Kesler et ai., 1995, 1996),
in which NajBr and CljBr data of fluid inclusions
plotting on the seawater evaporation trajectory were
taken to indicate fluids derived from seawater evaporation (e.g., Polaris, Viburnum Trend octahedral
galena, and Appalachian MVTs), and data plotting
on the halite-dissolution segment were interpreted as
indicating a halite-dissolution origin (e.g., Viburnum
Trend cubic galena, Illinois-Kentucky district).
The relations between CI-Br systematics and
brine-fonning processes are complex, as discussed in
previous studies (e,g., Hanor, 1994), Even under the
assumption that brines produced by seawater evaporation and halite dissolution have distinct CljBr and
NajBr ranges, it can be shown that the CljBrNajBr diagram may be inadequate in distinguishing
them. This can be illustrated using Fl and F2, a
highly evaporated seawater (high [BrD, and a fluid
derived from dissolution of halite by seawater (low
[BrD. Mixtures of these fluids strongly converge
toward the end member with higher [Br] (Fl, Fig. 1).
As a result, mixing of as little as 10-20% of evaporated seawater (F1) with as much as 80-90% of
halite-dissolution fluid (F2) will plot on the seawater
evaporation trajectory, and may be misinterpreted as
indicating seawater evaporation. If [Br] in Fl is
lower than shown in Fig. 1, the convergence of the
mixtures toward Fl will diminish, but nevertheless
still exists. Therefore, data plotting on the seawater
evaporation trajectory in a CljBr-NajBr
diagram
e
'-'
...
2000
§
U
1000
1000
1500
2000
NalBr
2500
3000
3500
4000
(molar)
1000
'k
0::
'0
-5
... 500
§
U
0
0
1000
500
NalBr (molar)
Fig, I. NajBr and CIjBr ratios of theoretical mixtures of an
evaporated seawater (n) and a halite-dissolution fluid (F2). The
composition of Fl is from McCaffrey et a!. (1987) and data of F2
are obtained by seawater dissolution of halite ([Br] between 72
and 238 ppm in McCaffrey et a!., 1987; a value of ISO ppm is
10-20% Fl with
80-90%
used). Note that the mixtures of
-
-
F2 have NajBr and CIjBr ratios lower than seawater. This may
lead to misinterpretation of the mixtures as an evaporated seawater.
can alternatively be explained by mixing of an evaporated seawater with a halite-dissolution fluid. Similarly, brines having CljBr and NajBr ratios higher
than seawater do not necessarily derive their salinity
entirely from halite dissolution, but can have a contribution from evaporated seawater.
3. Fluid mixing inferred from the Nadeficit-Caexcess
diagram
Regardless of their origin, evaporation of seawater or dissolution of halite, basinal brines must have
undergone significant fluid-rock interaction to account for their elevated concentrations of Ca (Hanor,
G. Chi. MM
Savard/Chemical
3000
1994). The Nadeficit-Caexcessdiagram of Davisson et
aI. (1994), where
(A)
2500
- 2000
'§.
.§ 1500
Caexcess= [Canuid - (Ca/C1)seawaterClnuid]
. 2/40.08
~
1000
~
brines are incompatible with those predicted from
the diagram and that the brine's initial composition
is better explained by mixing of halite-dissolution
fluid with evaporated seawater than by each of them
alone.
For example, the brines from the Smackover Formation in central Mississippi (Gulf of Mexico Basin)
plot along a linear trend which can be extrapolated to
the line of halite dissolution (Fig. 2A). The initial
composition estimated at the intersection of the linear trend and halite-dissolution line has a Nadeficit
value of - 101. If halite dissolution was the only
mechanism responsible for high salinity (average Cl
concentration = 165 gin, a Nadeficitvalue of - 700
would be obtained (line a), whereas brines originated solely from evaporated seawater would fall on
line b (Fig. 2A). The fact that the brine data are
between lines a and b probably implies that the
initial brine was a mixture. Previously, two opposite
interpretations have been proposed for this data set:
(1) an evaporated seawater origin linked to the
Louann evaporites, with variable degrees of dilution
by seawater (e.g., Carpenter et aI., 1974; Stoessell
and Moore, 1983), an interpretation mainly based on
the observation that the Br and Cl data of the brines
plot near the seawater evaporation trajectory (Fig.
2B); and (2) brines derived from dissolution of evaporites (e.g., Land and Prezbindowski, 1981), with Br
being preferentially released from halite (Land and
Prezbindowski, 1981; Land et aI., 1988).
A similar situation exists for the Viburnum Trend
MVT deposits. According to fluid-inclusion
leachate
;,
U
~
""""oJ'
,,0
h
~
Nadeficit= [(Na/ClLeawaterClnuid- Nanuid]. 1/22.99
and
may be used to infer the initial composition of brine;;
and the nature of fluid-rock interaction. Davisson
and Criss (1996) showed that most basinal brines
plot on a linear trend with a 1: 1 slope, indicating
fluid-rock interactions characterized by 1 Ca for 2
Na exchange. They further inferred that most basinal
brines originated from dissolution of halite because
their extrapolated initial composition falls on the
segment of halite dissolution. However, it can be
shown that the Cl concentrations of most basinal
123
Geology 143 (1997) 121-125
".a
'
8""/
?:_,i'
,G'"
".".11
O\~x~b
"."..,,\.%;::
".
"",,~
". ~".
500
".
.500
-1000
".'"
".'" ~".'"
~". ".
oW
.
seawater
-500
500
evaparallan
1000
1500
2000
2500
3000
Na-deficit (meqll)
1000
I
o~'"
- -:\""~"
<J>'~'
(B)
800
li0
'oJ<~C
600
"$ii>">"
E.
~ ~,\~ '
400
U
200
0
0
200
400
600
800
1000
NalBr (molar)
Fig. 2. (A) Nadeficit-Caexcess diagram of brines from the Smackover Formation in central Mississippi (data from Carpenter et a\..
1974). Lines a and b indicate the predicted position of a fluid
with a [CI] = 165 g/l (average for Mississippi Smackover brine)
originating from halite-dissolution
or seawater evaporation, respectively. The regression line for the Mississippi Smackover
brine can be best explained by mixing of a halite-dissolution fluid
with evaporated seawater. (B) Na/Br and CljBr ratios of brines
from the Smackover Formation in central Mississippi (data from
Carpenter et a\., 1974). All data plot below seawater composition
(SW) and near the seawater evaporation trajectory.
data of Crocetti and Holland (1989) and Viets and
Leach (1990), the octahedral galena stage (mainstage) ore fluid is characterized by CljBr and Na/Br
ratios lower than those of seawater, whereas the
ratios of the cubic galena stage fluid are higher (Fig.
3A). It was therefore inferred that the octahedral
galena stage mineralizing fluid was mainly derived
from evaporated seawater, and the cubic galena stage
fluid mainly from halite-dissolution (Kesler et aI.,
1995). However, as discussed in the previous section, such an inference may not be valid because the
mixture of evaporated seawater and halite-dissolution fluid may also have CljBr and Na/Br ratios
lower than those of seawater. On a Nadeficit-Caexcess
124
G. Chi, MM. Savard/Chemical
diagram (Fig. 3B), the cubic and octahedral galena
have different fields, and the scattering of the points
makes the interpretation difficult. This may be partly
related to the fact that the absolute concentrations of
the elements cannot be measured and an average
salinity of 23 wt% NaCI equivalent is assumed for
all inclusion data. If the composition of mineralizing
fluids was controlled by fluid-rock interactions similar to basinal brines (1 Ca for 2 Na exchange), then
Fig. 3B would suggest that neither cubic nor octahedral galena fluids were entirely derived from halite
dissolution or seawater evaporation. With a CI concentration of about 150 gjl (salinity 23 wt%), a
halite-dissolution fluid would plot on line Q, whereas
evaporated seawater would plot on line b (Fig. 3B).
'°;"'"
.",.""
"<i""'\\'
0 Cubic galena stage
1500 -I . Octahedral galena stage
. i<~c
:;
~
"0
51000
EF'
§U
~<>\.,o"
~"""~"~&,,,o
500
0
500
0
1000
NalRr (molar)
1500
2000
2500
(B)
2000 -1 0 Cubic galena stage
. Octahedral galena stage
~1500
E
jlooo
%
U
500
seawater
-500
-1000
-500
0
500
evaporalion
1000
1500
2000
2500
. Sphalerite
2000
~~
e Syn- In posH"e calcite
2500
Na-deficit (meqll)
Fig. 3. (A) Na/Br and CVBr ratios of fluid-inclusion leachates
from octahedral and cubic galena of the Viburnum Trend MVT
deposits (data from Crocetti and Holland, 1989; Viets and Leach,
1990). Octahedral galena data plot below seawater composition
(SW) and near the seawater evaporation trajectory, whereas cubic
galena data plot above SW. (B) Naddicit-Caexcess diagram of
inclusion fluids from octahedral and cubic galenas of the Viburnum Trend MVT deposits (data from Crocetti and Holland, 1989;
Viets and Leach, 1990; assumed salinity = 23 wt%). Lines a and
b indicate the predicted position of a fluid with a [CI] = 150 g/l
originating from halite-dissolution and seawater evaporation, respectively.
#
~'
~
!
O1-iJ>~
. e C/d-,,~'
~ 1500
1000
~
u
t
:J
'"
U
500
0
-500
-1000
seawater evaporation
-500
0
500
1000
1500
2000
2500
Na-deficit (meq/l)
Fig. 4. Nadeficil-Caexcess diagram of inclusion fluids from sphalerite and syn- to post-ore calcite of the Gays River MVT deposit
(data from Savard and Chi, 1998; assumed salinity = 24 wt%).
Lines a and b indicate the predicted position of a fluid with a
[cl] = 150 g/I originating
evaporation, respectively.
2000
(A)
Geology 143 (J997) 121-125
from halite-dissolution
and seawater
It is possible that both octahedral and cubic galena
fluids were initially a mixture of halite-dissolution
fluid and evaporated seawater, although the cubic
galena fluid is likely dominated by a halite-dissolution fluid.
Data of fluid-inclusion decrepitates analyzed by
the SEMjEDA method from ore-stage minerals of
the Gays River MVT deposit (Savard and Chi, 1998)
also plot on a linear trend on the Nadeficit-Caexcess
diagram (Fig. 4). Based on the previous reasoning,
the original Gays River mineralizing fluid may be a
mixture of halite-dissolution and seawater evaporation brines. Seawater evaporation and halite-dissolution brines may have descended from the Windsor
evaporites to the underlying Horton Group, where
they probably mixed and underwent various degrees
of fluid-rock interaction. In addition, based on
fluid-inclusion homogenization temperature-salinity
relationship (Chi and Savard, 1995), in-situ fluid
mixing may have taken place at the site of mineralization; this would partly explain the scattering of
data on the diagram (Fig. 4).
4. Discussion and conclusions
Evaporated seawater and halite-dissolution fluids
were often treated separately in inferring origins of
basinal or MVT mineralizing brines. This sometimes
led to opposite interpretations as to brine sources.
G. Chi, M.M. Savard
/
125
Chemical Geology /43 (/997) /2/-/25
Although mixing of brines with seawater or meteoric
water has been proposed to explain some of the
geochemical variations, the possibility of mixing of
the two saline end members has been ignored. In
fact, halite-dissolution fluid and evaporated seawater
are not mutually exclusive: both are related to seawater evaporation, and they can coexist in the same.
basin. Basin processes such as sediment compaction.
and tectonic activity will likely cause migration and
mixing of different fluids.
We have shown that the NajBr-CljBr
diagram
may lead to multiple interpretations with respect to
origins of brines. CljBr and NajBr ratios lower
than seawater do not necessarily imply that salinity
is entirely from evaporated seawater, but can have
variable and even predominant contributions from
halite-dissolution fluids. Similarly, brines having
CljBr and NajBr ratios higher than seawater can
have a contribution from evaporated seawater. The
interpretation of basinal brines as products of mixing
of halite-dissolution fluid and evaporated seawater is
supported by Nadeficit-Caexcess diagrams. Such an
interpretation may also be drawn on MVT mineralizing fluids.
Acknowledgements
We thank Dr. Dave Morrow of the Geological
Survey of Canada for reviewing a first draft of the
manuscript, and B. Nesbitt for helpful discussion.
Gays River carbonate-hosted Zn-Pb deposit and its surrounding barren areas, Nova Scotia. Atl. Geol. 31,.141-152.
Crocetti, e.A., Holland, H.D., 1989. Sulfur-lead isotope systematics and the composition of fluid inclusions in galena from
the Viburnum Trend, Missouri. Econ. Geol. 84, 2196-2216.
Davisson, M.L, Criss, R.E., 1996. Na-Ca-CI
relations in basinal
fluids. Geochim. Cosmochim. Acta 60, 2743-2752.
Davisson, M.L, Presser, T.S., Criss, R.E., 1994. Geochemistry
London, pp. 151-174.
Kesler, S.E., Appold, M.S., Martini, A.M., Walter, LM., Huston,
T.J., Kyle, J.R., 1995. Na-CI-Br
systematics of mineralizing
brines in Mississippi Valley-type deposits. Geology 23, 641644.
Kesler, S.E., Martini, A.M., Appold, M.S., Walter, LM., Huston,
T.J., Furman, F.e., 1996. Na-Cl-Br
systematics of fluid
inclusions from Mississippi Valley-type deposits, Appalachian
Basin: constraints on solute origin and migration paths.
Geochim. Cosmochim. Acta 60, 225-233.
Land, LS., Prezbindowski, D.R., 1981. The origin and evolution
of saline formation water, lower Cretaceous
carbonates,
south-central Texas, U.S.A. J. Hydrol. 54, 51-74.
Land, LS., Kupecz, J.A., Mack, LE., 1988. Louann salt geochemistry (Gulf of Mexico sedimentary basin, U.S.A.): a
preliminary synthesis. Chern. Geol. 74, 25-35.
McCaffrey, M.A., Lazar, B., Holland, H.D., 1987. The evolution
path of seawater and the coprecipitation of Br- and K+ with
halite. J. Sediment. Petrol. 57, 928-937.
Savard, M.M., Chi, G., 1998. Cation study of fluid inclusion
decrepitates in the Jubilee and Gays River Zn-Pb depositscharacterization of mineralizing brines. Ecoo. Geol. (in press).
Stoessell, R.K., Moore, e.H., 1983. Chemical constraints and
origins of four groups of Gulf Coast reservoir
Am. Assoc. Pet. Geol. Bull. 67, 896-906.
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