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CHEMICALGEOLOGY

fNCWDlNG

ISOTOPE GEOSCIENCE

Chemical Geology 143 (1997) 121-125

Sources of basinal and Mississippi Valley-type mineralizingbrines: mixing of evaporated seawater and halite-dissolution

brine 1

Guoxiang Chi *, Martine M. SavardGeological Survey of Canada-Quebec 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 toevaporation of seawater or dissolution of halite, although brines originating from the two processes are not mutuallyexclusive in basins. This study shows that the NajBr-CljBr diagram cannot distinguish between evaporated seawater andhalite-dissolution fluid. Using the Nadeficit-Caexcessdiagram which was previously proposed to characterize fluid-rockinteractions of basinal brines, it is shown that most basinal brines including MVT mineralizing fluids of the Viburnum Trenddeposits were probably initially a mixture of halite-dissolution fluid and evaporated seawater. Using the same diagram, wesuggest that the mineralizing brines of the Gays River MVT deposit were derived from an aquifer of clastics underlying athick succession of evaporites, where halite-dissolution fluid and evaporated seawater could have mixed. @ 1997 ElsevierScience B.V.

Keywords: Basinal brines; Mixing; MVT deposits; Evaporated seawater; Halite dissolution

1. Introduction

Origins of basinal brines and Mississippi Valley-type (MVT) mineralizing fluids have been the sub-ject of decades of research because of their impor-tance in understanding the evolution of sedimentarybasins, large-scale fluid migration, and genesis ofmineral deposits. The high salinities of brines werein most cases attributed to evaporation of seawater ordissolution of halite (Hanor, 1994), and a number of

. Corresponding author. E-mail: gchi@gsc.nrcan.gc.ca

I Geological Survey of Canada Contribution Number 1997060.

0009-2541/97/$17.00 @ 1997 Elsevier Science BY All rights reserved.Pll SOO09-2541 (97)00096-X

previous studies have aimed to distinguish betweenthese two origins for basinal brines (e.g., Land andPrezbindowski, 1981; Stoessell and Moore, 1983)and for MVT mineralizing fluids (e.g., Kesler et aI.,1995, 1996). These studies sometimes led to oppo-site conclusions for a given brine, i.e., seawaterevaporation vs. halite-dissolution origins.

We propose that brines derived from seawaterevaporation and halite dissolution are not mutuallyexclusive. Their mixing is expected in sedimentarybasins, and can better explain the geochemical char-acteristics of brines. In particular, we evaluate theapplicability of the Na/Br-CljBr (Walter et aI.,

122 G. Chi, M.M. Savard j Chemical Geology 143 (1997) 121 -125

40001990) and Nadeficit-Caexcessdiagrams (Davisson andCriss, 1996) as to recognition of brine sources.

2. The NajBr-CljBr diagram: an alternativeinterpretation

The NajBr-CljBr diagram was proposed by .Walter et ai. (1990) to characterize brines in theIllinois Basin. According to the principle of thediagram, brines derived from evaporation of seawa-ter past halite saturation have NajBr and CljBrratios lower than seawater, whereas brines producedby dissolution of halite likely have NajBr and CljBrratios higher than seawater. The NajBr-CljBr dia-gram is very suitable for fluid inclusion studieswhere absolute concentrations of elements are diffi-cult to obtain because it only requires measurementsof element ratios. The diagram has been applied tostudies of MVT deposits (Kesler et ai., 1995, 1996),in which NajBr and CljBr data of fluid inclusionsplotting on the seawater evaporation trajectory weretaken to indicate fluids derived from seawater evapo-ration (e.g., Polaris, Viburnum Trend octahedralgalena, and Appalachian MVTs), and data plottingon the halite-dissolution segment were interpreted asindicating a halite-dissolution origin (e.g., ViburnumTrend cubic galena, Illinois-Kentucky district).

The relations between CI-Br systematics andbrine-fonning processes are complex, as discussed inprevious studies (e,g., Hanor, 1994), Even under theassumption that brines produced by seawater evapo-ration and halite dissolution have distinct CljBr andNajBr ranges, it can be shown that the CljBr-NajBr diagram may be inadequate in distinguishingthem. This can be illustrated using Fl and F2, ahighly evaporated seawater (high [BrD, and a fluidderived from dissolution of halite by seawater (low[BrD. Mixtures of these fluids strongly convergetoward the end member with higher [Br] (Fl, Fig. 1).As a result, mixing of as little as 10-20% of evapo-rated seawater (F1) with as much as 80-90% ofhalite-dissolution fluid (F2) will plot on the seawaterevaporation trajectory, and may be misinterpreted asindicating seawater evaporation. If [Br] in Fl islower than shown in Fig. 1, the convergence of themixtures toward Fl will diminish, but neverthelessstill exists. Therefore, data plotting on the seawaterevaporation trajectory in a CljBr-NajBr diagram

'k3000~

'0e'-' 2000...

§U

(mg/l) CI Br NaFI 245000 5860 8710F2 200500 117 128063

1000

1000 1500 2000 2500 3000

NalBr (molar)

40003500

1000

'k0::

'0

-5... 500§U

0

0 500

NalBr (molar)1000

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 F2are obtained by seawater dissolution of halite ([Br] between 72

and 238 ppm in McCaffrey et a!., 1987; a value of ISO ppm isused). Note that the mixtures of -10-20% Fl with -80-90%

F2 have NajBr and CIjBr ratios lower than seawater. This maylead to misinterpretation of the mixtures as an evaporated seawa-ter.

can alternatively be explained by mixing of an evap-orated seawater with a halite-dissolution fluid. Simi-

larly, brines having CljBr and NajBr ratios higherthan seawater do not necessarily derive their salinityentirely from halite dissolution, but can have a con-tribution from evaporated seawater.

3. Fluid mixing inferred from the Nadeficit-Caexcessdiagram

Regardless of their origin, evaporation of seawa-ter or dissolution of halite, basinal brines must have

undergone significant fluid-rock interaction to ac-count for their elevated concentrations of Ca (Hanor,

G. Chi. MM Savard/Chemical Geology 143 (1997) 121-125

1994). The Nadeficit-Caexcessdiagram of Davisson etaI. (1994), where

Nadeficit= [(Na/ClLeawaterClnuid- Nanuid]. 1/22.99and

Caexcess= [Canuid - (Ca/C1)seawaterClnuid]. 2/40.08

may be used to infer the initial composition of brine;;and the nature of fluid-rock interaction. Davissonand Criss (1996) showed that most basinal brines

plot on a linear trend with a 1: 1 slope, indicatingfluid-rock interactions characterized by 1 Ca for 2Na exchange. They further inferred that most basinalbrines originated from dissolution of halite becausetheir extrapolated initial composition falls on thesegment of halite dissolution. However, it can beshown that the Cl concentrations of most basinal

brines are incompatible with those predicted fromthe diagram and that the brine's initial compositionis better explained by mixing of halite-dissolutionfluid with evaporated seawater than by each of themalone.

For example, the brines from the Smackover For-mation in central Mississippi (Gulf of Mexico Basin)plot along a linear trend which can be extrapolated tothe line of halite dissolution (Fig. 2A). The initialcomposition estimated at the intersection of the lin-ear trend and halite-dissolution line has a Nadeficitvalue of - 101. If halite dissolution was the onlymechanism responsible for high salinity (average Clconcentration = 165 gin, a Nadeficitvalue of - 700would be obtained (line a), whereas brines origi-nated solely from evaporated seawater would fall online b (Fig. 2A). The fact that the brine data arebetween lines a and b probably implies that theinitial brine was a mixture. Previously, two oppositeinterpretations have been proposed for this data set:(1) an evaporated seawater origin linked to theLouann evaporites, with variable degrees of dilutionby seawater (e.g., Carpenter et aI., 1974; Stoesselland Moore, 1983), an interpretation mainly based onthe 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 evap-orites (e.g., Land and Prezbindowski, 1981), with Brbeing preferentially released from halite (Land andPrezbindowski, 1981; Land et aI., 1988).

A similar situation exists for the Viburnum Trend

MVT deposits. According to fluid-inclusion leachate

123

3000(A)

h~~ """"oJ'

,,0

8""/ "",,~?:_,i',G'"

2500

- 2000'§..§ 1500~1000~;,U 500

a '".".".11

". O\~x~b"..,,\.%;::".". ~".

".'" ~".'"".'" ~". ".

". oW .seawater evaparallan

.500

-1000 500 1000 1500 2000

Na-deficit (meqll)

-500 2500 3000

o~'"- -:\""~"

<J>'~'

'oJ<~C

"$ii>">"~,\~ '~

400 600

NalBr (molar)

800 1000

Fig. 2. (A) Nadeficit-Caexcess diagram of brines from the Smack-

over Formation in central Mississippi (data from Carpenter et a\..

1974). Lines a and b indicate the predicted position of a fluidwith a [CI] = 165 g/l (average for Mississippi Smackover brine)

originating from halite-dissolution or seawater evaporation, re-spectively. 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 brinesfrom 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 (main-stage) ore fluid is characterized by CljBr and Na/Brratios 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 derivedfrom evaporated seawater, and the cubic galena stagefluid mainly from halite-dissolution (Kesler et aI.,1995). However, as discussed in the previous sec-tion, such an inference may not be valid because themixture of evaporated seawater and halite-dissolu-tion fluid may also have CljBr and Na/Br ratioslower than those of seawater. On a Nadeficit-Caexcess

1000

I (B)800

li 6000E.

400U

200

0

0 200

124 G. Chi, MM. Savard/Chemical Geology 143 (J997) 121-125

2500diagram (Fig. 3B), the cubic and octahedral galenahave different fields, and the scattering of the pointsmakes the interpretation difficult. This may be partlyrelated to the fact that the absolute concentrations ofthe elements cannot be measured and an averagesalinity of 23 wt% NaCI equivalent is assumed forall inclusion data. If the composition of mineralizingfluids was controlled by fluid-rock interactions simi-lar to basinal brines (1 Ca for 2 Na exchange), thenFig. 3B would suggest that neither cubic nor octahe-dral galena fluids were entirely derived from halitedissolution or seawater evaporation. With a CI con-centration of about 150 gjl (salinity 23 wt%), ahalite-dissolution fluid would plot on line Q, whereasevaporated seawater would plot on line b (Fig. 3B).

2000(A) '°;"'"

.",."""<i""'\\'

. i<~c

~ ~<>\.,o"

EF' ~"~&,,,o~"""

0 Cubic galena stage

1500 -I . Octahedral galena stage

:;"051000

§U

500

00 500 1000

NalRr (molar)1500 2000

2500

(B)

2000 -1 0 Cubic galena stage

. Octahedral galena stage

~1500E

jlooo% 500

U

seawater evaporalion

-500-1000 2000 25000 500 1000 1500

Na-deficit (meqll)

-500

Fig. 3. (A) Na/Br and CVBr ratios of fluid-inclusion leachatesfrom 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 ofinclusion fluids from octahedral and cubic galenas of the Vibur-

num 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, re-

spectively.

2000. Sphalerite

e Syn- In posH"e calcite~~

#O1-iJ>~

. e ,,~'C/d-~'

~ 1500~! 1000

~u:J 500

t '"U

0 seawater evaporation

-500-1000 500 1000 1500

Na-deficit (meq/l)

2000 2500-500 0

Fig. 4. Nadeficil-Caexcess diagram of inclusion fluids from spha-lerite 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 from halite-dissolution and seawater

evaporation, respectively.

It is possible that both octahedral and cubic galenafluids were initially a mixture of halite-dissolutionfluid and evaporated seawater, although the cubicgalena fluid is likely dominated by a halite-dissolu-tion fluid.

Data of fluid-inclusion decrepitates analyzed bythe SEMjEDA method from ore-stage minerals ofthe Gays River MVT deposit (Savard and Chi, 1998)also plot on a linear trend on the Nadeficit-Caexcessdiagram (Fig. 4). Based on the previous reasoning,the original Gays River mineralizing fluid may be amixture of halite-dissolution and seawater evapora-tion brines. Seawater evaporation and halite-dissolu-tion brines may have descended from the Windsorevaporites to the underlying Horton Group, wherethey probably mixed and underwent various degreesof fluid-rock interaction. In addition, based onfluid-inclusion homogenization temperature-salinityrelationship (Chi and Savard, 1995), in-situ fluidmixing may have taken place at the site of mineral-ization; this would partly explain the scattering ofdata on the diagram (Fig. 4).

4. Discussion and conclusions

Evaporated seawater and halite-dissolution fluidswere often treated separately in inferring origins ofbasinal or MVT mineralizing brines. This sometimesled to opposite interpretations as to brine sources.

G. Chi, M.M. Savard / Chemical Geology /43 (/997) /2/-/25

Although mixing of brines with seawater or meteoricwater has been proposed to explain some of thegeochemical variations, the possibility of mixing ofthe two saline end members has been ignored. Infact, halite-dissolution fluid and evaporated seawaterare not mutually exclusive: both are related to seawa-ter evaporation, and they can coexist in the same.basin. Basin processes such as sediment compaction.and tectonic activity will likely cause migration andmixing of different fluids.

We have shown that the NajBr-CljBr diagrammay lead to multiple interpretations with respect toorigins of brines. CljBr and NajBr ratios lowerthan seawater do not necessarily imply that salinityis entirely from evaporated seawater, but can havevariable and even predominant contributions fromhalite-dissolution fluids. Similarly, brines havingCljBr and NajBr ratios higher than seawater canhave a contribution from evaporated seawater. Theinterpretation of basinal brines as products of mixingof halite-dissolution fluid and evaporated seawater issupported by Nadeficit-Caexcess diagrams. Such aninterpretation may also be drawn on MVT mineraliz-ing fluids.

Acknowledgements

We thank Dr. Dave Morrow of the GeologicalSurvey of Canada for reviewing a first draft of themanuscript, and B. Nesbitt for helpful discussion.

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