Stable Isotope Evidence for the Petrogenesis of Carnallite in the Middle Devonian Prairie Evaporite Formation, Saskatchewan 1

G.D. Koehler2-, T.K. Kyser2- and T. Danyluk3

Koehler, G.D., Kyser, T.K. and Danyluk, T. (1990): Stable Isotope evidence for the petrogenesis of carnallite in the Middle Devonian Prairie Evaporite Formation, SaSkatchewan; in Summary of Investigations 1990, Saskatchewan Geological Survey; Sas­katchewan Energy and Mines, Miscellaneous Report 90-4.

The potash-bearing members of the Prairie Evaporite Formation in Saskatchewan are mainly composed of three chloride salts: halite (NaCl), sylvite (KCI), and car­nallite (KM9C'3.6H20). Other constituents, termed in­solubles, typically make up less than 1 O percent of the rock and consist of iron-oxides (haematite and goethite), dolomite, quartz, and clay. Sylvite is mined to produce potash whereas halite and carnallite are con­sidered to be undesirable by-products.

The Prairie Evaporite Formation consists of a thick s&­quence of evaporites that extend from northern Alberta, through central and southeastern Saskatchewan, and into Manitoba and the northern USA (Figure 1). The for­mation reaches a thickness of 200 m near Saskatoon and thins toward a dissolution edge at the eastern mar­gin. The Prairie Evaporite grades upward from basal

layered halite and anhydrite into massive halite. This suc­cession is capped by four potash-bearing members in the upper eom. These are, in ascending stratigraphic order: the Esterhazy, White Bear, Belle Plaine, and Patience Lake Members (Holter, 1969). Carnallite occurs along the northern and northeastern rim of the Prairie Evaporite Formation (Figure 1 ), and is generally more abundant in the lower potash members (ibid.).

1. Thermodynamics of Carnallite-Sylvite Relationships

The occurrence of sylvite in evaporite sequences is problematic for the interpretation of ancient evaporite deposits because unlike carnallite, precipitation of sylvite directly from evaporating seawater is not predicted from

chemical models (Holser, 1979; Harvie et al., 1980). Car-nallite, however, may under­

0 PCS Lonigcin Mine

go incongruent dissolution to sylvite by reaction with under­saturated fluids, a process which has been invoked to explain the occurrence of syl­vite in several other evaporite deposits (see Lowenstein and Spencer 1990, for review). There is considerable petrographic evidence that sylvite and carnallite in the Prairie Evaporite Formation are genetically related, but commonly younger than co­existing halite (Wardlaw,

(D Alwinsol Wifowbrook

G) IMC K-2 Miie

© PCS Roconville Mine

200

\

.)~M'\~-N.OAI< , \ ._

\

1968; Fuzesy, 1983; Baadsgaard, 1987) and may have been reversibly trans­formed from one to the other (Wardlaw, 1968). Further, stratigraphic relationships b&­tween carnallite- and sylvite­rich zones as described by Wardlaw (1968) indicate that sylvite generally overlies car­nallite where both are present, the reverse of an ex-

Figure 1 - lsopach map ofth6 Prairie Evaporite Formation. Tht1 shadfld area indicates the occur· pected sequence from simple rence of camallite (Holter, 1969) (modified from Baadsgaard, 1987). evaporating seawater. This

(1) Protect funded by the Saskatchewan Potash Producers Association and NSERC Operating Grant (2) Department Of Geologleal Sciences. University of Saskatchewan, Saskatoon, 5askatcheWan, S7N owo (3) Potash Corporation ol Sllsl<atchewan, Sune 500, 122 1st Ave. s .. Saskatoon, Saskatchewllf'I, S7K 763

218 Summary of Investigations 1990

evidence argues against a syndepositional origin for most of the syfvite and carnallite, and favours a sub-sur­face diagenetic process.

The behaviour of carnallite and syfvite in response to burial and diagenesis can be modelled with chemical thermodynamics. The reaction between carnallite and syfvite can be represented by:

KMgCl3.6H20 = KCI + Mg2+ + 2cr + 6H20

carnallite = sylvite + solution

Carnallite precipitating from evaporating seawater forms from a solution that has an ion activity product (IAP) within the camallite stability field at 1 bar pressure and approximately 30°C, shown as point A in Figure 2. In­creasing the pressure as the carnallite is buried stabi­lizes the sylvite + solution field and moves the carnallite­sylvite boundary to higher values of K, so that at a pres­sure of 1 kilobar, which corresponds to a lithostatic load equivalent to 2-3 km depth, the IAP of the solution that precipitated the carnallite at 1 bar may now lie in the syl­vite + solution field. If equilibrium is maintained, the car­nallite will react to form further syfvite + solution. Thus, provi~ed the temperature does not rise substantially, in­creasing pressure as a result of burial will favour the breakdown of carnallite to sytvite. As burial proceeds and the. geotherm recovers, the temperature may be­come high enough to favour the formation of carnallite from sylvite + solution.

2. Isotopic Properties of Carnallite Carnallites may record, in their hydration waters, the stable isotopic compositions of fluids that were respon­sible for the diagenesis and recrystallization of the Prairie Evaporite Formation. To this end, measurement of the fractionation of hydrogen and oxygen isotopes be­tween the hydration water of carnallite and the brine from which it formed is fundamental.

O and 18

0 fractionations were measured between the hydration water of carnallite and brine for both carnallite ~rown in the laboratory, and for natural samples grow­ing over a number of years in brine pools at PCS Lanig~n and _P~ Rocanville mines. At 2s°C, laboratory experiments indicate that hydration water in carnallite is depleted in D by about 40 permil4 and enriched in 180 by about 2 permil relative to the brine from which it forms. Preliminary results from laboratory experiments, where carnallite was precipitated at higher temperature ~41 °C), suggest that the fractionation of hydrogen isotopes between the hydration water of carnallite and brine may be slightly temperature dependant. Natural carnallites from brine pools at PCS Lanigan show similar depletions in D, and enrichment in 180 relative to the w~ter in the brine pools. However, carnallite growing in bnne pools at PCS Rocanville show depletions of O by ~b?

8ut 20 permil and depletions rather than enrichments

in O by about 2 permil (Figure 3a, b, c). Both the brine pools are at similar temperatures so the difference in the apparent carnallite-water D and 180 fractionations

3.8

Carnallite 3.7

3.6 ~ bl)

3.5 0 ...J I kbar

3.4

3.3 Sylvite + solution 1 bar

3.2 20 25 30 35 40 45 so

Temperature 0c Figum 2 • Phase relationship between the equilibrium constant (K) and temperatu,e for the ,eaction KMgC'3°6H2D c KCI + Mg2+ + 2cr + 6H2D, at various pressures. The Ion activity product (/AP) is related to the product of the concentration of er and Mg2+ in the solution and is equal to K along the boundary between cama/lite and syfvite. Thermodynamic data from Pabalan and Pitzer (1987).

must be due to differences in the chfmical composi­tions of the brines, most notably, Ca +. The concentra­tion of Ca2

• in brine at Rocanville is 4 times that at Lanigan. These fractionation factors are applicable for systems in which the amount of water in the fluid is sub­stantially greater than the amount in the carnallite.

In addition to equilibrium fractionation of hydrogen and oxygen isotopes between fluid and carnallite, the isotopic composition of carnallite also can be affected by the rate of exchange of isotopes with any later post­depositional fluid. Fast isotopic exchange between car­nallite and brine will result in measured stable isotopic compositions that are influenced by the last fluid in con­ta~ with the carnallite. On the other hand, if carnallite is resistant to isotope exchange with fluids, the stable isotopic compositions of the brine from which the carnal­lite was originally crystallized may be retained in its hydration water. Because the rate of isotope exchange is unknown, isotope exchange experiments were con­ducted in the laboratory under conditions of high fluid/carnallite ratios.

After 67 days of contact at 25°C, hydration water in car­nallite was about 10 percent exchanged with brine saturated with respect to carnallite (Figure 4). This rate of isotope exchange is extremely rapid; carnallite in the potash deposits will be in isotopic equilibrium with any diagenetic fluid after contact for about one year. There­f~:>re, carnallite will record the stable isotopic composi­tion of the most recent diagenetic fluid with which it inter­acted and isotopic composition will be the most sensi­tive indicator of recent hydrologic activity affecting the potash deposit.

(4) Sta~ l~top~ compositions are reported In the O notation whlcn Is defined as: o ('loo ) z ({Rsample/ Rocean ....Ce<} -1) x 1000 Where R Is the 0 / or 0/ 0 ratio. Toe units Of dlttereoce are referenced to as permll (> / oo). equivalent to differences In the ratios of parts per tllousand.

Saskatchewan Geological Survey 219

a) b) -50 ,-.--- --------,

Q -100 C-0

6 !:,.

6

Q -100 C-0

0 -100 C-0

• •• ~

- l 50_9 -8 -7 -6 -5 -4 -150 -9 -8 -7 -6 -5 -4

-150 -9 -8 -7 -6 -5 -4

lfb b1b 0

1b F/guf9 3 • Relationship between the isotopic compositions of hydration water in camallite (open symbols) and brine from which car­nallite precipitates (closed symbols). a) Camallite grown in the laboratory at 25"C. b} Brine pool at PCS Lanigan. c) Brine pool at 408-00-07, PCS Rocanville. Errors for the r3o measurements are ±5 and for '3180 values an, ±3 per mil.

3. Stable Isotopes in Natural Carnallite Samples of carnallite were collected from the Esterhazy Member of the Prairie Evaporite Formation, from the Alwinsal Willowbrook well drill core (Baadsgaard, 1987), ore at PCS Rocanville, carnallite pods at PCS Rocan­ville, and from carnallite pods at IMC K-2. The hydration water in the carnallite was analyzed for hydrogen and oxygen isotopic compositions to constrain the origin of fluids responsible for the formation of carnallite in dif­ferent locations within the basin. The Alwinsal Wil­lowbrook samples and ore samples from Rocanville have stable isotopic compositions that, with a few excep­tions, fall below the line formed by the stable isotopic compositions of waters from formations above the Prairie Evaporite Formation (Elk Point Basin Trend) (Wittrup et al., 1987), but are roughly parallel to it (Fig­ure 5a). The stable isotopic compositions of the parent brine from which these carnallites form can be es­timated using the hydrogen and oxygen isotopic frac­tionations measured in the laboratory and in brine pools. The estimated stable isotopic compositions of the fluids in equilibrium with these carnallites tend to fol­low the Elk Point Basin Trend, suggesting that these car­nallites formed from waters that have their origins in for-

20 O II

N O = • = I-,

• • • ~ -20

~ A A

•

•

mations above the Prairie Evaporite and do not repre­sent primary precipitation from ancient seawater.

Estimated fluid compositions that lie above the Elk Point Basin Trend, such as those which formed the carnallite pods at PCS RocanvHle and IMC K-2 and some of the Alwinsal Willowbrook samples, most likely indicate that these carnallites formed by incorporating most of the water in the brine as the brine interacted with sylvite to form carnallite (Figure 5b). This mechanism is different from the direct precipitation of carnallite from a brine be­cause most of the water responsible for carnallite precipitation is incorporated into the hydration water of the mineral. In this process, the fractionation of the stable isotopes of hydrogen and oxygen between the hydration water of carnallite and brine will be dependant on the amount of water incorP.orated into the mineral and, as a result, the do and d180 values of the carnal­lite will more closely resemble those of the brine respon­sible for the formation of the carnallite.

4. Conclusions

The above thermodynamic and stable isotope con­straints indicate that primary carnallite, formed from evaporated seawater during Late Devonian time,

• -

dehydrated much later to form sylvite under the influence of increasing lithos­tatic pressure during burial (Figure 6). Maximum burial of the Prairie Evaporite probably occurred during the Cretaceous, as reflected by the

C-0 Equilibrium fractionation <::! -40 ~- -• · ••• · • - · - - - - · -- • · · -- - - • - - · - - - · • A Brine

temperatures determined from fluid in­clusions in halite (Chipley and Kyser, 1989) and by the preponderance of Rb­Sr ages on sylvite which correspond to the Cretaceous (Baadsgaard, 1987). • B Brine -

0 AC Brine

-60 0 20 40 60 80

Time (days) Flflure -4 - Rate of hydrogen isotope exchange between hydration water in camallite and brine at 2S"C. Cama//ite was allowed to exchange wHh 3 brines having diffe19nt stable Isotopic compositions. Exchange Is evidenced by initial values approaching the equilibrium value of -37. L\dDeam-H20 is equal to the difference r3DeamaJJtt• -r3o-....

220

Reversion of sylvite to secondary carnal­lite may be a result of uplift during the Tertiary. From thermodynamic con­siderations, sylvite + solution may react to form carnallite as lithostatic pressure decreases, or as a result of interaction of sylvite with Mg-rich brines (Figure 6). Rt>Sr ages obtained from Alwinsal Wil­lowbrook carnallites suggest that they

Summary of Investigations 1990

a) 0 0 Al wins&! Willowbrook b) 0

O Alwinsal Willowbroolc

-20 /),, PCS Rocanville °"' -20 D PCS Rocanvillt pod•

!:::,. Ore zone - Rocanville

-40 -40 O Pods - Rocanville v IMC K-2pod

-60 -60 + Pods IMC K-2

oD -80 -80

-100 oD -100

-120 -120

-140 -140

-160 -160 0

-20 -16 -12 -8 -4 0 -180 -20 -16 -12 -8 -4 0 -24

o uo 18 o O

Figure 5 • (a) Stable isotope compositions of hydration water in camal/ite from th& Alwinsa/ Willowbrook well, ore at PCS Rocan­ville, pods at PCS Rocanvl//e, and pods from IMC K-2 (open symbols), and the calculated isotopic composition of the water from which th&y formed (closed symbols) assuming the isotopic fractionations measured from laboratory experiments and an infinite reservoir of water. Natural variation in the isotopic composition of recent global precipitation produces th& Meteoric Water Line (MWL). (b). Calculated isotopic compositions of brines that could have precipitated the camal/lte from the Elk Point Basin under conditions wh&re variable percentages of water in the brine enters the carnal/it&. Unes paral/el to the Elk Point Basin Trend denote the possible percentage of the total water in the brine incorporated into the camallite, assuming the water is from basinal brines that follow the Elk Point Basin Trend. Those samp/es of camalfite that have incorporated a high percentage of the available water in the brine into their stJtJcture (eg. >50%) most likely formed from the conversion of sylvite to carnallite.

were formed during the last 30 Ma (Baadsgaard, 1987). The hydrogen and oxygen isotopic composition of the carnallites indicate that they have been precipitated by waters that had their origin in formations above the Prairie Evaporite.

Carnallite contains an isotopic record of fluid events in its hydration water, the age of which can be determined using a variety of radiogenic methods. Thus, detailed study of carnallite will provide a history of fluid flow in the Prairie Evaporite Formation and afford a greater un­derstanding of the fluid events and diagenesis related to

the present structure and geology of the

r-x/,t .¥n.9.11llJ3YY\ Halite

Dominantly Hante-carnaflite rock

(about 370 Ma)

potash deposits.

5. References

Increasing pressure ~ Burial-

release of Mg-rich brines

~ Conversion of carnallite to

Halite

Mg-r ich b asina! brines

~

sylvite during burial

( about 100-60 Ma)

Uplift and reaction of existing sy lvite with Mg- rich basinal brines to form secondary carnalfite

(< 30 Ma)

Baadsgaard, H. (1987): Rb-Sr and K-Ca isotope systematics from potassium horizons in the Prairie Evaporite Formation, Saskatchewan, Canada; Chem. Geol., v66, p1-15.

Chipley, D.L. and Kyser, T.K (1989): Fluid in­clusion evidence for the diagenesis of the Patience Lake Member of the Prairie Evaporite Formation; Sed. Geol., v64, p287-295.

Fuzesy, L.M. (1983) : Petrology of the potash ore in the Middle Devonian Prairie Evaporite For­mation of Saskatchewan; in McKercher, R.M. (ed.), Potash Technology, Pergamon Press, p47-57.

Harvie, C.E., Weare, J .H., Hardie, LA. and Eugster, L.P. (1980): Evaporation of seawater: calculated mineral sequences; Scl., v208, p498·500. Figure 6 • Simple model of cama/lite - sylvite relationships In the Prairie Evaporit9

Formation.

221 Summary of Investigations 1990

Holser, W.T. (1979): Mineralogy of evaporltes; In Burns, R.G. (ed.), Maline Minerals, Reviews in Mineralogy, Mineral. Soc. NO., Washington D.C., v6, p211-294.

Holter, M.E. (1969): The Middle Devonian Prairie Evaporlte For­mation of Saskatchewan; Sask. Oep. Miner. Resour., Rep. 123, 134p.

Lowenstein, T.K. and Spencer, R.J. (1990): Syndepositional origin of potash evaporltes: petrographic and fluid In­clusion evidence; NO. J. Sci., v290, pt-42.

Saskatchewan Geological Sul\18y

Pabalan, R.T. and Pitzer, K.S. (1987): Thermodynamics of con­centrated electrolyte mlxturff and the prediction of mineral solubilities to high temperatures for mixtures in the system Na-K-Mg-Cl·S04-QH-H20; Geochim. Cos­mochim. Acta, v51 , p2429-2443.

Wardlaw, N.C. (1968): Carnallite-sylvite relationships in the Middle Devonian Prairie Evaporite Formation, Sas­katchewan; Geol. Soc. NO. Bull., v79, p1273-1294.

Wittrup, M.B .• Danyluk, T. and Kyser, T.K. (1987): The use of stable isotopes to determine the source of brines in Sas­katchewan potash mines; in Gilboy, C.F. and Vigrass, LW. (eds.), Economic Minerals of Saskatchewan, Sask. Geol. Soc., Spec. Publ. No. 8, p159-165.

222