Stable Isotope Evidence for the Petrogenesis of Carnallite in the
Middle Devonian Prairie Evaporite Formation, Saskatchewan1
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 1O 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.
layered halite and anhydrite into massive halite. This succession is capped by four potash-bearing members in
the upper
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.).
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
1. Thermodynamics of Carnallite-Sylvite
Relationships
eom.
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). Carnallite, however, may undergo incongruent dissolution to
sylvite by reaction with underPCS Lonigcin Mine
saturated fluids, a process
(D Alwinsol Wifowbrook
which has been invoked to
explain the occurrence of sylG) IMC K-2 Miie
vite in several other evaporite
deposits (see Lowenstein and
PCS Roconville Mine
Spencer 1990, for review).
There is considerable
petrographic evidence that
200
sylvite and carnallite in the
Prairie Evaporite Formation
are genetically related, but
commonly younger than coexisting halite (Wardlaw,
1968; Fuzesy, 1983;
Baadsgaard, 1987) and may
have been reversibly transformed from one to the other
(Wardlaw, 1968). Further,
\
stratigraphic relationships b&.)~M'\~- tween carnallite- and sylviterich zones as described by
N.OAI< ,
\ ._
Wardlaw (1968) indicate that
\
sylvite generally overlies carnallite where both are
present, the reverse of an exFigure 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
0
©
(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
3.8
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:
2
KMgCl3.6H20 = KCI + Mg + +
carnallite = sylvite + solution
3.6
~
bl)
0
2cr + 6H20
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.
18
Carnallite
3.7
O and 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
4
depleted in D by about 40 permil 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? ut 20 permil and depletions rather than enrichments
8
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
...J
3.5
I kbar
3.4
Sylvite + solution
3.3
1 bar
3.2
20
25
30
35
40
so
45
Temperature
0
c
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 concentra2
tion of Ca • 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.
o
(4) Sta~ l~top~ compositions are reported In the O notation whlcn Is defined as:
('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
b)
a)
Q -100
Q -100
6
!:,.
-9
--------,
-8
-6
-7
C-0
~
6
-5
••
0 -100
C-0
C-0
-150
-50 ,-.---
-4
- l 50_9
-8
-7
-6
-5
-4
-8
-6
-7
-5
-4
1
0b
1
bb
lfb
-150
-9
•
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 '3 180 values an, ±3 per mil.
mations above the Prairie Evaporite and do not represent primary precipitation from ancient seawater.
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
N
==
O
II
•
•
•
•
I-,
~
•
-20
•
~
A
A
C-0
Equilibrium fractionation
<::! -40 ~- -• · ••• · • -· - - - -· -- • · · -- - - • - -· - - -·
0
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 d 180 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
temperatures determined from fluid inclusions in halite (Chipley and Kyser,
1989) and by the preponderance of Rb• A Brine
ages on sylvite which correspond to
- Sr
• B Brine
the Cretaceous (Baadsgaard, 1987).
•
-
AC Brine
Reversion of sylvite to secondary carnallite may be a result of uplift during the
0
20
40
60
80 Tertiary. From thermodynamic considerations, sylvite + solution may react
(days)
to form carnallite as lithostatic pressure
decreases, or as a result of interaction
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 of sylvite with Mg-rich brines (Figure 6).
stable Isotopic compositions. Exchange Is evidenced by initial values approaching
Rt>Sr ages obtained from Alwinsal Wilthe equilibrium value of -37. L\dDeam-H20 is equal to the difference r3DeamaJJtt• lowbrook carnallites suggest that they
-60
Time
r3o-....
220
Summary of Investigations 1990
a)
0
-20
-40
oD
0
Al wins&! Willowbrook
/),,
PCS Rocanville °"'
D
PCS Rocanvillt pod•
v
IMC K-2pod
b)
0
O
-20
-40 O
-60
-60
-80
-80
oD
- 100
-120
-140
-140
-160
-160
-12
-16
-8
-4
0
ouo
+
Pods - Rocanville
Pods IMC K-2
-100
- 120
-20
Alwinsal Willowbroolc
!:::,. Ore zone - Rocanville
- 180
-24
0
-20
-16
- 12
-8
-4
0
o18O
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.
r-x/,t .¥n.9.11llJ3YY\
Halite
~
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
potash deposits.
Dominantly Hante-carnaflite rock
(about 3 70 Ma)
BurialIncreasing pressure
release of Mg-rich brines
~
Halite
Conversion of carnallite to sylvite
during burial
( about 100-60 Ma)
Mg-r ich b asina! brines
~
Uplift and reaction of existing
sy lvite with Mg- rich basinal brines
to form secondary carnalfite
5. References
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, p287295.
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;
Figure 6 • Simple model of cama/lite - sylvite relationships In the Prairie Evaporit9
Scl., v208, p498·500.
Formation.
(< 30 Ma)
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