Three Major Processes that Occur During the Deposition and
Preservation of Potash Salts of the Prairie Evaporite in Saskatchewan
D.M. Lane
Lane, D.M. (1989): Three major processes that occur during deposition and preservation of potash salts of the Prairie Evaporite
in Saskatchewan; in Summary of Investigations 1989, Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 89-4.
This study examines the general interrelationships of
gypsum, anhydrite, carnallite, sylvite and limestone in
the Saskatchewan sub-basin of the Middle Devonian Elk
Point basin. Its object is to demonstrate the roles played
by three major processes in the deposition and preservation of sylvite and carnallite in the basin. These processes are:
1. Dehydration of gypsum to anhydrite,
2. "Brine Pool" conversion of carnallite to sylvite and
3. ton exchange or chemical reaction of carnallite with
limestone of the Dawson Bay and Souris River formations.
Each of these processes is involved in volume reductions in the Prairie Evaporite and the conversion of carnaflite to sylvite.
stones of the Dawson Bay formation is mentioned in
Buchinski (1988).
Anhydrite occurring within the Prairie Evaporite below
the potash-bearing members has an aggregate thickness of over 75 m, which would have required deposition of some 121 m of original gypsum. During dehydration of the gypsum some 46 m of subsidence of overlying beds could result, excluding the effects of the
released water of hydration.
WATER
4 8 VOLUME S
GYPS UM
100 VOLUM ES
ANH YDRITE
1. Background
A new era of potash mining in Saskatchewan is thought
to have commenced in 1989 with the conversion of a
conventional underground potash mine to a solution
mine, the commercial production of calcium chloride
brine from another mine, and the massive injections of
calcium chloride into the Dawson Bay Formation to control water inflow into another potash mine.
Saskatchewan's conventional underground potash
mines are maturing after twenty years of production.
The new "era" demands a thorough review of existing
geological concepts and data. Applications of these concepts or processes to Saskatchewan Prairie Evaporite
sediments may be of benefit to the potash mining industry. Significant impact may also be realized in the
petroleum and chemical industries through enhanced industrial diversity.
Considerable new data are available since the comprehensive major regional work on the Prairie Evaporite
in Saskatchewan was completed and published by Holter (1969). These data include numerous deep test
holes for potash and hydrocarbon exploration that
penetrate the entire Prairie Evaporite section.
Sonnenfeld (1984) provides some of the basic elements
used in the application of three interrelated processes
presented in this report. The work of Shearman (1983)
is considered a principal reference for one of the major
processes. Brine pool concepts used in the present
study have been presented by various authors including
Valyashko in Baar 1972 and brine chemistry involving
the reaction of Prairie Evaporite carnallites with lime-
Saskatchewan Geological Survey
6 2 V OLUM E S
GYPSUM
Ca S04. 2Hz O
- - . - . ANHYDRITE
+
WATER
CoS0 4
Figure 1 - Illustration of the volume reduction in the dehydration of gypsum to anhydrite.
When carnallite converts to sylvite in brine pools, there
is a volume reduction ratio of 4.6:1 and liberation of the
water of hydration. Volumetric (thickness) reduction of
the Patience Lake Member, involved in the postulated
conversion of carnallite to sylvite in the Pacid Allan 4-2932-28-W2 well, would be about 23 m. Figure 2 shows
that, locally, sylvites occur over thick anhydrites in wells
1 and 5 whereas carnallites are predominant over thin
anhydrites in well No. 3.
A generalized interpretive model (not shown here) of
events related to the conversion of Patience Lake carnallite to sylvite and to the preservation of the sylvinitic
beds formed in the Saskatoon potash mining area is
presented and defined in a forthcoming report.
High salinity brines occurring in "closed" reservoirs appear to be products of a reaction between carnallites
and limestones of the overlying Dawson Bay and Souris
River Formations. End products of the reaction include
calcium chloride, dolomite and sylvite.
157
I
I
< 50
150
3
38
3A
2
ZONES OF POTA SSIUM MINE:RALI ZATION 1, Z,3 A ,3 8 .
ZONE BOUNDAR IES . - · ····- · ···- ··- · ---- - ---BOUNDARY SEPARATING SYLVITE .&BOVE F ROM
CARNAL LI TE BELOW .
l]J • 100 •.<, KCI
[l;J • 100 % KCI M;CI, eH, O -
=
M INERALOGICA L DATA FOR 4 - 29 I NFERRED FR OM GAMMA - RAY A ND
NEUTRON LOGS.
0
100 rtE T
V( AllCA.L St Al l
U
21
II
27
17
16
"
.SC .t,l(
+
p OT ASH MINE
LEGE ND :
BORE HOLES IN WHICH ONLY LOWER
o
WINN I PEGOSIS
BORE HOL ES IN WHICH UPPER WINNIPEGOS IS
AND RATN E R PRESENT
BA NKS EN COUNTERED
CO NTOUR IN TERVAL 50 F EET
--~
c:J
APPROXIMATE ZERO EDGE OF UPPER WJNNI PEGOS IS
O - 50 '
c:J
5 0 ' -100 '
c::::J 100' -
200'
Bj;7 j
200'
B ANKS
+
T H I CKNESS
Figure 2 - Cross-section and map showing relationships of thick local anlrjdrites to syMnitic or cama/litic
158
areas.
Summary of Investigations 1989
2. Conclusions
1. All potash occurring within the Saskatchewan sul>basin of the Elk Point was originally deposited as carnallite.
2. Local subsidence or downwarping of strata overlying
the Prairie Evaporite in the potash-bearing areas can
be attributed to a combination of the effects of
dehydration of gypsum to anhydrite and the conversion of carnallite to sylvite rather than compaction of
Winnipegosis lime muds.
3. Subsurface brines from the Winnipegosis, Dawson
Bay and Souris River Formations have a high potential for commercial applications such as the reduction of C02 from stack gases, production of calcium
chloride, calcium chlorate, calcium carbonate (for
paper coating) and a variety of other uses. Saskatchewan water analysis data for the Winnipegosis,
Dawson Bay and Souris River Formations have
been compiled. Salinities (TDS) have been mapped
and potentially commercial occurrences of magnesium and calcium brines are identified in a
separate study in preparation.
Saskatchewan Geological Survey
4. The concepts presented in this study should assist
in the interpretation of geophysical data for both oil
and potash exploration and development.
3. References
Barr, CA (1972): Actual geologic problems in Saskatchewan
potash mining, Annual Western Meeting of CIM, Saskatoon, sop, 37 Figures.
Buchinski, K.W. (1988): The occurrence, recovery, and commercial application of calcium chloride brine from a Saskatchewan potash mine; in 90th Annual General Meeting,
CIM, Saskatoon, 16p
Holter, M.E. (1969): The Middle Devonian Prairie Evaporite of
Saskatchewan; Sask. Dep. Miner. Resour., Rep. 123, 134p.
Shearman, D.J . (1983): Syndepositional and late diagenetic alteration of primary gypsum to anhydrite; in Sixth Symposium of Salt; The Salt Institute, Alexandria, Virginia, v1 ,
p41-50.
Sonnenfeld, P. (1984) : Brines and Evaporites; Academic
Press, Inc., Orlando, Florida, 613p.
159