J. Lonnee et Is. Al-aasm, Dolomitization and fluid evolution in the Middle Devonian Sulphur Point Formation, Rainbow South Field, Alberta: petrographic and geochemical evidence, B CAN PETRO, 48(3), 2000, pp. 262-283
Petrographic and geochemical studies of the Middle Devonian (Givetian) Sulp
hur Point Formation in the vicinity of the Rainbow South Field, northwester
n Alberta, reveal that dolomitization was a direct result of precipitation
by chemically distinct fluids, and that recrystallization of these dolomite
s significantly altered their original chemical signatures. Sulphur Point c
arbonates were deposited in a restricted peritidal environment. Lithofacies
include grainstones, sparsely fossiliferous packstones, mudstones, algal m
udstones, and intraclast breccia mudstones. Multiple episodes of calcite ce
mentation and dolomitization have affected these rocks to varying degrees.
Five dolomite types were identified: 1) dolomicrite, 2) fine-crystalline ma
trix dolomite, 3) medium-crystalline matrix dolomite, 4) saddle dolomite an
d 5) fracture-lining dolomite.
Dolomicrite (2-20 mu m) replaced both micrite and calcite cement in the mud
-supported facies before early compaction. A trend toward more negative del
ta(18)O values of -9.22 to -3.10 parts per thousand Vienna Pee Dee Belemnit
e (VPDB) with respect to postulated Middle Devonian marine carbonate values
suggests that dolomicrite was recrystallized by later fluids. Geochemical
modelling of the isotope and trace element trends in the dolomicrite suppor
t this interpretation.
Both fine- and medium-crystalline matrix dolomites (40-200 mu m) are usuall
y fabric destructive. However, some intervals have retained lamination and
algal structures. Matrix dolomite was formed during intermediate burial, as
suggested by its association with dissolution seams, high Fe and Mn concen
trations, and delta(18)O values of -12.20 to -8.34 parts per thousand VPDB.
This evidence, in addition to the presence of high salinity fluid inclusio
ns (similar to 18 wt% NaCl equivalent), indicates that matrix dolomite was
precipitated by basinal fluids between the Mississippian and Late Jurassic.
The precipitation of saddle dolomite (0.5-2.0 mm) is genetically related to
fractures and breccia zones where it partially to completely occludes the
fractures, breccias and vugs that were developed through the dissolution of
the earlier matrix dolomites. Geochemical and petrographic evidence sugges
ts that saddle dolomite was precipitated from a hot, slightly saline (10.5
to 13.3 wt% NaCl equivalent), calcium-rich fluid that was funnelled upward
along faults and fractures that developed during the Late Cretaceous to ear
ly Tertiary Laramide Orogeny. Strontium isotope modelling confirms that sad
dle dolomite was precipitated from a two-component hydrothermal fluid incor
porating a significant quantity of Middle Devonian brines and radiogenic ba
sement fluids.
Fracture-lining dolomite (0.2-1.0 mm) was the last dolomite phase to precip
itate, and is intimately associated with blocky calcite, quartz, sulphide m
ineralization and pyrobitumen. Isotopic and fluid inclusion evidence imply
precipitation from slightly saline brines (similar to 8 wt% NaCl equivalent
) at elevated temperatures. Extremely low Fe and Mn concentrations, negativ
e delta(13)C values (similar to -5 parts per thousand VPDB), and significan
t volumes of H2S gas suggest that fracture-lining dolomite was precipitated
from syn- to post-laramide fluids during thermochemical sulphate reduction
.