Dolomite Perspectives on a Perplexing Mineral

Burial Diagenesis Model—Dolomite can form 
in environments where pore-fluid chemistry is 
dominated by subsurface diagenetic processes or 
where interactions between water and rock have 
modified the original pore waters. Such environ-
ments are removed from active surface sedimen-
tation by intermediate to deep burial and are 
characterized by chemically reducing conditions. 
Burial dolomites form in the subsurface after 
lithification of lime sediments. These dolomites 
can either directly precipitate as cement or form 
as replacements in permeable intervals flushed 
by warm to hot magnesium-enriched basinal and 
hydrothermal waters. Since burial dolomite 
replacement occurs after lithification of a car-
bonate host, this dolomite may crosscut deposi-
tional facies as well as formation boundaries.
33
In 
addition to structural position, oxygen and stron-
tium [Sr] isotopes are useful in determining how 
such dolomites originate. These dolomites tend 
to have negative 
δ
18
O oxygen isotope values, 
indicating precipitation from fluids at some- 
what higher temperatures than those of earlier 
platform dolomites. The recrystallization of previ- 
ously formed dolomites by basinal fluids can 
reset the crystal characteristics, producing crys-
tals with low 
δ
18
O values, modified 
87
Sr/
86
Sr ratios 
and saline high-temperature fluid inclusions.
34
In these subsurface environments, dolomitiza-
tion of limestone is facilitated by higher tempera-
tures as burial depth increases. In turn, higher 
temperatures enable dolomitization by solutions 
with lower Mg/Ca ratios than the previously men-
tioned hypersaline brines. Temperatures of 60° to 
70°C [140° to 158°F] are sufficient for burial dolo-
mites to form, and these conditions can usually be 
met within just a few kilometers of the surface. 
With sufficient temperature increase, many sub-
surface waters can become dolomitizing solutions, 
including residual evaporite brines, seawater and 
shale-compaction waters. In the latter case, pore 
water is expelled from fine-grained sediments dur-
ing burial and compaction. Clay minerals release 
Mg
+2
, which may pass through carbonates, result-
ing in their dolomitization. 
However, dolomitization in the deep subsur-
face is not extensive because pore fluids and ions 
are progressively lost with continued compac-
tion. The case for shale compaction is another 
contentious topic. Some experts hold that the 
precipitation of chlorite within shales may be a 
local sink for Mg. As with other models, large vol-
umes of Mg-bearing fluids are necessary for this 
model to be viable.
Hydrothermal Model—One fairly popular 
model, hydrothermal dolomitization (HTD), 
stems from an older idea that has been reincar-
nated in refined form. HTD commonly forms mas-
sive dolomites that are localized around faults 
(above right)
. Hydrothermal dolomite is formed 
by deep basinal waters as they travel upward 
through relatively permeable conduits, such as 
faults and thrust planes, or even zones beneath 
impermeable seals. As waters circulate down-
ward in basinal convection cells, they warm in 
accordance with the local geothermal gradient. 
With heating, they become more buoyant, move 
upward and flow outward along faults and bed-
ding planes. 
Buoyancy and viscosity affect the ascent rate 
and geometry of the rising fluid. Where buoyancy 
forces are stronger, the rising fluid forms a 
concentrated, predominantly vertical plume. 
Within this plume, temperatures, flow rates and 
chemical potential may be expected to decrease 
from the center toward its margins. For relatively 
cool systems, in which viscosity dominates, fluids 
rise slowly and plume geometry is determined by 
the ratio of vertical to horizontal permeability.
35
Deep waters become hydrothermal—meaning 
they are at least 5°C [9°F] higher than the ambi-
ent formation temperature—as they are transmit-
ted upward into cooler, shallower parts of the 
basin. Pressures of hydrothermal fluids also tend 
to be higher than ambient fluid pressures. 
Hydrothermal fluids, therefore, are those that 
ascend to cooler strata before their heat has had 
time to dissipate appreciably into the formation. 
They flow rapidly upward through permeable 
conduits, rather than migrating slowly through 
low-permeability strata. Active faults make the 
best conduits because they have not been miner-
alized. Some faults may even breach the seals of 
deeper aquifers, tapping geopressured fluids that 
flow at a high rate up the faults.
36
A similar process—fault-related hydrother-
mal alteration—has long been recognized by the 
mining industry as an important aspect of car-
bonate diagenesis. However, until recently, this 
process was largely overlooked in the evaluation 
of carbonate reservoirs. As a result, some fea-
tures that were probably produced by faulting 
and hydrothermal fluid flow have been inter-
preted as having formed in meteoric mixing 
zones, deep burial and other environments.
37
25. Warren, reference 2.
26. Land, reference 23.
27. Warren, reference 2. 
28. Epicontinental shelves are flooded continents, created 
through flooding by ancient seaways.
29. The term “dorag” is said to be loosely translated from 
the Farsi language, and is used to infer “mixed blood
or hybrid.” 
Badiozamani K: “The Dorag Dolomitization Model—
Application to the Middle Ordovician of Wisconsin,”