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IMPLICATIONS OF HYPOGENIC TRANSVERSE SPELEOGENESIS
speleogenesis is converging groundwater flow to zones
where ascending cross-formational communication is
greatly enhanced by speleogenesis, a condition commonly
seen as the most important for flow-induced accumulation
of transported mineral matter. In this way, speleogenesis
facilitates the interaction of waters of contrasting
chemistries and different geochemical environments. Thus,
it often creates geochemical thresholds or transitional
environments favorable to precipitation and accumulation
of mineral ores, such as sulfide metals, sulfur, certain types
of uranium deposits, etc.
Sulfur deposits
Large sulfur deposits are associated with gypsum
and/or anhydrite and formed by the oxidation of H2S
generated by reduction of dissolved sulfates in the
presence of hydrocarbons. The overall process results in
epigenetic replacement of sulfate rocks by calcite and
sulfur ores. Although general geochemical conditions and
processes for the formation of epigenetic sulfur deposits
are well established (Feely and Kulp, 1957; Ivanov, 1964;
Yushkin, 1969; Ruckmick et al., 1979; Kirkland and
Evans, 1980; Machel, 1992; Kushnir, 1988), the genetic
models for many deposits are still debatable, mainly in
aspects of their paleohydrogeology. Proper flow models
are crucial for understanding ore genesis since it is the
appropriate conditions in a flow system that allow
particular geochemical processes to operate and produce
massive mineral accumulations. To generate large sulfur
deposits, such hydrogeological systems must accommodate
sources for dissolved sulfate and hydrocarbons, their
interaction in an anaerobic environment to reduce sulfates
to H2S, and proper conditions for sulfide to oxidize to
native sulfur at the boundary between the reducing and
oxidizing environments.
Intense karstification is ubiquitously reported for
virtually all epigenetic 2 sulfur deposits. Klimchouk (1997c)
generalized that karstification is the intrinsically
accompanying process for the formation of epigenetic
sulfur because it supplies the dissolved sulfates needed for
large-scale sulfate reduction. In turn, sulfate reduction
serves to maintain the dissolutional capacity of
groundwaters with respect to gypsum and anhydrite. Even
more importantly, speleogenesis opens up crossformational
hydraulic communication paths and
establishes flow patterns that provide the spatial and
temporal framework within which the processes of the
sulfur cycle take place. In this way, it controls geochemical
environments and the migration of reactants and reaction
products between them.
2 Note that the term epigenetic (not epigenic!) is used here in the
connotation of changes in the mineral content of a rock because
of outside influence, occurring later than deposition of the host
rock.
Epigenetic sulfur deposits form within multi-story
confined aquifer systems. Most of them are associated with
areas where the upper confining sequence is considerably
scoured by erosion, e.g. within fluvial valleys or
paleovalleys, which induces upward discharge in gravitydriven
flow systems and transverse speleogenesis in sulfate
beds. In mixed systems, where there is a prolific aquifer
beneath a sulfate sequence, transverse speleogenesis can be
supported or enhanced by the buoyancy component. Three
regional examples below, from western Ukraine, northern
Iraq and the Delaware Basin in the USA, are particularly
illustrative of the role of speleogenesis in the formation of
sulfur deposits.
Western Ukraine. The Miocene gypsum sequence is
exposed along the southwestern margin of the eastern
European Platform, in the transition zone between the
platform and the Carpathian Foredeep. Gypsum extends
from northwest to southeast for 340 km in a belt ranging
from several kilometers to 40-80 km wide. It is the main
component of the Miocene evaporite formation that girdles
the Carpathian folded region to the northeast, from the
Nida River basin in Poland across western Ukraine and
Moldova to the Tazleu River basin in Romania. The
Miocene succession comprises deposits of Badenian and
Sarmatian age. The Lower Badenian unit, beneath the
gypsum, includes carbonaceous, argillaceous and sandy
beds (10-90 m thick), which comprise the main regional
aquifer. The Middle Badenian gypsum sequence is up to
40 m in thickness and overlain by the Ratynsky evaporitic
(a few meters thick) and epigenetic (up to 30 m thick)
limestone. The latter variety has low δ 13 C signatures
ranging from -32 to -65‰, a diagnostic feature for
bioepigenetic calcite recognized in major sulfur deposits
around the world. This calcite contains sulfur ore in the
deposits and locally replaces the gypsum entirely. The
Ratynsky limestone and the lower parts of the overlying
Kosovsky Formation comprise the upper (supra-gypsum)
aquifer, overlain by the upper confining clays and marls of
the Kosovsky Formation.
The sulfogenic province in the western Ukraine lies
mainly within the confined zone of the Miocene aquifer
system, which is recharged on the northeast where the
confining sequence is eroded and the lower aquifer is
exposed at higher elevations. The confined flow zone
extends to the southwest, where it is dammed by the
tectonic boundary with the Carpathian Foredeep.
Discharge occurs throughout the confined flow zone via
tectonic faults or karst breakdown structures. In the
adjacent interior parts of the platform, the Miocene aquifer
system is presently unconfined due to intense Plio-
Pleistocene uplifts and deep erosional entrenchment.
Extensive maze caves are known there from the same
gypsum sequence, five of which are the longest gypsum
caves in the world. They are shown to be the foremost
examples of artesian transverse speleogenesis, being
formed by dispersed recharge from the sub-gypsum aquifer
(Klimchouk, 1990, 1996c, 2000b).
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