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