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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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