Download PDF - Speleogenesis

88
NCKRI Special Paper No. 1
The most common patterns for hypogenic caves
formed in confined settings are 2-D or multi-story mazes in
which conduits are densely packed, or complex 3-D
systems. Hypogenic systems evolve to facilitate crossformational
hydraulic communication between common
aquifers, or between laterally transmissive beds in a
heterogeneous soluble formation, across the cave-forming
zones. The latter commonly represent originally lowpermeability
units where vertical flow predominates.
Caves receive either diffuse or localized recharge from the
underlying aquifer or deeper parts of a succession. They do
not have direct genetic relations with the overlying surface.
This type of karstification commonly results in more
isotropic conduit permeability pervasively distributed
within highly karstified areas measuring up to several
square kilometers but the actual pattern depends on the
initial permeability structure. Localization of highly
karstified areas depends on the distribution of head
gradients in the multiple aquifer system (which is partly
guided by erosional topography), heterogeneities in initial
permeability of various beds in the system and on the
nature and distribution of permeability in a feeding aquifer
(source of cave-forming fluids). Although being vertically
and laterally integrated throughout conduit clusters,
confined conduit systems do not transmit flow laterally for
long distances relative to the regional scale. White (1988)
fittingly compared the organization of artesian maze
systems with swamp hydrology.
Huntoon (2000) noticed that well-developed artesian
karst porosity and storage in karst aquifers behave
similarly to their counterparts in porous media, with the
distinction that the “pores” are very large. Ubiquitous
conduit porosity that develops through areas of transverse
speleogenesis accounts for rather high aquifer storage.
Discharge of artesian karst springs is commonly very
steady, being moderated by high karstic storage developed
in soluble units and by the hydraulic capacity of a whole
artesian system.
5.2 The role of hypogenic speleogenesis in
the formation of mineral deposits
The last two decades have seen rapidly growing
recognition of the significance of fluid migration and
groundwater flow systems in the genesis of mineral
deposits; important reviews include Baskov (1987), Sharp
and Kyle (1988) and Tóth (1988, 1999). A recent overview
on the role of speleogenesis is provided by Lowe (2000).
This section refines and reinforces some key aspects from
the perspective of the new hypogene karst concept
presented in this paper, and refers to some particularly
instructive examples.
Sedimentary basins around the world that contain
soluble carbonate and sulfate formations often host major
epigenetic and stratabound deposits of metals (lead, zinc,
barium, fluorine, copper, uranium, etc.) and sulfur, which
appear to be associated with discharge segments of
regional groundwater flow systems. The association of
many such deposits with deep-seated karst features and
high-permeability karstified zones was widely noted in the
relevant literature that discussed their origin, geology, and
hydrogeology. However, an important point was
commonly missed, hindering more adequate understanding
of mineral deposition. In contrast to the common views
that karst porosity simply hosts mineral deposits, the
refined concept of hypogene karst suggests that processes
of deep-seated karstification and the formation of mineral
deposits are dynamically linked (Klimchouk, 2000a).
Mineral deposition not only fills or lines cavities or karst
breccia zones, using them as spaces or guiding
discontinuities, but it occurs because speleogenesis alters
the regional flow system to converge at certain localities
and creates necessary transitory reactive and depositional
environments and geochemical thresholds.
Another aspect of the same problem is that, in
interpreting the paleohydrogeology of mineral deposits
associated with groundwater flow systems, high karst
porosity is commonly taken as a given property of the
hydrostratigraphic framework, assuming that “karst was
always there” (either as paleokarst or with no consideration
of its origin at all) to converge flow and/or host
mineralization. Genetic and paleohydrogeology models for
karst-related deposits almost always imply a
hydrostratigraphic framework with highly permeable
karstified aquifers and intervening non-karstic beds of low
to moderate permeability. This is contrary to the common
“initial” hydrostratigraphic framework that preceded
hypogene speleogenesis.
The result of these misconceptions is the lack of
recognition of the true genetic relationships between
speleogenesis and ore formation, and some unresolved
issues in models of ore genesis. This situation is mainly
due to the fact, discussed in the introduction, that karst and
cave science itself has so far failed to appreciate the
significance of hypogenic/deep-seated speleogenesis, and
to integrate the emerging relevant conceptual framework,
mechanisms and methodology into the general karst
paradigm. Karst and cave scientists had not yet offered
hydrogeologists and ore and petroleum geologists the
appropriate conceptual and terminological arsenal.
The place of hypogenic speleogenesis within a basinal
flow domain is discussed in Section 2 and shown in Figure
1. It is regularly associated with discharge segments of
regional or intermediate flow systems. However, the
arguments of this paper and substantial evidence
worldwide strongly suggest that this association is largely
because speleogenesis creates these discharge segments,
and makes them recognizable at the regional scale. Hence,
in basins containing soluble formations, the primary result of