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IMPLICATIONS OF HYPOGENIC TRANSVERSE SPELEOGENESIS
the hypogene karst concept presented in this book, it can
be easily seen that these conditions are associated with
confined transverse speleogenesis and are largely created
by it.
At the regional scale, nearly horizontal flow mostly
occurs in the basal aquifer (Lamotte and Bonneterre
Formations), which is confined by the thin Davis Shale.
The hydrostratigraphy is shown in part 3 of Figure 60. The
main deposits are aerially associated with a highpermeability
lens, the Viburnum Trend, which is 20 km
wide and about 100 km long. It is situated over the
basement arch, but where the topographic elevations are
the lowest. The Viburnum Trend affects regional flow
patterns, creates a large discharge zone and focuses flow
upon it, and induces a hydrothermal anomaly. Ore
mineralization patterns at the deposit-scale are controlled
by sandstone pinchouts, karstic channels and breccia zones
(which are also karstic channels in this case).
It is beyond the scope of this paper to offer an
elaborate speleogenetic/ore origin conceptual model for
MVT deposits of the Ozark Dome, but the hydrogeologic
conditions described above seem to be generally favorable
for hypogene speleogenesis, which could be a major factor
in the formation of ores:
In the basal aquifer system regional lateral flow of
brines occurred mainly through sandstone units, with
unkarstified carbonate strata serving as intervening beds.
The Ozark Dome area and local pinchouts of the basal
sandstone within it were favorable sites for transverse
speleogenesis to commence through the overlying
carbonate Bonnetere Formation, due to the combined
ascending potential of both the regional topography-driven
flow system and local thermal anomalies induced by the
direct rise of hot fluids from fractured Precambrian
basement into the carbonates.
Multiple dissolution mechanisms for speleogenesis
could operate; various mixing effects (particularly invoked
in these settings), dissolution due to increased calcite
solubility in cooling hydrothermal paths, sulfuric acid,
dedolomitization, etc. (see Section 3.6); this should be the
subject of a separate region- and deposit-specific analysis.
Transverse speleogenesis changed vertical permeability
and opened migration paths across carbonate units and the
confining bed (through fracturing and collapsing in
response to growing cave porosity below), enhancing flow
and regional discharge and inducing various reactions at
geochemical thresholds that commonly occur along crossformational
paths. Some of these thresholds could favor
ore deposition in previously created karst porosity at some
stages.
Fluid migration, speleogenesis and ore deposition were
transient processes, adapting to the regional dynamics of
landscape evolution as well as to deposit-scale dynamics of
porosity and permeability changes. A number of
geochemical models for ore formation can be adapted to fit
the above hydrogeologic/speleogenetic scheme.
Figure 60. 1 = Geologic section A–A’ in southeast Missouri, USA, showing geology of the Southeast Missouri Ore District. In the Viburnum
Trend, several deposits formed in the reef facies of the Bonneterre Dolomite, and ore-bearing solutions appear to have migrated up from the
Lamotte Sandstone. (from Kaiser et al. 1987). 2 = Lithostratigraphy of the Ozark Dome region; 3 = Hydrostratigraphy of the Ozark Dome
region: 1 = the basal sandstone-carbonate Cambrian-Ordovician aquifer, with “high-permeability lenses” within it, rested on fractured
Precambrian basement; 2 = less permeable Ordovician carbonates and shale; 3 = Permian shale (Adapted from Garven et al., 1999). Note
similarity of litho- and hydrostratigraphy on these sections with those of the speleogenesis model of Brod (1964) (see Figure 39)
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