Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
high permeability structures that provide access for
hydrocarbons to reducing zones and sulfates. Such breccia
structures are heterolithic, being formed at different times
from the Triassic-Jurassic through the present, based on
the composition of heterolithic material and distribution in
the basin (Wallace and Crawford, 1992). Epigenetic calcite
bodies and sulfur deposits have been emplaced along
cavities and breccia structures since the late Cenozoic
Basin and Range extensional tectonism (Kirkland and
Evans, 1980; Miller, 1992; Wallace and Crawford, 1992).
Influx of oxygenated waters to interact with H2S to form
native sulfur occurred through shallow subsurface
carbonate beds in otherwise evaporitic sequences (such as
those in the Rustler), as well as through various disruptions
of evaporites, lateral ramifications of the hypogenic karst
structures beneath some barriers, and epigenic karst
features induced by the development of hypogene features
in the deeper zones. the Culberson ore body is an example
of how most of the sulfur accumulations occur at the top of
the Salado Formation, immediately beneath the vertically
heterogeneous Rustler Formation. Some deposits,
however, formed in the lower section of the Castile,
immediately above the Bell Canyon aquifer. The type
example is the Pokorny deposit (Klemmick, 1992), where
the formation of sulfur-bearing calcite bodies was
apparently guided by contact-type buoyancy-driven
speleogenesis.
Figure 59. Diagrammatic representation of hypogenic karst features in the Delaware Basin and adjacent reef structures, New Mexico and
west Texas, USA. Adapted from Martinez et al. (1998) for hypogenic features.
MVT lead-zinc deposits
MVT (Mississippi Valley Type) carbonate-hosted ore
deposits are considered by various researchers to be the
result of the mobilization, transport, and accumulation of
metal ions by regional groundwater flow (Baskov, 1987;
Garven et al. 1999; Tóth, 1999). Despite this general
understanding, geologists continue to debate the
mechanisms of fluid flow and chemical theories for ore
deposition. The relevance of hypogene speleogenesis to the
origin of MVT ore deposits has been recognized by some
workers (e.g. Ford, 1986; Ghazban, et al., 1991; Hill,
1996) but there is much broader potential here. To
illustrate this, below is a summary of the hydrogeological
characterization (based on Garven et al., 1999) of the
world's most important lead-zinc ore district, located in
southeast Missouri, USA.
The sulfide ore districts in the Mississippi Valley
region occur in Upper Cambrian and Lower Ordovician
dolomite strata that blanket the Precambrian rocks on the
Ozark Dome. Deposits are concentrated in a dolomitic reef
facies of the Cambrian Bonnetere Formation, with oremineralization
patterns controlled by pinchouts of the
underlying Lamotte Sandstone (against the Precambrian
granite) and collapse brecciation trends. It is believed that
deep sulfate brines were topography-driven mostly
northward from the source (the Arkoma foreland and
underlying basement), with focusing of flow, heat and
chemical mass within the carbonate formations. The
general interpretation is that ore formation was
concentrated on the Ozark Dome because of regional
groundwater discharge, aquifer pinchouts, and favorable
conditions for geochemical deposition related to
permeability, cooling, and fluid mixing. In the context of