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Metallic Sulfide Deposits in the Winnfield Salt Dome, Louisiana: Evidence for Episodic Introduction of Metalliferous Brines During Cap Rock Formation
Mark R. Ulrich (1), J, Richard Kyle (1), Peter E. Price (2)
Winnfield Dome is a shallow piercement salt structure which penetrates late Jurassic through early Tertiary siliciclastic and carbonate strata of the North Louisiana Basin. Quarrying operations in the calcite and anhydrite portions of the cap rock have exposed zones of metallic sulfides and barite. Within the anhydrite portion of the cap rock, sulfides form laminar zones parallel to the tightly intergrown, layered anhydrite; sulfides occur either as cement between anhydrite grains or as coarse euhedral crystals in open space. The sulfide layers are up to several centimeters thick and extend for tens of meters. These layers locally thicken into laminated, flat-bottomed concave downward massive sulfide mounds. Monoclinic pyrrhotite is the dominant sulfide; sphalerite, galena, or barite may also be present. In weathered samples hydrating to gypsum, marcasite is common and appears to be an alteration product of pyrrhotite.
A roughly laminated massive sulfide lens is exposed at the calcite/anhydrite transition zone. This sulfide body has a maximum thickness of about 5 m (16 ft) and extends for about 45 m (150 ft). The edges of the lens taper to a layer about 30 cm (1 ft) thick which separates the overlying calcite from the anhydrite. The massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite, gypsum, and anhydrite. Colloform sphalerite, galena, marcasite, pyrite, and barite line vugs and fractures; pyrrhotite is found locally within the sulfide lens. Examination of rotary drill samples suggests that massive sulfide concentrations, similar to the exposed example, are common along the calcite/anhydrite transition zone.
These sulfide concentrations are believed to have originated from the interaction of metalliferous basinal brines with reduced sulfur trapped within the cap rock. Textural relationships and variations in chemical compositions between the sulfide layers in the anhydrite portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations. Anhydrite grains that are completely surrounded by sulfides are euhedral and undeformed, similar to the anhydrite disseminated throughout the salt mass. Anhydrite grains outside the mineralized areas are deformed and tightly intergrown. These textures suggest that mineralizing fluids were introduced episodically along the salt/anhydrite interface at the zone of salt dissolution, before that portion of the anhydrite zone was compressed and accreted to overlying anhydrite cap rock. Therefore, the earliest-formed sulfides originating by this mechanism occur at the top of the anhydrite cap rock zone while the last sulfides to form are found at the base. Extensive sulfide concentrations along the anhydrite/calcite contact suggest that this contact also acted as a permeable zone allowing metalliferous brines into the cap rock. Textural and compositional relationships suggest that sulfides that formed along the anhydrite/calcite contact are locally superimposed on sulfides that formed at the salt/anhydrite contact.
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