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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 9. (September)

First Page: 1221

Last Page: 1221

Title: Metallic Sulfide Deposits in Winnfield Salt Dome, Louisiana: Evidence for Episodic Introduction of Metalliferous Brines During Cap Rock Formation: ABSTRACT

Author(s): Mark R. Ulrich


Winnfield dome is a shallow piercement salt structure that penetrates Late Jurassic through early Tertiary siliciclastic and carbonate strata of the North Louisiana basin. Quarrying operations in the calcite and Previous HitanhydriteNext Hit portions of the cap rock have exposed zones of metallic sulfides and barite. Within the Previous HitanhydriteNext Hit portion of the cap rock, sulfides form laminar zones parallel to the tightly intergrown layered Previous HitanhydriteNext Hit; sulfides occur either as cement between Previous HitanhydriteNext Hit 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, galen , 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 to Previous HitanhydriteNext Hit 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 that separates the overlying calcite from the Previous HitanhydriteNext Hit. The massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite, gypsum, and Previous HitanhydriteNext Hit. Colloform sphalerite, galena, marcasite, pyrite, and barite line the 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 to Previous HitanhydriteNext Hit 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 Previous HitanhydriteNext Hit portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations. Previous HitAnhydriteNext Hit grains that are completely surrounded by sulfides are euhedral and undeformed, similar to the Previous HitanhydriteNext Hit disseminated throughout the salt mass. Previous HitAnhydriteNext Hit grains outside the mineralized areas are deformed and tightly intergrown. These textures suggest that mineralizing fluids were introduced episodically along the salt and Previous HitanhydriteNext Hit interface at the zone of salt dissolution before that po tion of the Previous HitanhydriteNext Hit zone was compressed and accreted to overlying Previous HitanhydriteNext Hit cap rock. Therefore, the earliest formed sulfides originating by this mechanism occur at the top of the Previous HitanhydriteNext Hit cap rock zone, whereas the last sulfides to form are found at the base. Extensive sulfide concentrations along the Previous HitanhydriteNext Hit-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 Previous HitanhydriteNext Hit-calcite contact are locally superimposed on sulfides that formed at the salt-Previous HitanhydriteTop contact.

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