PACROFI VI - Electronic Program

Fluid Inclusions in Vein Minerals (Calcite, Quartz, and Gypsum) of the Southeast Piedmont, U.S.A.

David A. Vanko (dvanko@gsu.edu), Melissa A. Huner, Gwendolyn B. Rhodes, and Sandra M. Dowling
Department of Geology, Georgia State University, Atlanta, Georgia, USA, 30303

Metamorphic basement of the Southeast Appalachian Piedmont was cored at the Savannah River Site (SRS) in South Carolina. Veins in the core yield evidence of the history of fluid-rock interaction and possible mineralogic controls on ground water geochemistry. Early-stage vein minerals are dominated by quartz and epidote. Late-stage vein minerals in brittle fractures include zeolites (primarily laumontite with some stilbite), calcite, gypsum, and quartz. Calculated heterogeneous equilibria involving these minerals and water indicate a strong mineralogic control on deep ground waters within the crystalline Piedmont basement.

The metamorphic basement beneath the SRS is covered by a NW-thinning wedge of Cretaceous sediments that are 700' to 1200' thick. Core samples from 14 bore holes in the northern SRS, north of a major basement fault bordering the Triassic Dunbarton basin, comprise deformed and metamorphosed mafic to intermediate high-level plutonic and volcanic rocks.

A prolonged history of fluid-rock interaction extending from the peak of lower amphibolite grade metamorphism about 300 Ma ago is recorded in numerous and widespread veins. Early foliation-parallel "metamorphic" veins are characterized by quartz, epidote, calcite, chlorite, apatite, and k-spar, largely conforming to the structure and mineralogy of the host metamorphic rocks. FI are numerous in quartz and include a primary generation of L+L+V H2O-CO2 inclusions.

Later cross-cutting vein sets include successive generations involving quartz, calcite, laumontite and gypsum. Rarities include stilbite, barite and kutnahorite. FI in late-stage vein minerals are aqueous L+V or single-phase L-only inclusions. Calcite veins are differentiated between higher-temperature (150-300oC), brightly cathodoluminescent grains with L+V FI, and lower-temperature (70-85oC), moderately cathodoluminescent (with zoning) grains with L-only FI, based upon petrographic and isotopic data.

Gypsum veins are observed in two cores, although water chemistry indicates more widespread gypsum saturation. Ice melting temperatures of L+V FI in gypsum exhibit a mode at -0.2oC, corresponding to nearly fresh water [we measured the T of gypsum-saturated ice water has Tm(ice) of -0.03oC].

Chemical modeling of water analyses suggests that most samples are equilibrated with calcite, laumontite, feldspar, quartz, and gypsum. This illustrates that present-day groundwater chemistry is effectively buffered by the minerals occupying the permeable and porous fracture systems.

Preliminary sulfur isotopic analyses for gypsum and barite give d34SCDT values between +8.9 and +16.1 permil, suggesting the possibility of a Triassic seawater origin for the sulfate.