Metamorphic basement rocks of the Southeast Appalachian Piedmont have been cored at the Savannah River Site (SRS) in South Carolina. Fractures and veins in the core samples yield evidence on possible mineralogic controls on ground water geochemistry. 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 characterised by quartz, epidote, calcite, chlorite, apatite, and k-spar, largely conforming to the structure and mineralogy of the host metamorphic rocks. Later cross-cutting vein sets include successive generations involving quartz, calcite, laumontite and gypsum. Two
rarities are barite and kutnahorite.
Veins are distributed heterogeneously with more late-stage veins in bore holes close to the buried Triassic border fault. Gypsum is restricted to samples from two bore holes close to the fault, where late fractures are filled by coarse, low-temperature gypsum. Ice melting temperatures of fluid inclusions in gypsum exhibit a mode at -0.2C, corresponding to nearly fresh water [gypsum-saturated water has Tm(ice) of -0.1C].
Chemical modelling of water analyses suggests that all samples are equilibrated with calcite, laumontite and feldspar. Most samples are undersaturated with respect to gypsum, though two samples from bore hole DRB-6 are gypsum-saturated. This is one of the two bore holes where gypsum veins were observed in core samples.
Preliminary sulfur isotopic analyses for gypsum and barite give 34SCDT values between +8.9 and +16.1, suggesting the possibility of a Triassic seawater origin for the sulfate.