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