This project focuses on understanding the origin of a unique chlorite-quartz-garnet rock that occurs in the area near Mineral, Virginia. The mineral assemblage is atypical for any standard protolith rock type, suggesting an unusual bulk composition. These rocks are immediately adjacent to other amphibolite facies rocks and occur at the unconformable contact between the Chopawamsic and Arvonia Formations. Petrographic observations and whole-rock major and trace element compositions are reported for seven samples, showing elevated MgO+Fe2O3 (17.28 to 33.59) against all rock types. Chemical Index of Alteration (CIA) values (77.79 to 97.99) support a weathering profile hypothesis and a depletion in Ca, Na, and K. However, values for the Mafic Index of Alteration (MIA) (43.60 to 65.84) are similar to unweathered rocks, suggesting enrichment in MgO and Fe2O3. This is possible in the alteration zones of a volcanogenic massive sulfide (VMS) deposit, formed by the interaction of seawater with hot magma, supported by high values of the Chlorite-Carbonate Pyrite Index (CCPI) (93.16 to 99.86) and the Ishikawa Alteration Index (AI) (95.67 to 99.76). Sample trace element compositions normalized to MORB values show negative Nb-anomalies. Rare-earth element (REE) chondrite-normalized plots show relatively flat patterns. Whole-rock major and trace element compositions, negative Nb-anomalies, REE chondrite-normalized plot patterns, the presence of rutile, and local and regional geology like the Virginia Gold-Pyrite Belt and nearby ferruginous quartzites suggest this unique rock likely originated as a basalt or basaltic-andesite that underwent hydrothermal alteration prior to metamorphism rather than formation via weathering processes.
Peters, Jonathan, "Petrogenesis of an Unusual Chlorite-Quartz-Garnet Rock, Chopawamsic Terrane, Virginia" (2022). Geology Senior Theses. William & Mary. Paper 28.