GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF METAMORPHOSED MAFIC AND ULTRAMAFIC ROCKS OF SCHUYLER QUADRANGLE, NELSON AND ALBEMARLE COUNTIES, VIRGINIA

In Albemarle and Nelson Counties, Neoproterozoic metamorphosed mafic and ultramafic rocks—metagabbro, amphibolite, and talc-chlorite schist—outcrop as NE–SW striking lenses in distinct belts. These rocks exhibit significant potential for carbon mineralization, and the talc-chlorite schists display elevated critical mineral concentrations. This study examines these units’ petrology, mineralogy, and geochemistry to assess their origin.

Field examination reveals that metagabbro is green-grey, medium to coarsely crystalline, and non-foliated whereas amphibolite is dark gray to black, medium coarsely crystalline, and moderately schistose. Talc-chlorite schist is green to dark gray, fine to medium crystalline, and ranges from relatively hard with a weak foliation to soft, soapy, schistosity. Petrographic analysis shows metagabbro consists of amphibole, epidote, and plagioclase feldspar, with minor relict pyroxene and opaque minerals. Amphibolite contains amphibole, pyroxene, epidote, chlorite, and plagioclase feldspar, with minor quartz, biotite, and opaques. Talc-chlorite schist is composed of chlorite, talc, and calcite, with minor quartz and some magnetite. Geochemically, talc-chlorite schist has SiO₂ concentrations of 35-46 wt.%, while amphibolite and metagabbros range from 44-47 wt.%. The samples, on a Middlemost (1994) TAS plot, fall in the peridot-gabbro and gabbro fields, suggesting ultramafic origins. Mg-rich samples are talc-chlorite schists, whereas Fe-rich ones are metagabbros and amphibolites. Talc-chlorite schist shows elevated concentrations of Ni (713-1430 ppm), Co (65-115 ppm), and Cr (1710-3590 ppm), whereas amphibolite and metagabbro are Ni (66-73 ppm), Co (42.7-53.2 ppm), and Cr (128-134 ppm). Overall, the samples have LREE enrichment (~20-80X chondrites), slight negative Eu anomalies, and decreasing HREE (~8-40X). The amphibolite and metagabbro show the most elevated LREE (~70-80X). One talc-chlorite sample has enriched Lu (0.19 ppm), possibly indicating REE incorporation in phosphates such as apatite or monazite.

Mineralogical and geochemical similarities between the ultramafic rocks of Lynchburg Group to the north and those in the Ashe and Alligator Back metamorphic suites to the south suggest a regional-scale magma enrichment in Ni, Co, and Cr.