THE INFLUENCE OF CARBONATE PORPHYROBLASTS ON CARBON MINERALIZATION POTENTIAL OF ULTRAMAFIC ROCKS IN CENTRAL VIRGINIA

Ultramafic chlorite talc schists of the Lynchburg Group, and Ashe and Alligator Back Metamorphic Suites in Virginia are considered ideal candidates for carbon mineralization due to their high Ca and Mg contents. These rocks are exposed in discrete areas totaling ~1800 km², representing a promising carbon reservoir. Low permeability of these rocks poses a challenge for in situ carbon mineralization. Within these rocks are uncommon carbonate porphyroblast-bearing talc schists that might improve the carbon mineralization potential. These rocks consist primarily of talc, chlorite, and minor muscovite and hornblende. Carbonate porphyroblasts (≤1.3 mm) range from 10-20% in modal abundance and are rarely intergrown with biotite. Relict coarse plagioclase feldspar (< 1mm) is rare and fine pyrite (<0.25 mm) has a modal abundance of 1-2%. Optical mineralogy reveals carbonate porphyroblasts are likely dolomite, euhedral to subhedral, colorless to light brown (PPL), often embayed, display uncommon gray and speckled extinction, and have lamellar twinning along the short diagonal. Weathering of carbonate to orange-brown clays and voids following pyrite dissolution is observed. Using ICP-MS geochemical data, we created a PerpleX model of mineral stability in the SiO₂-MgO-CaO system at pressures <0.05 kb and CO₂/H₂O ~1.5 saturated conditions. The results suggest dolomite and magnesite are stable ≤180-200°C and at higher temperatures antigorite and tremolite are stable. Wang et al. (2013) conducted experiments at similar conditions and demonstrated that dolomite crystal margins are precipitation sites of finely crystalline carbonates. This suggests that carbonate porphyroblasts may serve as focal points for simultaneous dolomite dissolution and mineralization within otherwise low permeability rocks. Pyrite dissolution and incipient microfractures following precipitation of tremolite and antigorite may increase secondary permeability to facilitate in situ carbon mineralization. These rocks outcrop sporadically along a 600 km strike and therefore may be more prevalent than is reported.

Wang, X, et al. (2013) Reactivity of dolomite in water-saturated supercritical carbon dioxide: Significance for carbon capture and storage and for enhanced oil and gas recovery. Energy Convers. Manag, 65, 564-573.