MINERALOGY OF A CARBONATE VEIN, BALSAM GAP OLIVINE MINE, NORTH CAROLINA

Carbonate-bearing veins occur in metadunites of the Blue Ridge of Georgia and North Carolina. and cut a slightly hydrated noncarbonated assemblage of olivine (Ol) + chromite (Chr) ± orthopyroxene + chlorite (Chl) ± tremolite (Tr). The purpose of this study is to determine the mineralogy of the carbonate, and associated phases, and to relate their petrogenesis to the host metadunite.

An exceptionally coarse-grained carbonate vein from the Balsam Gap olivine mine, Macon County, NC, was selected for study. Very coarse-grained carbonate grains (up to 0.5 m) form the center of the vein. Included in the carbonate is coarse-grained (Tlc), (Srp), (Ol), and (Chl). A zone of coarse-grained Ol+Chl+Tlc+Tr with lesser amounts of carbonate marks the transition from the carboante-rich vein interior to the enclosing metadunite.

The host metadunite is composed of a polygonal fabric of Ol (Fo 90-92) + Chr. Rare grains of hydrous phases include Chl (mg# 93-96, Cr 0.2-0.6 apfu), Tlc (mg# 97-100) and Tr (mg# 95, IVAl 0.4-0.7) . A meshwork of fine-grained Srp (mg#95-97, Ni 0.01 apfu) outlines olivine grains in some samples.

Magnesite (Mg 094-0.97, Fe 0.02-0.06 apfu) is the major carbonate phase, but small amounts of dolomite (Ca 0.43-0.5, Mg 0.49-0.54 apfu) also occur in the vein. Nickle sulfides, mainly pentlandite, also occur in the carbonate.

Compositions of minerals common to the metadunite and vein are generally similar (e.g.metadunite Ol = Fo 90-92, vein Ol = Fo 94-95; metadunite Chl mg# 93-96, vein Chl mg# 94-95; metadunite Tr mg# 95, vein Tr mg# = 95-97 ), but the vein minerals range to higher Mg contents. Trace element compositions of the silicate phases show some differences (metadunite Chl Cr = 0.2- 0.6 apfu compared to vein Chl Cr = 0.4-0.6 apfu; metadunite Srp Al = 0.1 - 0.3, Ni = 0.1 apfu; vein Srp Al = 0.0 - 0.1, Ni = 0-0.1 apfu).

This pattern suggests the veins represented pathways for H2O/CO2/S-bearing fluids that altered the olivine of the metadunite. Fe from the altered olivine went into the magnesite and, along with Ni, into the pentlandite. Differences in chlorite compositions are related to the formation of Chl from Chr (elevated Al and Cr) in the metadunite. Fluid interaction with the metadunite is probably responsible for the introduction of Tlc, and possibly Tr, into the metadunite.