Evaluation of Bakersville Eclogite for Abrasive-Grade Garnet

1. Abrasive-grade garnet study: Mitchell Co., North Carolina EVALUATION OF THE BAKERSVILLE ECLOGITE, MITCHELL COUNTY, NORTH CAROLINA FOR ITS SUITABILITY AS AN ABRASIVE-GRADE GARNET MENSAH-BINEY, Robert, BALDWIN, Robert, North Carolina State University’s Minerals Research Laboratory, 180 Coxe Ave., Asheville, NC 28801, REID, Jeffrey C., North Geological Survey, 1612 Mail Service Center, Raleigh, NC 27699-1612. A heavy mineral garnet concentrate, suitable as an abrasive, was produced from a 2 kg sample of garnet-rich material from the Bakersville eclogite, Mitchell County, North Carolina. The bench scale laboratory process consisted of crushing, grinding, and gravity separation to separate the heavy minerals from the light minerals. The sample was ground to pass US 30-mesh and the minus 30-mesh fraction was subjected to sink float separation using a heavy liquid with density of 2.96 g/cc. The sink fraction (heavy mineral concentrate) composed mainly of garnet was subjected to mineralogical evaluation to determine garnet grade. The results of the heavy liquid separation process indicated that about 85% by weight of the original material reported as heavy mineral concentrate. The garnets are red, subhedral, and average 2 mm in diameter. Microprobe compositions are Py32Alm47Grs22Sps<1 [rims] and Py27Alm49Grs24Sps<1 [cores] (Willard and Adams, 1994)1. 1Reference - Willard, R.A., and Adams, M.G., 1994, Earth and Planetary Science Letters, v.123, pp. 61-70. (Left - top) -- Location map showing location in North Carolina and field trip stop 1-5 from the 1997 Carolina Geological Society field trip guidebook (Stewart and others). Sample for this study not collected from this specific locality. (Left) -- Typical garnet separated from the Lick Ridge Eclogite. Scale shows grain phi size and mm (top scale rule markings) (Above) -- Photograph of garnet-rich rock (eclogite) used in this mineral separation study. (Left) – Geologic map of eclogite-bearing terrane northeast of Bakersville, N. C. Geologic map from Adams (1995). Explanation: Zas – AMS [Ashe Metamorphic Suite] metasedimentary rocks, predominantly metapelite and metagraywacke. Zaa – AMS amphibolite. Consists of amphibolite and garnet amphibolite. Zae – AMS eclogite. Consists of eclogite and retrogressed ecologite. Ymg – Laurentian basement rock. Mafic gneiss and felsic gneiss. mz – Mixed rock mylonite zone. Consists of mylonite derived from rocks of hanging wall and footwall. • PROCESSING STEPS FOR GARNET SEPARATION • Crush raw ore through a jaw crusher to ¾” • Re-crush product in Chipmunk to ¼” • Crush in Roll Crusher to 10 US Standard mesh • Grind in rod mill to 30 US Standard Mesh • Screen ground product on 200 US Standard Mesh to remove –200 mesh fraction • Attrition scrub 30 x 200 mesh fraction at 70% solids for 5.0 minutes with NaOH • Deslime on 200 mesh sieve to remove –200 mesh slime • Perform heavy liquid separation (sink-float) using 2.96 g/cc liquid • Sink product is garnet and other heavy minerals possibly magnetite • Perform particle size analysis on the sink fraction (garnet) (Above) -- SEM backscatter image of typical garnet from the Lick Ridge Eclogite. Inclusions are predominantly quartz (dark) and rutile (white). Subtle zoning in the garnet is evident in the shades of gray with a lighter inclusion-rich core and darker mantle. Sample BAK 11509 – Courtesy of Dr. Kevin Stewart UNC-CH. (Above) -- Detail of garnet-rich rock (ecolgite) as shown in the poster panel above. The red appearing mineral is garnet. Base map is from the Bakersville 7.5-minute quadrangle. Dry particle size analysis of garnet (+200 mesh sinks) Results (continued) Results (continued) • Selected information about other garnet resources in western North Carolina • North Carolina Geological Survey (www.geology.enr.state.nc.us Minerals Information) • Mineral file contents – Archdale office (see “garnet”) • Mineral file contents – Asheville regional office (see “garnet”) • Carolina Geological Society (www.carolinageologicalsociety.org) • Stewart, K.G., Adams, M.G., and Trupe, C,H., editors, 1997, Paleozoic structure, metamorphism, and tectonics of the Blue Ridge of western North Carolina • U. S. Geological Survey • McFaul, E.J., Mason, G.T., Jr., Ferguson, W. B., and Lipin, R.R., 2000, U. S. Geological Survey Mineral Databases – MRDS and MASMILS, U. S. Geological Survey Digital Data Series DDS-52 (two disks). Table of particle size analysis of garnet. Bar chart of particle size analysis of garnet. Cumulative weight percent retained curve for garnet (+200 mesh sink).