Margarite, corundum, gahnite and zincohogbomite in a blackwall, Raleigh Terrane, Eastern Piedmont Province, USA

We report an unusual occurrence of margarite, corundum, gahnite and zincohogbomite from the Raleigh terrane in the Piedmont Province of Virginia. The assemblage occurs in a chlorite-rich blackwall associated with a small metamorphosed ultramafic rock body. The blackwall is dominated by chlorite, but also contains distinctive clusters (1-4 mm across) that typically consist of ragged Zn-rich spinel grains surrounded by masses of randomly oriented margarite. Also spatially associated with spinel are smaller grains of corundum and hogbomite. In most cases, spinel and hogbomite are sufficiently enriched in Zn to be called gahnite and zincohogbomite, respectively. Some hogbomite grains are distinctly banded in back-scattered electron images, primarily reflecting variations in Zn-content. Textures suggest that hogbomite formed at the expense of spinel (although locally the reverse relationship holds), and hogbomite compositions mimic those of spinel. Margarite appears to be a later phase, and textures imply formation via reactions with spinel, corundum and, possibly, chlorite. This occurrence of Zn-rich spinel and hogbomite is clearly related to the bulk composition of the blackwall rock, which contains >2500 ppm Zn. The origin of this Zn-rich composition is unclear, but could be due to a small amount of sphalerite or zincian-staurolite in the protolith.