MARGARITE, GAHNITE, AND PROBABLE ZINCOHÖGBOMITE IN A CHLORITIC BLACKWALL ASSOCIATED WITH METAMORPHOSED ULTRAMAFIC ROCK IN THE RALEIGH TERRANE, EASTERN PIEDMONT PROVINCE, VIRGINIA

Chlorite-rich “blackwalls” represent rocks of unusual bulk composition formed at the contact between metamorphosed ultramafic rocks and adjacent country rocks. Such rocks reflect considerable metasomatic alteration, particularly loss of Si to the ultramafic rock. We report here a rare mineral assemblage in a blackwall occurrence associated with a small (<20 m across) ultramafic body in the Raleigh Terrane about 5 km north of Lawrenceville, VA. The ultramafic rock consists primarily of variable amounts of talc and chlorite, and probably represents a metamorphosed harzburgite or pyroxenite based on normative mineralogy (Mg# = 0.9, 1243 ppm Ni, 1690 ppm Cr). The blackwall is dominated by chlorite, with trace amounts of ilmenite, rutile and epidote. It also contains distinctive clusters (1-4 mm across) that typically consist of ragged Zn-rich spinel grains surrounded by masses of randomly oriented margarite. Spinels contain from 24 to 35 wt% ZnO, leading to the following compositional range: Gahnite_49-80Spinel_8-21Hercynite_12-29. Also spatially associated with gahnite are much smaller grains of probable zincohögbomite (based on preliminary probe analyses) that contain up to ~4 wt% TiO₂. Margarite compositions typically contain ~10% paragonite component. To the best of our knowledge, gahnite and zincohögbomite have never been reported from blackwalls, and margarite has only rarely been reported. Gahnite is typically produced from sphalerite or staurolite in other settings, but there is no evidence for the presence of these minerals in the field area (although immediately adjacent country rocks are not exposed). However, a whole-rock analysis of the blackwall contains 2594 ppm Zn. Thus, gahnite may have been produced directly during blackwall formation as a consequence of the bulk composition – the origin of this Zn-rich composition remains unclear. The texture of the gahnite-margarite clusters implies that margarite may have been derived via a reaction with gahnite (and perhaps chlorite). Such a reaction probably requires additional Ca-metasomatism following blackwall formation.