Chromite from the Morgan Run melange, Maryland Piedmont, exhibits anomalously high ZnO concentrations of over 19 wt% ZnO. The chromite is complexly zoned, optically and chemically, and occurs as an accessory phase in a laminated quartz-magnetite rock, and in ultramafite, adjacent blackwall zones, and the surrounding metapelite.
From core to edge, four zones can be identified in some of the chromite grains: a central transparent zone that is a solid solution along the gahnite-chromite join, a two-phase “ferritchromit” zone, a narrow anisotropic zone, and an outer magnetite zone. The occurrence of “ferritchromit,” without an outer magnetite zone, surrounding a gahnite-chromite core hosted by metapelite, demonstrates that “ferritchromit” is not a reaction zone that forms between chromite and magnetite. Rather, “ferritchromit” most likely forms as a precipitate that is in local equilibrium with a solution that is deriving nutrient from an irreversibly dissolving chromite core.
Geologic evidence suggests that Zn was introduced into this system early in a multistage hydrothermal event that extended from sea-floor hydrothermal alteration to regional metamorphism. The Zn (and Cu in the associated chalcopyrite) was most likely leached from basalts that probably underlay the host strata during the time of sea-floor hydrothermal activity.