Inner shelf sediments off Chesapeake Bay. III - Heavy minerals
Abstract
The heavy minerals in the sand sized fraction of 112 grab samples collected off the Virginia coast were analyzed for their variations in mineralogy. The main purpose was to characterize the heavy mineral suite and to delineate potentially important economic areas.
The heavy minerals comprise between 0 and 18 weight percent of the total samples averaging 5.3 + 3.8%. Dominant heavy minerals are garnet, magnetite-ilmenite, hornblende and epidote. Less abundant are kyanite, sillimanite, andalusite, apatite, tourmaline, rutile and zircon. Occasionally phosphatic shell fragments dominate not only the heavy mineral suite but the total sample. In general weight percent of the heavy mineral fraction varies inversely with the mean grain size. Potentially economic concentrations of zircon, rutile and ilmenite occur along the 60-foot isobath off Wachapreague Inlet. Garnet, Hornblende and the opaques dominate the coarser fractions.
Distribution of the total heavy mineral assemblage indicates concentrations parallel the present-day shore-line in water depths of between 30 and 60 feet because of hydraulic fractionat ion. Based on heavy mineral suites and concentration variations, two major sources are hypothesized: a dominant contribution from Chesapeake Bay and its tributaries, and a secondary addition from the vicinity of the Delaware River. The possible ancient strand line concentrations suggested by surface samples may yield economic deposits but their true economic potential must be determined by sampling at depth.
