IMAGING OF HEAVY MINERAL SAND CONCENTRATIONS USING MARINE MAGNETICS OVER ATLANTIC COASTAL PLAIN SEDIMENTS

Magnetic anomaly survey data can be used to map concentrations of heavy mineral sands (HMS). A key advantage of this method is that nearly continuous coverage can be obtained along survey tracks, providing information at a much lower cost than sampling. There are challenges, however, because the magnetic susceptibility of HMS concentrations tends to be low (< 1x10^-3 SI) and associated anomalies in turn tend to be subtle. Nonetheless, magnetic surveys have been successful at delineating larger deposits, even for airborne platforms flown at low altitude (< 50 m flying height). Marine magnetic surveys allow reduced distance between the source material and sensor, suggesting that smaller HMS deposits may be detectable. Marine magnetic data collected in Chesapeake Bay, MD, and in the inlets near Charleston, SC, when filtered to highlight short-wavelength anomalies, show low amplitude (< 10 nT) anomalies that likely represent heavy mineral sand concentrations. Comparisons to sidescan and/or shallow seismic data suggest that the heavy mineral sands are concentrated in areas undergoing seabed erosion, which in turn suggests the importance of entrainment sorting for the formation of heavy mineral sand concentrations. The magnitude of the anomalies is generally higher in the Chesapeake Bay area than near Charleston, which may be attributable to both differences in size of the HMS concentration as well as differences in composition. In particular, analyses of heavy mineral separates of samples from nearby beaches indicate higher Ti, Fe and magnetic susceptibility for heavy minerals from near Chesapeake Bay. These differences reflect larger scale variations in onshore heavy mineral concentration observed in the southeastern U.S. Due to other area uses, Chesapeake Bay and the Charleston inlets are not likely to host resources that are currently economic. However, these methods and analyses can be applied elsewhere along the Atlantic Coastal Plain to address questions including locations of HM concentrations, regional variations in composition (or lack thereof due to mixing), and seabed erosion.