Titanium geophysics: The application of induced polarization to sea‐floor mineral exploration

Abstract

Titanium is abundant in the Earth's crust, but it can be economically extracted from only a limited group of minerals, principally rutile [Formula: see text] and ilmenite [Formula: see text], both found mainly in fossil beach‐complex placer deposits. Both minerals have only a weak magnetic susceptibility, insufficient to permit correlation between magnetic surveys and known titanium‐rich deposits. However, ilmenite shows an unusually strong induced‐polarization (IP) response, whereas the IP response of rutile is relatively weak. IP spectral signatures for ilmenite acquired in laboratory and field settings are also distinctly different from those of other polarizing materials, for instance pyrite. A nonfloating, towed‐streamer IP system was designed and deployed in surveys off the coasts of Virginia and Georgia. When the cable lies on the sea floor, calculations indicate that only about 8 percent of the injected current actually finds its way into the underlying sediment. Partly because of this high transmitted‐current to injected‐current ratio, a stationary‐streamer IP noise envelope of about 2–4 milliradians (mrad) phase shift and a towed‐streamer noise envelope of 4–6 mrad were measured. Two surveys were undertaken, one of which covered about 30 traverse km of the Atlantic continental shelf (ACS) and crossed two vibracore sites where geologic control could be obtained. Many IP anomalies were observed, with some ranging as high as 20+ mrad; about one‐third of the shallow bathymetric lows (probable paleochannels) showed anomalous IP results. Modeling suggests that these anomalies may have been caused by significant heavy‐mineral placer bodies containing as much as 20 percent ilmenite. Identification of an anomaly in towed‐streamer (conventional IP) mode data will probably be posibble only if the deposit contains more than 1 to 2 weight percentage of ilmenite. Attempts to use the spectral IP method to identify ilmenite directly with the marine IP streamer in a stationary sampling mode gave equivocal results. Although the spectral IP method appears to work well on land, in the marine application the ilmenite was present in only small quantities at the vibracore sites investigated, and the resulting weak signal was partially masked by the noise present in the IP measurement. To make spectral measurements, the streamer must be positioned accurately over a polarizing source. This task is difficult, because the deposits tend to be made up of numerous discrete, kilometer‐long bodies, perhaps no more than 50 m wide and usually only 5 to 15 m thick. Such deposits cannot be adequately tested by a vibracore survey designed to sample every 300 or 1000 m, even if the survey is augmented with a high‐resolution seismic profile. This work suggests that the large IP response of ilmenite may permit rough quantification of sea‐floor placer mineral sources from a shipborne platform while the ship is in motion. Polarizing mineral species might even be identified by using spectral IP measurements. Applications of the technology to identifying other marine mineral deposits, such as smokers at ocean‐floor spreading centers and cobalt‐rich manganese crusts, are logical extensions of this research.