Geophysical Signature of Seafloor Massive Sulfide Deposits
Maurice A. Tivey
Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution
Email: mtivey@whoi.edu
The geophysical response or signature of seafloor massive sulfide (SMS) deposits will likely be a
major factor in the future exploration, detection and quantification of seafloor massive sulfide
deposits. To date, the detection and location of such deposits has typically been a peripheral
result of the search for active hydrothermal vent systems, which characteristically produce a
buoyant plume of hot and chemically distinct fluid in the water column that is easily detectable
by oceanographic (CTD and optical backscatter) and chemical sensors. When such plumes are
not present, the detection of inactive and extinct vent systems becomes a much more difficult
challenge. Lateral scale is an important factor in the search for SMS deposits as their plan view
area of only a few tens to a hundred meters is virtually undetectable from the sea surface. The
detection of vent systems requires measurements to be made within a few hundred meters of the
seafloor, but such surveys using towed sleds, autonomous vehicles or remotely operated vehicles
are limited in survey extent to a few sq. kms. This mandates the use of a nested survey strategy
and exploration models to predict the likely occurrence of vent systems and mineralization to
optimize survey time.
Hydrothermal systems and associated SMS deposits have geophysical signatures that range from
distinctive seafloor morphology (i.e. a chimney field or mound-like features) and optical
properties (e.g. visual imaging, iron staining and microbial mats, etc) to contrasts in the physical
properties of the host rock and mineralized material. Seafloor morphological cues in the form of
vent chimneys and collapsed chimneys that form mounds can be imaged by sidescan sonar and
high resolution multibeam bathymetric mapping with sub-meter lateral resolution. Contrasts in
the physical properties of the host rock such as magnetism can also be used to detect and map
hydrothermal areas. The hot, corrosive, hydrothermal fluids destroy the magnetic minerals
(magnetite) in the host rock resulting in discrete pipe-like zones that generate magnetic anomaly
lows. Magnetite can also form as consequence of hydrothermal activity and thus also produce
magnetic anomaly highs in some cases. SMS mineralization is highly conductive so that
electromagnetic (EM) methods can be used to detect the lateral extent of mineralization. High
frequency acoustic chirp systems may also be useful in sedimentary environments but have not
been widely used in volcanic terrains. The depth extent of mineralization is more difficult to
evaluate without more time consuming in situ techniques. While the detection of vent systems
on a broad scale will likely rely on bathymetric, visual and magnetic signatures for some time to
come, advances in technology will be needed to improve the ability to more quantitatively
evaluate mineralization at depth. For example, full tensor magnetic field measurements provide
better 3D imaging of anomalies and the depth extent of sources. Metallic sulfides and minerals
such as barite have high densities that are at the detection limit using gravity methods – advances
in sensor technology and improvements in vehicle dynamics will help in this regard. The EM
method will undoubtedly be a key technology along with improvements in other methods
including optical, acoustic and seismic methods. The role of bio-geo-chemical sensors could be
especially important in the search for inactive and extinct vent deposits.