Geophysical Methods for the Mapping of Submarine Massive Sulphide Deposits

Abstract

Abstract In 1977, the first black smoker was discovered on the East Pacific Rise. Since this discovery, many more hydrothermal vent occurrences have been discovered in the deep ocean. These vents are associated with compact high grade copper, gold, and zinc deposits that are being actively explored for by national and private organizations. Exploration for these deposits usually begins with ship borne sonar mapping and towed water chemistry samplers. From bathymetric maps, potential targets for more detailed mapping with underwater vehicles are defined. Remotely operated vehicle (ROV) and autonomous underwater vehicle (AUV) mounted turbidity, pH, and oxidation reduction potential (ORP) sensors, magnetometers, electromagnetic (EM) systems, high-resolution multibeam echosounder (MBES) bathymetry, sidescan sonar and subbottom profiler surveys are used to delineate the extent and nature of the submarine massive sulfide (SMS) deposits. EM surveying can be used to determine the resistivity of near-surface and subsurface structure. SMS deposits fall into two categories; zinc rich non-conductive and conductive copper-gold deposits. One system is an ROV-mounted EM system that operates near the seafloor (Kowlaczyk, 2008). It comprises a transmitter coil wrapped around the ROV and an electric field sensor mounted close to the ROV. This system has successfully mapped high-conductivity zones corresponding to high grades of copper and gold. This system has detected blind mineralization at a depth of several meters, but does not penetrate beyond five or ten meters into the ocean floor. It does outline the limits of the system accurately, and has been used to direct resource-definition drilling of SMS deposits. Modelling shows that buried SMS deposits can be mapped using a controlled source electromagnetic (CSEM) system. A CSEM system consists of an electrical transmitter and one or more electric field receivers that are either on the seafloor or towed behind the transmitter. SMS deposits are conductive and will channel electrical current. Since the conductivity of the ocean is constant, electrical fields are a proxy for current density. Modelling shows that SMS targets produce a readily detectable electrical field anomaly. Using 3D inversion of the CSEM electrical field data, subsurface SMS deposits can be mapped. SMS deposits can also be mapped using seismic methods. The depth of water and the deposit geometry make it difficult to deploy standard systems. A new system using a vertical cable array and a surface source has successfully mapped SMS deposits in 3D. This paper will review the use of EM and seismic methods successfully used to map SMS deposits.