Feature ArticlesNew Technologies for Deep-Ocean Seafloor Resource Exploration
By Ian Stevenson
Nautilus Minerals Inc.
Over the past two years, Nautilus Minerals has made notable discoveries of high-grade base and precious metal mineralized systems during its commercial exploration of the seafloor for copper, gold, silver and zinc seafloor massive sulfide (SMS) deposits. The identification, adaption and improvement of key high-resolution geophysical techniques have helped the company achieve a high rate of success.
SMS deposits are typically located in water depths greater than 1,000 meters and in close proximity to tectonic plate boundaries and submarine volcanic activity. Cold seawater that has entered the Earth’s crust is heated to high temperatures by volcanic and magmatic processes, forming hot hydrothermal fluids that can become enriched with economically valuable metal concentrations. These hot fluids are driven up to the seafloor by convection, producing hydrothermal plumes, or “black smokers.” SMS deposits are formed by the precipitation of metal-bearing sulfide minerals as black smoker chimneys, which range from mere decimeters to a few meters in diameter and can reach up to 20 meters in height. The continued process of chimney build-up, extinction, renewal and collapse can lead to the formation of sulfide mounds, which can be hundreds of meters in lateral extent.
Preliminary terrane selection of potentially prospective areas is aided by the wealth of data collected over the past 20 years by marine scientific research groups, which has resulted in more than 200 discoveries of active or recently active hydrothermal systems worldwide. Follow-up exploration campaigns typically survey the seafloor at progressive scales of definition—from a regional scale (greater than 10,000 square kilometers) to a basin-wide scale (1,000 to 10,000 square kilometers) down to a prospect scale (less than 10 square kilometers). The latter involves mapping, ground truthing and sampling activities.
At the regional scale, vessel-based multibeam echosounder (MBES) and tow-yo techniques are used to rapidly assess large areas of seafloor and define prospective areas for detailed survey. MBES acquisition provides useful detail on the nature of the topography and acoustic backscatter response of the seafloor. This data provides information on the presence of key macroscale features, such as seafloor structural and volcanic features which may indicate geologically recent seafloor volcanic activity and associated hydrothermal activity. Tow-yo surveys typically involve towing a conductivity, temperature and depth sensor; an oxidation/reduction potential sensor; and a light-scattering sensor, as well as a Niskin bottle water-sampling carousel, through the water column to detect water column anomalies and water geochemistry signatures associated with hydrothermal plume activity. Data from both of these techniques are used to vector prospective target areas.
Deep-tow side scan sonar (SSS) acquisition has been used in previous exploration programs to provide the next level of detail in seafloor mapping. Although this offers a five to 10-fold improvement in mapping resolution compared to vessel-based MBES backscatter acquisition, survey efficiency is greatly reduced due to the slower survey speeds required because of the long tow-cable used, limited tow-body control and reaction time, and longer start and end-of-line turns. Data resolution is also compromised due to the need to fly the tow-body approximately 100 meters above the seafloor to avoid collision with rugged volcanic terrain.
At the prospect-scale end of the exploration pipeline, remotely operated vehicles (ROVs) have been consistently utilized to provide detailed geological observations of the seafloor. These operations entail methodical grid-traversing of the ROV at low altitude above selected seafloor target areas to delineate potential SMS mineralization through direct observation of features such as chimneys and hydrothermal alteration indicators, using video cameras mounted on the ROV. Follow-up grab-sampling of chimney samples is undertaken using the ROV’s five-function manipulator arm. These samples are later assayed for metal content.
The final stage in the exploration pipeline entails undertaking follow-up geophysical surveys and drilling operations to further delineate the resource and to ultimately determine the depth extent and grade of mineralization for recoverable reserve definition. To date, Nautilus Minerals has drilled a total of 184 holes with a total length of 1,640 meters. The results culminated in the world’s first NI-43-101-compliant SMS resource statement at the Solwara 1 deposit in the Bismarck Sea off Papua New Guinea.
High-Resolution Geophysical Mapping
Over the past five years, progressive testing and implementation of various high-resolution geophysical techniques as part of an ROV mapping payload has enabled significant gains in the ability to better delineate SMS deposits at the prospect-scale.
Over the past three years, an ocean floor electromagnetic system (OFEM), developed in conjunction with Vancouver, Canada-based Ocean Floor Geophysics Inc. and Teck Resources Ltd. (Vancouver), has been successfully used to define conductive copper-sulfide-bearing SMS mineralization to a depth of less than six meters below the seafloor, even within areas covered by a superficial veneer of sediment.
Fluxgate magnetometers mounted on an ROV have also been used to map the local magnetic field. In addition to providing useful data on “blind” structural trends, which may be hidden under sediment cover and influence mineralization control, the data have also been used to successfully map magnetic “burn-holes,” which are attributed to demagnetization of the sulfides and the associated alteration halo caused by the destruction of magnetite by hot, acidic hydrothermal fluids.
These features are important indicators of the presence of potential SMS systems and can show up as low-magnetization anomalies up to a few hundred meters across, contrasting with higher magnetization of the unaltered seafloor.
High-resolution MBES surveys from an ROV platform flying at a 40-meter altitude above the seafloor enables mapping of the seafloor at decimeter-scale resolution. At these altitudes, the resolution is sufficient to confidently map the presence of SMS chimney structures. High-resolution SSS has also been used to successfully map chimney fields at the lower altitudes afforded by ROV platforms.
The deployment of higher-resolution (higher source-frequency) MBES and SSS systems from an ROV has highlighted that the size and bathymetric nature of SMS targets are typically beyond the current resolution limits of both vessel-based and deep-tow sonar technology, and it has demonstrated that these targets can be confidently delineated using MBES and SSS systems. This higher resolution mapping capability is critical for discovering inactive SMS systems, which exhibit a seafloor superficial bathymetric expression, but do not have an associated regional hydrothermal plume characteristic that can be used as a targeting vector.
Exploration Success Rate
To date, Nautilus Minerals’ exploration programs have focused on the territorial waters of Papua New Guinea in the Bismarck Sea and in the Tongan exclusive economic zone.
Despite the infancy of the SMS industry, Nautilus Minerals has been able to utilize existing geosurvey and other technologies to achieve a very high success rate in generating and converting exploration targets into high-grade SMS prospects.
Nautilus Minerals has 17 SMS discoveries (one discovery, Solwara 15, has yet to be sampled) and two sulphate-rich discoveries (which remain prospective) on its exploration licenses in the Bismarck Sea. The sulphate discoveries to date have anomalous metal values associated with them, and sulphate caps can potentially indicate the presence of economically valuable materials at depth.
On its prospecting licenses in Tonga, Nautilus has an additional 21 SMS occurrences and 32 further exploration (“plume”) targets requiring detailed mapping and geophysics.
Nautilus Minerals is currently assessing further technological opportunities to improve its exploration efficiency and effectiveness.
AUVs. Autonomous underwater vehicles (AUVs) bridge the gap between vessel-based MBES acquisition and detailed prospect-scale ROV intervention.
In addition, AUVs provide the operational flexibility to survey with a variety of sensor payload configurations at various scales of resolution. AUVs can provide higher resolution mapping than what is possible with conventional deep-tow technology and offer an estimated fivefold improvement in survey productivity over ROV technology. The execution of high-resolution geophysical surveys with an AUV platform would enable ROVs to be deployed more effectively to undertake selective follow-up ground truthing and sampling operations.
Nautilus Minerals currently has 166,217 square kilometers in granted exploration tenements and a further 364,079 square kilometers under application. In the last two years, the total line kilometers of exploration geosurvey has increased by more than 50 percent each year.
The adoption of AUV technology is seen as key to keeping pace with the geosurvey requirements of a rapidly increasing exploration portfolio. Primary AUV geophysical mapping payload sensors for SMS mapping applications include: magnetometers, MBESs, SSSs and sub-bottom profilers. Developments with strap-down gravity system deployment on an AUV platform are also being monitored as emergent technology with the potential capability to enable high-spatial-resolution gravity detection of SMS targets.
3D Orebody Delineation. A variety of marine geophysical techniques have been successfully used to map out the spatial extent of SMS systems, including MBES, SSS, magnetometers and OFEM. However, most of these systems are suited to mapping the lateral extent of the SMS system and not the depth extent. To date, the vertical extent of these systems has mainly been inferred through ROV-based drilling programs, which are both high-cost and time consuming and still only provide pinpoint realizations of the mineralized extent. There is therefore an increasing need to define the SMS orebody in 3D using geophysical techniques.
Desktop modeling is being pursued to examine the feasibility of adopting 3D seismic reflection survey techniques for SMS orebody delineation. Geoacoustic analysis of existing drill core material suggests that there are significant acoustic impedance contrasts between massive sulfide, altered footwall volcanics and fresh host-rock volcanics to warrant pursuit of this technique. Furthermore, this technology has been successfully adopted for 3D delineation of terrestrial volcanic massive sulfide deposits in recent years. These deposits represent the onshore analogues of ancient SMS deposits and are often characterized by similar geological rock types and associated seismic attributes.
The successful adoption of 3D seismic technology will provide the ability to more confidently define the 3D nature and extent of SMS mineralization zones and should significantly aid in the optimization of drilling programs, reduce drilling costs, and also provide critical geological and geotechnical detail for future mine planning and mine-extraction operations. This is an important consideration, bearing in mind that future mining operations will depend on remotely operated technology, due to the inherent nature of the deposit.
A variety of high-resolution geophysical techniques have been successfully adopted by Nautilus Minerals in the search for SMS systems over the past six years. Despite the infancy of the SMS industry, Nautilus Minerals has been able to achieve a high exploration success rate and continues to grow its SMS resource pipeline. Notwithstanding this, continual evaluation and adoption of promising technologies will continue to realize significant gains in exploration efficiency and optimization and constantly advance the SMS exploration frontier.
Ian Stevenson is the chief geophysicist at Nautilus Minerals Inc. He has worked in the marine minerals exploration and mining industry for the past 21 years, specializing in the development and application of high-resolution geophysical techniques and AUV mapping payloads. He holds a Ph.D. in geophysics from the University of Reading.
Jonathan Lowe is exploration manager at Nautilus Minerals. Prior to joining Nautilus in 2007, he worked for BHP Billiton in global minerals exploration for 12 years. He holds a B.Sc. in geophysics from Curtin University of Technology and an MBA in technology management from La Trobe University.
Sean Plunkett is a senior geophysicist for Nautilus Minerals and has been heavily involved in a wide range of seafloor and water-column interrogation techniques utilized in SMS exploration. He holds a B.Sc. in geophysics from Curtin University of Technology.