Research cruise preparations

January 2016

At the National Oceanography Centre in Southampton, we are busy preparing for our 2 month cruise to the Mid Atlantic Ridge scheduled for July and August, 2016. Although our cruise was postponed by 9 months due to external vessel operational reasons, the delay coincides with a similar delay for the work-package 1 cruise by our GEOMAR partners, and as a result the logical order in which both cruises are run has been restored. The Geomar cruise will include the acquisition of some vital site survey information to help constrain the targets for our cruise. Our cruises will both study the ‘TAG’ segment of the Mid-Atlantic Ridge at 26°N.The objective of our cruise is to develop tools to: • test new technologies to characterise the in-situ physical, geophysical, mineralogical and chemical structure and composition of eSMS deposits. • To collect samples, by deep-sea drilling, of eSMS deposits from tens of metres below the seafloor. On June 30th 2016, the NERC research vessel RRS James Cook will sail from Southampton Oceanography Centre, England, over 2000km to 26°N on the Mid-Atlantic Ridge. We will be carrying a large array of tools developed for the project including new electro-magnetic surveys sensors and a seafloor drilling rig that will be optimised for drilling up to 55m long cores into the seafloor.

Target Area

Existing bathymetry for the MAR at 26°N shows the axial valley (deep areas more than 3000m coloured dark-green and blue) the ridge flanks (shallow areas less than 3000m coloured yellow and orange) and the location of the 3 x 4 km study area (in set square). This map is derived from ship-board swath sonar and has a resolution of 100m square which is too low to identify a single sulphide mound. High-resolution (2m square gridded) bathymetry of the study area shows the location of the active TAG hydrothermal mound (in the south), a number of eSMS deposits (circled), plus and a broad area of massive sulphides called the Alvin Zone in the north (cross-hashed). The map clearly shows a shallow area, less than 3000m (coloured orange) that is part of the eastern flank of the Mid-Atlantic Ridge, and a deeper area greater than 3000m (coloured green and blue). Individual circular sulphide mounds, 100m to 200m in diameter are clearly visible.
The high-resolution data were collected in the 1990’s and kindly provided by Susan Humphries of Woods Hole Oceanographic Institution. They were reprocessed by Tim LeBas

Target Sulphide Mounds

3D bathymetry of the study area indicates the roughness of the seafloor and the proposed study targets (white circles). No vertical exaggeration.
The image is based on the high-resolution (2m square gridded) bathymetry of the study area and has been propjected from the south. The ridges running approximately N-S are formed by lines of small volcanoes and fault structures, and pose an operational difficulty when we tow our near-bottom instruments over them.

Example – the active ‘TAG Mound’

3D bathymetry of the hydrothermally active TAG mound. This is not a research target for Blue Mining, but previous studies including drilling in the 1990’s will provide a comparison to our results from the hydrothermally inactive mounds. A major difference between the active mound and inactive SMS is the presence or absence of anhydrite. This mineral is a calcium sulphate and dissolves at temperatures lower than ~120°C. At the active TAG mound, anhydrite is a major component of the gangue rock subsurface, but it is probably dissolved at the inactive mounds. We are also interested If the fate of copper and gold in the inactive SMS after high-temperature fluids flow has ceased.

Example – the inactive ‘New Mound’

3D bathymetry of the hydrothermally inactive ‘New mound’. This is one of our main research targets for Blue Mining, and will be included in the geophysics and drilling campaign. The mound is quite rough and will require a seafloor survey to establish flat benches to place the 6 tonne drilling rig on. The mound is ~200m in  diameter and is thus of a similar size to the active TAG mound. In addition to this mound, we will survey and sample other areas of exposed seafloor sulphide including the MIR zone. Drilling targets will also be identified from the geophysics surveys and especially the EM and magnetic study.

Methodologies for assessing extinct seafloor massive sulphides

The Blue Mining approach involves integrating remote-sensed geophysical data derived from a variety of newly developed techniques including resistivity, electromagnetics, electrical inductance, seismic reflection ad refraction, with characterization of the physical materials recovered by sampling and by drilling.
The collection of these data and samples, in 2500m of water, and down to 55m below the seafloor, poses significant challenges.
The composite image shows, from top left to bottom right, deep-towed sonar mapping tools, an autonomous underwater vehicle, the HyBIS robotic underwater vehicle, sampling using a robotic arm from hy7BIS, the seafloor drill and finally a model of the TAG SMS for comparison.

Mapping the exposed surface of eSMS deposits

Approach using a nested box strategy: We aim to use autonomous and remotely operated underwater vehicles fitted with sonar swath bathymetry, sub-bottom profilers, cameras and spectrometres to map the seafloor extent of mineralisation, sub-seafloor metaliferous horizons, and changes in mineral composition. The Autonomous Underwater Vehicle (Abyss 6000 – GEOMAR cruise) will identify more detailed areas for later study by the robotic underwater vehicle (HyBIS – NERC) which will ultimately survey sites for seafloor drilling (RD2 – NERC). We will always avoid active high-temperature hydrothermal chimneys and their surrounding deposits.

Mapping the sub-surface of eSMS deposits

To date, only three or four SMS deposits have had any seafloor drilling to determine their sub-surface structure. Three of those drilling expeditions were made by the scientific international ocean drilling programme (IODP) and one was made commercially for Nautilus Minerals Inc. The unique approach of Blue mining is to combine drilling results with sub-seafloor imaging by geophysical means to derive a method that combines all the information into a singular interpretative system for both the geology and geophysics. The geophysical tools include near-bottom seismic reflection and refraction. These will require the use of a deep-towed seismic streamer and ocean bottom instruments to seismically map the extent of sub-seafloor mineralisation and sub-seafloor deposit structure (see below). The data will be acquired during the Geomar cruise in early 2016.

Near-bottom electromagnetic techniques

In addition our approach we will develop novel near-bottom electromagnetic techniques to identify resistivity and electrical induction properties of the sub-surface. The aim of the geophysics is to map the extent of sub-seafloor mineralisation, deposit structure and composition down to a depth of 200m.

Deep-towed EM system

As part of work-package 1, a deep-towed EM system will be deployed, that will look more widely at the nature of the SMS field. For this the ship tows an electromagnetic source ‘DASI’ and its receivers (50m long electrode streamer and 3-axis receiver ‘Vulcan’) at a water depth of 3km and at a distance behind the ship of about 3km. Seafloor ocean bottom electromagnetic instruments also detect the electrical field and allow us to ‘see’ many hundreds metres below the SMS deposit and into the stock-work zone or feeder zone, yielding information on the processes of SMS deposit formation.Ship-deployed near-bottom electromagnetic source (DASI), deep-towed receivers (VULCAN) and ocean instruments (OBEMs).

Sampling the sub-surface of eSMS deposits

To obtain sub-surface samples of the deposit we have developed a drilling rig that lands on the seafloor. The RD2 can drill up to 55m deep holes, recovering cores and logging the holes using instruments lowered in to the holes. The drilling rig will be landed on seafloor at a depth of 3500m and powered by cables from the ship. Each hole could take 3 days to drill, requiring the ship to remain stationary. The core will be described and sampled on board and before being archived and returned to shore in cold-store containers. Ores and fluids will be analysed as well as the sediments overlying the deposits.
The core samples will be examined once back at shore and analysed for their physical properties, mineralogy and geochemistry. We will use the physical property data to better interpret the geophysical data we acquire during the cruises. The geochemistry of the samples will also help us understand the processes that affect mineral deposits after they have stopped forming and the hydrothermal fluids have cooled.
Together, the geophysical and geological data will allow us to better predict the resource potential of extinct SMS deposits, reducing risk to operations and to the environment by allowing targeting of the richest horizons only.

Case Study

A case study on How to Find Buried and Inactive Seafloor Massive Sulfides using Transient Electromagnetics formed the base of this Mid-Atlantic Ridge cruise.
An abstract (written by Sebastian Hoelz for the Underwater Mining Conference in St. Petersburg, Florida) contains the first results for the successful test of geophysical exploration technology to be used in the cruise.