Friday, 19 August 2016

JC138 Blue Mining Cruise to the TAG hydrothermal Field, Mid-Atlantic Ridge: Part II...by Paul Lusty




Recovery of a gravity core onto the deck of
the RRS James Cook.
We are now on a nine day transit back to Falmouth after a long and highly productive cruise to the TAG hydrothermal field on the Mid-Atlantic Ridge at 26oN, 45oW. This was a very complex cruise due to the number of novel exploration and resource assessment technologies we deployed, and the extensive sampling we undertook in extreme water depths and below the seafloor. Despite our ambitious objectives and many technical challenges we have acquired a substantial new dataset comprising rocks and sediments, high resolution images and video of the seafloor and geophysical data. This will significantly enhance understanding of the distribution and style of mineralisation on this part of the Mid-Atlantic Ridge.

Sediment coring using both gravity and mega-coring devices has been an important part of the programme. These samples provide valuable information on the composition and geochemistry of sediments overlying and proximal to the sulphide mounds, and on a regional scale in order to determine potential vectors to the mineralisation. When the cores arrive on the ship they are quickly moved to the cold laboratory to minimise degradation. Pore waters are extracted for analysis once back ashore and the cores are then split for logging and subsampling. The cores typically comprise pelagic carbonate ooze and variably oxidised clays, locally containing sulphide-rich layers, which vary in thickness with distance from the hydrothermal source.


Changing a core barrel on RD2 on the seabed in water depths of about 3500m.
Core can be seen in the barrel in the left-hand image.
Images from L-R: L - Gravity core from close to the MIR Zone of the TAG hydrothermal field. R - RD2 drill core from the
oxidised zone of the Rona sulphide mound. The lighter silica-rich material on the RHS of the image is the jasper layer
which was challenging to drill through.

Whilst the sediment cores provide information on the composition of the upper 2-3 metres of the seafloor a key aim of this project was to sample the inner parts of the extinct massive sulphide mounds. This is necessary to improve understanding of the internal structure, mineralogical composition and metal distribution in these deposits, which is vital for determining their future resource potential. This can only be achieved through drilling. BGS with its rock drill 2 (RD2) drilling system has an almost unique capability in this area. Operating the rock drill in water depths of >3500 metres was a major test for the equipment, drilling crew and the coordinating scientists. It represents the record for the deepest operation of a seafloor lander style rig, with water depths well in excess of those encountered in the Atlantis Massif area where RD2 was previously deployed on International Ocean Discovery Programme (IODP) Expedition 357. The rig has the capability of drilling up to 55 metres below the seafloor using a carousel system.


A weathered sulphide chimney encountered during a
HyBIS dive on new mound.
Unfortunately due to a combination of technical issues, the very challenging seafloor environment (significant sediment cover and steep slope angles) and drilling conditions the deepest hole we drilled in the TAG field reached about 12.5 metres below the seafloor. This hole was drilled in a relatively newly discovered sulphide mound, which we have named ‘Rona Mound’ after the late Professor Peter Rona, a marine geologist who helped pioneer deep seafloor exploration notably along the Mid-Atlantic Ridge. Core recovery (about 27 per cent average from eight holes) from the holes that we have drilled with RD2 has been lower than expected due to a number of reasons. The contrasting geology encountered, ranging from soft sediment in the upper parts of the seafloor, transitioning into a consolidated highly oxidised zone, containing very hard jasper-rich layers, and below that massive sulphide made for challenging drilling conditions. This required the testing of a variety of drill bits (profiles and hardness), sometimes drilling open holes before changing to coring, and significantly varying drilling parameters including torque, bit weight and flushing pressures in an attempt to optimise recovery. Stability of the six tonne drilling rig on the soft seafloor sediment and hole stability also influenced recovery. Despite these challenges we have recovered some very impressive core samples, representing the upper part of the extinct massive sulphide mounds, from the seafloor sediments, through the oxidised zone and into fresh massive sulphide. These cores represent by far the most significant rock samples collected from the subsurface of the TAG hydrothermal field. In addition to geochemical and mineralogical study these samples will be used for physical properties experiments to characterise the elastic wave and electrical properties of the sulphide deposits. This will aid the interpretation of the seafloor seismic data and controlled source electromagnetic geophysical imaging of the deposits and host rocks acquired by this project.

Inside the RD2 control container during seafloor drilling operations.
In addition to generally advancing scientific understanding of extinct seafloor massive sulphide deposits a key aim of the Blue Mining project was to demonstrate novel methodologies and tools (such as a self-potential exploration tool for autonomous underwater vehicles, automated seafloor image analysis systems, RD2) to explore for and assess these potential mineral resources, particularly under sediment or lava cover. The experience gained and lessons learnt about operating in this extreme environment during the two project cruises will be vital for advancing deep ocean mineral exploration and resource assessment technologies beyond their current technology readiness levels.

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