Summary:
A very large nickel, manganese, cobalt, and copper resource occurring as polymetallic nodules is located on the seafloor in the Clarion-Clipperton Zone (CCZ) of the north-east Pacific Ocean.
Regional geological domains
Based on analysis of bathymetric data together with BC data, TMC recognized and mapped eight geological domains within NORI Area D:
• Abyssal plains: these constitute the majority of the Clarion-Clipperton Zone (CCZ) and are characterized by gentle slopes of 0° to 6º, and nodules lying on soft sediment;
• Abyssal hills: these are topographically higher features, oriented NNW-SSE, and are parallel to one another;
• Abyssal hills (hard): abyssal hills where the hill crests are associated with the occurrence of hardgrounds;
• Slopes > 6°: these are associated with the flanks of abyssal hills, where the slope is 6° or greater, and are likely associated with hardgrounds and/or volcanic debris and volcanic outcrop development typically associated with NNW trending faults. These steep slopes are considered to have low nodule prospectivity but have not been fully tested with sampling or photography;
• Slopes > 6° (hard): these are associated with the flanks of abyssal hills where the slope is 6° or greater, and are associated with hardground development, typified by outcropping (harder) Neogene-age sedimentary rocks. These steep slopes are considered to have low nodule prospectivity, based on limited box core sampling, AUV SBP data and photography;
• Volcanic outcrops: these are associated with volcanic growth-faults along the abyssal hill flanks, which trend NNW-SSE, and are elongated, narrow bodies;
• Volcanic cones: these are typically grouped in chains and follow the east-southeast "Hawaiian trend". These are isolated features and were not sampled, however, due to their volcanic origin, steep slopes (>6°) and dominant high-intensity backscatter, they are also considered to have low nodule prospectivity;
• Volcanic high: this is a macro-scale topographic feature situated in the SE corner of NORI Area D.
Geological investigations and the mapping of geological domains are less advanced in the other NORI and TOML areas, compared to NORI Area D. Features such as abyssal plains, abyssal hills, volcanic cones, and slopes > 6° have nonetheless been identified in these areas using MBES surveys. In addition, sediment drifts ponded in depressions, covering approximately 7% of TOML Areas B to F, have been interpreted from MBES data. There is also evidence for sediment accumulation near the base of some seamounts.
Nodule formation and sedimentation
Seafloor polymetallic nodules are composed of nuclei and concentric layers of iron and manganese hydroxides and formed by precipitation of metals from seawater. The metal accumulation rates are slow, and it takes a few million years to form a nodule (Skowronek et al, 2021).
In the NORI and TOML areas, most of the polymetallic nodules lie on the seafloor, often partly covered with soft sediment. In other locations, some nodules have been recorded as completely buried but the frequencies of such subsurface occurrences are very poorly defined (e.g., Kotlinski and Stoyanova (2006).
Nodule facies
Nodule size, shape and texture can be quite variable within a single sample. As a consequence, it is difficult to apply practical classification schemes based on these characteristics to broad areas of the seafloor. In order to characterize nodule occurrences on the seafloor at a larger, more practical scale, NORI identified three broad facies of nodule distribution. These are based on nodule coverage and the range of nodule sizes, as interpreted from camera imagery.
Type 1 nodule facies is typically characterized by >50% nodules (by area of coverage). The majority of these nodules are typically medium-sized and are closely packed, with many nodules in contact with their neighbors.
Types 2 and 3 are characterized by larger nodules, and the nodules are typically separated (i.e., there are noticeable sediment gaps between individual nodules).
Nodule distributions can be mapped by measuring the backscatter (return signal) response from multi-beam echo sounding (MBES) from vessels on the ocean surface. Type 1 nodule facies correlates with moderate-amplitude backscatter areas and is the most common facies. Type 2 and 3 nodule facies typically correlate with higher-amplitude backscatter areas.
Diagenetic crusts
Minor amounts of ferro-manganese crust were observed in photo-profiles collected from TOML areas. Two types of crusts were logged in a few locations by TOML and have been recognized by other workers (e.g., Menot et al., 2010):
• Massive crust is five to ten centimeters thick and is typically found in blocks tens of centimeters wide but occasionally as pavement;
• Crustal-nodules are small to medium sized (<20 cm) discrete fragments of ferro-manganese that can grade into nodules.
In total, crusts were logged in ~0.6% of the photo-profiles, with crustal nodules more common (~0.5%) and massive crusts being present in only ~0.1% of the photographs Neither type was collected in box cores during the TOML CCZ15 campaign, and their extent is deemed insignificant in terms of the Mineral Resource estimate.
Moisture content of nodules
The current estimate of moisture content of in situ nodules in NORI Area D is 28%, based on data from Campaign 6, 7 and 8 box cores and sampling of 3000 t of nodules recovered during the collector system test in 2022.
The current estimate of the moisture content of nodules in NORI-A, NORI-B and NORI-C is 24% and in the TOML areas is 28%.
Density of nodules
TOML measured the density of 76 individual nodules or batches of nodules from TOML Area B, C, D and F (AMC Consultants, 2016). The batches of nodules included fragments and sand resulting from attrition during transport and handing. The mean density of 34 individual nodules was 1.95 t/m³ (wet) and that of 27 batches of nodules was 2.0 t/m³ (wet).
Abundance of nodules in NORI and TOML
In detailed sampling of nodules by depth in the box cores from NORI Area D, on average, 96% of the nodules were recovered between the surface and a depth of 5 cm. The nodules recovered below 5 cm depth from NORI Area D were generally interpreted to have been pushed into the soft clay by the BC frame. There were only two box cores where nodules deeper than 15 cm were confidently observed in situ, but these nodules were so friable that they crumbled when attempts were made to remove the surrounding clay and were not recoverable.
The abundance of buried nodules in NORI-A, B, and C is poorly known at this time. Buried nodules were not included in Mineral Resource estimates.
In the TOML areas, nodules buried more than 10 cm beneath the surface were observed but are not very common. A total of 16 out of the 113 box cores taken during CCZ15 had buried nodules, however all of these were located in Area D and F. If just Areas D and Fare considered then buried nodules were found in about 23.8% of samples which is a similar ratio to that described by Kotlinski and Stoyanova (2006). Buried nodules tend to be of much lower abundance and larger than the average nodules found at the surface.
TOML Areas - Measurement of nodule sizes
The surveyed TOML areas host a range of nodule sizes at each location and there is variation on a scale of several kilometers across the surveyed areas. Within TOML-D, there is a mixture of sizes with some very large nodules found in the BC samples. Bigger nodules were recovered from the BC than were measured on the photo-profiling lines because partial cover by sediments resulted in an underestimation of nodule length.
The characteristics of nodule size and nodule size variation appear similar in NORI Area D and TOML-B, C, D, E and F. For the IA, it is reasonable to assume that mining systems designed for NORI Area D would be appropriate for the TOML areas.