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Sedimentary Characteristics Based on Sub-Bottom Profiling and the Implications for Mineralization of Cobalt-Rich Ferromanganese

Sedimentary Characteristics Based on Sub-Bottom Profiling and the Implications for Mineralization of Cobalt-Rich Ferromanganese

Deep–Sea Research I 158 (2020) 103223

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Deep-Sea Research Part I

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Sedimentary characteristics based on sub-bottom profiling and the implications for mineralization of -rich ferromanganese crusts at Weijia Guyot, Western Pacific Ocean

Bin Zhao a,b,c,**, Yong Yang a,b,*, Xiangyu Zhang a, Gaowen He a,b, Wenchao Lü a,b, Yuping Liu a, Zhenquan Wei a,b, Yinan Deng a,b, Ning Huang a

a Guangzhou Marine Geological Survey of China Geological Survey, Guangzhou, 510760, China b Ministry of Natural Resources Key Laboratory of Marine Mineral Resources, Guangzhou, 510075, China c Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, 511458, China

ARTICLE INFO ABSTRACT

Keywords: Cobalt-rich ferromanganese crusts on have attracted much attention due to high economic potential Western Pacific ocean of various metals. Studies showed that seamounts in the Western Pacific Ocean are rich in cobalt-rich crusts, and Weijia Guyot Weijia Guyot (Ita Mai Tai) is one of the most promising one. Cobalt-rich crusts were drilled from this in Sub-bottom profiling our previous investigation. This study evaluates promising areas of cobalt-rich crusts on the whole guyot. Sedimentary characteristics Combined the sub-bottom profiles, topography, scientific ocean drilling data with related studies, this paper Cobalt-rich ferromanganese crusts Metallogenic promising areas studied sedimentary characteristic and their implications for cobalt-rich crusts mineralization prospective areas on the summit of Weijia Guyot. Three types of stratum reflection characteristics were identified: pelagic deposits, oolitic , and lagoonal mud. Reflection sequences in Chirp sub-bottom profiler records are well matched to stratigraphy obtained at Deep-Sea Drilling Project Sites 200 and 202. Cobalt-rich crusts promising minerali­ zation areas were delineated based on the water depth, slope gradient and pelagic pinch-out line, with the area approximately 576.4 km2. This estimated number is 10% higher than the results from previous studies (approximately 436.6 km2). It provides great implication for exploration and mining-lease-block selection in the future.

1. Introduction zones of the highest abundance of cobalt-rich crusts in surveyed regions. The mineralization and distribution of crusts are influenced by As cobalt-rich ferromanganese crusts (short for cobalt-rich crusts) multiple ore-controlling parameters. Several evidence show a relation­ constitute important submarine solid mineral, a series of investigations ship between the coverage of cobalt-rich crusts/nodules and slope gra­ and studies have been carried out since the 1980s (Craig et al., 1982; dients of seamounts in the Pacific Ocean (Yamazaki and Sharma, 1998). Halbach and Manheim, 1984; Halbach et al., 1987; Glasby et al., 1987; Cobalt-rich crusts are enriched in areas where the slope gradients are � Yamazaki, 1993; Yamazaki et al., 1996; Yamazaki and Sharma, 1998; greater than 15 , and coexist with sediments when the gradients are � � Verlaan et al., 2004; Hein et al., 2009; Asavin et al., 2010; He et al., between 4 and 15 , according to photo and video evidence. Ma et al. � 2011; Du et al., 2017; Zhao et al., 2019a). Previous studies have indi­ (2014) indicated that low slope gradients (3–7 ) contribute to the cated that cobalt-rich crusts occur on sediment-free surfaces of mineralization of crusts: as the slope increases, the thickness of crusts seamount slopes and summits. Crusts generate growing economic in­ gradually decreases, according to submarine dredging, photos, videos, terest owing to potential of metal production, including , and gradient data from central Pacific Ocean seamounts. Kim et al. cobalt, , rare earth elements (REE), tellurium and group (2013) and Yang et al. (2016a) analyzed the correlation between photos, elements (PGE)(Hein et al., 2000; Hein, 2000; Verlaan et al., 2004). To videos and geological sampling data with acoustic backscatter data, and identify promising areas of mining exploration, it is vital to delineate concluded that acoustic backscatter results can be used to determine the

* Corresponding author. Guangzhou Marine Geological Survey of China Geological Survey, Guangzhou, 510760, China. ** Corresponding author. Guangzhou Marine Geological Survey of China Geological Survey, Guangzhou, 510760, China. E-mail addresses: [email protected] (B. Zhao), [email protected] (Y. Yang).

https://doi.org/10.1016/j.dsr.2020.103223 Received 3 September 2019; Received in revised form 10 December 2019; Accepted 11 January 2020 Available online 18 January 2020 0967-0637/© 2020 Elsevier Ltd. All rights reserved. B. Zhao et al. Deep-Sea Research Part I 158 (2020) 103223 regional spatial distribution of cobalt-rich crusts. However, similar et al., 2017; Hein et al., 2009; Zhao et al., 2019a). backscatter intensity characteristics may implicate different surficial The Weijia Guyot was drilled, dated, and surveyed with gravity, deposits. Geological prospecting investigations carried out by Russian seismics, sub-bottom profiling, and magnetic methods (Heezen et al., scientists in the eastern segment of the Magellan Seamounts during the 1973; Heezen and MacGregor, 1973; Koppers et al., 1998; Asavin et al., cruise of the R/V Gelendzhik in 2003–2010 (Mel’nikov et al., 2010, 2010; Lee et al., 2003, 2005). Geochemical composition of cobalt-rich 2012), Asavin et al. (2010) and Novikov et al. (2014) attempted to ferromanganese crusts from Weijia Guyot contains mainly oxides of delineate the most promising seamount areas for future mining by use of Mn (up to 22.9 wt%), Fe (up to 21.4 wt%), S (up to 0.42 wt%), Co (up to geo-acoustic studies, shallow drillings, and sampling of cobalt-rich 8960 ppm), Ni, Cu, Zn, REE, Mo, Pt, and other trace and rare elements crusts. (Asavin et al., 2010; Wang et al., 2017). In addition, REE and PGE are He et al. (2005a) and Zhao et al.(2019a) explained interconnections found rich on the southern and southwestern slopes (Wang et al., 2017). between sub-bottom profiling and deep-sea video recordings in Geological sampling and geophysical surveys undertaken by China Western Pacific seamounts. They found the crust distribution can be Ocean Mineral Resource R&D Association has revealed the central revealed by synchronous application of sub-bottom profiling and video summit of the Weijia Guyot is constituted of covered by calcareous recordings. The lower boundary of the sediments corresponds with the pelagic oozes, while carbonate sediments cover the edges (Yang et al., upper boundary of crusts. Summarizing, the slope gradients, water 2016b; Wei et al., 2017; Wang et al., 2017). depths, seafloor topography, sediments distribution and other As any of previous research identified the cobalt-rich crusts from parameters are main factors which control the distribution of mineral Weijia Guyot as a metallogenically prospective, we analyzed high pre­ resources on seamounts. Several detailed research attempt to find the cision topographic data, sub-bottom profiles and archival materials and best method to delineate areas prospective with cobalt-rich crusts on publications, for purpose of detailed sedimentary characteristics of seamounts. guyot summits. The paper focus on architecture of sediments and deals Different studies shown the spire seamounts indicate higher crust with implications for exploration and mining-lease-block selection at abundances and coverage, than guyots (Chu et al., 2006). However, Weijia Guyot. restricted by the limitations of current mining technology, mining op- erations focus on the summit region of guyots, ridges, and plateaus on 2. Geological setting flat or shallowly inclined surfaces, such as summit terraces, platforms, and saddles, which show relatively smooth small scale changes with The Weijia Guyot is located at the southern end of the Magellan topography (Hein et al., 2000; Hein, 2000). Seamounts, in the Western Pacific Ocean (Fig. 1a). The Magellan Sea­ The cobalt-rich crusts which are widely developed on the surface mounts are adjacent to the Mariana Trench and East Mariana Basin of guyots in the Pacific Ocean have been studied for more than half (EMB) to the west and southwest, respectively. The Malkuswick Islands a century (Asavin et al., 2010). The investigation of cobalt-rich crusts are located north and Marshall Islands southeast to the guyot (Fig. 1a). in China started around 1997 and has been carried out by more An L-shaped flank ridge extends west and south. The area of Weijia than twenty expeditions in the Central Pacific and Western Pacific Guyot summit is approximately 1459.7 km2. Plateau elevations are sea-mounts. The Weijia Guyot (Ita Mai Tai) studied in this paper is one generally situated at depths ranging from 1400 m to 2200 m. The most of these seamounts (He et al., 2001, 2005a; 2005b; Yang et al., 2016b; Wei

Fig. 1. (a) Location map of study area; (b) Bathymetric map of Weijia Guyot, PB-Pigafetta basin, OFZ-Ogasawara fault zone, the gray solid lines are depth contours (meters); KFZ-Kashima fault zone, EMB-East Mariana Basin; the black dash survey lines modified from Lee et al. (2009); the solid black lines are the sub-bottom profiles used in this study; the map came from the latest data measured by “Haiyangliuhao” of Guangzhou Marine Geological Survey (GMGS).

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shallow part of the summit (about 1300 m) is located central eastern mainly composed of (organic-clastic and oolitic lime­ � part of guyot. The slope gradients of the summit plateau vary from 0 to stones), planktonic limestones and micritic limestones, indicating ages � � 2 in central areas and increases up to 4 towards the edge (Lee et al., of / to (Mel’nikov et al., 2012). 2005; Mel’nikov et al., 2012; Wang et al., 2017). The Magellan Seamounts are located on seabed of Pacific 3. Data and methods Plate and consist of several seamounts formed by volcanic activity during (Lee et al., 2003). The Ogasawara Fault Zone (OFZ), Sub-bottom profilingserve as an invaluable technology to study the cutting the Magellan Seamounts, was formed in the Middle and Late depositional and erosional processes in deep-sea environments (Lee Jurassic, and consist of three separate faults: OFZ1, OFZ2 and OFZ3 et al., 2002; Zhao et al., 2018). In marine geological survey, sub-bottom (Nakanishi et al., 1989; Abrams et al., 1992; Koppers et al., 1998). The profilingcan effectively identify pelagic sediments, basement outcrops, OFZ divides the Western Pacific region into the EMB and the Peifetta slide deposits and debris zones (He et al., 2005a; Lee et al., 2005; Basin (PB) (Lee et al., 2005, Fig. 1a). The 40Ar/39Ar chronology, gravity Mel’nikov et al., 2010; Zhao et al., 2019a). and magnetic anomalies results indicated the first volcanic eruption in Sub-bottom profiles and seabed bathymetry data used in this study study area occurred during Aptian (Early Cretaceous), and ranged from were collected by the R/V “Haiyangliuhao” of GMGS, in 2015 and 2017. 120 to 118 Ma BP. The second volcanic event occurred during Eocene The applied multibeam echo sonar system was the Simrad EM122, pro­ (Wedgeworth and Kellogg, 1987; Koppers et al., 1998; Lee et al., 2003). duced by Kongsberg, Denmark. The sub-bottom survey lines were In 1971, the Deep Sea Drilling Program (DSDP) carried out drilling mainly conducted by NE-SW-direction tracks, with approximately 4.5 studies on the Weijia Guyot (Heezen et al., 1973). Three sites were km spacing (Fig. 1b). The sub-bottom profiler was Parasound P70, pro­ drilled (DSDP20-200, DSDP20-201 and DSDP20-202, Fig. 1b), with a duced by ATLAS (Germany), with an emission frequency of 20 kHz. The total depth of 132 m, 96 m and 153.5 m, respectively. The DSDP drillings original data were processed by section connection, coordinate con­ enabled to recognize early Eocene- sediments at the top of version, high value interference suppression, waveform processing, the Weijia Guyot. The bottom part consists of early Eocene- band-pass filtering and re-sampling (Zhao et al., 2019a). The interpre­ , covered by -Quaternary foraminif­ tation of sediment thickness was processed with DECO Geophysical eral oozes, often containing fragments. The pre-Eocene oolitic RadExPro Software (Gaynanov, 2010), which primarily identifies the limestones were found at depths of 83 m and 106 m (Fig. 2, Heezen boundary of pelagic sediments and the underlying strata. et al., 1973; Heezen and MacGregor, 1973; Hesse, 1973). In geological interpretation, time-depth conversion of sub-bottom Three sedimentary units were identified on the summit of Weijia profiles usually base on a constant velocity when examining shallow Guyot due to multichannel seismic profiling: (1) calcareous pelagic sediments with sound velocities similar to seawater (�1500 m/s) (Lee oozes with parallel layered reflectioncharacteristics; (2) reef limestones et al., 2005). However, the simple constant substitution method is not with high amplitude reflections, and (3) shallow-water lagoonal sedi­ accurate because of the difference in the propagation speed of sound ments with sub-parallel layered reflection characteristics (Lee et al., waves. Wedgeworth and Kellogg (1987) calculated the velocity of 2009). The strata on the summit of Weijia Guyot, according to pelagic sediments in the Weijia Guyot, combining DSDP data and related geo-acoustics, dredging and shallow drilling results, are divided into studies on surrounding seamounts, and concluded that the average three stratigraphic units: (1) late Miocene-Quaternary, (2) late Miocene, velocity in Magellan Seamounts is 1630 m/s, while the and (3) early Eocene-late Miocene (Mel’nikov et al., 2010). average velocities of limestones and shallow muds are about According to integrated acoustic profiles and detailed sediments 3800 m/s and 2000 m/s, respectively (Wedgeworth and Kellogg, 1987; sampling data, the basement exposed to margin of Weijia Guyot is Heezen et al., 1973; Jones, 1973). The equation of time-depth conver­ sion is: 1 Ts ¼ ⋅TWT � V 2

Where, Ts ¼ thickness of sediments, TWT ¼ two-way travel time, V ¼ sound wave velocity. Sedimentary thickness data was gridded in Golden Software Surfer 12. The interpolation method is ordinary kriging.

4. Results

4.1. Stratum reflection characteristics

Two sub-bottom profiles were selected for reflection characteristics analysis: line DY41SP1610, which passes through the DSDP20-200, and DY41SP1605 crossing the northwest of DSDP20-202 (Figs. 1b and 3). Stratum reflectionof Weijia Guyot were divided into three types. Type I indicates sharp reflections of parallel layers and non-distracted hori­ zontal continuity, extending from the center of the seamount till the edges, where sediments pinch-out (Figs. 3 and 4). Type II, below Type I, exhibits sub-parallel layer reflections, with a thick layer reflection interface, several transparent, weak inter-layering, and a blurred, discontinuous bottom reflection boundary (Figs. 3 and 4). Type III’s reflectionprofile is almost transparent, and associated with the volcanic Fig. 2. Stratigraphy of 200 and 202 sites, modified after Heezen et al. (1973); basement, sometimes contiguous with Type II (Figs. 3b and 4b). Heezen and MacGregor (1973) and Hesse (1973). The deposition age of oolitic limestones is not accurately determined. E.Pleist. ¼ Early Pleistocene, L.Plioc. ¼ Late Pliocene, E.Plioc. ¼ Early Pliocene, L.Mioc. ¼ Late Miocene, E.Mioc. ¼ Early Miocene, E.Eoc. ¼ Early Eocene, Paleoc. ¼ .

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Fig. 3. Sub-bottom profilereflection characteristics (DY41SP1610 and DY41SP1605), with three main stratum reflectiontypes identified.The horizontal axis “FFID” means file number; The vertical axis “TWT” is two-way travel time, where “ms” stands for milliseconds. The location of sub-bottom profiles is shown in Fig. 1.

Fig. 4. Comparison of DSDP Site 200 and 202 stratigraphy with sub-bottom profiler reflections. Two boreholes did not reached the volcanic basement. DSDP stratigraphy histograms legend see Fig. 2, location of profilessee Fig. 1. Location of profilessee Fig. 3. The sub-bottom profilesindicate good matching with lithology from DSDP cores.

4.2. Sedimentary characteristics geophysical data and geological sampling, the sediments deposition on Weijia Guyot began at least in Early Cretaceous Aptian (125-112Ma BP) The DSDP sampling revealed that the top of Weijia Guyot is covered (Koppers et al., 1998). The strata of the guyot include Early Cretaceous by an early Eocene-Quaternary sediments. Any of three DSDP wells (Aptian-Turonian), Late Cretaceous (-), late reached the volcanic basement, and only loose basalt fragments in for­ Paleocene-Eocene, Miocene and unconsolidated Pliocene-Quaternary aminiferous mud were found. Two layers of oolitic limestone were sediments (Mel’nikov et al., 2010, 2012). The Cretaceous and Paleo­ reached by the DSDP20-202, and the estimated age is pre-Eocene gene sediments indicate similar lithology, mostly consisting of reef (Heezen et al., 1973; Heezen and MacGregor, 1973). According to limestone, and fine clastic rocks. The strata are

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missing, similarly to other seamounts in the Magellan Seamounts subsided to a considerable depth (Lee et al., 2009). (Mel’nikov et al., 2010), which possibly generated by internal wave Lee et al. (2005) has carried out a sub-bottom profiling survey on motions (Mitchell et al., 2015), or Eocene/Oligocene (E/O) events Weijia Guyot (Fig. 1b, the black dashed lines in north-south-direction, (Zhang et al., 2015). Except for the Miocene and subsequent sediments, with approximately 13 km of spacing). However, the time-depth con­ such as pelagic oozes (Type I), other types represent shallow-water version of sub-bottom profileswas based on a constant velocity of 1500 sedimentary facies (Heezen et al., 1973; Heezen and MacGregor, m/s. It was difficult to accurately match the DSDP drilling results and 1973; Hesse et al., 1973, Figs. 3–5). The oolitic limestones (Type II) are the time-depth conversion, which lead to an ambiguity in the litholog­ widely distributed in the margin of Weijia Guyot, and found in the ical interpretation. Considering the calculation from Wedgeworth and adjacent Gelendzhik Guyot. The Type II sediments represent Kellogg (1987), this paper we estimated the sedimentary thickness in the Aptian- lithofacies. The age of oolitic limestones, combined with vicinity of DSDP drillings. The results show, the oolitic limestones and DSDP drillings data, indicates Early Cretaceous (Aptian-Albian) stage. lagoonal muds found during the DSDP exploration match with the Considering the reef limestone deposited in -Maastrichtian to interpretation of sub-bottom profiles (Figs. 2 and 4). For instance, the early Paleogene, it can be ascertained that the Weijia Guyot subsided pre-Eocene oolitic limestone (Type III) are found at depth of 83 m and hundreds of meters below the sea level during this period (Mel’nikov 106 m in DSDP20-202, and at depth of 132 m in DSDP20-200 (Table 1, et al., 2012). Fig. 4), which proves the reliability sedimentary facies analysis. In The Type I lithofacies is interpreted as a pelagic ooze, corresponding addition, calculation results revealed that the sedimentary thicknesses with DSDP 200–202 drilling results (foraminiferal ooze, foraminiferal above the volcanic basement at DSDP20-200 and 202 are approximately and foraminiferal mudstone) (Fig. 4, Heezen et al., 1973; 212.4 m and 174.8 m respectively. This indicates that the DSDP wells Heezen and MacGregor, 1973; Hesse, 1973; Lee et al., 2005), which is did not touch but almost reach the volcanic basement. lenticular on the whole top of the guyot (Fig. 5), thick in centre and thinner in the margins. This kind of pelagic sediments developed widely 5. Discussion on the Magellan Seamounts, especially on the guyots (Lee et al., 2005, 2009; Zhao et al., 2019a), indicating deep-sea environment. The li­ Cobalt-rich ferromanganese crusts are oxides and (oxy)hydroxides thology of Type II is much more complex, including sub-parallel layers formed on the surfaces of submarine rocks or debris. Crusts are mainly represented by reef mudstones, interlayered by transparent zones distributed on guyots or top and slope of marine terraces above the without reflections, or weak reflections of reef complexes and oolitic carbonate compensation depth (CCD). Crusts form within the Oxygen limestones (Badley, 1985; Lee et al., 2009). It is evident, the two layers Minimum Zone (OMZ) or below the OMZ, with depths between 500 m of oolitic limestones drilled in DSDP (Heezen and MacGregor, 1973) and 3500 m, especially enriched in the depth of 800–2500 m (Halbach correspond with Type II in sub-bottom profiles (Fig. 4b). The Type II and Manheim, 1984; Halbach et al., 1987; Glasby et al., 1987; Hein represents shallow lagoon deposition environment with reef complexes et al., 2013). (Lee et al., 2009; Mel’nikov et al., 2010, 2012). The Type III is repre­ The mineralization and distribution of crusts are influenced by sented by solid and homogenous limestones (Fig. 4a), with hardly any multiple ore-controlling factors, such as water depth, seabed topog­ signal observed between individual seismic layers (Erlich et al., 1990; raphy, oceanographic conditions, surface biological productivity, cur­ Zhao et al., 2019b), and indicating weaker hydrodynamic conditions, rents, rate of sedimentation, tectonic activity and basement type (Chu compared to Type II. In summary, Type I sediments overlay Type II and et al., 2005; Ma et al., 2014). Generally, the correlation between crust locally show unconformity contacts; The Type III underlies Type II, or thickness and depth is negative, and the crust within shallower water form unconformities directly on the volcanic basement (Fig. 5). The would be thicker and within deeper water would be thinner (He et al., Type II sediments were formed in a relatively turbulent hydrodynamic 2011). The cobalt-rich crusts coexist with sediments, where seafloor � � environment (shallow water), and did not change until the guyot slope gradients are between 4 and 15 (Yamazaki and Sharma, 1998).

Fig. 5. Geological interpretation of sub-bottom profiles (DY41SP1610 and DY41SP1605). For the location of sub-bottom profiles see Figs. 1 and 3.

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Table 1 Time to depth conversions.

Points Stratum Type TWT (ms) Velocity (m/s) Ts (m) Points Stratum, Type Velocity (m/s) TWT (ms) Ts (m)

A I 1961.0 1630 43.4 E I 1630 1940.4 41.9 B 2014.3 F 1991.8 B II 2014.3 2000 61.3 F III 3800 1991.8 111.5 C 2075.6 G 2050.5 C III 2075.6 3800 107.7 G II 2000 2050.5 21.4 D 2132.3 H 2071.9 Total 212.4 174.8

TWTs of A, B, C, D, E, F, G and H are two-way travel times from the point to sea level. When calculating the deposition thickness, as pelagic cap (Type I) in Fig. 4a for example: TAB ¼ 1/2 � TWT (B-A) � 1630 m/s ¼ 1/2 � (2014.3 ms–1961.0 ms) � 1630 m/s ¼ 43.4 m.

� � Usually, the slope between 3 and 7 is the best for crust mineralization. and edge of the summit, are the most promising areas, in terms of finding As slope increases, crust thickness gradually decreases (Ma et al., 2014). cobalt-rich crusts. Especially predominated are northern and southern Topographic flat and low-lying areas are prone to sediments accumu­ margins. lation. The central summit parts of large seamounts, where high rates of The summit of Weijia Guyot mainly consists of pelagic sediments, sedimentation are observed, are not favorable for long term growth of represented by oolitic limestones, lagoonal muds, and volcanic rock crusts (Chu et al., 2005; Ma et al., 2014; Zhao et al., 2019a). In summary, outcrops. Presented study indicate the spatial distribution and depth topography have a fundamental influence on crusts distribution and range of pelagic oozes. The pinch-out lines and thickness of sedimentary mineralization of Weijia Guyot. Sedimentation and bottom current cover on the summit of Weijia Guyot is estimated. The highest thickness scouring play a part of retarding and promotion respectively. Bottom of pelagic sediments cover is over 70 m, and located in the eastern- current is the key factor to ensure long-term growth of crusts on lower central margin (Fig. 7). No pelagic sediments are indicated in the gradient (Ma et al., 2014). western, northern and southern margins of the guyot (Fig. 7, Subareas 1, Moreover, crusts are prone to collapse during the growth process in 2, 3 and 4). Distinguished subareas overlay volcanic basement, and are steep terrain areas, which reduce stable formation. The growth of cobalt- preferable for the crust development. rich crusts usually requires a stable hard substrate (Bulmer and Wilson, Different studies demonstrated, the strong bottom current on the 1999). Outcrops, such as seafloor and associated substrate, are guyots of Magellan Seamounts are mainly distributed on the summit favorable for crust formation (Bulmer and Wilson, 1999; Chu et al., margins and ridge branches. Usually, the flowrate of bottom currents on 2005; Hein et al., 2009). In addition, research revealed shallow-buried the summit centers decreases rapidly (Zhao et al., 2019a). Strong bottom crusts under deposits with considerable thickness, and the shallowest currents prevent sediments to accumulate, and in consequence condu­ sedimentary coverage is less than tens of centimeters (Yamazaki, 1993; cive crusts formation (Chu et al., 2005; Ma et al., 2014). Similarly, the Yamazaki et al., 1996). In these cases, mentioned relations are essential strong bottom currents cause no pelagic deposition occur beyond the for effective identification of basement outcrops and crust enrichment pinch-out lines. zones, including sampling strategy design, mining areas delineation and The cobalt-rich crusts described in this study, were obtained by resources estimation. shallow drillings, carried out by R/V “Haiyangliuhao” of GMGS. The Based on high precision multibeam bathymetry data, a slope crusts show thickness from 10 to 20 cm (Fig. 8, SD1 and SD2). According gradient map of Weijia Guyot is presented (Fig. 6). The summit plat­ to the pelagic sediments distribution at the summit of Weijia Guyot and � � � � form’s slope vary from 0 to 3 in central part, and increases up to 4 ~7 outcrops location, the most promising subareas were delineated (Sub­ towards the edges. The western part of summit is crowned by dome- areas 1, 2, 3 and 4; Fig. 7). The sediments around Weijia Guyot, such as shaped and conical hills. Furthermore, the depth range of whole sum­ reef complexes, mudstones or limestones, are mostly hard substrate, and mit platform is 1500–2500 m (Fig. 6), which corresponds with the are preferable for the crusts growth. The shallow drillings described in optimal depth for cobalt-rich crusts mineralization. In accordance with the paper, confirm and indicate the location of hard background areas. the cobalt-rich crusts mineralization characteristics, the western part

Fig. 7. Pelagic deposit (Type I) thickness on the summit of Weijia Guyot. Subareas located outside the red line are stable hard basement rocks, i.e. mudstone, limestone and volcanic basement. Circled numbers 1–4 are outcrops and depositional rarefaction areas. SD1 and SD2 locate sites of recovered cores � shown in Fig. 8. Projection: UTM, Datum: WGS84, Zone: 57 , Central meridian: � Fig. 6. Slope gradient map of Weijia Guyot, the white solid lines are depth 159 E. (For interpretation of the references to colour in this figure legend, the contours (meters). reader is referred to the Web version of this article.)

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Fig. 8. Shallow drilling cores from Weijia Guyot, collected by R/V “Haiyan­ gliuhao” of GMGS. The location of drilling sites is presented on Fig. 1.

The estimated area of potential crust resources is approximately 576.4 km2, accounting for 39.5% of the total area of Weijia Guyot, which is reportly approximately 1459.7 km2 (Fig. 9). Asavin et al. (2010), based on studies of dredged samples and seabed topography, Fig. 9. Cobalt-rich crust promising areas on the summit of Weijia Guyot; Pro­ � � calculated the percentage of the area likely occupied by cobalt-rich jection:UTM, Datum: WGS84, Zone: 57 , central meridian: 159 E. crusts at approximately 436.6 km2 (29.9%). In that case, the delinea­ tion was similar to presented in this study, only with few exceptions: interpretation and then wrote the main manuscript text; Yong Yang and Subareas 2 and 3 showed that the pelagic deposit pinch-out line was in Zhenquan Wei provided geological data, involved in geological inter­ contact with limestones and mudstones, which are crusts growth pre­ pretation and part of the text work; Xiangyu Zhang and Ning Huang vailing hard substrate areas (Fig. 9). According to results of prepared the figuresin this article; Gaowen He and Wenchao Lü provided DY41SP1610 line, which crosses Subarea 4, and DY41SP1605, the preliminary thoughts and manuscript review; Yinan Deng responsible for limestones and mudstones are identified at the pelagic oozes pinch-out part of the text work; and Yuping Liu provided RadExPro software lines. These are potential sites for crusts-benefit substrate rocks. The training. All authors reviewed the manuscript. Subarea 2 indicated similar conditions, therefore we expect limitations in determining promising areas, if we only rely on limited sampling, and Declaration of competing interest slope data. In addition, taking into consideration the far greater time­ scale and costs associated with geological sampling compared to No potential conflict of interest was reported by the authors. sub-bottom profiling,it seems that effective sub-bottom profilingis still an economical and rational method, regardless high potential error range. For now, all current profilingroutes indicated promising areas for Acknowledgements cobalt-rich crusts occurrence. Thanks for the support for all of the expedition members of R/V “Haiyangliuhao” (GMGS). Thanks to Professor Weilin Ma from the Second 6. Conclusions Institute of Oceanography of Ministry of Natural Resources, for his valuable suggestions. Thanks to Doctor Neil Mitchell from University of Based on a sub-bottom profilesand topographic data, combined with Manchester, and three other anonymous reviewers, for their rigorous the DSDP lithological data and related studies, this paper provide sedi­ and scientificcomments and suggestions on the manuscript. Thanks for mentary characteristics and implications for mineralization of cobalt- the financialsupport from National Natural Science Foundation of China rich ferromanganese crusts, on the summit of Weijia Guyot, Western (No. 41606071 and No. 41803026), China Ocean Mineral Resources Pacific Ocean. Research and Development Association (No. DY135-C1-1-06, No. Three types of stratum reflections are identified: (1) pelagic oozes, DY135-C1-1-03, No. DY135-C1-1-05, No. DY135-S1-1-04, and No. (2) oolitic limestones, and (3) mudstones, matching properly with DSDP DY135-E2-2-06), and Ministry of Natural Resources Key Laboratory of coring results. Calculation results revealed that the sedimentary thick­ Marine Mineral Resources (No. KLMMR-2018-A-09). nesses above the volcanic basement at DSDP20-200 and 202 are approximately 212.4 m and 174.8 m respectively. This indicates that the DSDP wells almost reach the volcanic basement. References The cobalt-rich crusts promising mineralization areas were delin­ Abrams, L.J., Larson, R.L., Shiply, T.H., Lancelot, Y., 1992. The seismic stratigraphy and eated on the summit of Weijia Guyot, based on water depths, slope sedimentary history of the East Mariana and Pigafetta basins of the Western Pacific. gradients and pelagic oozes pinch-out lines. The delineated area is Proc. Ocean Drill. Program Sci. Results 129, 551–569. ’ approximately 576.4 km2, 39.5% of the total summit area of Weijia Asavin, A.M., Kubrakova, I.V., Mel nikov, M.E., Tyutyunnik, O.A., Chesalova, E.I., 2010. Geochemical zoning in ferromanganese crusts of Ita-Mai Tai guyot. Geochem. Int. 48 Guyot. (5), 423–445. 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