多金属结壳生长间断期与磷酸盐化事件的关系

磷酸盐化的发生会造成多金属结壳主要成矿元素的贫化,总体上抑制结壳的生长。磷酸盐化作用呈幕式在太平洋内发生,已有研究将其发生期次与多金属结壳中的磷酸盐化层位及其P、Ca等元素地球化学特征对应,为结壳的年代厘定提供证据。然而,由于磷酸盐化多次发生且在结壳多个层位中显现,首先需要厘定出较为可靠的多金属结壳年代框架,才能与磷酸盐化事件相互佐证。本研究通过定年范围较大,同时可以识别结壳中存在的生长间断的Co-Os法定年体系,为中、西太平洋6块多金属结壳定年,并参考肉眼识别的磷酸盐化矿化特征和地球化学特征佐证年代框架,同时分析磷酸盐化作用对结壳成矿的影响。39～34 Ma和27～21 Ma的两次大规模磷酸盐化期次全部对应多金属结壳的大规模间断期,而71 Ma、55 Ma和31 Ma等小规模磷酸盐化事件多数可在多金属结壳的层位和地球化学特征中识别,对应P和Ca百分含量的飙升和峰值特征。这一方面证实了磷酸盐化的发生整体抑制了结壳的生长,且具有广泛的影响范围和明确的时代区间,另一方面则佐证了多金属结壳年代框架的准确性。     

九州-帕劳海脊南段共生多金属结核与富钴结壳地球化学特征及其资源意义

深海多金属结核与富钴结壳富含Mn、Co、Ni、Cu及稀土等元素,极具资源潜力,因而广受关注。选择采集自九州-帕劳海脊南段水深约3 000 m的6个多金属结核和2个富钴结壳样品进行地球化学特征研究。结果显示本区样品Mn/Fe比值为0.88～1.07,Co及稀土元素含量高,与库克群岛多金属结核元素特征相似,表现出典型的水成成因特征。与其他地区水成结核、结壳相比,样品具有较高的Ca含量,可能和其所在位置海脊部位水深较浅、位于碳酸盐补偿深度（CCD）以上有关。样品表现出Y负异常、Nd含量高等水成结核的特点,但微弱的Ce正异常特征与其他地区水成多金属结核/壳的强烈Ce正异常特征不同,这表明其生长于弱氧化环境。本区结核分布在CCD以上,最小溶氧层（OMZ）以下,是一种水成成因、贫氧生长的新型多金属结核,其物质来源、成矿模式将为揭示多金属结核的形成分布规律提供一条全新的认识途径。同时研究区结核、结壳Co及稀土元素含量较高,且分布位置较浅,易于开采,具有一定的资源潜力。     

西北太平洋多金属结核铂族元素地球化学特征

Polymetallic nodules and cobalt (Co)-rich crusts are enriched in platinum-group elements (PGEs),especially platinum (Pt) and may be important sinks of PGEs.At present,little information is available on PGEs in polymetallic nodules,and their geochemical characteristics and the causes of PGEs enrichment are unclear.Here PGEs of polymetallic nodules from abyssal basin in the Marcus-Wake Seamount area of the Northwest Pacific Ocean are reported and compared with the published PGEs data of polymetallic nodules and Co-rich crusts in the Pacific.The total PGEs (ΣPGE) content of polymetallic nodules in study area is 258×10~(–9) in average,markedly higher than that of Clarion-Clipperton Zone (CCZ) nodules (ΣPGE=127×10~(–9)) and lower than that of Co-rich crusts in the Marcus-Wake Seamount (ΣPGE=653×10~(–9)),similar to that of Co-rich crusts in the South China Sea(ΣPGE=252×10~(–9)).The CI chondrite-normalized PGEs patterns in different regions of polymetallic nodules and cobalt-rich crusts are highly consistent,with all being characterized by positive Pt and negative Pd anomalies These results,together with those of previous studies,indicate that PGEs in polymetallic nodules and Co-rich crusts are mainly derived directly from seawater.Pt contents of polymetallic nodules from the study area are negatively correlated with water depth,and Pt/ΣPGE ratios in nodules there are also lower than those of the Corich crusts in the adjacent area,indicating that sedimentary water depth and oxygen fugacity of ambient seawater are the possible important controlling factors for Pt accumulation in crusts and nodules.     

南海多金属结壳(核)铁锰矿物/海水界面效应与成矿元素的富集

海底铁锰结壳和结核是重要的海底矿产资源,蕴含着丰富的金属元素并且具有巨大的经济价值。本文主要以南海多金属结壳（核）为研究对象,采用X射线粉晶衍射（XRD）、激光拉曼光谱、红外光谱分析（FTIR）以及X射线光电子能谱对铁锰矿物的矿物学和谱学特征进行了系统的分析和研究。粉晶衍射和拉曼光谱分析结果表明,南海多金属结壳的矿物组成为水羟锰矿、石英和长石,结核的矿物组成为钡镁锰矿、水羟锰矿、石英和长石,铁相矿物均为无定形铁氧化物/氢氧化物,并且锰相矿物和铁相矿物的结晶程度均较差。红外光谱分析结果显示多金属结核和结壳中的铁锰矿物具有大量表面羟基,这些含质子表面羟基官能团,可为海水中各成矿元素的络合提供丰富的活性位点。XPS分析表明多金属结核和结壳中铁锰矿物表面以Fe、Mn和O元素为主,其中Fe呈正三价态,Mn以正四、正三价为主,可能还含有少部分正二价态。对比南海多金属结壳（核）与太平海山结壳,南海多金属结壳（核）具有更为显著的表面羟基氧（-OH）含量,而太平洋海山结壳则以晶格氧（O^2-）为主,表明太平洋海山结壳铁锰矿物结晶程度较南海多金属结壳（核）高。综合研究表明,在海底铁锰结壳和结核中（氢）氧化锰/铁矿物与海水之间界面效应对金属离子的富集机理主要有：（1）金属离子与矿物表面羟基进行络合反应,形成以配位键相连的羟基络合物,或与表面的质子交换生成稳定的内层络合物;（2）矿物的带电表面与金属离子通过静电作用形成双电层,生成外层络合物;（3）金属离子与矿物结构中的Mn、Fe离子同晶置换而成为结构阳离子。     

亚洲大陆边缘海底峡谷的形态、分布及演化进程

基于SRTM15_Plus水深数据,利用地形表面流水分析与等高线几何分析相结合的方法对亚洲大陆边缘海底峡谷进行识别,分析峡谷的形态、分布及演化进程。共识别出海底峡谷531条,按照平面形态分为直线型、蛇曲型和树枝型三种,数量分别为239条、75条和217条。直线型峡谷主要分布在白令海盆地、千岛盆地、南海等海域;蛇曲型峡谷主要分布在孟加拉湾东部俯冲带海域;树枝型峡谷主要分布在日本海沟、伊豆小笠原海沟和南海海槽交界处,南海以及苏门答腊俯冲带海域。根据峡谷的形态与分布,结合研究区内构造背景,讨论了峡谷“直线型-蛇曲型”的形态演化进程,并依据树枝型峡谷主干谷的形态信息,推测存在“蛇曲型-树枝型”、“直线性-树枝型”的形态演化进程。     

浅谈深海多金属硫化物采矿的环境影响和保护措施

深海矿产资源开发及其环境保护问题近年来重新成为一个研究热点。深海环境极其复杂,人类对深海生物及其生态系统的了解仍十分有限,而深海采矿不可避免会对海洋生态环境造成影响,如不加强对采矿环境影响的研究并制定相关的保护措施,未来的深海采矿对海洋环境的破坏将无法估量。本文从深海采矿发展由来、多金属硫化物矿区环境、深海采矿技术发展、采矿环境影响和保护措施等几方面全链条系统地梳理了前人的研究成果,提出了深海采矿环境影响评价的发展方向,从而为今后开展深海采矿环境监测与保护提供参考。     

Mineralogy and composition of manganese crusts and nodules and sediments from the Manihiki plateau and adjacent areas: Results of HMNZS Tui Cruises

Forty‐four sediment samples and 23 Mn crusts and Mn nodules have been analyzed for several elements in order to characterize their geochemical nature. Most of the Mn crusts and Mn nodules are of hydrogenous origin, although one crust (U375) from the flanks of the Rarotonga volcano is thought to be of hydrothermal origin. δ‐MnO₂ is the principal Mn mineral in the crusts, whereas todorokite and birnessite occur in addition to δ‐MnO₂ in the nodules. The sediments can be divided into carbonates, with calcite as the dominant phase, and deep‐sea clay, consisting mainly of illite and smectite.     

西太平洋Kocebu海山铁锰结壳稀土元素地球化学特征

西太平洋麦哲伦海山区是全球重要的铁锰结壳资源分布区,具有丰富的稀土元素资源潜力。本文对采自麦哲伦海山区Kocebu海山的11个铁锰结壳表层样（＜1 mm）进行稀土元素地球化学研究,探讨其含量特征、成因和影响稀土元素富集的环境因素。结果表明：Kocebu海山铁锰结壳表层样品ΣREY（Rare earth elements and yttrium）平均含量为1 366 mg/kg,低于前人在麦哲伦海山区其他海山以及邻近的马尔库斯–威克海山区的分析结果;样品轻稀土富集和Ce正异常（平均值为1.45）特征以及稀土元素成因图解、配分曲线和分配系数曲线等均表明该海山结壳属于水成成因;海水中稀土元素含量和溶解氧含量是控制结壳生长的关键环境参数,二者在Kocebu海山所在海区的浅水环境中含量较低;结壳ΣREY含量偏低与采样点水深较浅导致的海水稀土元素含量和溶解氧含量较低密切相关,受碎屑矿物的稀释作用影响较小。在开展铁锰结壳地球化学特征研究和资源勘探评价时应充分考虑采样水深的分布范围,局部水深样品的分析结果可能导致研究结果出现较大偏差。     

Minor and rare elements in manganese crusts and nodules and sediments from the Manihiki plateau and adjacent areas: Results of HMNZS Tui cruises

Six manganese crusts, 13 manganese nodules, and 16 sediments were analyzed by instrumental neutron activation analysis. Data were generated on selected major and minor elements but geochemical evaluations are based only on Fe, Sc, U, Th, and the rare earth elements (REE). Manganese crusts and manganese nodules have comparable trivalent REE contents and show a shale‐like distribution pattern. Both crusts and nodules are characterized by a positive Ce anomaly but the anomaly is higher in nodules. REE contents in manganese nodules show a linear dependence on the Fe content, and it is concluded that these elements are incorporated in the Fe‐rich (δ‐MnO₂) phase. In the crusts, the REE correlate with Sc and are therefore assumed to be associated with the clay minerals. Uranium contents are significantly higher in the crusts than in nodules whereas Th is slightly higher in the nodules. There is a clear positive correlation between U and Th in nodules but there are too few data to make a similar conclusion for crusts. Compositional data suggest a division of the sediments into two groups. The carbonate sediments have much lower REE contents and a more pronounced negative Ce anomaly than the clays, while both show a lithogenous component as indicated by a slight negative Eu anomaly.     

太平洋徐福海山富钴结壳稀土元素和铂族元素赋存状态研究

利用等离子体发射光谱法、等离子体质谱法以及相态分析手段,对太平洋徐福海山富钴结壳稀土和铂族元素地球化学特征、赋存状态及富集机制进行研究。结果表明,富钴结壳稀土和铂族元素明显富集,其稀土总量为1 842～2 854 μg/g,铂族总量为144～1 180 ng/g,老壳层中稀土和铂族元素含量明显高于新壳层,这可能与老壳层发生磷酸盐化作用有关。轻稀土元素含量明显高于重稀土元素含量,呈现Ce正异常而Eu无异常,具有明显Ce富集特征。铂族元素之间发生了明显分异作用,PPGE含量明显高于IPGE,表现出明显Pt正异常而Pd负异常,具有明显Pt富集而Pd亏损特征。稀土元素赋存状态显示,新壳层中稀土元素主要赋存于铁氧化物相,其富集比例为65.40%～70.05%,老壳层中稀土元素主要赋存于残渣态,富集比例为62.27%～65.77%,这可能与残渣态中磷酸盐有关。铂族元素赋存状态显示,新壳层和老壳层中铂族元素都主要赋存于铁氧化物相,其富集比例为63.66%～69.51%,残渣态(29.20%～34.68%)对铂族元素也有一定的富集能力。富钴结壳稀土和铂族元素的富集推测为铁氧化物胶体粒子的吸附作用,受海水氧化性环境影响,可溶性Ce³⁺和Pt²⁺分别被氧化成不溶性Ce⁴⁺和Pt⁴⁺,沉淀被吸附到铁氧化物相中,从而造成富钴结壳稀土和铂族元素的富集。     

Mineralogical and geochemical constraints on the origin of ferromanganese crusts from the Rivera Plate (western margin of Mexico)

Marine hydrogenetically precipitated ferromanganese crusts are widespread in the Pacific Ocean. They occur as pavements coating volcanic or sedimentary hard-rock substrates, mainly on the slopes of seamounts, plateaus and hills in ocean basins and continental margins. We studied three ferromanganese crusts from one dredge haul from the Rivera Plate (western margin of Mexico), which are up to 15 mm thick and grow directly on a substrate of pillow basalt fragments. They consist of laminated, botryoidal, porous aggregates mostly of Fe and Mn oxyhydroxides and up to 10% silicates. XRD analysis showed the predominance of poorly crystallized mineral phases in the crusts that include Mn-feroxyhyte and vernadite, and an authigenic smectite-like clay. Detrital minerals probably derive from granodiorites of the eastern wall of the neighboring Middle America Trench. Scattered barite grains occur on the crust surface and suggest plume fall-out derived from hydrothermal vents, although a possible pelagic source cannot be dismissed. Ratios between major (Fe, Mn, Si) and trace (Co, Ni, Cu) elements reveal that such crusts are predominantly hydrogenetic in origin, although they show a hydrothermal influence that increases in the outer layers. Iron contents range from 16.2 to 25.2 wt.%, and manganese from 3.4 to 14.5 wt.%. The Fe/Mn ratio ranges from 1.6 to 7.0. The SiO₂/Al₂O₃ ratio ranges from 4.3 to 6.6, indicating the presence of biogenic silica (radiolarians). The concentrations of copper (up to 383 ppm) and cobalt (up to 534 ppm) are significantly below those of the typical hydrogenetic crusts, whereas Ni (up to 1320 ppm) is about the same or slightly lower. REE normalized profiles and ΣREE values (486 to 732 ppm) match those reported for hydrogenetic crusts, but suggest a hydrothermal contribution for the later crust layers. The inferred crust growth rates using the Co-chronometer (44 and 229 mm/Myr) are higher than those reported for pure hydrogenetic crusts (mostly 1 to 6 mm/Myr), thus indicating an increasing hydrothermal influence. The highest growth rate (229 mm/Myr, corresponding to the outer crust layers) suggests a regional hydrothermal input of iron and manganese to seawater. The intensification of regional submarine hydrothermal activity began about 13,000 yr ago, and may be related to the tectonic activity in the complex junction of the Rivera, Cocos, Pacific and North America plates.     

西太平洋采薇海山群富钴结壳铂族元素地球化学特征与来源

为探讨富钴结壳铂族元素的来源,以西太平洋采薇海山群富钴结壳样品为研究对象,利用X射线衍射法、等离子体发射光谱法和质谱法,对富钴结壳样品进行了矿物组成、主量元素和铂族元素含量分析。研究表明,富钴结壳的主要结晶矿物为水羟锰矿,次要矿物有石英、斜长石、钾长石和碳氟磷灰石,同时含有大量非晶态铁氧/氢氧化物。富钴结壳中Mn和Fe含量最高,且明显富集铂族元素,铂族总量为142～1 352 ng/g,其中Pt为114～1 268 ng/g,占铂族总量的80%以上。老壳层的铂族元素含量高于新壳层,老壳层发生了磷酸盐化作用。富钴结壳的铂族元素之间发生了分异作用,Pd组（PPGE）含量高于Ir组（IPGE）。铂族元素配分模式显示Pt正异常和Pd负异常,具有Pt富集而Pd亏损特征,推测富钴结壳的铂族元素来源于洋壳蚀变过程中海山玄武岩和海水之间的水−岩反应。富钴结壳的铂族元素与CaO、P₂O₅、Ba和Cu正相关,推测铂族元素赋存于磷酸盐矿物相中。富钴结壳生长剖面从上到下,由新到老,铂族元素含量呈逐渐增加趋势,海水氧化性环境和高海洋生产力可增进磷酸盐化作用,从而进一步促进了铂族元素的富集。该研究对于揭示富钴结壳铂族元素的来源具有一定的参考价值。     

Kr and Xe isotopic compositions of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

Kr and Xe nuclide abundance and isotopic ratios of the uppermost layer of Fe-Mn Crusts from the western and central Pacific Ocean have been determined. The results indicate that the Kr and Xe isotopic composi-tions, like that of He, Ne and Ar, can be classified into two types： low3He/4He type and high3He/4He type. The low3He/4He type crusts have low84Kr and132Xe abundance, while the high3He/4He type crusts have high84Kr and132Xe abundance. The82Kr/84Kr ratios of the low3He/4He type crusts are lower than that of the air, while the83Kr/84Kr and86Kr/84Kr ratios are higher than those of the air. The Kr isotopic ratios of the high-er3He/4He type crusts are quite similar to those of the air. The128Xe/132Xe,130Xe/132Xe and131Xe/132Xe ratios of the low3He/4He type sample are distinctly lower than those of the air, whereas the129Xe/132Xe,134Xe/132Xe and136Xe/132Xe ratios are higher than those of the air. The low3He/4He type samples have the diagnostic characteristics of the MORB, with excess129, 131, 132, 134, 136Xe relative to130Xe compared with the solar wind. The128Xe/132Xe,130Xe/132Xe and131Xe/132Xe ratios of the high3He/4He type samples are slightly higher than those of the air, and the129Xe/132Xe,134Xe/132Xe and136Xe/132Xe ratios are qiute similar to those of the air. The noble gases in the Fe-Mn crusts are derived from the lower mantle, and they are a mixture of lower mantle primitive component, radiogenic component and subduction recycled component. The helium isotopic ra-tios of the low mantle reservoir are predominantly controlled by primitive He （3He） and U and Th radiogenic decayed He （4He）, but the isotopic ratios of the heavier noble gases, such as Ar, Kr and Xe, are controlled to different extent by recycling of subduction components. The difference of the noble isotopic compositions of the two type crusts is the result of the difference of the noble isotopic composition of the mantle source reservoir underneath the seamounts the crusts occurred, the noble gases of the high3He/4He type crusts are     

A comparative analysis of compositional variations in and between marine ferromanganese nodules and crusts in the South Pacific and their environmental controls

Regional variation of Mn, Fe, Co, Ni and Cu in ferromanganese oxides (nodules and crusts) in the central south Pacific is related to primary productivity, oxygen minimum layer, and calcium carbonate compensation depth. The largely latitudinal influence of these environmental parameters on nodule and crust composition reflects their predominantly latitudinal variation. Primary productivity is the principal regional environmental control, influencing diagenetic enrichment of these elements in nodules through its effect, mediated by the CCD, on supply and concentration of labile organic matter vs. carbonate remains to the sediments. It influences hydrogenetic enrichment of these elements in nodules and crusts through its effect, mediated by the oxygen minimum layer (mainly in the case of crusts), on their export from surface waters.The elements’ varying susceptibility to being scavenged or organically bound influences the contrasting composition of diagenetic vs. hydrogenetic ferromanganese oxides, which is further influenced by depth. Hydrogenesis is the fundamental process governing nodule and crust formation, superimposed on which is diagenesis under specific circumstances; both are subject to intermittent interruption, diminution and augmentation by changes in environmental parameters. Application of regionally operative environmental controls locally explains local compositional variations and helps refine exploration criteria for economically viable nodules and crusts.     

马里亚纳海沟挑战者深渊初期多金属氧化物的 矿物学、地球化学特征及其成因环境研究

对大洋27航次在西太平洋马里亚纳海沟挑战者深渊获取的3个多金属氧化物样品进行了X射线矿物衍射分析、穆斯堡尔谱分析及地球化学元素分析,研究其矿物、地球化学特征差异。结果表明,所取样品处于多金属氧化物发育的初始阶段,具有独特的矿物地球化学特征:(1)相较于太平洋CC区及中太平洋海盆获取的多金属结核样品,本研究样品的矿物组成中含有异常高的石英、斜长石以及黏土矿物,而水羟锰矿和钙锰矿含量较低。(2)样品中铁相矿物主要为正方针铁矿(91.6%),另含少量纤铁矿(8.4%),推测是纤铁矿向更加稳定的正方针铁矿衍变的结果。(3)由于样品中深海黏土组分以及氧化物核心物质的混入,加上吸附金属氧化物时间较短,导致SiO₂和Al₂O₃含量均高于正常结核,而Fe、Mn、Cu、Co、Ni等其余金属元素含量较低。(4)由于形成时间较短,样品中稀土元素含量相对较低,ΣREE仅约为0.4×10^-3(一般太平洋CC区及中太平洋结核中稀土含量均大于1.0×10^-3);加之海水氧化还原作用的降低以及研究区海底热液活动的影响,Ce元素未表现出多金属结核中常见的正异常。     

中太平洋海山区尖顶和平顶海山结壳成矿与分布的比对研究

尖顶海山和平顶海山是洋底海山的两种类型,同时也是富钴结壳资源赖以成长的主要载体,两者在地形上具有显著差别,资源状况和分布也不尽一致。以地形分类为基础,就中太平洋海山区两类海山结壳的成矿特征、宏微观构造、矿物和化学组成以及空间分布等进行了比对研究。研究认为,尖顶海山体积虽小,但地形演化连续,底层流及环境氧化性较强,有利于结壳的长期稳定生长,结壳厚度总体较大,富成矿元素和生物组分,远景成矿区域连续绵长。平顶海山山体巨大,顶部和斜坡陡崖区不利于结壳成长,底流活动只在山顶边缘和山脊地带表现突出,环境氧化性整体较弱,后期构造活动相对频繁,结壳总体厚度较小,铁,碎屑和热液组分含量较高,远景矿区相对狭窄。     

东太平洋多金属结核中铁、锰元素的分形演化及其意义

运用分形理论,通过建立不同的相空间,计算了东太平洋多金属结核中的铁、锰元素的时间序列在各相空间中的关联分维,并与其在沉积物岩心中的关联分维进行了对比.通过研究发现,多金属结核中铁、锰元素和沉积物中Fe₂O₃及MnO在分形演化过程中均存在分维吸引子,认为沉积物中的Fe₂O₃和MnO可以用3个变量来描述,它们可能代表了火山、生物和陆地的三种来源,而描述结核中的铁、锰元素却需要14个变量,反映多金属结核中铁、锰元素的来源和/或赋存非常复杂,至少有14个因素在起作用,可能暗示了结核在形成过程中存在复杂的生物化学作用,为多金属结核的生物成因提供了间接的数学证据,同时也合理解释了结核生长结构所具有的自相似韵律现象.     

The role of basement tectonic reactivation on the structural evolution of Campos Basin,offshore Brazil: Evidence from 3D seismic analysis and section restoration

The structural analysis of regional 3D seismic data shows evidence of long-term tectonic inheritance in Campos Basin, offshore Brazil. Main Lower Cretaceous rift structures controlled themselves by strike-slip deformation belts related to Proterozoic orogenic events, have been episodically reactivated during the divergent margin phase of Campos Basin, from the Albian to the Miocene. Balanced cross-sections of major salt structures indicate that such tectonic reactivations have been controlling thin-skinned salt tectonics, triggering pulses of gravitational gliding above the Aptian salt detachment. Additionally, major basin features like the Neogene progradation front and the salt tectonic domains are constrained by the main Proterozoic orogenic trends of the Ribeira Belt (NE–SW) and the Vitória-Colatina Belt (NNW–SSE). As the basement involved structures observed in Campos Basin can be attributed to general geodynamic processes, it is suggested that basement tectonic reactivation can be as relevant as isostatic adjustment and detached thin-skinned tectonics on the structural evolution of divergent margin settings.     

东海陆架HY126EA1孔有孔虫壳体的氧、碳同位素记录

东海陆架HY126EA1孔有孔虫壳体氧同位素组成具有明显的阶段性变化.通过与深海氧同位素组成曲线对比,可将氧同位素组成变化分为5期,分别对应着冰消期、末次冰期极盛期、末次冰期间冰段、末次冰期初期和末次间冰期.碳同位素组成的变化与氧同位素组成的变化基本一致.δ18O和δ13C的相关性在不同气候期存在明显差异,主要受陆源水和海水交换复杂程度的控制.     

A comparison between manganese nodules and cobalt crust economics in a scenario of mutual exclusivity

The past 20 years have been characterised by limited interest in the economic viability of deep seabed mining with the exception of those mineral ores rich in precious metals such as polymetallic sulphides. This paper goes against the tide. After reviewing the most significant literature, it compares the economic feasibility of mining polymetallic manganese nodules and cobalt-rich ferromanganese crusts in a scenario of mutual exclusivity. It uses a new indicator, the cobalt-nickel price ratio, whose aim is to verify at what metal prices the net present value of the two mining projects equalises. Previous research has shown that the turning margin between manganese nodules and cobalt crusts is a cobalt price of $50/kg. The present paper revises this by showing that the choice between mining crust and nodules depends on fluctuations in the price of nickel. A Monte Carlo simulation proves that cobalt crust mining could be reasonably preferred to manganese nodules at cobalt prices of $40-60/kg if the price of nickel fell below $11/kg. Within this condition, investors would receive the minimum acceptable internal rate of return of 15%, which is at a level of risk closer to the land-based mining. However, when the price of nickel passes the $11/kg threshold, the equilibrium between the two ventures can be reached only at a cobalt price greater than $100/kg, causing cobalt crust to become uncompetitive. Finally, the paper, recognising that prices are not unique drivers, introduces legal, political, technological and environmental concerns to show that the final choice between the two mining ventures cannot be merely driven by economic issues.