COBALT CRUST

Mr. Malcolm Afonso

On the slopes of submarine mountains around the world, minerals precipitate out of the seawater to form thin crusts on rocky surfaces. The crusts are commonly called ferromanganese crusts, reflecting the fact that their major constituents are iron (Fe) and manganese (Mn), although a host of other minerals occur in them in smaller amounts, including cobalt - which is why they are also often called ‘cobalt-rich crusts’ or ‘cobalt-rich ferromanganese crusts’.

Introduction

There are two primary practical interest in Fe-Mn crusts

Their economic potential for Co, but also for Mn, Ni, and Pt and possibly also Ti, REE, Te, P, and others

Use of crusts as recorders of the past 60 Ma of oceanic and climatic history.

WHY DO WE NEED TO STUDY Co-CRUSTS?

Up untill 1970s Fe-Mn crusts were usually not distinguished from Fe-Mn Nodules however there are distinct differences between Fe-Mn nodules and crusts, other than just form.

Nodules commonly form by both diagenetic and hydrogenetic processes and thus their composition reflects input from both sea water and sediment pore water sources, the latter being substrate contribution, which is not found in crusts.

Generally, crusts and nodules have different mineralogical (vernadite vs. todorokite and vernadite) and chemical compositions( eg: High Co vs. High Cu) because of their genetic differences as well as differences in water depth of formation, although there is much overlap

Classification

Cobalt-rich ferromanganes crust are found throughout the global oceans on the flanks and summits of seamounts (submarine mountains), ridges and plateaus, where seafloor currents have swept the ocean floor clear of sediment for millions of years. They form at water depths of 600m to 7000m. Crusts of sufficient mineral content to be of economic interest commonly occur at depths of about 800m to 2500m. Their thickness varries from 1mm to about 260mm. These seamounts can be huge, some as large as mountain ranges on the continents. Only a few of the estimated 30,000 seamounts that occur in the Pacific, where the richest deposits are found, have been mapped and sampled in detail. The Atlantic and Indian oceans contain far fewer seamounts but have been far less sampled.

Based on grade, tonnage and oceanographic conditions, the central equatorial Pacific region offers the best potential for crust mining, particularly the exclusive economic zones around Johnston Island and Hawaii (United States), the Marshall Islands, the Federated States of Micronesia and international waters of the mid-Pacific.

Occurrence and Distribution

Cobalt-bearing ferromanganese crusts sampling points in the world's oceans

Bathymetric map of the Pacific showing the location of seamounts

Occurrence

Occurrence

Formation

Formation of cobalt-rich ferromanganese crusts. Adapted and modified from Hein 2004.

O2 Minimum zone1)Oxidation of Mn2+ and formation of colloids

2) Precipitation of Oxides/ oxyhydroxides

Even though Fe-Mn crusts form by hydrogenetic precipitation, the exact mechanism of metal enrichment in the water column and at crust surface are poorly understood. The ultimate sources of metals to the oceans are river and aeolian input, hydrothermal input, weathering of basalts, release of metals from sediments, and extraterrestrial input. Scientific models show that most hydrogenetic elements in crusts occur as inorganic complexes in sea water. Hydrated cations (Co,Ni,Zn,Pb,Cd,Tl, etc.) are attracted to the negatively charged surface of Mn oxyhydroxides, whereas anions and elements that form large complexes with low charge-density (V,As,P,Zr,Hf, etc.) are attracted to the slightely positive charge of Fe Hydroxide surfaces.

Mixed Fe and Mn colloids with adsorbed metals precipitate onto hard-rock surfaces as poorly crystalline or amorphous oxyhydroxides, probably through bacterially mediated catalytic processes.

The metals are adsorbed because of the crusts’ very slow growth rates (1 to 5 millimetres per million years) and the enormous specific surface area(average 325 square metres per cubic centimetre of crust) (Hein et al. 2000)

Formation

Crust surface directly exposed on the seafloor are botryoidal, with botryoids varying in size from microbotryoidal (millimeter size) to botryoidal (centimeter size).

Crust profiles vary according to thickness and regional oceanographic conditions. Thin crusts(<40 mm) are usually black and massive, botryoidal or laminated. Thicker crusts (40-80 mm) commonly have atleast two distinct layers, a lower black, massive, dense layer that is phosphatized and an upper black to brown layer that is more porous, with laminated, mottled, botryoidal, and/or columnar textures.

The various textures probably reflect bottom water conditions at the time of precipitation of the oxides. Mottled, columnar, botryoidal, and laminated textures probably represent progressively decreasing energy in the depositional environment.

Texture

Texture

Ferromanganese crusts have a simple mineralogy. They are composed predominantly of vernadite (manganese oxide, or MnO2) and non-crystalline iron oxyhydroxide (FeOOH). Ferromanganese crusts also contain minor amounts of detrital minerals, such as quartz and feldspar and authigenic carbonate fluorapatite.

Cobalt, the trace metal of greatest economic interest, can be up to 2 percent, but usually averages 0.5 to 0.8 per cent by weight.

Ferromanganese crusts contain the highest concentrations of the rare metal tellurium, which is used in the solar cell industry to produce thin-film photovoltaics – the best material for converting sunlight into electricity.

Other important metalic and rare earth elements found in crusts include titanium, cerium, nickel, platinum, phosphorus, thallium, zirconium, , tungsten, bismuth and molybdenum.

Mineralogy

Future exploration and mining

To locate areas likely to be productive, prospective miners will first have to develop detailed maps of crust deposits and a comprehensive, small-scale picture of seamount topography, including seismic profiles. Once sampling sites are identified, dredge hauls, core samplers, sonar and video cameras can be deployed to ascertain crust, rock and sediment types and distribution. Large, well equipped research vessels will be needed to operate bottom acoustic beacons and towed equipment, and to handle a large number of samples. Manned submersibles or remotely operated vehicles will be required in later stages. For environmental assessment, current-meter moorings and biological sampling equipment will have to be deployed.

For successful crust mining, it is essential to recover the crusts without collecting too much substrate, which would substantially dilute the ore quality. One possible method of crust recovery consists of a bottom-crawling vehicle attached to a surface vessel by a hydraulic-pipe lift system and an electrical umbilical Some innovative systems that have been suggested include water-jet stripping of crusts from the rock, chemical leaching of the crusts while they are still on the seamounts

Future exploration and mining

Cronan, handbook of marine mineral deposits,2000, CRC press, london. pp 239

E. Baker and Y. Beaudin, Secretariat of the Pacific Community (2103) Deep Sea Minerals: CobaltrichFerromanganese Crusts, a physical, biological, environmental, and technical review. Vol. 1C, SPC

International seabed authority, Cobalt bearing ferromanganese crustssampling points in the world's oceans v2-Mar08

Referrences