What Are Polymetallic Nodules And How They Are Formed?

Polymetallic nodules or manganese nodules are small potato shaped, porous, black earthy colored rock concretions found in the sea bottom. They form deposits on the top or within the 100m of the deep sea sediments. Marine nodules grow radially and thus exhibit concentric layered structure. The rate of growth of nodules is very slow being about average rate of 1–10 millimeters per million years. The nodules of Eastern Tropical Pacific and Central Indian Ocean are of great economic interested due to high enrichments of minerals. Polymetallic nodules contain minerals like nickel, copper, cobalt, lead, cadmium, vanadium, molybdenum, titanium. They are also found in shallow seas and freshwater lakes but they have less concentration of valuable minerals.

polymetallic nodules
Polymetallic Nodules at Ocean Floor. Image via www.mining-technology.com

Conditions Necessary for the Formation

  1. Low sedimentation rate.
  2. Availability of Nucleus around which accretion of oxides can take place.
  3. Oxidizing Environment.
  4. Bottom currents of low velocity.
Polymetallic nodules, in cross-section
Polymetallic nodules, in cross-section. Image via earthzine.org

Formation of Polymetallic Nodules

There are 3 principal processes by which they are formed

  1. Digenetic Process

Formation of Polymetallic nodules due to elements precipitate from pore water present within the sediment or on the sediment surface is called Digenetic process.

  • The nodules formed by this process are rich in Manganese, copper, nickel, zinc content.
  • Manganese and iron content is greater than 2.5 %.
  • Growth rate is faster (2500mm/million years)
  • The generally show Dendritic Structure.
  • Rough Surface texture.
  1. Hydrothermal Precipitation

As Manganese is soluble in reducing and acidic condition and its solubility is not temperature dependent. Manganese can not only be transported by hydrothermal solution but also remains dissolved until hydrothermal fluids are oxidized upon contact with Oxic sea water, they are enriched in hot hydrothermal fluids which occur either at periphery of hydrothermal vent or during its waning phase.

Manganese will oxidize and precipitate as Manganese Oxide upon mixing of these fluids with cold oxic sea water forming Manganese crusts or irregularly shaped precipitates within or at sea floor.

  1. Hydrogenetic Process

Formation of Manganese and Iron oxide colloids in oxic seawater and their precipitation onto the sea floor is called Hydrogenetic process.

  • The nodules formed by this process are rich in Iron, copper, tin, lead.
  • Growth rate of layers is 1-5 mm/ million years.
  • Hydrogenetic layers are typically composed of finely laminated to columnar structure.
  • And form smooth surface texture.
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