Types of Ocean Deposits 

HYDROTHERMAL METALLIFEROUS SEDIMENTS

Ancient examples of VMS deposits can be found in the geologic record in both Late Archean (< 2800 Ma) rock successions and Paleozoic rock successions. Ancient VMS deposits generally occur in clusters with one or more "giant" deposits in association with numerous smaller deposits. They occur at specific stratigraphic horizons, commonly boundaries between contrasting lithologies within volcanic successions. Within the Archean rock successions it is the Granitoid-greenstone belts that contain the economic mineralisation.

VMS deposits may form as a result of mid-oceanic ridge basaltic magmatism, placing them at sites of extensional tectonic regimes, or as a result of oceanic arc and back-arc basin magmatism, placing them in subduction related tectonic environments. Metal concentrations have been found to be higher at subduction related VMS deposits (in back-arc or arc related basins) than at mid-oceanic ridges. As a result, fossil volcanogenic deposits demonstrate that back-arc and arc environments with highly differentiated lavas are the most favourable settings for the formation of large VMS deposits. The Kuroko VMS deposits of Japan are the best known ancient analogues of modern Black Smokers.

MANGANESE NODULES ON THE OCEAN FLOOR

Ferromanganese nodules and encrustations occur both overlying basalt lava at mid-oceanic ridges where they are considered to be hydrothermal deposits, and overlying sediments away from ridge crests where they are considered hydrogenous or authegenic in origin. They are of economic interest because of their high copper, nickel and cobalt content, these metal abundances being particularly high where nodules are widely dispersed on the sea floor. The absence of manganese nodules from deep sea cores indicates that they do not normally survive burial by younger sediments.

MINERAL DEPOSITS OF OPHIOLITE SEQUENCES

Ophiolites are thought to represent slices of oceanic crust that have been thrust or obducted onto a continental margin during collision. Ophiolites are characterised by a sequence of rock types, consisting of deep sea sediments overlying basaltic pillow lavas, sheeted dykes, gabbros and ultramafic peridotites. Most large ophiolite bodies are no older than Triassic and the scarcity of Proterozoic ophiolites suggests that they are normally destroyed by erosion within a period of several hundred million years. This presumably represents the erosion rate of high-level nappes, flanking the core of orogenic belts, in which the larger ophiolite bodies usually occur. 

 

STRATIFORM MASSIVE SULPHIDES

The Troodos ophiolite complex of Upper Cretaceous age in Cyprus has become the type-example of stratiform massive sulphides associated with basaltic pillow lavas and cherts. The sequence of mafic and ultramafic igneous rocks overlying pelagic sediments originally formed at a submarine spreading ridge during the late Cretaceous. The massive sulphides and related stockwork mineralisation all occur within the pillow lava sequence, with over 90 deposits known from within the Troodos Complex. The Semail ophiolite in Oman provides another example, with all mineralisation associated with the volcanic centres strung out along the uppermost part of the ophiolite on top of the pillow basalt unit. The Semail ophiolite was formed at the same time as the Troodos Complex, and emplaced during the same Tethyian collision event.

PODIFORM CHROMITE DEPOSITS IN ULTRABASIC ROCKS

Although 97 % of the world's chromite reserves occur in non-ophiolite, layered mafic and ultramafic intrusions, over half of current world production comes from ophiolite-hosted chromite deposits. Lenticular podiform chromite or chrome-spinel deposits are known from many Alpine-type ultrabasic bodies, interpreted as on-land oceanic lithospere. The chromite bodies occur only within the harzburgite lithologies (including dunite) and not within lherzolite units (both peridotite rock types). The harzburgite unit forms the basement to the cumulate rocks, is generally tectonised, lacks recognisable cumulus magmatic textures, and is considered to be a depleted mantle source. Upper Mesozoic and Cenozoic examples include the ophiolite sequences in Cyprus, Oman, Greece, Turkey, Cuba and the Philippines. Lower Paleozoic deposits are known from Newfoundland, Proterozoic deposits from the Eastern Desert of Egypt and Saudi Arabia.


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