Chromite Deposits And Their Ultramafic Host Rocks In The Oman Ophiolite

Brown, Michael Anthony (1984). Chromite Deposits And Their Ultramafic Host Rocks In The Oman Ophiolite. PhD thesis The Open University.



Chromite deposits in the Oman ophiolite are invariably associated with dunite bodies occurring dominantly within the upper part of the basal harzburgite unit of the ophiolite, the "Mantle Sequence". The host harzburgite and the dunite/chromitite suite show textural and geochemical evidence of origin by different genetic processes.

Harzburgite has tectonite textures from polyphase deformation and is homogeneous on a gross scale in its mineralogy and chemistry. Harzburgite mineral compositions suggest an origin as a refractory residue from one or more partial melting episodes, low fo2 conditions, and show no systematic spatial variation. Large variations in accessory chromite are attributable to re-equilibration with variable bulk-rock alumina contents.

Dunite-chromitite bodies are less deformed than the harzburgite and contain primary igneous textures. Chromitite spinel compositions are least affected by silicate-chromite re-equilibration and show systematic variation with depth throughout the Mantle Sequence compatible with crystallization from a rising hydrous magma diapir. Pulses of chromite precipitation may have been initiated on periodic release of magma from the diapir into the overlying chamber. Coupled with crystal settling and/or boundary layer crystallization this a possible mechanism for formation of massive chromite bodies and explains the congregation of large chromite deposits within a narrow depth zone at the top of the Mantle Sequence.

The presence of alkali-rich minerals and volatile phases in interstices and/or primary inclusions in massive chromite deposits are significant. Clearly alkalies and volatiles are intimately associated with the processes of transport and concentration of chromium, possibly involving immiscible liquids, but little is known of the phase relations of such complex systems at present.

Oman chromitites are not commercially very attractive for large scale investment but could be worked on a small scale and represent a further diversity in a world market dominated by two countries.

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