Geology and lithostratigraphy of the ultramafic-mafic rocks and associated mineralizations, Wadi Khamal Area, West-Central Arabian Shield

الملخص EN

Field, petrography and major elements geochemistry of the ultramafic -mafic rocks of Wadi Khamal area led to the recognition of the following rock groups, namely : 1) ophiolitic suite, melagabbronorite and cumulate gabbronorite to the south of Wadi Mahalah (southern ultramafic-mafic rocks) and 2) Wadi Khamal anorthositegabbro complex to the north.

The ophiolitic rocks in the southern area include serpentinites, peridotites and pyroxenites.

The serpentinites contain large blocks of amphibolites, schists, gneisses, and metagabbros.

These rocks were intruded by melagabbronorite and cumulate gabbronorite.

On the AFM diagram, these rocks plot along the MgO-FeO* side in the tholeiitic field except those of the melagabbronorite which plot within the calcalkaline field.

The Wadi Khamal complex consists of four major units; marginal gabbro unit, anorthosite unit, central gabbroic unit and northern gabbronorite unit.

The marginal gabbro unit and anorthosite unit have a calc alkaline differentiation trend while those of the central gabbro unit (olivine gabbro, melagabbronorite and Fe-Ti oxides-apatite gabbronorite) and the northern gabbronorite show a distinct different trend parallel to the FeO*-MgO side line and belong to the tholeiitic differentiation trend.

Based on major element geochemistry, the southern ultramafic–mafic rocks were derived from a more mafic liquid source rather different from magma source (s) which formed the Khamal anorthosite-gabbro complex.

The Fe-Ti oxides-apatite gabbronorite of the central unit of the Khamal complex hosts two main types of Fe-Ti oxide ores.

1) Massive bands of nelsonite ore consist mainly of magnetite, ilmenite, apa tite and minor amounts of silicates.

It occurs in the form of bands and lenses of variable sizes intercalated with the magnetite-ilmeniteapatite gabbronorite, and 2) massive magnetite-ilmenite ore, which is less abundant compared to the nelsonite.

The latter occurs as dyke-like bodies with nelsonite selvages within the anorthosite, or as bands or lenses enclosed within the nelsonite and close to the contact with the overlying anorthosite cap.

The contact of magnetite-ilmenite ore with the nelsonite bands is gradational.

The close spatial association and mineralogical similarities between the Fe-Ti oxides–apatite gabbronorite and the nelsonite, as well as the variable Fe-Ti oxides / apatite ratios suggest that the nelsonite was formed as accumulations due to in situ fractional crystallization of Fe, Ti, P and volatiles-rich gabbroic magma.

The formation of the massive magnetite-ilmenite ore is attributed to high fO2 and mobility which promote later injection into the anorthosite host rocks.