Geology Reference
In-Depth Information
2. Large mafic-ultramafic intrusions often display excellent igneous layering and
lamination. Individual layers may range petrologically from the extremes of
anorthosite and economically-important chromitite, through a wide variety of
peridotites, pyroxenites and gabbros;
the layers should be described in terms
of grain size, modal mineralogy and texture
. Penetrative, disruptive and 'sedi-
mentary' slump structures are frequently displayed by the layering, especially
where strong density contrasts (for example, between peridotite/gabbro and
anorthosite) exist between the alternating layers. In general, large mafic-
ultramafic intrusions become more felsic upwards, and residual liquids from
their fractional crystallisation may evolve towards granitic compositions.
3. Most ophiolite complexes are igneous masses of elongate outcrop caught
within known continental sutures. They are generally most ultramafic towards
the base, and from the bottom upwards should comprise combinations of:
a. a basal 'mantle' sequence of deformed dunite, harzburgite and pyroxenite,
b. a
layered
mafic-ultramafic
sequence
that
becomes
more
leucocratic
upwards,
c. a sheeted complex of dolerite dykes and
d. basaltic pillow lavas overlain by Fe-Mn umbers.
The effects of hydrothermal metamorphism should be recognised in the
uppermost 3 - 4 km; high-level mineralised stockworks and, less frequently,
massive exhalative sulfide ore bodies may also be found.
4. In ancient greenstone belts highly mafic volcanics can be found in the form of
komatiites. These have characteristic spinifex textures and can be associated
with mineral deposits.
