Geology Reference
In-Depth Information
sea water through cracks and fissures when the ocean crust was newly formed.
The pillow lavas and dykes of ophiolites frequently show minerals of the zeolite
and greenschist facies of metamorphism, whereas other dykes, lower in the
sequence, and the layered gabbroic rocks, may show amphibolite facies minerals.
This means that, on a mineralogical scale, dark-green amphibole, originally at
deeper levels, and pale-green chlorite, originally at shallower levels, should
be recognised - both of which grew at the expense of pyroxene. White zeolite
minerals may also be found in cavities or, more rarely, replacing feldspars in the
dyke-pillow lava sequence. During hydrothermal metamorphism, in general, it
is common for base metals to be remobilised and concentrated into ore deposits
(see Chapter 10), and the upper regions of ophiolite sequences may contain
stockworks
along which mineralising fluids are able to penetrate and which are
often lined with chalcopyrite and other sulfide minerals. Where a major fracture
focused the flow of mineralising fluids into a sea-bed discharge zone, reaction
between the hot, metal-enriched fluids and sea-water may have caused massive
'exhalative' sulfide ore bodies to be precipitated at the upper surface of the ocean
crust; such sulfide ore bodies are also known from ophiolites. More commonly,
however, the metals were precipitated as Fe-Mn-rich hydroxide umbers, carrying
lesser amounts of copper, lead, zinc and other transition metals. So ophiolites
can be economically important, not only for their deep chromite layers, but also
for their high-level stockworks, massive sulfide mineralisation and the related
sedimentary umbers.
8.4 Komatiites
Komatiites
are a rare but important ultramafic rock type found predominantly
in the Archaean. They contain very high MgO contents above 18%, as high
as 32%. Komatiites are commonly associated with Ni-Cu-PGE-bearing sulfide
deposits (for example, Yilgarn craton, Western Australia; Abitibi greenstone belt,
Canada), and have also been used extensively in helping to determine mantle
potential temperatures as they are thought to represent very hot primary melts.
The rocks occur as lava flows and shallow intrusions, and are characterised by
some key textural relationships. Figure 8.14 shows a schematic section through
a komatiite lava with specific zones recognised by different olivine textures. The
lower B2 zones contain mainly euhedral olivine crystals which were present as
phenocrysts during the eruption of the komatiite and have settled out on emplace-
ment. The most characteristic texture of the komatiite is the bladed interlocking
olivine texture known as spinifex (after the long grass of the same name), where
long olivines rapidly grow from the top of the flow downwards. Such textures
are thought to form due to the high temperatures and super saturation of Mg in
the melt. It should be noted that not all of the zones defined in Figure 8.14 will
be present in a Komatiite, and careful mapping may be required as komatiites
are commonly very weathered due to their old age and MgO content.
