Dwars River’s UG1 Exposure
Chromitite layers occur frequently within the Critical Zone of
the Bushveld Complex. In particular, Dwars River in the eastern
Bushveld is well known for the intimate association of chromitite
and anorthosite layers.
An explanation has been deduced for bifurcations of chromitite
layers and other features frequently found in exposures of the UG1
and its footwall. It is proposed that the unconsolidated footwall
anorthosite experienced liquefaction possibly caused by major magma
influx and associated seismicity.
This resulted in structures analogous to those seen in sediments
that undergo liquefaction, including sand volcanoes or boils.
Bifurcations of chromitite layers occur where there is
‘background sedimentation’ of chromite at the same time
as periodic extrusion of plagioclase plus magma slurry at the
magma–cumulate-pile interface. These extrusions form
‘waterlilies’ of anorthosite that build up on top of
each other to form domes. Simultaneously, chromite accumulates and
forms multiple layers, which vary in thickness from those formed by
a single layer of chromite grains to those over 0.5 m thick.
Subsequent post-depositional structures suggest continued
liquefaction in the footwall rocks, mobility of residual
anorthositic magma and brittle deformation of chromitite
layers.
The Bushveld Igneous Complex (or BIC) is a large igneous
intrusion within the Earth's crust which has been tilted and eroded
and now outcrops around what appears to be the edge of a great
geological basin. Located in South Africa, the BIC contains some of
the richest ore deposits on Earth. The reserves of platinum group
metals (PGMs), platinum, palladium, osmium, iridium, rhodium, and
ruthenium are the world's largest, and there are vast quantities of
iron, tin, chromium, titanium and vanadium. Gabbro or norite is
also quarried from parts of the Complex and rendered into dimension
stone.
The Bushveld Igneous Complex is divided into an eastern and
western lobe, with a further northern extension. All three sections
of the system were formed around the same time — about 2
billion years ago — and are remarkably similar. Vast
quantities of molten rock from the earth's mantle were brought to
surface through long vertical cracks in the earth's crust —
huge arcuate differentiated lopolithic intrusions — creating
the geological intrusion known as the Bushveld Igneous Complex.
This vast Igneous Complex is a huge, layered mafic to ultramafic
igneous body in which some layers consist of 90% chromite in the
rare rock type, chromitite.
Chromite is a spinel group mineral, iron magnesium chromium
oxide, in which Mg is invariably present and Al and Fe can
substitute for Cr. Chromite is commonly associated with olivine,
magnetite, serpentine, and corundum. It occurs in peridotite,
layered ultramafic intrusive igneous rocks, and metamorphic rocks
such as serpentinites. Chromite is the only ore of chromium, and
ore deposits of chromite formed as a result of early magmatic
differentiates. Dwarsrivier chromite layers alternate with
anorthosite layers in the Bushveld Igneous Complex.
Anorthosites are typically pale gray, and are phaneritic,
intrusive igneous rocks comprising predominantly plagioclase
feldspars (90-100%) with minimal mafic constituents (less than
10%). The plagioclase feldspars exist as solid solutions between
the albite and anorthite end members, and are classified according
to their percentage of anorthite (An0-100). Modified anorthosites
have less than 90% but more than 78% of plagioclase, for example
gabbroic anorthosite, whereas anorthositic rocks have 78–65%
of plagioclase, such as anorthositic gabbro. Anorthosites can occur
as Archean bodies of layered rock in which calcium-rich plagioclase
anorthosites layers alternating with adjacent layers that are rich
in chromite, amphiboles, and pyroxenes. These comparatively
uncommon Archean anorthosites occur in Greenland (2), Canada, and
the Africa's Bushveld ( the Bushveld Igneous Complex ). The mafic
minerals in anorthosites are usually pyroxenes, ilmenite,
magnetite, and olivine. However, the mafic minerals in Proterozoic
anorthosite can also include clinopyroxene, orthopyroxene, or, more
rarely, amphiboles. Other rare minerals include biotite, apatite,
zircon, scapolite, and calcite.
To claim a find on this Earth Cache you need to email the
owner the answers to the following questions:
1. The black chromite layers split open. In which direction do they
split?
2. How wide is the gap in the rocks at the waterfall?
3. On the nearby plaque, what is the date displayed.
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