TOWARDS A MAGMATIC REACTION MODEL
FOR IRON-RICH ULTRAMAFIC PEGMATITES
IN THE RUSTENBURG LAYERED SUITE
D.L. Reid
University of Cape Town
I. Veksler, J. Keiding, R. Trumbull
GFZ German Research Centre for Earth Sciences
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Iron-rich ultramafic pegmatite
WHY STUDY IRUP??
1.Replace mineralized Merensky Reef
2.Structural problems (FOG, Gas release)
3.Ultra ferrous differentiates
4.Liquid Immiscibility
5.Solubility of sulphides and PGE
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Iron-rich ultramafic pegmatite
WHAT IS IRUP??
1. Iron-Rich Ultramafic Pegmatite
2. Iron Rich Silicates: Ferroaugite, Fayalitic Olivine
3. Iron Oxide: Magnetite
4. Sulphides: Fe – Cu – Ni – PGE - ?
5. Others (Metal Alloys?)
6. Coarse Grain Sizes
7. Post-Dates Layering
8. Morphologically Complex
9. Emplacement obscured by Replacement
10.Nature of Source(s)
11.Direction of Movement?
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Iron-rich ultramafic pegmatite
HW Melanorite
IRUP
Merensky Cr
FW Leuconorite
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IRUP replaces the Merensky Reef
and thus removes the PGE
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Iron-rich ultramafic pegmatite
IRUP
HW Melanorite
Merensky Cr
FW Leuconorite
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Iron-rich ultramafic pegmatite
IRUP
FW Leuconorite
PARTIAL REPLACEMENT OF FW LEUCONORITE BY IRUP
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Iron-rich ultramafic pegmatite
IRUP
FW Leuconorite
IRUP morphology may look intrusive
but more likely replacement
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Iron-rich ultramafic pegmatite
Troctolite
Transition Zone
FW Leuconorite
IRUP
SECTION THROUGH AN IRUP VEIN
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Iron-rich ultramafic pegmatite
Iron-Rich Ultramafic Pegmatites (IRUP)
post layering but late magmatic
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Iron-rich ultramafic pegmatite
IRUP can be very coarse grained
and sometimes associated with
granitic pegmatites
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Iron-rich ultramafic pegmatite
DIRECTION OF RESEARCH
1.Liquid Compositions?
2.Immiscibility?
3.Source?
4.Mechanism of Emplacement?
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Iron-rich ultramafic pegmatite
LIQUID COMPOSITIONS?
1.Geochemical Comparison with known
or computed liquids
2.Melt inclusions
3.Experimental Melting and
Crystallization
4.Experimental Melt-Solid Interaction
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Iron-rich ultramafic pegmatite
F
Comparison with liquids
Immiscible Liquid
Pairs
Feld IRUP
Upper Zone
Lineage
IRUP
Norites
A
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Pyrox
M
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Iron-rich ultramafic pegmatite
IMMISCIBILITY?
1.Necessary?
2.Melt Compositions?
3.Unmixing Processes?
4.Recognition?
5.Implications?
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Iron-rich ultramafic pegmatite
SOURCE?
1.Upper Zone?
2.Geochemical Signatures
3.Whole Rock
4.Clinopyroxene
5.Trace Elements and Isotopes
6.Microbeam Techniques
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Iron-rich ultramafic pegmatite
LA-ICP-MS REE in WR and Cpx
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Iron-rich ultramafic pegmatite
Calculated REE in source magma
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Iron-rich ultramafic pegmatite
Rustenburg Layered Suite
Sr Isotope Stratigraphy
Nd Isotope Data
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Iron-rich ultramafic pegmatite
EMPLACEMENT?
1.Intrusion?
2.Infiltration Reaction?
3.Mass Balance?
4.What happened to the PGE?
5.From above? below? lateral?
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Iron-rich ultramafic pegmatite
PETROGENESIS OF THE IRON RICH PEGMATITES
CRYSTALLIZED LIQUIDS?
Compositions too extreme
CUMULUS ENRICHED CRYSTAL MUSHES?
Intrusive forms unlikely
REACTION PRODUCTS?
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Iron-rich ultramafic pegmatite
PETROGENESIS OF THE IRON RICH PEGMATITES
REACTION PRODUCTS?
Influx of Reactive Melt
Melt derived from the Upper Zone
Transport in UCZ is sideways and upwards
Melt (1) + Host Cumulate = IRUP + Melt (2)
In Situ Crystallization
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Iron-rich ultramafic pegmatite
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Iron-rich ultramafic pegmatite
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Iron-rich ultramafic pegmatite
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Iron-rich ultramafic pegmatite
This is why the
work is important
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Iron-rich ultramafic pegmatite
This is why the
work is important
Answers probably
beneath our feet
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Iron-rich ultramafic pegmatite
But also inside
this head
This is why the
work is important
Answers probably
beneath our feet
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