L6 - PGE Magmatic Ore Deposits

What are some similarities across all the different magmatic ore deposit types?

1. Phase separation

2. Density segregation

What is the difference between sulphide and oxide ores?

Oxides - ore is a crystalline solid

Sulphides - ore is carried in immiscible phases

What are some Platinum Group Metals (PGM)?

1. Platinum (Pt)

2. Palladium (Pd)

3. Rhodium (Rh)

4. Iridium (Ir)

5. Ruthenium (Ru)

6. Osmium (Os)

What are some PGM characteristics?

1. Dense Metals

2. Resistant to corrosion

3. Caralyzers (catalytic converters)

What are some applications to PGM?

• Autocatalyst - control vehicle pollution

• Chemical - catalysts for chemical synthesis electrode coatings

• Dental - alloys for dental restorations

• Electronics - Computer hard disks multi-layer ceramic capacitors, crucibles to grow single crystals

• Fuel Cells

• Glass

• Investment

• Jewellery

• Petroleum

• Sensors

• and more

Is it common to find economic PGE deposits?

It is extremely rare

What two districts supply a majority of the world’s PGE?

1. Bushveld

2. Noril’sk-Tainakh (not primarily PGE, but a by-product form Ni-Cu ores)

Where do Platinum Group Elements (PGE) reside in magmatic ore deposits?

• Dissolved is base metal sulfides

• As native metals

• In platinum group minerals (PGM): Pt-Pd sulphides, PGE arsenides, and PGE-tellurides

• A variety of PGE-Fe alloys (through metallic bonding)

What are 4 of the 109 recognized PGM?

1. Speryllite PtAs₂ ← Important platinum arsenide

2. Braggite (Pt, Pd)S

3. Cooperite PtS

4. Merenskyite PdTe₂

What is Tulameenite and what is its chemical formula?

A platinum-copper alloy

• Pt₂CuFe

• discovered 1973

Found in the PGM placer deposit in BC, Canada

What are the two main types of PGE deposits?

1. Reef Type

2. Magmatic Breccia Type

Describe a reef type PGE deposit

• Stratiform sulphide-bearing ore zones

• Present as thin layers in intrusions

• Tend to occur above ultramafic rocks

• well below the upper fractionated rocks

What are two examples of reef type PGE deposits?

• Merensky reef - Bushveld

• J-M reef - Stillwater

Describe a Magmatic Breccia Type PGE deposit

• large zones of sparsely disseminated sulphides

• in a mafic magmatic host

• has a high proportion of breccia clasts (both cognate and exotic)

What is the typical age for PGE deposits?

Most are Paleoproterozoic to Neoarchean in age

What percentage of PGE are combined to sulphides of PGE deposits?

<3% = sulphide poor

How do PGM typically form?

in very minute quantities by exsolving from the base metal sulphides (BMS) during slow cooling of the intrusions

What is the only sulphide mineral that contains a significant amount of any PGE?

Pentlandite (Pd)

What is an example of a Magmatic Breccia Type PGE deposit?

Platreef, Bushveld

Lac des Iles, Ontario

What is the PGE grade of the Merensky Reef?

5-7 g/t (Pd/Pt ~0.5)

What is the range of thickness of the Merensky Reef?

300cm to several 10s of cm

What is the average thickness of the Merensky Reef?

90 cm

What is the relative sulphide and PGE content in the Merensky Reef?

Sulphide poor (2-4 vol% sulphide), but PGE-rich

What is a “pothole“ in a PGE deposit?

When layers moves up and down relative to stratigraphy (very steep changes)

• strike remains the same

• a type of magmatic unconformity

What is the typical lithology of rocks in the Merensky Reef footwall?

anorthosite/leuconorite

What marks the bottom of the footwall ?

A chromite seam

What is the hanging wall of the Merensky Reef typically composed of?

melanorite (plag + opx) / plag bearing orthopyroxenite

What is the average grade of PGE is the J-M Reef (of the Stillwater Complex)?

~18 g/t or ppm

What type of rocks host PGE-rich sulphide zone in the J-M reef?

Occurs in troctolitic rocks of the Lower Banded Series

What is troctolite?

Olivine and plagioclase rock

How thick is the J-M Reef?

1-3 m thick

What percentage of BMS is found in the J-M Reef?

0.5-2%

Where do the PGM normally occur in the J-M Reef?

PGM mainly occur as discrete grains on the margins of BMS (separated/exsolved out during slow cooling)

What is the gradational cumulative rock change of the J-M Reef?

Hanging Wall - Norite (plag-opx cumulate)

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Anorthosite - Plag cumulate

Troctolite - Plag - Olivine Cumulate

Pegmatitic Melatroctolite - olivine cumulate

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Footwall - Gabbronorite - Plag-cpx-opx cumulate

Why might olivine be black in color?

alteration to serpentine, chlorite, talc, tremolite ± magnetite

What is the source for PGE?

Peridotitic mantle

What is the transport for PGE?

transported in solution in the magma

What is the trap for PGE deposits?

upper crustal magma chambers in a “reef“

• form a horizon in a layered intrusion

• Nucleate sulphide

What is the most effective way to reach sulphide saturation?

Magma mixing

Why is magma mixing the most effective way to reach sulphide saturation?

• Magma is already Ni-poor and therefore is more likely to reach oversaturation.

• When more magma is added (containing sulphides) the system quickly becomes oversaturated

Do PGE deposits require early saturation of sulphide melt and an external source of sulfur?

Neither is needed to create a PGE deposit

What type of deposits require early saturation of sulphide melt and an external source of sulfur?

Ni-sulphide deposits