L5 - Ni-Cu (PGE) Magmatic Ore Deposits

What are the uses for Nickel?

1. A key component of stainless steel because it is corrosion resistant

2. Alloys (Ni-Cu, Ni-Cr, Ni-Fe, Ni-Ti)

3. Electroplating

4. Batteries (ie. Toyota Prius)

What is the percentage of Ni in stainless steel?

~8-10%

What are the typical grades for Ni-Cu deposits?

• Ni is typically 0.7- 3%

• Cu grades are 0.2-2%

What is the typical tonnage range in Ni-Cu deposits?

100,00s to 10s of million

What are two giant Ni-Cu deposits?

1. Sudbury (Ontario)

2. Noril’sk (Russia)

What are the characteristics of Pyrrhotite (po)?

• Bronzy yellow in hand sample; creamy pinkish brown in polished section

• Variably magnetic (may not have this property)

What are the characteristics of Pentlandite (pn)?

• Yellowish bronze in hand sample; light cream to yellow in polished section

• Not magnetic

• Present as exsolved lamellae or “flames“ in pyrrhotite

What are the characteristics of chalcopyrite (cp)?

• Brassy yellow in hand sample; brass yellow in polished section

• High reflectance in polished section (not as high as pyrite)

• Twinning is common

What is pyrrhotite’s formula?

Fe_1-xS

What is Pentlandite’s formula?

(Fe,Ni)₉S₈

What is Chalcopyrite’s formula?

CuFeS₂

When does an immiscible sulphide liquid form?

at sulphide saturation

What are 3 ways sulphide liquid saturation of basaltic magma can be reached?

1. Extensive FC (S does not enter silicate structures/exsolution)

2. Magma Mixing (even mixing of two sulphide-bearing magmas)

3. Assimilation of S-bearing crustal rocks (magma melts sulphur rich country rock)

Where is nickel located in sulphur unsaturated melts?

Ni substitutes for Fe in Olivine formula/crystal lattice

Does NI, Cu, and the PGE prefer to bond with silica for sulphur?

They are chalcophiles and therefore prefer to bond with sulphur

What is the source of Ni-Cu(PGE) deposits?

peridotitic mantle

What is the transport mechanism of Ni-Cu(PGE) deposits?

S, Ni, Cu, and PGE are transported in solution in ascending magmas

What is the max ppm of S that basaltic magmas can dissolve at low pressures (close to the surface)?

1000-2000 ppm S

How much ppm in S does the peridotic mantle contain?

~250 ppm of S

Are Ni-Cu(PGE) deposits restricted to certain interval of geologic time?

Usually in Archean to Proterozoic but not restricted to a certain time

• Only need mafic to ultramafic magmatics

What is the only example of a Ni-Cu(PGE) deposit created by a meteorite impact?

Sudbury, Ontario

What is the only example of a Ni-Cu(PGE) deposit created by a convergent margin?

Turnagain, BC

How does partial melting begin with a mantle plume?

1. The plume reaches the lithosphere and spreads out

2. The thinnest section of the lithosphere (divergent zone) undergoes partial melting by decompression

3. faults etc can create magmatic freeways. Assimilation of crustal S

Define komatiites

High-temperature, ultramafic (>18 wt% MgO) volcanic rocks with spinifex-textured olivine crystals (dendritic, branching crystals oriented perpendicular to flow contacts)

What wt% of MgO does ultramafic rocks have?

>18%

What temperature do ultramafic magmas range up to?

Up to 1600’C

What direction do spinifex textures grow?

Spinifex textures grow downwards in the presence of a strong thermal gradient

When were the last major komatites?

1.88Ga (paleoproterozoic)

What allowed the formation of komatiites in early Earth?

Hotter Mantle = extensive mantle melting (30-50%)

What percent of partial mantle melt occurs in komatiite settings?

30-50% - extensive mantle melting

What is the most common mantle derived magma today?

MORB = partial melting of the mantle

What percent of partial mantle melt occurs in MORB settings?

8-10%

Why might some Komatiite basalt be present in the phanerozoic? In other words, what process creates these melts?

Related to melting in hotter mantle plumes

Examples: Gorgona, Karoo, and Baffin

What are some locations of Komatiites worldwide?

1. Barberton - South Africa 3.5 Ga

2. Abitibi - Ontario/Quebec: 2.7 Ga

3. Kambalda - Australia: 2.7Ga

4. Pechanga - Finland: 2.4Ga

5. Thompson - Manitoba: 1.9 Ga

How does Komatiite flow? What are some physical parameters at play(T, V, Thickness, and Length)?

• Very High Temp.

• Very Low Viscosity (less silica)

• Flow thickness = > 100 m in channel flows

• Flow length = 10s to100s of kilometers

How might Komatiite flows create a Ni ore forming environment?

1. Komatiite flow causes thermal and mechanical erosion of the surrounding country rock (includes/assimilates sulphides)

2. Sulphide liquids saturate and sink to the bottom of the flow

3. Liquid sulphide collects chalcophiles (Ni, Cu and PGE) from magma that flows by

Sulphides can also accumulate in fault-induced traps and other uneven surfaces

What are the 4 ore textures in komatiite-hosted Ni deposits?

1. Massive

2. Net-textured

3. Disseminated

4. Blebby

Define and describe the Massive ore texture in komatiite-hosted Ni deposits?

Massive = Ore consisting of 80% or more magmatic sulphide

Define and describe the Net-textured ore texture in komatiite-hosted Ni deposits?

• Ores containing 30-70 modal % sulphide

• Forms a continuous network, enclosing silicates

Define and describe the Disseminated ore texture in komatiite-hosted Ni deposits?

Ore where the sulphide or oxide minerals are distributed homogeneously throughout the rock but separated from each other

Define and describe the blebby ore texture in komatiite-hosted Ni deposits?

Disseminated ore variety consisting of 1-3 cm spherical blabs of sulphide

Why is net-texured ore less likely then disseminated or massive ore textures?

ore deposits normally are caused by the accumulation of saturated liquid sulphides = massive most likely

Do Komatiite NC deposits occur only extrusively as flows?

Can be both intrusive and extrusive as long at their is a flow and country rock is assimilated

What arethe 3 Xenophases?

1. Xenomelts

2. Xenocrysts

3. Xenovolatiles

What are Xenomelts, xenocrysts, and xenovolatiles?

are foreign melts, crystals, and volatiles derived by partial or wholesale melting of country rocks or country-rock xenoliths.

• collectively called xenophases

What are the $ big Canadian Ni-Cu-PGE Deposits and mining camps?

1. Sudbury

2. Thompson

3. Raglan

4. Voisey’s Bay

Please memorize summary