L7 - Granites and Pegmatites

What plot/ternary diagram is used to classify granitic rocks?

QAP Diagram

What are some characteristics of granitic rocks?

• Composed of Kspar, Plag, qtz, bt/hbl

• light in color

• coarse grained

What is anatexis?

The partial melting of crust

• through weight of the overlying crust

What is a migmatite?

a “mixed rock“ with dark metamorphic “melasome“ and light granitic “leucosome“

What is the weight percentage of water in granite?

3-6 wt% of H2O

What minerals present in a granitic rock contain H2O and other hydrous elements?

• Micas: biotite and muscovite

• Amphiboles: Hornblende

Where do incompatible and compatible elements end up during crystallization?

Compatible - are incorporated into the crystal lattice (depleted in the melt)

Incompatible - stay in the melt (melt becomes enriched in the element)

Do incompatible elements prefer the volatile or non-volatile melt?

incompatible elements are compatible with volatile melts

What are some rare metal bearing minerals?

1. Spodumene (Li)

2. Petalite (Li)

3. Lepidolite (Li)

4. Pollucite (Cs)

5. Cassiterite (Sn)

6. Columbite-Tantalite (Nb-Ta)

7. Pyrochlore-microlite (Nb-Ta)

8. Tapiolite (Ta)

9. Niobian Rutile (Nb)

What are 2 Rare Earth Element (REE) bearing minerals?

• Bastnasite: (La,Ce,Y)CO₃F

• Monazite: (Ce,La,Th)PO₄

Where do the world’s resource of REE come from?

China

Define a pegmatite

An intrusive rock that are distinguished from other igneous rocks by extremely coarse, but variable grain sizes.

• commonly of granitic composition

What are some other pegmatite characteristics other than their grain size?

• Skeletal, graphic (intergrown) or other strongly directional growth habit

• Occur as sharply bounded, homogeneous to zoned bodies within igneous or metamorphic host-rocks

How do pegmatites help concentrate REEs?

REEs are concentrated by extreme fractionation of granitic magmas

• T = 550’C - 250’C

Where do pegmatites commonly form? How is this different from REE-enriched pegmatites?

Pegmatites commonly form very close to the source (intrusion)

• REE-enriched pegmatites are further away from the intrusion and very fractionated

• typically linked to nearby granitic plutons

Are not mineralized

• lack beryl, lithium minerals, and complex Nb-Ta-Sn oxides

What is a miarolitic pegmatite and what pattern is a characteristics?

Miarolitic = featuring internal crystal-lined cavities or pockets ("miaroles") that form when exsolved aqueous vapour and volatile-rich fluids are trapped during late-stage crystallization

Pattern = zoning pattern

What is the line called where miarolitic cavities occur?

the “center line“ or “pocket line“

• where crystals from the hanging wall meet the footwall crystals

What do miarolitic cavities in pegmatite provide evidence for?

evidence of vapour saturation

What is the depolymerization effect? How does this relate to miarolitic cavities?

Depolymerization effect = reduces solidus temperatures, lowers viscosity, and reduces the rate of nucleation of silicate minerals

• Volatiles act as fluxes and disrupt the polymeric linkages of silica-rich melts

What pressures are miarolitic cavities present in?

abundant only in very shallowly emplaced granites

1-2 kbar

How old is the Tanco Pegmatite?

2.670 Ga

What critical elements are enriched in the Tanco pegmatite?

LCT- type mine

• Lithium (L)

• Cesium (C)

• Tantalum (T)

How many kilometers of drillcore was recovered from the Tanco Mine?

150 Km

How thick are the Tanco intrusions?

10s to 100s of meters thick

What rock types are in the exterior vs. the interior margin of the zoned pegmatites?

exterior/outer margins = simple rock types (graphic granite and apalite)

interior/inner margins = ore-bearing zones (fractionated interiors)

What was the magma source of the Tanco pegmatite?

Magma is derived from the partial melting of a pelitic metasedimentary source

How many mineral species have been identified in the Tanco pegmatite?

105 mineral species

Where are apatites usually recovered in the Tanco Mine?

typically in the lower parts of the intrusion, but are abundant everywhere

What are two examples of lithium aluminosilicates?

• Petalite

• Spodumene

What is SQUI or SQI?

a pseudomorphic intergrowth of spodumene + qtz

What is a NYF Pegmatite?

An ore-bearing pegmatite containing Niobium (Nb), Yttrium (Y), and Fluorine (F)

Can you see a record of the complete crystallization history in: a single pegmatite? A single crystal?

Why or why not?

Yes, for both

Extreme fractionation causes very large concentration changes in trace elements

What is the source of REE in pegmatites?

Source of rare metals is from pre-existing contenental crust that is partially melted to form granitic melts

What is the transport system of metals in pegmatites?

metals are dissolved and transported in granitic magmas

• Stalling and fractionation lead to enrichment in incompatible elements

What is the trap in pegmatites?

The pegmatite body itself

• crystals with unusual chemistry and size crystallize from the low-temperature, volatile-rich magma (± aqueous fluid)