Week 3 Geology

Question 1

What deposits are found in layered magic intrusions

Answer 1

PGE, Ti,V (oxide deposits)

Question 2

What deposits are found in ultramafic lavas - komatiites

Answer 2

Ni-Cu sulphide-rich deposits

Question 3

What are different types of Ni deposits

Answer 3

Ni Laterites - 60-70% of all Ni resources (USGS).
Ni sulphides hosted in layered intrusions, flood basalts, and in Archean lavas (KOMATIITE hosted)
Ni associated with meteorite impact related.

Question 4

What are Komatiites

Answer 4

Age: 3.5 -1.9 Ga lavas
The youngest Komatiite lava is of Craterous age aprox 90Ma from Gorgona island, Colombia. Composition, whole rock MgO % is > 18; low TiO2 wt. %.

Question 5

Summary of part 1

Answer 5

Three types of Ni deposits-sedimentary (laterites), magmatic (mafic intrusions and Archean lavas) and others - impact related.
Archean lavas are old (Archean) high-MG lavas.
Komatiites are characteristic by their “spinifex” top and cumulate tubes.

Question 6

Distribution of sulphides in a komatiite flow

Answer 6

Sulphides are very weak phases, weaker than silicates.
During deformation events, sulphide ore can be remobilised any from its primary position.

Question 7

Deformation of sulphides

Answer 7

Sulphides are very weak phases, weaker than silicates.
Remobilisation along fault zones.

Question 8

Summary part 2

Answer 8

Sulphide liquid and enriched in Ni and Cu, and to less extent PGE.
Sulphide ore can be massive (>50 % vol.), net (50-30% vol.) and disseminated (<30% vol).

Question 9

What are the main ingredients to form sulphide deposit

Answer 9

High temperature (high Mg) magma.
Long-lived magma chamber.
Availability of sulphur.

Question 10

Summary - Part 3

Answer 10

The main ingredients to form Ni-(Cu-PGE) deposits are:
a) Primitive high - Mg magmas,
b) Long lived magma chamber,
C) Availability of sulphur - In most cases crustal sulphur.

Question 11

Explain the formation of granite

Answer 11

Through fractional crystallisation of the basaltic melt, mantle dominated system. I-type granite.
2. Partial melting of crustal rocks, crust dominated system, S type granite.

Question 12

Formation of Granite through crustal melting

Answer 12

Dehydration reaction -releasing H20 from crystal structure, triggers melting of crustal rocks.

Question 13

What is a key feature of granite pegmatites

Answer 13

key features of the pegmatites are very large crystals - up to a couple of metres.

Question 14

Why do pegmatites have such coarse grain size ?

Answer 14

Because they contain H20, F, P and B, which depress the solidus temperature, lower the density, and increase rates of ionic diffusion, enabling pegmatites to form thin dikes and massive crystals.

Question 15

What is solidus temperature

Answer 15

Temperature at which the whole rock is solid

Question 16

Summary part 1

Answer 16

Granites are felsic magmas, formed wither by fractional crystallisation (I-type) or crustal melting i.e. anatexis (S-type).
I-type granites are formed in diverse geodynamic environments, but S-type are mostly associated with mountain building (convergent boundaries).
Pegmatites are coarse late melts associated with granites.

Question 17

What are the types of pegmatites ?

Answer 17

Lithium-caesium-tantalum (LCT), Niobium-yttrium-fluorine (NYF), Mixed or “hybrid”

Question 18

Explain Lithium-caesium-tantalum (LCT) pegmatites in more detail

Answer 18

They are enriched in Li,Cs,Ta,Be,B,F,P,Mn,Ga,Rb,Nb,Sn and Hf
LCT are associated with S-type granites.

Question 19

Explain Niobium-yttrium-fluorine (NFY) in more detail

Answer 19

They are enriched in Be,Sn,B,Nb,Ti, rare earth elements (REE), among others, but are depleted in Li,Cs and Rb.
NFY are associated with I-type granites.

Question 20

Explain mixed or “hybrid” rare-element pegmatites

Answer 20

They are considered to be by products of contamination of NFY pegmatites by metasomatic fluids rich in Li,B,Ca and Mg (Martin and De Vito 2005).

Question 21

Explain some information about lithium deposits

Answer 21

Identified reserves of Li (2019) 17Mt (Source USGS)
Estimated resources of Li (2019) approx 80Mt
Highest grade (0.3-1.4% Li) in pegmatite hosted deposits.
Easier to exploit sue to high grade.
Size of deposits: 0.1 Mt of Li average.
Represents 50% of current production.
But les than 40% of resources.

Question 22

What are the three types of lithium deposits.

Answer 22

Pegmatites, Brines (associated with arc settings), Sedimentary - clay deposits. Lowest grades in Brine but largest resources - 60% percent of known resources.

Question 23

What are some of the ore minerals in LCT Pegmatites

Answer 23

Spodumene, Lepidolite, Beryl, Torumaline

Question 24

What do LCT pegmatites mostly intrude into

Answer 24

Metasedimentary or metavolcanic rocks, and les commonly other igneous rocks (granites or gabbros).
Li in many LCT pegmatites was likely derived, in part, from clays in the metasedimentary source ricks of the parent S-type granites.

Question 25

Explain the relationship with structures

Answer 25

Shallow Crustal levels:intrude along faults: intruded along faults, fractures, foliation, and bedding.
At deeper crustal levels: concordant with the regional foliation, and form lenticular or ellipsoidal bodies.
They can be located along or near major deep-crustal faults or emplaced into anti forms located at the junctions of multiple fold systems.

Question 26

What is an alteration halo

Answer 26

Presence of Alteration Haloes - highly mobile alkali elements (e.g. Li) and volatile components (B and F) tend to alter the adjacent country rocks during LCT pegmatite emplacement

Question 27

Summary Part 2

Answer 27

LCT Pegmatites can be found in orogenic hinterlands, that is, in the cores of mountain belts. Associated with collisional phase of super continental cycle.
Li is coming from clays, source rocks of the parent S-type granites.
Good indicator of exploration geologists - presence of Alteration Haloes.