Week 3 Geology Flashcards
What deposits are found in layered magic intrusions
PGE, Ti,V (oxide deposits)
What deposits are found in ultramafic lavas - komatiites
Ni-Cu sulphide-rich deposits
What are different types of Ni deposits
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.
What are Komatiites
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. %.
Summary of part 1
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.
Distribution of sulphides in a komatiite flow
Sulphides are very weak phases, weaker than silicates.
During deformation events, sulphide ore can be remobilised any from its primary position.
Deformation of sulphides
Sulphides are very weak phases, weaker than silicates.
Remobilisation along fault zones.
Summary part 2
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).
What are the main ingredients to form sulphide deposit
High temperature (high Mg) magma.
Long-lived magma chamber.
Availability of sulphur.
Summary - Part 3
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.
Explain the formation of granite
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.
Formation of Granite through crustal melting
Dehydration reaction -releasing H20 from crystal structure, triggers melting of crustal rocks.
What is a key feature of granite pegmatites
key features of the pegmatites are very large crystals - up to a couple of metres.
Why do pegmatites have such coarse grain size ?
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.
What is solidus temperature
Temperature at which the whole rock is solid
Summary part 1
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.
What are the types of pegmatites ?
Lithium-caesium-tantalum (LCT), Niobium-yttrium-fluorine (NYF), Mixed or “hybrid”
Explain Lithium-caesium-tantalum (LCT) pegmatites in more detail
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.
Explain Niobium-yttrium-fluorine (NFY) in more detail
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.
Explain mixed or “hybrid” rare-element pegmatites
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).
Explain some information about lithium deposits
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.
What are the three types of lithium deposits.
Pegmatites, Brines (associated with arc settings), Sedimentary - clay deposits. Lowest grades in Brine but largest resources - 60% percent of known resources.
What are some of the ore minerals in LCT Pegmatites
Spodumene, Lepidolite, Beryl, Torumaline
What do LCT pegmatites mostly intrude into
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.
Explain the relationship with structures
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.
What is an alteration halo
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
Summary Part 2
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.
