Magmatic ore deposits

Question 1

carbonatites and ores

Answer 1

Rare igneous rocks with more than 50% CO3 minerals, usually dolomite, calcite, and siderite. They are found around the globe and are important for LREE deposits.

Ores are generally complex fluorocarbonates with igneous textures, sometimes weathering can enhance the grade of ROM. The grade is often very high (5-10%) but the shell may only be 10-100 Mt

Question 2

Carbonatites: LREE characteristics

Answer 2

ions with large ionic charge (+3 to +5) and relatively small ionic radius. they are not compatible with the silicate minerals in the mantle

Question 3

Geometry and geography of carbonatites

Answer 3

These are usually steep dipping, roughly cylindrical intrusions. they have metasomatic alteration surrounding the intrusion. In the areas around the intrusion there is fenitized (qtz to K-spar) granites. They are most often found within rift valleys.

Question 4

Carbonatite Examples

Answer 4

Mountain Pass CA

Question 5

Carbonatite genesis

Answer 5

These arise from partial melting of carbon-rich peridotite which “extracts” carbonatite. At higher T the magmas become carbonate-bearing alkaline magmas. The LREE’s are generally incompatible with many minerals and can be sequestered within these partial melts. These are usually accompanied by (Na,K)CO3 liquids forming at depths over 90 km. Interestingly the carbon in the melts is organic.

Question 6

Stratiform Chromite deposits

Answer 6

These are large, layered ultramafic-mafic intrusions. The most notable being the Great Dyke, Zimbabwe, Bushveld Complex, and the Stillwater Complex. Chromitite layers are usually thin cumulates. They are considered oxide-related magamatic ore deposits.

Question 7

Ophiolites: Podiform Chomite ores

Answer 7

Ophiolites are chunks of oceanic crust and upper mantle that have been thrusted on the continental crust, they include the remnants of MOR sequences.

Question 8

Chromite ores in ophiolites

Answer 8

These are highly irregular tubes and cylinders of chromatite. They are usually small, <1000 tonnnes.

Question 9

Does chromitite form from oxidation or settling?

Answer 9

No. The Great Dyke shows gradual composition changes indicating that it was not a singular event (like the injection of water) that formed chromitites.
Settling is discounted because chromitites are found in orthopyroxene without a stratigraphic pattern.

Question 10

Hypothesis for chromite formation

Answer 10

Principally, it is suggested that as magmas with chromate are cooling magmatic injections (higher in chromate) create instability and lead to the oversaturation of chromate. Principally it is a function of mixing

Question 11

Podiform chromitites genesis

Answer 11

It is hypothesized that the podiform chromites within the ophiolites form when intruding mantle lava interacts with the wall dunite and precipitates the chromite

Question 12

What deposits form from immiscible sulfide melts?

Answer 12

Base metal, Ni-Cu in basaltic intrusions
Komatiites; base metal, Ni-dominated deposits
PGE magmatic sulfides in layered UMF to MF intrusions

Question 13

Explain the importance of this chart and what it says about the distribution of PGE, Cu, and Ni in magmas.

Answer 13

Because sulfur saturation in mafic-UMF silicate magmas is very low (<1000 ppm) and because it has a relatively low MP it is an early component to be in the liquid phase. The graph shows that the PGE’s are the most chalcophilic and will partition into the first sulfide melt (@~20% overall perdiditite melt). Cu and Ni are in higher grades in these melts but Cu is not as chalcophilic and Ni can substitute into olivine meaning that as the melt increases so does the presence of Ni.

Question 14

Common sulfide minerals in magmatic systems

Answer 14

Chalcopyrite, pentlandite, pyrrhotite, magnetite

Question 15

Base metal intrusions (sulfide basaltic Ni-Cu): summary, localles, processes, and commodities.

Answer 15

These are the sulfidic mafic to UMF intrusions which have high degrees of melting and hence have Cu-Ni as the primary commodities. These are the massive sulfide to gangue sequences. Famous examples of this include Sudbury, Duluth, and noril’sk. The process is the melting of silica leading to sulfide immiscibility.
Commodities: Cu-Ni because it is in intermediate heat/partial melting.

Question 16

What physical process drives the precipitation of sulfides from mafic to UMF melts (basaltic Ni-Cu deposits)?

Answer 16

The key process is the interaction of the magmas and host rocks or sediments and then the unmixing of the dense sulfide melts. In Sudbury, the flash melting of the greenstone belt enabled immiscible sulfides to cumulate, in Noril’sk the flood basalts interacted with gypsum (sulfate) to become oversaturated, and in Duluth it is a contact between the intrusive bodies of magma and the country rock.

Question 17

Geometry of the base metal intrusions

Answer 17

They have three zones; massive (@ base because of density segregation), net textured (“groundmass” supported opx in sulfides), disseminated stringers, and above this is gangue (basalts…). They are generally in the bottom of troughs and similar structures but are highly heterogenous and can take many shapes.

Question 18

Magmatic systems are the dominant source for:

Answer 18

mafic intrusions: Ni, Cr, PGE, V, Cu, Ti, Co
Alkaline magmas: diamonds, REE, Nb, Zr, sapphire, and phosphates
granites: Li, Be, Cs, Ta, Rb, U, aquamarines, topaz

Question 19

Magmatic systems: source, sink, and processes

Answer 19

Source: partial melting of the lower crust and upper mantle
Sinks: Various points in the crust
Chemical processes: Chemical assimilation, fractional crystallization, reaction with country rocks
Physical processes: churning and mixing

Question 20

How do elements partition as a function of felsic melt portion?

Answer 20

The more felsic the melt the more the large elements (Be, K, W, Pb, U) will be incorporated. In comparison UMF magmas will have (Mg, Cr, Ni, Pd)

Question 21

Why are magmas not inherently ore?

Answer 21

A look at the phase diagram of a solid state from basalt to FeS, chromite, magnetite, and PGM, has the eutectic very near basalt. This means that there is very little of the ores in the partial melts that create magmatic systems.

Question 22

What processes are responsible for concentrating ores in magmatic systems?

Answer 22

fractional crystalization - either the most compatible (crystal sink) or the least compatible (final melt). This occurs within pegmatites, carbonatites.
liquid immiscibility - Mixing with country rocks or magma injections oversaturates the magmas. This is responsible for the LMI’s, komatiites.
Gravimetric unmixing is essential - sulfide melts have a higher density, floating hypothesis for chromitites - basaltic Ni-Cu systems

Question 23

Komatiite; summary, ores, localles, processes

Answer 23

These are UMF lavas with greater than 18 wt% MgO erupting at temperatures over 1600 C. They are characteristic of the archean age with spinefix texture
ores: High Ni, low PGE, low Cu
Localles: kambalda in WA
processes: komatiite mixes with sediment (silica rich) to precipitate sulfides.

Question 24

Mafic Nickel deposits

Answer 24

These are layered komatiites. The komatiites react with silica-rich ground, partially melting the ground which then creates an immiscible sulfide melt that sequesters the nickel. Because komatiites represent the highest end of the melt spectrum, the Cu and PGE are diluted.

Question 25

What is the evidence of komatiite deposits and other mafic intrusions mixing with host rocks?

Answer 25

The ores from these deposits contain unique radioisotopes that are not common within the mantle. The country rock and the ores have the same anomalies.

Question 26

Layered Mafic Intrusions: deposits, geometry, commodities, and process

Answer 26

Deposits: stillwater, bushveld, great dyke, duluth
geometry: Large UMF to felsic intrusions with a “stratigraphy” based on settling. PGE ore occurs in “reefs” - thin (<1 m) veins
Commodities: Chromitites, Ni-Cu, and PGE
Processes: crystal sinking -> “stratigraphy”; magma mixing -> creation of small amounts of sulfide melt for reefs

Question 27

What is the geochemical evidence for an event related to PGE precipitation in LMI?

Answer 27

In Bushveld and Stillwater the PGE reef occurs in sequence and there is a change in the isotopic composition of the rocks

Question 28

Calc-Alkaline Rocks

Answer 28

This refers to the class of mainly igneous rocks that have high proportions of Na2O and K2O relative to SiO2 derived from small degrees of partial melt

Question 29

Process Types: Magmatic Systems

Answer 29

Degree of partial melting: carbonatites and PGE ores are from low degrees of partial melting that concentrate incompatible elements
Fractional crystalization: pegmatites
Immiscible magmas: The introduction of felsic materials or excess sulfur into the magma chamber create sulfide melts and pyrhotite
Physical transport - diamonds

Question 30

Peraliminous

Answer 30

This is where the ratio of Ca+Al>Si

Question 31

Pegmatites: geography, geometry, commodities, and processes

Answer 31

Geography: Anywhere with intrusive granites -> Notably Black hills and Kola Penninsula
Geometry: These are internally zoned small ore bodies (< 100m X 1 km) that become more coarse towards the center. They are generally lenticular or oblong.
Commodities: These represent the very final stages of crystalization and thus have the incompatibles like Li, Cs, Ta, Nb, Y, F, Tn, Sn, Cs, U, and Rb
Processes: fractional crystalization, decompression, SiO4 is depolymerized by the large ions which enables it to be molten far below the solidus

Question 32

Kimberlites: def., characteristic texture, processes, geography, and geometry

Answer 32

Kimberlites are UMF diatremes, dykes, pipes from alkaline, volatile rich, K rich partial melts of the upper mantle (1-2%) with a grab bag of entrained rocks
Texture: Similar to a groundmass supported breccia with the key indicators being ilmenite and the g10 garnets (purple pyrope)
Processes: Rapid transport of deep mantle rocks to the surface due to decompression of a volatile rich magma.
geography: ONLY on Archean+ cratons
Geometry: champaigne glass shaped

Question 33

Why are diamonds only in cratons?

Answer 33

A cold geotherm is needed to put the carbon in a PT state where it is not graphite and some eclogites from areas like the Himilayas (high P) have microdiamonds

Question 34

2. Layered mafic intrusions (LMIs) have high Ni content overall, but the PGE-bearing reefs that are mined in them are Ni-poor compared to other (ultra)mafic magmatic deposits - why?

Answer 34

This is because the PGE’s have a partition coefficient several magnitudes of order greater than Ni and are also found at much lower grades than Ni. This has two major consequences; first, PGE’s are easily diluted in immiscible sulfide melts and will rapidly become entrained within sulfide melts, second, with higher temperatures more Ni is incorporated into the melt and so is sulfur resulting in higher nickel grades but a dilution of PGE.

Question 35

Where would you search for Komatiite Ni-Cu(-PGE) deposits? Why? What about LMI’s?

Answer 35

I would search on the Archean cratons because komatiites represent a magma derived from a time when the Earth’s mantle was significantly hotter than today. I would also search for LMI’s in or near cratons because as the Earth has become more differentiated through cooling, mafic intrusions are less common.

Question 36

what is the significance of fractional crystallization and what is meant by “D”

Answer 36

Fractional crystalization is important for concentrating incompatible elements. It is most important within pegmatites where the rocks represent the very final stages of cooling. D is the bulk partition coefficient and represents the ratio of solid to liquid.

Question 37

why might multiple magmatic processes be important in combination to make a
mineral deposit?

Answer 37

A singular process may not concentrate elements of interest/value enough to be economic. For example, carbonatites are considered to be the result of both partial melting of carbon-bearing peridotite and fractional crystalization of the intrusion which allows for REE grades into the multiple percents

Question 38

What is the most common and important Ni ore mineral?

Answer 38

pyrhotite: It represents the quenching of immiscible sulfide melts.

Question 39

What is the parallel between copper smelting and magmatic mineral deposits?

Answer 39

In copper smelting you melt sulfides to form an iron rich silicate matte and at the base you have a copper sulfide which sinks due to density and concentrates the copper.

Question 40

What is the average grade within magmatic deposits?

Answer 40

LMI - ~9% Ni 13% Cu high sulfide
PGE reefs - low sulfide
Chromitites - low base metal 20-100% cr2O4
Komaatiites - 10-15% Ni, ~1% Cu, ~0 PGE, high sulfide
carbonatites - low base metals, ~10% REE
Pegmatites - Depends

Question 41

How do alpine chromitites differ from LMI-related chromitites?

Answer 41

alpine chromitites (podiform) are those that form in MOR’s and have magnesian chromitite ores. This is preferred in sume uses of chromitites because during the mineral processing the common ferric chromitities have limited use.

Question 42

What is the flotation model for chromitite formation?

Answer 42

The chromitite model suggests that individual crystal nuclei float to the upper parts of the melt where they congregate and then sink as blebs.

Question 43

Anorthosite Fe-Ti: processes, locations, minerals, geometry, geography

Answer 43

aThese are oxidized mafic systems where an immiscible oxide melt (exsolved) creates ilmenite, magnetite, and appatite. They are found within the massive anorthites that came about during the mid-proterozoic Laurentia. They have a similar shape to LMI’s (bowl shaped with the layers sloping inwards) and are large 10’s km on the surface and ~10 km thick

Question 44

What evidence is used to indicate the source of carbon within diamonds?

Answer 44

There are three main isotopes, ¹³C, ¹⁸O, and ³⁴S these indicate that the materials come from both subducted (organic) carbon and from primitive carbon stored within the mantle.

Question 45

How does the geochemical fractionation and zoning vary within a pegmatite as a function of distance from the granitic intrusion vary?

Answer 45

As the distance from the source granite increases the fractionation becomes more intense and the pegmatites become more zoned.

Question 46

What is a rare metal granite and how does it compare to a pegmatite?

Answer 46

These are larger granitic bodies that are oftentimes also a subsection of a massive intrusion that are typically enriched in metals like Sn and W. Unlike pegmatites they are not zoned and take a more regular/homogeneous form meaning there is not the complex crystal zoning common within pegamtites