Magmatic Ore Deposits: Carbonatites, Pegmatites, and Sulfides
Magmatic Ore Deposits
Carbonatites
Carbonatites are the only rocks on Earth that are composed of molten carbonates. These are mantle-derived molten carbonates, meaning that somewhere in the mantle underneath volcanoes in some rare areas of the planet, there is enough carbonate around so that when the mantle evolves and comes to the surface, it is composed of pure calcite. Oldoinyo Lengai is the only carbonatite volcano on Earth, located in the mountains of East Africa. Multiple generations of carbonatites can be found overprinting one another. They are confined to rift zones, and intrusions are concentrically zoned.
Important commodities that can be obtained from carbonatites include:
- REE (in apatite, pyrochlore, perovskite, brookite)
- Zr (zirconium is important to the nuclear industry as a good nuclear moderator)
- Hf (in zircon or baddelyite)
- P (in apatite; phosphorus is the most important commodity)
- Nb, Ta (occurs in pyrochlore, tantalite, columbite)
- U, Th (occurs in minor quantities only, but sometimes from zircon, tantalite, columbite)
- Cu (in chalcopyrite, bornite; this can only be found in Palabora, South Africa)
Pegmatites
Pegmatites are things that crystallize as the product of extreme magmatic fractionation. These are very coarse-grained rocks. They occur as relatively small features: small dykes, sills, irregular pockets, and masses. Features that represent very small segregation of very evolved liquid, so they have specific igneous morphology. Elements associated with the deposit that are mined are highly incompatible. They have a partition co-efficient much less than 1.
Things that we mine from them include:
- Tantalum and niobium
- Alkali metals including lithium (Canada produces a large portion of the world’s lithium, although there are lithium-bearing deposits in South Africa), rubidium, and cesium.
All of these elements have one thing in common: they are incompatible in minerals due to being sluggish and having large ionic radii. They don’t fit into the structure of minerals easily. They also tend to have high ionic charge or are strongly electronegative, so they also don’t like to get into mineral structures because they have charges that cannot be balanced by normal silicate mineral structures.
Pegmatites are generally classified into four groups based on the primary elements that are mined and based on the temperature and pressure of pegmatites:
- Miarolitic gem-bearing: consists of quartz crystals, beryl, topaz, tourmaline
- Rare element: consists of spodumene, amblygonite, petalite, lepidolite, pollucite, beryl, columbite-tantalite, microlite, wodginite, uraninite, cassiterite, xenotime, gadolinite.
- Muscovite class: consists of muscovite, feldspar, and uraninite.
- Abyssal class: consists of feldspar and quartz.
Rare element pegmatites are further subdivided into NYF and LCT types.
Nickel-Copper Sulfide Deposits
Nickel-copper sulfide deposits are sulfide concentrations that occur in certain mafic/ultramafic intrusions of volcanic flows. Nickel-copper sulfide deposits are further subdivided into astrobleme-associated nickel-copper, rift & continental flood basalt-associated nickel-copper. Nickel is the main economic commodity, copper may be either a co-product or byproduct, and PGEs are usually byproducts. Other commodities recovered in some cases include gold, silver, cobalt, selenium, and tellurium. These minerals are associated with sulfides, which generally make up more than 10% of the ore. Furthermore, all subtypes of nickel sulfide ores usually consist of a simple sulfide assemblage: pyrrhotite, pentlandite, and chalcopyrite, either as massive sulfides, sulfide matrix breccias, or disseminations of sulfides.
Deposits of this type are associated with mafic/ultramafic rocks, such as gabbros, peridotites, anorthosites – all those rock types that can be made through a combination of mixing things like plagioclase, olivine, pyroxenes, maybe some biotite, maybe some hornblende. So they are mineralogically simplistic, but they are complex systems because:
- They are, regardless of being sourced by partial melting of the mantle,
- They all form over very large periods of time, formed from the periodic filling and refilling of magma chambers.
They are very abundant. They occur all over the place, and that tells us that basically, areas where the mantle can be breached and the mantle can melt to produce basalts are ultimately the source of the rocks that host these deposits. We do not find these deposits associated with hotspots. Hotspot magmatism and mid-oceanic ridges do not generate these deposits because, in hotspot magmatism, the part of the mantle being melted is just not very rich in nickel and PGE, and we do not melt much of the mantle during hotspot magmatism also because of the volumetrically small amounts of magma generated. Mid-oceanic ridges do not produce these systems because rocks are all being extruded onto the seafloor, and we need the rocks to be contained within the crust in a magma chamber.
In terms of grades, there are three commodities that we mine:
- PGE: which have unique catalytic properties and are quite valuable, typical grades can range from a few ppm to some very high-grade systems where we can have 15-20 ppm.
- Nickel grades and copper grades: are typically very low. Economical nickel grades are typically in the 0.5 to 2 wt% window, and copper is also usually quite low, usually no more than 1% in terms of bulk.
Pyrrhotite pentlandite, when it grows from the magma as single sulfide crystals, it does not have the composition of pyrrhotite-pentlandite. In fact, it starts off with a composition that is kind of like pyrrhotite, but it has a lot of nickel in it. And the name of that starting composition that grows from the magma, we…