• Carbonatites (calcite carbonatite (sovite), dolomite carbonatite (beforsite), ferroan or ankeritic calcite-rich carbonatite (ferrocarbonatite)) occur in a continental environment; rarely in oceanic environments (Canary Islands) and are generally related to large-scale, intra-plate fractures, grabens or rifts that correlate with periods of extension. Carbonatite lava flows and pyroclastic rocks are unmineralized

• Associated rocks are magnetite-olivine-apatite phlogopite rock, nephelinite, syenite, pyroxenite, peridotite and phonolite.

• Processes:

– Magmatic mineralization may be linked either to fractional crystallization or immiscibility of magmatic fluids

– Metasomatism and replacement

• A fenitizationhalo (alkali metasomatized country rocks) commonly surrounds carbonatite intrusions. Typical minerals are sodic amphibole, wollastonite, nepheline, mesoperthite, antiperthite, aegerine-augite, pale brown biotite, phlogopite and albite. Most fenites are zones of desilicification with addition of Fe, Na and K.

• Carbonatites relatively easily undergo supergene alteration and it is an important factor for concentrating residual mineralization (anatase, pyrochlore, apatite, crandallite-group minerals containing REE)

See also the review: Fenites associated with carbonatite complexes: A review

By H.A.L. Elliott et al. 2018 in Ore Geology Reviews

Carbonatites form at great depth of about 100km and they are part of the alkaline rock group.

If you are interested particularly the origin and behavior of Indian carbonatites kindly go with this link DOI:10.1515/geo-2020-0007

To answer your question specifically regarding the mineral assemblages and proportions.

The three minerals together (apatite, olivine, magnetite) are characteristic of early stage crystallisation from carbonatite melts. You did not say so, but I am assuming they coexist with calcite.

These are cumulate minerals that form in equilibrium with the carbonatite melt, but the composition of carbonatite melt most likely contains lots of alkalis (Na, K) which are not preserved is the solid mineral assemblage. The varying proportions simply reflect different proportions of cumulate minerals, and I don't think there is enough understanding of carbonatitic magma systems to be able to say anything smart about that yet.

The fact that these are associated with ultramafic alkaline rocks suggests that you are looking at a very deep system which has not fractionated yet to form more evolved carbonatites and silicate alkaline melts.

Dear Damepaia,

about 20% of the carbonatites worldwide donˋt have associated silicate rocks. Please see the review of Jones et al 2013 for the processes, Mitchell 2005 for the association, the contributions of Michael Anenburg and our recent Earth Science Review paper on the role of fluids in carbonatitic systems for more details.

Thank you all for your kind information. Can you also please help me with some references or comments on how carbonatite takes in the REE minerals, which may or may not be economical. Thanks again.

Dear Damepaia,

Although the scientific community has made significant advances in understanding the origin and evolution of carbonatites in the last decades, not every question has been answered yet.

Regarding your first question: Why are carbonatites associated with alkaline rocks?

Ben has already correctly explained that about 20% of all carbonatites are not associated with alkaline rocks. These 20% are mostly found in rather higher crustal levels, which indicates that especially the deeper crustal levels exhibit a higher proportion of alkaline rocks associated with carbonatites. This is probably simply caused by the fact that both types of rocks share certain zones of weakness as ascent channels. However, since carbonatites have a much higher volatile content, much lower viscosity and liquidus temperature, carbonatite melts ascend much faster than silicate melts and can still be active at higher crustal levels, while the magma activity of silicate magmas has already ebbed. Whether there is actually a genetic link between the two rock types has not yet been clarified in detail for all occurrences. Some occurrences probably only show an accompanying ascent of both melts within the same ascent channel (see Giebel et al. 2019a, Lithos; Gittins & Harmer 2003, Per. Mineral.). For others, liquid immiscibility in depth is assumed.

Regarding your second question: How do carbonatites take in the REE minerals?

Michael (Anenburg) can certainly offer you a detailed explanation on this :)

Michael's last papers provide a lot of information.

In general, it can be assumed that carbonatite melts are already significantly enriched in REEs. These REEs become more and more enriched with the evolution of carbonatite melts. Thus, it can be observed that early carbonatites (calcite carbonatites) are rather poor in REEs (or better distinct REE minerals) and late carbonatites, which are found e.g. in the inner region of a carbonatite complex as dolomitic or ankeritic (even more evolved) carbonatites, are relatively enriched in REE minerals (in fact ankerite carbonatites show the highest potential). It has been shown that silicate contamination of carbonatite melts results in an increased early incorporation of REEs into apatite (see Anenburg et al. 2020, Science Advances; Giebel et al. 2019b, Journal of Pertrology). This process consumes REEs from the melt and can thus inhibit subsequent REE enrichment in higher evolved melts. Under ideal conditions (if there has been no strong contamination by silica), alkali REE carbonates such as burbankite are likely to form from a carbonatite melt. However, as these minerals are very unstable, there is commonly a replacement of burbankite (there are very typical replacement assembalges: e.g., ancylite + baryte, strontianite + REE-F-carbonates, cordylite or sometimes monazite) and remobilisation of REEs by fluids, followed by distal precipitation of new REE phases. Very often this is monazite which forms at the expense of apatite. Often there is also a later post-magmatic redistribution and reprecipitation of a range of different REE minerals (see Moore et al. 2015, Ore Geology Reviews; Giebel et al. 2017, American Mineralogist).Therefore, the REE mineralisation of carbonatites can be seen as a multi-stage process in which redistribution occurs again and again.

I hope that makes something easier to understand.

Best,

Hannes

I'll just add to what Hannes and Ben said - one of the problems with carbonatites is that we just don't know. Carbonatites and their related magmatic and hydrothermal studies are exceptionally hard to study. If you read some of the recent papers on the topic, you will see that not all people agree.

The last major review paper on carbonatites was published in 2005 as far as I know, and much have been learned since. The recent review paper by Walter, Giebel, and others, is a good up-to-date summary on fluid behaviour in carbonatite systems and it provides some very good ideas on what we can learn from that. There's an Elements Magazine issue on carbonatites coming out later this year which will have several papers reviewing various aspects of carbonatites. Next year there will another in-depth review on carbonatites as well. So in the not too distant future, I think it will be easier to have some foundation in the literature on which to base future studies on.

Although it makes it very hard for beginners, it is what makes is so exciting - there's lots of new things to do. If you have any interesting rocks or ideas, feel free to contact me directly, and I think others here would also love to chat. Thinking as a team is always better than thinking alone!

Thank you one and all respected sir. I am looking into your suggestions and comments.

Here is something interesting and confusing for me a starting researcher. Please give your valuable comments and feedback.

From my field evidences in my study area, most carbonatites doesn't seem to have olivine as an "early stage crystallising mineral from a carbonatite melts". The margins of the "olivine xenocryst" seems to be resorbed was reported. However, I have observed that olivine are abundant in carbonatites that are closed to the ones (carbonatite) having ultramafic enclaves. (Are these enclaves mantle xenoliths? How do I confirm it?) These enclaves occurs as mingling type with carbonatite; elsewhere some of the enclaves are nodular nodules (aggregates of nodules / or nodules within a nodular body) and in the same are location we have rock enclave that are surrounded by concentric layers of calcite and forsterite. What is happening here? Am I seeing huge mantle derived xenolith carried by the carbonatite? Kindly advice my next step.

Dear Damepaia S M Pdah,

carbonatite petrogenesis is one of the most debated issue among scientific community, but as Michael Anenburg said in the next future i think it will be a more clear situation regarding carbonatite petrogenesis (i suggest to you to read Michael Anenburg papers, an expert in carbonatites).

I published a recent scientific review paper regarding oceanic carbonatites, maybe it could be usefull for you (attached).

Turning to your last phrase "i'm seeing huge mantle derived xenolith carried by the carbonatite?". This would be the only evidence (actually missing) to constrain the mantle origin of carbonatites. We found evidence of carbonatite magmatism within mantle xenolith but not yet mantle xenolith within carbonatite.

Best regards,

Gabriele Carnevale

I would like to add an amendment in terms of a frontier area of carbonatites. As an economic geologist and field geologist since round about 45 years I look at a topic like that from a different angle mainly from geodynamic setting, the element portfolio and diagnostic rocks nearby using them as an ore guide. I know several regions, e.g., the East African Rift System where I studied the Kangankunde REE-Strontianite carbonatite and the Mt. Malosa nephelin-syenite pegmatite nearby in more detail as a genetic couple of mineral deposits. Apart from the EARS there are other regions endowed with a similar pegmatite- carbonatite association, e.g., Morocco and Namibia. The findings may shed some light on the origin of carbonatites entering the "kitchen" from the backyard not through the frontdoor. See common minerals associations in:

DILL, H.G. (2010) The “chessboard” classification scheme of mineral deposits: Mineralogy and geology  from  aluminum  to zirconium.- Earth Science Reviews, 100: 1-420.

DILL, H.G. (2015) Pegmatites and aplites: Their genetic and applied ore geology.- Ore Geology Reviews 69: 417-561.

The schematic element compilation and geodynamic - chemical cross section attached are from Dill (2015)

Hi Damepaia.

What would you think if I ask: "What is the origin of igneous rocks"? You would certainly think that this is a too generic question, as it is, indeed. To speak of "carbonatites" as a single entity is wrong. There are carbonatites and carbonatites and carbonatites (see Mitchell, 2005, Canad. Mineral., 43, 2049-2068).

There are three main types of carbonatites (rich in Ca, Mg and Fe) plus a few rich in alkalis (such as the only active carbonatite volcano, the Oldoinyo Lengai). Each of them is associated to specific silicate rocks (or come alone, without any associated silicatic magma). The origin is quite variable for each of them.

There are carbonatitic liquids that can be considered as primary (i.e., in equilibrium with mantle paragenesis) and carbonatitic liquids that can be considered as evolved (i.e., far from being in equilibrium wiht mantle minerals). It is the same for "normal" silicate magmas, with examples of basalts (some of which are considered primary or, at least, poorly evolved melts) and rhyolites/trachytes/phonolites (which are strongly evolved melts). The REE Kd for carbonatititc magma is not different from that of silicatic magma. In other words, REE can concentrate in basaltic or carbonatitic melts at upper mantle conditions. It is only the presence of evolution of the carbonatitic or the mixed carbonatitic/silicate melts that can allow more and more concentration of REE in the residual carbonatitic liquids. These can unmix or exsolve from hybrid magma compositions. Of course the REE content of a rhyolitic/trachytic/phonolitic melt is always higher than the REE content of a basaltic melt. An interesting and recent article dealing with this aspect can be found in Nabyl et al. (2020, Geochim. Cosmochim. Acta, 282,297-323).

Then, as Johannes Giebel writes, there are several processes that can spread the REE concentration in different secondary minerals, rendering the finding of general rules to explain economic importance of carbonatites very difficult.

A reply on Gabriele Carnevale comment. I agree with him that evidence of true mantle xenoliths in carbonatites is still missing. In literature there are several articles in literature reporting the presence of mantle xenoliths in such kind of melts (several cases from central Italy, such as Polino and San Venanzo and central Spain, such as Calatrava). However, all the cases I have investigated can be much easily explained with the presence of normal liquidus olivine, not mantle-derived olivine xenocrysts. Unfortunately petrography is not considered with the due respect, and several authors prefer to concentrate on geochemical aspects.

Below I report some text from my article in press in Lithos (Innocenzi et al., 2021. "The pyroclastic breccias from Cabezo Negro de Tallante (SE Spain): Is there any relation with carbonatitic magmatism?":

The origin of carbonatitic melts has been related to several processes, often interconnected, such as: 1) low-degree direct partial melting of a carbonated peridotitic mantle (e.g., Litasov and Shatskiy, 2018; Wallace and Green, 1988); 2) partial melting of carbonated eclogite (e.g., Poli, 2016); 3) unmixing from silicocarbonatite magma at mantle depths (e.g., Lee et al., 2000; Martin et al., 2013) or 4) prolonged fractionation of strongly evolved CO2-rich melts (e.g., Schmidt and Weidendorfer, 2018; Tappe et al., 2017). Another possibility to form carbonatitic or “pseudocarbonatitic” magma is related to the digestion of marine and continental sedimentary carbonates by basic-ultrabasic melts and then unmixed at relatively shallow magma chamber depths (e.g., Gozzi et al., 2014; Hegner et al., 2020; Lustrino et al., 2019, 2020; Shaw, 2018). The origin of the most abundant carbonatitic magma (Ca‑carbonatites) is particularly puzzling. Indeed, despite experimental petrology indicates dolomitic to magnesitic melts as the only carbonates in equilibrium with a peridotitic matrix, calcite carbonatites are considered the typical carbonate melts produced in the upper mantle at a depth ranging from ~60 to ~250 km (e.g., Hammouda and Keshav, 2015). The overall incompatible element budget of Ca‑carbonatites, considered a key aspect in igneous petrogenesis (e.g., Woolley and Kempe, 1989) has been now reconsidered and strongly connected to the silicate magma associated to them (e.g., Nabyl et al., 2020).

Above all, did you read: Srivastava et al., 2004 (Mineral. Petrol., 80, 241-263) and Melluso et al. , 2010 (Canad. Mineral., 48, 205-229)?

Cheers,

Michele

Dear Damepaia,

Unfortunately, interpretations of the formation of fossil carbonatites are chaotic and often misleading. The main problem is resulted in that compositions of carbonatite rocks are far not identical to that of carbonatite melts. I suggest you considering this in your studies. Regards, Tibor

Hi Damepaia, good morning.

I personally agree with most of written notes above, carbonatite petrogenesis is one of the most debated geological issue last 100 years. There are at least three main types of carbonatites and many subgroups. And I am quite sure, it is also because those rocks are to much exotic and to much overprinted by younger process and people like sensation and uniqueness.

But it is also because people/scientists often focus on details and easily overlook a number of essential clues and do not pay attention to broader geological aspects.

Carbonatites and associated primary mineral parageny are often composed of mineral phases that are highly unstable and easily soluble in the shallow crust or under near-surface conditions.

It has been increasingly recognized that secondary processes such as regional fluid flow, weathering or regional low-grade metamorphism often overprint the primary textural, geochemical and mineralogical composition of the carbonatites (e.g., Lastochkin et al. 2011 and a lot of presentations before and after this paper).

These processes result in the remobilization or re-precipitation of some elements but they also modify the original isotope patterns of the primary minerals and fluid inclusions. Due to the unique and often simple mineralogical composition of carbonatites (calcite, ankerite, dolomite, olivine, sodic pyroxenes, alkali amphiboles, biotite, apatite, and magnetite), these secondary processes can often remain unrevealed, or are barely detectable for rocks which pretend to represent an unmodified carbonatite at hand specimen scale (Smith et al., 2018).

Yet, the extent and significance of these changes for mineral exploration have not yet been elaborated in a great detail and real effects of secondary processes remain poorly understood.

Neither the exact causes nor geological implications of this mobilization have been identified to date.

Moreover, the textural, geochemical and mineralogical characteristics of metamorphosed carbonatites also provide clues to the nature of tectonic and hydrothermal–metasomatic processes operating in convergent boundaries or plate-collision zones, respectively.

Turning to another important question "i'm seeing huge mantle derived xenolith carried by the carbonatite?" We found evidence of mantle xenolith within calcite carbonatites (Ulugei Khid in Mongolia, Feng Zhen in China), e.g.:

Article A Paleoproterozoic mantle source modified by subducted sedim...

Article Cold deep subduction recorded by remnants of a Paleoproteroz...

Article Recovery of an oxidized majorite inclusion from Earth's deep...

Article Primary rare earth element enrichment in carbonatites: Evide...

Wish you good luck in your own reasearch.

Best wishes,

Jindra

A rare type of igneous rock formed from unusual processes.

Wissale Boukirou

Please read my blog posts:

1) The superciritical fluid and the origin of carbonatites - https://earth.geoscientifix.com/2018/08/the-superciritical-fluid-and-origin-of.html

2) A carbonate source for carbonatites: Whether mantle is progressively enriched in δ13C over geological time scale?

https://earth.geoscientifix.com/2018/09/a-carbonate-source-for-carbonatites_5.html

Hello Damepaia, You may found some answers for your questions here: Carbonatites: Classification, Sources, Evolution, and Emplacement

Annual Review of Earth and Planetary Sciences

(https://doi.org/10.1146/annurev-earth-032320-104243)

carbonatites were thought to form by melting of limestone or marble by intrusion of magma (this is the most probable reason for associating them with the alkaline rocks), but geochemical and mineralogical data discount this. For example, the carbon isotopic composition of carbonatites is mantle-like and not like sedimentary limestone