Dear Dr. Müller,

I know that in some cases every black vein or dyke in underground mines or drill holes is denominated as lamprophyre. I myself prefer for the use in the field for mapping and drill core examination the classification scheme found in the attached file which is in my opinion very logical and easy to handle.

Mesodiabases or dolerite dykes are some of the common terms used for mafic dykes showing a basaltic composition.

I copy into this thread what only recently I wrote in a different thread where you also delivered a contribution:

I studied lamprophyres which formed prior (metalamprophyres) and in the aftermaths of pegmatites mainly kersantites, minette, camptospessartites and spessartites (lamprophyres sensu stricto). These rocks formed from magmas rather unfractionated from subcrustal sources (Métais and Chayes 1963 , 1964 ; Rock 1991 ; Wooley et al.1996 ; Le Bas 2007 ; Seifert 2008 ; Vasyukova et al. 2011 ). In this case of magma ascent I would neither claim fractionation nor magma mixing as the potential reason for such zoning effects. All dykes of this kind formed along deep-seated fault zones and rather “clean” magmas. The endocontact alteration begins at the level of emplacement where they can either undergo metamorphism or strong wall rock interaction (metasomatic process) which predominantly concern some trace elements. The felsic pegmatites are not the result of a simple fractionation of pegmatites but some of their rare elements such as Nb, T and Be have been derived from deeper sources. The same hold true for U where lamprophyres have also been sidelined for quite a long time. I do not know the local geology etc. and I will not push you into a wrong direction but in this case a strong metasomatic exchange reaction seems to me a plausible process.

To avoid further confusion the term subcrustal = mantle.

With kind regards

H.G.Dill

Dear Dr. Müller,

Let's see what the Glossary of Geology has to say about the terms "lamprophyre" and "mafic dykes":

Quotation:

"lamprophyre A group of dark-colored, porphyritic, hypabyssal igneous rocks characterized by panidiomorphic texture, a high percentage of mafic minerals (esp. biotite, hornblende, and pyroxene), which form the phenocrysts, and a fine-grained groundmass with the same mafic minerals in addition to feldspars and/or feldspathoids; also any rock in that group, e.g. minette, vogesite, kersantite, spessartite, camptonite, monchiquite, fourchite, alnoite. Most lamprophyres are highly altered. They are commonly associated with carbonatites. Cf: leucophyre.

Adj.: lamprophyric.

lamprophyric Said of the holocrystalline-porphyritic texture exhibited by lamprophyres, which phenocrysts of mafic minerals are contained in a fine-grained crystalline groundmass."

End of Quotation.

The terms "mafic dyke" or "mafic dykes" are not mentioned in the

Glossary of Geology Second Edition by Robert L. Bates and Julia A. Jackson, Editors. American Geological Institute Falls Church, Virginia (1980).

With kind regards,

G. Grundmann

Dear Dr. Müller,

The best way to discuss the most confusing lamprophyre group is through representative examples of original illustrative material.

I show digital images (width 2 mm, each) of two representative examples of lamprophyres (please see attached) from my thin section collection here:

(1) The first image shows an amphibole-bearing vogesitic lamprophyre (locality: Andlautal, Vosges, France) under the polarization microscope (one nicol). The brownish phenocrysts are amphiboles, which show a clear zoning. Main minerals of the groundmass are plagioclase. Historic thin section manufactured by Thomas Uebel, Bonn, Germany.

(2) The second image shows a pyroxene-bearing monchiquitic lamprophyre (locality: Eichelspitze, Kaiserstuhl area, Baden-Württemberg, Germany) under the polarization microscope (one nicol). The bright phenocrysts are pyroxenes, which also show zoning, the opaque phenocrysts are magnetite/ilmenite in a very fine-grained groundmass. Historic thin section manufactured by Thomas Uebel, Bonn, Germany.

With kind regards,

G. Grundmann

Dear Dr. Müller

To decipher the enigma of lamprophyres in economic geology it is important to take a tripartite approach: Geology-Mineralogy/petrography-Geochemistry.

1. Similar to the mafic dykes lamprophyres used to be bound to deep-seated fault zones and they are post- (to rarely synkinematically /metalamprophyres) in origin

2. They have to be properly denominated and put into one of these pigeonholes which allow them the distinction from mafic dykes present in similar geodynamic settings. It is mandatory to also have a look at the country and host rocks around. Often these magmatic rocks denote a bimodal distribution with mafic dykes and felsic country rocks such as magmatic rocks of the granitoid clan and even rhyolites around or parallel to the same structure. To my experience the couple mafic dykes-felsic dykes seems to be more common than lamprophyres and felsic dykes.

3. Geochemically these rocks may have been considered in some cases relevant for Nb/Ta, U, Be and Ti and also may give rise in case of anomalously high Al and C contents to sapphires and diamonds. They are bound to lamprophyre dykes classified as ouachitite, a biotite monchiquite devoid of olivine with a glassy or analcite-bearing groundmass (Yogo Gulch, Montana-USA). Both associations attest to the subcrustal/mantle origin of lamprophyres.

Kind regards

Harald G.Dill

The 1980 Glossary of Geology was probably written before the widespread use of the word "mafic". Back in those days the same dykes were likely to be called "basic dykes", but "basic" became unfashionable. "Mafic dyke" is a good general term for most black dykes,until you have more specific information.

The Glossary of Geology shows very well that the term mafic was proposed and introduced over 100 years ago.

Quotation:

"mafic Said of an igneous rock composed chiefly of one or more ferromagnesian, dark-colored minerals in its mode; also, said of those minerals. The term was proposed by Cross, et al. (1902, p. 561) to replace the term femag, which they did not consider to be euphoniuos. Etymol: a mnemonic term derived from magnesium + ferric + ic. It is the complement of felsic. Cf: femic; salic; basic. Partial syn: ferromagnesian."

End of Quotation.

Kind regards,

Guenter Grundmann

Dear all: I have found and studied quite a few lamprophyres in northern Venezuela, and diabase and basalt dykes and sills in southern Venezuela‘s Avanavero-Roraima LIP. Definitey most of those found in Paraguaná Península and offshore islands are lamprophyres of the minette (phogopite phenocrysts) and spessartite (hornblende phenocrysts, beautifully zoned); of course all have the panidiomorphic texture and very fine and altered groundmass, but no felsic phenocrysts; some have also amphibole-carbonate ocelli. These dykes are of Paleocene The largest one is about 20 m thick, and has a 600 m outcrop!

The LIP is Mesoproterozoic, and the diabase mega-sills and dykes have typical ophitic textures, and very seldom have large plagioclase megacrysts, so they aren’t lamprophyric!!! All these are “mafic” intrusions, but only the Caribbean related ones are truly

lamprophyres. Regards, Sebastian.

May I request anyone to show the microphotograph of ocellar calcite in lamprophyre please??

You are quite right. Lamprophyres are porphyritic rocks that contain only mafic phenocrysts, any felsic minerals (plagioclase, alkali feldspar, quartz) are confined to the groundmass. The predominant mafic phenocrysts are mica and/or amphibole and lesser cpx. Olivine and opx phenocrysts are not characteristic of lamprophyres (see 1991 book by N. ROCK). These are characteristic of picritic rocks (WOOLLEY & AL. 1996). Now, mafic dikes like dolerites and diabases (altered) have an ophitic or subophitic texture.

Ioannis

Dear Ioannis:

In the I.U.G.S. classification of lamprophyres kimberlite is considered as a lamprophyre, so lamprophyres sensu lato can contain also olivine phenocrysts, and obviously kimberlites have lots of these, besides opx and cpx ones, and sometimes even pyropic garnet. In Venezuela, I have studied lamprophyres and kimberlites from several parts of the country. The kimberlites are sill-like intrusions in the Cuchivero Province of the Guiana Shield, they have of metric thickness (2-3 m) and some km in length. Their ages are 830-700 Ma, thus they relate quite surely to the breakup of supercontinent Rodinia. Some are quite fresh, and show rounded olivine and cpx phenocrysts, with large phlogopite leaflets and phogopitised garnets, in a shattered carbonate-rich groundmass, which contains small apatites, rutiles, perovskites, and spinels. Others are thoroughly serpentinised and choritised. These are diamond-bearing kimberlites, exploited in alluvial terraces with land-stripping primitive mining techniques.

Another set of dykes crops out specifically in the islands offshore belonging to the Leeward Lesser Antilles, and in the Paraguaná Peninsula. Lamprophyre dykes are abundant there, and have usually beautifully zoned hornblende phenocrysts (with some augite ones), or magnesian-biotite phenocrysts. Since they have been dated Paleocene-Eocene the groundmass, probably vitreous in part, has been totally devitrified to sericite, albite, calcite, chlorite, and other minerals. Most of these lamprophyres are minettes and spessartites, and also contain a some ocelli made mainly of carbonate, with some amphibole. Ocelli are another quite distinctive characteristic of this litologic type. Regards, Sebastián, UCV, Caracas, Venezuela.

Dear Dr. Guenter Grundmann,

I saw your second picture showing a "lamprophyre" containing pyroxenes + magnetite as phenocrysts but devoid of amphibole and biotite. The definition of lamprophre on Wikipedia or by IUGS gave me an impression that there must be amphibole and/or biotite phenocrysts. Is that right? The thing is that I collected a set of closely associated mafic–intermediate dikes from North China, that are very similar in ages, trace elements and Sr-Nd isotopes. For the mafic ones (46–54 wt% SiO2), some of them only contain phenocrysts of clinopyroxene and altered olivine (without amphibole or biotite), and some of them contain phenocrysts of amphibole, clinopyroxene and altered olivine. Phenocrysts of the intermediate dikes (55–59 wt% SiO2) are dominanted by biotite. All these dikes contain some ocellis mainly of carbonate. I would like to call all these dikes as lamprophyre. Hope it will not cause any misunderstanding.

With best regards,

Jia Chang

Dear Rameshchandra Phani: You asked for a photomicrograph of carbonate ocelli in a lamprophyre, here I attach some, and also panoramic views of the whole rock, and inversely zoned hornblende, all from the lamprophyres of Paraguaná Peninsula. Regards, Sebastián.

Dear Sebastián Grande, thanks for very clear and nice photographs.

Dear Sebastian

Thank you for your interesting reply. The IUGS (WOOLLEY ET AL. 1996, LE MAITRE ET AL. 2002) supports the view that kimberlites, lamproites and lamprophyres are better considered independentally although there is some interrelationship between these exotic rocks e.g. in the Indian Shield (RAO ET AL. 2020). You may say that a kimberlite is of the lamprophyric facies when your rock is a volatile-rich porphyritic minor intrusion (MITCHELL 1994). As I mentioned before the characteristic mafic phenocrysts in lamprophyres are the hydrous ones (mica and or amphibole) but minor non-hydrous mafic phenocrysts (like cpx and olivine) also occur sometimes. Opx is not characteristic and is unique in a few Bohemian Massif minettes. The Cuchivero sill is not a typical archetypal Group I kimberlite but rather an orangeite (Group II kimberlite) since it contains phlogopite and cpx phenocrysts. The other dikes you mention in Venezuela are lamprophyres.

Best regards

Ioannis

Dear Ioannis, you are right that kimberlites, lamproites and lamprophyres should be considered independently. However, in places, alkaline lamprophyres can have characteristics that are transitional to lamproites. For example, the phlogopite-phyric Truro lamprophyres in South Australia. The attached photographs were taken under crossed nicols. Note the zoning of the phlogopite phenocrysts.

Dear Daniel thank you for the chance to analyse a much debated matter. The lamproite proper rocks (lamproites sensu stricto) frequently contain phenocrysts of Al-poor Ti-rich phlogopite whereas in the Truro lamprophyres (the phlogopite is Al-rich). It is a difficult task to distinguish the two types of phlogopite (for detailed description of lamproites see LE MAITRE ET AL. 2002). Now, it is my personal opinion that there are rocks like alkali minettes, cascadites, cocites, kajanites and prowersites that are distantly related to Mediterranean-type lamproites (lamproites sensu lato). These lamproites have some distinct characteristics such as that they contain crustal plagioclase (PRELEVIC ET AL. 2007).

Best regards

Ioannis

Dear Rameshchandra Phani,

Here is an example of an analcime + carbonate (calcite) ocelli in a camptonite in the context of the rock. One needs to go to the bottom of the interface and select sync views before moving around (due to a bug of the site). It takes a bit of practice before getting it right. There are more carbonates around in the sample but those seem to be due to steatisation and subsequent dolomitisation.

https://viewer.gigamacro.com/compare/5Nkm0DDU8g4xJUoQ/QHRDQfPoetLn4wLb?x1=21884.65&y1=-4826.79&res1=4.36&rot1=0.00&x2=21824.85&y2=-4846.07&res2=4.37&rot2=0.00&comparemode=layeredxray&comparelink=true®1x=22229.94®1y=-4837.25®1resolution=4.360®1rotation=0.00®2x=22170.93®2y=-4856.56®2resolution=4.370®2rotation=0.00

Dear Ciprian Popa, Sebastian Grande,

Thanks a lot for your pictures. Fascinating textures. Apologies for replying after more than a year.

Dear Ioannis, Thank you for the discussion.

Regards,

Rameshchandra Phani.

Dear colleagues

Good day to you all. One of the most famous debates in Igneous Petrology is the relation between the diamondiferous rocks (i.e. lamprophyres, lamproites, orangeites and kimberlites). In 1991, the late geologist Nick Rock considered them to have similar petrological and geochemical signatures and were included in one group named the "Lamprophyre Clan". Recent publications have shown that relations do exist (e.g. see The "Lamprophyre Clan" Revisited 2022 paper in ResearchGate). The Version of Record is available online at: https://doi.org/10.1007/s12594-022-2153-4. One can also read the Version of Record through the Springer SharedIt link https://rdcu.be/cVljF (note that you need to use Wi-Fi in order to open the Springer SharedIt link).

On the other hand igneous petrologist Roger Mitchell, who disagreed with the idea, proposed the "Lamprophyre facies" concept which includes rocks that formed under volatile-rich conditions. Which one is correct? GPT-4 was also asked. The answer was that both terms can be correct, but they represent different perspectives in the study of these rocks. What is your opinion? Please comment.

Best regards

Ioannis Kamvisis