Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

Round 1

Reviewer 1 Report

I found your manuscript interesting, particularly as I have worked at the Longonjo carbonatite, also in Angola, but the topic will also be of interest to many readers of Minerals. However, I found a number of difficulties with the manuscript so cannot recommend publication at this point. I have made numerous changes to the text in the annotated pdf file that I hope will help with English grammar and clarity of meaning. More fundamental issues of science are listed below. I hope you will persist with getting your work published and that the following comments will assist.

Organisation should be improved. The various components of the breccia that are the focus of this study are described first in terms of textural relationships (section 5) but are differentiated as well by chemical compositions - before the chemistry of the pyrochlore and other minerals is presented. This is unsatisfactory. I recommend that section 5 and section 6 (Mineral chemistry) be combined so that the five or more generations of pyrochlore can be fully defined in textural and compositional characteristics. Combining the two sections this way should lead to significant shortening of what is already quite a long manuscript. Some of the interpretations of images showing textural relationships between different minerals are unconvincing to me. In fact, I would interpret some in the opposite way to those of the authors. Although most of the paragenesis seems reasonable, textural evidence of replacement relationships is notoriously contentious, and this issue may not be easily resolved. The various generations of pyrochlore appear to be mislabelled in some images (e.g. Fig3a, 4a). The abbreviation IMCL for Intrusive Mixed carbonatite-Lamprophyre complex is inconsistently applied in various parts of the manuscript. The use of mesostasis as a synonym for groundmass is inappropriate. The English grammar and sentence construction need serious attention; in places leading to confusion or lack of clarity.

Comments for author File: Comments.pdf

Author Response

ANSWERS TO REVIEWER#1

 

Line 43. The author remarks that 40 citations are not required. In fact, many of these citations will be cited again along the text, but they are synthetic about the general characteristics of carbonatites. We propose to preserve the format.

 

Line 72. We broke sentence in two and rewrote both.

 

Line 88. The reviewer proposed to remove the sentence about dating of the rocks. We agree that without indicating these dates the information can be poor, and the second referee proposed to introduce the full data. Therefore, we had introduced these data instead removing the sentence.

 

Line 96. We introduced the number of polished samples (130).

 

Line 116. Reference to figure was introduced.

 

Line 129. We changed “mesostases” for “rocks”. In the rest of the manuscript we changed “mesostase” for “groundmass” because we used this word in a wrong way.

 

Line 131. IMCL bodies are not breccia, we considered them as mixture heterogeneous bodies.

 

Line 134. We added lamprophyric mesostase

 

Line 149. Figure 1, Geological map. We introduced the sample symbols and changed “Fracture” for “Fault”. In the text, we highlighted that magmatic breccias where we found IMCL bodies are locally extremely heterogeneous. Therefore, we put the yellow start to indicate the location of IMCL bodies we studied. We are not be able to know if IMCL bodies could outcrop in other areas, mainly because the sampling of external breccias was very difficult because they occur at highly vertical cliffs.

 

Line 168. We renamed phoscorite xenoliths to olivine phoscorite xenoliths.

 

Line 171. As observed in the new Figure 3, we observed two types of ICML bodies based on groundmass composition. However, we did not observe their contact in any sample or outcrop. In fact, the breccia and ICML bodies surrounding the central unit are a highly mixed unit. For that reason, we did not distinguish rock contacts. We cannot answer you if alnöite spheroids are contained into aillikite spheroids. We only could observe alnöite spheroids into an aillikite groundmass.

 

Line 174. We changed “significant” for accessory mineral

 

Line 180. We agree. We changed “REE carbonates” by “REE fluorocarbonates”

 

Lines 186-188. This texture is probably better explained as a skeletal crystallization of pyrochlore that should be mantled by late primary zirconolite and magnetite. Changed in the text and figures.

 

Line 190 and 193. We agree. We deleted the compositional description of secondary pyrochlore because it is fully explained at section 6 (Mineral chemistry).

 

Line 204. We changed the awkward sentence for the suggested one.

 

Line 233: Changed the legend of Figure 4a. Perhaps can be interesting to compare with figure 5b; zirconolite could be formed in a late pulse during the pyrochlore growth.

 

Line 244: We introduced the coma.

 

Line 270. We changed “Zirconolite” by “Calzirtite. 

 

Line 273: We think that this is explained in the text in the first paragraph “Unlike the apatite-magnetite phoscoritic xenoliths, the phoscorite xenoliths contain major apatite, magnetite, ilmenite and serpentinized olivine with trace amounts of phlogopite-tetraferriphlogopite, calcite and richterite but do not present pyrochlore”.

 

Line 281. The section 5.2 corresponds to a short introduction of the IMCL bodies of aillikite groundmass where we only wanted to list the type of xenoliths identified. However, at section 5.2.1 we explained the petrography of aillikite groundmass in detail.

 

Line 288. Nb perovskite is too difficult to see because Nb rims are very thin in aillikite groundmass and most of perovskite is replaced by rutile. Therefore, we change the position of Figure 5a to illustrate the major minerals.

 

Line 290. We added “Richterite plus dolomite”.

 

Line 290. In order to avoid a wrong interpretation, we changed the Figure 5a. In the new one, we want to show magmatic pyrochlore in a carbonatite xenolith cemented by carbonatite groundmass.

Figure 5e: We changed red labels for yellow ones.

 

Line 294. We agree with your explanation. Pyrochlore I and zircon are replaced by zirconolite formed at their contact. Finally baddeleyite is the latest mineral, related to richterite.

 

Line 328. We changed “external rims and inner cores”.

 

Line 336: Similarly as aillikite spheroids, alnöite spheroids, Nb perovskite forms very fine rims surrounding primary perovskite. In Figure 5e, Nb rims were not visible. We changed the position of “Figure 5e” in order to illustrate the major minerals well identifiable.

 

Line 337. We reanalyzed the replacement texture illustrated at Figure 6e. On the left hand of the photo, magnetite and perovskite I cocrystallize and display symplectitic texture. 

 

Line 338. We added a schematic diagram showing the units and their relationships composing IMCL bodies of carbonatite and aillikite groundmass. However, we did not observe rock contacts among these two domains and their extension. As we said at line 149, external breccias occur in a vertical cliff so their sampling was extremely complicateand it did not allow to distinguish better the studied domains.

The new figure is renamed as “Figure 3” At the beginning of section 5. Therefore, the following figures were also renamed.

 

Line 342: We agree that most of the pyrochlore of IMCL bodies has similar composition as the other units of Bonga. We changed the sentence for: “Pyrochlore types present compositions which may differ from the generations previously defined in other studies of the Bonga carbonatites carried out on the plug or the concentric dykes”.

 

Line 348. Figure 6b: Changed. The relationship between apatite and richterite is not enough representative of replacement, which is seen well in other images. However, the most remarkable in this image is the replacement of pyrochlore by zirconolite.

Figure 6c: We agree that Nb-rich perovskite (PrvII) is not distinct in this photo. It corresponds to Nb rich band, weakly brighter than type I perovskite. For that reason, we deleted “PrvII” from the figure 6c.

 

Line 363. We referred Figure 4, 5 and 6 into the Mineral Chemistry section.

 

Line 371. We removed the repeated sentence about vacancies in the magmatic pyrochlore I.

 

Line 406. We referred Figure 7 and 8 in the Mineral chemistry section. Color of pyrochlore IV symbol was changed in order to differentiate them from pyrochlore I.

 

Line 423. Many pyrochlore symbols are in the same location. We added detailed compositional diagrams for each unit at the Supplementary Material folder in order to present data more clearly.

 

Line 449. We changed the Figure 9 to improve perovskite III symbol.

 

Line 465. We changed “relevant” for “significant”.

 

Line 525. We agree. We changed the sentence to be more logical.

 

Line 536. We agree with you. The sentence is too long. We changed the sentence for “The simultaneous removal of Nb, Zr, and Ti in the different units of the IMCL mixtures during late hydrothermal processes may explain the widespread precipitation of late zirconolite and ilmenite.”

 

Line 546. We agree with you. We removed it.

 

Line 566. We agree. We changed the sentence for “It is also another argument to indicate that these units were formed late during the carbonatite processes after the crystallization of early F rich magmas.”

 

Line 582. We agree. We added “… but potentially lead to an unfragmented exploration target.”

 

Line 614. We added the following reference: Lee et al (2006) [32].

 

Line 617. We changed “highly viscous magmas” for “low viscous magmas”.

 

Line 629. We explained why mingling is produced mainly by the intrusion of immiscible lamprophyre magma into the carbonatite magma chamber. We added the sentence: “Hence, successive intrusion of immiscible lamprophyre magmas and its partial crystallization and degassing caused when entering into contact with a cooler magma, could have favored the overpressure of the magma chamber leading to its rupture (Refs.....). This cracking of the host rock would have favored degassing and displacement of carbonatite magmas through the margins of the magma chamber, which could be enriched in pyrochlore and may also form cone sheets intruding from the magma chamber towards the surface, enriched in dense phases”.

We also added new references to support this thesis. We changed the following references.

 

Line 645. We agree. We added “overpressure, probably as a result of fluid exsolution in the magma chamber”.

 

Line 650. We agree with your comment. Most of carbonatites will have Na and Ca pyrochlore in early stages. Early magma could be sodium rich as suggested in discussion section.

 

Line 652. We rewrote the section 7.3 in the past tense as suggested.

 

Line 657. We added “carbonatite plug”.

 

Line 659. We added “also due to fenitization”.

 

Line 703. Isotopic data told us that fluids had meteoric origin. We added “meteoric”.

 

Line 742. One of the key question of the paper is that we consider that mingling may lead to eruptive processes and therefore are a key mechanism of carbonatite breccias. We agree, however that mingling process may help to produce the exsolution of fluids as we indicated in the references.

Author Response File: Author Response.pdf

Reviewer 2 Report

An interesting manuscript, which used pyrochlore composition to look at the evolution of a carbonatite in Angola.

My comments are all minor and appended to the pdf file. There are a few places where more information is needed - give the ages up front, and give the d13C and d18O values, before discussing the possibility of alteration. By and large it is well-written and I have suggests only a few minor modifications to the text. The figures/tables are all appropriate.

Comments for author File: Comments.pdf

Author Response

ANSWERS TO REVIEWER #2

All the suggestions made by this referee and annotated on the pdf manuscript were introduced in the fina text. We are acknowledged with this referee because all these corrections helped improve the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript is considerably improved this time around. I find the new Figure 3 very useful. There are still some inconsistencies in the abbreviation (IMCL). All ar highlighted in the annotated manuscript, along with some remaining grammatical corrections.

There is still some doubtful labelling of Figures (in my mind) showing microscope and SEM views. I have indicated where this is so in the annotated manuscript; please check and make sure the labels are correct.

Otherwise, an interesting read. Thank you.

Comments for author File: Comments.pdf

Author Response

 

ANSWERS TO REVIEWER #1 (Second round)

 

Line 20: We agree, so we added “proxy of magmatic-hydrothermal evolution of the complex”.

Line 20: We changed “could lead” to “led”.

Line 23: We changed “could reduce” to “reduced”.

Line 28: We changed “producing” to “produced”.

Line 32: We changed “be produced” to “have been deposited”.

Line 76: We changed “highest concentrations of rare elements” to “highest concentrations of Nb and REE”.

Line 129: We changed “carbonatite core” to “carbonatite plug”.

Line 132: We changed “mixture” to “mixed”.

Line 134: We changed “will allow” to “allows”.

Line 166: We changed “to be calculated” to “to calculate”.

Line 166: We changed the” to “of”.

Line 167: Yes, we wanted to say that ICML are 100 m width.

Line 196: We rewrote the text. Type III pyrochlore is overgrowing type II pyrochlore, which is brighter. Primary pyrochlore is fully replaced by secondary generations.

Line 199: We rewrote “generation” to “generations”. We spoke about three previous pyrochlore generations.

Line 212: Ok, rhabdophane-(Ce) is observed at Figure 4d. We changed it in the manuscript.

Line 218: We changed “Figure 5a” label to represent mineral composition of carbonatite xenoliths. “Calcite is more than 95% of the modal content and is accompanied by minor amounts of other minerals such as apatite, magnetite and at least two generations of pyrochlore (Figure 5a).”

Line 225: We changed the “has” to “forms”.

Line 225, figure 5b: We changed the text. Type I pyrochlore is not observed in this image because it was totally replaced by type II pyrochlore.

Line 252: We changed the “into” to “in the”.

Line 277: We changed the “Phoscorite xenoliths” to “Olivine phoscorite xenoliths” and to distinguish them from magnetite-apatite phoscorites.

Line 286 to 288: We introduced suggested changes.

Line 297, figure 5d: Yes, Pcl I and Pcl II are labelled correctly. We based on SEM-EDS data. Pcl I is darker and Pcl II is brighter. In this case Pcl II replacement is irregular forming patches.

Line 343: We defined Type I and Type II perovskite both at 5.2.1 section of “Aillikite Groundmass” and 5.2.4 sections of “Alnöite spheroids”. We also referred Type III (Na-and Nb-rich perovskite) and Type IV (LREE perovskite) generations.

Line 344 and figure 6f: We studied Figure 6f in detail and observed that magnetite could crystallize later and as you suggested, it may replace perovskite. It is observed on the left hand of the image. We changed the explanation in the manuscript.

Line 435: At section 5.2.4 of Alnöite spheroids, we added a description for Type I to IV perovskite to characterize each generation.  

Line 636:  We changed “absent” to “undetected”.

Line 649:  We changed “could had” to “could have had”.

Line 651:  We added “were concentrated”.

 

All changes are highlighted in yellow at the attached document. 

 

 

Author Response File: Author Response.pdf