Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)
and Joan Martí Molist 4
Round 1
Reviewer 1 Report
General comments :
Overall, the manuscript describes with great detail the mineralogy and in-situ elemental geochemistry of mineral phases in the Monte Verde alkaline complex, Angola, and discusses the distribution of HFSE and REE elements along with some brief economic considerations.
The mineralogy and paragenetic sequences are complex in these (fantastic) materials. However, the reader is quickly overwhelmed and lost by this complexity because the paper uses only text descriptions and BSE images. My opinion is that an effort should be made to clarify things especially by using graphical support for the presentation of paragenetic associations and sequences, perhaps with a focus on the observations that are critical for metallogenic aspects. A table with mineral data should also be included (group, formula, etc.). BSE figures could be improved in terms of contrast and labelling. It also appears that some thin-sections have poor polishing quality, which is not ideal for getting high-quality micrographs (and micro-analyses).
I think this study would have greatly beneficiated from the use of cathodoluminescence for imaging those minerals that are luminescent such as feldspars, calcite, apatite, fluorite, zircon, strontianite and likely other minerals. CL is also useful for revealing growth zones and other crystal textures, which helps in deciphering replacement, recrystallization processes, etc., but also REE activation and fenitization. But this requires access to a CL equipment and extra analytical work, which is not deemded mandatory in the frame of this review.
While this is seemingly not the primary scope of the paper, much less detail is given regarding field and macroscopic relations, which unfortunately limits a lot some topics in the discussion, for example in section 4.1 (Evolution of the Monte Verde complex). I think that such data could have been useful for discussing the economic potential as well.
Although I myself don’t feel comfortable with judging the English, I think the phrasing could be substantially improved in some sections of the manuscript.
Detailed comments, suggestions and corrections :
See annotated manuscript in pdf format
Comments for author File: 
Comments.pdf
Author Response
COMMENTS REVIEWER 1
(Yellow marks on the Final Revisited Manuscript)
All corrections referred to English was changed as suggested.
Line 13: “Carbonatite Complex”, is this terminology appropriate given the scarcity of carbonatite in this alkaline complex? We agree with your suggestion. Therefore, the Monte Verde Complex was renamed as “carbonatite-alkaline complex”.
Line 18: “Replaced” was changed by “overgrown”.
Line 20: “Types” was changed by “generations”.
Line 21: “Types” was changed by “generations”.
Line 28: “Hidden deep” was changed by “buried”.
Line 43: “Roughly described” was changed by “poorly constrained”.
Line 48: “Remarkable” was changed by “significant”.
Line 69: “Performed” was changed by “studied”.
Line 71: “Being studied” was changed by “further analysis”.
Line 77: Each mineral was analysed using different operating conditions at specific programs.
Line 79: We put a table (renamed as Table 1) in order to improve the readability of standards and crystals used by WDS detector. Counting time on peak and background, acquisition method and beam diameter were detailed at Section 3. To avoid spectral interference concern with REE, Ce and La were measured using Lα lines, while Pr, Nd, Gd and Sm were measured using Lβ lines and thus no peak overlap correction was required. The same applies to U, which was measured using the Mβ line.
REE standards are synthetic glasses containing each REE available from P&H Development Ltd.
https://www.pandhdevelopments.com/reeg6.html
Line 106: Figure 1, geological map. We checked the map scale and we confirmed that it was wrong. Therefore, we changed the map scale.
Line 118: We added “unaltered”.
Line 122: We deleted “and with no sphericity”.
Line 126: We changed “depending” by “are distinguished based”.
Line 127: “hosting” was changed by “host” and we added “with respect”.
Line 134: “Plutonic facies, breccias and fenites” was changed by “rocks”.
Line 141: “Remarkable” was changed by “significant”.
Line 161: Rephrased as “cancrinite (Ccn) pseudomorphs after nepheline”.
Line 163: A table with name, group and formula was provided in order to recognize all minerals.
Line 165: “With a sequence” was deleted.
Line 166: “Replacing” was deleted.
Line 168: “Smectites” was substituted by “smectite”.
Line 173: Figure 3f: “FosfLaCe” was renamed as “PhospLaCe”.
Line 189: “Describing” was changed by “forming”
Line 192: The thickness of fenites was added to the text.
Line 196: We rephrase the highlighted sentence to “However, a Na-rich alteration assemblage formed by albite, richterite, nepheline and sodalite overprinted the K-rich one. On its turn, both associations were replaced by restricted quartz and calcite”.
Line 203: “Produce” was changed by “produced”.
Line 206: We rephrase the highlighted sentence to “a first generation has very finely grain-sized and hematite inclusions whereas a second type does not contain Fe oxides”.
Line 211: We renamed “mesostase” to “groundmass” at 3.2.3.1 section title.
Line 224: “They are cut on its turn or overgrown” was changed by “They are cut or overgrown”.
Line 226: A graphical paragenetic sequence for carbonatite breccia unit was provided. The new Figure 5 displays the sequence of crystallization of breccia formed by carbonatite fragments cemented by alkaline groundmass, whereas new Figure 6 shows breccia composed by fenitized granite clasts surrounded by a carbonatite groundmass.
Line 234: “Calcite rounded inclusions” was changed by “inclusions of rounded calcite grains”.
Line 237: “Calcite-crystals” was changed by “calcite crystals”.
Line 239: “Two types of alkaline mesostase appear surrounding carbonate clasts” was changed by “Two types of alkaline groundmass are observed in the alkaline magmatic breccia”.
Line 240 and 243: “these tend” was changed by “which tends”.
Line 252: “Replace” was changed by “Replaced”.
Line 271: We modified contrast and brightness of Figure 4a to better visualize primary pyrochlore replacement for Sr-rich pyrochlore rims. However, we did not have available a magnification micrograph.
Line 280: We increased contrast of Figure 4e in order to show how zircon replaced richterite crystals in the foidolitic type groundmass of alkaline breccias.
Line 288: “The last” was changed by “the latter”.
Line 299: “Replacing” was substituted by “overgrowing”.
Line 318: We added “they”.
Line 319: “Interstitially among” was changed by “interstices between”.
Line 327: We provided probe size in section 2.
Line 329: We changed section 3.3.1.1 title by using “Primary” instead “magmatic”. Also, we put brackets surrounding “type I”.
Line 334: “Lesser” was changed by “Less”.
Line 336: “Totally absent” was changed by “undetected”.
Line 346: We added “for”.
Line 355, Figure 6: Evolution trends were added to ternary diagrams of pyrochlore generations. Most of them are observed in most carbonatite complex, as Lumking and Ewing (1995) and Melgarejo et al. (2012) previously suggested.
Line 359: “A site changes” was changed by “A-site occupancy”.
Line 362: “Variation” was changed by “subtype”; “Impoverished” by “depleted”;
Line 367: “Principal” was changed by “Predominant”.
Line 396: We added “Eudialyte type”.
Line 398: “ZrO2” was changed by “Zr”.
Line 399: “Entry” was changed by “enter”.
Line 403: The following sentence “In addition, rims have high Ce2O3 and La2O3 contents (1.55 and 1.61 wt% respectively) substituting Na2O” was changed by “In addition, rims have high Ce and La contents (1.55 and 1.61 wt% Ce2O3 and La2O3 respectively) substituting for Na” as suggested.
Line 406: “This group of minerals” was changed by “Minerals of this group”.
Line 428: Figure 8. We deleted commas and the second dots in the references used in the Figure. We also arranged datasets in order to match correctly in both plots. The “B” tag in the second diagram (Mn2+ vs. Nb5+) was hidden one dataset.
Line 434: Few REE carbonate concentrations locate” was changed by “REE carbonates were found”.
Line 440: “Ba and Sr may entry considerably at the B-site” was changed by “Ba and Sr may enter into B site”.
Line 453: “Presents” was changed by “Exhibit”.
Line 483: This is not really a reconstruction but rather a few points regarding chronology. A sketch would have helped. Anyway, this section seems a little bit short in discussion and supporting evidence (but I understand this is not the aim of the paper). Yes, this was not the aim of the paper, but we are preparying other additional papers on this subject.
Line 490: “It is assumed the existence of magma mingling between a carbonatitic and an undersaturated alkaline magma” was changed by “the existence of magma mingling between a carbonatitic and an undersaturated alkaline magma is assumed”.
Line 510: We added “replacing and overgrowing”.
Line 511: “REE entry to equilibrate charge deficit” was changed by “REE increase to balance charges”.
Line 518: “Which can be able at high temperature to accommodate these elements” was changed by “which can accommodate these elements at high temperature”.
Line 519: “hosting” was changed by “host”; “Replaced” was changed by “displaced”.
Line 532: “Can be released” was changed by “could originate”.
Line 536: “Must calculate the possible three-dimensional geometry of this complex”. Our idea was that a possible carbonatite body of great dimensions may occur at depth. When comparing with other carbonatite complexes studied by our team in Angola, we concluded that only the most superficial part of the complex is outcropping, and for that reason carbonatite rocks are scarce and they almost do not outcrop.
Line 538: We added “Breccia bodies”; “Set” was changed by “hosted”.
Line 539: “Far of” was changed by “away from”.
Line 546: “Extensions” was changed by “Areas”.
Line 547: “Hidden external” was changed by “More distal”; “below” was changed by “beneath”.
Line 550: “Metallogenetic objective” was changed by “metallogenetic prospects”.
Line 572: “Because it is difficult to make a deduction of the distribution of this process in the carbonatite” was changed by “because it is difficult to make a deduction of how hydrothermal processes distribute REE minerals in the carbonatite”.
Line 579: “Existing” was changed by “documented”.
Line 580: “Indicative” was changed by “indication”.
Line 585: “Reserves” was changed by “resources”.
Line 582: In Tchivira, the flanks of the carbonatite intrusions are strongly mineralized in fluorite and secondary generations of pyrochlore, which replaces the primary carbonates along with quartz and ankerite” was changed by “In Tchivira, the flanks of the carbonatite intrusions are strongly mineralized in fluorite and secondary generations of pyrochlore replacing the primary carbonates along with quartz and ankerite”.
Line 585: We added “magmatic product”.
Line 588: “Existing” was changed by “Known”.
Line 590: We added “possible existence”.
Line 596: “By main zircon” was changed by “mainly by zircon”.
Line 597: “Mostly of them” was changed by “Most of them being”.
Line 600: “In spite of containing” was changed by “Although they contain”.
Line 613: “Contains” was changed by “exhibits”; we added “the magmatic one”.
Line 615: We added format with subscript.
Line 623: Yes, those concentrations were for Nb2O5 and REE2O3. We changed it in the text.
Line 624: “At the Lovozero massif at the Kola Peninsula” was changed by “In Lovozero massif, Kola Peninsula”.
Line 636: We added “buried”.
Line 640: “2km of radius” was changed by “5 km of radius” after fixing the map scale of Figure 1.
Line 645: “Structures” was changed by “bodies”.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Nb and REE distribution at the Monte Verde carbonatite – alkaline – agpaitic complex (Angola)
Sandra Amores-Casals1,2*, Antonio Olimpio Gonçalves3, Joan Carles Melgarejo1 and Joan Martí Molist4
This manuscript studied the alkaline rocks and the carbonatite breccias at Monte Verde. The study presented a detailed work on the accessory minerals in those rocks. The pyrochlore in those rocks has been grouped into five generations and REE minerals study has also provided fruitful results for the magmatic processes. The evaluation volatile activities in the magmatic system can provide extra information of the evolution of Monte Verde volcanic system. The manuscript is well written and is publishable with minor revision.
Following are the points for authors to consider:
Line 64-67: Extensions of both Cretaceous and Quaternary have been mentioned. Are you suggesting that a potential relation exists between them? Is there any expectation here to say that the Lucapa graben was under extensional condition since Cretaceous and lasted till Quaternary? If not, it may need to rearrange the sentences. Otherwise, is it possible that the alkali system in the Lucapa graben were formed from Quaternary extension?
Line 69: “performed” change to “made”
Line 79-96: 1) what are those REE-3 standards? Oxide or phosphate? 2) There is no need to repeat same standard for same elements.
Line 84: (PbS should be PbS.
Line 68-96 section: Since no data table is listed in the manuscript, it might need to give some explanation how you work on your probe data, for example, in some mineral chemistry tables, there is a sum REE value column; is that for the total of analyzed light REE? Since no HREE is analyzed in the work, it is better to use make a note of what the “sum REE” means in the table.
Line 109: “according” should be “according to”.
Line 111-112: “also present” should be “have”.
Line 116: “have replaced extensively” should be “have extensively replaced”.
Line 116-117: “increasing the alteration near the carbonatite and plutonic alkaline rocks” should be “the alteration is more intensive near the carbonatite and plutonic alkaline rocks”.
Line 217: “At” should be “In”
Line 221: “may” should be “can”
Line 230: “At” should be “In”
Line 292: “A) Rosenbuschite-mosandrite (Ros-Mos) poikilitic crystals replaced by rinkite (Rin)…”, why these need to be a replacement relationship, could they all crystalized together in the same stage and formed the poikilitic texture?
Line 294-295: “B) Perovskite (Prv) and loparite (Lpr) replaced by titanite (Ttn) and niobian titanite (Ttn-Nb) with cancrinite (Ccn) and albite (Ab).” Why Prv and Lpr was the original phase and been replace titanite? Is it possible that Prv and Lpr was formed by a deuteric reaction? In this image, loparite seems showing veins within perovskite.
Line 310: “… may be replaced by loparite–(Ce) veins and Nb-Ti zoned titanite” seem not matching with the figure caption in B). Please check.
Line 463: “ … rare REE rare minerals …” should be “… rare REE minerals …”. It looks like that lack of HREE minerals exist in the rock, what might cause that?
Line 482: “Although the volume of geological data is relatively reduced because of the reduced dimension” should be “Although the volume of geological data is relatively restricted because of the limited dimension …”
Line 485: “Carbonatitic breccias”, is this equivalent to the “3.2.3.2. Breccia with carbonatite matrix”? If so, try to use the same term.
Line 491: “the existence of magma mingling…”, in the rocks of this study, in there any evidence of this magma mingling? Can carbonatite magma be a separated magmatic event than the alkaline magma?
Line 494: “use to be formed…”, awkward, want to say “can be formed”? or “used to be forming”?
Line 539: “small outcrops of carbonatite breccias occur up to 2 km far of the central alkaline plug”, can this caused by landslide or any surface effect?
Figures: The labels on images can be considered cleaning up, do not need to repeat same label.
Table: May consider providing representative mineral chemistry table in the manuscript.
Author Response
COMMENTS REVIEWER 2
(Purple marks on the Final Revisited Manuscript; our comments are here in red).
All corrections referred to English was changed as suggested.
Line 64-67: Extensions of both Cretaceous and Quaternary have been mentioned. Are you suggesting that a potential relation exists between them? Is there any expectation here to say that the Lucapa graben was under extensional condition since Cretaceous and lasted till Quaternary? If not, it may need to rearrange the sentences. Otherwise, is it possible that the alkali system in the Lucapa graben were formed from Quaternary extension?
Giuliani et al. (2017)[1] demonstrated that Catanda carbonatite lavas formed during Quaternary extension of Lucapa graben.
Line 69: “performed” change to “made”.
We changed it.
Line 79-96:
1) What are those REE-3 standards? Oxide or phosphate?
REE standards are synthetic glasses containing each REE available from P&H Development Ltd.
https://www.pandhdevelopments.com/reeg6.html.
2) There is no need to repeat same standard for same elements.
We put a table (renamed as Table 1) to improve the readability of standards and crystals used by WDS detector.
Line 84: (PbS should be PbS.
We changed it.
Line 68-96 section: Since no data table is listed in the manuscript, it might need to give some explanation how you work on your probe data, for example, in some mineral chemistry tables, there is a sum REE value column; is that for the total of analyzed light REE? Since no HREE is analyzed in the work, it is better to use make a note of what the “sum REE” means in the table.
Sum REE corresponds to a sum of light rare earth elements: La, Ce, Pr, Nd, Sm and Gd analysed. We changed excel files adding what means “sum REE”.
Line 109: “according” should be “according to”.
We changed it.
Line 111-112: “also present” should be “have”.
We changed it.
Line 116: “have replaced extensively” should be “have extensively replaced”.
We changed it.
Line 116-117: “increasing the alteration near the carbonatite and plutonic alkaline rocks” should be “the alteration is more intensive near the carbonatite and plutonic alkaline rocks”.
We changed it.
Line 217: “At” should be “In”
We changed it.
Line 221: “may” should be “can”
We changed it.
Line 230: “At” should be “In”
We changed it.
Line 292 and 307: “A) Rosenbuschite-mosandrite (Ros-Mos) poikilitic crystals replaced by rinkite (Rin)…”, why these need to be a replacement relationship, could they all crystalized together in the same stage and formed the poikilitic texture?
Rinkite-(Ce) was found as secondary phase forming Zr-poor, Ce-rich rims surrounding primary rosenbuschite. It appears in nepheline trachyte dykes but also in melteigite rocks as observed in the figure below, as darker rims. We raised contrast of image shown in Figure 5a in order to better differentiate rinkite patches overgrowing rosenbuschite.
Line 294-295: “B) Perovskite (Prv) and loparite (Lpr) replaced by titanite (Ttn) and niobian titanite (Ttn-Nb) with cancrinite (Ccn) and albite (Ab).” Why Prv and Lpr was the original phase and been replace titanite? Is it possible that Prv and Lpr was formed by a deuteric reaction? In this image, loparite seems showing veins within perovskite.
We have been revisiting the texture. We think that perovskite and titanite were primary minerals and a secondary alkali-, Nb-rich fluid replace both minerals forming Nb-rich titanite and loparite. This explanation would be more plausible than the previous one. We changed texture description at the manuscript.
Line 310: “… may be replaced by loparite–(Ce) veins and Nb-Ti zoned titanite” seem not matching with the figure caption in B). Please check.
We changed the figure caption in 5B and the manuscript. Therefore, loparite and Nb-titanite replace primary perovskite and titanite.
Line 463: “ … rare REE rare minerals …” should be “… rare REE minerals …”.
We changed it.
It looks like that lack of HREE minerals exist in the rock, what might cause that?
Alkaline rocks of Monte Verde do not contain xenotime, zircon, monazite, fergusonite, allanite which usually accumulate HREE. According Chakhmouradian & Wall ( 2012)[2], HREE behave more mantle-compatible than the LREE during partial melting and occur relatively rarer in nature. Peralkaline feldspathoid rocks such as urtite-melteigite-ijolite series form Monte Verde complex, which in turn volcanic alkaline rocks of similar affinity cut them. Generally, this type of association may present LREE-rich minerals such as loparite, LREE phosphates, zircon and titanosilicates or eudyalite and monazite or REE carbonates are lacking (Chakmouradian & Zaitsev, 2012)[3]. In addition, carbonatites usually contain Na-Ca-Sr-Ba-REE carbonates showing a strong enrichment in LREE than HREE.
Line 482: “Although the volume of geological data is relatively reduced because of the reduced dimension” should be “Although the volume of geological data is relatively restricted because of the limited dimension …”
We changed it.
Line 485: “Carbonatitic breccias”, is this equivalent to the “3.2.3.2. Breccia with carbonatite matrix”? If so, try to use the same term.
Yes, we would like to say “the breccia with carbonatite matrix”.
Line 491: “the existence of magma mingling…”, in the rocks of this study, in there any evidence of this magma mingling? Can carbonatite magma be a separated magmatic event than the alkaline magma?
The existence of magmatic breccia at the Monte Verde complex suggests mingling of carbonatite and silicate alkaline magmas. In fact, recently we studied highly heterogeneous magmatic breccia of the Bonga complex (near to Lubango, Angola). We proposed magma mingling to explain its formation. In fact, alnöite and aillikite spheroids together with carbonatite fragments and groundmass allowed interpreting mingling of carbonatite and lamprophyre magmas (Amores-Casals et al., 2019)[4]. At the Monte Verde complex, we did not find such textures but also magmatic breccias occur.
Line 494: “use to be formed…”, awkward, want to say “can be formed”? or “used to be forming”?
We wanted to say “can be formed”
Line 539: “small outcrops of carbonatite breccias occur up to 2 km far of the central alkaline plug”, can this caused by landslide or any surface effect?
Our idea is that a possible carbonatite plug may occur at depth. In fact, we only see the most superficial part of the complex. Carbonatite breccia with calciocarbonatite fragments evidence it.
Figures: The labels on images can be considered cleaning up, do not need to repeat same label.
We changed it.
Table: May consider providing representative mineral chemistry table in the manuscript.
Because of the great variety of mineral phases we considered to put all mineral chemistry results in the Supplementary material. They were separated by pyrochlore, perovskite-loparite, eudialyte, Na-, Zr-, Ti-rich sorosilicates (rosenbuschite, rinkite, etc), rhabdophane and REE silicates and REE carbonates files. However, we provided a new Table 2 containing name, group and formula of all minerals mentioned.
[1] Giuliani, A.; Campeny, M.; Kamenetsky, V.; Alfonso, J.C.; Maas, R.; Melgarejo, J.C.; Kohn, B.P.; Matchan, E. L.; Mangas, J.; Gonçalves, A.O.; Manuel J. Southwestern Africa on the burner: Pleistocene carbonatite volcanism linked to deep mantle upwelling in Angola. Geology, 2017, 45, 971-974.
[2] Chakhmouradian, A.R., Wall, F. (2012). Rare earth elements: Minerals, mines, magnets (and more). Elements, vol. 8, 333-340.
[3] Chakhmouradian, A.R., Zaitsev, A.N. (2012) Rare earth mineralization in Igneous rocks: Sources and processes. Elements, 8, 347-353.
[4] Amores-Casals, S., Melgarejo, J.C., Bambi, A., Gonçalves, A,O., Morais, E.A., Manuel, J., Neto, A.B., Costanzo, A., Marti-Molist, J. (2019). Lamprophyre-carbonatite magma mingling and subsolidus processes as key controls on critical element concentration in carbonatites-The Bonga complex (Angola). Minerals, 9 (10), 601.
Author Response File: Author Response.docx
Reviewer 3 Report
It is an interesting study that provides information about the Nb and REE distribution at the Monte Verde carbonatite. I think the article is suitable to the Journal providing the authors carry out the improvements to the manuscript (see below). This version of the article suffers from several weaknesses, both from a formal and a substantial viewpoint. Consequently, many revisions are needed to improve even more the manuscript in order to meet the requirements of the Minerals Journal.
The abstract needs to be completed with some results and conclusions. The relevant literature is not fully cited (e.g. Edahbi et al 2017; 2018,Nadeau et al 2015; 2016):https://link.springer.com/article/10.1007/s11356-018-1309-7; https://www.mdpi.com/2075-163X/8/2/55; https://www.sciencedirect.com/science/article/pii/S0169136815300809
I suggest to add a part of sampling to briefly discuss in addition the geological context, the methodology used and the representativeness of studied samples. I suggest to add some mineralogical characterization (e.g. XRD, QEMSCAN or MLA) of the studied materials in order to: Identify and quantify the REE bearing minerals. Evaluate the degree of liberation of REE bearing minerals. Evaluate the mineralogical grain size of REE-bearing minerals, etc. I think you must summarize the bulk rock major and trace element using a table for example because it so difficult to get an idea about the concentrations values with the used graphs. In the results and discussion part, the manuscript is confused in the way it is written. The authors comment their results but they did not discussed them enough (e.g comparison with other carbonatites such as Canadian carbonatites; mobile-to-immobile element ratios; model for the evolution of Monte Verde carbonatite). Also, the results are insufficient and partially interpreted. I suggest also separating results part to the discussion part. The authors discuss only Nb but what about other radioactive elements associated with carbonatites such as thorium and uranium? The manuscript needs to be reorganized in order to make the aim of this study more clear.Author Response
COMMENTS REVIEWER 3
(Green marks on the Final Revisited Manuscript)
a) I suggest to add a part of sampling to briefly discuss in addition the geological context, the methodology used and the representativeness of studied samples.
We could agree with the referee, because we included in many papers sent before these data, but the referees proposed to remove them in all the cases. However, we introduced in the new version some data about the number of studied rocks that were used to ensure the representativeness of the samples. We believe that the number of samples is enough representative, and the referee must take into account the problems found to get these samples in a remote area in Angola.
We added the following text at the line 69: “Sampling and mapping of Monte Verde complex was done during 2014 field work. 95 samples of plutonic alkaline rocks, alkaline dykes, breccia and fenitized granites were selected and collected”.
b) I suggest to add some mineralogical characterization (e.g. XRD, QEMSCAN or MLA) of the studied materials in order to: Identify and quantify the REE bearing minerals. The relevant literature is not fully cited (e.g. Edahbi et al 2017; 2018,Nadeau et al 2015; 2016):
https://link.springer.com/article/10.1007/s11356-018-1309-7; https://www.mdpi.com/2075-163X/8/2/55; https://www.sciencedirect.com/science/article/pii/S0169136815300809
Minerals were characterized with EPMA (complemented by RAMAN when necessary), because in many of the cases XRD is not a solution because the grain size of the crystals is too small. To our knowledge, QEMSCAN or MLA can have problems with complex rare phases, and can have difficulties when identifying polymorphs or minerals of complex groups; these techniques are not available in Barcelona but we prefer to characterize minerals with EDS + EPMA + Raman (when possible). For that reason, references suggested of Edahbi and Nadeau could not fit in our study.
c) Evaluate the degree of liberation of REE bearing minerals.
This is not an objective of the contribution; it could be a possible subject for further investigation.
d) Evaluate the mineralogical grain size of REE-bearing minerals, etc.
The quantitative study of the distribution of sizes is not an objective of the current paper; of course, it would be necessary in case of renewed interest in economic processing of the ores in the area. However, to our mind, many job is necessary yet with classical techniques.
e) I think you must summarize the bulk rock major and trace element using a table for example because it so difficult to get an idea about the concentrations values with the used graphs.
The bulk major rock characterization of the rocks of the area is not an objective of the paper; however, other contributions in this direction are in preparation for the future. We added sequences of crystallization and a resume table of all minerals present in the Monte Verde units in order to facilitate reading.
The mobility of elements in carbonatite/alkaline environments is not the same as in other classical environments.
The evolution of the Monte Verde carbonatite itself is hard to be discussed because there are only outcrops of carbonatite breccias, and drilling data are not available so far.
f) In the results and discussion part, the manuscript is confused in the way it is written. The authors comment their results but they did not discussed them enough (e.g comparison with other carbonatites such as Canadian carbonatites; mobile-to-immobile element ratios; model for the evolution of Monte Verde carbonatite).
The discussion focused more on the similarities with the Angolan carbonatites, which are currently under study.
g) Also, the results are insufficient and partially interpreted. I suggest also separating results part to the discussion part. The authors discuss only Nb but what about other radioactive elements associated with carbonatites such as thorium and uranium?
We think that the results are separated from the discussion, although the complexity of the general discussion requires some partial interpretations in the section of results.
Nb is not a radioactive element, and its study is as a critical element, which is the objective of the paper. Although some elements as U are critical for USA, this is not the case for EU. Moreover, the tenors in these elements are very low and probably in most of the cases can be a penalty more than a resource.
Author Response File: Author Response.docx
Reviewer 4 Report
I wish to congratulate the authors. The work is well written, clear, concise and informative. You really did a good job. At the moment there are various research groups that are addressing this crucial issue and I advise you to update yourself on the latest developments. I have advised a couple of papers but in particular one that is updated for the bibliography from the various groups, especially the Chinese ones. Your work would have more emphasis if you update the bibliography and your information about other similar outcrops elsewhere. few notes in the annotated pdf attached herewith
Comments for author File: Comments.pdf
Author Response
COMMENTS REVIEWER 4
(Blue marks on the Final Revisited Manuscript)
All corrections referred to English changed as suggested.
Line 15: Eburnean granites: We wanted to say that fenitized granites intruded during the Eburnean orogeny.
Line 15: “The ensemble” was changed by “This rock association”.
Line 15: Are those tuffisites in diatremes?
No, we had not found tuffisite facies nor diatremes.
Line 24: So far extrusive rocks have not been cited, where do they occur?
They occur as late dykes cutting plutonic alkaline rocks, fenites and magmatic breccia as we detailed at Line 16.
Line 33: These are not pegmatites ss. may be pegmatoid facies may be appropriate but it is better to define what rock-type they are. There is a specific terminology for peralkaline very coarse grained rocks.
“Pegmatites” was changed by “Pegmatoid facies”.
Line 41: I sow tuffiste samples from Catanda and they are tuffistes (pelletal lapilli plus sovite fragments...do you have the same ??). for tuffisites see.
No, we did not see any tuffisite facies at the Monte Verde complex. Only few magmatic breccia outcrops suggest volcanic activity.
Line 44: You cited extrusive rocks...where are they?
Extrusive rocks identified mainly cut nepheline syenites and phonolite rocks (urtites, melteigites, ijolites) of the central part of the complex, but also fenitized granites and few carbonatite breccia. We found dykes corresponding to nepheline trachytes and alkaline feldspar-, sodalite-rich trachytes with scarce tinguaites and latites.
Line 102: Ok. We added “sub-volcanic”.
Line 116: Eburnean is a formational name but is not reported in figure 1.
We deleted “Eburnean”.
Line 139: Do you mean hydrothermal phase? It seems that this sequence is very typical of this kind of magma evolution and rock association please see discussion and references in:
10.1016/j.oregeorev.2019.103041.
Ok, we added two references of the Italian alkaline-carbonatite complexes at 5.3 Section of the Discussion. At line 571 we added:
“….the textural evidence and the mineral association with quartz suggest a hydrothermal origin for them, as it was also suggested in Tchivira [25]. Other carbonatites worldwide, for instance, Italian carbonatites [56,57], the Amba Dongar complex, India [71-73] or occurrences from Sichuan Province, China [74-76] may display similar late REE mineral association.
Line 186: Eudyalite bearing rocks are interesting please see: Karup-Møller S.; Rose-Hansen J. (2013): New data on eudialyte decomposition minerals from kakortokites and associated pegmatites of the Ilimaussaq complex, South Greenland. In: Geological Society of Denmark. Bulletin, Vol. 61, 2013, p. 47-70.
The unknown primary phase observed at the andradite-bearing nepheline syenite decomposed to zircon, wöhlerite, calcite, albite and pyrochlore, so its composition could correspond to a zirconosilicate such as eudiayte as we detailed in the manuscript. We added the reference of eudyalite decomposition of Illimaussaq Complex and a brief comment at Line 611 (Section 5.4).
“In addition, andradite-bearing nepheline syenite contain pseudomorphic eudialite completely replaced by a secondary association of zircon, wöhlerite, pyrochlore, calcite and albite. Eudialyte decomposition is described at kakortokite series of the Illimaussaq complex and resulted in the formation of catapleiite and two phases A1 and A2 where most of the REE concentrate [84].
Line 209: Fenites usually consider barren rock if you have evidence that they are mineralized this is important please see the papers of Francesc Wall.
Fenitized granites have zircon and Ba-rich pyrochlore but both minerals are scarce and trace phases. This is not an important mineralization; we agree that fenites are barren rocks.
Line 257: I recommend making the maximum effort to define these brecciae that deserve a better textural description. Please consult the specific papers and refer the reader to summary papers regarding these interesting rocks. Many observations can be deduced from two main papers and from the bibliography contained in them, (the second also contains updated references to outcrops similar to yours with similar evolution and concentration of Nb and REE) I suggest reading them and quoting them because they contain an updated bibliography on the topics treated by you:
DOI: 10.1016 / j.gr.2016.07.001
10.1016 / j.oregeorev.2019.103041
We added the suggested references at the 5.1 Section inside the discussion.
Line 492: “The association of carbonatites and alkaline rocks is a well-known fact and has been documented also in the currently active natrocarbonatite volcano at Oldoinyo Lengai in Tanzania; in this case, the existence of magma mingling between a carbonatitic and an undersaturated alkaline magma is assumed [54]. In addition, very recent studies about Italian carbonatite alkaline complexes display immiscible separation of silicate and carbonatite liquids [56,57]“
Line 323: Amazing resemblance to recently described outcrops from Italy, China and India. I think that to give importance to the afficormaneto that described in your paper and to the processes that you hypothesize, it is important to put them in the petrological context of these rocks worldwide.
We added some reference at the Section 5.3 to give a petrological context worldwide apart from Angola. [Line 571: Other carbonatites worldwide, for instance, Italian carbonatites [56,57], the Amba Dongar complex, India [71-73] or occurrences from Sichuan Province, China [74-76] may display similar late REE mineral association.]
Line 404: Other important deposits of agpaitic rocks are the Khibina and Lovozero complexes on the Kola Peninsula (Russia), Mont Saint Hilaire in Canada, the Tamazeght complex in Marrueco and Narra Karr in Sweden. All are characterized by extreme enrichment in alkali metals, halogens (F, Cl, Br) and the HFSE lithophilic elements (Zr, Ti, Y, Nb, Ta and rare earth elements) and many host exploitable resources of those elements.
Thank you for your comment. In the 3.3.3. Section of eudialyte mineral chemistry, we compared our results with eudialyte data from Illimaussaq, Tamazeght and Mount Saint Hilaire alkaline complexes.
Line 663: I suggest citing a couple of new paper dealing with this topic and especially the paper 10.1016/j.oregeorev.2019.103041
Yes, we added new references suggested they are very useful. Thank you.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The new version of this paper is a little improved. However, the authors did not answer my questions adequately. Some of their answers contain contradictions, which is unacceptable and completely wrong “XRD is not a solution because the grain size of the crystals is too small, etc.”
Overall, this paper in the way it is written is not suitable for publication due to its superficiality, and it is not in the range of papers published in Minerals.
The authors need to review the used methodology deeply and add other results, as I mentioned in my first report (first round).
Author Response
REPLY TO REVIEWER 3 (SECOND ROUND)
OUR ANSWERS IN GREEN
Open Review
(x) I would not like to sign my review report
( ) I would like to sign my review report
English language and style
(x) Extensive editing of English language and style required
( ) Moderate English changes required
( ) English language and style are fine/minor spell check required
( ) I don't feel qualified to judge about the English language and style
We sent the paper to an English native speaker, who is also geologist, for review. The English language and style have been carefully edited.
Comments and Suggestions for Authors
The new version of this paper is a little improved. However, the authors did not answer my questions adequately. Some of their answers contain contradictions, which is unacceptable and completely wrong “XRD is not a solution because the grain size of the crystals is too small, etc.”
We think that our answers were probably not CLEAR enough and could be misinterpreted. we used the above corrections of referee #3 to modify our answers. we hope that our answers ARE now more EXPLANATORY.
Overall, this paper in the way it is written is not suitable for publication due to its superficiality, and it is not in the range of papers published in Minerals.
The authors need to review the used methodology deeply and add other results, as I mentioned in my first report (first round).
We think that the paper submitted is a standard petrographic paper, and the methodology used to characterize the minerals is the same used in hundreds of equivalent papers published in international journals similar to Minerals, as well as in other papers in this journal. Among them, we could cite the following:
Nb distribution in carbonatites:
Melgarejo, J.C.; Costanzo, A.; Bambi, A.C.J.M.; Gonçalves, A.O.; Neto, B.N. Subsolidus processes as a key factor on the distribution of Nb species in plutonic carbonatites: The Tchivira case, Angola. Lithos, 2012, 152, 187 – 201. Bambi, A.J.M; Costanzo, A.; Gonçalves, A.O.; Melgarejo Draper, J.C. Tracing chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of F. Mineralogical Magazine 2012, 76, 377 – 392. Torró, L.; Villanova, C.; Castillo, M.; Campeny, M.; Gonçalves, A.O.; Melgarejo, J.C. Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola. Mineralogical Magazine 2012, 76, 393 – 409. Chakhmouradian, A.R; Reguir, E; Kressall, R.D.; Crozier, J; Pisiak, L.K; Sidhu, R; Yang, P. Carbonatite-hosted niobium deposit at Aley, northern British Columbia (Canada): Mineralogy, geochemistry and petrogenesis. Ore Geology Reviews, 2015, 64, 642-666. Lee, M. J.; Lee, J. I.; Garcia, D.; Moutte, J.; Williams, T.; Wall, F.; Kim, Y. Pyrochlore chemistry from the Sokli phoscorite-carbonatite complex, Finland: Implications for the genesis of phoscorite and carbonatite association. Geochemical Journal, 2006, 40, 1 – 13. Zheng, L.; Gu, X.; Zhang, Y. Pyrochlore chemistry from the Bonga carbonatite type Nb deposit, Huila Province, Angola: implications for magmatic-processes of carbonatite. Acta Geologica Sinica, 2014, 2, 487-488. Amores-Casals, S; Melgarejo, J.C; Bambi, A.C.J.M.; Gonçalves, A.O.; Morais, E.; Manuel, J.; Costanzo, A.; Martí Molist, J. Lamprophyre - carbonatite magma mingling and subsolidus processes as key controls on critical elements enrichment in carbonatites: the Bonga complex (Angola). Minerals, 2019, 9, Accepted. Chakhmouradian, A.R.; Mitchell, R.H. Compositional variation of perovskite-group minerals from the Khibina complex, Kola peninsula, Russia. Canadian Mineralogist, 1998, 36, 953 – 969. Kogarko, L.N.; Williams. C.T.; Woolley, A.R. Chemical evolution and petrogenetic implications of loparite in the layered, agpaitic Lovozero complex, Kola Peninsula, Russia. Mineralogy and Petrology, 2002, 74, 1 – 24. Mitchell, R.H.; Chakhmouradian, A.R. Compositional variation of loparite from the Lovozero alkaline complex, Russia. Canadian Mineralogist, 1996, 34, 977 – 990.
REE-minerals characterization and distribution in carbonatites, pegmatite and others.
Dalsin, M.I.; Groat, L.A.; Creighton, S; Evans, R.J. The mineralogy and geochemistry of the Wicheeda Carbonatite Complex, British Columbia, Canada. Ore Geology Reviews, 2015, 64, 523-542. Doroshkevich, A.G.; Viladkar, S.G.; Ripp, G.S.; Burtseva,M.V. Hydrothermal REE mineralization in the Amba Dongar carbonatite complex, Gujarat, India. Canadian Mineralogist, 2009, 47, 1105–1116. Marien, C.; Dijkstra, A.H.; Wilkins. The hydrothermal alteration of carbonatite in the Fen Complex, Norway: mineralogy, geochemistry, and implications for rare-earth element resource formation. Mineralogical Magazine, 2018, 82, Supplement S1, S115-S131 Miloš René. Nb–Ta–Ti Oxides in Topaz Granites of the Geyersberg Granite Stock (Erzgebirge Mts., Germany). Minerals 2019, 9(3), 155; https://doi.org/10.3390/min9030155 Koerber, J.A; Thakurta, J. PGE-Enrichment in Magnetite-Bearing Olivine Gabbro: New Observations from the Midcontinent Rift-Related Echo Lake Intrusion in Northern Michigan, USA. Minerals 2019, 9(1), 21; https://doi.org/10.3390/min9010021 Zozulya, D.; Lyalina, L.; Macdonald, R.; Bagiński, B.; Savchenko, Y.; Jokubauskas, P. Britholite Group Minerals from REE‐Rich Lithologies of Keivy Alkali Granite—Nepheline Syenite Complex, Kola Peninsula, NW Russia. Minerals 2019, 9, 732. Uher, P; Ondrejka, M; Bacik, P; Broska, I; Konecny, P. Britholite, monazite, REE carbonates and calcite: Products of hydrothermal alteration of allanite and apatite in A-type granite from Stupné, Western Carpathians, Slovakia. Lithos, 2015, 236 – 237, 212-225. Chikanda, F; Otake, T; Ohtomo, Y; Ito, A; Yokoyama, T; Sato, T. Magmatic-hydrothermal processes associated with Rare Earth elements enrichment in the Kangankunde carbonatite complex, Malawi. Minerals, 2019, 9, doi:10.3390/min9070442. Guastoni, A; Nestola, F; Giaretta, A. Mineral chemistry and alteration of rare earth element (REE) carbonates from alkaline pegmatites of Mount Malosa, Malawi. American Mineralogist, 2009, 94, 1216-1222. Broom-Fendley, S; Brady, A.E, Horstwood, M.S.A; Woolley, A.R.; Mtegha, J; Wall, F; Dawes, W; Gunn, G. Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi. Journal of African Earth Science, 2017, 134, 10-23. Mouchos, E; Wall, F; Williamson, B. High-Ce REE minerals in the Parnassus-Giona bauxite deposits, Greece. Applied Earth Science IMM Transactions section B 126, 2017, DOI: 10.1080/03717453.2017.1306281 Yang, X,M; Yang, X.Y.; Zhang, P.S.; Le Bas, M.J. Ba-REE fluorocarbonate minerals from a carbonatite dyke at Bayan Obo, Inner Mongolia, North China. Mineralogy and Petrology, 2000, 70, 221-234. Chakhmouradian A.R., Mitchell R.H. Primary, agpaitic and deuteric stages in the evolution of accessory Sr, REE, Ba and Nb-mineralization in nepheline-syenite pegmatites at Pegmatite Peak, Bearpaw Mts, Montana. Mineralogy and Petrology, 1999, 67, 85-110.Agpaitic rocks
Andersen , T; Elburg, M; Erambert, M. The miaskitic to-agpaitic transition in peralkaline nepheline syenite (white foyaite) from the Pilanesberg Complex, South Africa. Chemical Geology, 2016, 455, 166-181. Chakrabarty, A.; Pruseth, K. L.; Sen, A. K. Composition and Petrogenetic significance of the Eudialyte group minerals from Sushina, Purulia, West Bengal. Journal of Geological Society of India, 2012, 79, 449 – 459. Christiansen,; Johnsen, O.; Makovicky, E. Crystal chemistry of rosenbuschite group. Canadian Mineralogist, 2003, 41, 1203 – 1224. Schilling, J.; Wu, F. Y.; McCammon, C.; Wenzel, T.; Marks, M. A; W., Pfaff, K.; Jacob, D. E.; Markl, G. The compositional variability of eudialyte group minerals. Mineralogical Magazine, 2011, 75, 87 – 115.
None of these authors use the methodologies suggested by referee #3 to characterize the mineral associations because all these papers are petrographic. they use exactly the same methods used in this paper. Another similar paper in this Special Volume has been accepted. In contrast, the papers mentioned by referee #3 do not use the methodologies used in our paper, because the purpose of the papers mentioned by referee #3 is more focussed on ore beneficiation.
COMMENTS REVIEWER 3 (revising the first Round suggestions)
(Green HIGHLIGHTS on the Final Revised Manuscript)
a) I suggest to add a part of sampling to briefly discuss in addition the geological context, the methodology used and the representativeness of studied samples.
We added the following text IN line 69:
“Sampling and mapping of Monte Verde complex was done during 2014 field work. 95 samples of plutonic alkaline rocks, alkaline dykes, breccia and fenitized granites were selected and collected”. About 76 polished thin sections were prepared and studied in order to identify mineral phases and textural features using transmitted and reflected optical microscopy with plane polarized light.
We renewed the methodology section by adding some more information on the methodology used. in particular, we explained the number of samples studied and the analyses:
A total of 21 more mineralized samples were selected to be studied in detail with SEM-BSE-EDS. Qualitative analyses and detailed characterization of mineral textures were done using a Quanta ESEM Quanta Q-200 FEI XTE 325/D8395 Scanning Electron Microscope (Thermo Fisher Scientific, Waltham, MA, USA) coupled with an INCA Energy 250 EDS microanalysis system, located at the Scientific and Technological Centers of the University of Barcelona. Operating conditions were 20 keV, 1 nA beam current, and 10 mm working distance.
7 samples were selected in order to quantify Nb- and REE-bearing phases using a JEOL JXA-8230 electron microprobe (JEOL USA, Peabody, MA, USA) with five wavelength dispersive spectrometers (JEOL USA, Peabody, MA, USA), also located at the Scientific and Technological Centers of the University of Barcelona. Operating conditions used were the following: pyrochlore 20 kV and 14.4 nA; Na-Ti-Zr-REE silicates 20 kV and 14.7 nA; perovskite and loparite 20 kV and 14.9 nA; REE carbonates and phosphates 20 kV but 6.5 nA. For all elements, counting time was 10 seconds on peak and 10 seconds on background except for Ce, La, Ba (20 seconds per peak and 20 seconds per background) and for Na and F (30 seconds per peak and 30 seconds per background). The acquisition method used was two background measurements at each side of the peak. The beam diameter was 5 microns. Standards and lines used are listed in table 1. A total of 681 analyses were obtained. A selection of these results can be found in the supplementary material.
b) I suggest to add some mineralogical characterization (e.g. XRD, QEMSCAN or MLA) of the studied materials in order to: Identify and quantify the REE bearing minerals. The relevant literature is not fully cited (e.g. Edahbi et al 2017; 2018,Nadeau et al 2015; 2016):
https://link.springer.com/article/10.1007/s11356-018-1309-7; https://www.mdpi.com/2075-163X/8/2/55; https://www.sciencedirect.com/science/article/pii/S0169136815300809
These references were cited in the last version of the manuscript, at the Discussion section:
Other carbonatites worldwide, for instance, Italian carbonatites [56,57], the Amba Dongar complex, India [71-73, the occurrences from Sichuan Province, China [74-76], the Montviel deposit, Canada [77,78,79,80], may display similar late REE mineral associations.
Minerals were characterized with EPMA (complemented by RAMAN when necessary). XRD was not the best solution in our case because these facts:
the grain size of the crystals of Rare-element minerals is too small to extract easily single grains from their mixtures, in order to havepure substances to proceed identification with x-ray diffraction in most of the cases, the proportion of the rare minerals is too low (normally below 2%) to obtain a good detection of these minerals when using xrd in whole rock samples; therefore, most probably the spectra of minerals of critical elements can be overlapped by the spectra of other major rock-forming minerals. a whole rock diffraction of selected areas (i.e. using quantitative x-ray diffraction as we carried out in other research published in this volume) could be proposed. However, in our circumstances, it would not be representative enough of the real rock composition, only of the composition of this domain. therefore, quantitative xrd is not the ideal solution to calculate the proportions if the minerals, because the rocks are extremely heterogeneous. in situ punctual x-ray analyses could be a solution to identify the small scattered crystals, but it is not a routine technique and can be better substituted by raman or other punctual techniques, as emp, when necessary. To our knowledge, QEMSCAN or MLA can have problems to discriminate within complex rare phases, and can have difficulties when identifying polymorphs or minerals of complex groups. hence, we prefer to characterize these complex minerals in petrographic papers with EDS + EPMA + Raman (when necessary). many other authors use also these methods for characterization qemscan and mla are more suitable to investigate the mineral associations when the objective is the study of the optimal mechanisms of ore processing, and this is not the aim of the paper. unfortunatEly, such equipments are not available in Barcelona
c) Evaluate the degree of liberation of REE bearing minerals.
This is not an objective of the contribution. it could be a possible subject for further investigation in case of a more detailed study of the economic possibilities of the deposit. this paper is the first contribution on the mineralogy of the area.
d) Evaluate the mineralogical grain size of REE-bearing minerals, etc.
The quantitative study of size distribution is not an aim of the current paper. of course, it would be necessary in case of further interest in economic processing of the ores in the area. However, in our opinion, it IS necessary TO carry OUT more work with classical techniques. HOWEVER, WE INTRODUCED ADDITIONAL DATA ON THE PETROGRAPHIC ASPECTS OF THESE MINERALS
e) I think you must summarize the bulk rock major and trace element using a table for example because it so difficult to get an idea about the concentrations values with the used graphs.
The major bulk rock characterization of the rocks of the area is not an objective of the paper. however, other contributions in this direction are in preparation for the future. We added crystallization sequences and a summary table of all minerals present in the Monte Verde units in order to facilitate reading.
f) In the results and discussion part, the manuscript is confused in the way it is written. The authors comment their results but they did not discussed them enough (e.g comparison with other carbonatites such as Canadian carbonatites; mobile-to-immobile element ratios; model for the evolution of Monte Verde carbonatite).
The discussion focused more on the similarities with Angolan carbonatites, which are currently under study. comparison of the monte verde mineral associations with many other occurrences worldwide is provided. however, We NOW introduced a mention to the data obtained in the Canadian carbonatites.
The evolution of the Monte Verde carbonatite itself is defined based on the available information in the outcrops, because drilling data are not available so far. hence, it is hard to be discussed because there are only outcrops of carbonatite breccias, and we can only establish the sequence of intrusion based on the observations in the breccias and in the outcropping alkaline rocks.
we did not carry out whole-rock analyses to have data for an in depth discussion on the mobile-immobile elements or the petrogenesis of the rocks. this was not the objective of the paper.
g) Also, the results are insufficient and partially interpreted. I suggest also separating results part to the discussion part. The authors discuss only Nb but what about other radioactive elements associated with carbonatites such as thorium and uranium?
We think that the results are standard for a paper of these characteristics. we do not deal with the economic processing of the ores. the paper wants to attract the attention of a possible prospective area, discussing the occurrence of some mineral species that can be used as pathfinders of mineralized bodies in depth.
We think that the results are absolutely separated from the discussion, although in the discussion section we remind the readers of some of the more outstanding results, just to introduce and follow the discussion. there is no discussion in the results section, and most of the citations are in fact in the discussion section, to be used to compare our results with those of other authors from different regions.
the paper deals with critical elements. uranium and thorium are not critical elements for the european union. in fact, they may be more a penalty. hence, these elements were mentioned in the original version of the paper only in the results section. however, we now see that these elements can be critical in america. therefore, we introduced a short discussion on the possible interest of these elements.
Author Response File: Author Response.docx
Round 3
Reviewer 3 Report
The authors answer my comments.
