Review Reports - Biogeochemical Redox Processes Controlling the Element Cycling: Insights from Karst-Type Bauxite, Greece

Round 1

Reviewer 1 Report

Dear editor,

First, I would like to congratulate the authors for their work. It is pleasant to read. The paper is short and straightforward which is much appreciated. The structure of the Ms fits well with most scientific publications. The scope of this study is to discuss the intervention of bacterial/microbial activity to the bauxite ore grade and scavenging of some elements. This is done with field observations and geochemistry of the section of Parnassos-Ghiona (Greece) of 2 m-thick bauxite. The study is well presented and the discussion very interesting with references.

However, the work needs more detailed pictures of microbial/bacterial activity. This is not clear if the micro-organisms are recent or fossils as supposed in the abstract and conclusions. The pictures are not very detailed about the identity of the micro-organisms they have found and what they interpret to be micro-organisms textures. This is one major input of the study and the pictures do not convince me. These observations must be illustrated more thoroughly. One interesting feature is the presence of sulphide veins forming after the bauxite ore, which needs to be stressed in the text and could be explained by such microbial activity. In line 217, they point out the importance of S isotopes to trace microbial/bacterial activity, but not such data is available. I am wondering why the authors did not perform this analysis, as it is a world-recognized diagnostic of microbial/bacterial activity and this would support other proxies. At least they must explain why they did not. I am also wondering about the correlation of elements in Fig. 3. In a statistic point of view, I am not sure correlation ratio (R²) of 0.5-0.6 are valid. Otherwise, the discussion is very interesting with an overview of the control of micro-organisms in other bauxite ore deposits around the world. The authors present concentrations as mg/kg but it is more convenient/conventional to present as wt.% or ppm. I do not really understand the interest of studying the leachate residue to the topic.

Following that, this paper brings new information about poorly documented deposits, which will be of interest to Minerals audiences. However, it cannot be published like this and needs minor revision. You will find specific comments directly in the text (attachment).

English

Overall, the manuscript is well written although some typos remain. I have made some suggestions.

Title

The title represents the content of the manuscript.

Shape of the manuscript

The shape of the document is consistent with Minerals and most scientific publications.

Figures

Number of figures is sufficient but an emphasis is needed to show bacterial filaments and other micro-organism features. Pictures in Fig. 1b must be increased. Quality needs to be checked. All figures must be cited orderly.

Comments for author File: Comments.pdf

Author Response

Reviewer 1: Comments and Suggestions

First, I would like to congratulate the authors for their work. It is pleasant to read. The paper is short and straightforward which is much appreciated. The structure of the Ms fits well with most scientific publications. The scope of this study is to discuss the intervention of bacterial/microbial activity to the bauxite ore grade and scavenging of some elements. This is done with field observations and geochemistry of the section of Parnassos-Ghiona (Greece) of 2 m-thick bauxite. The study is well presented and the discussion very interesting.

Dear reviewer 1, Thank you very much for the consideration of our manuscript and your constructive comments.

However, the work needs more detailed pictures of microbial/bacterial activity. This is not clear if the micro-organisms are recent or fossils as supposed in the abstract and conclusions. The pictures are not very detailed about the identity of the micro-organisms they have found and what they interpret to be micro-organisms textures. This is one major input of the study and the pictures do not convince me. These observations must be illustrated more thoroughly.

Reply: The identity of the micro-organisms is not available for the Parnassos bauxite deposit and the description of that type of bio-mineralization is not clear indeed, although it is reported that “Aggregates of hydrous sulfate minerals: halotrichite, szomolnokite and voltaite (Fig.2i-k) are neo-formed minerals (lines 132-136), their TOC content (line 154) and the conditions of their formation (155-158) are given. In the revised manuscript more images (Fig. 2) and detailed description is provided.

I do not really understand the interest of studying the leachate residue to the topic.

Reply: The authors believe that a contribution on the bio-geochemical redox processes controlling the element cycling related to karst-type bauxite, should be complete, including the presentation of the composition of water leachates from representative samples of bauxite ore and red mud, both being potential sources of contamination by heavy metals/metalloids, and the discussion of the sub-section: Mining/smelting processes and environmental implications.

One interesting feature is the presence of sulphide veins forming after the bauxite ore, which needs to be stressed in the text and could be explained by such microbial activity. In line 217, they point out the importance of S isotopes to trace microbial/bacterial activity, but not such data is available. I am wondering why the authors did not perform this analysis, as it is a world-recognized diagnostic of microbial/bacterial activity and this would support other proxies. At least they must explain why they did not.

Reply: The importance of S isotopes to trace microbial/bacterial activity is emphasised indeed, but they are already provided in our previous publication, which is cited [11].

I am also wondering about the correlation of elements in Fig. 3. In a statistic point of view, I am not sure correlation ratio (R²) of 0.5-0.6 are valid.

Reply: Considering that the analytical data are relatively limited (small sample), the positive and negative correlations between two variables show changes in the same or opposite direction and we use r (not R²).

The authors present concentrations as mg/kg but it is more convenient/conventional to present as wt.%

Reply: We use as units wt.% to express the content in solids for major components, mg/kg and μg/kg for their trace element content (Table 2), and μg/L to express the concentration of elements in leachates (Table 3) because they are liquids. We commonly use these units instead their other way as ppm and ppb (respectvely) with the same mean.

Comments noted on the manuscript

Lines 15 and 60: You should at least cite scientific papers dealing with microorganism activity in weathering crust deposits

Reply: The following chapter in Book is cited now: Berthelin J. (1988) Microbial Weathering Processes in Natural Environments. In: Lerman A., Meybeck M. (eds) Physical and Chemical Weathering in Geochemical Cycles. NATO ASI Series (Series C: Mathematical and Physical Sciences), vol 251. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-3071-1_3.

Lin : You should at least cite the very important work done by Combes in Southern France and Spain in this list. They are benchmark publication.

Reply: The following publication by Combes is cited now.

Combes P. ‐J. (1984) Regards sur la géologie des bauxites; aspects récents sur la genése

de quelques gisements à substratum carbonate – A look at the geology of bauxite; recent data on the genesis of some deposits in carbonate rock. Bulletin des Centres de Recherches Exploration‐production Elf‐Aquitaine, 8 (1), pp. 251‐274.

Line 103: precise the type of fluid. I guess meteoric

Reply: It was corrected.

Figure 1. The images on the right of (b) are way too small. Modification of Fig. 1 is needed.

(b) should be located on (a) (close up to B3?). Quality of (a) can be better.

Reply: The quality of the Fig. 1(a) was improved. It was clarified that Fig. 1(b) is a profile from the B3 Formation.

Lines 124-125: This is key feature of the study and this must be clearly shown on several images. I cannot clearly see right now.

Reply: I hope the new images and their description to make clear the difference between fossilized microoranisms, which are revealed on the polished sections, like Fig. 2g (white allows) and in unpolished parts, and the present days mineralization (Fig.2j-m) deposited on the surface of ores.

Fig.2a: each image caption should be placed before the corresponding text.

Reply: It was moved. We thought that according to the MINERALS instructions it used to be placed before the image caption.

Fig 2b. I cannot see any of this at this resolution

Reply: You are right; It was re-written to show what we can see.

Line 130: I'm wondering about the process that can precipitate pyrite (reducing conditions) after bauxitisation (oxidizing conditions). This must be discussed.

Reply: It is discussed (lines 228-235) in the section of the Discussion, to avoid interpretation here.

Line 150, ….from 7.2 to 16 mg/kg in the typical brown-red colored ore: must be expressed as ppm.

Reply: Since ppm=mg/kg, the authors prefer the express the unit as mg/kg, instead ppm, as in their previous publications.

Figure 6: this is a very weird presentation of the REE content. This is quite incomplete to interpret the data correctly. Nowadays, I have never seen REE patterns with only 6 elements. Please present the complete

Reply: It is clarified now that the chondrite normalized REE-patterns for bauxite samples from the studied profile of the Parnassos-Ghiona bauxite exhibit a similar trend to those analyzed for the total of REE and presented in previous publication [11], in order to avoid repetition. The aim of this presentation was is just to compare the REE patterns of the described bauxite types showing these anomalies too.

Lines 198-200: Cannot understand. Leachate from the analysed samples?

Reply: The authors believe that a contribution on the bio-geochemical redox processes controlling the element cycling related to karst-type bauxite, should be complete, including the presentation and discussion of the sub-section 3.1. Mining/smelting processes and environmental implications. This is the reason of the analysed representative samples from bauxite ore and red mud, both being potential sources of contamination.

Line 201: this should move to discussion or introduction. Not a result but interpretation.

Reply: It was ommited here.

Line 209: re-write

Reply: the driving force can be written as controlling factor

Line 217: not done in this study

Reply: Unclear comment, because in the mentioned sentence there is the relative reference [11] where sulfur isotopes are given.

Line 266: no such image to support your statement? Please refer to fig.

Reply: Unclear comment, because in the mentioned sentence “Scanning electron microscopy images showing biofilms contained abundant PGE nanoparticles and microcrystalline aggregates have shown that microorganisms can influence the mobility of PGE [39]” there is the relative reference [39].

Line 331: You should add more artwork/picture of that. First time present day micro-organism are mentioned.

Reply: Please check again, it is mentioned: Figure 2i-k includes Back-scattered electron images showing “Aggregates of hydrous sulfate minerals: halotrichite, szomolnokite and voltaite”, description (lines 132-135), “Hydrated Al and Fe-sulfates, under a needle-like morphology like halotrichite [Fe2+Al2(SO4)4.22(H2O)], fine-grained voltaite [K2Fe2+5Fe3+ 3Al(SO4)12.18(H2O)] and szomolnokite [(Fe2+SO4.(H2O)], were found growing on grey-color bauxite samples under normal atmospheric conditions or present days (Fig. 2i-k). In addition, 0.66 wt.% O.M. was determined in a whitish material grown, after a few weeks on the surface of grey bauxite samples, exposed to air oxidation, in the room conditions (20-25°C) and moderate air humidity (atmospheric water).

Line 443: here you should also mention the relation between pyrite crossbedding oxides.

Reply: It was added.

Many thanks once again for your valuable comments

Kind regards, Maria Economou-Eliopoulos

Author Response File: Author Response.pdf

Reviewer 2 Report

This article presents a very interesting research about bio-geochemical redox processes in karst-type bauxites of Parnassos-Ghiona deposit. A very comprehensive data on geochemical and mineralogical characterisation of different bauxite layers is presented by authors. According to my opinion this paper will be accepted with pleasure by Minerals readers.

The abstract fully reflects the content of the article and clearly directs the reader to the specific topic presented in the article.
The introduction emphasizes the role of microorganisms in the transformation of organic and inorganic matter during the geological processes of formation of certain minerals and ore deposits. According to the geochemical and mineral processes, bauxitization of the parent material is accompanied by complicated and multiphase biogeochemical processes, especially in predisposed zones. An example of previously mentioned events has been studied in various varieties of karst-type bauxite from the well-known and previously studied Parnassos-Ghiona deposit.

Corrections, line 55: Montenegro; line 56: Doğankuzu.

The applied analytical methods are presented clearly and in detail.

Suggestion: Briefly describe the method of bauxite sampling.

The geological characteristics of the bauxite zone Parnassos -Ghiona are briefly presented. Presenting more detailed stratigraphic position of bauxite formations would be useful for readers.

The term: “bauxite formation” instead of “bauxite horizon” is recommended; “Late” instead of “Upper” (Jurassic, Cretaceous); "Early" instead of "Lower" (Jurassic, Cretaceous) in the text, as well as on Fig. 1. The term "Middle" Cretaceous should be avoided (it is not a correct epoch).

Correction of the chapters numbering from 3 to 6 is necessary.

Very nice presentation and comment of Back-scattered electron images, especially of fossilized microorganisms.
Geochemical characteristics are presented through a description of the similarities and differences of the studied bauxite levels, documented in the table; a correlation matrix was made as well (for 20 elements and 2 oxides) and graphically shown correlation relations of characteristic elements (As, Pd, Ce, Sc). The vertical distribution of the 12 elements / oxides trough bauxite profiles, is shown clearly and distinctly in Figures 4 and 5. Chondrite normalized REE-patterns of the studied bauxite levels are shown in Figure 6.

Correction of the Fig. 6: add marks a, b and c is necessary.

Suggestion: Paragraph from line 198 to 205 as well as table 3 should be shown under a subtitle as they refer to water-leaching experiments of red mud.

Correction, lines from 219 to 225: "Also, Cretaceous white bauxite of high quality is the type of the economic importance in the Montenegro (Bijele Poljane) bauxite deposits [16], Cretaceous bauxites in Serbia [proposed reference: Timotijević, S. Cretaceous Bauxites of Serbia. Special Editions of the Geoinstitute no. 27, Belgrade, Serbia, 2001, pp. 1–183. ], Cretaceous reddish, yellowish and greyish bauxites at Ayranci, Karaman, Turkey [31], and Late Permian grey bauxites at the Kanirash deposit, NW Iran [17] and elsewhere, suggesting an important role of microorganisms to bio-geochemical processes."This article is an interesting discussion that relies on the presented results of bauxite samples mineralogical analyses, especially on PGE content and Pd/Pt ratio in studied bauxites and Fe-Ni laterites, including implications of possible origin of parent material, its transport to karstic areas and epigenetic processes supported by microorganisms.

According to my opinion, the special value of the article is a review of the processes of mining and smelting as well as the environmental implications of bauxite exploitation and alumina production, that is to say, the formation of bauxite residue that can be a threat to the environment.

The conclusions are clear and represent the essence of the text presented in the article.

Suggestion: Complete the conclusions with special pointing to water-leaching experiments of red mud and environmental implications.

Author Response

Reviewer 2

Comments and Suggestions for Authors

The abstract fully reflects the content of the article and clearly directs the reader to the specific topic presented in the article.
The introduction emphasizes the role of microorganisms in the transformation of organic and inorganic matter during the geological processes of formation of certain minerals and ore deposits. According to the geochemical and mineral processes, bauxitization of the parent material is accompanied by complicated and multiphase biogeochemical processes, especially in predisposed zones. An example of previously mentioned events has been studied in various varieties of karst-type bauxite from the well-known and previously studied Parnassos-Ghiona deposit.

Dear Reviewer 2, Thank you very much for the consideration of our manuscript, and your constructive comments. All suggested corrections were done.

Corrections, line 55: Montenegro; line 56: Doğankuzu.

Reply: They were corrected.

The applied analytical methods are presented clearly and in detail.

Suggestion: Briefly describe the method of bauxite sampling.

Reply: The method of the bauxite sampling is described now.

The geological characteristics of the bauxite zone Parnassos -Ghiona are briefly presented. Presenting more detailed stratigraphic position of bauxite formations would be useful for readers.

Correction of the chapters numbering from 3 to 6 is necessary.

Reply: Thank you very much for these corrections, all done.

Very nice presentation and comment of Back-scattered electron images, especially of fossilized microorganisms.
Geochemical characteristics are presented through a description of the similarities and differences of the studied bauxite levels, documented in the table; a correlation matrix was made as well (for 20 elements and 2 oxides) and graphically shown correlation relations of characteristic elements (As, Pd, Ce, Sc). The vertical distribution of the 12 elements / oxides trough bauxite profiles, is shown clearly and distinctly in Figures 4 and 5. Chondrite normalized REE-patterns of the studied bauxite levels are shown in Figure 6.

Correction of the Fig. 6: add marks a, b and c is necessary.

Reply: These marks were added.

Suggestion: Paragraph from line 198 to 205 as well as table 3 should be shown under a subtitle as they refer to water-leaching experiments of red mud.

Reply: This paragraph is presented as a separate sub-section now.

Correction, lines from 219 to 225: "Also, Cretaceous white bauxite of high quality is the type of the economic importance in the Montenegro (Bijele Poljane) bauxite deposits [16], Cretaceous bauxites in Serbia [proposed reference: Timotijević, S. Cretaceous reddish, yellowish and greyish bauxites at Ayranci, Karaman, Turkey [31], and Late Permian grey bauxites at the Kanirash deposit, NW Iran [17] and elsewhere, suggesting an important role of microorganisms to bio-geochemical processes."This article is an interesting discussion that relies on the presented results of bauxite samples mineralogical analyses, especially on PGE content and Pd/Pt ratio in studied bauxites and Fe-Ni laterites, including implications of possible origin of parent material, its transport to karstic areas and epigenetic processes supported by microorganisms.

Reply: Thank you for this correction. Following the provided information this paragraph was corrected.

The conclusions are clear and represent the essence of the text presented in the article.

Suggestion: Complete the conclusions with special pointing to water-leaching experiments of red mud and environmental implications.

Reply: Done. A relative conclusion was added.

Many thanks once again

Kind regards

Maria Economou-Eliopoulos

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Authors,

Congrats to your research, this is a logically developed manuscript importance for broad scientific community and for decision-makers in economy and politics. The manuscript is well written I have not suggestion to improve English style of manuscript. The resolution of figures are fine, the morphology microbial structures can be well observed. The geochemical data are well presented.

Author Response

Comments and Suggestions for Author

Dear Authors,

Dear Reviewer 3,

Thank you very much for consideration our manuscript, and your comments.

Kind regards

Maria Economou-Eliopoulos

Author Response File: Author Response.pdf