Zun-Kholba Orogenic Gold Deposit, Eastern Sayan, Russia: Geology and Genesis

Round 1

Reviewer 1 Report

Title: Zun-Kholba orogenic gold deposit, Eastern Sayan, Russia: geology and genesis

The southeastern part of Eastern Sayan hosts the Oka ore province which includes some large gold districts (Ilchir, Bokson-Gargan, Oka, and Khamsara) hosting numerous gold deposits The Zun-Kholba gold deposit is the largest past producer in Eastern Sayan, having yielded more than 50 tonnes of Au during past mining operations. But there is no consensus about the genesis of Zun-Kholba gold deposit. This manuscript present a new point of view on the deposit geology and genesis through geological observations, mineralogical, stable (S, O, C) isotope, and fluid inclusion.The topic is interesting. However, this manuscript should be improved further. I therefore recommend major correction before its publication.

 

Major comments

Abstract

1. The abstract need to show the highlights of the study. It is not only about presenting the result.

Introduction

2. The authors wanted to developed metallogenic model of the Zun-Kholba gold deposit. To achieve this goal, previous studies on mineral resources and or precipitation processes of Zun-Kholba gold deposit must be thoroughly reviewed. Previous studies have employed various techniques to constrain the metallogenic model.

 

Regional geology and tectonics

3. The Zun-Kholba gold deposit are structurally controlled. Thus, regional tectonic evolution and structural framework are of interest and of importance in better understanding gold genesis. Unfortunately, the tectonic and structural background is not sufficiently presented in this section. There are numerous regional faults and folds shown in Figure 1 and the descriptions, which should be described and their genesis should be addressed, so that you can link gold genesis to regional tectonic evolution and structural elements in subsequent Discussion.

 

Deposit geology

4. The authors should add the description of mineralogy of the ores and stages of mineralization

 

Sampling and analytical methods

5. In the part of sampling and analytical methods, the content of the fluid inclusion study was miss. And The authors should describe sample types.

 

Ore bodies structure

6. Please add the evidence of field geological characteristic, such as the photos of ore body and quartz-sulfide vein.

 

Mineralogy of the ores

7. The authors could choose better photos (Fig. 2). It's hard to get any information (alteration haloes, lithologies, structures) from the photos and it better to mark the minerals
8. Line 371-375 The authors suggested that single hydrothermal ore-forming system presence, later mineral assemblages overprint earlier ones. Please add more evidence.
9. Line 378-379 “Ore-forming minerals distribution at the deposit shows that the appearance of some minor minerals is caused by ore-hosting substrate influence.”
10. Are there some different mineral assemblage of different wall rock. It is better to add some indicative photographs in figures.

 

Fluid inclusion study

11. There are three-stage of The Zun-Kholba gold deposit. So the authors should distinguish different types of quartz. And the fluid inclusion has been destroyed in the figures, which can let results are imprecise. Please give a reasonable explanation.

 

Mineral geothermometry

12. A summary for the contents (range, mean) of important elements of sphalerite and arsenopyrite should be added in this part.

 

Disscusion

P-T conditions of ore-formig mineral deposition

13. The authors proposed “This can best be explained by mixing of meteoric water with the deeper hydrothermal fluids at the shallow depths. However, it cannot be ruled out that the temperature reduction is caused by cooling of the fluids as they move upward”. Please add more references

 

Deposit genesis

14. “One of the main factors for the ore formation is probably a change in fluid chemistry due to fluid-rock interaction, which is the result of the metasomatic replacement of the mélange rocks”. This argument is unconvincing and Please add more evidence.
15. In the results, the authors described the ore bodies structure but it was not included in discussion.
16. The authors have advocated that the metamorphic fluid released during regional metamorphism. You need to document that regional metamorphism of greenschist facies or higher grade occurred at time of gold deposition. The authors should presented it on this issue, which can make your arguments on gold genesis particularly the a metamorphic devolatilization model more convincing.

Minor comments:

1. 2 It’s better to change ore bodies and for another color, which could make it more clearly.
2. Line 117-118 The authors said there are the rare lamprophyre and porphyry dikes of undetermined age in the Zun-Kholba gold deposit area, however it was lost in the Fig.2.
3. 4 Please mark the minerals.
4. 7 What’s the mean of bule and gray. Please add legend.
5. Fig. 18 FIA study should be addressed.

Author Response

See attached file

Author Response File: Author Response.docx

Reviewer 2 Report

I found this paper detailed and interesting, presenting a wide range of useful data. The introduction to orogenic gold deposits is concise, with suitable citations. I was unfamiliar with the setting, and was pleased that there was a good account of the large-scale to local setting and geological history. The Late Paleozoic shear deformation seems crucial, ultimately, to the story of the Zun-Kholba deposit. Do we get a clear enough impression of the extent and importance of the Kholba shear zone? The more local description of parts as a transpressive duplex, and with lens-like zones of tectonic melange, is informative.

Nevertheless I found the described sequence  of geological/tectonic development a bit confusing. First para of section 2 states that the ophiolites are the oldest rocks of CAOB – but their age is given in ref 14 as 1020 Ma, out of sequence with a description of Late Neoproterozoic cover sequence two sentences earlier. Maybe it would be best to outline the regional context and development first, as done in para 2, and then describe, again in age sequence, the units and rock types applicable to the Eastern Sayan study area. Effectively this involves transferring material from para 1 to para 3, so that the account homes in on the Urik-Kitoi disctrict and Zun-Kholba deposit.

[around line 131] Needs rewriting to be clear. There is evidently a deformed lamprophyre dyke and some undeformed porphyry dykes in the melange and ore zones. The lamprophyre must be inferred (rather than presented) to be a single body if it occurs as lens-like fragments.

Fragments of ancient VMS in the ophiolitic component of the melange are interesting and turn out to be important in the overall model, apparently affecting both chemistry and isotopes of the sulfides.

[line 198-9] “Near the surface, there are vein and vein-like bodies, whereas ore bodies transform with depth into mineralized shear zones.”. Interesting; even over this limited depth range, perhaps reflects fluid migration out of stressed domains.

[line 249] Formation of listvenite implies a CO2 component in fluids (liberated from associated carbonate units?)

Table 1 has a simple and useful layout

Lines 317-318 describe noteworthy systematic associations

You could make more use of the quartz microstructures. On figure 9 I note that the recrystallized grain size appears to be around 20 microns, mostly BLG style but parts of photo (b) and (c) show evidence for the beginning of internal subgrain rotation. This would fall around 400°C on a usual scale (e.g. Stipp et al. 2002, J Struct Geol 24, 1861-84). You could note somewhere that this is similar to and consistent with the temperatures deduced for ore formation.

Lines 374-375: Yes, important to recognize coexisting associations for PT interpretation

Geothermometry, lines 564 onwards. What are the likely overall uncertainties on T measurements? Bearing in mind that there are only three 'points' for the electrum thermometer, I might have preferred to say that the data suggest (rather than show) an increase in T with depth. The arsenopyrite thermometer values were “determined using the Fe-As-S phase diagram. This doesn't really give the reader enough information. The calibration appears to be based on figure 7 of Kretschmar & Scott 1976, which is a rationalization of more scattered experimental data in their figure 4. It doesn't seem possible to determine the absolute or relative uncertainty of the T estimates without further information.

Pressure estimations, lines 602-3: This seems a reasonable estimate, imprecise but certainly indicating a relatively shallow process. In later discussion [e.g. line 646] I was glad to see it stated that the calculated [maximum] depth was close to the true depth below surface - I was thinking along the same lines when considering the data. Nevertheless, the recorded pressure in a hydrothermal system is not easy to relate to depth in an evolving partially open fluid system.

You make a valid conclusion that there is no connection between ore formation and any magmatic event.

With respect to lines 731 onwards, dealing with metamorphism and devolatilization, this is the least thoroughly argued part of the story. The reference cited in line 733 is inappropriate and doesn’t seem relevant, referring to a generic example of shear zone initiation in a homogeneous lithology from the Alps. The paper is about microstructural development and doesn't account for heat generation and resulting devolatilization. In fact it implies effects that are due to infiltration of a fluid, not generating it.

Your model seems to require that these rocks, or deeper ones [line 737], are undergoing prograde metamorphism. Is this consistent with the timing and regional context? The granitoid rocks that apparently dominate at the present surface are not a likely source of fluid. Clearly the Late Paleozoic shearing [lines 759-761] is the key period. Was deformation accompanied by devolatilization in these episodes? Is there actual evidence here or elsewhere in the Eastern Sayan that this transpressive episode was accompanied by prograde metamorphism at deeper exposed levels?

Minor points, and comments on figures

I was initially confused by your use of ‘level’ in the ore deposit, when it first appeared in line 131, as a common convention is that it stands for depth below surface. A little later you make it clear that it is absolute elevation above sea level, the surface being at about 2300 m. Maybe remove the first reference (line 131) to a specific level, so that your definition is clear when it arrives.

There is text on Figure 2a that is illegible.

Figure 3: Ornaments on lithological units are not distinguishable at this scale. The original diagram must have been large, and it may have to be enlarged or re-drawn.

Figure 6: chalcopyrite, sphalerite and galena are not labelled on panel (c). Should they be?

Figure 12 is hard to read and interpret: colours are not distinctive, arrows don't appear to point to peaks, can't even be sure that the legend is correct. What, for example, is the red curve apparently recording >10^7? (and what are the units, counts/s?).

Minor English-language points and typos.

Abbreviations like “don’t”, “can’t” and “hasn’t” are used in conversation but not in formal writing: write “do not”, “cannot”, and “has not”. In line 166, change “don’t’ have influence to” to “have no influence on”.

It’s not usual to begin a sentence with “But …” (e.g. line 52). “However, …” is better.

Line 57: discussible. Replace with “open to discussion”.

Line 173: replace “electronic” with “electron”

Line 192: replace “Ore bodies structure” with “Structure of the ore bodies”

Line 196: replace “deformations” with “deformation”

Line 220: replace “most powerful” with “largest” or “greatest”

Line 246: replace “are undergone to” with “have undergone”

Line 294 (Fig.5 caption): replace “crashing” by “crushing”

Line 351: replace “by the ore-forming fluids flow” by “by the flow of ore-forming fluids”

Line 378-379: rephrase this sentence as “The distribution of ore-forming minerals in the deposit shows that the appearance of some minor minerals is influenced by the ore-hosting substrate”

Line 790: replace “take” with “took”

References, line 855: Dewey, J.E. should be Dewey, J.F.

Author Response

Authors are grateful all the reviewers for useful comments, which made it possible to improve the quality of the manuscript.

Answers to comments in attached file

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript no. 1603799 presents a very interesting study of a gold deposit from Russia. The manuscript is a classic ore geology article. The study is complex and well structured.

The methods are plenty and various and give a clear view on the mineralogy and geochemistry of the deposit. The deposit is described in detail regarding the mineralogy, mineral associations and textural relations. The results are very interesting and correlates well between them. The authors established temperature values for the deposit formation and therefore new ideas on the genesis were concluded.

The English is good but a small language polishing is necessary. I recommend that a native-speaker or an English professor colleague to take a second look on the grammar and spelling.

Minor comments:

• the last paragraph from the introduction (lines 61-63) needs to be re-written so that the objectives of this study are clear;
• Figure 6 – pyrrhotite abbreviation (Po) is not mentioned in the figure caption.

Author Response

Authors are grateful all the reviewers for useful comments, which made it possible to improve the quality of the manuscript.

Answers to comments in attached file

Author Response File: Author Response.docx

Reviewer 4 Report

After reading the manuscript, I have to say that the manuscript can be accepted after major revision.

The authors wrote a very interesting manuscript where they presented interesting information about geology and genesis in Zun-Kholba orogenic gold deposit, Eastern Sayan, Russia. I am very glad to see how the authors dealing with this problematic and introduce some novelty in the manuscript but there are some important concerns that must be addressed prior publication. Please, find some general and detailed comments below:

My main concerns are related to the sampling methodology and low number of samples. The authors need provide more detailed sampling methodology. It is not clear how they performed sampling. I am not completely convinced that authors can ensure a representativity with such low numbers of samples, and consequently they variability. I wonder if authors collect more samples what their result will be. Same or completely different? Thus, I highly recommend to authors to improve this chapter, because all further work depend on it.

Analytical precision needs significant improvement. The authors should provide more detailed description of QA/QC.

The authors should correct/rounded the numbers to two digits through the whole manuscript, as I proposed bellow. The authors must be aware that systematic error of analytical instruments is approximately 10%.

 
0.01234 = 0.012

0.1234= 0.12

1.234 = 1.2

12.34 = 12

123.4 = 120

1234 = 1200

Introduction - should be significantly improved. The authors need to citate more similar research all over the world.

Figures – some figures (7, 12, 13, 16) have quite low resolution, this should be corrected. If possible, add a scale at figures 4,9,10.

Table 2 – please use standard statistical symbols (average can be mean, median…)

Author Response

Authors are grateful all the reviewers for useful comments, which made it possible to improve the quality of the manuscript.

Answers to comments in attached file

Author Response File: Author Response.docx

Reviewer 5 Report

This manuscript reports a great number of solid data that the Authors used to model the formation of a gold deposit poorly studied. The manuscript is well organized and the conclusion are fully supported by the data. Therefore it can be published in Minerals after minor revision. Here are my few comments.

Line 24. Check the symbol of Celsius. Line 49. Add one short sentence to summarize what is the devolatilization model. Lines 315-316. Being compositional data I would prefer to see the data presented in wt%. Furthermore I suggest to add a discriminating diagram to show the two types of analyzed gold. Finally if other elements such as Cu and Fe have been analyzed, please insert their values. Table 2. Please insert the title ppm or the scale of the used values. Fig. 13 insert the scale (ppm?) in the axes.

Author Response

Authors are grateful all the reviewers for useful comments, which made it possible to improve the quality of the manuscript.

Answers to comments in attached file

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The author has revised it as requested, and I agree to accept it.

Reviewer 4 Report

After reading the revised manuscript, I have noticed following: the authors concerned my remark about rounded numbers but not on the way I suggested and explained. Next remark about additional references that consider a similar topics world widely is not considered, corrected, and understood properly.

However, considering the total improvement, the manuscript can be accepted for publication.