Syn-Sedimentary Exhalative or Diagenetic Replacement? Multi-Proxy Evidence for Origin of Metamorphosed Stratiform Barite–Sulfide Deposits near Aberfeldy, Scottish Highlands

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This is a well-written manuscript that interpret novely the origin and forming model of the Aberfeldy Barite-sulfide deposit using mineralogy, lithology, and isotopic geochemistry. The authors presented good data and concluded that the deposits formed from mixing hydrothermal fluids with seawater in marine basins. I agree with their idea that the Aberfeldy barite deposits provide an example of classic model of SEDEX mineralization. So I recommend to publish the manuscript as soon as possible.

Author Response

No comments were made by this reviewer.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper by Moles et al. “Syn-Sedimentary Exhalative or Diagenetic Replacement? Multi-Proxy Evidence for Origin of Metamorphosed Stratiform Barite—Sulfide Deposits near Aberfeldy, Scottish Highlands” unravels the exhalative and diagenetic features of the Proterozoic barite-sulfide deposit from Scotland. The paper is urgent, presents a lot of data, and is well written.

In general, I agree with arguments about the genetic aspects of mineral formation processes, but there are some issues, which I’d like to address to make the paper more interesting and easily readable.

The manuscript is extremely long and I recommend its reduction. There are a lot of places with “discussion” in Results, which can be combined with similar points in section Discussion.

The presentation of Results is a little bit confusing, since the data presented are a mixture of new results and those (especially, in isotopic subsections), which have already been published. Move this information either to Intro (what have been done before) or to Discussion (comparing new results with previously published data). The same concerns the explanation of “the purpose of measuring the triple oxygen composition...” All these explanations should be placed either in Introduction supporting the main goal of the manuscript or to Discussion.

I didn’t quite understand the comparison with cold-seep barite. Just because of MSR involved in formation of barite or do you suggest your barite is a product of cold seeps? In some cases, MSR can also be involved as a major agent for the formation of hydrothermal chimney barite (e.g., Eickmann et al. 2014, Geobiology, 12, 308–321). Did you try to compare your results with present-day Red Sea brine-pool stratiform deposits?

  Section 5.7 - I'm not sure this is a necessary section: further research either can be done or not due to a number of reasons. You can save some space removing most of this section and providing some future bullet points in conclusions, since the paper is long too much.

Comments for author File: Comments.pdf

Author Response

Thank you for your comments on the manuscript and a very useful review. We have implemented the changes that you recommend, or in a few cases we present our justification for not making the changes. Details follow:

Lines 113-115 = references are missing - we have added three, including one that was not previously in Reference list. Subsequent references/citations are renumbered.
Lines 407-408 and 410-413 (3 sentences) = 'discussion' - The heading of this section is 'Sulfate pseudomorphs' and the text reports the previously noted inference that these features are texturally similar to sulfate crystals that grew in sediment during diagenesis, citing morphologically similar features in published accounts. We prefer to leave these sentences here, and in the Discussion to consider the processes by which the sulfate crystals formed in the sediment.
Lines 446-450 start of barium carbonates results = 'not results of this study': This text has been moved to section 2 preceding description of non-barian carbonates in the deposits. The remainder of the paragraph has been deleted as it duplicates text elsewhere in the paper.
Line 478 = 'discussion' - This is not discussion: the comment links to statements earlier (in section 3) on the cause of cathodoluminescence in barium carbonate minerals, and is important to retain in the Results section as it explains why norsethite is probably more abundant in the rocks than was observed by this method.
Lines 493-495 = 'discussion' - We agree that this interpretation should not be placed here, and have moved the text to Discussion, at the end of section 5.2.1.
Lines 508-509 = 'discussion' - This part of the sentence has been deleted.
Lines 543-544 ='discussion' - This part of the sentence has been deleted.
Lines 546-548 = 'discussion' - Sentence moved to Discussion section 5.2.2.
Lines 552-553 = 'discussion' - these are all features of metamorphic alteration of sphalerite compositions, previously reported in Moles 1983 and 1985. The sentence 'The lack of evidence for depositional growth zoning...' is moved to Discussion section 5.2.2. However, the phrase 'suggests homogenization of the matrix sphalerite...'  is necessary in the explanation of why encapsulated crystals are so important in our study. Therefore, no change has been made to this sentence.
Lines 588-602 = 'discussion' - Although not a 'result', this reference to barite Sr contents in general is necessary to provide a context for the range in barite Sr contents that we report in this section. Therefore, no change is made.
Lines 590-592 = 'not results of this work' - Our work builds on this previous work which should be acknowledged. Nevertheless we have deleted this sentence in the interests of shortening the paper. We have also deleted the following sentence to help shorten the paper, as the observation is self-evident from Fig. 9a.
Lines 599-602 = 'already published results, not new' - The first sentence has been merged with the previous sentence to clarify that this is part of the results of the current study. The second sentence 'Further EMPA...' is not published, it is new data: the word 'unpublished' has been inserted to clarify this.
Line 649 4.4 Stable isotope geochemistry = 'too much already published results' - We have moved this text to section 2, under a new heading 2.4. Also, the previous Fig. 11 has been moved to section 2 and renumbered Fig. 4. Subsequent figures are renumbered accordingly.
Line 697 4.4.2 = 'the same' - This section is renumbered 4.4.1. The first paragraph of this section explains the categories into which we have divided the previously published isotopic data, which differs from previous publications, and 'sets the scene' for the new data presented in the following paragraphs. We prefer to retain this text here rather than move it to an earlier section, therefore no change is made.
Line 771-790 first two paragraphs in section 4.4.3 = 'not results!' - This introduction to triple oxygen isotope systematics and how results are interpreted has been moved to the new section 2.4 following the text on previous sulfur and oxygen isotope studies of Aberfeldy barite and sulfides.
Line 810 section 4.4.4 = 'the same' - Moles (1985b) is an unpublished PhD thesis by the first author: the carbonate 13C and 18O data from this thesis is published here for the first time here. We have modified the first sentence accordingly. The fourth paragraph in this section is Discussion and has been moved to the end of section 5.4.
Line 1301 = 'why do you compare with cold-seep barite? Do you suggest your barite is a product of cold seeps?' - No, this is for comparison and we subsequently conclude that Aberfeldy barite did not form by cold seep/biogenic remobilisation processes. We have inserted sentences to clarify why we describe modern cold seep barite occurrences here.
Line 1301 = 'It seems that, e.g., brine-pool stratiform deposits from Red Sea with barite could be more relevant for the comparison.' - We agree that it would be useful to refer to Red Sea brine pools and other modern occurrences of euxinic marine waters, and have added two sentences on this topic.
Line 1315 - 'it would better start from major barite, not barium carbonate' - This is a sensible suggestion. We have reversed order of topics to start with evidence for barite formation and diagenetic alteration before discussing barium carbonates and their significance.
Line 1366 - section 5.7 Proposed further research: Reviewer thinks this is unnecessary and can be cut to shorten the paper. On the contrary, the authors feel strongly that the opportunities for further analytical studies of the Aberfeldy mineralization should be emphasized in this paper. The methods described here have the potential to add greatly to the interpretation of the mineralization in terms of syn-sedimentary or diagenetic origin. It is quite a short section (<1 page) and we would much prefer this text to be retained.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript by Moles et al., deals with an important barite-rich deposit in the Scottish Highlands. It is typified by a high complexity due to the superposition through time of syndepositional hydrothermal mineralization followed by a series of diagenetic processes and lastly of metamorphic transformations, all this resulting in a very diversified mineral assemblage.

The manuscript is very well organized, written, and illustrated. It summarizes a great deal of data emerging out of a multidisciplinary approach starting with field work and drillcore analysis, normal and cathodoluminescence petrography, microprobe and laser ablation ICP mass spectrometry trace element analysis, and C, O, S and Sr isotope geochemistry, partly already published and partly newly acquired.

This detailed analysis allowed to show that the primary isotopic composition of the thick barite beds was not altered by diagenesis and, even in more transformed beds, small, micron-size, crystals (barium carbonates, barite, sphalerite), being encapsulated in pyrite, survived later diagenetic and metamorphic changes and preserve the original chemical and textural features. In my opinion this aspect, i.e. preservation of pristine features in metamorphic rocks, should be more emphasized.

I have made some comments/suggestions directly on the text which, if accepted, imply a very minor revision of this very interesting and well documented case history.

Comments for author File: Comments.pdf

Author Response

Thank you for your very positive comments on our manuscript and suggested improvements. We have implemented these as detailed below. We have also added a final sentence to the Conclusions on thepoint you mention should be emphasized: the preservation of pristine features in metamorphic rocks that enable inferences on the depositional and early diagenetic processes that formed the mineralization.

Line 248 Fig. 3 caption error - Yes this should be (j); the correction has been made.
Line 250 - add label for celsian in the photo - We have done this, and labelled other minerals in the image. We have also labelled the barite rock shown in (i) and deleted the descriptive text from the caption as this is no longer necessary.
Line 398 - 'it would be interesting to add a picture in which bedding/lamination are clearly visible'. - In our opinion, lamination is clearly visible in a hand specimen and several thin section photos of the silicified sediments, particularly Fig. 3g. The text has been modified to include links to all figure components that show laination.
Line 1205 - 'maybe better specify why sulfate diffuse downard whereas sulfide upward'. The reviewer has identified a top/bottom issue in this sentence which is confusing as the upward/downward vectors differ depending on whether the top or bottom of the barite bed is undergoing alteration. Therefore we have deleted the second part of this sentence, and instead refer the reader again to Fig. 16 at the end of this paragraph. We have also taken the opportunity to add another citation to the previous sentence.
Line 1265 - 'C isotopes should be expressed with respect to PDB not SMOW' - This is my error, SMOW has been changed to PDB.
Line 1331 - 'don't you think that in organic-rich sediments the original seawater sulfate should have been completely reduced to sulfide during the very early, shallow burial diagenesis as a result of microbial activity?' - this is a good point. We have added a sentence: 'Marginal increases in δ34S are pronounced in barite adjacent to sediments that lack or-ganic carbon (calcareous mica schist: Fig. 11a) and therefore would have retained more dissolved sulfate than organic-rich sediments.'