Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review Comments

This paper uses the Lead Trial gold prospect in central Scotland as a case study to compare and evaluate the contributions of standard exploration methods (rock geochemistry, soil geochemistry, magnetic surveys) and gold particle compositional studies during early-stage exploration. The study analyzes 703 gold particles from six sampling sites, employing SEM-BSE imaging and EPMA to determine alloy compositions and inclusion assemblages, identifying three gold compositional types (Types 1–3) and establishing a spatial distribution model of mineralization. This paper addresses a topic of practical value, presents a substantial dataset, and the approach of comparing gold particle compositional studies with standard exploration methods is novel. However, the manuscript exhibits significant shortcomings in structural organization, quantitative rigor of data analysis, and the evidentiary basis of the genetic model, requiring substantial revision before it can meet the standard for publication.

 

1. Major Comments

（1）The manuscript structure does not conform to journal conventions, and the logical hierarchy is confused.

The manuscript lacks an independent "Materials and Methods" section. Descriptions of standard exploration methods are dispersed across Section 3 (Historical Exploration) and Section 4 (Modern Exploration), while gold particle methodology is nested within Section 5.1. Furthermore, Section 6 is titled "Interpretation" and Section 7 is titled "Discussion," yet their functions overlap—both contain interpretation and discussion of results. The typical structure should be: Introduction → Geological Setting → Materials and Methods → Results → Discussion → Conclusions. The current structure makes it difficult for readers to distinguish facts (results) from inferences (interpretations).

（2）The alloy grouping (Alloys A–D) lacks statistical support.

The authors divide all gold particle Ag contents into four alloy groups (A–D) based on "continuous compositional ranges separated by breaks in the slope" (lines 338–339) of a cumulative percentile curve. This classification relies entirely on subjective visual judgment, with no statistical method provided for validation. For group identification in such continuous data, commonly used methods include Gaussian Mixture Models (GMM), Kernel Density Estimation (KDE), or at minimum, frequency histograms.

The Alloys A–D classification forms the foundational basis for all subsequent interpretations and the genetic model throughout the paper. If the group boundaries lack statistical significance, the entire Type 1–3 classification system and the derived spatial mineralization model (Figure 9) will lack credibility.

（3）The evidence chain supporting the Deposit Model is weak.

The deposit model is one of the core academic contributions of this paper, yet the current strength of evidence is insufficient to distinguish the "vertical zonation" hypothesis from alternative hypotheses (e.g., different fluid generations, different wallrock controls, etc.). In this manuscript, the core argument of the spatial mineralization model proposed by the authors (Figure 9) is that Type 1 gold (Alloys A+B) is hosted at deeper levels, while Type 2 and 3 gold (Alloy C) is hosted at shallower levels, representing vertical zonation within a single magmatic-hydrothermal system. However, the direct spatial evidence supporting the "vertical zonation" hypothesis is very limited: (a) there are only two in situ sampling sites (Lead Trial at ~100 m above the valley floor, and Tullichglass represented by valley-floor boulders); (b) Alloys A and B are found only in alluvial gold (lines 342–343), and their "deeper" position is entirely speculative—based on the unverified premise "provided that Alloys A and B are present at lower elevations" (Line 545); (c) the only evidence of Alloy A in an in situ sample is a single minute sub-particle within one grain from Tullichglass (Fig. 6H, lines 425–426), which has extremely weak statistical significance.

（4）The "comparative evaluation" of standard exploration versus gold particle studies lacks quantitative rigor.

The manuscript's title is "Evaluating the Potential of Gold Compositional Studies," and the core proposition is a comparative evaluation of the contributions of the two methodological approaches. However, Figure 11 uses only qualitative categories ("Proven/Inferred/Not possible") for comparison, without providing: (a) a time-cost comparison (lines 713–717 mention "two summer seasons" vs. "under two weeks," but do not quantify analytical time or total project duration); (b) an economic cost comparison; (c) quantification of information gain (e.g., what critical information did gold particle studies independently provide that standard methods could not? What proportion does this represent?).

（5）The discussion of the absence of inclusions in Tullichglass samples is Insufficient. 

The inclusion occurrence rate in the 48 gold particles from Tullichglass is 0% (Table 2), compared with 45% at Lead Trial. In lines 452–457, the authors relate the inclusion characteristics of alluvial gold to Tullichglass (rather than Lead Trial), concluding that "the relatively low abundance of sphalerite in the mineralization at Tullichglass is consistent with the inclusion signature of the alluvial gold." However, the logical basis of this inference is: Tullichglass gold particles contain no inclusions → therefore sphalerite content in its mineralization is low. This is a non-causal argument—inclusion absence may have multiple explanations (e.g., small gold grain size resulting in insufficient cross-sectional observation area, polishing quality, etc.) and cannot be directly equated with the absence of a mineral from the paragenetic assemblage.

Authors should discuss possible reasons for the 0% inclusion rate at Tullichglass (is a sample size of 48 particles sufficient? Does the smaller grain size (~100 µm, line 321) reduce the probability of capturing inclusions in cross-section?). Besides, consider supplementing with systematic thin-section observations of sulfides in Tullichglass mineralized quartz veins as an independent constraint.

 

2. Minor Comments

（1）Sections 2 and 3 contain substantial overlapping content.

Section 2.3 "Deposit Geology" already describes vein characteristics, mineral assemblages, alteration types, and gold deportment (lines 132–142). However, Section 3.1 "Studies of in situ and float mineralization" repeats nearly identical content—re-describing brecciated vein textures (line 178), galena and sphalerite (lines 180–181), chalcopyrite (line 181), wallrock alteration (lines 182–183), and the association of gold with galena (lines 184–185). The only distinguishing material in Section 3.1 is the description of Tullichglass boulders and a brief discussion of the limitations of previous petrographic work. I’d like to recommend authors to merge Sections 2.3 and 3.1 into a single "Deposit Geology and Mineralization" section.

（2）Section 3 is titled "Historical Exploration" but contains modern research content

The Section 3 heading implies a review of historical exploration activity. The first part (lines 148–163) does describe work from the 1860s to 1990s. However, Section 3.1 (lines 171–207) and Section 3.2 (lines 208–220) actually describe recent MSc/PhD research and regional gold compositional results—these are not "historical" in the conventional sense. This creates confusion regarding the chronological framework.

（3）The Abstract is overly long and contains excessive methodological detail

The abstract spans approximately 220 words across 17 lines, but roughly 40% is devoted to methodological detail ("standard geochemical, geophysical and petrographical approach"; "703 gold particles from local in situ and alluvial occurrences"). The core innovative contribution is insufficiently distilled. Additionally, the final sentence—"Although increasingly common in studies with an academic focus, gold compositional studies remain generally underutilised in commercial exploration"—is an editorial comment rather than a research finding.

（4）The Conclusions introduce new claims not adequately supported in the body text

Lines 768–769 assert that "in many alluvial gold districts the approach can provide insights into the nature of source mineralization in the absence of any other geological context." This is a new generalization about the method's universal applicability, yet the manuscript is based on a single case study (Lead Trial) and does not systematically demonstrate applicability across "many alluvial gold districts." Conclusions should not introduce claims that have not been adequately argued in the Discussion. Therefore, either move this claim into the Discussion and support it with additional literature (e.g., quantitative comparisons with other successful case studies), or adopt more cautious language in the Conclusions.

（5）Some Typographical Errors

Line 99: "ead Trial" → Should be "Lead Trial" (missing letter "L").

Line 320: "Gold recovered from crushed ore at Dunan" → The location "Dunan" is not introduced or defined anywhere in the text. Based on context, it should be "Tullichglass".

Line 49: "[8.9]" → Should be "[8,9]" (incorrect citation format).

Lines 118–119: "Tay Nappe32" → The numeral "32" should be a superscript reference number; formatting error.

Line 413: "Figures 6K and F" → Based on the caption and context, should be "Figures 6K and L".

Line 594: "study rea" → Should be "study area".

Inconsistent units throughout: "g/T" and "g/t" are used interchangeably (e.g., lines 237, 241, 244). These should be standardized to "g/t".

"In situ" vs. "in-situ": Inconsistent hyphenation throughout. Should be standardized to "in situ" (no hyphen), consistent with Minerals journal style, except when not used as an adjective.

Lines 774–780 (Author Contributions): Placeholder template text "For research articles with several authors, a short paragraph specifying their individual contributions must be provided." remains and should be deleted.

Line 781 (Funding): Contains the editorial instruction "Please add:" before "SW and LS were in receipt of NERC studentships." The "Please add:" should be deleted.

Author Response

Responses to Reviewer 1

We thank the reviewer for the detailed consideration of our work and for highlighting some areas where we can better communicate our findings. However, we do not agree with all of the comments, and the rationales for our own views are included below

1. Major Comments

（1）The manuscript structure does not conform to journal conventions, and the logical hierarchy is confused.

The manuscript lacks an independent "Materials and Methods" section. Descriptions of standard exploration methods are dispersed across Section 3 (Historical Exploration) and Section 4 (Modern Exploration), while gold particle methodology is nested within Section 5.1. Furthermore, Section 6 is titled "Interpretation" and Section 7 is titled "Discussion," yet their functions overlap—both contain interpretation and discussion of results. The typical structure should be: Introduction → Geological Setting → Materials and Methods → Results → Discussion → Conclusions. The current structure makes it difficult for readers to distinguish facts (results) from inferences (interpretations).

We considered the structure that we have adopted to be the most efficient to convey the various strands of information contributing to the paper. As the reviewer indicates the paper describes a novel multi-faceted approach, and this does not easily conform to the linear narrative of a ‘one size fits all’ Journal convention.  However, we have reviewed our structure in accordance with directions from the Journal editor for ‘Major Revisions’ and made several changes:

1. Making a clear distinction between historical information/previous work and any information flowing from contemporary exploration.
2. Adding a bespoke Materials and Methods section

• Combining outputs of exploration and gold particle work under a single ‘Results’ section’ with appropriate sub- headings

1. Integrating the Interpretation section into the Discussion Section.

（2）The alloy grouping (Alloys A–D) lacks statistical support.

The authors divide all gold particle Ag contents into four alloy groups (A–D) based on "continuous compositional ranges separated by breaks in the slope" (lines 338–339) of a cumulative percentile curve. This classification relies entirely on subjective visual judgment, with no statistical method provided for validation. For group identification in such continuous data, commonly used methods include Gaussian Mixture Models (GMM), Kernel Density Estimation (KDE), or at minimum, frequency histograms.

The Alloys A–D classification forms the foundational basis for all subsequent interpretations and the genetic model throughout the paper. If the group boundaries lack statistical significance, the entire Type 1–3 classification system and the derived spatial mineralization model (Figure 9) will lack credibility.

We disagree with the reviewers comments here. The criticism is one of established methodology within gold particle studies rather than specific to this study. Of course established techniques may be critically evaluated, but the nature of data from alluvial gold compositional studies must be understood before criticisms such as this are proposed.  There are two elements to this. Over the years we have analysed populations of gold particles from many ‘single’ sources, i.e. particles liberated from matrix. These populations invariably show a continuum of compositions with respect to silver for the majority of particles  but often with 1-5% outliers, whose composition is impossible to predict from the bulk of the population. Secondly, there is the possibility of some population overlap near the cusp of sub portions of the curve which is also be unpredictable depending on the individual case.  If two ‘known’ populations from specific sources were mixed, the resulting Ag profile would indicate clearly the nature of the major contributors to each sub population, but could not pick out the outliers from either. We are forced to deal with imperfect data sets because that is a consequence of the nature of the material we study. Defending established approaches that have been discussed elsewhere is beyond the scope of the current contribution. In summary, our approach is based on an understanding of natural gold  which is why the standard methodology involves the collection of as large a number of gold particles to provide the most robust platform possible for application of the standard methodology, i.e:

1. Identification of multiple sub populations within a single larger population of gold particles through consideration of Ag profiles,
2. Correlation of interpretation of alloy data with independent information derived principally from inclusion suites of the ‘sub populations’ or in some cases concentrations of minor alloying elements.

If no correlations are present there would be limited or no argument for gold compositional classification. We acknowledge that this approach is based on ‘weight of evidence’ rather than sophisticated approaches to population characterization via statistical methods, but it is entirely appropriate given the compositional characteristics of natural gold. In conclusion we do not accept that the quality of the overall interpretation would be improved by the reviewer’s suggestion, and have not made any modifications

（3）The evidence chain supporting the Deposit Model is weak.

The deposit model is one of the core academic contributions of this paper, yet the current strength of evidence is insufficient to distinguish the "vertical zonation" hypothesis from alternative hypotheses (e.g., different fluid generations, different wallrock controls, etc.). In this manuscript, the core argument of the spatial mineralization model proposed by the authors (Figure 9) is that Type 1 gold (Alloys A+B) is hosted at deeper levels, while Type 2 and 3 gold (Alloy C) is hosted at shallower levels, representing vertical zonation within a single magmatic-hydrothermal system. However, the direct spatial evidence supporting the "vertical zonation" hypothesis is very limited: (a) there are only two in situ sampling sites (Lead Trial at ~100 m above the valley floor, and Tullichglass represented by valley-floor boulders); (b) Alloys A and B are found only in alluvial gold (lines 342–343), and their "deeper" position is entirely speculative—based on the unverified premise "provided that Alloys A and B are present at lower elevations" (Line 545); (c) the only evidence of Alloy A in an in situ sample is a single minute sub-particle within one grain from Tullichglass (Fig. 6H, lines 425–426), which has extremely weak statistical significance.

We have several comments here,but accept that some modifications are required.

Firstly, establishing a deposit model is not one the major objectives of this paper as the reviewer states. The objectives of the paper have been clearly  set out: i.e. to evaluate what extra information may be gained by application of gold compositional studies during a synchronous exploration campaign. The reviewer acknowledges in this in their comments contained in ‘4’ below. Nevertheless, commentary on a deposit model is possible up to a point, and this aspect of the paper demonstrates clearly the information that may be gleaned from gold compositional work that is not possible from standard exploration approaches.

Vertical zonation appears the best interpretation of the data available. If the alternative scenarios the reviewer suggests had been important then we would expect to see evidence form variation in the float  identified throughout the study area, but none were observed  (L276, original manuscript). The Tullichglass boulder  locality is not as the reviewer states on the valley floor, but 50m up slope, and therefore its mineralogy is and location are compatible with our preferred hypothesis  we have emphasized that this model is the one that best fits interpretation of current data sets, whilst highlighting limitations. Re observation of alloy A at Tullichglass : we accept that one particle is not strong evidence: but nevertheless it exists. We have altered the text to acknowledge this limitation. (L. 460)

（4）The "comparative evaluation" of standard exploration versus gold particle studies lacks quantitative rigor.

The manuscript's title is "Evaluating the Potential of Gold Compositional Studies," and the core proposition is a comparative evaluation of the contributions of the two methodological approaches. However, Figure 11 uses only qualitative categories ("Proven/Inferred/Not possible") for comparison, without providing: (a) a time-cost comparison (lines 713–717 mention "two summer seasons" vs. "under two weeks," but do not quantify analytical time or total project duration); (b) an economic cost comparison; (c) quantification of information gain (e.g., what critical information did gold particle studies independently provide that standard methods could not? What proportion does this represent?).

These comments suggest that such quantitative data would in some way be generically applicable and enlightening, We disagree, because of the wide variation in exploration activities and associated expenditure between different localities/projects, together with the different challenges to gold particle studies encountered in specific studies. For example in the current study access to the field area was by vehicle and on foot, whereas similar studies in North America require daily helicopter support and perhaps the establishment of exploration camps. We believer that it is sufficient to indicate where large cost differences between the two approaches may originate, such that the reader may map those on to their specific projects as appropriate. Consequently the reviewer’s request that we identify a ‘proportion’ of information derived from both techniques would be so full of caveats to be ultimately meaningless. We have added a comment to the effect that our comments are general in L729, to make clear the information implicit in the original text .

（5）The discussion of the absence of inclusions in Tullichglass samples is Insufficient. 

The inclusion occurrence rate in the 48 gold particles from Tullichglass is 0% (Table 2), compared with 45% at Lead Trial. In lines 452–457, the authors relate the inclusion characteristics of alluvial gold to Tullichglass (rather than Lead Trial), concluding that "the relatively low abundance of sphalerite in the mineralization at Tullichglass is consistent with the inclusion signature of the alluvial gold." However, the logical basis of this inference is: Tullichglass gold particles contain no inclusions → therefore sphalerite content in its mineralization is low. This is a non-causal argument—inclusion absence may have multiple explanations (e.g., small gold grain size resulting in insufficient cross-sectional observation area, polishing quality, etc.) and cannot be directly equated with the absence of a mineral from the paragenetic assemblage.

Authors should discuss possible reasons for the 0% inclusion rate at Tullichglass (is a sample size of 48 particles sufficient? Does the smaller grain size (~100 µm, line 321) reduce the probability of capturing inclusions in cross-section?). Besides, consider supplementing with systematic thin-section observations of sulfides in Tullichglass mineralized quartz veins as an independent constraint.

We acknowledge that the size of the sample population from the Tullichglass locality is less than ideal, and in a perfect world we would have had access to further material, potentially yielding an inclusion signature for comparison with other samples. Our task is to interpret the alluvial signatures in terms of the in-situ samples at our disposal. Sphalerite inclusions are rare in the alluvial sample, which is  clearly  inconsistent with the gold from Lead Trial, but potentially consistent with gold from Tullichglass. We are not implying that gold from Tullichglass cannot contain inclusions: inclusion abundance is variable (demonstrably so) such that the methodology strives to generate a sample size to mitigate inclusion rarity. Unfortunately in this instance, that aim was not successful, and we were constrained by the gold particle abundance within the veining of float boulder accessible for sampling. We have altered the text to better emphasize the possible (and impossible) relationships, and linked these clearly to our interpretations.  (L466)

The reviewer again asks questions about fundamentals of gold compositional work with respect to inclusion abundance in relation to particle size and sample preparation which in effect suggest poor rigour in methodological approaches. Gold compositional studies do have such limitations, which have been previously reported in reviews of the methodology. These limitations do not fatally undermine gaining information form the data sets whose interpretation provides insights that cannot be gained form other approaches.

We have access to SEM mages of mineralized vein material from Tullichglass, but did not include them because they did not add information over and above that available from optical examination of the same material.

2. Minor Comments

（1）Sections 2 and 3 contain substantial overlapping content.

Section 2.3 "Deposit Geology" already describes vein characteristics, mineral assemblages, alteration types, and gold deportment (lines 132–142). However, Section 3.1 "Studies of in situ and float mineralization" repeats nearly identical content—re-describing brecciated vein textures (line 178), galena and sphalerite (lines 180–181), chalcopyrite (line 181), wallrock alteration (lines 182–183), and the association of gold with galena (lines 184–185). The only distinguishing material in Section 3.1 is the description of Tullichglass boulders and a brief discussion of the limitations of previous petrographic work. I’d like to recommend authors to merge Sections 2.3 and 3.1 into a single "Deposit Geology and Mineralization" section.

We have refined our manuscript structure to provide an overview of deposit geology, before recent exploration, and moved outcomes of recent exploration into the new Results section

（2）Section 3 is titled "Historical Exploration" but contains modern research content

The Section 3 heading implies a review of historical exploration activity. The first part (lines 148–163) does describe work from the 1860s to 1990s. However, Section 3.1 (lines 171–207) and Section 3.2 (lines 208–220) actually describe recent MSc/PhD research and regional gold compositional results—these are not "historical" in the conventional sense. This creates confusion regarding the chronological framework.

We acknowledge this inconsistency and have made appropriate changes in the context of our response to Section 2.

（3）The Abstract is overly long and contains excessive methodological detail

The abstract spans approximately 220 words across 17 lines, but roughly 40% is devoted to methodological detail ("standard geochemical, geophysical and petrographical approach"; "703 gold particles from local in situ and alluvial occurrences"). The core innovative contribution is insufficiently distilled. Additionally, the final sentence—"Although increasingly common in studies with an academic focus, gold compositional studies remain generally underutilised in commercial exploration"—is an editorial comment rather than a research finding.

The design of the abstract was in part influenced by comments of previous reviewers for this Journal who specifically requested such detail. In this instance we will follow any forthcoming steer form the Journal Editor. We have removed the final sentence as the Reviewer suggests

（4）The Conclusions introduce new claims not adequately supported in the body text

Lines 768–769 assert that "in many alluvial gold districts the approach can provide insights into the nature of source mineralization in the absence of any other geological context." This is a new generalization about the method's universal applicability, yet the manuscript is based on a single case study (Lead Trial) and does not systematically demonstrate applicability across "many alluvial gold districts." Conclusions should not introduce claims that have not been adequately argued in the Discussion. Therefore, either move this claim into the Discussion and support it with additional literature (e.g., quantitative comparisons with other successful case studies), or adopt more cautious language in the Conclusions.

Of course we acknowledge the comments about the remit of the conclusion. We have made alterations to the text in the Discussion to better link the findings of this Case study, with general observations made in others, where gold studies and exploration work were not synchronous.

（5）Some Typographical Errors

Line 99: "ead Trial" → Should be "Lead Trial" (missing letter "L").

Line 320: "Gold recovered from crushed ore at Dunan" → The location "Dunan" is not introduced or defined anywhere in the text. Based on context, it should be "Tullichglass".

Line 49: "[8.9]" → Should be "[8,9]" (incorrect citation format).

Lines 118–119: "Tay Nappe32" → The numeral "32" should be a superscript reference number; formatting error.

Line 413: "Figures 6K and F" → Based on the caption and context, should be "Figures 6K and L".

Line 594: "study rea" → Should be "study area".

Inconsistent units throughout: "g/T" and "g/t" are used interchangeably (e.g., lines 237, 241, 244). These should be standardized to "g/t".

"In situ" vs. "in-situ": Inconsistent hyphenation throughout. Should be standardized to "in situ" (no hyphen), consistent with Minerals journal style, except when not used as an adjective.

Lines 774–780 (Author Contributions): Placeholder template text "For research articles with several authors, a short paragraph specifying their individual contributions must be provided." remains and should be deleted.

Line 781 (Funding): Contains the editorial instruction "Please add:" before "SW and LS were in receipt of NERC studentships." The "Please add:" should be deleted.These errors have been addressed

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

General comments

This manuscript presents a petrographic, crystallochemical, and geochemical study of alluvial gold grains from a Scottish site, with the objective of linking placer particles to mineralized vein outcrops. The authors distinguish two types of Au–Ag alloys containing 23–32% Ag, characterized by different silver contents and specific sulfide inclusion assemblages. These observations may indeed reflect lateral variations in vein assemblages and fluid evolution.

Overall, the manuscript is well written, easy to follow, and logically structured. The scientific question, analytical approach, and interpretation are clearly presented from the introduction through to the conclusions.

However, several points deserve further consideration.

First, while the recognition of lateral variations in vein assemblages is interesting, this alone does not establish any relationship with the auriferous potential of the different assemblages. The discussion would benefit from addressing whether one assemblage may be more favorable for gold concentration or preservation than the other, and what metallogenic implications could arise from these differences.

Second, it is somewhat regrettable that more advanced analytical techniques were not employed. In particular, chemical mapping of gold grains using micro-XRF, SEM-EDS, or electron microprobe imaging could have provided valuable information on internal zoning, remobilization features, or compositional heterogeneity. Isotopic approaches could also have strengthened the genetic interpretations.

The metallogenic interpretation also requires greater caution. The authors largely rely on literature-based models, but some aspects appear insufficiently supported by the presented data. In particular, it is difficult to accept the inference that these vein systems were produced by mantle-derived fluids. Likewise, the proposed relationship with magmatic fluids remains highly speculative when compared with similar hydrothermal assemblages documented elsewhere in Western Europe. The term “magmatic-hydrothermal” should be avoided unless direct evidence for a magmatic contribution is demonstrated, as it otherwise becomes largely meaningless from a genetic perspective.

It should also be emphasized that Bi-tellurides are typical of relatively low-temperature hydrothermal systems (commonly around 250–300 °C) and are widespread in many European ore systems without necessarily implying any direct magmatic affiliation. Similarly, molybdenum may result either from leaching of earlier granite-related pre-concentrations or from later hydrothermal remobilization processes.

I would also draw attention to the quality of the figures. Several figure captions and labels appear to have been degraded during formatting and insertion into the Minerals template, and parts of the figures have become difficult or impossible to read. This issue should be carefully corrected before publication.

Finally, the term “geophysical” should be removed from the abstract, as no geophysical dataset or interpretation is actually presented in the manuscript.

Conclusion

In conclusion, this manuscript could be accepted for publication after minor revisions. The main improvements required concern:
(i) a more cautious reconsideration of the metallogenic model and genetic interpretations, particularly regarding the inferred magmatic or mantle contribution;
(ii) revision and improvement of figure quality and readability; and
(iii) if possible, the addition of chemical mapping data at the scale of individual gold grains, which would significantly strengthen the study.

Comments for author File: Comments.pdf

Author Response

Comments and Suggestions for Authors

 

We thank the reviewer for these detailed comments, which in the main we found very helpful.

General comments

This manuscript presents a petrographic, crystallochemical, and geochemical study of alluvial gold grains from a Scottish site, with the objective of linking placer particles to mineralized vein outcrops. The authors distinguish two types of Au–Ag alloys containing 23–32% Ag, characterized by different silver contents and specific sulfide inclusion assemblages. These observations may indeed reflect lateral variations in vein assemblages and fluid evolution.

Overall, the manuscript is well written, easy to follow, and logically structured. The scientific question, analytical approach, and interpretation are clearly presented from the introduction through to the conclusions.

However, several points deserve further consideration.

First, while the recognition of lateral variations in vein assemblages is interesting, this alone does not establish any relationship with the auriferous potential of the different assemblages. The discussion would benefit from addressing whether one assemblage may be more favorable for gold concentration or preservation than the other, and what metallogenic implications could arise from these differences.

We agree, and this subject is already discussed in L533-539, and is revisited during discussion of relative importance of gold types in L 686.

Second, it is somewhat regrettable that more advanced analytical techniques were not employed. In particular, chemical mapping of gold grains using micro-XRF, SEM-EDS, or electron microprobe imaging could have provided valuable information on internal zoning, remobilization features, or compositional heterogeneity. Isotopic approaches could also have strengthened the genetic interpretations.

The purpose of the ms was to compare the standard methodological approaches of gold particle characterization with standard exploration approaches. Whilst it is true that other analytical techniques could add layers of detailed information to gold particle characterization, the important outcomes of gold particle compositional heterogeneity, and implications for gold paragenesis are available from combinations of BSE imaging and parallel analysis by EMP such as reported here. A more academic study focussing on element remobilization might have required more detailed information on Hg distribution, and perhaps Cu distribution in alloy in Type 1 gold, but this information would not have added value in an exploration context,. Isotopic approaches are currently outwith gold characterization methodology in exploration contexts, but a recent paper including S isotope data of sulphides from Lead Trial has already been cited (Webb et al 2025), which provide a clear indication of the nature of the ore fluids.

The metallogenic interpretation also requires greater caution. The authors largely rely on literature-based models, but some aspects appear insufficiently supported by the presented data. In particular, it is difficult to accept the inference that these vein systems were produced by mantle-derived fluids. Likewise, the proposed relationship with magmatic fluids remains highly speculative when compared with similar hydrothermal assemblages documented elsewhere in Western Europe. The term “magmatic-hydrothermal” should be avoided unless direct evidence for a magmatic contribution is demonstrated, as it otherwise becomes largely meaningless from a genetic perspective.

It should also be emphasized that Bi-tellurides are typical of relatively low-temperature hydrothermal systems (commonly around 250–300 °C) and are widespread in many European ore systems without necessarily implying any direct magmatic affiliation. Similarly, molybdenum may result either from leaching of earlier granite-related pre-concentrations or from later hydrothermal remobilization processes.

Several comments here.

In our view the term ‘magmatic- hydrothermal’ is a useful descriptor to differentiate deposit models form the metamorphic fluid models proposed for many orogenic gold deposits. As the deposit models for gold mineralization in this region are not wholly clear, we find it useful to use this term to acknowledge a magmatic influence/input o the ore fluid, as this immediately provides limitations on a deposit model.

 A magmatic – hydrothermal model was suggested for Lead Trial on the basis of an earlier S isotope study, (Webb et al. 2025:  cited) which showed sulphide isotope signatures at Both lead Trial and nearby Tomndashan (Intrusion hosted mineralization) commensurate with magmatic fluids. The ‘mantle influence’ is attributed to a recent study by Lyell et al. 2025  (also cited).

We accept that Bi tellurides may be  present in a range of polymetallic gold- bearing mineral assemblages. Our inferences are based on global studies of inclusion signatures within gold particles, and the comparison of the data reported here with our previously reported compositional templates, entirely derived through or own work (presumably the ‘literature- based models’ referred to by the reviewer). On the basis this approach  a magmatic- hydrothermal origin is the most likely, and completely in agreement with the isotopic data as indicated by reference described above, as we state in L696.

I would also draw attention to the quality of the figures. Several figure captions and labels appear to have been degraded during formatting and insertion into the Minerals template, and parts of the figures have become difficult or impossible to read. This issue should be carefully corrected before publication.

Absolutely, we are in discussion with the editorial office concerning figure corruption during importation.

Finally, the term “geophysical” should be removed from the abstract, as no geophysical dataset or interpretation is actually presented in the manuscript.

This is not actually the case, as the interpretation of the ground based magnetic surveys were spatially consistent with float trains. (Table 2), and was an influence in determining drill targets.

Conclusion

In conclusion, this manuscript could be accepted for publication after minor revisions. The main improvements required concern:
(i) a more cautious reconsideration of the metallogenic model and genetic interpretations, particularly regarding the inferred magmatic or mantle contribution;
(ii) revision and improvement of figure quality and readability; and
(iii) if possible, the addition of chemical mapping data at the scale of individual gold grains, which would significantly strengthen the study.

Reviewer 3 Report

Comments and Suggestions for Authors

This publication is an excellent methodological study of gold deposit research. It demonstrates a multi-tool research approach that allows for a comprehensive discussion of the deposit's genesis. It requires no further additions and should be published in its current form. The complex, multi-tool methodology requires teamwork, which is evident in the publication. The conclusions at the end of the publication are outlined at a high level of generality. It seems that this complex methodology could be presented in sections as a step-by-step guide for other researchers who would like to use it when considering similar issues.

Author Response

No response is required for this review

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I thank the authors for having considered my comments.
I do not have any further comments.