Targeting High-Grade Mineralization via a Synthesis of Compositional Profiles of Alluvial Gold with Structural and Paragenetic Models

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is devoted to a topical issue – the relationship between ore and placer gold. To solve this problem, the authors proposed a comprehensive approach based on both the use of native gold properties (composition and set of mineral inclusions) and the structure of the region. A case study of one of the gold-bearing regions of Scotland shows convincing evidence of the relationship between ore and placer gold. However, the manuscript does not pay sufficient attention to some general questions related to the present topic.

In particular, when discussing the possible relationship between ore (in situ) and placer gold, it is necessary to take into account the change in the properties of gold particles during their transition from hypogene ore through eluvium and deluvium to alluvium. For example, some particles (low-grade) may dissolve in the oxidation zone, while small ones will be carried away by the water flow from the placer. In addition, it should be borne in mind that particles from the upper horizons of ore bodies entered the placer, the properties of which may differ significantly from the properties of particles from the lower horizons. It is also possible that gold particles from near-ore metasomatites entered the placer.

It should also be noted that the silver content in native gold particles is a fairly convergent feature; the same silver content can be observed in ore gold of different mineral associations. Therefore, along with studying mineral inclusions in gold particles, it would be very useful to involve information on the content of other isomorphic impurities (Cu, Sb, Hg, Pd). I would like to recommend that the authors use the analysis data taking into account the size (weight) of gold particles when constructing Ag profiles in order to ensure equal representation of particles of different sizes in the curve.

Other comments can be found in the atteched file 

Comments for author File: Comments.pdf

Author Response

Reviewer 1

1. In particular, when discussing the possible relationship between ore (in situ) and placer gold, it is necessary to take into account the change in the properties of gold particles during their transition from hypogene ore through eluvium and deluvium to alluvium. For example, some particles (low-grade) may dissolve in the oxidation zone, while small ones will be carried away by the water flow from the placer.

We agree that the pathway of host- placer particle may include residence in unconsolidated material. In the case of the study area, there was most weathering of the near surface outcrop prior to Quaternary glacial events. The degree to which small gold particles could be removed by chemical processes cannot be established, but their removal by fluvial processes is almost certain, as described in the text. Visual studies of sections of around 280 gold particles from the Eas Anie burn near the Cononish mine undertaken by Spence- Jones (2001) did not reveal any internal microfabrics consistent with particle modification in the surficial environment. As the geomorphological setting of the Eas Anie sampling site is entirely typical of others in this study we are confident that our analyses suites of placer gold reflect the hypogene alloy characteristics of those particles. We have added text in section 4.2 to clarify this

1. In addition, it should be borne in mind that particles from the upper horizons of ore bodies entered the placer, the properties of which may differ significantly from the properties of particles from the lower horizons. It is also possible that gold particles from near-ore metasomatites entered the placer.

We are mindful of Ag variation in gold particles from a single orebody: indeed that subject lies at the heart of this paper. In this case the variation in Ag within the orebody is related to paragentic stage: all of which may be present in the same ‘horizon’ of vein at Eas Anie. There may indeed be some systematic variation in Ag content in each paragenetic stage relating to the position of the gold in the vein, but this would be hard to characterize without systematic sampling underground over a greater vertical extent than afforded by the mine itself.  In addition it is impossible to infer systematic alloy composition variation from the placer samples given the range of Ag values recorded overall (8->50 wt %). The potential for ‘near ore metasomatites’ is very small given the cm scale potassic alteration adjacent to the known in situ occurrences.

 

• It should also be noted that the silver content in native gold particles is a fairly convergent feature; the same silver content can be observed in ore gold of different mineral associations. Therefore, along with studying mineral inclusions in gold particles, it would be very useful to involve information on the content of other isomorphic impurities (Cu, Sb, Hg, Pd). 

The standard analytical suite is designed to detect Au, Ag, Cu, Hg. All measurements of Cu and Hg were below the LOD values of 200 pmm and 3000ppm respectively. We have noted that Cu and Hg values were all below LOD, but the Methodology section has been amended to specify the LOD vales.. Regarding Sb and Pd we are aware that these element can on occasion be present as a component of natural gold alloys. A screening of a sub sample of gold particles from Cononish using La-ICP-MS revealed absence of Pd, and variable Sb between 0 and 80 ppm. Using the standard analytical protocols we generally obtain a LOD value of 200ppm for Pd, and would expect something similar for Sb. Therefore these elements were not included in the analytical protocol. We agree with the reviewer that Sb has the potential to act as a discriminant in some cases because of our previous work that identified Sb in LA-ICP-MS analyses of gold from many environments worldwide. However in order to to gain any  quantitative data it is necessary to employ  analysis times incompatible with the size of the sample suites and analytical  facility availability.

1. I would like to recommend that the authors use the analysis data taking into account the size (weight) of gold particles when constructing Ag profiles in order to ensure equal representation of particles of different sizes in the curve.

Our workflow does not include characterization of the mass of individual gold particles by weighing. Furthermore, manipulation of the sample suite used to generate cumulative plots as suggested here would preclude direct comparison between populations, which is the main function of that approach. We have reported that the size distributions of gold particles in observed in thin section and those derived from placer populations are near mutually exclusive: which we have ascribed to the hydrodynamic behaviour of gold that controls residence time in fluvial settings and capacity to recover gold by hand panning. We have not made changes according to the Reviewer’s  suggestion because we do not consider small variation in the particle size of gold be particularly instructive , and hence our workflow does not include this sub stage.

 

In addition, Reviewer 1 made several comments in the text:

P7 : add location of the analytical facility: amended

Fig 3 labelling issue: resolved

Fig 4 caption: modified to include reference for mineral abbreviations

Page 9: comments re Sb analysis and LOD values. See discussion re Sb above. LOD values added to the text.

Fig 5: negative gradient on plot. This is simply an artefact of the line smoothing of software when plots have limited data points. We have modified the plot

Fig 5 labelling: caption changed

Page 11 comment on the size of gold particles: see discussion above

Page 13: image of Y bearing minerals. We acknowledge that an image would be useful to demonstrate that these inclusions are coeval with gold. However we don’t consider it essential: and have sought advice from the editors.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The article is devoted to the gold compositional studies which promote to resolve genetic relationships between alluvial and in-situ occurrences based on the assumption that the P-T-X conditions at the sites of mineral deposition is reflected in common compositional signatures of gold. It contains results of the new original research.

The authors propose approach combining the alluvial gold signature, a structural model and (if possible) in-situ gold signature and paragenesis, as a powerful exploration tool.

The manuscript is clear, relevant for the field and presented in a well-structured manner.

Only 25% of 60 cited references are recent publications (within the last 5 years), but this does not reduce the quality of the publication. All cited references are relevant. 

Self-citation is within reasonable limits.

The conclusions of the article are scientifically sound and supported by factual data.

All tables and graphic illustrations are appropriate, informative, easy to interpret and understand. The data illustrations correspond to the text of the manuscript.

The reviewer has some comments on the text of the article:

1. p.2, second paragraph. Authors wrote

“only a relatively small number of deposit-scale studies have differentiated between gold compositions in different paragenetic stages. Even fewer have linked the findings to a wider geological evolution”.

As the studies which have differentiated between gold compositions both in different paragenetic stages and linked the findings to a wider geological evolution are of a great geological interest for determination of the links between placers and in-sity gold occurrences. So, it will be suitable to list these articles. One of the studies which researched evolution of gold-bearing system (primary in-situ mineralization – intermediate hosts – placer deposits) in dependence of different hydrothermal mineralization is:

Lalomov A.V, Naumov V.A., Grigorieva A.V., Magazina L.O. Evolution of the Vagran gold-bearing placer cluster (Northern Urals) and prospects for revealing bedrock mineralization // Geology of Ore Deposits. 2020. No. 5. P. 407–418. DOI: 10.1134/S1075701520050049

 2. p.2, second paragraph:

The authors submit that “Most importantly, the bulk of the gold values were contained within the second paragenetic stage”, but it is not obvious on the figure 2, where Au-Ag alloys are listed at approximately the same rank in stages 2, 3 and 4 – the line is of the same width.

3. notes on the Methodology section:

The authors used generally accepted methods of studying gold particles in polished sections to study the chemical composition of grains and determine inclusions. This allowed them to draw important conclusions about the paragenetic evolution of gold-bearing mineralization. Unfortunately, the authors did not use another classic method used in the study of placer gold – the study of morphology of the gold particles. According to this technique, after mounting of the gold particles on double sided adhesive tape, they are first photographed in BSE, and only then poured with resin and polished.

This makes it possible to study not only the chemical composition? inclusions and internal structure of the grains, but also the morphology. First of all, the roundness and the nature of the surface, which indicates the range of transfer from the primary source.

In the proposed work, it is assumed that all the gold in the studied placer occurences has the proximal character. It is quite possible, but in some cases, even in small watercourses, we find gold of different types. Thus, in the work cited by the authors (Lalomov et al., 2023) in a small placer of the Sukhoi Log Creek (draining the field of the giant quartz–sulphide veinlet-disseminated mineralization deposit of the same name), 60% of gold grains have a proximal character, and 40% have the distal characteristics with evidences of intermediate hosts. The study of morphology to clarify the nature of gold transfer (distal or proximal) is especially important for placer occurences that do not have a specific source (Glen Corallan, Allt Auchtertrye, Crom Allt).

Also, the work does not say anything about gold-rich rims which indicate. a complex history of the gold particles, most likely related to intermediate hosts.

 4. Note to figure 3

It is necessary to indicate the indexes A, B and C on the figure 3.

5. Note to figure 5

What means the yellow field on the figure 5C? The explanation is in the text, but it is absent in the symbols of the figure. 

This is a decent article., presenting a new comprehensive approach to the study of primary and placer gold relationships, as well as structural controls on gold mineralization. The paper is not without its shortcomings - not all possible methods have been used, the local nature of placer gold is not always substantiated. There are a number of technical comments on the manuscript design. However, with minor editing and without re-review it can be published in Minerals.

Author Response

Reviewer 2

1. p.2, second paragraph. Authors wrote

“only a relatively small number of deposit-scale studies have differentiated between gold compositions in different paragenetic stages. Even fewer have linked the findings to a wider geological evolution”.

As the studies which have differentiated between gold compositions both in different paragenetic stages and linked the findings to a wider geological evolution are of a great geological interest for determination of the links between placers and in-sity gold occurrences. So, it will be suitable to list these articles. One of the studies which researched evolution of gold-bearing system (primary in-situ mineralization – intermediate hosts – placer deposits) in dependence of different hydrothermal mineralization is:

Lalomov A.V, Naumov V.A., Grigorieva A.V., Magazina L.O. Evolution of the Vagran gold-bearing placer cluster (Northern Urals) and prospects for revealing bedrock mineralization // Geology of Ore Deposits. 2020. No. 5. P. 407–418. DOI: 10.1134/S1075701520050049

We have added this reference

 

2. p.2, second paragraph:

The authors submit that “Most importantly, the bulk of the gold values were contained within the second paragenetic stage”, but it is not obvious on the figure 2, where Au-Ag alloys are listed at approximately the same rank in stages 2, 3 and 4 – the line is of the same width.

We agree with the reviewer and have altered the figure accordingly

ACTION RC modify figure

3. notes on the Methodology section:

The authors used generally accepted methods of studying gold particles in polished sections to study the chemical composition of grains and determine inclusions. This allowed them to draw important conclusions about the paragenetic evolution of gold-bearing mineralization. Unfortunately, the authors did not use another classic method used in the study of placer gold – the study of morphology of the gold particles. According to this technique, after mounting of the gold particles on double sided adhesive tape, they are first photographed in BSE, and only then poured with resin and polished.

This makes it possible to study not only the chemical composition? inclusions and internal structure of the grains, but also the morphology. First of all, the roundness and the nature of the surface, which indicates the range of transfer from the primary source.

We have a different opinion on the value of morphological study on gold particles to the reviewer. We have been involved in detailed morphological studies of alluvial particles, in several studies and have a clear view of those cases where study is warranted. The lead author was a co-supervisor of two Canadian MSc studies based at UBC which investigated relationships between the Corey shape factor  (derived from 2 images of each particle mutually at 90˚, subsequently processed using Image J) of individual gold particles and transport distance. For gold particles a few km from their source, this approach worked very well when a gradient factor was applied, but for gold closer to source the dominant factor was the original morphology of hypogene particles: which in many cases was tabular. In addition, in upland areas such as the study area where watercourse gradients are high, the milling effect during transport can be highly variable even over limited transport distances. This has the effect of generating highly variable morphological modifications simply according to chance interactions with large clasts in high energy environments. A further large study carried out as a consultancy project in an upland area of Northern Ireland generated detailed morphological measurements which was integrated with compositional data and structural considerations . The results were frankly meaningless: we speculated both because of the high energy environments and because it seemed likely that the ‘sources’ comprised multiple occurrences within a single watercourse. As the geomorphological environment in the current study area is most similar to that of that previous study in Northern Ireland study, the approach of systematically determining gold morphology was not adopted. Nevertheless, we accept that some morphological features are very useful in providing information regarding proximity to source- and have added comments in the results section to describe in general the features of gold particle populations.

3. notes on the Methodology section:

In the proposed work, it is assumed that all the gold in the studied placer occurences has the proximal character. It is quite possible, but in some cases, even in small watercourses, we find gold of different types. Thus, in the work cited by the authors (Lalomov et al., 2023) in a small placer of the Sukhoi Log Creek (draining the field of the giant quartz–sulphide veinlet-disseminated mineralization deposit of the same name), 60% of gold grains have a proximal character, and 40% have the distal characteristics with evidences of intermediate hosts. The study of morphology to clarify the nature of gold transfer (distal or proximal) is especially important for placer occurences that do not have a specific source (Glen Corallan, Allt Auchtertrye, Crom Allt).

In part we have answered this comment above, and have amended the text to describe general morphological textures. We acknowledge that gold from different origins can be present in the same alluvial setting, and have successfully applied compositional approaches to establish this (see Discussion re Cononish 1 and Cononish 2). The ‘evidence of residence in intermediate hosts’ specified by the reviewer we interpret to refer to compositional modification. We systematically observe internal compositional heterogeneity both to observe features derived from evolving hypogene settings and to check for microfabrics indicative of compositional modification. These are described in section 4.2, and they do not suggest particle modification that would influence our current discussion.

3. notes on the Methodology section:

Also, the work does not say anything about gold-rich rims which indicate. a complex history of the gold particles, most likely related to intermediate hosts.

We believe that the routine compositional characterization of gold rims is unwarranted because they are such a common feature globally. However, the reviewer has made a very useful comment because the absence of gold rims is itself significant. Images of sectioned gold particles from the various localities often reveal particles that either do not exhibit a silver depleted rim or a short potions of thin rim. Unfortunately we do not have systematic data describing the nature/presence of rims for the entire data set and we agree that such information could add interesting information. However, evidence of rim formation during residence in an intermediate environment is not in itself an indicator of transport distance: only local geomorphological  environment.

We have added a comment in section 4.2 to note that in many cases rims are absent. This subject has also been integrated into the discussion section.

4. Note to figure 3

It is necessary to indicate the indexes A, B and C on the figure 3.

This has been corrected

5. Note to figure 5

What means the yellow field on the figure 5C? The explanation is in the text, but it is absent in the symbols of the figure.

A note has been added to the caption

 

Reviewer 3 Report

Comments and Suggestions for Authors

Review of the manuscript by Chapman et al.

This is a very interesting manuscript regarding the alluvial and in situ gold mineralization from three different localities (Allt Ghamhnain, Eas Anie, and Halliday) and adjacent areas of the Cononish Au-Ag deposit in central Scotland. A synthesis of structural considerations is combined with spatial variation in compositional signatures of gold mineralization, aiming to provide a powerful exploration approach. I have no further comments and think this manuscript is well prepared and could be accepted after minor revisions. Below are my comments aiming to provide a final polish to your manuscript:

Line 146-149: A sketch about the geographical location and/or marking a bigger city on map A would be useful, so that foreign readers could easily position the study location. Coordinates of maps are further missing in Figure 1. Please add coordinates.

Line 241-243: Where is located the laboratory for performing SE and BSE imaging? Please clarify the laboratory's name, city, and country.

Line 262: Please add label A, B, and C in Figure 3. Replace 4A, 4B, and 4C by 3A, 3B, and 3C, respectively.

Line 274-275: The mineral abbreviations should be added, while a brief description of figure 4A to 4F should be given in figure caption.

Author Response

Reviewer 3

 

Line 146-149: A sketch about the geographical location and/or marking a bigger city on map A would be useful, so that foreign readers could easily position the study location. Coordinates of maps are further missing in Figure 1. Please add coordinates.

Edinburgh and Perth are already marked on map A. We have added coordinates to Fig 1B

Line 241-243: Where is located the laboratory for performing SE and BSE imaging? Please clarify the laboratory's name, city, and country.

We have amended the text to include this information.

Line 262: Please add label A, B, and C in Figure 3. Replace 4A, 4B, and 4C by 3A, 3B, and 3C, respectively.

Line 274-275: The mineral abbreviations should be added, while a brief description of figure 4A to 4F should be given in figure caption.

We have addressed the issue of mineral abbreviations in the text.