Gold in Irish Coal: Palaeo-Concentration from Metalliferous Groundwaters

Round 1

Reviewer 1 Report

I have reviewed the manuscript with great interest. However, the obtained results are of limited value. I did not learn anything except that a few inclusions of gold and selenium were found. The authors have incorrectly described the tests and results. There is no in-depth analysis of the geological structure of the deposit, no cross-section, basic information about the deposit is not provided. The authors generalized all types of samples. In addition, such observations require reflected light microscopy, which was not performed. Furthermore, chemical analysis of individual samples was not conducted. The content of gold or selenium in selected samples is unknown. The authors discuss platinum, yet the results for this element are not presented. Discussion of the results and conclusions are not supported by the results obtained, at least such results were not presented in the manuscript. In large part the findings are based on literature data.  It should also be borne in mind that the authors discuss the potential of waste from the combustion of the examined coal. However, the authors mention that there are no mines in the studied area.  Therefore, the estimated amounts of the recovered elements are questionable. The other major concern is the English language usage. Even though the English is grammatically correct, the text must be completely rewritten in a more scientific style. In conclusion, the manuscript in its current form is not appropriate for publication in the Minerals and requires major revisions before further consideration.

Specific comments

Introduction

Lines 35, 35, and 39 - please consider providing detailed and relevant references. The authors should indicate countries where the studies on the presence of gold in the coal were undertaken.

Line 54 The authors have demonstrated that Au is associated with siderites. Most of the siderite is removed during processing, so it does not reach the fly ash because it is not burned with coal.

Line 59 It' is not entirely clear what 5-10% Ag means. Please clarify.

Lines 78-80 Citations are the same as above.

3. Study site

There is no information on the coal deposit and the type of coal.  There is no information on the depth of the sampled seam, its thickness, and the type of overburden and roof rocks. The results of proximate and ultimate analysis are required. What is the vitrinite reflectance of the studied coal? What is the ash content? What is the oxide composition of the ash? 

Line 103 „To the authors knowledge, no gold has been reported in these Irish coal deposits.” The sentence must be rewritten in a more scientific style.

109  It is unclear what authors mean with “coal detritus”. Please clarify. This concept is used when discussing the organic matter content in rocks. In this case, we are discussing the coal deposit

4. Materials and Methods

There is no information on the number of samples taken.

5. Results

There is no clear identification of the samples, no information on sampling locations. The results for individual samples are not presented.

Figure 4 – which samples are these? 

What is the Se and Au content from ICP-MS analysis? There is no information on the percentage share of mineralization. Were these individual inclusions?

Line 235 Most of the coal is of Carboniferous-Permian – In the northern hemisphere, a significant proportion of coal is the younger Paleogene and Neogene.

Line 237 “The purity of the coal-hosted gold measured here”. The authors have not conducted analysis confirming this statement.

Line 240 – “ over an order of magnitude more than in the coal-bearing gold”. Please specify what is the content of gold in the examined coal.

Line 246 6.2. Gold and Platinum Source – The authors discuss platinum yet the results for this element are not presented

Line 279 “The high tonnages of coal and the waste derivatives ….” Do the authors mean the potential of the closed mine?  It seems that one should not generalize here.

Author Response

Reviewer 1 comments

C: There is no in-depth analysis of the geological structure of the deposit, no cross-section, basic information about the deposit is not provided.

R: Further information about the study area and regional geology, including generalised stratigraphy, thermal maturity and cross section, added, with new figure included. Worked seams information also included in text and figure. More references included.

C: The authors generalized all types of samples. In addition, such observations require reflected light microscopy, which was not performed.

R: Sample codes, localities and gold presence added to text and table, and descriptions expanded. Our previous attempt to use reflected light microscopy showed pyrite but does not have the scale of resolution to detect micron-scale inclusions.

C: Chemical analysis of individual samples was not conducted. The content of gold or selenium in selected samples is unknown. The authors discuss platinum, yet the results for this element are not presented.

R: Gold was not detected above 0.2 ppm ICP-MS levels of detection. Gold analysis was conducted by LA-ICP-MS on samples where gold was recognised by SEM-EDS, using spot analyses. Two samples showed gold above 0.01 ppm level of detection, and this data has been added to the table and text.  Platinum was not quantitatively detected by ICP-MS or LA-ICP-MS above levels of detection but was detected qualitatively by EDS as a percentage of the grain. Grains are too small to measure quantitatively to any reasonable degree of accuracy by LA-ICP-MS. This explanation has been included in text.

C: It should also be borne in mind that the authors discuss the potential of waste from the combustion of the examined coal. However, the authors mention that there are no mines in the studied area.  Therefore, the estimated amounts of the recovered elements are questionable.

R: The section in question points towards a wider, more global potential for coal and coal combustion products as a viable resource, in active coal mining regions that may have similar chemical and physical attributes to areas like the Leinster and South Wales Coalfields. The authors agree that the current wording of the section is confusing and misleading and have removed the estimates of resource availability and emphasised that this is a global potential rather than one specific to this particular study site.

C: Introduction - Lines 35, 35, and 39 - please consider providing detailed and relevant references. The authors should indicate countries where the studies on the presence of gold in the coal were undertaken. Two papers regarding Au in coal that you should pay attention to:

Seredin, V., Finkelman, R.B., 2008. Metalliferous coals: a review of the main genetic and geochemical types. Int. J. Coal Geol. 76, 253–289

Dai, S., Wang, X., Seredin, V.V., Hower, J.C., Ward, C.R., O'Keefe, J.M.K., Huang, W., Li, T., Li, X., Liu, H., Xue, W., Zhao, L., 2012. Petrology, mineralogy, and geochemistry of the Ge-rich coal from the Wulantuga Ge ore deposit, Inner Mongolia, China: new data and genetic implications. Int. J. Coal Geol. 90–91, 72–99.

R: References added, and localities moved from later section to the introduction here.

C: Introduction - Line 54 The authors have demonstrated that Au is associated with siderites. Most of the siderite is removed during processing, so it does not reach the fly ash because it is not burned with coal.

R: This line has been removed from the introduction, and the siderite removal has been added to the final wider context section. Instead of targeting fly ashes, it is proposed that removal and recovery of pyrite and siderite-rich material during processing may present a concentrated source of trace elements such as gold.

C: Introduction - Line 59 It' is not entirely clear what 5-10% Ag means. Please clarify.

R: Reworded – “silver contents often comprise 5-10% of the alloy”, as detected by EDS.

C: Introduction - Lines 78-80 Citations are the same as above.

R: Text adjusted.

C: Fig. 2. A scale bar is needed. Also legends are also needed.

R: Adjusted figure to include scale bar. Carboniferous coals indicated in key, and sample locality described in caption.

C: Study site - There is no information on the coal deposit and the type of coal.  There is no information on the depth of the sampled seam, its thickness, and the type of overburden and roof rocks. The results of proximate and ultimate analysis are required. What is the vitrinite reflectance of the studied coal? What is the ash content? What is the oxide composition of the ash?

R: More detail added to Study Site section for nature of coal, including seams and thicknesses, and additional coal properties added to results, including vitrinite, vitrinite reflectance, carbon content, mineral matter, inertinite, liptinite and birereflectance. Regional and local geology added, including figure with stratigraphic section, thermal maturity and cross section. Coal seams added and described.

C: Study site - Line 103 „To the authors knowledge, no gold has been reported in these Irish coal deposits.” The sentence must be rewritten in a more scientific style.

R: Rewritten, “The presence of gold has not been previously reported in Irish coalfields.”

C: Study site - 109 It is unclear what authors mean with “coal detritus”. Please clarify. This concept is used when discussing the organic matter content in rocks. In this case, we are discussing the coal deposit

R: Reworded – coal spoil heaps were sampled.

C: Materials and Methods - There is no information on the number of samples taken.

R: Sample number (n) has been added to section, and more sample information added to Table 1.

C: Results - There is no clear identification of the samples, no information on sampling locations. The results for individual samples are not presented.

R: Added details to samples in Table 1, including locality specifics. All results are presented, and specific samples described in text and captions.

C: Results - Figure 4 – which samples are these?

R: Sample code added to figure caption.

C: Results - What is the Se and Au content from ICP-MS analysis? There is no information on the percentage share of mineralization. Were these individual inclusions?

R: Mineralogy expanded with modal abundance given, ICP-MS for Au not given due to being below level of detection (<0.2 ppm, LA-ICP-MS Au shown where available in Table 1 and text). Se content shown in Table and in text.

C: Results - above world average coal concentrations. What does this mean?

R: Value and reference given, based on study of Yudovich and Ketris.

C: Discussion - In these sections, something must be clearly addressed regarding diagenesis and deposition. For coal/peat, we usually refer peat deposition to a syngenetic process, but diagenesis is usually referred to as the rank advance. See more in the following paper:

Dai, S., Bechtel, A., Eble, C.F., Flores, R.M., French, D., Graham, I.T., Hood, M.M., Hower, J.C., Korasidis, V.A., Moore, T.A., Püttmann, W., Wei, Q., Zhao, L., O’Keefe, J.M.K., 2020. Recognition of peat depositional environments in coal: A review. Int J Coal Geol, 219.

R: Information added to emphasise this is likely later burial diagenesis, with the overprint of increasing thermal maturation and fluid migration through time resulting in the mineralogical and geochemical features that are evident in the study. Explanation and reference added to text.

C: Discussion - Line 235 Most of the coal is of Carboniferous-Permian – In the northern hemisphere, a significant proportion of coal is the younger Paleogene and Neogene.

R: Changed to “Many widespread areas”.

C: Discussion - Coal ash is usually referred to as coal combustion production.

R: Reworded here and throughout manuscript.

C: Discussion - Line 237 “The purity of the coal-hosted gold measured here”. The authors have not conducted analysis confirming this statement.

R: Reworded – “The purity of the coal-hosted gold in this study (inferred from EDS analysis)”

C: Discussion - Line 240 – “over an order of magnitude more than in the coal-bearing gold”. Please specify what is the content of gold in the examined coal.

R: Reworded – “Analyses of gold in the bedrock of SE Ireland show silver contents often comprise 5-10% of the alloy [44], compared to the <0.2% silver component observed in this study”

C: Discussion - Line 246 6.2. Gold and Platinum Source – The authors discuss platinum yet the results for this element are not presented.

R: Platinum detected by EDS, but not quantified (below levels of detection) – brackets added to title to downplay the significance of this observation, and a sentence has been added to show how platinum was detected as part of the alloy, but at low concentrations.

C: Discussion - Line 279 “The high tonnages of coal and the waste derivatives ….” Do the authors mean the potential of the closed mine?  It seems that one should not generalize here.

R: Reworded to reflect global significance, rather than this specific mine. Subtitle also adjusted to reflect this.

Author Response File: Author Response.pdf

Reviewer 2 Report

see attached

Comments for author File: Comments.pdf

Author Response

Reviewer 2 comments

C: I would like to see a little more discussion about the odd mineralogy of the gold in this occurrence, especially the Au-Pt alloy which is Ag-free. As I understand it, this is likely to be a new mineral. Other Au minerals contain Ag and the platinoid element usually associated with the Au in natural occurrences is Pd rather than Pt. The sensitivity, of course, is to contamination since Au-Pt alloys may be sourced by commercial catalysts. So it would be worth the authors discussing this in more detail, perhaps including accounts the occurrences of other Ag-free Au-Pt natural alloys.

R: Although rare in nature, natural gold-platinum alloy precipitate grains (free of silver) have been identified in parts of the UK (Leadhills, Scotland, and Devon, England). It may be that these alloys precipitated from groundwaters or mineralising solutions of mixed sources, one or multiple sources that are rich in gold and platinum. Incidentally, Leadhills gold also contain high selenium and similar concentrations of copper, and are noted for their lack of silver content. Gold-platinum-copper alloys have also been described in Russia, in the placer of the Konder alkaline-ultrabasic massif (Nekrasov et al 2001), and in the alluvium along the River Bolshoy Khailyk, western Sayans (Barkov et al. 2018). These examples have been added to the text.

C: It may also be worth referring to the recent extensive review by Sanyal et al. (2019) (Sanyal, S.K., Shuster, J. and Reith, F., 2019. Cycling of biogenic elements drives biogeochemical gold cycling. Earth-Science Reviews, 190: 131-147) and the book on the subject by Boris Osovetsky (Natural Nanogold published by Springer 2017).

R: Reference added for the growing consensus of mobility of gold in bio/sedimentary environment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report