General comments

This manuscript presents a summary of (U-Th)/He goethite ages stemming from 33 published studies and 12(?) unpublished studies around the world. The goal of the manuscript is to provide a database that can be used by referred to and added to by future studies.

 

However, while I see the value in providing such a database, particularly one that is accessible and can be added to in the future, I question whether the manuscript really contains enough information to form a publication without more information and some degree of interpretation of the results.

 

The manuscript would be a great resource to learn about all the environments goethite can form in and what dating it can reveal, e.g. deposition and weathering of sediments, or enrichment of ore deposits. However, it doesn’t provide any insights into these processes using the data that has been compiled. Again, I can appreciate that for a short communication in a journal focused on geochronology, this might not seem important, but it felt to me that with this dataset more effort could be put into telling the reader what it shows.

 

I also think a lot more information needs to be clarified from the start, particularly terminology that won’t be familiar to all readers of Geochronology, such as terms like ‘supergene’.

 

The figures are quite poor and lack important information. They are also rather low resolution so it is quite difficult to discern key features within them, and often difficult to read labels and axes. They often lack labels and detail that would make them much more useful to the reader. I discuss each figure separately below.

 

Overall, I think this database is a useful contribution to the community but I think it would benefit from a more thorough introduction and a bit more of an attempt to discuss the trends that the database reveals and what they might tell us about “changes in global environmental conditions through time” as mentioned in the Summary.

 

Specific comments

Clarify what is meant by “supergene” from the start. Surface weathering…. Deposits?

 

Line 23: Can you be more specific than “millions of years”?

 

Line 26: Please give some examples of the “various settings”.

 

Line 32: I think the concept of supergene enrichment of ore deposits needs to be introduced before it is raised in this sentence: “further show the influence of paleoclimate on weathering and supergene ore genesis”.

 

Line 33: What are pisoliths?

 

Line 39: Without a more comprehensive introduction, it is unclear how the goethite (U-Th)/He database will elucidate changes in the frequency of precipitation, nor how that can be tied to environmental conditions.

 

Line 59: Define “laterites” and “ferricretes”. This could form part of a more comprehensive introduction before diving into the detail of the database. This would also help to explain why access to open-pit mining operations is helpful.

 

Figure 1: In the text, various country names are provided but they are not labelled on the map. Please label or highlight all countries discussed in the text, particularly those listed in Figure 4.

 

Lines 72–73 (Fig. 1 caption): Add the age ranges for the Miocene, Pleistocene, and Pliocene groupings in parentheses so it is easier to compare them with the ages provided in the figure. Are the groupings in the figure related to Miocene, Pleistocene, and Pliocene? If not, explain why you have grouped them as you have. Be clearer that almost all of the ages are Cenozoic. Rather than talking about 65 Ma, be clear that this is the Cenozoic/Mesozoic boundary, so very few are Mesozoic (big climate changing event at this boundary?).

 

Line 89: Briefly tell us what banded iron formations are.

 

Line 93: Give formulae of hematite and magnetite. Give examples of iron-bearing carbonates. Also might be worth mentioning that hematite and magnetite can and have also been dated using (U-Th)/He method but that you are just focusing on goethite here?

 

Line: 98: Explain the concept of “lateritized”. A brief introduction to laterization would be a useful part of an expanded introduction.

 

Line 100–101: What do you mean by “absolutely” and “relatively” iron enriched? Can you clarify?

 

Line 103: When you say the primary hematite is inherited from the bedrock, do you mean it is a primary within it? If so, maybe just say “primary hematite within the bedrock” rather than “inherited from it”?

 

Lines 103–104: This is confusingly worded. I would rewrite this section along the lines of: “Goethite dominates the mineralogy of cangas (Monteiro et al., 2014), whereas older, primary hematite within the bedrock dominates in the lower horizons (Monteiro et al., 2018a), and goethite is restricted to veins or local replacement of minor carbonates or silicates (e.g. siderite or grunerite).”

 

Lines 127–128: What does it mean if iron cementation “intensifies”? Do you mean the abundance of goethite increases? And how does that relate to no goethites younger than ~5 Ma? Presumably the lack of goethites after ~5 Ma is counter to this intensification? If so, maybe replace “notably” with “although” or “however”.

 

Lines 131–132: What are massive sulfide deposits and iron-oxide copper gold deposits? (presumably IOCGs are a type of massive sulfide deposit?) These could be briefly introduced in a revised Introduction, as suggested previously. Presumably it’s the Fe sulfides (e.g. pyrite) producing the Fe for the goethite? But this isn’t made clear.

 

Line 141: Weathering of which sulfides will produce Fe2+ and Cu2+? Does the sulfide being weathered have any bearing on what Fe-oxide is produced (goethite vs hematite?).

 

Line 188: You mention here “geochronology of goethites and hematites” but this is the first time mention has been made of dating hematite. The potential to (U-Th)/He hematite, and how this compares with goethite, should be raised earlier.

 

Line 201: It would be good to see this area on a map. Or at least say how far it is from the mouth of the Amazon River to Rio de Janeiro?

 

Lines 230–231: What is the significance of the different coloration? They give different ages, so presumably their colors are meaningful? Do they get darker each time they are re-weathered? Is that why black ones are the youngest group…?

 

Line 265: Replace “of” with “the” so the sentence reads “Massive goethites lack the growth bands notable in colloform goethites”.

 

Figure 2 doesn’t do justice to the range of goethite textures discussed in the text. To get a true understanding of how goethite presents itself, textural relationships with other phases, and implications for dating (e.g. grain size, mixing with other U-Th-bearing minerals), there should be thin section and, ideally, SEM images showing some examples. And where are these samples from? Are they samples that form part of the database?

 

Line 277: Can you show an illustration of a single grain with multiple generations of goethite? Is anything visible in such a grain to tell? Zonation in SEM? If there is no way to tell prior to dating, this is also useful information.

 

Line 284: How rapid is rapid? Can you be more specific? And is this based on textural relationships/thin rims or (U-Th)/He dating?

 

Line 286 (section 4.5 title): Since both are discussed, should this title be “Goethite replacing wood fragments and soft tissue organisms”?

 

Line 316: Give formulae of uraninite and coffinite.

 

Line 325–326: The possible presence of microscopic U-Th-bearing minerals, such as monazite, should be addressed previously in the manuscript. How big an issue can it be? How can it be avoided? (again, an improved Fig. 2 showing SEM images would help to explore this issue).

 

Line 327: Which geological environments are likely to have simultaneously enriched U and Th contents?

 

Lines 339–341: “The high Th contents of goethite cements and pisoliths reveal significant sources of detrital Th minerals (e.g., monazite or thorite) in the sedimentary units….” Are you referring to Th that has been remobilized from monazite and thorite and incorporated into the goethite as it grew or inclusions of monazite and thorite grains incorporated within the goethite? This is unclear but has potential implications for (U-Th)/He dating if such inclusions are not accounted for.

 

Figure 3: It is very difficult to discern the axes on the plots but I am assuming they are Th (ppm) vs U (ppm). I suggested labelling each panel (a) – (i) so they can be referred to more clearly in the text.

 

Lines 357–358: The USA histogram in Figure 4 doesn’t look “right-skewed” to me, although I’m not certain what “right-skewed” means in this context.

 

Lines 359–360: “Goethites from Morocco cluster at ~95-50 Ma…” but there appears to be a bimodal population, with a dip around 75 Ma. How do you account for that? Or, if you can’t account for it, then at least make note of it.

 

Line 360: When you say “Globally, young goethites”, how do you define “young”?

 

Lines 386–387: “Some of the oldest goethites contain significantly low eU concentrations while relatively young goethites contain hundreds of ppm eU”. Any thoughts on why this might be?

 

Line 402: “Future methodological developments” is rather vague. Such as? Along what lines?

 

Technical corrections

Line 38: Remove the word “aim” and just say that you “assess the influence….”.

 

Try not to switch between present and past tense throughout the manuscript, e.g. Lines 47–48 “For cases where the mass of the analyzed grain was not provided and parent element amounts are reported, it was impossible to calculate concentrations in ppm”

 

Line 50: Rather than “bedrock”, perhaps better to say “lithology”?

 

Line: 57: Change “map distribution” to “spatial distribution”.

 

Line 61: Replace “similar geographic positions” to “similar latitudes”.

 

Line 70: In the Figure 1 caption, I would perhaps qualify the first sentence to say “…distribution of dated goethites included in this study” in case you missed any.

 

Line 74: Label “Amazon and Quadrilatero Ferrifero” and “Hamersley Province and Flinders Ranges” on the map in Figure 1.

 

Line 80: Define “duricrust”.

 

Line 99: Refer here to Fig. 1, where you need to have labelled these specific locations.

 

Line 101: replace “transition” with “transitions”.

 

Line 101: Define “saprock” – either here or in the introduction.

 

Line 106: What is “hypogene”? – another definition for the introduction.

 

Lines 102–114: Monteiro (2014) and Monteiro (2018a) are cited 7 times within these 12 lines. Try to consolidate them so it isn’t so repetitive.

 

Line 130 (and 139): Define “Gossan”. Since this is typically applied to ore deposits, it’s important to explain it. Add it to the broader introduction.

 

Line 138: Define “mottled zone”.

 

Line 145: Replace “too fine grained and not suitable….” with “too fine grained to be suitable…”

 

Line 150: “Goethites precipitated at depth” – what kind of depth?

 

Line 161: Define “limonite”.

 

Line 189: “targeting samples on Mars…” rather than “…at Mars”.

 

Line 220: Typo – Should be “Morocco” rather than “Morroco”.

 

Line 294: Remove “very” from “very crystalline”.

 

Line 300: Replace “following” with “followed”.

 

Line 332: Replace “data was obtained” with “data were obtained…”. And remind the reader that Lynn Peak is in Australia.

 

334: If you give a value for U then you should also give a value for Th (rather than just “very little”).

 

Line 338: “A few goethites plot in-between (Fig. 3)” – I suggest labeling the panels (a) to (i) so they can each be referred to more clearly at various points within the text. And in this instance, point specifically to where the “in-between” data lies so it is clear to the reader.

 

Lines 343–344: Is this fossilized tree trunk goethite the same as shown in Fig. 2g? Or at least from the same locality? If so, refer to the figure.

 

Line 355: Correct “Artic” to “Arctic”.

 

Lines 376–377: Try to rephrase these two sentences so that you’re not citing Flowers et al. (2007) twice in quick succession.

 

The Excel database:

In column A (Authors), provide year as well. Can you add a link to each reference as well for easy access?

 

For Calculated Age, what level of uncertainty is being reported? 2 sigma?

Review of: Short communication: A database of the global distribution of (U-Th)/He ages and U, Th contents of goethites

By: Hevelyn S. Monteiro, Kenneth A. Farley, Paulo M. Vasconcelos

This manuscript provides a database of (U-Th)/He ages (almost 3000 ages) and U-Th contents of goethite samples from different countries and weathering environments in the world. Authors attempt to assess the influence of environmental conditions on changes in the frequency of precipitation and preservation of goethite in weathering profiles. I agree with the authors that their data set may contribute to understanding goethite (U-Th)/He dating of supergene weathering processes. I think this work is suitable for Geochronology, and they have interesting data set to contribute. However, in the paper's current form, I think with a modest and careful effort to revise the manuscript, this will make a nice addition to this journal. My only concerns are: (1) Authors provide a large amount of information on ages and U-Th contents of goethite, but discussion and interpretation are limited; (2) the figures need to be improved; and (3) implication on paleoenvironmental and paleoclimatic studies.

1. “Introduction” -This section authors proposed that goethite dating can provide information on global environmental conditions, but they do not clearly explain the links between goethite formation and environmental conditions. I also suggest authors may add some explanation on why goethite is important and profit to (U-Th)/He dating; why other iron-oxides (lepidocrocite, hematite, magnetite, limonite) are not widely used to (U-Th)/He dating? Is it related to well preservation, crystallinity, high U-Th contents, or specific paleoclimatic condition?

2. “Geological Environments” and “Types of goethite”-Authors spend the great length to describe the geological environment, occurrence, geochemical characters, and Eh-Ph of the goethite. I think those two sections should be combined and reduced, if authors add a summary table that list the location, elevation, profile, depth, paleoclimatic conditions, goethite occurrence (colloform, massive, infilling, pisolitic….), crystallinity, mineralogical associations (goethite+hematite+clay, goethite+magnetite+ilmenite, goethite+Mn-oxides, goethite+gibbsite+gold, goethite+malachite+azurite+cuprite+native copper+chalcocite, ….), geochemical composition (Al, Cu, Ni, P, REE, Co, U, Th, U/Th…), and (U-Th)/He ages of different geological environments, then it is easy to compare the goethite in different geological environments.

3. “U and Th concentrations in globally distributed goethites”- The U-Th concentrations in goethite are very interesting, I hope author may give more interpretation of these results. First, authors only use the reported data in ppm to plot, thus some information may loss. In this case, I suggest authors may plot with element molar ratios, such as U/Sm, Th/Sm, and U/Th…Because we find the U and Th concentrations are variable within a weathering profile and even within a single hand-sample, but U and Th commonly show a strong positive correlation, this indicates they have similar U/Th ratios in certain surficial environments, thus it possibly reflects the source of weathering rocks or others. Second, I suggest U vs Th concentrations plates of distinct geological environments may plot in a single plate, it is easy to compare the U-Th distributions at different condition. I also suggest authors may add U/Th ratios vs deep or Sm/Th ratios, and then compare and discuss the geochemical characters of goethite in different geological environments.

4. “The global distribution of goethite (U-Th)/He ages”-The (U-Th)/He age distribution and age vs eU plots of goethite samples from distinct environments are significant. First, the age distribution plots (Figure 4) show the ages from the most regions (Canada, USA, China, Tunisia, French Guiana, Suriname) are almost younger than 20 Ma, some regions (Brazil, Australia, Switzerland) show goethite formed since >60Ma, but goethite from Morocco formed at 40-100 Ma. I expect authors do not just show the age distribution, but add some explanation on why these regions have distinct age distribution patterns. Second, although eU vs (U-Th)/He age plots do not show positive correlation, goethite with old ages (>~80Ma) have very low eU, especially samples from lateritized continental sediments show low eU in the old age sample but high eU in the young age sample. Is it possible that high eU goethite may damage the mineral’s structure, and thus the He loss of high eU samples cause the young age or U loss of sample cause the goethite have low U and high ages?

5. “Summary”-I expect to what kind of goethite samples are suitable for (U-Th)/He. Also, authors may add some approaches for goethite (U-Th)/He age interpretation in future works; such as, more geochemical works (such as U, Th, Al, P, Si, Ti, Ni, Cu, REE…) on goethite is benefit to understand weathering processes, the researches on the crystallinity of goethite are used to evaluate the He retentive, and stable isotopes (C, N, H, O, Fe, Cu, Zn…) may reflect the paleoclimatic conditions during the goethite formation.