Platinum-Group Minerals of Pt-Placer Deposits Associated with the Svetloborsky Ural-Alaskan Type Massif, Middle Urals, Russia

Round 1

Reviewer 1 Report

Comment to the manuscript by Sergey Yu. Stepanov et al., submitted Submitted to Minerals 423962, December 2018.

Running Title: Platinum group minerals from the placers of various genetic types associated with the Svetloborsky Ural-Alaskan massif, middle Urals, Russia

This manuscript presents a description of the mineralogy, physical features, and mineral chemistry of PGM nuggets from placer deposits related with the Svetlibor ultramafic massif in the middle Urals of Russia. The authors report the results of a study of samples taken from different types of placers located at increasing distance from the load source, and try to identify the physical and chemical changes that may have occurred in the nuggets during their transfer from the erosion site to re-deposition. The recovered nuggets have an average size much higher than 50 μm, judging from the SEM images, and mainly consist of Pt-Fe alloys, with a few other accessory PGM (alloys and sulfides). Weak erosion features such as furrows and scratches on crystal faces affect the PGM nuggets, while mineralogy and composition are essentially preserved, during transfer to placers located at a short-distance (less than 1 km) from the load sources. In contrast, relevant alteration of the morphology occurs during the transport of the material to placers several km far away the load source. This is manifested by rounding of grain-shape and increase of the relative abundance of isoferroplatinum with increasing distance from the source, in contrast with tetraferroplatinum and tulameenite that tend to disappear. This is due to selective erosion of the two latter that originally are found as external coats or rim-replacement of isoferroplatinum. The process, however, does not cause any substantial change in the mineral chemistry of the PGM nuggets compared with composition of PGM in the load deposits. The presented data support the model that the high-grade chromite-platinum massive ore in the lode dunite of Ural-Alaskan complexes are the major contributor to the Pt-nuggets >1 mm in the Ural placers. In view of problems concerning the origin of PGM nuggets in placer deposits, the results of this study do not lead to a conclusive answer, but certainly provide further argument for debate, and may contribute to the on going discussion of question still unexplained.

The manuscript is well organized and properly formatted nevertheless, a minor revision is necessary. I give a short list of discrepancies that should be emended.

Line 2. The title is too long. It can be rewritten:  “Platinum group minerals from Pt-placer deposits of the Svetloborsky Ural-Alaskan massif, middle Urals, Russia”...or something like that. Mention of the different types of placers in the title is probably over abundant.

Line 161-163. Not so many readers are familiar with the “blow-off” method fro the separation of heavy minerals. Could you give a few words of explanation?

Line 181. “...with wavelength detectors...”, do you mean that the analyses were carried out in “WDS-mode”?

Line 188-192. Here we suppose that there are two types of mineralization: 1) idiomorphic  cubic   PGM grains (<50 μm) disseminated in fresh chromite, and 2) fairly massive Pt-ore up to more than 1 mm, cementing chromite crystals. Furthermore explain what are the “...xenomorphic grains .... with dissolved surfaces .... found in the dunite...”.

Line 207. “....are found in a single amount” unclear statement!

Line 246. Please write : “...ferrorhodsite”.

Line 292. “...single amount...” unclear!

Table1 to Table5. Please control the analytical totals of every single analysis: they can be wrongly rounded by the computer (for example an.9 in Table 2 sums up to 101.19 not 101.20).

Line 382 and others. Please do not use the misleading term “platinoid” that means “being similar to...”, and may be referred to PGE or PGM indifferently. In the case on line 382, is better to use “PGM associations” or “PGM assemblages”.

Table5 and Fig.13b. According to my knowledge “Ferroiridsite” is not a mineral species approved by the IMA Commission for new mineral names, like “Ferrorhodsite”. Please make a proper control.

Line 427-433. The Ir-oxide and the unnamed Rh-Fe-Ni sulfide phase are wrongly referred to be shown in Fig. 13a and Fig. 13b, instead of Fig. 14a and Fig. 14b.

How did you identify the IrOx in Fig. 14a? Were you able to assess Ir-O chemical bond? The mode of occurrence of this mineral perfectly included in isoferroplatinum is quite puzzling. At what temperature did both phases crystallize? Do you have any explanation? The same question applies to the Rh-Fe-Ni sulfide in Fig. 14b.

Line 462-465. There are 5 symbols in the diagram of Figure 15, but 6 symbols are described in the caption. What’s going wrong here?

I have no further comments. The manuscript is interesting and represents a good contribution that can be published with minor revision on a special volume dedicated to Platinum group minerals.

Author Response

Dear reviewer,

We agree with almost all Your remarks. The text is revised in accordance with them.

Kind regards and gratitude for Your work,

Palamarchuk Roman.

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript ID: minerals-423962 - Review

Dear Authors,

this is an overall fine work, and I liked to read it. The science is sound, the Figures are of high quality, and the paper follows a read thread which the reader can follow.

Most (and many) corrections I made were concerned with the English which needs some drastic improvement.

Accordingly, my recommendation is minor revision regarding text and Figures,
but major revision regarding the English.

In this context it became clear that the authors used some expressions from Russian textbooks not too familiar to geologists from western countries, or they used translations that do not fit that well to what they wanted to say. Further, they often use various synonyms for the same thing which is confusing.
I tried my best to provide some alternatives on the PDF file of the manuscript (appended).

Following some points where changes or clarifications are needed:

1.         You talk of PGM, platinoids, platinum, platinum mineralization, ….
Only PGE (platinum-group elements) and PGM (platinum-group elements) should be used.
Avoid platinoids (=PGE, not PGM !) which is outdated.
металл платиновой группы (metall platinovoy gruppy) = platinoid (PGE).
PGM = "PG mineral".

2.         Ural-Alaskan type is used throughout, but I think Uralian-Alaskan type is correct ?

3.         eluvial - deluvial - alluvial - proluvial. This complicates the matter too much. There are 4 sampling points: primary lode, eluvial (downhill), proximal (ca. 1 km) placer, and distal (ca. 50 km?) placer. This would make understanding much easier.

4.         Abstract (and text)
i) Sampling: - lode, eluvial, proximal and distal placers (give distances here).
ii) Primary PGM (+/- all Pt-Fe alloys with various inclusions ...) - appearance.
And then - what happens physically during transport - eluvial - alluvial.
iii) with that: changes in grain sizes during transport (?).
Do the PGM get smaller or do they grow ? (There are some advocates of the latter theory!).
iv) Chemical changes with transport - apparently none ?
If so, please make no big story out of that.
#
State all your findings. You have no direct proof of zonality of the lode ores - leave that comment. And - what should really happen chemically to Pt-Fe alloys ?

5.         The Urals - so far away from the rest of the world! Therefore, it would be fine if the authors could add some photos from the field - primary mineralization to placers (new Fig. 3).

6.         In my opinion, 3.1 is fine with good results – but please make some clear statements here. There is nothing to hide.

7.         No need to save paper - make Figures 3 and 4 to full-page Figures - otherwise some details get lost (e.g. furrows in Fig. 3 - not really visible).

Table …

8.         Remarks/data on changes (reduction in grain sizes or growth to nuggets?) of
PGM grain sizes with transport distance should be made or given.

9.         It seems that >97% of the PGM - primary, eluvial and placer - are Pt-Fe alloys. If so, it should be stated clearly. And all "Pt-Fe minerals" should be named Pt-Fe alloys - discerning between isoferroplatinum. tetraferroplatinum, ferroan platinum, tulameenite, ferronickelplatinum, .... complicates the matter more than needed. Anyway, it seems also that out of the Pt-Fe alloy group, ~95% are compositionally close to isoferroplatinum (Pt₃Fe). Please give some at least approximate numbers.

10.     Besides Pt-Fe alloys, there are only inclusions of Os-Ir-Ru alloys (common?),
and PGE sulfides (rare ?). Should be made clear.

11.     If available, some more detail should be provided on the Ir-oxide.

12.     Figures: This is an electronic journal, so there is no need to save paper.
Figs. 2a, b, c need separate legends each – not one for all – that is confusing.

Further remarks regarding scientific matters are provided (besides numerous editorial points) on the original PDF file of the manuscript (appended).

Additional References:

McClenaghan, M.B. & Cabri, L.J. (2011). Review of gold and platinum group element (PGE) indicator minerals methods for surficial sediment sampling. Geochemistry: Exploration, Environment, Analysis, Vol. 11, pp. 251–263. DOI 10.1144/1467-7873/10-IM-026

Oberthür, T., Weiser, Th.W. & Melcher, F. (2014). Alluvial and Eluvial Platinum-Group Minerals (PGM) from the Bushveld Complex, South Africa. South African Journal of Geology117(2), 255-274.

Oberthür T (2018). The fate of platinum-group elements and minerals in the exogenic environment ─ from sulfide ores via oxidized ores into placers: Examples Great Dyke, Zimbabwe, and Bushveld Complex, South Africa. Minerals, 2018, 8, 581, 28 pp. doi:10.3390/min8120581.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

We agree with many of Your remarks.

Reworked figures, tables and a significant amount of text. Newly written abstract and added a number of new paragraphs. Also added 3 items in the list of references and one new figures.

Two comments that we ignored were the Ural"ian"-Alaskan and chromspinels/chromite. These terms are used differently. In the document, I briefly substantiated our positions on these issues. If you think that it is correct to use the Uralian-Alaskan type and chromite, of course we will  correct these terms.

Kind regards and gratitude for Your huge work,

Palamarchuk Roman.

Author Response File: Author Response.pdf