Paleoproterozoic Layered Intrusions of the Monchegorsk Ore District: Geochemistry and U–Pb, Sm–Nd, Re–Os Isotope Analysis
Round 1
Reviewer 1 Report
Manuscript ID (minerals-1965056)
Paleoproterozoic Layered Intrusions of the Monchegorsk Ore District: Geochemistry and U-Pb, Sm-Nd, Re-Os Isotope Analysis
By Smol'kin and Mokrushin
In this manuscript authors determine the genetic relations based on the geochemical analysis of intrusive rocks of Paleoproterozoic intrusions and comagmatic dykes of the Monchegorsk ore district, as well as the results of the isotope analysis.
The result of this good work is remarkable. The manuscript is organized and to the point. I think that the manuscript looks suitable for an international audience.
The main comment is
1- The manuscript is too long, and the authors must reduce it by cutting and summarizing the topic without going into deep details.
2- Several redundant sentences and/or repeated concepts are present throughout the paper (especially, the first three sections), please reduce and summarize in order to make the manuscript more fluent and easier to access for readers not familiar with the geology of the area.
3- If possible, reduce the number off figures used.
4- although the paper contains high quality geochemical analysis (however the authors depend mainly on the isotopic data to discuss the source of magma for layered intrusion).
5- I would strongly suggest that authors have someone with strong skills in English scientific writing to review your revisions for proper grammar and syntax before submitting a revised version Grammers and typosetting of English. (I tried to fix some of them, see the attached pdf file)
6- (Minor comments were provided in the attached pdf file)
I think that the manuscript would have the quality of being accepted after minor revision for publication in Minerals.
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Article: Paleoproterozoic Layered Intrusions of the Monchegorsk Ore District: Geochemistry and U-Pb, Sm-Nd, Re-Os Isotope Analysis
Journal: Minerals-MDPI.
Dear, respected authors and editors
This article is very interesting work about the ore-bearing layered mafic-ultramafic intrusions, which host chromite, sulphide Cu-Ni-PGE, low-sulphide PGE, and Ti-V ores in the Monchepluton and the main ridge complex. The authors suggested a genetic model for the emplacement and crystallisation as two different complexes (the mafic-ultramafic Monchepluton and the gabbro-anorthosite Monchetundra massif) with age ranges of 2.50-2.44 and 2.51-2.45. However, they are overlapping in the age of crystallization and are spatially close.
The authors interpreted the changes in the ore types, rock set and composition, isotopic signatures, and differentiation trends within the complex and among them, as a result of magma cycles and repeated pulsing from deep-laid reservoirs. However, they have the same parental source of komatiitic basalts withthe high-Mg magmatism. I agree with the main conclusion of the work. But I have some issues should be raised in this work before publishing.
1. You suggested that the variation in geochemical features, isotopic signatures, and related ore deposits within complexes, is due to the feeding of new magma batches, and magma differentiation and conditions. You also suggested that the parental magma was contaminated with a source enriched in sulfur (granulite-amphibolite and amphibolite-gneiss complexes), during feeding process. Explain this in different section, and how this can affect the magma path.
2. To make this genetic model easy in reading, can you provide us with a schematic model illustration.
3. What is your other evidence that the magma feeders of the two complexes are different except for the depth, and they become spatially close owing to movement along the deep-laid fault in the Svecofennian Orogeny (2.04–1.90 Ga). It is supposed to make it clearer.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
It is uncommon to have the opportunity to review a paper displaying the impressive geochemical dataset the authors show here. At first glance, one tends to believe any interpretation and conclusion -so compelling is geochemical evidence.
Unfortunately, there is a number of serious pitfalls in the authors' use (or misuse) of their data. First, these pitfalls make the manuscript very difficult to follow; second, it is soon realized the the conclusions are either weaky supported or entirely unsupported at all.
It is to be noted that the studied area, containing a large volume of igneous rocks in a tectonically active, Paleoproterozoic area, is complex in terms of general Geology, Petrology, Geochemistry and ore genesis. Therefore, I am sure that even the apparently routine task of displaying the previously available geological information (on which Geochemistry must be based) is no simple task. On the other hand, I also understand that the introductory chapter of a scientific paper cannot be too long. However, (foreign) readers need to have a better look on previous descriptions of the general Geology of this complex area. Please do an effort to clarify the general description presented on Page 2 in the manuscript. Also, please include here the pertinent references to previous works.
The field descriptions of the studied rocks, as shown in Figs 1 and 2, do not help to clarify the structural relationships of the successive stages of magma emplacement - neither the relationships with country rocks, apart from showing that the Archaean and the igneous, Proterozoic rocks show large-scale intrusive contacts in the studied area.
As an example, if cross-cutting relationships of late faults (red dashed lines in Fig.2) are carefully examined the timing of late faulting is confusing. For instance, one of these faults produces relatively important displacement between the Sopcha and Nyuz-Poas igneous sub-units wheras it does not displace the intrusive contact at all -neither the underlying Archaean rocks. In contrast, another of these faults sharply separates the Archaean rocks from blastocataclasitic (metagabbroic?) rocks, these latter dated at 2505-2501 Ma; however, this same fault produces no displacement of the different layered parts of the overlying Sopcha group, dated at 2505-2493 Ma.
To me, this indicates that there were several faulting stages in the studied area; but this should be better clarified, because in any case these stages developed within a short time span, affecting in different ways to igneous rocks also generated within a reduced time interval, (and possibly cogenetic?). In other words, when the authors interpret the radiometric ages of the different igneous rocks under study, any interpretation must be faced to large-scale structural evidence. This work is still to be done.
The petrographic description of the studied rocks, although presented in some length (pp. 5-9), is confusing because of the (too) many igneous units included in the description. Probably due to this excessive number, each unit is poorly described: in most cases, just the rock name and a very limited mineralogical description. From time to time, one can find references to deformation and metamorphism (page 6 3rd paragraph), which are absolutely out of context. In other instances, references to mineral deposits can be found (P. 6 4th paragraph)... To summarize, the description is confuse and not written to the reader. I am persuaded that the authors know very well the area under study; but clearly, they have not fulfilled their most important task, i. e., to communicate with potential readers worldwide in an ordered, understandable way. Contrarily, they constantly mix the igneous mineralogy of rocks, the ore deposits therein, the superimposed regional metamorphism, the possible contact metamorphism produced in country rocks and even more issues they are interested in.
The final result is that, when arriving to the geochermical evidence, which is undoubtedly the aim of the work, one cannot go back to the petrographic descriptions in search for some support of the geochemical interpretations the authors do. In general, these interpretations could be plausible, or could be not; but the reader has no criteria to decide, because of the very dispersed, vague and confusing petrographic descriptions the authors had previously offered.
As an example among many others, I reproduce here a representative paragraph: "... A combination of two consecutive trends on a MgO-SiO2 diagram is typical for the Monchepluton and also for the Burakovsky Pluton, the Kivakka and other Karelian lay-ered intrusions [40]. The first trend shows sequential SiO2 increasing, while the second one shows relatively stable SiO2 content (Figure 7). Both trends are determined by the change of paragenesis CrSp + Ol + Opx to paragenesis Opx + Pl. Unlike the volcanics, the Monchepluton provides no direct relation between MgO and Fe2O3tot due to considerable spread of Fe2O3tot content within the megacycle limits. The reason for this is the variation of dark-colored mineral ferruginosity in each rhythm due to multiple interchanges of de-creasing and increasing of the phase crystallization temperatures...". This is the first time in the text where this reaction is even mentioned (in spite of its obvious interest).
My final advice is that, in order to make justice to their excellent geochemical dataset, the authors must completely reorganize their manuscript. In doing this, they should highlight the field and petrographic evidence which is essential in support of their own geochemical interpretation. I also guess that they probably should focus their work in a part of the rocks they describe, presenting finally their results in more than one paper. But this is just my guess.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
I agree with the authors in that showing additional petrographic information will result in some enlargement of their manuscript. However, I contend that a very moderate enlargement could be acceptable in order to make the information more easily accesible to the reader. Even more so as a number of the more general papers the authors cite are in Russian - a language hard to understand to many potential readers worlwide. In my opinion, the authos should try at least to make some effort in order to help these readers. This does NOT mean, of course, that they should reproduce all of the (many) previously published geological data.
Apart from this, I consider that if these carefully selected data are incorporated to the new version, this could be now published without no longer delay.
Author Response
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Author Response File: Author Response.pdf
