Trace Element Distribution in Magnetite Separates of Varying Origin: Genetic and Exploration Significance
Round 1
Reviewer 1 Report
Comments on the manuscript:
attached please find the review of the manuscript ‘Trace element distribution in magnetite of varying origin: Genetic and exploration significance’ by Demetrios G. Eliopoulos and Maria Economou-Eliopoulos.
General comments:
attached please find the review of the manuscript ‘Trace element distribution in magnetite of varying origin: Genetic and exploration significance’ by Demetrios G. Eliopoulos and Maria Economou-Eliopoulos. The authors investigated the trace element content of magnetite using SEM/EDS and ICPMS (wet whole rock chemistry, not in situ data) in order to use them for discrimination between porphyry-Cu-Au-Pd-Pt and those lacking precious metals. The authors further suggest that trace elements of magnetite, including REE and PGE in magnetite-bearing deposits, may provide valuable evidence for their origin and exploration. This is of interest for a wide international readership. However, in my opinion the present version of the manuscript does not deserve publication in an international journal such as Minerals. This is mainly due to the uncommon approach of the authors. They need to explain why they use magnetite separate trace element analyses (whole rock analysis) instead of in situ methods. What is the advantage of their approach, since there are many disadvantages? For instance, how can the authors to be sure to deal with the actual trace elements of the magnetites and not with those of their solid mineral inclusions ? This has to be explained in detail (see detailed review below). Therefore, I suggest major revisions before the manuscript can be published.
Detailed comments:
Abstract:…..lavas, black coastal sands and metamorphosed Fe-Ni-laterites deposits, were investigated using SEM/EDS and ICP-MS analysis. The aim of this study is to establish potential relationships between composition, physico/chemical conditions, magnetite origin and exploration for ore deposits. Trace elements, hosted either in the magnetite structure or as inclusions and co-existing mineral, revealed differences between magnetite of magmatic and hydrothermal origin,…..
Comment: How can this method (magnetite separate analyses, whole rock analyses) reveal the differences between magnetite from different environments ? The trace elements the authors actually measure could exclusively or mainly occur in mineral inclusions or even submicroscopic (nano) inclusions of magnetite (see Dedetius et al. 2018, CMP, 173:46). Please add that to text….
line 23: …. It should be: ….PGE have not been found……
Lines 70-87: Whole rock analysis:
Comment: The authors need to explain why they use magnetite separate trace element analyses (whole rock analysis) instead of in situ methods. What is the advantage of this approach, since there are many disadvantages? For instance, how can the authors to be sure to deal with the actual trace elements of the magnetites and not with those of their solid mineral inclusions ? Furthermore, there may be several generations of magnetites with and without inclusions. How do the authors differentiate ? See comment in Abstract. Furthermore how representative are the whole rock analyses. How much material (in gram) and how many samples were analyzed ?
Lines 102-103: Thus there are different generations of magnetite and thus their trace element budget will be different. This must be discussed in detail.
Line 111: Table 1: Please give the number (n) of samples investigated. The reader should know how representative the analyzed samples are.
Line 136: …..should be: …. of fine-grained minerals associated with……And please add the number of minerals you analyzed.
Lines 181-184: Also, the presence of abundant primary two-phase aqueous fluid inclusions in the apatite, associated magnetite and sulfides, have been attributed to their hydrothermal origin….
Comment: It is impossible to refer to the origin of any mineral just by the presence of primary two-phase aqueous inclusions….Needs to be explained
Lines 193, 201, 202: ..Amphibole….since the authors have the EDS analyses, please explain. Which amphibole we are dealing with.
Lines 245: …..The dismembered ophiolite masses…..
Comment: What are ophiolite masses ? Please be precise.
Lines 275-278: Table 4: Please give the number of samples analyzed. At least the chromite is chemically zoned. Thus please give rim and core analyses. What about the magnetite ?
Lines 290-291:….. although a good separation of magnetite from very fine inter-grown silicates and/or sulphides. (Figs 2-10) was not achieved…
Comment: Consequently. The authors do not know whether certain trace elements occur in the magnetites or their mineral inclusions ? see also Lines 324-326, same story. Which mineral is causing the enrichment or decrease of trace elements such as REE ?
Line 350: Table 5
Comment: How do the EDS and ICP-MS analyses of certain elements compare ?
Line 373: Table 6
Comment: Are these average values ? Are these ICP-MS values or are you also using the EDS values for certain analyzed elements ?
Line 388: should be: Figure 14. Plot of the (Pt+Pd) versus Cr content of magnetite in porphyry-…..
Lines 401-402: please delete. This is already described in the Introduction.
Line 536: should be: ….Au-rich residue is left (?) in…..
Line 560: should be: ….is a common feature, recorded….
Lines 566- 572: The authors describe the use of stable isotopes such as hydrogen and oxygen of quartz in order to gain information on the fluid trapped in quartz and thus on the
origin of the.
Comment: This can also be achieved with O and Fe isotope measurements of the magnetite instead which distinguishes between magmatic and hydrothermal magnetite. This would even supplement the trace element of the magnetite (cf. Jonsson et al. 2013, Nature Sci. Report, 3; Günther et al., 2017, Chem. Geol. ) and is much more convenient for the actual investigated mineral.
Author Response
Reviewer 1
General comments:
Comment: The authors investigated the trace element content of magnetite using SEM/EDS and ICPMS (wet whole rock chemistry, not in situ data) in order to use them for discrimination between porphyry-Cu-Au-Pd-Pt and those lacking precious metals. The authors further suggest that trace elements of magnetite, including REE and PGE in magnetite-bearing deposits, may provide valuable evidence for their origin and exploration. This is of interest for a wide international readership. However, in my opinion the present version of the manuscript does not deserve publication in an international journal such as Minerals. This is mainly due to the uncommon approach of the authors. They need to explain why they use magnetite separate trace element analyses (whole rock analysis) instead of in situ methods. What is the advantage of their approach, since there are many disadvantages? For instance, how can the authors to be sure to deal with the actual trace elements of the magnetites and not with those of their solid mineral inclusions ? This has to be explained in detail (see detailed review below). Therefore, I suggest major revisions before the manuscript can be published.
Reply: Thank you for the consideration of our manuscript and your constructive comments. The reason of the investigation of trace element analyses (whole rock analysis) magnetite separates instead of in situ methods is the lack of the required facilities for in situ analysis at the University of Athens. Although a separation of magnetite from very fine intergrown silicates and/or sulphides (Figs 2-10) was not achieved, they are a relatively small portion, allowing the discrimination of magnetite types (please, see detailed reply below).
Abstract:…..lavas, black coastal sands and metamorphosed Fe-Ni-laterites deposits, were investigated using SEM/EDS and ICP-MS analysis. The aim of this study is to establish potential relationships between composition, physico/chemical conditions, magnetite origin and exploration for ore deposits. Trace elements, hosted either in the magnetite structure or as inclusions and co-existing mineral, revealed differences between magnetite of magmatic and hydrothermal origin,…..
Comment: How can this method (magnetite separate analyses, whole rock analyses) reveal the differences between magnetite from different environments? The trace elements the authors actually measure could exclusively or mainly occur in mineral inclusions or even submicroscopic (nano) inclusions of magnetite (see Dedetius et al. 2018, CMP, 173:46). Please add that to text….
Reply: In the proposed reference, the high-resolution studies of minerals applying TEM techniques provides a new and very important way to elucidate valuable information on the physico-chemical processes and mechanisms during formation of ore deposits and post-depositional changes of ore minerals (Deditius et al 2018, Contr. Miner. Petrology). According to these authors hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, showed three generations of magnetites which vary significantly in terms of their Si, Al, Ca, Mg, Ti, Mn, Na and K minor and trace element contents (V content was remarkably constant) and focused on the complex process of the incorporation of trace elements during growth of magnetite. This reference is discussed now, but since there is not possibility for that type of investigation, our study is focused on differences in the trace element content in the whole massive and disseminated magnetite (concentrates) from various geotectonic settings covering a wide range of formation conditions (Table 5) as a contribution to the discrimination of magnetite on varying origin and exploration of ore deposits.
The proposed reference is included in the section of the discussion now.
line 23: …. It should be: ….PGE have not been found……
Reply: It was corrected.
Lines 70-87: Whole rock analysis:
Comment: The authors need to explain why they use magnetite separate trace element analyses (whole rock analysis) instead of in situ methods. What is the advantage of this approach, since there are many disadvantages? For instance, how can the authors to be sure to deal with the actual trace elements of the magnetites and not with those of their solid mineral inclusions ? Furthermore, there may be several generations of magnetites with and without inclusions.
How do the authors differentiate ? See comment in Abstract. Furthermore how representative are the whole rock analyses. How much material (in gram) and how many samples were analyzed ?
Reply: As it was mentioned above, the reason of the investigation of trace element analyses (whole rock analysis) magnetite separates instead of in situ methods is the lack of the required facilities for in situ analysis at the University of Athens. We agree that in situ analysis of magnetite and co-existing minerals can provide information about the real trace element content within magnetite. Alternatively magnetite derived from large (weighing approximately two kg) samples, is considered to be a good approach. Selected elements allow conclusions on differences in terms of the trace element content between massive and disseminated magnetite (concentrates) from various geotectonic settings covering a wide range of formation conditions (Table 5) aiming to the discrimination of magnetite on varying origin and exploration of ore deposits.
Thus, although there are advantages of the in situ analysis of magnetite, the portion of the mineral inclusions is relatively small portion, as indicated by the provided SEM/EDS images. In addition, elements which are potentially derived from sulphides (Cu, Zn) and/or silicates (Si, Mg, K) are not a mai topic for discussion. In our opinion, these data provide information allowing the discrimination with respect to the origin and exploration of ore deposits. The V, Ti and Ga contents in individual types of magnetite are plotted, and show different fields for magmatic and hydrothermal magnetite, and furthermore between hydrothermal magnetite associated with ophiolites and calc-alkaline rocks (Figure 12). Also, the discrimination between porphyry-Cu-Au-Pd-Pt and those lacking precious metals that is based (a) on the high abundance of magnetite (oxidized nature of magmas may control the Pd and Pt potential and abundance of magnetite in porphyry systems, and (b) the presence of Cr in magnetite associated with calc-alkaline rocks, which is clear in the SEM/EDS analyses.
Comment: Lines 102-103: Thus there are different generations of magnetite and thus their trace element budget will be different. This must be discussed in detail.
Reply: The potential difference in the trace element content in magnetite of different generation requires investigation using in situ analysis and high-resolution TEM techniques, which are not available currently. For this reason the present study is not focused on the trace element distribution during the evolution of a magnetite mineralization, but on selected trace elements in the magnetite from the Lavrion mine and the Skouries deposit which differ clearly from those in hydrothermal magnetite associated with ophiolites. Specifically, in the plots of Al versus Ga and (V+Ti) versus Ga there is a discrimination between (a) magmatic and hydrothermal magnetite and (b) between hydrothermal magnetite associated with calc-alkaline tocks and that associated with ophiolites (Fig. 12).
Comment: Line 111: Table 1: Please give the number (n) of samples investigated. The reader should know how representative the analyzed samples are.
Reply: In the section of the analytical methods is now clarified that n=2-3 samples.
Line 136: …..should be: …. of fine-grained minerals associated with……And please add the number of minerals you analyzed.
Reply: It was corrected.
Lines 181-184: Also, the presence of abundant primary two-phase aqueous fluid inclusions in the apatite, associated magnetite and sulfides, have been attributed to their hydrothermal origin….
Comment: It is impossible to refer to the origin of any mineral just by the presence of primary two-phase aqueous inclusions….Needs to be explained
Reply: It was a reference and it is deleted now as unnecessary.
Lines 193, 201, 202: ..Amphibole….since the authors have the EDS analyses, please explain. Which amphibole we are dealing with.
Reply: It was clarified that the amphibole is tschermakite, according to previous publication by Koutsovitie et al., 2012 [34).
Lines 245: …..The dismembered ophiolite masses…..
Comment: What are ophiolite masses ? Please be precise.
Reply: It was clarified now.
Lines 275-278: Table 4: Please give the number of samples analyzed. At least the chromite is chemically zoned. Thus please give rim and core analyses. What about the magnetite ?
Reply: The investigation of the compositional variation in zoned chromite grains in Fe-Ni-laterites was the topic of several previous publications and about 100 samples from the Balkan Peninsula have been analyzed. However, the trace element contents in separated magnetite (23 samples) from various geotectonic settings are presented for first time.
Core and rim analyses are given now (Table 4), as well as various morphological forms and textural relationship between magnetite and associated minerals (Fig. 10).
Lines 290-291:….. although a good separation of magnetite from very fine inter-grown silicates and/or sulphides. (Figs 2-10) was not achieved…
Comment: Consequently. The authors do not know whether certain trace elements occur in the magnetites or their mineral inclusions ? see also Lines 324-326, same story. Which mineral is causing the enrichment or decrease of trace elements such as REE ?
Reply: As was mentioned above, the discussion of the present study is focused on plots of selected trace elements in various magnetite types. Elements which are commonly major components of sulphide (Cu, Zn) and/or silicate (Si, Mg, K) inclusions are not the topic of the trace element application to the origin of magnetite and exploration of ore deposits. The distribution and plots of selected elements for individual types of magnetite, such as V, Ti versus Ga, showing different fields for magmatic and hydrothermal magnetite, and furthermore between hydrothermal magnetite associated with ophiolites and calc-alkaline rocks (Figure 12). Specifically, trace element in magnetite from the Lavrion and the Skouries mines differ clearly from those in hydrothermal magnetite associated with ophiolites. Also, besides the common association of magnetite with calc-alkaline intrusions in the Lavrion and Skouries, they exhibit differences in terms of the morphology and chemistry.
Line 350: Table 5
Comment: How do the EDS and ICP-MS analyses of certain elements compare ?
Reply: In our opinion, the presented SEM/EDS analyses of magnetite provide information about the minor and trace elements associated with magnetite. For example, the relatively high V and Ti contents in ICP-MS analyses ranging from 1500 to 6700 ppm V and 2.5 to wt% to 13.3 wt% Ti in magnetite from magmatic ophiolitic rocks and black coastal sand derived from weathered volcanic rocks (Table 5) are consistent with the corresponding SEM/EDS analyses showing 0.5 to 1.8 wt% V2O5 and 0.6 to 6.9 wt% TiO2 in magnetite (Table 1).
Also, magnetite from the Skouries porphyry-Cu deposit showing relatively high (280-1060 ppm Cr) in the ICP-MS analyses (Table 5) contains high (0.6-2.1 wt% Cr2O3) in SEM/EDS analyses (Table 1). Chromium, Ni and Co contents in magnetite may reflect the origin and evolution of hydrothermal fluids.
Line 373: Table 6
Comment: Are these average values ? Are these ICP-MS values or are you also using the EDS values for certain analyzed elements ?
Reply: They are ICP-MS values. It is clarified now.
Line 388: should be: Figure 14. Plot of the (Pt+Pd) versus Cr content of magnetite in porphyry-…..
Reply: Figure 14. This is a plot of the (Pt+Pd) versus Cr content in porphyry-Cu-Au-Pd-Pt and porphyry-Cu-Mo deposits from literature data. Although Pt and Pd contents in separated magnetite are not available, the level of the Cr ICP-MS values in magnetite (Table 5) are comparable.
Lines 401-402: please delete. This is already described in the Introduction.
Reply: It was deleted and this sentence was re-written.
Line 536: should be: ….Au-rich residue is left (?) in…..
Reply: It was corrected.
Line 560: should be: ….is a common feature, recorded….
Reply: It was corrected.
Lines 566- 572: The authors describe the use of stable isotopes such as hydrogen and oxygen of quartz in order to gain information on the fluid trapped in quartz and thus on the origin of the.
Comment: This can also be achieved with O and Fe isotope measurements of the magnetite instead which distinguishes between magmatic and hydrothermal magnetite. This would even supplement the trace element of the magnetite (cf. Jonsson et al. 2013, Nature Sci. Report, 3; Günther et al., 2017, Chem. Geol. ) and is much more convenient for the actual investigated mineral.
Reply: This recommendation is a very good idea for a next step of investigation. There is abundant material and everybody has the possibility for O and Fe isotope measurements is welcome.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors,
I've red your manuscript “Trace element distribution in magnetite of varying origin: Genetic and exploration significance” describing geochemical composition of magnetite from various rocks and various origin and geotectonic settings with interest. I consider it is well organised and written, the conclusions are supported by analythical data obtained with modern methods of investigation. I think your paper will be interesting for scientists related to economic geology, ore deposits and geochemistry and may be bublished in "Minerals". I have some minor comments and suggestions that are shown in attaced PDF file.
Comments for author File: Comments.pdf
Author Response
Reviewer 2
Lines 37-42, Comment: This sentence seems to be very long. It is hard to read it. Probably it is better to divide it into two sentences.
Reply: It was done.
Table 3. Please, check location of Rhonite and Amphibole in the Table. Probably they should be moved slightly upper and left and right, correspondingly. It seems to me that vertical line is not at the proper place.
Reply: It was corrected.
Line 268, comment: and occurring along cracks ? Reply: It was corrected.
Line 289, comment: delete "and" Reply: It was done.
Line 293, comment: missed Mn Reply: It was corrected.
Line 312, comment: coma after "alkaline)" Reply: It was done.
Lines 314-316, comment: Please, check the word order is probably incorrect.
Reply: It was re-written.
Line 317, comment: as well as actinides Reply: It was corrected.
Line 333. Comment: check, please Reply: This sentence was corrected.
Line 334, comment: (Fig. 9 [41]) Reply: It was corrected.
Lines 343-346, comment: Long sentence, difficult to read. better to divide into two sentences. Reply: It was re-written and corrected.
Line 347, comment: LREE? Reply: It was corrected.
Line 356, comment: Eu/Eu* ? Reply: It was corrected.
Line 357, comment: missed ) Reply: It was corrected.
Line 383, comment: feature? Reply: It was corrected.
Line 412, comment: (Fig. 8a,d) ? Reply: It was corrected.
Line 426, comment: than those in the volcanic? Reply: It was corrected.
Line 465, comment: indicated. Reply: It was corrected.
Thank you very much for the consideration of our manuscript and your constructive comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I have read now the response of the authors and the revised manuscript, and I am mostly satisfied with their response and undertaken changes. However I still think that one of the my main concerns -that the analyzed trace elements of the magnetites could have been those of their mineral inclusions (e.g. REE => apatite inclusions)- has not been satisfactorily treated. In order to overcome this issue I strongly suggest when appropriate to change 'magnetite' to 'magnetite separates' along the text. This would have two advantages: the reader immeadiately would know what to expect from the paper content (for instance see the title, I assumed that in situ analyses were undertaken) and, in addition, the interpretions would be sound and straightforward, since this is what the authors actually analysed.
Author Response
Dear Reviewer,
Thank you very much for your constructive criticism and suggestions. The correction of 'magnetite' to 'magnetite separates' in the title, throughout the text, and Graphical Abstract was done.
Kind regards
Maria Economou-Eliopoulos
Author Response File: Author Response.pdf
