Multisensor Satellite Data and Field Studies for Unravelling the Structural Evolution and Gold Metallogeny of the Gerf Ophiolitic Nappe, Eastern Desert, Egypt

Comment

The current work integrated remote sensing data, and field investigations to delineate the structural control of gold mineralization at the Gabal Gerf area and highlight its gold potentiality. The manuscript is interesting and well-organized, however, several issues should be considered.

Authors’ Response

Many thanks for your kind consideration and comments to the manuscript. We followed your comments carefully and applied them properly. Please refer the following point by point:

Comment 1

“The southern sector of the SED, particularly where the study area lies is characterized….” This paragraph is related to SED. Please add references if it is not your field observations. Also, as the study area belongs to SED, I highly recommend reviewing the recent studies (and adding them to this paragraph) related to gold mineralizations and their structural control within the SED, which receives much more interest from the readers and researchers.

Authors’ Response

We appreciate your comment. We confirm that the first and second authors have many papers about the structural architecture of the SED. Anyway, we added a reference to this section. In regard to, the papers about the mineralization in the SED. We invite you to review the references cited in the whole paper and reference list, most of the recent publications were added. ex.  Zoheir, et al. (2018a, b) [ref. 7 and 8], Zoheir et al., (2019a,b) [ref. 9 and 10], Abu El-Leil  et al. (2019) [ref. 88], Abd El-Wahed et. al. (2021) [ref. 13], Eldosouky et al., (2021) [ref. 76]. We added two of them in this section (thank you)

Comment 2

Please justify using more than one optical sensor, for example, Sentinel 2 and Landsat 8.

Authors’ Response

Thank you very much for your comment. Exactly, we used more than optical sensor for some reasons. Firstly, the availability of Landsat-8, Sentinel-2 and ASTER. Secondly, we want to use the benefits of the different spectral ranges of each sensor where the study area has heterogeneity in the different rock units. The using of different satellite resources increase the discrimination process between the different rock units. The different processing techniques were applied on the visible to near infrared and short waves infrared of Landsat-8 and Sentinel-2 to produce upgraded geologic map of the study area. where the ASTER data is used mainly to extract the abundance of alteration minerals from its visible to near infrared and thermal bands. Finally, we think that the use of multisensory data can confirm the results of each others and increase the accuracy degree of geological and mineralogical mapping. Many workers in literatures used more than optical satellite sensors in their geological mapping in the Eastern Desert of Egypt.

Comment 3

As a suggestion, is it possible to link remote sensing results, with the field, petrography, and geochemical analysis over different locations in the study area to ensure consistency?

Authors’ Response

Thank you very much for your suggestion. The present study aims to define the geological and structural controls on the gold mineralization in one of the most important areas of gold mineralization. The Egyptian government paid an attention to this area in the last years in the mineral exploration especially the gold. The authors achieved the objective the current study through the integration of the image processing of satellite data, structural analysis and intensive field measurements and analysis. The filed work is performed through two field trips in December 2020 and January 2023 for about 30 days. And as you see the manuscript is already lengthy and we try to concise some parts. But we have already a petrographic study for all rock units and alteration zone and used it to make a validation for the geologic and mineralogic mapping. In this context, we added Photomicrographs plate of some rock units in the study area to make the consistency and use it in the verification process (supplementary figure 1 in the revised manuscript) and added a short note at the end of the geologic setting. Also, we invite you to review an enough figures of 3, 13, 16, 17 of field photographs for the rock units and structural elements. This field photographs help also in the verification of geologic and mineralogic mapping. In addition, the authors did not prefer to add the geochemical analysis in the present study because it is away from the article subject. The paper discusses the distribution of gold mineralization and its relation to the structural setting in the study area not the genesis of the gold.

Comment 4

“If The Landsat-8 and ASTER data were resampled ” please specify the resampling method.

Authors’ Response

We appreciate your comment. The resampling process added in detail in the lines 371 to 373 to explain it clearly. The Landsat-8 and ASTER data were resampled to 10 m of Sentinel-2 for improving the spatial resolution and obtaining high spatial resolution to enhance the small areas of the hydrothermal alteration zones. The resampling of Landsat-8 and ASTER are performed using the Gram-Schmidt spectral pan-sharpening module included in ENVI based on nearest neighbor algorithm. Thank you for giving us the opportunity to explain the fusion of the optical data.

Comment 5

Please justify using radar data for lineament extraction, this paper (https://doi.org/10.1016/j.rsase.2021.100617) and references therein could help.

Authors’ Response

We appreciate this nice article. We confirm that we used it in justifying the lineaments extraction and cited then added in the reference list.

Comment 6

Please justify using Maximum Likelihood Classifier (MLC), these papers, and references therein (https://doi.org/10.1016/j.rsase.2021.100643, https://link.springer.com/article/10.1007/s12517-022-09948-w) could help, It could be also better if you provide several examples of recent studies implemented MLC over similar terrains. Also, the lower PA (61%) and UA (77%) of metavolcanics should be interpreted.

Authors’ Response

We appreciate these nice articles.  We confirm that we used them as examples of machine learning classification in the Eastern Desert of Egypt and cited. Added to the reference list. In regard to you mentioned about the lower PA (61%) and UA (77%) of metavolcanics. We invite you to review the lines from 625-630 interpreted the low accuracy of this rock units. The arc metavolcanics have the high value of misclassification due to the mixture of signatures with other rock units. This misclassification is regards the arc metavolcanics in the study area that are represented by small dismembered masses in a vast area of wadi deposits.

Comment 7

Thanks for the efforts exerted in the remote sensing part but I strongly recommend adding some field photographs for these extracted alteration minerals if possible. Detecting these features over different scales of observations (i.e. field, petrography, etc.) strongly supplements remote sensing results.

Authors’ Response

Thank you very much for your suggestion. we have already added a petrographic study section for all rock units and alteration zone and used them to make a validation for the geologic and mineralogic mapping. In this context, we added Photomicrographs plate of some rock units in the study area to make the consistency and use it in the verification process (please see the supplementary figure 1 in the revised manuscript and added a short section at the end of the geologic setting for the petrographic study. Also, we invite you to review an enough figure of 3, 13, 16, 17 of field photographs for the rock units and structural elements. This field photographs help also in the verification of geologic and mineralogic mapping. Please check Figures 16 and 17 of altered zone in Koarbai and Madari gold mines.

Comment 8

I suggest adding a detailed paragraph linking the extracted (automatic) structural lineaments with the main structural trends representing D2, D3,..and how well they match each other?

Authors’ Response

Thank you very much for your suggestion. We invite you to review the relation between the orientation of the extracted lineaments and the main structural trend in section 7.2. Integration of mineralization to lineament and structural elements. We discussed the corresponding of the two types and their relation to minealization.

Comment 9

Could you please explain the basis for alteration zones class ranking and what you think if the favorable host lithology and alteration zones have the same influence?

Authors’ Response

We appreciate your comment. In accordance to, the geologic characteristics of the study area and the known mineralization zones. We prefer to give the host lithology the highest rank to drive the machine learning to give the lithology the priority in choosing the pixel. The gold mineralization in the study area mostly confined with the contacts of serpentine with metavolcanics and metagabbros and/ or Quartz veins as well as the alteration zone in these rock units. On the other hand, the area covered with a vast area of granitic rocks and these rocks also can altered to clay minerals which give the similarity to alteration zone. Therefore, we give the lithology little bit highest rank than alteration zone to give more accuracy in localization of the gold occurrences. We already make a trial with the same rank of lithology and alteration zone but the validation of the model shows the good result when the lithology is highest in ranking. The evidence from the field work and the existing gold occurrences are the base of the whole ranking.

Comment 10

Figure 19b, what the y-axis stands for, if the number of field verification points, so please add the label.

Authors’ Response

We appreciate your notice. The figure axes are annotated

Comment 11

In a small paragraph, the authors must more effectively demonstrate the mineral potential of these mineral deposits if they are to be taken into consideration as exploration targets. The strengths and limitations of the current study should be highlighted. Over the study area, Future research directions or applicable recommendations should be clearly stated.

Authors’ Response

Thank you for this comment which guide us to add sentences in the discussion and conclusion to explore the effectiveness of the gold potentiality map in the future mineral exploration in the study area. Especially this area had an attention from the Egyptian government and Egyptian Mineral Resources Authority ‘EMRA’.

Comment 12

Check the spelling because small errors, misspellings may occur, for example: "...complete suc,cession of Neoproterozoic basement...", "...Allaqi-
Heiani-Onib-Sol Hamed-Yanbu suture suture...", "...signatures for OH-bearing, clay, Iron oxides...", "...CO3 acid group" (note CO3 should be considered as carbonate group consisting of carbonate ions, and not carbonate acid), "the metamorphic sequence inthe Gerf area include both the ophiolitic sequence..", "..Gerf area contains several prominent mountain peaks of mountain peaks such as...". "The Contacts between the rocks forming the Gerf...", "the syn-tectonic foliated granodiorites (Figure 3g...", "...U-pb zircon SHRIMP age...", "Post tectonic Syenite intrusions are the youngest rock unit in the Gerf...", "whereas the Sentinel-1 and ALOS PALSAR datasets...", "To build the training classes file,The results of the above-mentioned image processing techniques...", "...lineaments (faults/fracture zones) [79,80]. Where the hydrothermal or.., "...ultramafic-mafic rocks and the surrounding SiO2 rich...", "...about 53% of the study area. one of these new promising sites...",

Authors’ Response

We appreciate your time and effort to detect these misspellings. We confirm that all you mentioned are considered and corrected. Moreover, the English of the manuscript is revised.

Comment 13

This sequence is not clear to me: " Moreover, the study aims to produce a gold."

Authors’ Response

We appreciate your comment. This sentence has been omitted.

Comment 14

"This sequence is intruded by syn- to late-tectonic granitoids and were proposed to have igneous origin." Do you mean I type granitodes?

Authors’ Response

We appreciate your comment. Yes we mean I type granites. The granites of igneous origin

Comment 15

If you do not use any additional sources or literature to draw the Figure 2 geologic map, I mean that is based on only your recent work resulting in a totally new unpublished map then ok, otherwise please cite them. In the first case I should emphasize that this map is a result of your comprehensive work presented in this paper.

Authors’ Response

We appreciate your comment. Exactly the presented geologic map has a considerable change from the published geologic maps of the study area. Especially in the rock names, contacts and the structural elements are presented here as a first contribution. In meantime we have to say that the base of this geologic map is geologic map of EGMSA (2002) and we cited the reference in the figure caption.. Thank you again.

Comment 16

In Fig 13 you should change l to n in the legend: "(l) Quartz prophyroclast indicating sinistral sense of movement"

Authors’ Response

We appreciate your comment. Done

Comment 17

"kinked textiles" probably you wanted to write texture.

Authors’ Response

We appreciate your comment. No it is right it means fabric: a type of structural evidence to thrusting

Comment 18

Somehow I see underlined lowercase letter o from the numero symbol as a degree symbol in the manuscript. You should change to uppercase o.

Authors’ Response

We appreciate your important notice. We confirm that we change them to o. Thank you very much

Comment

In its premise, this is an interesting manuscript that combines remote sensing, gold exploration and structural geology. Its scope fits well with Remote Sensing profile. Therefore, I recommend publication of the manuscript after revision. However, in my opinion, a major revision will be necessary for the paper to meet journal standards.

Authors’ Response

Thank you for giving us the opportunity to revise our manuscript. We are grateful to your careful review and subjective comments. In the revised manuscript, we have endeavored to comprehensively update the manuscript to meet the quality level pointed out by your considered comments.

Comment 1

Ad 1. The English of the manuscript is generally communicative. I highlighted the rare unintelligible sentences in yellow. However, I have the impression that the text was translated using a computer translator. This is manifested by the following symptoms:

-          Many sentences are grammatically defective. Especially often they lack a verb.

-          Some words are duplicated.

-          Some words obviously do not fit the context. For example, instead of "sinistral sense of shear" is “sinistral sensation of shear”.

I understand that the authors must use a computer translator, but the effects of its work should be verified by a person using English as a first language. I made about 320 corrections in the annotated manuscript, but firstly I am not a native speaker myself, and secondly, careful linguistic proofreading goes beyond the reviewer's duties and exceeds my time capacities.

Authors’ Response

We appreciate your comment and your efforts to correct the English of the manuscript. We confirm that we made all the correction in the annotated PDF and the manuscript reviewed by a professor whose native language is English. Thank you again for giving us the opportunity to improve the English language of the manuscript.

Comment 2

Ad. 2. The section on remote sensing is the core of the article and it is well documented with figures presenting the research results. However, the geological part is just as long and complicated. Here, I would see the possibility of significant shortening of the manuscript. Structural analysis is based on qualitative descriptions supported by field photographs of varying quality and diagrams in Figure 12 presenting a limited number of measurements. I am not suggesting that the structural history presented is bad, but it is hardly well documented. In my opinion, the scope of structural geology should be limited to identifying and describing structures that are recognisable on remote sensing maps.

Authors’ Response

We appreciate your comment. Exactly we made significant shortening in the geologic and the structural analysis sections. Some sentences have been omitted, and others become more specific. We also confirm that the structural analysis based on the remote sensing data and intensive field work. The aim of this section to define the main structural elements which control the alteration zone and gold mineralization in the study area. we have to document the deformation history and how the main thrusts and shear zone control the gold mineralization in the study area. Therefore, we considered the tectonic evolution diagram (fig. 15) one of the most important contributions of the present study and presented here for the first time. We think this section after shortening it will me more concise and informative. Finally, we confirmed in this study that the reference old working gold occurrences are associated with the high deformation and fracturing zones bounding the sheared ophiolitic belt and faulted contacts such as Korabai and Madari gold mines. This means that the gold mineralization in this region mostly on the structural contact and all produced maps confirm this finding. Therefore, the detailed structural interpretation is very important to readers to understand the tectonic setting of this region.

Comment 4

I would also like to point out that describing the Gerf nappe complex as a "large-scale porphyroclast" is a bit naive and not entirely professional. The correct term should be "block".

Authors’ Response

We appreciate your comment. Yes the correct term is block and already changed in the manuscript. Thank you

Comment 5

Many detailed corrections and comments are in the annotated manuscript which is an integral part of my review. I hope that my comments will be helpful in preparing a version of the manuscript acceptable for Remote Sensing.

Authors’ Response

We appreciate your kind consideration about the manuscript and its presentation. We followed your constructive comments to improve the quality of the manuscript. The English language is improved, and some sections have been shortened. We hope the revised manuscript meets the high standards of Remote Sensing

Comment

I am very glad to review your paper. This is a successful application of RS into detection of ore-related structure. Maybe, you can add more comparative analysis on the results from different data processing methods.  disadvantages and advantage of different methods? the better combination of different methods for different problems?

Authors’ Response

We appreciate your kind consideration about the manuscript. We confirm that we take your considerations in account in the revised manuscript. We highlighted the results of the different processing methods. Re-draw some important figures which give new findings. Re-formatted the conclusion to explore the role of remote sensing and the integration between the results of all methods. Thank you for your time and efforts and we invite you to review the revised manuscript.