Mapping New IOCG Mineral Systems in Brazil: The Vale do Curaçá and Riacho do Pontal Copper Districts
and Carla Braitenberg 4
Nils Holzrichter
Round 1
Reviewer 1 Report
This contribution aims to characterize the structure and geodynamics of the emplacements of the IOCG mineral system in Brazil, particularly The Vale do Curaçá and Riacho do Pontal Copper Districs.
In the current form the contribution only addreses the detailed structure of the distribution of the known Mineral system, it is not aiming to NEW so in the current form the title does not represent the main goal of the contribution.
Large effort need to be dedicated to rive the English grammar and syntaxis, preferable by a native English speaker.
There is a miss-match between what the authors say they will do (see introduction subsection: Purpose and Scope) and what they conclude in the contribution.
An relevant points needs to be addressed is the information concerning the Seismic S-wave tomographic models. The authors present an image of a depth section of the velocity S-wave map. They don't mention the reference velocity and they don't iindicate the source of the tomographic model, The references 64,64, 66 are papers that address the topinc of the formation of mineralization in Africa. The resolutions aspect of the model are not presented and/or discuss. This are critical to support the points the authors raise related to the generation of the mineralized areas.
Fig 5, 6 need a similar figure with the mapped geology at the same scale these would reinforce the correlation between anomalies and geologic domains
Figure 11/12, The figure caption needs to explain how this imaged is obtained. Furthermore If it is the composite result of the gravity and the magnetics the resolution power of the methodologies needs to be addressed at some point in the text or the reader should be addressed to the appropriate reference material.
In order to increase the impact and value of this contributiona the manuscripts needs a major revisions. With special emphasis in the English.
Comments for author File: 
Comments.pdf
Author Response
Dear Assistant Editor Lumia Lu,
We thank the editor and the reviewers very much for their time spent on the review and for the constructive comments which were used to improve the manuscript.
The manuscript addresses integration of geophysical and geologic data and identifies important geological, structural and metallogenetic characteristics related to the Proterozoic Riacho do Pontal and Vale do Curaçá iron-oxide copper gold (IOCG) mineral systems in northeast Brazil.
The revision of the original manuscript focused on clarifying the reviewer’s observations and incorporating all their suggestions. A point-by-point reply to all the questions raised by the reviewers have been made.
We worked hard on the figures in order to find a better representation of the data and results and accomplished the recommendations of the reviewers.
The reviewers put the changes directly in the manuscript, so we copied the major comments here into the letter, to make our replies more evident.
Sincerely
Sérgio Roberto Bacelar Huhn
11/11/2020.
Point by point replies to Reviewer #1:
Reviewer # 1:
1.1 Comments and Suggestions for Authors:
Rev #1: In the current form the contribution only addreses the detailed structure of the distribution of the known Mineral system, it is not aiming to NEW so in the current form the title does not represent the main goal of the contribution.
Reply: to our view the Vale do Curacao district is known as a Copper district, not known as a IOGC district. Therefore we think the title is justified, so we propose to keep the title. If reviewer #1 insists on this point we can change the title.
Rev #1: Large effort need to be dedicated to rive the English grammar and syntaxis, preferable by a native English speaker.
Reply: sorry, we have now improved English by native speaker.
Rev #1: There is a miss-match between what the authors say they will do (see introduction subsection: Purpose and Scope) and what they conclude in the contribution.
Rev #1: An relevant points needs to be addressed is the information concerning the Seismic S-wave tomographic models. The authors present an image of a depth section of the velocity S-wave map. They don't mention the reference velocity and they don't iindicate the source of the tomographic model, The references 64,64, 66 are papers that address the topinc of the formation of mineralization in Africa. The resolutions aspect of the model are not presented and/or discuss. This are critical to support the points the authors raise related to the generation of the mineralized areas.
Reply: sorry, now we have added the original references on the seismic tomographic.
Rev #1: Fig 5, 6 need a similar figure with the mapped geology at the same scale these would reinforce the correlation between anomalies and geologic domains.
Reply: thank’s for the suggestions, we have added such a figure.
Rev #1 :Figure 11/12, The figure caption needs to explain how this imaged is obtained. Furthermore If it is the composite result of the gravity and the magnetics the resolution power of the methodologies needs to be addressed at some point in the text or the reader should be addressed to the appropriate reference material.
Reply: Thank’s for the suggestions, we have added some details on the inversion method.
Rev #1: In order to increase the impact and value of this contribution a the manuscripts needs a major revisions. With special emphasis in the English.
Reply: yes, we have improved text and English, following the suggestions
Reviewer 2 Report
Dear authors,
your paper presents an extensive investigation of a mining area in South America, using gravity and magnetic data for interpreting the underground and the underlying structures to assess an exploration potential.
My expertise is gravity and magnetic processing, modelling and interpretation, thus, I can mostly judge on these parts and will do this in the following. The geological description and background information sounds reasonable to me.
The paper needs revision, because some parts of the potential field discussion are confusing and maybe wrong, or affected by some slopiness presenting the results:
### Figure 4b:
Must be wrong, colorbar tells gravity anomalies from 24.95 to 25.28. Looks a bit like the anomaly values of the magentic divided by 1000. At least Figure 6b shows a different colorbar.
### Figure 11
Colorbar shows Mgal which means megagal. Below the colorbar, the legend says, that it shows isosurfaces of SI >0.005 and Grav > -12000 mGal (sic!).
That is- wrong in two ways, the first is, that -1200 mGal is way to much, probably also mixed up with nT and also this value is not found in the text. The caption ways -40 mGal which could make sense, however, this has to be corrected.
Second, (also through the text, see below), there is a mix up in properties. The isosurface shown for magnetics is defined by SI, which is a rock property, but in gravity it is defined as a mGal value which is the unit of the resulting field. To be consistent, if magnetic body isosurfaces are defined by rock properties, for gravity the analogue must be density.
Other way around, if mGal in the legend is correct, then magnetics should be nT. However, from my understanding SI and density should be disucssed.
Lines 495 - 497
These trends are positioned in the zone of contact between domains with high magnetic (SI > 0.005) and gravimetric anomalies (> –10.000 nT) and show contrasts in density and magnetite Content.
Same as above, first discussion with SI and mGal which makes no sense, and -10000 nT given as value for gravimetric anomalies.
Figure 12
Same problem as in figure 11 in the legend and also in the caption, where again SI and mGal are mixed up.
There are often this error through the document, this should be checked carefully, at the moment, this discussion is not appropiate for publication.
Additional comments about study design
In line 389 you discuss the negative gravity anomalies as effect of isostatic compensation. All inversion and anomalies which are discussed are in a depth of up to 5 km. Why do you use Bouguer anomalies and do not correct for isostatic compensation to get rid of the crustal undulation effects which are not of interest for you ? Does it affect your inversion and results if you remove the effect of the crust mantle interface ?
Your Bouguer anomaly runs from around -60 to -20 mGal. As its relative, that is actually ok, but this negative trend is distracting, when you talk about "high" anomalies which are acutally negative. Probably you would get rid of this problem, if you use isostatic residual anomalies. Otherwise I would suggest to remove the long wavelength background field, which are not important for your study.
In the northern part at the edge of your study above, north from Riacho Seco, there seems a strong edge effect in the gravity signal. I suggest to cut of the part or remove the edge effect, that would highly improve the dynamic range of your colorbar and enhance features in the actual investigated area.
Author Response
Dear Assistant Editor Lumia Lu,
We thank the editor and the reviewers very much for their time spent on the review and for the constructive comments which were used to improve the manuscript.
The manuscript addresses integration of geophysical and geologic data and identifies important geological, structural and metallogenetic characteristics related to the Proterozoic Riacho do Pontal and Vale do Curaçá iron-oxide copper gold (IOCG) mineral systems in northeast Brazil.
The revision of the original manuscript focused on clarifying the reviewer’s observations and incorporating all their suggestions. A point-by-point reply to all the questions raised by the reviewers have been made.
We worked hard on the figures in order to find a better representation of the data and results and accomplished the recommendations of the reviewers.
The reviewers put the changes directly in the manuscript, so we copied the major comments here into the letter, to make our replies more evident.
Sincerely
Sérgio Roberto Bacelar Huhn
11/11/2020.
Point by point replies to Reviewer #2:
Reviewer # 2
Rev#2 - Figure 4b: Must be wrong, colorbar tells gravity anomalies from 24.95 to 25.28. Looks a bit like the anomaly values of the magentic divided by 1000. At least Figure 6b shows a different colorbar.
Reply: Fig 4b: sorry, it was wrong. We have a correct the colorbar now.
Rev#2 - Figure 11: Sorry, this was a wrong legend. We have now corrected, and units are correct. It was a misunderstanding of the person who finalized the figures, and who did the inversions had not noticed.
Reply: The model has now correct units for both the magnetic and gravity fields, and the units of the inversion results, in density and susceptibility values.
Rev# 2 - Figure 12: Same problem as in figure 11 in the legend and also in the caption, where again SI and mGal are mixed up.
Reply: Sorry, it is the same problem. It is fixed now.
Rev# 2 - Additional comments about study design:
Reply: in principle you are right, but in this specific case, the topography is flat, and the Bouguer anomaly does not correlate with the topography. We are well aware of this problem, as shown in the manuscript Pivetta T., Braitenberg C. (2020). Sensitivity of gravity and topography regressions to earth and planetary structures. Tectonophysics, 774, 228299, doi:10.1016/j.tecto.2019.228299,
We have added now the topography map of the Vale do Curaça and Riacho do Pontal Copper Districts. Riacho do Pontal District has a topographic variation between 356 e 705m. It is a slightly wavy region. On the other hand, the Vale do Curaça region is predominantly a flat region having a topography between 356 m and 800m.
We have made the inversion for both Vale do Curaça and Riacho do Pontal area separately.
Reviewer 3 Report
The article provides a good overview of data on the geodynamic, tectonic, structural, lithological, and other factors for localization of the IOCG mineralization in the Copper Districts of Vale do Curaca and Riacho do Pontal. Also, hydrothermal alteration and its zoning patterns was analyzed. On the listed geological and geophysical data (gravimetric, magnetic, seismic tomography), the authors carried out three-dimensional geological modelling which can facilitate the spatial and temporal understanding of hydrothermal systems of Vale do Curaça and Riacho do Pontal Districts
The article is of undoubted scientific and practical interest for the problems of targeting for IOGC style ore fields; however, a review by experts on geophysical methods is required as the paper contain a number of specific details, which can be not clear for exploration geologists.
Author Response
Dear Assistant Editor Lumia Lu,
We thank the editor and the reviewers very much for their time spent on the review and for the constructive comments which were used to improve the manuscript.
The manuscript addresses integration of geophysical and geologic data and identifies important geological, structural and metallogenetic characteristics related to the Proterozoic Riacho do Pontal and Vale do Curaçá iron-oxide copper gold (IOCG) mineral systems in northeast Brazil.
The revision of the original manuscript focused on clarifying the reviewer’s observations and incorporating all their suggestions. A point-by-point reply to all the questions raised by the reviewers have been made.
We worked hard on the figures in order to find a better representation of the data and results and accomplished the recommendations of the reviewers.
The reviewers put the changes directly in the manuscript, so we copied the major comments here into the letter, to make our replies more evident.
Sincerely
Sérgio Roberto Bacelar Huhn
11/11/2020Point by point replies to Reviewer #3:
Reviewer # 3Rev#3 - The manuscript aims to make an important contribution to the understanding of IOCG deposits in northeast region of Brazil.Reply: We thank you for your positive comment about the paper.
Sincerely, Sérgio Bacelar
Round 2
Reviewer 1 Report
N/A
Author Response
Dear Reviewer,
We work hard to improve the English of the manuscript. As well as correcting suggestions for improvement in the figures of the manuscript.
I would like to thank you for all the recommendations made for improving the manuscript,
Sincerely,
Sérgio Bacelar
Reviewer 2 Report
Hello,
in Figure 7 a and b have a different size.
in Figure 11 to 13 the colorbar is labelled with Mgal, which means mega gal. Please change it to mGal.
Author Response
Dear Reviewer,
We improve the English of the manuscript. As well as correcting suggestions for improvement in the figures in the manuscript. We made all the modifications recommended by the reviewer in figures 7, 11, 12 and 13.
I would like to thank you for all the recommendations made for improving the manuscript.
Sincerely,
Sérgio Bacelar
