Integrated VNIR–SWIR Spectral, Mineralogical, and Geochemical Classification of Hydrothermal Alteration Zones in the Shadan Au–Cu System, Eastern Iran
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe article is interesting, it is devoted to the study of the area productive for copper and gold. First of all, the zones of hydrothermal changes were studied. On the one hand, they are important for low-temperature gold and copper deposits. On the other hand, their mineral composition is difficult to study due to the small size and complex composition of minerals, their fine accretions. The authors applied innovative spectral analysis methods along with X-ray diffraction and X-ray fluorescence. Seven associations have been identified according to the composition and conditions of education. In general, interesting theoretical and practical results have been obtained.
But at the same time, the essence of spectral methods was not disclosed by the authors and remained incomprehensible to the inexperienced reader.
Unfortunately, the results are not verified by an electron microscope or a microprobe.
And the photos of the thin sections are shown only in the appendix.
Confusion in the affilations: d and e are confused.
The authors use a large number of poorly understood abbreviations - they need to be deciphered at the first use.
Figure 1 - own or borrowed?
Conclusion should be broken down into separate short statements for easier understanding.
In the list of references, doi is indicated only for the article in print - it is advisable to indicate all articles.
Author Response
# Reviewer 1:
The article is interesting, it is devoted to the study of the area productive for copper and gold. First of all, the zones of hydrothermal changes were studied. On the one hand, they are important for low-temperature gold and copper deposits. On the other hand, their mineral composition is difficult to study due to the small size and complex composition of minerals, their fine accretions. The authors applied innovative spectral analysis methods along with X-ray diffraction and X-ray fluorescence. Seven associations have been identified according to the composition and conditions of education. In general, interesting theoretical and practical results have been obtained.
But at the same time, the essence of spectral methods was not disclosed by the authors and remained incomprehensible to the inexperienced reader.
Unfortunately, the results are not verified by an electron microscope or a microprobe.
Reply:
Many thanks for your insightful comment. While techniques such as Scanning Electron Microscopy (SEM) and Electron Probe Micro-Analyzer (EPMA) can indeed yield additional micro-scale textural and compositional constraints, the integrated use of VNIR–SWIR spectroscopy, XRD, XRF, and ICP-MS in this study provides a robust mineralogical and geochemical framework at the scale pertinent to alteration zoning and exploration targeting. The primary objective of this work is to characterize alteration systems and elucidate spectral–mineralogical relationships, rather than to perform detailed micro-textural or in situ compositional quantification.
And the photos of the thin sections are shown only in the appendix.
Reply:
The photomicrographs of the thin sections have been incorporated into the figures. Please refer to Figure 6.
Confusion in the affiliations: d and e are confused.
Reply:
Thank you! The affiliations have been revised accordingly.
The authors use a large number of poorly understood abbreviations - they need to be deciphered at the first use.
Reply:
Thank you! The abbreviations have now been defined at their first occurrence.
Figure 1 - own or borrowed?
Reply:
Thank you! The text has modified as follows;
“A simplified structural map of Iran highlighting the main tectonic units, modified from [21, 42].”
Conclusion should be broken down into separate short statements for easier understanding.
Reply:
Thank you! The conclusion has been broken into several points as follows;
“1- Integrated VNIR–SWIR spectroscopy, XRD mineralogy, and whole-rock geochemistry define seven alteration–lithological groups (G1–G7) forming a continuous gradient from deep propylitic to shallow argillic, oxidized, and carbonate-bearing domains in the Shadan Au–Cu system.
2- The vertical and lateral zonation defines a systematic mineralogical progression from quartz–illite–kaolinite–dominated central assemblages to peripheral chlorite–epidote halos and late-stage carbonate–Fe vein networks. This spatial architecture records progressive thermal decline and coupled evolution of fluid pH and redox conditions during multistage hydrothermal circulation, consistent with telescoping in a porphyry–epithermal system driven by uplift and structural reactivation.
3- Diagnostic absorptions (~0.9, 2.20, 2.33, 2.50 µm) correlate with XRD-verified mineral assemblages and major-element systematics; band shifts in Al–OH and Mg–OH record octahedral Al–Mg–Fe substitution and temperature-dependent phyllosilicate evolution.
4- Fe³⁺ and CO₃ features document supergene oxidation and late carbonate veining, marking fluid evolution from magmatic–hydrothermal to mixed meteoric–hydrothermal regimes.
5- Au–Cu enrichment spatially associated with mixed argillic–carbonate and oxide assemblages indicates structurally focused, redox-controlled ore deposition governed by permeability and fluid mixing.
6- The integrated spectral–mineralogical–geochemical model provides a transferable framework for alteration mapping and vectoring toward mineralized centers in post-collisional porphyry–epithermal systems.”
In the list of references, DOI is indicated only for the article in print - it is advisable to indicate all articles.
Reply:
Thank you. DOIs have now been added to all references.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis paper explains the prospection methods used in eastern Iran in order to locate hydrothermal alteration zones. By analyzing thirty-eight representative samples using XRD, XRF, ICP-MS, and reflectance spectroscopy, researchers identified seven distinct alteration-lithological groups (G1–G7). These groups represent a hydrothermal continuum ranging from acidic argillic and advanced argillic assemblages to neutral propylitic and late-stage carbonate-iron oxide overprints. The results reveal a vertically and laterally zoned system evolving from magmatic-hydrothermal conditions to low-sulfidation epithermal overprints. Diagnostic absorption features near 0,9 1.4 1.9 2.17 2.2 2.33 and 2.5 microns were strongly correlated with specific mineral phases as aluminum oxide, iron oxide and calcium oxide.
Some comments:
- Figure 2. Please explain if “adapted” means modified and if the authors have permission to use this map.
- Figure 4: please detail how this map has been obtained. How are alteration zones are defined. Which software has been used and which data origins have been used.
- Figure 8: please detail how this diagram has been developed. Which software or programming tool.
- Figure 9: The sa,e. I woulf change color of the fonts in order to clearly differenciate the releative band depth,
- Paper stablishes 38 rock samples. We suppose they are not close to each other, as they are taken in a quite wide area. Is this number enough? How can it be justified?
Author Response
# Reviewer 2
Figure 2. Please explain if “adapted” means modified and whether permission was obtained to use this map.
Reply:
Thank you! The text has modified as follows;
“Geological map of the Lut Block, modified from [42].”
Figure 4: please detail how this map has been obtained. How are alteration zones are defined. Which software has been used and which data origins have been used.
Reply:
To clarify the interpretation, the following text has been added to the manuscript:
“Lithological and alteration zones were delineated through the integration of field-based lithological mapping, petrographic observations, VNIR–SWIR spectral characteristics, and remote sensing interpretations, supported by available technical datasets provided by Karand Sadr Jahan Mines and Mineral Industries Co.”
Figure 8: please detail how this diagram has been developed. Which software or programming tool.
Reply:
Thank you! Figure 9 (formerly Figure 8) has been revised to clarify the construction of the diagram. In addition, the relevant information regarding the software or programming tool used has been incorporated into the manuscript.
“Radar diagrams were generated from processed VNIR–SWIR spectral datasets to facilitate comparative visualization of diagnostic absorption features across alteration groups. Spectral band depths were normalized to a 0–1 scale prior to plotting. Data visualization and analytical plotting were performed using standard Python-based tools (Matplotlib) under an institutional academic license (Sheffield Hallam University). “
Figure 9: I would change color of the fonts in order to clearly differentiate the relative band depth.
Reply:
Figure 10 (formerly Figure 9) has been revised to clarify the differentiate the relative band depth.
Paper stablishes 38 rock samples. We suppose they are not close to each other, as they are taken in a quite wide area. Is this number enough? How can it be justified?
Reply:
Thank you for this valuable comment! The authors have addressed this point by incorporating the following additions into the manuscript:
“The study employed 38 strategically selected samples, reflecting a targeted, rather than statistical, sampling strategy, optimized for characterizing and classifying hydrothermal alteration types rather than quantifying mineral abundances or geochemical gradients across the deposit. The sample selection prioritized representativeness over numerical abundance, ensuring that each alteration type—argillic, advanced argillic, silicified, Fe-oxide, oxidized argillic, propylitic, and carbonate–iron assemblages—was represented by 4–6 samples, capturing intra-type variability. This approach aligns with established protocols in hydrothermal alteration studies, where 30–50 well-chosen samples typically suffice to delineate alteration mineralogy and zonation in porphyry–epithermal systems [e.g., 11]. Each sample was analyzed using four complementary techniques (VNIR–SWIR spectroscopy, petrography, XRD, and whole-rock geochemistry), providing multi-dimensional validation. Consistency between spectral groupings and independently derived mineralogical and geochemical data confirms that the dataset adequately captures the principal alteration assemblages. Samples were spatially distributed along key structural trends, lithological contacts, and alteration boundaries identified in prior field mapping (Fig. 4). This transect-style design captures both hypogene and supergene domains without requiring dense sampling of homogeneous zones. And, the 38-sample dataset is consistent with previous integrated hydrothermal studies, where combined spectral–mineralogical–geochemical approaches typically use 25–50 specimens [e.g., 16], reflecting the diminishing informational returns of additional sampling once the main alteration facies and their spectral–mineralogical signatures are established. Therefore, the combination of strategic sample selection, multi-method validation, and comprehensive coverage of all alteration types provides a robust framework for spectral–lithological classification, supporting the interpretation of hydrothermal processes in the studied system.”
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors- Regarding the methods, I miss the details of the XRD measurements (where was it done? what was type of the XRD equipment?).
- It should shortly explain why are representative spectra shown for G4, 5, 6, and 7 samples, while for G1, 2, 3 samples there are six spectra each? (See Figure 7)
- Figure 8 seems to me not really demonstrative; I suggest modify the colors or may use another diagram type.
Author Response
# Reviewer 3:
Regarding the methods, I miss the details of the XRD measurements (where was it done? what was type of the XRD equipment?).
Reply:
Thank you for your constructive point! The following text has been added to the manuscript (please find Methodology section).
“Powder XRD analyses were performed on representative samples from each spectral group at the University of Tehran using a Bruker D8 Advance diffractometer (Bruker AXS, Germany) equipped with CuKα radiation (λKα₁ = 1.5406 Å) and a Ni filter to suppress CuKβ radiation. The instrument was operated at 40 kV and 40 mA. Diffraction data were collected over a 2θ range of 5–70° with a step size of 0.02° and a counting time of 1 s per step. The incident beam optics included a fixed divergence slit of 0.6° and a 0.1 mm receiving slit. Bulk samples were crushed and pulverized in an agate mill to <75 μm to minimize preferred orientation. Powder mounts were prepared using the back-loading technique to ensure random grain orientation. Mineral phases were identified by comparison with the PDF-4 database of the International Centre for Diffraction Data (ICDD). Diffractograms were processed to identify primary and secondary mineral phases associated with alteration, including illite, kaolinite, chlorite, quartz, calcite, dolomite, and hematite. Particular attention was given to mineral pairs with overlapping VNIR–SWIR spectral features (e.g., illite vs. kaolinite; calcite vs. dolomite), allowing XRD results to resolve spectral ambiguities and confirm diagnostic absorptions. These mineralogical constraints were used to validate group assignments and strengthen the interpretation of hydrothermal processes when integrated with petrography and spectral feature analysis [11].”
It should shortly explain why are representative spectra shown for G4, 5, 6, and 7 samples, while for G1, 2, 3 samples there are six spectra each? (See Figure 7).
Reply:
Great point! Thank you! The text has been amended as follows;
Section 3.1; “For spectral visualization, groups G1–G3 (n = 6 each) are shown with all individual spectra to capture intra-group variability in absorption features, while groups G4–G7 (n = 1–3) are represented by median spectra due to high spectral homogeneity and limited sample numbers. This dual strategy balances completeness and clarity, highlighting diagnostic absorption features without redundancy. All spectral assignments were validated against XRD mineralogy and whole-rock geochemistry, ensuring robust and reproducible alteration group definitions.”
Section 4.3; “Spectroscopic analysis of the seven alteration–lithological groups reveals distinct absorption features that correlate with mineral assemblages confirmed by XRD and petrography. It should be noted that the spectral representation in Figure 8 varies by group. Groups G1–G3 display all six individual spectra to document intra-group variability, whereas groups G4–G7 are represented by median spectra due to smaller sample sizes (G4, n = 1; G5–G7, n = 2–3) and the high spectral homogeneity within these groups. This strategy ensures that diagnostic absorption features are clearly conveyed without redundancy, while all spectral assignments are corroborated by independent XRD mineralogy and whole-rock geochemical analyses, confirming their robustness and reproducibility.”
Figure 8 seems to me not really demonstrative; I suggest modify the colors or may use another diagram type.
Reply:
Thank you! The Figure 9 (formerly Figure 8) has been modified as per the Reviewer’s request.
Best regards,
Author Response File: Author Response.pdf
