Review Reports - Zircon U–Pb and Lu–Hf Signatures for the Gold Porphyry Deposits in the Alta Floresta Gold Province (Amazonian Craton, Brazil): Implications for Regional Exploration

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I read with interest the paper "Zircon U-Pb and Lu-Hf signatures for gold porphyry deposits in the Alta Floresta Gold Province (Amazonian craton, Brazil): Implications for regional exploration" by Oliveira and co-authors.

After carefully reviewing the manuscript, I believe that, in its current form, it is not yet suitable for publication. The manuscript requires substantial revision, as several issues need to be addressed before it can be considered for publication. The main comments are outlined below, while additional specific observations and suggestions are provided directly within the manuscript.

1) The abstract needs to be rewritten and restructured. What are the reasons of this study?

2)The Introduction lacks a clear explanation of the motivations underlying this study. I strongly recommend expanding this section to better articulate the scientific rationale and broader significance of the work, making it relevant to a wider audience by including several examples from different regions worldwide. In addition, I suggest incorporating references to previous studies that have successfully applied U–Pb geochronology to zircon and detrital zircon (e.g., Sanità et al., 2025, Scientific Reports, https://doi.org/10.1038/s41598-025-18508-7), as well as key studies employing Lu–Hf isotope analyses. Providing this broader context would strengthen the scientific framework of the manuscript, enhance its impact, and improve its overall soundness.

3) method section: I suggest revising this section to adopt a more scientific and formal style. The current text is somewhat descriptive and divulgative; a more rigorous and technical presentation would improve the overall quality of the manuscript.

4) Could the authors explain the relatively high MSWD values? Do these values reflect analytical uncertainties or issues associated with the analytical procedure, or are they indicative of multiple zircon populations, suggesting a more complex geological history? A discussion of the possible causes and their implications would strengthen the interpretation of the results.

5) Can the auhtos plot Th/U against U-Pb ages got from detrital zircons? this would provide a qualitative inforamtion on the nature of zircons (i.e., magmatic vs metamorphic)

I suggest MAJOR revision

Comments for author File: Comments.pdf

Author Response

The comments and suggestions from Reviewer 1 have been addressed, with the exception of the suggestion to include one reference, which the authors deemed inappropriate for the topic under discussion.

Comments Reviwer 1

1) The abstract needs to be rewritten and restructured. What are the reasons of this study?

Response: We included a sentence (in green) to address the request. New Abstract.

The present work focuses on investigating U-Pb and Lu-Hf ages of zircon grains obtained from Au mineralized granitic rocks from Alta Floresta Gold Province (AFGP), in Amazonia craton, Brazil. Also detrital zircon grains from recent Au-rich sediments were analised for U-Pb ages and Lu-Hf isotopic signatures, aiming to apply for regional gold exploration in the AFGP to define targets consisting of deep-seated mineralized bodies using isotopic tracers. U-Pb ages of magmatic rocks, obtained from zircon grains, indicate crystallization ages ranging from 1989 Ma to 1827 Ma for rocks associated with gold deposits, and TDM model ages between 2.63 and 2.01 Ga and εHf values between +8.0 and -4.6. U-Pb ages obtained from 89 detrital zircon grains indicate ages peaks at 2.055-1,980 Ma, 1,889-1985 Ma, 1,790-1700 Ma, and 1,600-1550 Ma. The first group is related to the basement, represented by the rocks of the Cuiu-Cuiu Complex (TDM 2.0-1.9 Ga and εHf = +8.7). The second group concerns the mineralized magmatic rocks in the AFPG and the ages range from 1889 to 1825 Ma, TDM ages ranging from 2.90 to 2.75 Ga and the εHf values from +15 to -8. The age ranging from 1790 to 1700 is associated with anorogenic magmatism (TDM 2.0-1.9 Ga and εHf from +5.36 to -8.46). The youngest group is comprised of ages from 1,600 to 1550 Ma, probably correlated to Serra da Providencia suite with TDM of 1,6 to 1,50 Ma and εHf from +5,0 to -10. In this way, it was possible to identify the age range of detrital zircon grains concerning the ages of mineralized granites and characterize the Hf signatures associated with the mineralization processes. In this sense, the zircon grains associated with the magmatic processes responsible for the concentration of noble metals exhibited U-Pb ages and εHf values interpreted as having been generated in magmas origi-nating from a depleted mantle associated with hot, hydrous, Fe-rich, high-K calc-alkaline magma and subvolcanic emplacement, with rapid cooling at low pressure. Based on the model that correlates gold deposits with porphyritic magmatic processes, significant dis-coveries with world-class volumes may result from better characterization of the deposit types found in the Province.

2) The Introduction lacks a clear explanation of the motivations underlying this study. I strongly recommend expanding this section to better articulate the scientific rationale and broader significance of the work, making it relevant to a wider audience by including several examples from different regions worldwide. In addition, I suggest incorporating references to previous studies that have successfully applied U–Pb geochronology to zircon and detrital zircon (e.g., Sanità et al., 2025, Scientific Reports, https://doi.org/10.1038/s41598-025-18508-7), as well as key studies employing Lu–Hf isotope analyses. Providing this broader context would strengthen the scientific framework of the manuscript, enhance its impact, and improve its overall soundness.

Response: We provided a good number of examples with worldwide disribuitions (as presented in the paragraph below). The reference suggested by the reviewer is not suitable for the topic addressed in this section.

The Cu-Au porphyry systems exhibit a range of copper metal concentrations, accom-panied by gold byproducts, and occasionally, gold-only deposits. Porphyry gold deposits tend to be concentrated in localized belts around the world [8], such as Maricunga in Chile (Marte and Lobo deposits; [9], the Javorie belt in Slovakia (Biely Vrch deposits; [10], the Middle Cauca belt in Colombia (La Colosa deposits) [11], and western Anatolia in Turkey [12]. This notable limitation in the global distribution of porphyry gold deposits has been attributed by [10] to a combination of factors, including magma type (hot, hydrous, Fe-rich, high-K calc-alkaline, mantle-derived) and emplacement conditions (subvolcanic em-placement, rapid cooling at low pressure).

Comments:

Response: We included two new texts, the first with procedure details of U-Pb analysis, and the second describing Lu-Hf analysis steps.

Zircon analyses were performed at the Multi-user Environmental Laboratory (Mul-tiLab) at UERJ, using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) equipped with an Element-2 spectrometer. The data acquisition sequence on the equipment was as follows: (1) blank reading; (2) GJ-1 standard reading; (3) sequential reading of nine unknown grains; (4) 91,500 standard reading; (5) GJ-1 standard reading; (6) blank reading. Ablation of the grain surfaces was performed using laser pulses with a 30 μm diameter, and the vaporized material was transported in Ar (0.80 L/min) and He (0.55 L/min) for analysis using 700 cycles of 1'1'' each. The analyses included measure-ments of 204Pb, 206Pb, 207Pb, 208Pb, 232Th, and 238U masses. Hg is a common contaminant in He and Ar gases, and the 204Hg mass interferes with the 204Pb mass counts. Therefore, the 202Hg mass count was used to correct this isobaric interference.

Lu-Hf isotopic data were also determined for individual zircon grains using a Nep-tune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), with a detector array of 9 Faraday cups attached to a Photon-Machines excimer laser ablation mi-crosampling system. Analytical procedures includes the Hf standard, GJ-01 and 91.500, reported in that paper, yielded 176Hf/177Hf = 0.28216 ± 0.00002 (2 SD; n = 208, ∼± 0.7 ɛHf units) and 176Hf/177Hf = 0.28216 ± 0.00002 (2 SD; n = 208, ∼± 0.7 ɛHf units), respectively.

Comments:

Response: We included the following discussion in the manuscript:

The upper-intercept U-Pb ages yield MSWD values of 2.8 (sample AFPC 3PR, Figure 6A), 4.2 (sample AFPC 8PR, Figure 6B), 0.61 (sample AFPC 11, Figure 6C), 2.5 (sample AFPC 13R, Figure 6D), 5.3 (sample AFPC 7R, Figure 6E), and 0.86 (sample AFPC 25, Figure 6F). The MSWD values are relatively high (samples AFPC 8PR and AFPC 7R) due to the use of a 2-sigma-based error calculation. This more rigorous approach yields more reliable age values.

Suggestion:

5) Can the auhtos plot Th/U against U-Pb ages got from detrital zircons? this would provide a qualitative inforamtion on the nature of zircons (i.e., magmatic vs metamorphic)

Response:

We included ta new Figure qith the diagram solicited and included the discussion and interpretations in the manuscript.

The Th/U ratio versus ²⁰⁷Pb-²⁰⁶Pb age diagram shows a pattern characteristic of magmatic rocks, with Th/U values ranging from 20 to 1.3.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses a relevant topic and presents a potentially useful application of U–Pb and Lu–Hf zircon data for understanding mineralized granitoids and supporting regional exploration in the Alta Floresta Gold Province. The study is generally well structured and the dataset is of interest to the readership of Minerals. However, the manuscript requires major revision before it can be considered for publication. Several issues were identified, including inconsistencies among the Abstract, Results, Discussion, and Table 1; contradictory age ranges and isotopic interpretations; problems with section numbering; numerous grammatical and typographical errors; and citation and reference list inconsistencies. In addition, some interpretations appear stronger than the evidence presented and should be better justified or more cautiously interpreted. Detailed line-by-line comments and specific recommendations are provided in the annotated PDF

Comments for author File: Comments.pdf

Author Response

The comments and suggestions from Reviewer 1 have been addressed, as presented below:

Comments by Reviwer 2

Suggestions:

Correction in Abstract values of U-Pb ages.

And 1,50 Ga

Response:

The correct values are 1889 to 1825 Ma

Suggestions:

….with hot, hydrous, Fe-rich, high-K calc-alkaline magma and subvolcanic emplacement, with rapidcooling at low pressure..

Response: New text:

As suggested in the literature, the magma characteristics here reported is coherent with a hot, hydrous, Fe-rich, high-K calc-alkaline magma.

Suggestion:

Ctaron

Response:

Craton

SUGGESTION:

references [1], [2] [3] [4][5].

Response:

[1][2][3][4][5].

Suggestion:

….and to apply this information to regional gold exploration surveys as proposed by [7].

Response: New text:

….and to apply this information to regional gold exploration surveys as proposed by [7]. This investigation partially demonstrate a provenance discrimination towards an exploration effectiveness and predictive targeting must be further developeded.

Suggestion:

The emplacement depth of copper porphyry systems are more profound than that of gold-only porphyry deposits (<1 km), while copper porphyry deposits (grades of 0.0–1.5 g/t Au) are commonly emplaced at paleodepths exceeding 2 km [10][16].

Response: New text:

When considering Cu-rich and Au-rich deposits, we can, initially disregard grade variations for each deposit type. But, the emplacement depth of copper porphyry systems are more profound than that of gold-only porphyry deposits (<1 km), while copper porphyry deposits (grades of 0.0–1.5 g/t Au) are commonly emplaced at paleodepths exceeding 2 km [10][16].

Suggestion:

On the other hand, barren arc-related rocks composition plot Sr/Y < 35 and the range of SiO2 > 55 wt%.

Response: On the other hand, barren rocks associated with arcs exhibit Sr/Y ratios < 35 and SiO2 contents > 55 wt%.

Suggestion:

Methodology

Response: 4. Methodology

Suggestion:

The 89 ages obtained from detrital zircon grains (Tables C and D Supplementary Materials) indicate ages peaks at 2.055-1,980 Ma, 1,889-1985 Ma, 1,790-1700 Ma, and 1,600-1550 Ma (Figure 8). The first group is related to the basement, represented by the rocks of the Cuiu-Cuiu Complex with TDM 2.0-1.9 Ga and εHf = +8.7 (Figure 9). The second group concerns the mineralized magmatic rocks in the study region and the ages range from 1896-1825 Ma.

Response: checked

The 89 ages obtained from detrital zircon grains (Tables C and D Supplementary Ma-terials) indicate ages peaks at 2.055-1,980 Ma, 1,889-1985 Ma, 1,790-1700 Ma, and 1,600-1550 Ma (Figure 8). The first group is related to the basement, represented by the rocks of the Cuiu-Cuiu Complex with TDM 2.0-1.9 Ga and εHf = +8.7 (Figure 9). The second group concerns the mineralized magmatic rocks in the study region and the ages range from 1896-1825 Ma;

Suggestion:

As suggested by [45], the texture of zircon crystal cores observed in CL images can be used to assess magma fertility for porphyry-type deposits. In this way, fertile suites exhibit bi-modal zircon CL images show with cores weak zones. In contrast, infertile suites typically exhibit zoned zircon CL textures, with strong oscillatory zonation [46].%.

Response: New text

Catodoluminescense textures cannot alone demonstrate metal deposition, but the fer-tile zircon grains are interpreted as having grown in magmas responsible for porphyry-type mineralization and present weak oscillatory zonation, which matches with detrital zircon grains.

Suggestion:

Response: New text:

The U-Pb ages of detrital zircon grains in this study range from 1884 Ma to 1860 Ma for rocks associated with gold deposits. The 89 ages obtained from detrital zircon grains indicate age groups of 1,980 Ma, 1,880 Ma, 1,790 Ma, and 1,590 Ma (Table 1). The first group is related to the basement, represented by rocks of the Cuiu-Cuiu orogen (2.0-1.9 Ga [53]. Only the second group concerns mineralized magmatic rocks in the study region. The ages of 1790 and 1590 Ma are associated with anorogenic magmatism or a younger mag-matic arc as discussed by [54].

Suggestion:

Zircon grains from the oldest rocks indicated TDM model ages between 2.90 and 2.89 Ga (εHf +8.7).

Response: New text:

Zircon grains from the oldest rocks (U-Pb ages from 2.055 to 1.989 Ma) indicated TDM model ages between 2.00 and 1.90 Ga (and εHf from +8.7 to -1.32).

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

In the revised version of the manuscript, many issues have been addressed by authors. I still disagree with some points, but the paper can be considered ready to be published in the journal minerals.

Reviewer 2 Report

Comments and Suggestions for Authors

All my comments have been solved, and the manuscript has been improved for publication