Using Whole Rock and Zircon Geochemistry to Assess Porphyry Copper Potential of the Tonggou Copper Deposit, Eastern Tianshan

Round 1

Reviewer 1 Report

Overall the manuscript is well-written with a few sections where the wording needs to be clarified or corrected.  

There are several major issues with the document in that you base your Ce oxidation and zircon fractionation on only two points from the TG1 sample?  Were these the only viable results?  This must be clarified.  Zircon composition can be complex in large-scale batholith systems and further work may be required in order to truly characterize the system.  

I have provided an annotated copy of your manuscript with comments and edits for review.

The location of the Daheyan granodiorite needs to be presented in Figure 1 or on an additional figure to show correlation with the Tonggou deposit.  Some cosmetic corrections are also required.

In regards to the sample location of TG1 in the large granodiorite body and location of the Tonggou deposit it is confusing as to how they are related as there is no scale for distance.  Additionally while you note that porphyry mineralization is present the images do not clearly show porphyry veining or alteration as it appears massive and the figure noted with argillic alteration does not have argillic assemblage minerals.  Can the granodiorite truly be distinguished as the source of mineralization or is just a host unit?  Additional clarification is required in the regional geology and ore deposit section on this.

Additional discussion or expansion on analytical methods and standard results should be added in a supplemental data file.

Why so few analyses in the trace element in zircon section?  Need to explain why you chose the two samples from TG1 and four from Daheyan sample.  

The low Eu/Eu* values in your samples typically reflect reduced conditions or normal feldspar fractionation yet you note that the Ce anomaly indicate an oxidized melt.  Why the discrepancy?  How can this be explained?  Need to add this to the discussion.  

The paragraph on lines 260 to 272 needs to be modified.  You can not say that the Daheyan granodiorite did have mineralization, but then say that it did not form.  This needs to be clarified.

Section 6.4 needs further discussion and basing the results on only two samples and spots when other research studies have show the complexity in mineralized systems is difficult to accept.  Additional discussion of the results in the samples will benefit this section.  

Comments for author File: Comments.pdf

Author Response

Dear Sir or Lady,

Thank you for paying so much attention to my manuscript “Using whole rock and zircon geochemistry to assess porphyry copper potential of the Tonggou Copper deposit, Eastern Tianshan”.

I have revised my paper according to suggestions of reviewers.

Listed below are queries from reviewers relating to my manuscript. All queries have been answered. In addition, some new added contents have also been described.

The Original manuscript queries and answers:

(A) Comments from Reviewer #1

Answers:

1. I have referred these references, they are really useful, and I learn a lot. We appreciated that the reviewers for their advice on this section.

2. We have revised Figure 1, adding the location of China and Daheyan granodiorite, scale bar, legend, and box of Fig. 2, etc.

3. We have revised it in Figure 1. Tonggou vein mineralization has been discovered early, it contain 36 economic ore bodies, which are currently sources of Cu and Zn. Since then, we has discovered porphyry Cu mineralization at Tonggou district, north of Tonggou vein Cu-Zn orebodies. Porphyry Cu mineralization occurs in the top of granodiorite (TMG), vein Cu-Zn mineralization occurs in the Qijiaojing Formation. Tonggou porphyry Cu mineralized age is consistent with vein Cu-Zn mineralized age.

4. 4. We have revised this section, it is propylitic alteration, and thank the reviewers for their advice on this section. Tonggou vein mineralization (Cu-Zn) occurs in periphery of Tonggou porphyry Cu mineralization, it is similar to Cordilleran Polymetallic Mineralization (Catchpole et al., 2015). The hydrothermal alteration in the Tonggou vein mineralization includes silicification, epidotization, chloritization, and carbonation, it is similar to propylitic alteration of porphyry mineralization (Sillitoe, 2010). The chalcopyrite-epidote-chlorite-quartz assemblage commonly occurs as veinlets and veins in stage 2.

5. We have added the location of Daheyan granodiorite in Fig. 1. Tonggou and Daheyan granodiorite both exposed in Bogda Belt, they have a similar U-Pb ages.

6. We have added the Cathodoluminescence images of zircons from Tonggou granodiorite (Fig. 4), which show the size and location of zircon, and thank the reviewers for their advice on this section.

7. One sample from the Tonggou granodiorite have been measured.

8. We have revised it, thanks.

9. We have revised this figure. We appreciated that the reviewers for their advice on this section.

10. Firstly, we thank the reviewers for their advice on this section. Actually, twenty grains have been measured from TMG, and ten grains have been measured from DBG. Zircon grains commonly contain minute inclusions of REE-bearing minerals (e.g., apatite, titanite, monazite, and allanite), which may yield higher LREE and lower Ce⁴⁺/Ce³⁺ values than their true concentrations in zircon. In addition, the possible presence of such inclusions may show high Sr, Th, Al, and P content during LA-ICP-MS analysis. In this study, we just show a few of zircon data representatively, which have very low P, Ca, Sr, and Al. Tonggou porphyry Cu deposit is the first porphyry Cu mineralization found in the Bogda Orogenic Belt. We hope researchers will pay more attention to porphyry Cu mineralization in this Belt by writing this paper.

11. Although twenty grains have been measured from TMG samples, but we must select out valid data. We have shown the zircon texture in Fig. 4, and we will read references about zircon textures associated with porphyry mineralization, it is interesting, and we really want to learn and study it, thanks a lot.

12. Yes, typically oxidized magmas have Eu/Eu_N* ratios >0.4 in porphyry mineralized systems (Ballard et al. 2002, Dilles et al., 2015, Lee et al., 2017). However, some porphyry Cu deposits also show low Eu anomalies in CAOB, such as Tuwu-Yandong deposit. It has low Eu anomalies (Eu/Eu_N* <0.4), yet these zircon grains show high Ce⁴⁺/Ce³⁺ ratios from 74 to 332 (Shen et al., 2015). Under normal fractional crystallization, Eu/Eu_N* values in zircon will decrease rapidly as Eu is removed from the melt by plagioclase crystallization, resulting in larger negative Eu anomalies in zircon with time (Lee et al. 2017). In addition, due to assimilate and contaminate some wall rocks containing plagioclase during evolution of magma, some barren intrusions may show high Eu anomalies (Xin et al., 2008). We have added this section in chapter 6.4.

13. We appreciated that the reviewers for their advice on this section. Some other points have high Sr, Th, Al, and P content during LA-ICP-MS analysis. So, we have not list these data. These data in table 3 is slightly little, but it is valid and truth.

14. Firstly, these references are done very professionally, we really appreciate your reviews. We have calculated Ce anomalies using the equation proposed by Zhong et al (2019). We redescribed Ce/Ce* in chapters 5.4 and 6.4. But, the calculation of Eu/Eu* is problematic as despite the strong fit of the curves, the MREE (Sm, Eu, and Gd) values are lower than the analytical values (Lee et al., 2020). Thus, we prefer to use the traditional Eu/Eu* method. We have discussed this section in chapter 6.4.

15. We have revised it, and rewrite this section, thanks. We have changed the section to: The LA-ICP-MS zircon U-Pb age for TMG varys from 302.2 Ma to 303.0 Ma, which represents Tonggou porphyry mineralization. Thus, Tonggou porphyry Cu mineralized age is consistent with vein Cu-Zn mineralized age (Zhang et al., 2019), the similarity in ages of porphyry and vein mineralization in the Tonggou deposit indicate that vein Cu polymetallic mineralization is closely associated with porphyry Cu mineralization in temporal and spatial terms.

16. We have changed the section to: "Porphyry Cu mineralization has not been found in the Daheyan granodiorite. We offer two hypotheses to explain this fact; (a) porphyry Cu mineralization may have formed at the top of the Daheyan granodiorite, however, this mineralization was mostly eroded away; (b) although the Daheyan granodiorite had a high initial Cu value, porphyry Cu mineralization may be absent due to other factors such as water content and magma oxidation state."

17. We have revised this section. We appreciated that the reviewers for their advice on this section.

18. Thank you for your advice. Some references are really useful, and learn a lot about Ce and Eu anomalies in zircon. We have rewritten chapter 6.4, and added chapter 6.5. We have revised other mistakes that have marked in pdf.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled ‘Using Whole Rock and Zircon Geochemistry to Assess Porphyry Copper Potential of the Tonggou Copper Deposit, Eastern Tianshan’ adds data that may help understand the significance of the Tonggou Cu deposit. The manuscript is overall well structured, informative and easy to read, although some improvements in style could be made. Besides, some other issues need to be taken care off before publishing. 

Abstract:  

The English language needs some polishing and the important data numbers should be included here, not only the zircon U-Pb age. 

Introduction:  

The ideas are well arranged but the style could be improved, to relate an idea to the next one. 

Geological setting:  

A map of China to locate the area would help readers as myself that are not familiar with the region geography.  

Ore deposit: 

This description is rather long and not referenced properly. There are previous articles by some of the manuscript authors that explain the mineralization, thus, I would shorten this section a lot and refer to these papers. Also, I miss some dimensions and frequency of mineralized veins.  

Some photographs of Figure 2 have already been published in the Geoscience Frontiers 2019 paper: they should be removed. If the above paper is cited, there’s no need for all the pictures. 

Methods: 

The sampling location of the barren granodiorite is difficult to locate for someone not familiar with the geography of the area, as it doesn’t appear on the map. Therefore, consider adding Daheyan in Figure 1. The number of samples should be mentioned, for both, barren and ore bearing intrusives. 

The description of granodiorite samples may be better placed within the results section, together with the geochemistry.  Figure 3a and 3c are not illustrative of the grain size and mineralogy.  

The detection limits for the elemental analyses should be mentioned.  

As hydrothermal alteration is mentioned, how does it affect the geochemical data? Alteration of rocks usually implies a remobilization of many elements, so that only immobile elemental ratios are used. Check Pearce, 2014 for elements that are considered relatively immobile under mild hydrothermal alteration (HFSEs, REEs, transition elements, and Th). Are both intrusive rocks altered? 

There are several indexes to evaluate the alteration degrees:  Ishikawa Alteration Index (AI; Ishikawa et al., 1976), the chlorite-Carbonate-Pyrite Index (CCPI) and the Advanced Argillic Alteration Index (Large et al., 2001; Williams and Davidson, 2004). See the alteration box plot for an example (Large et al., 2001; Williams and Davidson, 2004). 

The reference material (standards) used for all the analysis must be reported. What standards were used for U-Pb zircon dating? What standards were used for zircon trace element geochemistry and what standards were used for U-Pb-Hf isotopes? 

Results: 

There is only on picture of CL image of zircon. A figure with the CL images of all analyzed zircons would better be included. It should also show all the spot analysis (e.g., U-Pb dating, Trace element geochem and U-Pb-Hf isotopes). Why is the band indicated in the picture of Fig. 4a chosen? It doesn’t have a scale of reference.  

Consider using Terra-Wasserburg plot instead of Concordia plots to show the discordant zircons and the effect of common lead together with the weighted average plot. 

Please, zoom in to the data in figure 4a. 

What software was used for the processing of zircon (U-Pb, Hf isotopes and trace element) data? 

Only 4 of the 8 geochemical analysis are reported in the table. The complete dataset should be published. 

Figure 6 b shows spider diagram with a wide variability on the Rb, Ba, K content. Could this be an effect of alteration? 

The paragraph of lines 223-232 is rather confusing to me. Is it the same think to calculate de Ce anomaly than the ratio Ce4+/Ce3+ ?  It should be better explained, and, anyway, the modeling could be placed in the discussion section, as it is not straight data.   

Discussion:  

Avoid the repetition of data already presented in the Results section. 

Include the reference to the Geosc. Frontier 2019 paper in line 237 as it is the one that publishes the date of vein chalcopyrite. Also, the paragraph should reference the Geol. Magazine 2020 paper, which contains ages for other intrusions of the Bogda Mountains; I think it should be mentioned that the granodiorite age corroborates previous ones of the Bogda Orogenic Belt. 

You may want to look at Loucks, 2014 in order to distinguish ore-forming magmas and ore-undproductive mamas using a sort of diagrams (e.g. Sr/Y vs. SiO2 and V/Sc vs. SiO2).  

Also, the fertility /prospectiviy plots proposed by Lu et al. 2016 may be useful.  

The statement of line 267 ‘does not occur’ is rather strong, I suggest to modify it to something like ‘may not occur’. In fact, I would be prudent and write the whole paragraph using a conditional tense.  

Figure 9 contain some modeling of fractional crystalization or partial melting curves that should be explained in the text.  

Conclusions: 

Rewrite to include the main data (conclusions 2, 3). 

Rewrite conclusion 4, it is too general and not clear. It may better be referred to the study area data that points to the potential for a rich porphyry copper deposit. It could be something like “The conditions of water-rich oxidizing magmas produced by fractional crystallization, usually associated with porphyry deposits, are observed in the studied area. Therefore, it has all the requirements to host a porphyry Cu deposit of economic interest”. 

Author Response

Answers:

1. We have polished this section and added some description with data

2. We thank the reviewers for their advice on this section. We have revised introduction.

3. We have added the location of China in Figure 1.

4. 4. Tonggou vein mineralization has been discovered early, it contain 36 economic ore bodies, which are currently sources of Cu and Zn. As we have described it in previous articles (e.g., Geoscience Frontiers 2019), we didn’t describe it in this section. Since then, we has discovered porphyry Cu mineralization at Tonggou district, north of Tonggou vein Cu-Zn orebodies. In addition, we found that magnetite and a few of hematite occur in vein mineralization, so I rewrite characteristic of minerals compared to previous article.

5. For Figure 2, comparing to previous article, we have added some pictures for porphyry and vein mineralization. In addition, we have replaced some pictures.

6. We have added the location of Daheyan granodiorite in Figure 1. We thank the reviewers for their advice on this section.

7. We have revised the Figure 3a and 3c, thanks for your advice. Before the U–Pb dating, Hf isotopic and trace element analyzes of zircons, we should described samples. Therefore, the description of granodiorite samples have been placed in this section.

8. The detection limits are different for major and trace elements, the major elements mainly vary from 0.005% to 0.01%, the trace elements mainly vary from 0.01 to 0.5. Therefore, we have not described it.

9. We thank the reviewers for their advice on this section. Samples of Tonggou mineralization granodiorite have suffered epidotization and chloritization, it effects the values of major elements (such as K, Na, and Ca). For some trace elements, it effects rarely. In this paper, we have discussed fractional crystallization with immobile elements (e.g., Zr, Hf, Nb, Ta, and a few of REEs). However, Daheyan barren granodiorite did not suffer altered.

10. We thank the reviewers for their advice on this section. We have calculated the Ishikawa Alteration Index (Ishikawa et al., 1976), as well as AAAI (Williams and Davidson, 2004). But Tonggou granodiorite has suffered epidotization, it have high values of CaO. In addition, the CCPI has been calculated (Nicholas and Garry, 2004), the ratios vary from 95.1 to 97.8, it is consistent with chloritization in Tonggou granodiorite.

11. In order to better control analytical uncertainty and to assure instrument stability, an internal standard zircon GJ-1 (603 ± 3 Ma) and external standard zircon 91500 (1064 ± 4 Ma) were carried out for twice and once before and after every five analyses of unknown samples, respectively. Uncertainties are quoted at 1σ for individual U-Pb dating and trace element analyzes and at 2σ (with 95% confidence level) for Hf isotopic analyzes, respectively. We have added this section in 4.2 Analytical method.

12. We have added the Cathodoluminescence images of zircons from Tonggou granodiorite (Fig. 4), which show the size and location of zircon, and thank the reviewers for their advice on this section.

13. We have revised Concordia diagram (using Terra-Wasserburg plot), and thank the reviewers for their advice on this section.

14. We have revised this figure. We thank the reviewers for their advice on this section.

15. ²⁰⁶Pb/²³⁸U ratios were calculated by ICPMSDataCal, while concordia diagrams and weighted mean calculations were made using Isoplot 3.0. We have described this section in 4.2 Analytical method.

16. In this manuscript, we compared Tonggou granodiorite and Daheyan granodiorite in Bogda Belt, they both formed in 302 Ma. Whereas, we have found porphyry Cu mineralization in Tonggou, but no found in Daheyan. Thus, this study presents zircon U–Pb, whole-rock geochemical, zircon Hf isotope and trace element data for the Tonggou mineralized granodiorite and Daheyan barren granodiorite. We compared the whole rocks and zircons geochemistry of Tonggou granodiorite and Daheyan granodiorite. Although the zircon U–Pb and whole-rock geochemical data of Daheyan barren granodiorite are published in Geological Magazine (2020), we can cite these data to illustrate some scientific issues.

17. It is true, Rb, Ba, K contents should be effected by alteration, and can be verified by Pearce (2014). We thank the reviewers for their advice on this section. In this figure, we just want to show that some compatible elements (Ba, Sr, Zr, and Hf ) from Tonggou mineralization granodiorite are lower than Daheyan barren granodiorite, and some mobile elements have been effected by alteration.

18. It is Ce^4+/Ce³⁺ ratios, we have revised it, and sorry for this mistake.

19. We thank the reviewers for their advice on this section. We have revised this section (discussion).

20. We have revised this section and added some description about U-Pb age of Daheyan.

21. 21. We thank the reviewers for their advice on this section. We have calculated the Sr/Y and V/Sc ratios, the Sr/Y ratios for Tonggou and Daheyan granodiorite vary from 34 to 38, and from 52 to 68, respectively; V/Sc ratios for Tonggou and Daheyan granodiorite vary from 8.0 to 9.1 and 4.2 to 4.8, Loucks (2014) consider that Sr/Y >40 and V/Sc > 10 are unambiguously prospective for large Cu deposits. Sr/Y and V/Sc ratios for Tonggou granodiorite are close to the range of ore-forming intrusions.

22. We thank the reviewers for their advice on this section. Lu (2016) suggest the best fertility indicators are zircon Eu/Eu* (>0.3), but Tonggou granodiorite has low Eu/Eu* values in zircons, it may be explained by removal Eu by plagioclase crystallization, resulting in larger negative Eu anomalies in zircon with time (Shen 2015; Lee et al. 2017). Low Eu anomalies in zircon is not in conflict with high Ce/Ce* values in zircon grains because a signifcant fraction of Eu in any magma is Eu2+ even under highly oxidized conditions (Shen et al., 2015).

23. Thank you for your views, we have revised this section, we agree with your advice using a conditional tense.

24. We rewritten this section, thanks.

25. We have revised conclusions 2, 3. Thanks a lot.

26. We have changed conclusion 4 to Ce^4+/Ce³⁺ ratios (159-286) for the TMG approach those of large-to intermediate porphyry deposits at the CAOB. The high Ce^4+/Ce³⁺ ratios reflect an oxidizing magmas produced by fractional crystallization, indicating that TMG has the potential to form a large porphyry Cu deposit.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for addressing the comments and suggestions and adding the zircon images.  I recommend reading through and making sure there are no additional minor errors are issues with the manuscript.