Sm–Nd Isochron Age Constraints of Au and Sb Mineralization in Southwestern Guizhou Province, China

Round 1

Reviewer 1 Report

This is a clearly written paper. I enjoyed reading it. Data are well presented with appropriate figures. However, the discussion is not convincing in some parts of the paper, as I discuss below:

My major criticism on this paper is that the author ignored or considered the geochronolocical data of previous studies as unreliable (line 210 and lines 212-219). I have a serious concern regarding the disqualifying the sericite Ar-Ar and arsenopyrite Re-Os ages by Chen et al. (2009 and 2015). The authors argue: “the reported Re–Os isochron ages of the Lannigou and Shuiyindong Au deposits are likely significantly mixed”. If they are mixed how did Chen et al. (2015) get isochrons in their Fig 7? Are they mixing lines giving similar ages by coincidence? Authors also questions the purity of arsenopyrite samples.  Chen et al. (2015), however, presented the dated samples as being pure in their Figs. 5 and 6.  I am not necessarily defending Chen et al. (2015), but Wang et al. seem to be ignoring Chen et al. (2015) as a whole. Also, Wang et al. state: “hydrothermal sericites from the Lannigou Au deposit are also easily mixed with detrital or authigenic micas from sedimentary rocks due to their extremely small size.” This is a presumption. Sericite can be mixed with detrital materials in the matrix of the host rocks, but this is unlikely to be case if it occurs as a vein mineral together with quartz. Do vein minerals not precipitate from circulating hydrothermal fluids? Having said that, it is possible that the Ar-Ar age reported by Chen et al. (2009) is older than the true sericite age because of the recoil effect of very-fine grain clay minerals (sericite could be everything: fine-grained muscovite, illite, mixed-layered illite-smectite). Chen et al. (2009) is not accessible for me; so, I could not check what sort of material they dated, and hence artificially older ages cannot be excluded. Why don’t the authors consider two different tectonic and fluid flow events such as Late Triassic/early Jurassic (arsenopyrite and maybe sericite ages) and Early Cretaceous (calcite and fluorites)? In my opinion, calcite and fluorites could represent a later fluid flow events that may be associated with ore mineralisation or not. The authors should discuss this briefly for the revision of the paper.

Some very minor issues and questions:

Lines 129-130: Is this error of 0.005% (2s) estimated for both 147Sm/144Nd and 143Nd/144Nd ratios?

Line 139: Are you sure that there is a positive Eu anomaly in Fig. 5a? It looks like to me that the enrichment of MREE makes a peak there. However, negative Ce anomaly on Fig. 5b is clear.

Line 145: Did you use 0.005% error for both 147Sm/144Nd and 143Nd/144Nd ratios to calculate the age and the MSWD value of 0.82 on the Isoplot program?

Author Response

Reviewer #1
we are grateful to reviewer #1 for his/her effort reviewing our paper and his/her positive feedback. Here below we address the questions and suggestions raised by the reviewer #1.

Question-1
My major criticism on this paper is that the author ignored or considered the geochronolocical data of previous studies as unreliable (line 210 and lines 212-219). I have a serious concern regarding the disqualifying the sericite Ar-Ar and arsenopyrite Re-Os ages by Chen et al. (2009 and 2015).
Reply:
    Recently, many researchers (Tan et al., 2019; Su et al., 2018; Xie et al., 2018) including Chen et al. (2019) believe that the presence of both pre-ore and ore-stage zoned pyrite demonstrates multiple generations of pyrite and means that interpretations of the ages are problematic. In addition, the low Re and Os contents of the pyrite and arsenopyrite in these deposits result in large analytical uncertainty and means that the age data are unreliable. Su et al., 2018 think that the reported 40Ar/39Ar total gas age on sericite is not reliable, because it has a highly irregular age spectrum with no plateau, which may reflect incompletely reset detrital muscovite and mixtures with hydrothermal illite of the main stage.
Those above are just one of the views, the true ages remain controversial. So, in this paper, we combined with regional low-temperature thermochronology, crosscutting relationships of stratigraphy or intrusions, and previous dating results to constrain the age of the low-temperature metallogenic events in southwestern Guizhou.
1\\ Tan, Q., Xia, Y., Xie, Z., Wang, Z., Wei, D., Zhao, Y., Yan, J., Li, S., 2019. Two hydrothermal events at the Shuiyindong Carlin-Type gold deposit in Southwestern China: insight from Sm–Nd dating of fluorite and calcite. Minerals 9(4), 230.
2\\ Xie, Z.J., Xia, Y., Cline, J.S., Koenig, A., Wei, D.T., Tan, Q.P., Wang, Z.P., 2018. Are there Carlin-type gold deposits in China? a comparison of the Guizhou, China, deposits with Nevada, USA, deposits, In: Muntean J.L. (Eds.), Diversity of Carlin-Style Gold Deposits, Reviews in Economic Geology. Society of Economic Geologists, Inc, Colorado, USA, pp. 187-233.
3\\Su, W.C., Dong, W.D., Zhang, X.C., Shen, N.P., Hu, R.Z., Hofstra, A.H., Cheng, L.Z., Xia, Y., Yang, K.Y., 2018. Carlin-type gold deposits in the Dian-Qian-Gui “golden triangle” of Southwest China, In: Muntean J.L. (Eds.), Diversity of Carlin-Style Gold Deposits, Reviews in Economic Geology. Society of Economic Geologists, Inc, Colorado, USA, pp. 157-185.
4\\Chen, M.H., Bagas, L., Liao, X., Zhang, Z.Q., Li, Q.L., 2019. Hydrothermal apatite SIMS ThPb dating: Constraints on the timing of low-temperature hydrothermal Au deposits in Nibao, SW China. Lithos 324-325, 418-428.

Question-2
Why don’t the authors consider two different tectonic and fluid flow events such as Late Triassic/early Jurassic (arsenopyrite and maybe sericite ages) and Early Cretaceous (calcite and fluorites)? In my opinion, calcite and fluorites could represent a later fluid flow events that may be associated with ore mineralisation or not. The authors should discuss this briefly for the revision of the paper.
Reply:

    Tan et al.,(2019) reported two episodes of hydrothermal minerals of fluorite (200.1 ± 8.6 Ma) and calcite (150.2 ± 2.2 Ma) at the Shuiyindong gold deposit. Fission track and (U-Th)/He ages of detrital zircons collected from the Shuiyindong, Taipingdong, Yata, and Getang Au deposits in Southwestern Guizhou also show two thermal events (i.e., 192–216 Ma, 132–160 Ma) (Huang et al., 2019).
But, up to date, no geological evidences in ore (e.g., mineral assemblage and paragenetic sequence) support the idea of two different metallogenic events in Southwestern China.
1\\ Tan, Q., Xia, Y., Xie, Z., Wang, Z., Wei, D., Zhao, Y., Yan, J., Li, S., 2019. Two hydrothermal events at the Shuiyindong Carlin-Type gold deposit in Southwestern China: insight from Sm–Nd dating of fluorite and calcite. Minerals 9(4), 230.
2\\Huang, Y., Hu, R.Z., Bi, X.W., Fu, S.L., Peng, K.Q., Gao, W., Oyebamiji, A., Zhaanbaeva, A., 2019. Low-temperature thermochronology of the Carlin-type gold deposits in southwestern Guizhou, China: Implications for mineralization age and geological thermal events. Ore Geology Reviews 115, 103178.

Question-3
Lines 129-130: Is this error of 0.005% (2s) estimated for both 147Sm/144Nd and 143Nd/144Nd ratios?
Line 145: Did you use 0.005% error for both 147Sm/144Nd and 143Nd/144Nd ratios to calculate the age and the MSWD value of 0.82 on the Isoplot program?
Reply:
We used 0.005% error for both 147Sm/144Nd and 143Nd/144Nd ratios.

Question-4
Line 139: Are you sure that there is a positive Eu anomaly in Fig. 5a? It looks like to me that the enrichment of MREE makes a peak there. However, negative Ce anomaly on Fig. 5b is clear.
Reply:
The calcites from the Zimidang Au deposit are characterized by positive Eu anomalies (δEu = 1.24–1.38).

Reviewer 2 Report

A review of a manuscript titled "Sm–Nd isochron age constraint of Au and Sb mineralization in southwestern Guizhou Province, China" by Zepeng Wang, Qinping Tan, and coauthors.

The manuscript presents new fluorite and calcite Sm-Nd dating results of a Carlin-type Au and Sb deposit, SW China.

The introduction paragraph is focused on previous research in the deposit, highlighting the ambiguous dating results of the previous attempts to date the mineralization, as well as the reasoning in the selection of methods that could resolve it. This is followed by an in-depth description of the geological setting of the study area. The Samples and analytical methods section could benefit from a more detailed description of the dated gangue minerals and the ore, as well as BSE images of the mineral relation, gangue mineral morphology, and CL images. Results are well presented and consistent. The main shortfall of the manuscript is the arguing of the meaning of the isochron ages. The observed trends in La/Ho vs Y/Ho diagram might be a result of the re-equilibration process, or changes in chemistry during a single growth episode, as demonstrated by Nadoll et al (2018), stressing the need for a detailed petrological description of the dated samples. This in turn may impact the isochron slope, if the former is the case. Also, the differences in the chemical composition of calcites and fluorites need to be discussed. What is the nature of the spread and how it originated during a single geological event?

I added a copy of the manuscript with some comments on minor issues and misspellings that needs to be corrected.

Overall, the manuscript is well written, presents interesting results, and should be interesting for the international community.

Comments for author File: Comments.pdf

Author Response

Reviewer #2
We thank the reviewer #2 for his/her effort to review our manuscript. The reviewer gives an accurate summary of our work and brings forward constructive questions. We have addressed them below.

Question-1
The main shortfall of the manuscript is the arguing of the meaning of the isochron ages. The observed trends in La/Ho vs Y/Ho diagram might be a result of the re-equilibration process, or changes in chemistry during a single growth episode, as demonstrated by Nadoll et al (2018), stressing the need for a detailed petrological description of the dated samples. This in turn may impact the isochron slope, if the former is the case. Also, the differences in the chemical composition of calcites and fluorites need to be discussed. What is the nature of the spread and how it originated during a single geological event?
Reply:
    The calcites and fluorites show consistent Y/Ho ratios indicating that the calcite and fluorite were deposited form same fluid, respectively. Therefore, the mixing of two end members having quite different 143Nd/144Nd and 147Sm/144Nd ratios can be rejected. The differences between calcites and fluorites in La/Ho vs Y/Ho diagram might be the result of different partition coefficients between fluids and minerals (calcite and fluorite).

Question-2、line 48-50.
I do not see a controversy in the dating results of previous researchers, rather than an ambiguity that has a logical explanation that the authors provide in the discussion section.
Reply:
    Many researchers have reported ages for Au and Sb deposits in southwestern Guizhou, that is, 235–195 Ma; 135–150 Ma; 83–106 Ma. Recently, many researchers consider that the ages 235–195 Ma and 83–106 Ma are not reliable. Those are just one of the views, the true ages remain controversial.
In this paper, we combined with regional low-temperature thermochronology, crosscutting relationships of stratigraphy or intrusions, and previous dating results to constrain the age of the low-temperature metallogenic events in southwestern Guizhou.

Question-3\line 81.
An odd classification of deposit size. If it is based on some criteria or other publications, it needs a reference and maybe the amounts of ore present in the "super-large" and "large" deposits.
Reply:
The classifications of deposit size are based on other publications, we have added references in the revised version of the manuscript.
1\\Zheng, L.L., Yang, R.D., Gao, J.B., Chen, J., Liu, J.Z., Li, D.P., 2019. Quartz Rb-Sr isochron ages of two type orebodies from the Nibao Carlin-Type gold deposit, Guizhou, China. Minerals 9(7), 1-15.

Question-4\line106.
"30 ten thousand tons" needs clarification
Reply:
We have clarified. The sentence has rewritten in the revised version of the manuscript. It is based on other reference (Chen et al., 2018). “the Qinlong-Dachang Sb deposit contains a proven and an inferred Sb resource of ca. 300,000 tonnes.”
1\\Chen, J., Yang, R.-D., Du, L.-J., Zheng, L.-L., Gao, J.-B., Lai, C.-K., Wei, H.-R., Yuan, M.-G., 2018. Mineralogy, geochemistry and fluid inclusions of the Qinglong Sb-(Au) deposit, Youjiang basin (Guizhou, SW China). Ore Geology Reviews 92, 1-18.

Question-5\line 113.
CL images of some of the analyzed fluorites would improve the quality of the paper, as well as BSE images of the relation between ore and analyzed gangue minerals (if possible). CL images of typical fluortes and calcites would be especially useful because of the remobilization trend observed in the La/Ho vs Y/Ho plot.
Reply:
Thanks for the reviewer’s constructive suggestions. The mineral crystal of fluorite and calcite are coarse in Sb and Au deposits. So, we ignored the microanalysis. In the future, new microanalysis (CL and BSE images) will be carried out to discuss the microstructure and paragenetic sequence.

Question-6/line116.
The sample numbers in the tables 1 and 2 suggest that more samples were analyzed in the study. If so, even if they might not be suitable for constructing the isochron, they might highlight the challenge of dating a low-temperature metalogenic event in the region.
Reply:
Not all samples are minerals, some samples are rocks.

Question-7/
line 154. Which chondrite values are used for the normalization? A reference is needed.
Reply:
We have added references in the manuscript.