The Formation Age and Geological Setting of the Huoqiu Group in the Southern Margin of North China Craton: Implication for BIF-Type Iron Prospecting Potentiality

Comments 1: The authors note peaks at 3.00 Ga, 2.70 Ga, and 2.55 Ga, which align with known Archean events. However, the discussion about the youngest detrital zircon ages in the Wuji and Zhouji Formations seems crucial. The inconsistency where the upper formation has older zircons than the lower is explained by erosion patterns, but is this explanation sufficient? Maybe suggest expanding on alternative hypotheses.

Response 1: Many thanks for this notation. Though there might be several alternative explanations, ours is the best and the most popular. Despite of this, the other possibilities is now mentioned in the revision.

Comments 2: In the discussion, the existence of >3.60 Ga crustal components is supported by references, which is good. However, when comparing with other regions, are there enough data points? The section on crustal growth ties the zircon ages to global events, which is a strength.

Response 2: In addition to the >3.6 Ga rocks in the Anshan-Benxi region, many areas in NCC have been reported to contain >3.60 Ga components. We here mentioned them as a cooperative support of the >3.6 Ga crustal components in the Huoqiu terrain. It may go too far from the focus (Huoqiu terrain) if we link the zircon ages to global events.

Because Huoqiu terrain is our focus, we add Th/U ratios, concordance values and cathodoluminescence (CL) image (Figure 4c, page 8, line 167) for specific detrital zircon to support the reliability of zircon U-Pb ages. Therefore, we think they can be regarded as new records for >3.60 Ga terrestrial crust in the Huoqiu region.

Comments 3: The classification of the BIF as Superior-type based on geochemical and isotopic data seems well-argued, but are all the necessary criteria for Superior-type BIFs addressed? They mention LREE depletion, Eu anomalies, and δ³⁰Si values, which are relevant.

Response 3: Thank you for correction and suggestion. The classification of the BIF as Superior-type is initially based on ore features, lithostratigraphic context and depositional environment (miogeosynclinal facies, i.e., shallow marine setting along passive continental margin). At present, element and isotope geochemical features are more popular criteria.  Because the Huoqiu BIF was ever considered as Algoma-type BIF, rather than Superior-type, we here address all the necessary criteria for Superior-type BIF, including LREE depletion, Eu anomalies.

Comments 4: The statement that the Zhouji Formation’s youngest zircon (2500 Ma) is older than the Wuji Formation’s (2383 Ma) needs deeper discussion. Consider alternative explanations (e.g., sedimentary recycling vs. source heterogeneity).

Response 4: Yes, done. Please also see Response 1.

Comments 5: >3.6 Ga Crustal Components: Compare the Huoqiu Terrane’s ancient zircons with other NCC regions (e.g., Anshan–Benxi) to contextualise their significance.

Response 5: Yes, done. Many Thanks.

Comments 6: Weak ~2.55 Ga Signal: Discuss why the Huoqiu Terrane lacks the strong ~2.55 Ga magmatism seen elsewhere in the NCC. Is this due to sampling bias or a regional tectonic quiescence?

Response 6: Many thanks for this notation. We consider that the absence of a ~2.55 Ga peak of detrital zircon ages is due to the lack of exposure of ~2.55 Ga igneous rocks in source regions, and we have added it in lines 355–358

Other comments: Address potential biases in detrital zircon datasets such as sampling strategy, zircon fertility of source rocks. Specify how many zircon grains were analyzed per sample and whether analyses were conducted via LA-ICP-MS or SIMS.

Response: Thank you very much. We have added Table 1 to show the samples and analytical methods.

Other comments: Please explain why this range was chosen over others?

Response: Thank you very much. We have added reference [40] for this range in page 5, line 149.

Comments 1: This study provides a critical reappraisal of the formation age, tectonic evolution, and metallogenic implications of the Huoqiu Group, integrating zircon U-Pb geochronology and Hf isotopic data to redefine the stratigraphic framework. The main problem with the paper lies in the oversimplified data organization and analysis, which fails to adequately integrate the data with the discussion section. This results in unconvincing arguments regarding the reclassification of the Huoqiu Group, as well as insufficiently compelling discussions about the depositional age and sedimentary environment of the BIF.

Response 1: Sorry, we do not fully agree with this comment. The lithologic or lithostratigraphic subdivision of the “Huoqiu Group” is unconvincing due to shortage of exposure. In this contribution we only assign the Huayuan Formation into “granitic gneiss”, on the basis of lithologic observation of drill core samples. Anyway, we re-wrote section 4.2 to further the necessity of re-division of the Huoqiu Group, particularly, lithologic difference between Huayuan Gneiss Complex and the Huoqiu Group.

Comments 2: Most studies determine the maximum deposition age of BIF by the youngest detrital zircon age group in the BIF-related sedimentary sequence. However, in this paper, the deposition age of the Huoqiu BIF was determined by the youngest single-grain zircon age. This age comes from LY105.461 in the Wuji Unit, and the original author did not provide more explanations for this age. This age may be part of a zircon age group together with other ages (Liu et al., 2016). It is more appropriate to use the youngest zircon age group to determine the maximum deposition age of the Huoqiu BIF. Therefore, when using this age in this paper, it is necessary to carefully consider whether it can represent the formation age of the Huoqiu BIF. I tend to argue that this BIF was deposited between 2.7 and 2.5 Ga. If you do not provide additional evidence to support a Paleoproterozoic age for this BIF, I have to reject your paper.

Response 2: From LY105.461, four detrital zircons yield the youngest zircon age group around 2.35 Ga, and we have added table 2 to introduce U-Pb data for these detrital zircons (Page 9, line 184–185). Also, we have added the cathodoluminescence (CL) image of spot 15 from sample LY105.461 (Figure 4c, page 8, line 167), which shows well-developed oscillatory zoning, and the Th/U ratio of is 0.82, indicating a magmatic origin. Therefore, we think these youngest zircon age group can be used to determine the maximum depositional age. Notably, the youngest zircon age of these 2.35 Ga group is 2343 ± 10 Ma (spot 17 from LY105.461), and it can further constrain the maximum depositional age of the Wuji Formation is 2343 Ma. The original author did not provide more explanations for these ages, we think their focus is to use U-Pb ages of detrital zircons to discuss Meso- to Neoarchean episodic crustal growth in the Huoqiu region, rather than the depositional age for the Huoqiu Group. To some extent, the youngest single age is similar to the value of the youngest age group (four zircon grains).  Also, we have reassessed ortho-metamorphic rocks in the Huoqiu Terrane are more likely para-metamorphic rocks according to wide ²⁰⁷Pb/²⁰⁶Pb age ranges and zircon CL images and descriptions (Page 9–10, line 200–213). As a result, the available evidence does not support the interpretation that the Wuji and Zhouji formations were deposited in the Neoarchean.

Comments 3: The manuscript requires professional language editing to eliminate structural redundancies and resolve lexical ambiguities. It is recommended to implement rigorous proofreading to enhance the clarity and precision of academic expression.

Response 3: Thank you for correction and suggestion. Have done

Comments 1: The Huoqiu is not shown in Fig. 1, please add it

Response 1: Thank you for pointing this out. Have done as Figure 1 in page 2, line 52–59.

Comments 2: It is better to add a strata column here, and add your sample's location in the strata column. The figure can also indicate the stratigraphic attribution believed by previous researchers and the stratigraphic attribution believed by this study. In this way, the results and significance of this study are clear at a glance.

Response 2: Agree. We have have added a strata column as Figure 3 in page 5, line 123–125.

Comments 3: The appearance of 2343 Ma here is too abrupt and needs to be introduced earlier.

Response 3: Thank you for pointing this out. We have changed it as “In this study, the youngest zircon age from the 2.35 Ga group is 2343 ± 10 Ma (spot 17 from LY105.461), indicating that the Wuji Formation was deposited no earlier than 2343 Ma.” (Page 9, line 181–183), and we have added U-Pb data of the four youngest detrital zircons in sample LY105.461 as table 2 in page 9, line 184–185

Comments 4: Why the two-stage model ages, up to 3.30 Ga (Fig. 6) can reflect a small contribution from older continental crust for 2.9 Ga crustal growth？I added the following sentence, which makes the discussion more logical.

Response 4: Thank you for this great suggestion and modification. We have accepted your revision (page 13, line 325–327).

Other comments: Other comments are provided in the annotated pdf file.

There are 111 comments in the Pdf file entitled “minerals-3667424-peer-review-v1-annotated by Dr. Liu”. Please revise them one by one.

Response: Thank you very much. We have revised following the annotations one-by-one.

Comments 1: According The primary objective of this paper is to establish a more precise age constraint for the Huoqiu iron deposit. Nevertheless, various methodologies have been employed by previous researchers to determine the age of this iron deposit. In the introduction section, it is imperative to address potential issues associated with the ages reported by prior studies. In comparison to past endeavors, authors should highlight the advancements and refinements incorporated in your approach.

Response 1: Thank you for pointing this out. We have address two potential issues ([1] volcanic interlayers in BIF orebodies; [2] the youngest minimum detrital zircon age) associated with depositional ages reported by prior studies and reassessed their results in the introduction section (Page 3, line 87–103). Systematically collecting and reassessing the detrital U-Pb age data of the Huoqiu Group can minimize sampling bias and avoid the neglect of key zircon ages. Actually, Liu et al. (2016) reported that detrital zircons in the para-gneisses from the Huoqiu Group yield ²⁰⁷Pb/²⁰⁶Pb ages of 2343–3997 Ma. However, they use these detrital zircon ages to discuss episodic crustal growth in the Huoqiu region, and ignore these key zircon ages as well as their importance for depositional age.

Comments 2: Resolving the debate regarding the depositional age of the Huoqiu BIF (2.35 Ga or 2.5-2.7 Ga) is the key point of this paper. Notably, the age assigned to the Huoqiu iron deposit in this paper relies on previous data, However, it is generally believed that the age of the Huoqiu BIF still belongs to the Neoarchean era. Currently, the evidence available to the authors is insufficient to support the conclusions of this paper. Consequently, The authors must provide more convincing evidence and conduct in-depth discussions regarding the age of the Huoqiu BIF.

Response 2: Thank you for pointing this out.

In page 10, line 223–236, we have reassessed the formation age of the Huoqiu Group from previous studies by volcanic interlayers dating. These samples in BIF orebodies are more likely para-metamorphic rocks according to wide ²⁰⁷Pb/²⁰⁶Pb age ranges and zircon CL images and descriptions. As a result, there are no the available evidences to support the interpretation that the Wuji and Zhouji formations were deposited in the Neoarchean.

Based on the youngest detrital zircon ages, metamorphic zircon ages and intrusive rocks ages, previous studies constrained the formation age to 2.75–1.84 Ga (Wan et al., 2010), 2.70–1.80Ga (Yang et al., 2014), 2.54–1.80 Ga (Hou et al., 2017). These age ranges are not contradictory with our findings, and we agree with their approaches in constraining depositional age of sediments. We just further constrain the age range based on compiling data. After collecting detrital U-Pb zircon ages, we have found four detrital zircons yield the youngest zircon age group around 2.35 Ga with concordance of 95–108% (Liu et al., 2016), and have added U-Pb data (Table 2), CL image, and Th/U ratio (0.27–0.82) to suggest that this youngest zircon age group is sufficient to determine the maximum depositional age. The original author did not provide more explanations for these ages, we think their focus is to use U-Pb ages of detrital zircons to discuss Meso- to Neoarchean episodic crustal growth in the Huoqiu region, rather than the depositional age for the Huoqiu Group. In Liu and Yang (2015), the same author also reported the youngest age populations (2288 Ma and 2280 Ma for ZYF1 and ZYF9) with apparently magmatic feature. Unfortunately, these ages have poor concordance and show significantly Pb loss, so the author didn’t use these ages. Therefore, we argue that the 2.35 Ga zircon population is reliable and our findings is consistent with the original authors’ ideas. We have mentioned this in page 9, line 183–187.