The Solubility of Antimony (Sb) in Liquid Hydrocarbons and Its Implication for the Ore-Forming Process of Orogenic Antimony-Gold Deposits in Southern Tibet

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript is well-written overall. This study aims to detect the transportability of antimony by liquid hydrocarbons in orogenic antimony ores. The authors designed experiments to dissolve antimony in n-odecanethiol and n-dodecane compounds at various temperatures and durations. I have no comment on the design of these experiments.  But the biggest question is to clarify why you chose orogenic antimony deposits in southern Tibet as your research background. It seems that organic matter might have played roles in the formation of other types of Sb deposits.

1. Previous studies have found that liquid hydrocarbons play a crucial role in the migration and enrichment processes of mineralizing elements, but why you still choose the orogenic type antimony gold deposit in southern Tibet as the research objective. Please clarify the reasons.
2. Lack of introduction to research progress on the mineralization process of gold deposits in southern Tibet, such as their respective viewpoints, focal points of debate, innovation in your own work and so on.
3. It seems like that your experiment cannot directly prove that liquid hydrocarbons have a promoting effect on the migration and enrichment of gold in ore forming fluids. Please explain more about the research implications of your work.
4. Why is your experimental temperature from 100 to 200 ° C, but Migdisov et al. examined the solubility of gold in a variety of natural crude oils at temperatures ranging from 100 to 300°C.

Author Response

Response to Reviewer #1:

Thank you for evaluating our manuscript. Your comments have greatly improved the quality of our research paper. We value your insights and suggestions, which have strengthened our study. Below, you will find our detailed responses to your comments. Your comments are in black font. Our responses are in blue and changes are in red. We appreciate your time and attention to our manuscript.

 

Reviewer #1: This manuscript is well-written overall. This study aims to detect the transportability of antimony by liquid hydrocarbons in orogenic antimony ores. The authors designed experiments to dissolve antimony in n-dodecanethiol and n-dodecane compounds at various temperatures and durations. I have no comment on the design of these experiments. But the biggest question is to clarify why you chose orogenic antimony deposits in southern Tibet as your research background. It seems that organic matter might have played roles in the formation of other types of Sb deposits.

 

Reply:

We feel great thanks for your professional review work on our article. As you are concerned, there are several problems that need to be addressed. Please find our detailed response to your comments below:

 

1. Previous studies have found that liquid hydrocarbons play a crucial role in the migration and enrichment processes of mineralizing elements, but why you still choose the orogenic type antimony gold deposit in southern Tibet as the research objective. Please clarify the reasons.

 

Reply:

We sincerely appreciate your question.

As you mentioned, previous studies have found a strong association between liquid hydrocarbons and many antimony-only deposits and antimony-gold deposits. Researchers and scholars have found organic inclusions containing liquid hydrocarbons in the antimony deposits and antimony-gold deposits they have studied and have found that antimony mineralization is closely related to oil and gas formations, but this is mainly concentrated in areas such as South China and the Qinling Mountains (Ye et al., 1997; Zhang et al., 1998; Yin et al., 2000; Yan et al., 2004; Zhang et al., 2018; Wang, 2019). Recently, some scholars demonstrated experimentally that liquid hydrocarbons have the ability to mobilize metals from source rocks and transport them in concentrations sufficient to contribute to ore-forming processes (Migdisov et al., 2017; Crede et al., 2019a, 2019b; Sanz-Robinson et al., 2019, 2020; Kubrakova et al., 2022). They used an improved experimental method with less error than before, the crude oils were ashed using a combination of thermal combustion and chemical oxidation, but their studies focused mainly on metals such as Zn, Au, and Pd, and lacked studies on Sb. These give us inspiration.

Orogenic antimony-gold deposits contribute significantly to the global antimony resource base. Situated within the Tethys Himalayan region, the antimony polymetallic mineralization belt in southern Tibet is a typical metallogenic area for antimony-only deposits and antimony-gold deposits. In the sediment-hosted orogenic antimony-gold deposits, the ore-forming ores and surrounding rocks generally contain organic matter (Zheng et al., 2004; Wang et al., 2020; Ding et al., 2023), and more importantly, there are liquid-phase organic inclusions in the minerals of some deposits (Zhai et al., 2018). Therefore, this makes it necessary to choose the orogenic antimony-gold deposits in southern Tibet as a target to study whether liquid hydrocarbons can be involved in transporting antimony.

 

2. Lack of introduction to research progress on the mineralization process of gold deposits in southern Tibet, such as their respective viewpoints, focal points of debate, innovation in your own work and so on.

 

Reply:

Thank you for kindly bringing up this critical suggestion. We greatly appreciate your input. We have taken your suggestion into consideration and have made additions related to it in the manuscript.

 

Revise:

We have further elaborated on these in the introduction section. You can find that at lines 95-110:

The origin of the ore-forming fluids in orogenic antimony-gold deposits continues to be a subject of debate. For instance, Yang et al [2] determined that the ore-forming fluids in the Shalagang antimony deposit are dominated by circulating atmospheric water, and the ore-forming fluids in the Mazala antimony-gold deposit are a mixture of magmatic and atmospheric water. Nie et al [27] concluded that the mineralization of the Mazala antimony-gold deposit was related to the tectonic-magmatic activity of the late Yanshan-early Himalayan period, with magmatic-hydrothermal fluids extracting metallic elements from the surrounding rocks, leading to mineralization. Zhai et al [14] reported that the Shalagang antimony deposit and the Mazala antimony-gold deposit include a mixture of constructed and metamorphic water in the ore-forming fluids. Nonetheless, the current research on the ore-forming fluids of orogenic antimony-gold deposits primarily emphasizes the investigation of their inorganic components. The liquid hydrocarbon components found in the fluid inclusions of the mineralized ores associated with the Shalagang antimony deposit and the Mazala antimony-gold deposit suggest that liquid hydrocarbons may be able to play an active role in the antimony mineralization process. Therefore, we start from the organic components in the ore-forming fluids to investigate whether the liquid hydrocarbons can participate in transporting antimony to mineralization.

 

3. It seems like that your experiment cannot directly prove that liquid hydrocarbons have a promoting effect on the migration and enrichment of gold in ore forming fluids. Please explain more about the research implications of your work.

 

Reply:

We sincerely appreciate your question. This helped us to revisit our manuscript and think more deeply about it.

As you say, our experiments did not perform a solubility study of gold and only reported the solubility of antimony in n-dodecane and n-dodecanethiol. For objective reasons, we were unable to extract liquid hydrocarbons from the rock samples we took. But since alkanes and organothiol compounds are ubiquitous and abundant components in natural oils, we can conclude that liquid hydrocarbons have great potential for transporting antimony mineralization. Further understanding the capacity of organic-based fluids to mobilize metals is not only important for mineral exploration, it is key information for assessing the cycling of metals in the Earth’s crust. In addition, in combination with previous research on gold and liquid hydrocarbons, such as those conducted by Migdisov et al. (2017) to examine the solubility of gold in various crude oils at temperatures of 100-300 °C and Crede et al. (2018) performed an in-situ XAS experiment at 25-250 °C to determine the morphology and structural properties of gold complexes in aqueous and oil-based fluids (S-free n-dodecane, S-bearing 1-dodecanethiol), we can show that there is a close relationship between liquid hydrocarbons and antimony and gold.

Antimony and gold deposits are widely distributed throughout the world, mainly in orogenic belts, and the two elements, gold, and antimony, often have symbiotic differentiation phenomena in space (Mo et al., 2013). Combined with the results of previous experimental studies, we can conclude that liquid hydrocarbons can play an important role in both transporting antimony and gold mineralization. This means that we can provide a possible explanation mechanism for the co-occurrence of antimony and gold ores in southern Tibet from the ore-forming fluid. Liquid hydrocarbons act as ore-forming fluids to extract antimony and gold mineralizing elements from the surrounding rocks, after which they are subsequently influenced by changes in temperature and pressure, leading to their migration, precipitation in favorable tectonic sites, and the eventual formation of mineralized bodies. Moreover, the ore-forming fluids of orogenic antimony-gold deposits are still mainly focused on the inorganic components therein, and our study starts from the organic components in the ore-forming fluids, which provides a new perspective for the study of antimony transport mechanisms in antimony-gold deposits in southern Tibet.

 

4. Why is your experimental temperature from 100 to 200 °C, but Migdisov et al. examined the solubility of gold in a variety of natural crude oils at temperatures ranging from 100 to 300 °C.

 

Reply:

Thank you for your inquiry. We greatly appreciate your insightful question.

We chose n-dodecane and n-dodecanethiol, representative components of liquid hydrocarbons, for antimony solubility experiments, whereas natural crude oil was used by Migdisov et al. The oil window extends from 80 to 160 °C (Peters et al., 2004). Pyrolysis experiments (Price and Wenger, 1992) and evidence from liquid hydrocarbons entrapped in black smokers (Peter and Scott, 1988), however, both suggest that oil remains stable to temperatures above 300 °C for protracted periods of time. So Migdisov et al. could choose higher temperatures for their experiments.

Antimony is a typical low-temperature mineralizing element. Combining the temperature ranges identified through micro thermometry of fluid inclusions in orogenic antimony deposits and considering the physical and chemical properties of n-dodecane and n-dodecanethiol, the boiling point of n-dodecane is 215-217 °C. Therefore, we chose an experimental temperature of 100-200 °C to avoid the risk of thermal degradation of n-dodecane and n-dodecanethiol.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The submitted manuscript focuses on an experimental study of the solubility of antimony in liquid hydrocarbons in a temperature range of 100°-200°C typical of hydrothermal fluids linked to the formation of orogenic antimony or antimony-gold deposits such as those of southern Tibet. The experimentation carried out highlights how hydrocarbons rich in thiols have a high capacity to solubilize antimony, and that thiols must be considered as ligands of primary importance in the mobilization of antimony in orogenic hydrothermal cells. The study provides some new and interesting elements on an experimental basis to the discussion on the formation of orogenic class of sediment-hosted mineral deposits, stimulating further investigations on this topic. I found the experimental part well-conceived and the discussion and conclusions well-supported by data. The manuscript is well-organized, and writing is generally fine, but there are some typos (also in the figures) and, above all, the last paragraphs must be checked with attention for English, with some sentence that should be rewritten. Some further references are required in the Discussion chapter. See the attached file for my detailed observations and suggestions.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Author Response

Response to Reviewer #2:

Thank you for evaluating our manuscript. Your comments have greatly improved the quality of our research paper. We value your insights and suggestions, which have strengthened our study. Below, you will find our detailed responses to your comments. Your comments are in black font. Our responses are in blue and changes are in red. We appreciate your time and attention to our manuscript.

 

Reviewer #2: The submitted manuscript focuses on an experimental study of the solubility of antimony in liquid hydrocarbons in a temperature range of 100-200 °C typical of hydrothermal fluids linked to the formation of orogenic antimony or antimony-gold deposits such as those of southern Tibet. The experimentation carried out highlights how hydrocarbons rich in thiols have a high capacity to solubilize antimony, and that thiols must be considered as ligands of primary importance in the mobilization of antimony in orogenic hydrothermal cells. The study provides some new and interesting elements on an experimental basis to the discussion on the formation of orogenic class of sediment-hosted mineral deposits, stimulating further investigations on this topic. I found the experimental part well-conceived and the discussion and conclusions well-supported by data. The manuscript is well-organized, and writing is generally fine, but there are some typos (also in the figures) and, above all, the last paragraphs must be checked with attention for English, with some sentence that should be rewritten. Some further references are required in the Discussion chapter. See the attached file for my detailed observations and suggestions.

 

Reply:

We appreciate your summary of the manuscript and encouraging comment. We also sincerely thank you for pointing out the critical problem and providing valuable feedback. According to your nice suggestions, we have made corrections to our previous draft, the detailed corrections are listed below.

 

1. The manuscript is well-organized, and writing is generally fine, but there are some typos (also in the figures).

 

Reply:

Thanks for your careful checks. We are very sorry for our carelessness. Based on your comments, we have checked our manuscript and made the corrections.

 

Revise:

Line 51: as well as alkali-rich intermediate-basic intrusive rock masses

Line 54: Figure 1. Metallogenic map of antimony ore belt in south Tibet (modified after [4,5]).

Line 60: 12. Gold-antimony deposit;

Line 68: and Mississippi Valley-type Pb-Zn deposits [18,19,20].

Lines 115-118: These two organic compounds are suitable for this experiment because of their physical characteristics, including their melting and boiling temperatures, as well as their stability under various redox and temperature conditions [34].

Reference:

34. Crede, L.S.; Rempel, K.U.; Hu, S.Y.; Evans, K.A. An experimental method for gold partitioning between two immiscible fluids: Brine and n-dodecane. Chemical Geology. 2018, 501, 35-50.

Line 123: Additionally, we performed an X-ray photoelectron spectroscopy (XPS) analysis to investigate the surface composition of the antimony lumps after the reaction.

Line 248: In a study by Kolpakova [37], the solubility of stibnite ores in H₂S-H₂O solution was examined across temperatures ranging from 100 to 300 °C.

The new Figure 3 and Figure 5 are as follows:

 

2. Two experimental groups were established: one group received a mixture of 0.25 ml n-dodecane and 0.25 ml n-dodecanethiol in the quartz test tubes, while the other group received only 0.25 ml of n-dodecane. Why the mixture? Please explain.

 

Reply:

Thank you for your inquiry. We greatly appreciate your insightful question.

The experiments conducted by previous scholars on the solubility of metals used crude oil (Migdisov et al., 2017; Sanz-Robinson et al., 2019, 2020; Kubrakova et al., 2022). And metal solubility studies were performed with 0.5 ml of crude oil; we referred to their experimental methodology. However, due to objective reasons we were not able to extract liquid hydrocarbons from the rock samples taken, so we selected alkanes and organothiol compounds, which are ubiquitous and abundant components in natural oils, for our experiments. We chose to mix n-dodecane and n-dodecanethiol in order to ensure, as far as possible, their representativeness as liquid hydrocarbons. The choice of n-dodecane alone for the experiments was to verify which of the n-dodecane and n-dodecanethiol groups would be effective in transporting antimony.

 

3. Revise the English and English geological terminology in the 4.2 paragraph

 

Reply:

We sincerely appreciate your excellent suggestion, which we have wholeheartedly embraced. We have tried our best to polish the language in the revised manuscript.

 

Revise:

Lines 300-301: Throughout this process, various geological features emerged in southern Tibet, including fold tectonics, reverse faults, metamorphic core complexes, and near east-west and near north-south fault systems (Figure 1).

Lines 303-308: Numerous antimony-only deposits and antimony-gold deposits are present within the Southern Tibetan Plateau, of which the Shalagang and Mazala deposits represent examples of antimony-only and antimony-gold types, respectively, both hosted in sedimentary rocks. The Triassic, Jurassic, and Cretaceous sedimentary units, where these two deposits are located, are mostly represented by siltstone, sandstone, quartz sandstone, carbonaceous slate, and marl. These units contain a large amount of sedimentary organic matter and often constitute the ore-forming wall rocks of the deposits.

Line 328: Regional metamorphic tectonic events impact the organic-rich rocks around the sediment-hosted orogenic antimony-gold deposits, leading to the liberation of organic components, including liquid hydrocarbons.

Line 340: The solubility characteristics of stibnite indicate that solutions of different properties can dissolve and transport high concentrations of antimony.

Lines 365-366: Consequently, in sediment-hosted antimony deposits, organic fluids, rather than aqueous fluids, primarily facilitate antimony transfer.

Line 375: An example of this is the synthesis of noble metal nanoparticles (NMNPs) to explain the enrichment and transportation of gold and platinum group elements (PGE) within naturally occurring organic (hydrocarbon) systems.

Line 377: Furthermore, it has been found that liquid hydrocarbons are more efficient in transporting precious metals when they are in the form of nanoparticles rather than molecules.

Line 379: Hence, a detailed investigation into the specific form of liquid hydrocarbons involved in the transportation of antimony during mineralization is required.

Line 383: The solubility of antimony in organic reagents with a high thiol concentration surpasses its solubility in fluids previously identified as antimony-rich mineralized.

Lines 384-385: The thermochemical sulfate reaction that occurs in the surrounding rocks of sediment-hosted orogenic antimony-gold deposits in a specific temperature range produces high concentrations of hydrogen sulfide.

 

4. Some further references are required in the Discussion chapter.

 

Reply:

We sincerely appreciate your valuable comments. As you suggested, we have added more references in the revised manuscript.

 

Revise:

Lines 344-345: Extensive research data on the antimony concentration in modern geothermal fluids has shown significant variability, with the majority having antimony contents around 0.1 ppm [56,57]. In contrast, fluids in New Zealand's geothermal areas exhibit antimony contents as high as 84.42-238.25 ppm [58], demonstrating the presence of fluids with elevated antimony levels in nature.

Reference：

56. Hu, X.W. Characteristics and discussion of stibnite solubility in different solutions. Mineral Resour. 1994, 201, 33-42.
57. Zhang, T.Y.; Li, C.Y,; Sun, S.J.; Hao, X.L. Geochemical characteristics of antimony and genesis of antimony deposits in South China. Acta Petrologica Sinica. 2020, 36, 44-54 (in Chinese with English abstract).
58. Wood, S.A.; Crerar, D.A.; Borcsik, M.P. Solubility of the assemblage pyrite-pyrrhotite-magnetite-sphalerite-galena-gold-stibnite-bismuthinite-argen-tite-molybdenite in H₂O-NaCl-CO₂ solutions from 200-350 ℃. Economic Geology. 1987, 82, 1864-1887.

Author Response File: Author Response.docx