Magmatic initial and saturated water thresholds determine copper endowments: Insights from apatite F-Cl-OH compositions

Abstract

Magmatic volatiles (H₂O, F, Cl), especially water, are critical in the formation of porphyry copper deposit, for its significance as a carrier for metals. However, accurately quantifying the water contents of deep ore-forming magma remain a challenge. Here, we used apatite and forward modelling methods to reconstruct magmatic water evolution histories, with special concern on the control of initial magmatic H₂O contents and water saturation threshold to porphyry mineralization. Samples investigated include granitoid rocks and apatite from highly copper-mineralized and barren localities. Generally, our research suggested that both ore-related and ore-barren magma systems are hydrous, the modeled magmatic water contents vary significantly among systems whether mineralized or not, and the major difference lies in the threshold of water saturation (6.0 wt.% for barren, and up to 10.0 wt.% for highly mineralized). Combined with whole rock geochemistry data (high K₂O and Sr/Y contents) and modeling result (high modeled water thresholds), we think the ore-related magmas are stored at deeper depth with higher water solubility. In conclusion, we propose that the level of magmatic water saturation plays a crucial role in the formation of porphyry copper systems. Fertile magma has higher water solubility to which deeper storage depth is a critical contributing factor, and can get significantly water enriched upon saturation.

Keywords

Porphyry copper deposit / Magmatic water threshold / Apatite / Gangdese belt

CRediT authorship contribution statement

Yingcai Sun: Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Data curation, Conceptualization. Qiushi Zhou: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. Rui Wang: Writing – review & editing, Validation, Supervision, Methodology, Conceptualization. Madeleine C.S. Humphreys: Writing – review & editing, Validation, Software, Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This study is financially supported by the National Key R&D Program of China (2022YFF0800902), National Natural Science Foundation of China (42225204, 42121002), Fundamental Research Funds for the Central Universities (grant numbers 2652023001; 2-9-2023-202), and the 111 project (B18048). M.C.S.H. was supported by funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 864923).

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