Contributions of trans-magmatic fluid in the formation of porphyry copper deposits: A case study from the Baoshan deposit, South China

Release time:2022-03-22

Impact Factor:2.3

DOI number:10.1016/j.chemer.2022.125881

Journal:Geochemistry

Key Words:The classical mechanism “source-transport-storage” of the formation of porphyry copper deposit has been advanced in recent studies, as the “source” is not the main factor for the mineralization in some Cu deposits, and the metallogeny may be affected by other variables factors during the magma-fluid transportation or storage. We recommend the essential role of trans-magmatic fluid in the ore-forming process, this fluid is released from the melting of the sedimentary overlying the subducted plate with high water and volatiles concentrations and high oxygen fugacity. The Baoshan granodioritic cryptoexplosion breccia representing the influence of hydrothermal events as well as the unaltered Baoshan granodiorite porphyry are conducted by LA-MC-ICP-MS analysis, to identify the contribution of trans-magmatic fluid. In case of the εNd(t) of whole rock do not increase with the MgO increasing and SiO2 decreasing, the large variations of zircons εHf(t) values in Baoshan granodiorite porphyry (BGP, −14.24 to −6.38) and Baoshan granodioritic crypto-explosion breccia (BGCB, −25.24 to −6.62) were considered to be the interaction of partial melting of ancient mafic lower crust and Neoproterozoic juvenile crust. However, the copper mineralization requires high oxygen fugacity and a large amount of water, according to the tectonic settings of Baoshan, we recommend that it is the trans-magmatic fluid trapping and concentrating Cu from the whole pluton during the upwelling driven by magma convection. The initial magma was stalled by the ductile-brittle transition at shallow depths of upper-crust. The trans-magmatic fluid leads to the pressure increases at the top of the initial magma, then the overlying rock is ruptured and cryptoexplosion produced. Moreover, after the cryptoexplosion, the sudden reduction of circumference temperature and pressure leads to the decrease in the oxygen fugacity of the ore-forming system, which will change the valence state of sulfur from S6+ to S2−. Finally, sulfur precipitates with chalcophile elements like copper in the metallogenic system and forms porphyry copper deposits with the low Sr/Y ratio. This study highlights the use of trans-magmatic fluid and ductile-brittle transition in the formation of the Baoshan porphyry copper deposits.

Abstract:The classical mechanism “source-transport-storage” of the formation of porphyry copper deposit has been advanced in recent studies, as the “source” is not the main factor for the mineralization in some Cu deposits, and the metallogeny may be affected by other variables factors during the magma-fluid transportation or storage. We recommend the essential role of trans-magmatic fluid in the ore-forming process, this fluid is released from the melting of the sedimentary overlying the subducted plate with high water and volatiles concentrations and high oxygen fugacity. The Baoshan granodioritic cryptoexplosion breccia representing the influence of hydrothermal events as well as the unaltered Baoshan granodiorite porphyry are conducted by LA-MC-ICP-MS analysis, to identify the contribution of trans-magmatic fluid. In case of the εNd(t) of whole rock do not increase with the MgO increasing and SiO2 decreasing, the large variations of zircons εHf(t) values in Baoshan granodiorite porphyry (BGP, −14.24 to −6.38) and Baoshan granodioritic crypto-explosion breccia (BGCB, −25.24 to −6.62) were considered to be the interaction of partial melting of ancient mafic lower crust and Neoproterozoic juvenile crust. However, the copper mineralization requires high oxygen fugacity and a large amount of water, according to the tectonic settings of Baoshan, we recommend that it is the trans-magmatic fluid trapping and concentrating Cu from the whole pluton during the upwelling driven by magma convection. The initial magma was stalled by the ductile-brittle transition at shallow depths of upper-crust. The trans-magmatic fluid leads to the pressure increases at the top of the initial magma, then the overlying rock is ruptured and cryptoexplosion produced. Moreover, after the cryptoexplosion, the sudden reduction of circumference temperature and pressure leads to the decrease in the oxygen fugacity of the ore-forming system, which will change the valence state of sulfur from S6+ to S2−. Finally, sulfur precipitates with chalcophile elements like copper in the metallogenic system and forms porphyry copper deposits with the low Sr/Y ratio. This study highlights the use of trans-magmatic fluid and ductile-brittle transition in the formation of the Baoshan porphyry copper deposits.

Co-author:Weijian Zhou, Yongjun Shao, Huan Li, Liang Cao, Guofeng Xu, Jiaxian Li

First Author:Tianyang Hu

Indexed by:Journal paper

Correspondence Author:Lei Liu*, Zhongfa Liu*

Discipline:Natural Science

First-Level Discipline:Geology

Number of Words:15000

ISSN No.:0009-2819

Translation or Not:no

Date of Publication:2022-03-22

Included Journals:SCI

Links to published journals:https://doi.org/10.1016/j.chemer.2022.125881