Porphyry Cu ore deposits host most of the Cu, Au, and Mo reserves worldwide and generally form under highly oxidizing conditions. However, it is unclear what causes the high redox state (i.e., the source or subsequent magmatic differentiation). Amphibole fractionation is a key step in the formation of porphyry Cu ore deposits. In this study, amphibole-rich lithologies and derivative intermediate-silicic porphyries from two porphyry Cu systems (Laiyuan and Tongling) in eastern China were investigated to constrain the magma redox state during amphibole fractionation. At Laiyuan, the amphibole-rich rocks are hornblende-bearing gabbroic fragments or lenses hosted in granodiorite, consisting of two crystal populations: group I—brown, anhedral hornblende cores, with clinopyroxene ± olivine relics; and group II—green hornblende rims and matrix hornblende. At Tongling, the amphibole-rich lithologies are clinopyroxene-hornblende cumulates or hornblende-bearing gabbro xenoliths hosted by intermediate-silicic porphyries. Based on the clinopyroxene-liquid thermobarometer and amphibole-liquid thermobarometer, the brown amphibole and clinopyroxene from two systems crystallized under 4.6−5.9 kbar and 990−1000 °C, but their green amphiboles crystallized under lower pressure-temperature conditions (∼630−850 °C and 0.5−1.4 kbar). The results of amphibole and zircon-based oxybarometers show that their parent melts have similar moderate oxygen fugacities of ΔFMQ +1.0 ± 0.5, comparable to those of primitive arc magmas, but after amphibole fractionation their residual intermediate-silicic melts had obviously increased oxygen fugacities of ΔFMQ +2.5 ± 1.0. Modeling results show that the increase in oxygen fugacity is probably caused by the preferred partitioning of ferrous Fe into amphibole and clinopyroxene during amphibole fractionation. Amphibole differentiation is a common process in porphyry Cu systems; such processes may facilitate the oxidation of causative magmas and, therefore, the occurrence of porphyry Cu deposits.
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Research Article|
January 22, 2025
Early Publication
Amphibole fractionation controls the redox state of magmas in porphyry Cu ore systems: Constraints from amphibole-rich rocks in eastern China
Jingguo Du;
Jingguo Du
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
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Xiaohua Deng;
Xiaohua Deng
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
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Yangsong Du;
Yangsong Du
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
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Gongwen Wang
Gongwen Wang
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
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Jingguo Du
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
Xiaohua Deng
1
School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
Yangsong Du
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
Gongwen Wang
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
Publisher: Geological Society of America
Received:
26 Mar 2024
Revision Received:
10 Nov 2024
Accepted:
26 Dec 2024
First Online:
22 Jan 2025
Online ISSN: 1943-2674
Print ISSN: 0016-7606
© 2025 Geological Society of America
GSA Bulletin (2025)
Article history
Received:
26 Mar 2024
Revision Received:
10 Nov 2024
Accepted:
26 Dec 2024
First Online:
22 Jan 2025
Citation
Jingguo Du, Xiaohua Deng, Yangsong Du, Gongwen Wang; Amphibole fractionation controls the redox state of magmas in porphyry Cu ore systems: Constraints from amphibole-rich rocks in eastern China. GSA Bulletin 2025; doi: https://doi.org/10.1130/B37687.1
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