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Iron isotope fractionation during skarn Cu-Fe mineralization

Xue, Song and Niu, Yaoling and Chen, Yanhong and Xia, Boyang and Wang, Peiyao and Gong, Hongmei and Wang, Xiaohong and Duan, Meng (2021) 'Iron isotope fractionation during skarn Cu-Fe mineralization.', Minerals, 11 (5). p. 444.

Abstract

Fe isotopes have been applied to the petrogenesis of ore deposits. However, the behavior of iron isotopes in the mineralization of porphyry-skarn deposits is still poorly understood. In this study, we report the Fe isotopes of ore mineral separations (magnetite, pyrite, chalcopyrite and pyrrhotite ) from two different skarn deposits, i.e., the Tonglvshan Cu-Fe skarn deposit developed in an oxidized hydrothermal system and the Anqing Cu skarn deposit developed in a reduced hydro-thermal system. In both deposits, the Fe isotopes of calculated equilibrium fluids are lighter than those of the intrusions responsible for the skarn and porphyry mineralization, corroborating the “light-Fe fluid” hypothesis. Interestingly, chalcopyrite in the oxidized-Tonglvshan skarn deposit has lighter Fe than chalcopyrite in the reduced-Anqing skarn deposit, which is best understood as the result of the prior precipitation of magnetite (heavy Fe) from the ore fluid in the oxidized-Tonglvshan systems and the prior precipitation of pyrrhotite (light Fe) from the ore fluid in the reduced-Anqing system. The δ 56Fe for pyrite shows an inverse correlation with δ 56Fe of magnetite in the Tonglvshan. In both deposits, the Fe isotope fractionation between chalcopyrite and pyrite is offset from equilibrium line at 350℃ and lies between the FeS-chalcopyrite equilibrium line and pyrite-chalcopyrite equilibrium line at 350℃. These observations are consistent with the FeS pathway towards pyrite formation. That is, Fe isotopes fractionation during pyrite formation depends on a path, from the initial FeS-fluid equilibrium towards the pyrite-fluid equilibrium due to the increasing extent of Fe isotopic exchange with fluids. This finding, together with the data from other deposits, allows us to propose that the pathway effect of pyrite formation in the Porphyry-skarn deposit mineralization is the dominant mechanism that controls Fe isotope characteristics.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.3390/min11050444
Publisher statement:© 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Date accepted:19 April 2021
Date deposited:20 April 2021
Date of first online publication:22 April 2021
Date first made open access:20 April 2021

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