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Two Origins of Illite at the Dexing Porphyry Cu Deposit, East China: Implications for Ore-Forming Fluid Constraint on Illite Crystallinity

Published online by Cambridge University Press:  01 January 2024

Zhangdong Jin ,
Jinchu Zhu ,
Junfeng Ji ,
Fuchun Li  and
Xinwei Lu
Zhangdong Jin*
Affiliation:
Lake Sediment and Environment Laboratory, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory for Research of Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China
Jinchu Zhu
Affiliation:
State Key Laboratory for Research of Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China
Junfeng Ji
Affiliation:
State Key Laboratory for Research of Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China
Fuchun Li
Affiliation:
State Key Laboratory for Research of Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China
Xinwei Lu
Affiliation:
State Key Laboratory for Research of Mineral Deposits, Department of Earth Sciences, Nanjing University, Nanjing 210093, China Institute of Geography, Chinese Academy of Sciences, Beijing 100101, China
*
*E-mail address of corresponding author: zhdjin@niglas.ac.cn
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Abstract

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Illite is a distinctive clay mineral formed by K alteration within hydrothermal alteration zones in porphyry Cu deposits. Based on differences in spatial distribution, Kübler index, number of swelling layers, and polytype, two kinds of illite are recognized within the Dexing porphyry Cu deposit, East China. One is a hydrothermal mineral within hydrothermally-altered granodiorite porphyry and altered tuffaceous phyllite near the contact zone with the granodiorite porphyry cupola. The hydrothermal illite is formed by illitization of plagioclase and/or micas during hydrothermal fluid-rock interaction. The considerable variation of their higher Kübler indices (0.17–1.41°Δ2θ) with swelling layer is affected by fluid/rock ratio or fluid flux. The other type of illite is a product of low-grade metamorphism within tuffaceous phyllite away from the porphyry cupola (>2 km), and has a lower Kübler index (0.06–0.13°Δ2θ), a 2M1 polytype, and no swelling layers. We suggest that, within the mineralized alteration zone, the lower the Kübler index, the stronger the degree of alteration, and the higher the copper grade. This is caused by a higher fluid/rock ratio in the middle-upper portions of the contact zone.

Keywords

AlterationHydrothermal FluidKübler IndexLow-grade MetamorphismPorphyry Cu Deposit
Type
Research Article
Information
Clays and Clay Minerals , Volume 50 , Issue 3 , June 2002 , pp. 381 - 387
Copyright
Copyright © 2002, The Clay Minerals Society

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