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Element Mobility During Syn-Deformational Melt Migration

Abstract

Metasomatism and element mobility are key controls in the formation of economic deposits, often influenced by deformation assisted fluid-migration in the Earth’s crust. Magmatic ore deposits in the upper crust are often genetically linked to the fluid migration and deformation processes occurring at depth. This study investigates the high strain shear zones in the lower crust through two key global cases in Fiordland, New Zealand and Akia Terrace, Greenland to investigate the deformation driven metasomatic processes in the lower crust. The two examples demonstrate the evidence of syn-deformation melt flow using field, microstructural, and geochemical analyses of lower crustal gneisses from high strain zones. Observations include recurring microstructural evidence that include anhedral grains with cuspate boundaries, euhedral grains with straight crystal facets, low dihedral angles, and polymineralic intergrowths. Additionally, geochemical signatures such as hydration of minerals, MgO depletion and enrichment of LREE, SiO₂, and Na₂O were observed. In addition, Cr and Ni depletion, and Zr enrichment are observed in Fiordland, New Zealand. The high-strain rocks display more evolved characteristics and stronger signatures of melt-rock interaction. 

The study results demonstrate that deformation driven melt migration significantly influences melt transfer and metasomatism in high-strain lower crustal regions, contrasting sharply with mylonitic rocks of upper crust.

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Author

Joyjit Dey

Joyjit Dey, PhD (Structural Geology), Master of Research (Structural Geology), MSc (Geology), AIG – Consultant 

Joyjit’s experience in structural geology to date involves using a multi-disciplinary/multi-scale approach that integrates key geological information from regional to deposit-scale litho-structural mapping, 3D modelling, core logging and geochemical analysis to highlight mineralisation control and support exploration.  

Joyjit has worked with a range of commodities – namely zinc, lead, gold, copper, iron, coal, and lithium – in a number of regions and mineral provinces within India, Saudi Arabia, Africa, Australia, the UK and Europe.