85-7 Mobilization and trapping of HREEs in mélange during collisional orogenesis, northeastern CT, USA: Implications for crustal HREE transport and mineralization

Session: The Power of Hard Rocks: Driving the Energy Transition and Serving Society

Presenting Author:

Authors:

Abstract:

Rare Earth elements (REEs) are required for a range of technologies, and demand for these elements continues to increase. It is therefore necessary to understand the processes affecting REE mobilization and transport in the lithosphere, including outside of ore-forming settings. Although deposits of other critical elements, such as gold, are often formed by orogenic fluid processes, economic-scale REE ore bodies are commonly associated with igneous activity, particularly (per)alkaline rocks and carbonatites. Previous work has suggested that trapping of heavy REEs (HREEs) by garnet in metamorphic systems may restrict the availability of these elements for ore-forming processes. Here, we examine a carbonate-silicate vein system associated with orogenic fluid flow that cuts a mélange formed during Appalachian orogenesis. We analyzed vein-host block transects in a range of block types, including garnet-bearing and garnet-free veins and altered selvages, and used mass balance calculations to quantify the extent to which various elements were gained or lost during fluid-rock interaction. All vein-selvage systems underwent volatile gains associated with CO₂ mineralization, but changes in most other individual elements differ among vein-selvage systems in different blocks. Local mass changes for certain elements can be large, with some garnet-bearing veins and selvages having undergone HREE increases of 100–1000%. Other vein-selvage systems, however, underwent little to no change in HREE content, or even lost HREEs. Considering data from all blocks in aggregate, results suggest relatively small mass changes for most elements in the mélange blocks overall. These small overall changes combined with large local changes imply that these local changes average out across the mélange blocks. Thus, most elements, including the HREEs, were likely primarily cycled within the mélange system. Our results therefore suggest that garnet growth may prevent fluid-mobile HREEs from leaving metamorphic systems, potentially inhibiting ore body formation at shallower depths. Our results also emphasize the importance of studying multiple blocks when evaluating mass transfer in mélange systems. If the results from a single block were generalized to the entire mélange, it would have been possible to reach a range of different conclusions depending on the block selected. Thus, we suggest that the analysis of multiple blocks is necessary to fully understand mass transfer processes in mélange systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7215

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.