Mineralogical controls on rare earth element redistribution and enrichment: A study from the Franklin Mining District, New Jersey

Abstract

Rare earth elements (REEs) are in high demand as they are crucial in our transition to renewable, green energy sources. As such, quantifying how REEs may substitute at the trace level (<1000 ppm or <0.1 wt%) and in which minerals will provide a framework for exploration of low concentration deposits where REEs may be a byproduct of primary mineralization, establish more efficient extraction techniques during mining, and help sustain our growing demand for critical elements. This study identifies REE concentrations within key minerals from the Franklin Mining District in Franklin, New Jersey, a locale with more than 400 unique mineral species. The area provides an ideal location to evaluate mineralogical and crystal chemistry controls and how they may influence trace-level REE substitution and partitioning.

Here, we identify silicates that contain VIII- to XII-fold coordination sites and large cations such as calcium (Ca) have enriched concentrations of light REEs (LREE) ([LREE] ranging from 160 to 730 ppm) when compared to the upper continental crust (UCC). In some cases, there is more than five times enrichment; beyond silicates no other mineral group analyzed showed LREE enrichment. On the other hand, heavy REE (HREE) enrichment was observed across multiple mineral groups: silicates (six of 32 minerals, [HREE] ranging from 11 to 110 ppm), carbonates (two of six minerals, [HREE] ranging from 16 to 18 ppm), and an oxide (one of five minerals, [HREE] = 18 ppm) demonstrated HREE enrichment. As of 2025, processing REEs contained within silicates is more costly and has lower recovery efficiencies at an industrial scale when compared to processing REEs from more common REE-bearing phases such as fluorocarbonates (bastanäsite) and phosphates (monazite). Across mineral groups, those that formed as a result of water-rock interactions (hydrothermal alteration and supergene processes) and that contained moderate-to large- (VI to XII) coordination sites were more likely to contain enriched concentrations of HREE compared to the UCC, in some cases being enriched by more than an order of magnitude. Thallium (Tl) isotopes, which are highly sensitive to changes in redox conditions and can reflect water-rock interactions, show no clear correlation with [REE] enrichment, but there is a general trend toward more positive Tl isotope compositions (denoted as ε²⁰⁵Tl) with REE-enriched minerals (ε²⁰⁵Tl ≥ 0 for all enriched samples, ε²⁰⁵Tl ranges from 0.0 to 5.7). However, Tl isotopes may be useful to track processes that remobilize and deplete REE. We show that areas which experienced syn- or post-depositional metamorphism tend to be depleted in REE, likely a reflection of their incompatibility, and have significantly lower ε²⁰⁵Tl values (ε²⁰⁵Tl ranges from −5.2 to −0.1). Coupled mineralogical [REE] and non-traditional stable isotope data demonstrate the need for a firm understanding of both mineralogical and crystal chemistry controls, such as bond type, coordination environment, and mineral type, along with large-scale, regional processes, such as metamorphism and metasomatism, to evaluate potential for REE trace-level substitution. As REE dependence continues to grow, secondary, trace-level REE resources may become more critical to meeting demand and, as such, crucial to further exploration and extraction.