113 Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones (Posters)

From ore materials to precious gemstones, minerals play an important role in almost every aspect of our lives. This session will focus on characterization of economic minerals employing spectroscopic techniques, geochemical analysis, and microanalytical techniques.

Monday, 20 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Oxygen and Hydrogen Isotope Evidence for Hydrothermal Fluid in Brazilian Paraíba Tourmaline-Bearing Pegmatites

Paper number: 113-10
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Pegmatite crystallization is complex, with the relative roles of magmatic processes and hydrothermal fluids still debated. Our prior Cu isotope work on Paraíba tourmaline gemstones established that elevated δ65Cu values (+1.64 ± 0.34‰; 1σ; n = 16) in blue tourmalines reflect hydrothermal introduction of Cu, likely mobilized through partial oxidative dissolution of sulfide minerals in the surrounding country rock, rather than derivation from a Cu-rich magmatic melt. Pink cuprian tourmalines, typically found in the core of crystals, have lower δ65Cu values (-0.95‰ and +0.41‰), likely due to redox variations during crystallization or limited Cu mobility during early growth. Building on the Cu isotope data, new O and H isotope data from Brazilian Paraíba tourmalines and minerals from the host pegmatite further constrain the nature and temperature of the fluids involved in crystallizing different color zones. Oxygen isotope measurements were determined via laser fluorination and hydrogen isotope measurements were on bulk tourmaline samples. Oxygen isotope values for pink tourmalines (δ18Otur = +10.8 ± 0.5‰; 1σ; n = 9) are in equilibrium with quartz (δ18Oqtz +14.0 ± 0.4‰; 1σ; n = 6) at approximately 500 °C, consistent with magmatic crystallization. In contrast, blue tourmalines (δ18Otur = +12.3 ± 0.3‰; 1σ; n = 9) are out of equilibrium with coexisting quartz, with elevated δ18Otur values (> +12‰) commonly interpreted as interaction with 18O-rich metasedimentary rocks. Preliminary hydrogen isotope data for blue tourmalines are broadly consistent with values for hydrous minerals in igneous systems. The combined O–H–Cu isotope trends indicate that externally derived hydrothermal fluids, likely interacting with the country rock of the Equador Formation, composed of mica-rich quartzites and metaconglomerates, were not only responsible for Cu introduction but also played a critical role in late-stage pegmatite evolution.

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

doi: 10.1130/abs/2025AM-413368

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