Times are displayed in (UTC-05:00) Central Time (US & Canada) Change

113-11 Oxygen and Hydrogen Isotope Evidence for Hydrothermal Fluid in Brazilian Paraíba Tourmaline-Bearing Pegmatites

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

Poster Booth No.: 256

Presenting Author:

Sarah Brooker

Authors:

Brooker, Sarah E¹, Barnes, Jaime D², Befus, Kenneth S³, Dutrow, Barbara L⁴, Mathur, Ryan⁵

(1) Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA, (2) Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA, (3) Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA, (4) Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, USA, (5) Department of Geological Sciences, Juniata College, Huntingdon, Pennsylvania, USA,

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 (δ¹⁸O_tur = +10.8 ± 0.5‰; 1σ; n = 9) are in equilibrium with quartz (δ¹⁸O_qtz +14.0 ± 0.4‰; 1σ; n = 6) at approximately 500 °C, consistent with magmatic crystallization. In contrast, blue tourmalines (δ¹⁸O_tur = +12.3 ± 0.3‰; 1σ; n = 9) are out of equilibrium with coexisting quartz, with elevated δ¹⁸O_tur 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 Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8748

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

Description

Session Format: Poster

Presentation Date: 10/20/2025

Presentation Room: HBGCC, Hall 1

Poster Booth No.: 256

Author Availability: 9:00–11:00 a.m.

Back to Session