36 Cordilleran Mineral Systems: Interdisciplinary Perspectives on Critical Metal Ore Deposits (Posters)

Explore the processes responsible for selective transport and accumulation of critical metals in the Earth’s crust: mineral systems science, economic geology, mineral exploration, geochemistry and isotopes, fluid inclusions, and mineral chemistry (bilingual session (English/Spanish)).

Friday, 24 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Boron Isotope Signatures in Hydrothermal Tourmaline from Three IOCG Deposits within the Lights Creek Stock in Northern California

Paper number: 36-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Lights Creek Stock (LCS) is a granitoid pluton located in the Northern Sierra Nevada, CA. The LCS hosts three IOCG deposits (Engels, Moonlight, and Superior) of which Superior and Engels were historically mined for copper. These formed through a long history of magmatic activity, hydrothermal fluid, and surficial water interactions. The three deposits have varying alteration assemblages and mineralization due to differences in host rock, depth and interaction with hydrothermal fluids, but tourmaline is found in various textures and assemblages throughout the LCS. Tourmaline is a borosilicate mineral that may form under a variety of conditions, from magmatic to hydrothermal, and the chemistry of hydrothermal tourmaline may be used to understand the chemistry of the altering fluids. Insight on the hydrothermal fluids within the system will help to determine the genesis of the IOCGs. Boron has two isotopes 10B and 11B (natural occurrence of 20% and 80% respectively) which encompass a range of -70% to +75% ∂11B, but tourmaline typically has a range of -26.8% to +35% ∂11B. Analyzing boron isotopes in tourmalines at LCS can help understand the fluid sources and fluid-rock interactions that formed the IOCG deposits. Four samples were analyzed, one from Engels and Moonlight and two from Superior drill cores at 37ft and 484ft below the ground surface. Samples were analyzed using the Stanford USGS Sensitive High Resolution Ion Microprobe Reverse Geometry (SHRIMP-RG). The samples showed a range of boron signatures from -1.7 to +11.17%. Engels signatures ranged from +0.21 to +3.38%, Moonlight from +0.06 to +4.88%, the shallow Superior sample had a range of -1.72 to +2.43% and the deep sample ranged from +1.12 to +11.17%. The heterogeneity of the boron signatures details mixing of boron fluid sources.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410970

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