242 Crustal Petrology, Part I

This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

Exploring zircon dark zones through cathodoluminescence and trace elements: New insights from the Ruby Star Granodiorite, Arizona

Paper number: 242-13
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

As the global energy transition requires a shift towards renewable energy sources surge, the demand for copper and other strategic metals is increasing. The main global copper sources are Porphyry Copper Deposits (PCDs). New PCDs are becoming more challenging to find, and major discoveries from the past decade are being found deeper in the subsurface. To improve exploration strategies, a holistic understanding of the processes that drive PCD emplacement is required, and the development of mineral geochemical vectors is promising.

Recent work has focused on accessory igneous minerals (e.g. zircon, apatite) within intrusions associated with PCDs as recorders of magmatic and hydrothermal evolution. Zircon textural features revealed by Cathodoluminescence (CL) record variations in the chemical evolution through its characteristic oscillatory growth zoning. This study specifically focuses on zircon “dark” growth zones that are commonly observed in porphyry intrusions, yet poorly studied (e.g., Bajo de la Allumbrera; Buret et al., 2016). In Bajo de la Alumbrera, zircon dark zones display anomalously high concentrations of uranium and rare earth elements (REE), and are interpreted as relics of a mafic recharge event (Buret et al., 2016).

To test petrogenetic models for zircon dark zone growth, and their possible usefulness as a geochemical vector for PCDs, zircon grains from the Ruby Star Granodiorite associated with world-class sierrita PCD were depth profiled via Laser Ablation Split Stream Inductively Coupled Mass Spectrometer (LASS-ICP-MS). Zircons from the Ruby Star Granodiorite commonly exhibit dark growth zones characterized by high U (7000 ppm), high REE (6000 ppm total REE), and low Ti content corresponding to crystallization at 650-750 ºC. After rotating these grains 90º and polishing into the depth profile to expose a thermochemical cross-section, we use SEM imaging, Raman Spectroscopy, and electron microprobe analyses to evaluate morphology, mineralogical structure, and major element chemistry of zircon grains across these dark growth zones. Here we present these findings and discuss their implications for the petrogenetic mechanisms forming zircon dark zones in porphyry intrusions associated with PCDs, and evaluate their potential for better understanding emplacement and as a geochemical indication to identify “fertile” intrusions.

Buret et al., 2016, EPSL

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

doi: 10.1130/abs/2025AM-413970

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