Near-Infrared Microscopy, Trace Element, and Sulfur Isotope Geochemistry of Pyrite from the Triangle Deposit (Canada): A Novel Approach to Investigate Hydrothermal Processes in Orogenic Gold Deposits

Abstract The Triangle deposit (Val-d’Or, Canada) is an orogenic gold deposit characterized by quartz-tourmaline-carbonate-pyrite veins hosting Au-Ag-Te-Bi-Pb minerals as inclusions in pyrite and as fracture fillings. Near-infrared microscopy combined with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element mapping identifies seven pyrite textural types: cores (Py-a), oscillatory zoning (Py-b), deformed banding (Py-c), disruptive textures (Py-d and Py-e), pyrite veinlets (Py-f), and halos around gold-bearing veinlets (Py-g). Pyrite shows low median Co (82 ppm), Ni (103 ppm), and As (66 ppm) contents, and in situ LA-ICPtandem mass spectrometry (MS/MS) analyses yield δ34S values from –5.6 to 14.8‰, with significant intra- and intergrain variability. Oscillatory zoning (Py-b) is consistent with repeated pulses within the fault-valve model, whereas disruptive textures (Py-d/e) record postfault-valve, late fluid-assisted recrystallization. Gold deposition occurred in three main stages: (1) Au-1, early Au-Ag-Te inclusions in Py-b formed during vein opening and phase separation; (2) Au-2, gold refinement by local remobilization of preexisting gold within Py-d disruptive textures; and (3) Au-3, late remobilization by sulfur-poor fluids producing Au-Ag-Pb-Bi-Te fracture fillings (Py-g). This multistage evolution is consistent with the fault-valve model, in which seismic and aseismic cycles control fluid flow, followed by postdeformation sulfide recrystallization and then fracturing and gold redistribution. These results highlight the dynamic nature of hydrothermal processes in orogenic systems and demonstrate the effectiveness of integrating near-infrared microscopy with in situ microanalytical techniques to resolve the complex evolution of mineralization in orogenic gold deposits.