An Optimized Analytical Workflow for Magnetite as an Indicator Mineral for Porphyry Cu DEPOSITS

Laser ablation-inductively coupled plasma-mass spectrometry is commonly used to measure trace element concentrations in magnetite, an indicator mineral that can serve as a geochemical vector to various types of ore deposits. Herein, we optimize the speed and efficiency of analytical workflows for discriminating hydrothermal ore-related magnetite grains in till samples. Four laser ablation-inductively coupled plasma-mass spectrometry workflows, varying in laser ablation time, calibration standards, and editing of the time-resolved signal, were evaluated for their ability to classify hydrothermal magnetite grains using a linear discriminant analysis model applied to till samples collected proximal to a porphyry Cu deposit. Calibration using Fe-poor NIST glass standards resulted in lower analytical accuracy in external standards than calibration using USGS basaltic glasses, indicating that higher-Fe basaltic glass standards provide a better calibration match for Fe-rich magnetite. Detailed inspection of the analytical data shows that a rapid workflow using only 10 s of laser ablation, 100 μm spot sizes, and no manual editing of the time-resolved signal produces nearly identical results for key classification elements (Fe, Mg, Al, Ti, V, Cr, Mn, Co, Ni) and yields comparable hydrothermal magnetite classification to more time-consuming workflows. The rapid workflow also estimates the proportion of hydrothermal magnetite in a bulk till sample to within ±4% of results obtained using more time-consuming analytical approaches. Mean similarity coefficients of ∼0.95 between the most and least efficient workflows indicate that our optimized workflow provides a rapid and reliable method for exploration programs seeking to discriminate hydrothermal magnetite as an indicator mineral in till or stream sediment samples, while substantially reducing analytical and data-processing time.