A Microscale Analysis of Hydrothermal Epidote: Implications for the Use of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Mineral Chemistry in Complex Alteration Environments

High-resolution, quantitative imaging of epidote from the Ann Mason fault block, Yerington district, Nevada, using scanning electron microscopy (SEM), X-ray fluorescence (XRF), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has revealed at least two types of epidote (type 1 and type 2), each with different major and trace element chemistry. Type 1 epidote is coarser grained, typically greater than 50 μm in diameter, and forms euhedral crystals that display twins and sector zones enriched in Fe, Sr, and Mn. Type 2 epidote is finer grained, typically less than 30 μm in diameter, irregularly zoned with respect to Fe and Al, and forms polycrystalline aggregates that include void space. Two sources of intragranular compositional variability are defined in this study—one related to different generations of epidote only visible on the microscale, and the other related to crystallographic features such as sector zones and twins. Intragranular compositional variations within Yerington epidotes highlight the potential importance of detailed sample characterization in complex alteration environments prior to undertaking mineral chemistry studies in the context of resource exploration. Based on statistical analysis of LA-ICP-MS spot data from one sample, the optimal number of spot analyses to adequately represent the range in element concentrations within zoned or twinned crystals is determined to be at least 40.