Mapping alteration in geothermal drill core using a field portable spectroradiometer

Hydrothermal alteration mineralogy in geothermal systems is commonly used to infer system temperature and past fluid flow patterns. Infrared spectroscopy is particularly good at identifying a wide variety of hydrothermal alteration minerals. The technique requires little sample preparation, and is especially helpful in discrimination among a wide range of phyllosilicate minerals that may be difficult to distinguish in hand sample or require lengthy preparation for XRD analysis. We have performed several pilot studies of geothermal drill core and chips to prototype rapid alteration characterization over large depths. These preliminary studies have established reliable methods for core/chip surveys that can quickly measure samples with high depth resolution and show the efficiency of the technique to sample frequently and provide alteration logs similar to geophysical logs. We have successfully identified a wide variety of phyllosilicates, zeolites, opal, calcite, iron oxides, and hydroxides, and note depth-associated changes in alteration minerals, patterns, or zones. Alteration mineralogy identified using these techniques shows good correlation with traditional petrographic microscope and XRD methods.