Using the Alunite Shift as an Indicator in Gold Exploration

Alunite characteristics can provide a good indication of gold ore potential in a high sulfidation epithermal system. Alunite is a potassium sulfate mineral that forms in hydrothermal systems. Sulfates such as alunite and jarosite are common in hydrothermal environments and have distinctive NIR spectra. Alunites have the potential to delineate zoning based on chemical and temperature changes in the hydrothermal system.

By measuring the shift in alunite from potassium rich (K-alunite) to sodic (Na-alunite) a geologist can have a better understanding of the mineral alteration and can use this data to vector to mineralization. The shift is typically seen at the 1480-1490nm wavelength in an NIR spectra and is thought to be a record of the mineralizing effect of magmatic and meteoric fluids.

The oreXpress is a field portable NIR spectrometer that provides mineral identification and analysis for in situ measurements. Lightweight and rugged, the oreXpress is used in the field with two lithium-ion batteries for 6-8 hours of scanning outcrops, pits, and benches. The system has no moving optical parts – solid state photodiode arrays prevent accidental breakage of misalignment.

With our mineral contact probe, a geologist can scan a large area and create a mineral map. With the optional EZ-ID mineral identification software of more than 1100 minerals and 2600 spectra, identifying typical minerals in hydrothermal systems like kaolinite, jarosite, dickite, pyrophyllite, and zunyite, is fast and accurate.

With the oreXpress and EZ-ID, the return-on-investment can occur very quickly by eliminating drill holes in unprofitable locations and concentrating on spots that are indicative of profitable mineral alteration.

Close-up of two alunite samples from core showing the shift from potassium rich alunite (blue) to sodium rich alunite (red).

Alunite sample. Alunites are important pathfinder minerals in both gold and copper systems.

EZ-ID mineral identification software can be usewd to identify alunite and other associated minerals including jarosite, kaolinitie, dickite, pyrophyllite and zunyite. With three libraries of 1100 minerals and 2600 spectra, EZ-ID matches the high resolution scan of your target sample to a known library spectra.