ALTERATION IN HAWAIIAN DRILL CORE USING A PORTABLE FIELD SPECTROMETER

The Humu’ula Groundwater Research Project (HGRP) drilled their first continuously- cored hole in the saddle region of the big island of Hawaii in March of 2013. Temperatures at the bottom of the hole were unexpectedly high and reached over 100C. Faculty, staff and students at UNR have conducted several pilot studies to identify alteration in geothermal drill core and cuttings. These initial studies established reliable methods for core/chip surveys using an Analytical Spectral Devices (ASD) FieldSpec instrument with a contact probe that measures from 0.4 to 2.5 μm. The pilot studies focused on what minerals could be identified at the 2 cm scale of the probe, and methods of mapping alteration mineralogy with depth. Alteration mineralogy can indicate both the presence of thermal fluids and the relative range of fluid temperatures. In pilot studies of four different geothermal systems spectral measurements were acquired every 1 to 10 feet. In each case, depth-associated changes in alteration mineralogy and zoning were found. These same techniques were applied to the HGRP drill core. Here, we used the spectral data as a survey tool to help identify and select sections of core for sampling and more detailed mineralogical analysis using traditional XRD and petrographic techniques. Over 780 spectra were collected on the core from depths of 3190 to 5785 feet over 3 days in May of 2014. Spectra were sorted in order of increasing depth. Many of the spectra can be classified along a gradation from unaltered mafic mineralogy (augite and olivine) to chlorite, possibly with some mica. This suggests a method to identify the degree of alteration using the relative strength of various absorption features. In addition to abundant chlorite, zeolites are also common. The majority of zeolites are spectrally similar to each other at these wavelengths, however analcime is uniquely identified in some sections and natrolite is also present. Clay minerals such as montmorillonite and kaolinite are commonly found in geothermal systems, but their spectral signatures were not identified in the HGRP survey, suggesting that alteration was initiated from higher temperature and moderate pH fluids. Detailed analysis is underway and we will present minerals maps with depth in the HGRP drill hole.