In tropical climates, postdrilling oxidation of sulfide-rich core can severely degrade drill core, producing low-temperature iron oxyhydroxides, sulfates, and clays. Variable growth of these secondary minerals in exposed drill core, combined with the hydration and degradation of primary hydrothermal minerals, may lead to the production of spurious results in near-infrared (NIR) spectroscopic studies. However, the NIR technique can remain an effective tool in assessing hydrothermal alteration, even in extremely degraded core.
We have assessed the usefulness of the NIR technique on degraded core at the Ladolam gold deposit, Papua New Guinea. Here, we seek to determine whether the primary alteration mineralogy had been significantly transformed by postdrilling oxidation over several years of weathering. In doing so, the study tested whether NIR analysis can be an effective tool in the discrimination of primary hydrothermal minerals in degraded core.
Our study was made possible using semiquantitative X-ray diffraction (QXRD) analyses of a drill hole in 2004, where samples were collected at 50-m intervals. We subsequently repeated NIR and QXRD analyses on the same drill core in 2012. After nine years of storage, the drill core had degraded considerably, with the growth of jarosite and other sulfates. Despite this, XRD results from 2004 and 2012 show no major differences in the primary alteration mineralogy.
Closely spaced NIR analyses were conducted at 1-m intervals to increase the chance of obtaining a spectrum of the primary mineralogy and to exclude secondary oxidation minerals. The drill core, where possible, was broken immediately prior to analysis to obtain a fresh surface. On average, over a 10-m interval, approximately 25% of the NIR spectra did not contain secondary minerals and relict primary alteration minerals could be detected. The remaining spectra were affected by the occurrence of secondary jarosite, gypsum, and/or residual water, but in most cases, the primary alteration mineralogy could be determined. We conclude that NIR analyses remain an effective tool in the construction of geological deposit models when logging degraded historic core, even for sulfide-rich core that has degraded in tropical environments.