Sulfide detection in drill core from the Stillwater Complex using visible/near‐infrared imaging spectroscopy

Abstract

This feasibility study examines the potential of imaging spectroscopy to estimate sulfide percentage in drill core from the Stillwater Complex, Montana. The Stillwater Complex is a layered mafic to ultramafic intrusion hosting ore‐grade platinum group elements within the zone known as the JM Reef. Stillwater Mine geologists indirectly infer the platinum/palladium grade by the presence and abundance of sulfide minerals. In order to discriminate between waste and ore rock, geologists visually inspect the core and working faces for minerals such as chalcopyrite, pentlandite, and pyrrhotite. Iron sulfide minerals have a strong ultraviolet absorption that blends into the blue portion of the visible region and produces their yellow luster. The spectral differences between these pathfinder minerals and the accessory minerals are sufficiently distinct to allow classification of this mineralogy using imaging spectroscopy even in the absence of a particular absorption feature. Five different sections of split core from the JM Reef were chosen for their representative mineralogical character. The surface of each sample was scanned with Montana Tech's prototype Airborne and Laboratory Imaging Spectrometer (ALIS) and the images were analyzed for sulfides. For validation, the amount of sulfides was independently determined visually with counting grids. The imaging spectrometer results correlate well with the point‐count percentage, although all five samples consistently fall below the point‐count average. This underestimation is possibly due to metal ion substitution, linear mixing at mineral boundaries, or anisotropic scattering due to the high spatial resolution of the spectrometer. The success of this experiment suggests possible machine vision applications in future mining operations, such as automation of core logging and downhole instrumentation.