Mapping alteration minerals at Malmbjerg molybdenum deposit, central East Greenland, by Kohonen self-organizing maps and matched filter analysis of HyMap data

Abstract

The Malmbjerg molybdenum deposit in central East Greenland is a world-class porphyry molybdenum deposit. The porphyry molybdenum deposit occurs within a granite stock intruded into sedimentary rocks. The Malmbjerg molybdenum deposit is associated with a pronounced zone of hydrothermal alteration. The reflectance spectra of rock samples from the Malmbjerg alteration assemblage show absorption features of a number of minerals including topaz, jarosite, goethite, muscovite, phengite, epidote, chlorite and smectite. This study investigated the Malmbjerg alteration assemblage using airborne imaging spectrometer data recorded by the HyMap imaging system. The HyMap data were analysed using an unsupervised classification based on Kohonen self-organizing maps and partial spectral unmixing based on the matched filter algorithm. The mapping results show the spatial distribution of jarosite, goethite, phengite, epidote/chlorite, smectite, topaz and non-altered sediments. The remote-sensing mapping results bring new information for the alteration assemblage at Malmbjerg, especially for the occurrence and the spatial distribution of a phengite zone in the altered sediments overlying the molybdenum deposit and for the propylitic alteration zone. The study is an example of detailed characterization by imaging spectrometry of alteration assemblages associated with porphyry molybdenum deposits. This research also shows the potential of imaging spectrometry as a tool for geological mapping and exploration in the Arctic regions of East Greenland.

Acknowledgments

This study was funded by the Danish Ministry of Science, Technology and Innovation and the Geological Survey of Denmark and Greenland (GEUS). I cordially thank the two anonymous reviewers for their helpful comments on the manuscript.