Petrophysical target characterization with lithogeochemical clustering: the Metsämonttu Zn–Pb–Cu deposit, southern Finland

Abstract

In mineral exploration, reliable and comprehensive petrophysical groundwork helps evaluate the success rate of applying geophysical methods to the target and thus improves the quality of decision-making related to geophysical surveys. Petrophysical data for potential field exploration can be efficiently collected from drill cores together with portable XRF data that provide insight into rock compositions. We acquired and analysed a multi-parameter portable-instrument drill core data set (2036 samples) with portable XRF, magnetic susceptibility and density measurements from the Metsämonttu Zn-Pb-Cu deposit, one of the base metal deposits in the Orijärvi Belt, southern Finland, with little post-mining exploration during the last 50 years. Our study applied a machine learning technique (Gaussian Mixture Model clustering) to the portable XRF data to provide a pseudo-lithological reclassification of the drill cores and used this as a basis for characterization of the deposit in petrophysical terms. We also used thermomagnetic measurements to inspect the magnetic mineralogy of the drill cores. Finally, we estimated the success of using potential field methods (magnetic and gravity) for brownfield exploration of analogous deposits in the area. With the portable XRF data clustering we were able to distinguish between barren mafic and felsic volcanic rocks, altered rocks and three different ore mineral enriched rock types. The petrophysical measurement data and thermomagnetic results were combined with the clusters to define the petrophysical characteristics for future mineral exploration with potential field methods. At Metsämonttu, the highest densities are related to the ore and iron sulphide-bearing rocks. High natural remanent magnetization intensities are related to the iron sulphide-bearing ore, and remanent magnetization is a significant magnetic anomaly source. Based on synthetic modelling results, both magnetic and gravimetric data could be used in direct exploration in the Metsämonttu area.