Spectrometric borehole logging in mineral exploration and mining

Abstract

Borehole logging tools based on pulsed neutron technology are used to infer geophysical parameters such as bulk density. If fitted with a gamma-ray spectrometer, they may also measure elemental mass fractions of a variety of formation elements. Spectrometric borehole logging tools of this kind are established in the oil and gas industry. With the demand for mineral resources surging, cost- and time-efficient measurement techniques are increasingly needed in mineral exploration. Their ability to detect multiple elements that are building blocks of minerals makes spectrometric borehole logging tools popular in mineral exploration and mining grade control. In this article, the potential of the OreLog pulsed neutron gamma spectrometric downhole logging tool is demonstrated at Erzberg Mountain (Styria, Austria). Erzberg hosts the largest known siderite deposit in the world, and open-pit mining is used to extract iron ore. The aim was to obtain a more detailed and faster geochemical characterization of iron ore deposits prior to blasting. Elemental logs were acquired with highest accuracies for Fe, Ca, and Mn, followed by Si, Mg, K, and S. The tool mapped a 2D cross section of formation element concentration within a few hours, which agrees with orogenetic fault and geologic contact lines. Mineralogical information retrieved from elemental logs confirmed the interlaced mineralogy of the site. It is dominated by a carbonate body, hosting an intergrowth of Mg-rich siderite (sideroplesite) and a solid solution of dolomite/ankerite/kutnohorite. In addition to the tool's potential for enhanced real-time mining control, the benefit of continuous depth-resolved elemental logs is seen in the possibility to deduce a more refined and reliable geochemical and mineralogical model of the deposit. The information-rich data stream opens applications beyond grade control and exploration, such as data assimilation with gravity models or geologic model building.