Unravelling geology is oftentimes a complicated occupation, especially in areas with a history of ore-forming processing. Lithologies can be hydrothermally altered, weathered, folded, metamorphosed, eroded, or all of the above. Correctly identifying the original assemblage of rock units and discriminating them into a short list of principal components can be very challenging.
Correctly purveying geology in a drill core log is of utmost importance for the subsequent creation of a geological model and resource estimation. The whole exploration and mining operation is based on refinements of these logs and datasets collected on their basis.
Drill core logs are usually drawn by those with the least experience: junior or contract geologists. Obviously, junior geologists should work under the supervision of a senior geologist. The senior’s time is spent calibrating the juniors’ logs so they are consistent and provide the desired information. While most seniors agree on how logging should be organised and supervised in an ideal world, in reality this turns out to be a pipe dream.
Senior geologists commonly work in the head office instead of the core shack. The senior geologist is usually too occupied with own tasks to spend enough time with juniors in the core shack or to prelog drill cores when they come in.
Geochemical discrimination is a common practice for distinguishing rocks, especially volcanic rocks. Drill core logs are in many cases adjusted or redrawn after the assay results come in. Providing the logger early on with objective and quantifiable information about the core, such as chemical data, greatly facilitates consistency and quality of the logs.
The entire geology team benefits from utilising the geochemical dataset produced by continuous XRF scanning with Minalyzer CS. Junior geologists have access to an objective dataset as an overview tool and guideline for geological logging. Senior geologists can quality control logs by comparing them with XRF results and high-resolution digital images.
Case study: Geochemical core logging
The merit of geochemical core logging can be illustrated with a real-world example. Figure 1 shows a down-core graph of TiO2 and Zr concentrations with 10-cm analysis length collected by continuous scanning XRF.
Figure 1 – Down-core TiO2 and Zr concentrations used for geochemical drill core logging.
The geology consists of a metamorphosed and deformed sequence of intermediate and felsic lavas, which have been heavily altered by hydrothermal fluids, especially along lithological boundaries. Logging on a purely visual basis would result in inconsistent results, since the ocular characteristics of these rocks are largely determined by the pervasive alteration fronts.
While different modes of alteration are an important feature, a geological model and sampling strategy should preferably be constructed around the underlying protolithology. Metamorphosed and altered rocks can in most cases be identified by concentrations and ratios of less-mobile elements, such as Ti and Zr. The data in Figure 1 clearly display different units in a lithologic suite, identified by distinct down-core transitions in contents and ratios of Ti and Zr. The high-resolution geochemical dataset is a brilliant tool for drill core logging, which provides both overview and detail. By setting up rock discrimination criteria based on geochemistry, rocks can be classified on a more objective basis, as exemplified in Figure 2.
Figure 2 – Down-core TiO2 and Zr concentrations with the identified rock units coloured in.
Thus, the senior geologist can quickly execute quality control of the drill core logs and choose areas of interest for group discussions or further studies. The risk for inaccuracies and inconsistencies between different geologists is minimised, which ultimately leads to a more unified and better understanding of the geology at an earlier point in the project’s timeline.