Geochemical Core Logging with Minalyzer CS


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.

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.

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.

Figure 1

Down-­core TiO2 and Zr concentrations used for geochemical drill core logging.

Figure 2

Down-­core TiO2 and Zr concentrations with the identified rock units coloured in.