Logging lithology through regolith can be a challenging exercise for geologists as the rocks have been strongly weathered to clays and oxides. However, with the aid of both a portable X-ray fluorescence (pXRF) instrument, visible near-infrared and short-wave infrared (VNIR-SWIR; TerraSpec 4 ASD) spectrometer, lithologies and alteration can be deciphered. The results from these instruments can be collected on site and in real time allowing the geologist to make decisions early and increase the chance of discovery. The results of these two entirely different methodologies can be complimentary and can give the geologist confidence in the interpretation of results.
In this example from an Archean Au prospect at Pioneer Resources Kalpini South prospect, the geology in the regolith was better deciphered using pXRF and VNIR-SWIR technology. The differences in minerals (e.g., weathering of mafic rocks to nontronite) and spectral features (kaolinite Al-OH peak) identified different rock types in the regolith which was further supported by their distinct geochemistry (Zr vs Ti and V). The results from the study showed that the top of the Archean is deeper than previously thought and as a result gives implications for the exploration and detection of a geochemical and alteration signal from Au mineralisation.
Four lithologies have been observed at the prospect (felsic, mafic, transported, and alluvium; Fig. 1 and 2). With the use of pXRF, all lithologies can be discriminated on Zr vs Ti and V diagrams (Fig.3A and B. Figure 2 shows a downhole log through a regolith profile where the interpreted location for the top of the Archean has changed dramatically from 6 m using geologist logs to 46 m using pXRF results. The proper detection of the Archean has large implications for exploration and the ability to detect a geochemical anomaly from nearby mineralisation.
The discrimination of lithologies in the regolith was further complimented with a VNIR-SWIR spectrometer. The mafic lithology altered to nontronite, a clay that is not easily identified visually. The alluvium, transported, and felsic lithologies are all altered to kaolinite (Fig. 1 and 2). Differences in the spectral features of kaolinite allow these rock types to be distinguished on an Al-OH peak position versus Al-OH width and Al-OH depth plots (Fig. 3C and D). The base of hematite delineates the bottom of the transported lithology and the top of Archean (Fig. 1 and 2). The spectral features of the kaolinite and the detection of nontronite and hematite make the VNIR-SWIR a valuable tool for the geologist to detect the lithology in the regolith at the Kalpini prospect.
Figure 1 – Schematic cross-section through the Kalpini prospect highlighting the geology and alteration using geologist logs supplemented with VNIR-SWIR and pXRF results. The top of the Archean is difficult to identify in the regolith environment, however, with pXRF and VNIR-SWIR results it was distinguished more clearly. In particular, the weathered mafic lithology in the north was not identified during the geologist logging but was highlighted by its distinct geochemistry and occurrence of nontronite using pXRF and VNIR-SWIR results respectively (see Fig. 2 and 3).
Figure 2 – Drill hole through regolith at the Kalpini prospect showing the initial geologists log and the final log supplemented with pXRF and VNIR-SWIR results. The lithology geochemsitry (see Figs. 3A and B), occurrence of nontronite, and kaolinite spectral features (see Figs. 3C and D) allowed the rocks lithologies and the top of Archean to be better distinguished. Abbreviations: Kao 1 – kaolinite 1, Alluv. – alluvium.
Figure 3 –A and B. Portable XRF Zr, Ti, and V results for discriminating lithologies can be used in the regolith environment where rock types are difficult to distinguish. C and D. In the regolith felsic, transported and alluvium lithologies are altered to kaolinite and can be distinguished using spectral features of the kaolinite Al-OH absorption feature. The mafic lithology is detected in the regolith by the occurrence of nontronite.