Characterization of altered mafic and ultramafic rocks using portable XRF geochemistry and portable Vis-NIR spectrometry

Abstract

The accurate characterization of mafic and ultramafic rocks is a challenging but necessary task given the spatial and genetic relationship of mineralization with specific lithologies (e.g. komatiite hosted nickel-sulfides preferentially associated with cumulate-rich ultramafic rocks). Rock classification is further complicated as most mafic and ultramafic rocks have undergone varying degrees of alteration. The accuracy and reproducibility of characterization can be significantly improved by using portable energy dispersive X-ray fluorescence (pXRF) chemical data with portable visible and near-infrared (pVis-NIR) mineralogical data. A new workflow using pXRF and pVis-NIR is presented and used to reliably characterize mafic and ultramafic rocks from the Yilgarn Craton, Western Australia. The workflow involves six steps: (1) Mitigate and identify compound processing and closure issues. For example, we used a pXRF with helium flush to reliably and rapidly measure light elements and mitigate closure, i.e. problems related to data failing to sum to 100%. (2) Identify and exclude geochemically heterogeneous samples. Heterogeneity may be unrelated to alteration and caused by veining or small-scale structure interleaving of different rock types. Geochemical heterogeneity was evaluated using skewness and kurtosis of SiO2 data. (3) Relate rocks from similar magmatic, weathering and alteration events. This was achieved by interpreting data grouping of Vis-NIR ferric and ferrous iron data via a 852 nm/982 nm reflectance v. 651 nm/982 nm reflectance plot and the ferrous abundance index. Unrepresentative data were omitted. (4) Correct XRF iron data, and characterize lithology and alteration. Values ascribed to regions in the TAS (total alkali silica) diagram were used to approximate FeO and Fe2O3. Subsequently, geochemical indices (e.g. Mg#) were used to characterize the alteration box plot. (5) Characterize fractionation in detail. Fractionation variation diagrams were used to interpret fractionation, e.g. MgO v. Al2O3, Ca/Al v. Al2O3, Ni/Cr v. Ni/Ti, and MgO v. Cr. (6) Identify and quantify talc alteration and serpentinization. This included the use of a new alteration plot (Mg# v. 1410 nmRAD/Albedo) to estimate serpentinization and identify relationships between serpentine, carbonate, chlorite and talc abundances. The results and observations contained in this contribution have important implications for progressive technologies such as core logging platforms that are equipped with pXRF and pVis-NIR instruments.