Quantitative mineral mapping of drill core surfaces I: a method for µXRF mineral calculation and mapping of hydrothermally altered, fine-grained sedimentary rocks from a Carlin-type gold deposit

<p>Mineral distributions can be determined in drill core samples from a Carlin-type gold deposit, using micro-X-ray fluorescence (<em>µ</em>XRF) raster data. Micro-XRF data were collected using a Bruker Tornado <em>µ</em>XRF scanner on split drill core samples (∼25 × 8 cm) with data collected at a spatial resolution of ∼100 <em>µ</em>m. Bruker AMICS software was used to identify mineral species from <em>µ</em>XRF raster data, which revealed that many individual sample spots were mineral mixtures due to the fine-grained nature of the samples. In order to estimate the mineral abundances in each pixel, we used a linear programming (LP) approach on quantified <em>µ</em>XRF data. Quantification of <em>µ</em>XRF spectra was completed using a fundamental parameters (FP) standardless approach. Results of the FP method compared to standardized wavelength dispersive spectrometry (WDS)-XRF of the same samples showed that the FP method for quantification of <em>µ</em>XRF spectra was precise (R² values of 0.98–0.97) although the FP method gave a slight overestimate of Fe and K and an underestimate of Mg abundance. Accuracy of the quantified <em><em>µ</em></em>XRF chemistry results was further improved by using the WDS-XRF data as a calibration correction before calculating mineralogy using LP. The LP mineral abundance predictions were compared to Rietveld refinement results using X-ray diffraction (XRD) patterns collected from powders of the same drill core samples. The root mean square error (RMSE) for LP-predicted mineralogy compared to quantitative XRD results ranges from 0.91 to 7.15% for quartz, potassium feldspar, pyrite, kaolinite, calcite, dolomite, and illite.</p> <p>The approaches outlined here demonstrates that <em>µ</em>XRF maps can be used to determine mineralogy, mineral abundances, and mineralogical textures not visible with the naked eye from fine-grained sedimentary rocks associated with Carlin-type Au deposits. This approach is transferrable to any ore deposit, but particularly useful in sedimentary-hosted ore deposits where ore and gangue minerals are often fine grained and difficult to distinguish in hand specimen.</p>