Lithological discrimination of altered volcanic rocks based on systematic portable X-ray fluorescence analysis of drill core at the Myra Falls VHMS deposit, Canada

Abstract

Portable X-ray fluorescence (pXRF) analyzers can rapidly and inexpensively provide chemical concentrations of a range of geologically significant elements, often with instrument detection limits below 10 s of ppm. We compiled conventional XRF lithogeochemical data from Myra Falls to assess Ti/Zr discrimination criteria for volcanic-hosted massive sulfide (VHMS) host rocks at Myra Falls. Robust and discrete Ti/Zr trends for coherent volcanic rock types were identified from this dataset. To test if pXRF analysis could reproduce these trends, two experiments were designed. Single-spot pXRF analyses of pressed powdered drill core samples and three-spot pXRF analyses measured directly on the flat, cut surface of drill core samples were compared to conventional XRF results from the same sample sets. Our results indicate that both pXRF sampling methods reproduce the laboratory based XRF results for Ti and Zr, and that there is no significant improvement in accuracy or precision between drill core powders and unprepared drill core samples. We propose a calibration, estimation of total measurement uncertainty, and data reduction procedure for systematic three-spot pXRF analysis of drill core samples to improve lithological logging of altered volcanic rock types. We suggest that pXRF analysis become a routine part of lithology logging providing, for some key elements, robust and time-efficient chemical analyses with results that can be used to define important, often cryptic, lithological boundaries at Myra Falls. Portable XRF therefore has the ability to improve geological and stratigraphic interpretations, which are vital for developing mineral exploration models, for VHMS deposits and other economic mineral systems.

Introduction

Lithogeochemistry is an effective tool used widely in petrologic, chemostratigraphic and hydrothermal alteration studies of the host rocks of volcanic-hosted massive sulfide (VHMS) deposits, with significant implications for exploration (MacLean and Barrett, 1993; Barrett and Maclean, 1999; Large et al., 2001; Franklin et al., 2005; Galley et al., 2007; Piercey, 2010). Conventional laboratory-based X-ray fluorescence (XRF) analysis is an accepted technique for the acquisition of accurate and precise lithogeochemical data from bulk samples. However, this and other lab-based methods suffer from poor spatial resolution due to the high analytical costs, extensive time lag between sample collection and obtaining laboratory results, and the destructive nature of the sample preparation. Field-portable X-ray fluorescence (pXRF) analyzers allow rapid, non-destructive analysis of drill core materials and are proving to be an effective, on site tool for the fit-for-purpose acquisition of lithogeochemical data in the mineral exploration industry (e.g. Morris, 2009; Peter et al., 2009; Gazley et al., 2011; Ross et al., 2014b; Le Vaillant et al., 2014).

Numerous investigations have been focused on the development of pXRF as an analytical tool in the field of lithogeochemistry. Specifically, studies have aimed to better understand the effect of grain size on analytical results (e.g., Potts et al., 1997; Forster et al., 2011); test the performance of different brands and models of pXRF analyzers (e.g., Goodale et al., 2012; Brand and Brand, 2014; Ross et al., 2014a); and evaluate the application of pXRF in discriminating lithology, alteration and mineralization in ore deposits (e.g., Morris, 2009; Gazley et al., 2011; Gazley et al., 2014; Le Vaillant et al., 2014; Mauriohooho et al., 2016).

Recent studies have focused on documenting the operation, calibration, data quality, and independent testing of pXRF analyzers in the mineral exploration industry (e.g. Fisher et al., 2014; Hall et al., 2014). The majority of previous pXRF studies were conducted on powdered rock samples, a sample preparation step that is time and labor consuming. A small number of studies have examined the applicability of pXRF analysis in VHMS deposits (Peter et al., 2009; Sack and Lewis, 2013; Piercey and Devine, 2014; Ross et al., 2014b; Ross et al., 2016); with only a few studies focused on the acquisition of pXRF lithogeochemical data from unprepared drill core samples. Bourke and Ross (2016) compared pXRF measurements on drill core samples with measurements on the corresponding drill core powders for 27 samples of fine- to medium-grained, mafic to felsic, unmineralized Precambrian volcanic and intrusive rocks from the Abitibi Greenstone Belt of Canada. Their study confirmed that averaging multiple measurements on different spots of the sample yielded, after correction, accurate and precise elemental results for many geological applications.

In this study, we compare conventional XRF lithogeochemical results with data obtained using a pXRF instrument from both powdered samples and the surface of drill core from the H-W member of the Myra Falls VHMS deposit, Canada. We propose a routine pXRF sampling methodology and data processing procedure, then present estimates of correlation with each sample medium to demonstrate that analysis from the surface of drill core samples produces accurate major and trace element concentrations for some key lithogeochemical elements. Our results are based on selected immobile elements and indicate that pXRF analysis can discriminate the protolith of hydrothermally altered volcanic rock types, and provide increased spatial resolution from systematic pXRF down-hole analyses with direct applications to mineral exploration.