Assessing Hydrothermal Alteration Intensity in Volcanic-Hosted Massive Sulfide Systems Using Portable X-Ray Fluorescence Analysis of Drill Core: An Example From Myra Falls, Canada

Abstract

Current portable X-ray fluorescence (pXRF) technology can rapidly and inexpensively yield concentrations of geologically significant elements, typically with instrument detection limits below several tens of parts per million. Based on conventional XRF whole-rock geochemical data, both the Ishikawa alteration index and the chlorite-carbonate-pyrite index increase with proximity to sulfide mineralization at Myra Falls. However, available pXRF technology is typically unable to detect all the elements required to calculate these alteration indices. As a result, there is a need to utilize the elements that are readily detectable using pXRF and apply these to hydrothermal alteration assessment. We propose that Rb/Sr ratios provide a robust proxy for the Ishikawa alteration index and demonstrate that conventional whole-rock XRF analytical results for Rb and Sr can be reproduced using pXRF analysis from drill core surfaces. At Myra Falls, the Rb/Sr ratios vary from <0.1 for least altered rocks, 0.1 to 0.5 for weakly altered rocks, 0.5 to 1.0 for moderately altered rocks, 1.0 to 2.0 for strongly altered rocks, and >2.0 for intensely altered rocks. Downhole profiles of alteration intensity generated from systematic pXRF analysis of drill core surfaces can be used to inform drilling and targeting decisions. The application of the Rb/Sr ratio as a proxy for alteration intensity extends beyond this case study and can be applied to other hydrothermal systems that produce phyllosilicate minerals as alteration products of feldspar.

Vectors to ore in replacive volcanogenic massive sulfide (VMS) deposits of the northern Iberian Pyrite Belt: mineral zoning, whole rock geochemistry, and application of portable X-ray fluorescence

“…360-335 Ma) within a transient transtensional intra-continental pull-apart basin generated on the South Portuguese Zone during the geodynamic evolution leading to the growth of the Variscan orogen in the late Paleozoic . Crustal thinning and magmatic intrusion triggered the hydrothermal circulation responsible for the massive sulfide mineralizing events Oliveira et al, 2004;Mitjavila et al, 1997;Tornos, 2006). A detailed review of the geology and mineralization processes of the IPB can be found in , Leistel et al (1998), Carvalho et al (1999), andTornos (2006) and is briefly summarized in Sect.…”

“…From oldest to youngest, these are the (1) Phyllite-Quartzite Group (PQ), (2) the Volcanic Sedimentary Complex (VSC), which hosts the mineralization, and (3) the Culm Group or Baixo Alentejo Flysch Group. The host VSC consists of a complex bimodal volcanic and shallow intrusive sequence of mantle-derived mafic magmas and crustal felsic magmas, interbedded with mudstone and minor chemical sediments (mainly chert and Fe-Mn-rich sediments) (Munhá, 1983;Mitjavila et al, 1997;Thiéblemont et al, 1998;Tornos, 2006). The VSC in the northern area of the IPB -where the Aguas Teñidas deposit is located (Fig.…”

Vectors to ore in replacive VMS deposits of the northern Iberian Pyrite Belt: mineral zoning, whole rock geochemistry, and use of portable XRF

“…Thus, efforts have been devoted to implementing the use of portable XRF as a fast, costeffective first-stage tool in lithogeochemical exploration in VMS and other mineral systems (e.g. VMS systems, Ross et al, 2014Ross et al, , 2016McNulty et al, 2018McNulty et al, , 2020; komatiite-hosted nickel sulphide deposits, Le Vaillant et al 2014; Au in greenstone belts, Glazley et al, 2011;laterites, Duee et al, 2019), analysing varied materials such as rocks, soils or tills (e.g. Hall et al, 795 2016).…”

“…Therefore, alternative ratios using more robust elements have been proposed to track hydrothermal alteration during exploration by p-XRF in VMS system. For instance, the Rb/Sr ratio is suggested to approximate the behaviour of the Alteration Index (McNulty et al, 2020). At Myra Falls, Rb/Sr ratios vary from <0.1 for least altered rocks, 0.1 to 0.5 for weakly altered rocks, 0.5 to 1.0 for moderately altered rocks, 1.0 to 2.0 for 855 strongly altered rocks, and >2.0 for intensely altered rocks (McNulty et al, 2020).…”

“…Prior to sample analysis, an assessment of the performance of our device was made, particularly on the effect of equipment warm-up, measuring time, distance to sample, water content, and number of analysis per sample following the examples and recommendations of previous works (e.g. Ge et al, 2005;Hall et 255 al., 2013Hall et 255 al., , 2014Bourke and Ross, 2015;McNulty et al, 2020;Laperche and Lemière, 2021); results and discussion of these aspects are presented in Supplementary Material 1.3. Measurements were made using the Cu/Zn mining mode with 30 s analysis time per filter (4 filters) after an initial 15 minute warm-up time at the beginning of each session.…”