Microscale data to macroscale processes: a review of microcharacterization applied to mineral systems

Abstract

Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.

Structural and geochemical ore-forming processes in deformed gold deposits: towards a multiscale and multimethod approach

“…11; e.g. Huston et al, 1995;Morey et al, 2008;Gourcerol et al, 2018b;Pearce et al, 2018;Voute et al, 2019;Cugerone et al, 2020;Combes et al, 2021). In that sense, more and more studies couple micro-textural or -structural ore characterisation via techniques such as HRXCT, 3D neutron tomography and EBSD with in situ geochemical analyses through hyperspectral X-ray computed tomography (e.g.…”

“…In a given deposit, sulphide mineral texture and geochemical signature may thus indicate gold input and crystallisation-remobilisation stages (e.g. Velasquez et al, 2014;Fougerouse et al, 2017;Pearce et al, 2018;Gourcerol et al, 2018aGourcerol et al, , 2018bGourcerol et al, , 2020Kontak et al, 2018;Large and Maslennikov, 2020;Combes et al, 2021) and could be used as pathfinders for high-grade gold ore (e.g. Kadel-Harder et al, 2020).…”

“…When coupled to the microstructural and -textural characterisation of ore-bearing mineral assemblages, such geochemical datasets allow to reassess ore genetic models (e.g. Velasquez et al, 2014;Gourcerol et al, 2018aGourcerol et al, , 2020Pearce et al, 2018) and integrate micro-deformation-induced geochemical ore-forming processes into the deposit-and district-scale deformation history (e.g. Dubosq et al, 2018;Sayab et al, 2020).…”

“…In support of conventional field and microscopy work, the analysis of microstructures and/or mineral geochemistry via laboratory analytical techniques is therefore critical to the understanding of orebodies as it offers information on structuralgeochemical ore relationships (Pearce et al, 2018). Through the example of the Galat Sufar South (GSS) gold deposit, northeastern Sudan, we show how the coupling of a classical field-and microscopy-based petrological-structural approach to laboratory 3D micro-imagery (high-resolution X-ray computed tomography, HRXCT), microstructural analysis (electron back-scattered diffraction, EBSD) and in situ geochemical investigation of the gold-bearing sulphide assemblage (laser ablation inductively-couple plasma mass spectrometry on pyrite, LA-ICP-MS) helps (i) zooming in structural control on mineralisation from the district and deposit-scales to the microscopic scale and (ii) bringing knowledge about deformation-induced geochemical gold mineralisation processes.…”

“…Through the example of the Galat Sufar South (GSS) gold deposit, northeastern Sudan, we show how the coupling of a classical field-and microscopy-based petrological-structural approach to laboratory 3D micro-imagery (high-resolution X-ray computed tomography, HRXCT), microstructural analysis (electron back-scattered diffraction, EBSD) and in situ geochemical investigation of the gold-bearing sulphide assemblage (laser ablation inductively-couple plasma mass spectrometry on pyrite, LA-ICP-MS) helps (i) zooming in structural control on mineralisation from the district and deposit-scales to the microscopic scale and (ii) bringing knowledge about deformation-induced geochemical gold mineralisation processes. Pearce et al (2018) reviewed principles and applications of numerous laboratory techniques, including HRXCT, EBSD and LA-ICP-MS used in this study, enabling the micro-characterisation of metallic ore systems from the tens of cm-to the nm-scales. As the reader may consult the latter reference for further information, this section briefly reviews the insights of these three techniques into geoscience and more especially into ore geology.…”