Applications of Micro-X-Ray Fluorescence (μXRF) Techniques to Ore Formation Questions in Economic Geology
Advances in x-ray optics over the past decade have enabled a new generation of benchtop micro energy-dispersive x-ray fluorescence (μXRF/EDS) instruments to analyze samples with an x-ray spot size of 25 microns or less and detection limits of tens to hundreds of parts per million. The analysis of samples from mineral (ore) deposits may illustrate some of the capabilities and limitations of these instruments applied to geoscience problems. The qualitative elemental analysis process allows the production of detailed elemental maps which, together with line scan profiles and other petrographic and geochemical evidence, leads to a better definition of key chemical, structural and textural features. Importantly, minimal sample preparation is required to identify mineral phases in different sample types. Standardless quantification based on international standards and internal references results in highly reproducible results that allow a semi-quantitative analysis of a wide range of samples. The main objective of this research project is to better understand mineralization processes in a variety of ore-forming environments through the visualization of key geologic parameters such as 1) mineralogical and chemical composition, 2) textures and textural relationship among ore and gangue, and 3) assessment of the timing of mineralization. For this purpose, samples were collected for the study from four deposits or mineralization environments: i) epithermal Au-Ag- (Sn-Se-Te) mineralization of Miocene age from the Rodalquilar deposit in Spain, ii) sediment-hosted Cu mineralization of pre-Cambrian age from the Khoemacau-Zone 5 deposit, Botswana, iii) metamorphic-rock hosted orogenic Au mineralization of uncertain age from Awak Mas, Indonesia, and iv) shallow geothermal and magmatic-hydrothermal alteration in crater lake sediments and altered volcanic rocks of the Adatara volcano in central Japan. Results from a representative sample of breccia vein from the Rodalquilar Au deposit show a complex sequence of alteration stages and mineralizing pulses that start with: i) barren wall rock hydrothermal alteration (Stage 1, white mica and alunite kaolinite) and continue with ii) strong silicification, pyrite and low grade Zn-Cu-Ag-Au-As mineralization (Stage 2), iii) fine grained, amorphous silica (banded) cavity fill with high-grade Au and Sn-W (Stage 3) and iv) late cavity fill of Se and pyrite. followed by Te. Silica precipitates through the evolution of the deposit, but by the time of Stage 3 silica is deposited as finely laminated bands of amorphous silica colloids, which implies formation temperature below 180C. Overall, these observations suggest mineralization from consecutive pulses of magmatic-hydrothermal fluids from cooling and crystallizing magma at depth and decreasing temperature. The study of multiple samples from the Khoemacau-Zone 5 Cu-Ag deposit in Botswana shows that the main-stage Cu mineralization is structurally controlled and associated with hydrothermal activity in an orogenic deformation setting. Chalcopyrite (CuFeS2) and bornite (Cu5FeS4) display morphology, geometry, and timing characteristics of mineralization associated with post-peak metamorphism and deformation (cleavage) and are associated with quartz, illite and, carbonate gangue. Mineralization at Khoemacau is multi-stage and formed over a protracted period, which has broadly been associated with the Damaran orogeny. By contrast, diagenetic (framboidal) pyrite mineralization is a much older age and unrelated to the main Cu-Ag mineralizing event that characterizes the Kalahari Copperbelt. Results from the Awak Mas Au deposit in Sulawesi, Indonesia, show that Au mineralization is also epigenetic and hydrothermal in origin and associated with sulfidation and albitization of the host rocks, which are significantly higher in Fe-rich hosts such as meta-hematitic mudstones and metavolcanic rocks. Mineralogical and textural relations indicate that Au mineralization occurred by sulfidation during a stage (Stage 3) of hydrothermal quartz veining that followed barren Stage-2 carbonate veins. The study of hot crater lake sediments from the Adatara volcano in central Japan presented an example of the type of sample that delivers a limited use of the μXRF instrument. The crater lake sediments are very fine-grained (<0.1 mm) and present a very homogenous mineralogical and chemical composition, consisting of native S, silica, pyrite, and minor clays (i.e., S, Si, Fe, K, Al). The conclusion is that a lack of mineralogical (chemical) heterogeneity, a lack of elements with a higher atomic weight, and a very fine-grained nature of a sample may detract heavily from effective use of μXRF applications. The results from this study illustrate the value of an μXRF/EDS instrument to the discipline of economic geology. Its application to well-selected and representative samples from three different deposit types highlights the potential to achieve a better understanding of the genetic processes of ore mineralization.
Buamono, Hannah A, "Applications of Micro-X-Ray Fluorescence (μXRF) Techniques to Ore Formation Questions in Economic Geology" (2022). ETD Collection for University of Texas, El Paso. AAI30001297.