Exploration of New Zealand mineral deposits and geothermal systems using X-ray diffraction (XRD) and reflectance spectrometry (SWIR): a comparison of techniques

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Simpson, M.P.; Christie, A.B. 2016 Exploration of New Zealand mineral deposits and geothermal systems using X-ray diffraction (XRD) and reflectance spectrometry (SWIR): a comparison of techniques. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/61 v, 45 p.; doi: 10.21420/G2NP4D

Abstract: Low-intermediate sulphidation epithermal Au-Ag deposits of the Hauraki Goldfield and geothermal fields of the Taupo Volcanic Zone, New Zealand, have host rocks that are similarly replaced by various hydrothermal alteration minerals. Common alteration minerals include quartz, adularia, chlorite, illite, calcite, pyrite, mixed layered illite-smectite, smectite, and kaolinite, and from geothermal fields the additional occurrence of alunite, dickite, pyrophyllite and topaz. Many of these alteration minerals can only be identified using analytical techniques, mainly X-ray diffraction (XRD) or short-wave infrared (SWIR) reflectance spectroscopy. Whilst XRD can detect a wider range of minerals, SWIR spectroscopy is a rapid (10’s scans), low-cost, field-based technique which requires no sample preparation and that permits the collection of large datasets of closely spaced samples impractical by XRD. However, SWIR mineral identifications can be less definitive and complicated by mineral mixtures. Spectral parameters (H2O/Al-OH depth ratios) can be used to distinguish smectite, mixed-layered illite-smectite and illite or approximate mineral compositions but can be compromised by the influence of other minerals. For example, the presence of gypsum which can form following sample exposure will result in an underestimation of the H2O/Al-OH depth ratio, but this problem can easily be avoided by physically removing this mineral prior analyses. Careful sampling and the filtering of the data according to mineralogy can minimise these complications. Thus, the overall trends in mineral and/or spectral parameter value from SWIR can, for some surveys, be more significant than individual results. For epithermal deposits and geothermal fields, both XRD and SWIR show broad scale zoned occurrences of smectite, mixed-layered illite-smectite and illite. SWIR H2O/Al-OH depth ratio values (clay rank / ‘crystallinity’) typically increase with depth, but no systematic variation is seen towards mineralised veins. However, SWIR did reveal the occurrence of NH4-illite / buddingtonite surrounding and proximal to the Favona vein. Further studies of alteration adjacent other significant veins is required to establish if the occurrence of NH4-illite / buddingtonite could be used as a mid-scale indicator mineral towards veins. It is also possible from SWIR to approximate the composition of illite and chlorite. Based on the limited data sets for the epithermal deposits and geothermal systems there seems little systematic variation in the composition of illite. Data for chlorite is lacking though its composition in geothermal fields has been noted on occasion to correlate with rock type. For the geothermal fields studied, XRD and SWIR have further detected kaolinite, alunite, dickite and pyrophyllite which have formed from acidic fluids; both steam-heated acid-sulphate and in the case of pyrophyllite that occurs in association with minor topaz and rare alumino-phosphate minerals from past magmatic-hydrothermal condensates. Since SWIR analyses can be routinely made at 5 m intervals the extent of an acid zone can be accurately determined and the appropriate well casing and cements selected in real time to exclude these acidic fluids. In summary, while XRD is a more definitive mineral identification technique that can identify a wider range of minerals, as well as accurately determine the amount of interstratification for mixed layered clays and coexisting clay associations (i.e. illite + smectite), SWIR requires no sample preparation, can obtain a scan in 10 seconds with analyses made in the field. While there is a danger in SWIR to rely on automated mineral identification software, careful evaluation of spectra and calculated spectral values can be very useful in the characterisation of epithermal and geothermal fields. Though the focus has been on epithermal Au-Ag deposits and geothermal systems in NZ, SWIR is being successfully used in the exploration of many other deposits with applications extending beyond exploration and include grade control and environmental applications. (auth)