Oral Presentation 26th ACMM “2020 Visions in Microscopy”

Hyperspectral cathodoluminescence mapping of natural apatite crystals (#15)

Karsten Goemann 1 , Jeffrey A Steadman 2
  1. Central Science Laboratory, University of Tasmania, Hobart, TAS, Australia
  2. Centre for Ore Deposit and Earth Sciences, University of Tasmania, Hobart, TAS, Australia

Apatite [Ca5(PO4)3(OH,F,Cl)] is a common accessory mineral in ore deposits, and panchromatic or colour cathodoluminescence (CL) images are frequently used to constrain the formation and geological history of rocks, in particular regarding fluid evolution and hydrothermal alteration [1,2]. Natural apatite contains trace elements such as the rare earth elements (REE) which substitute for Ca ions in the crystal lattice and can produce characteristic sharp CL peak emission series (e.g. Nd3+, Sm3+, Dy3+) or broader emissions (Mn2+, Eu2+, Ce3+), see e.g. [2,3]. Hyperspectral CL maps with a spectral range of 300-1000nm were acquired using a JEOL JXA-8530F Plus electron microprobe equipped with a JEOL xCLent V CL spectrometry system on a range of natural apatite crystals from drill holes within a 5 km radius around the Ernest Henry iron oxide copper gold (IOCG) deposit in NW Queensland, Australia. In spite of that proximity the resulting CL maps show a wide range of textural features and colours ranging from yellow over green and blue to violet, with additional peaks in the near infrared and near ultraviolet. The corresponding CL spectra are complex, requiring the fitting of up to >40 different sharp and broad Gaussian peaks for a complete spectral description. By comparing the spectra with the available literature data [2,3] we can assign some of the peaks to REEs like Nd3+, Sm3+, or Dy3+ with reasonable confidence, whereas many of the broader peaks are either ambiguous due to conflicting literature data or substantial peak overlaps (such as potential candidates for Mn2+, Eu2+, Ce3+), or can't be assigned at all. The maps will be compared with laser ablation ICP-MS spot analyses and maps in an attempt to further constrain the nature of the spectral variations.

  1. Farhad Bouzari, Craig J.R. Hart, Thomas Bissig, and Shaun Barker (2016): Hydrothermal Alteration Revealed by Apatite Luminescence and Chemistry: A Potential Indicator Mineral for Exploring Covered Porphyry Copper Deposits. Economic Geology 111, pp 1397-1410.
  2. U. Kempe and J. Götze (2002): Cathodoluminescence (CL) behaviour and crystal chemistry of apatite from rare-metal deposits. Mineralogical Magazine 66, pp 151-172.
  3. Michael Gaft, Renata Reisfeld, and Gerard Panczer (2015): Modern Luminescence Spectroscopy of Minerals and Materials, second edition. Springer Mineralogy, 606p. ISBN 978-3-319-24765-6.