Report on the activities in the ruby project 2011-2013

This project on the evaluation of Greenlandic rubies is part of a collaboration project between the Ministry of Industry and Mineral Resources (MIM), Greenland, and the Geological Survey of Denmark and Greenland (GEUS), Denmark. The investigations are mainly aimed at the characterisation of the Fiskenæsset complex rubies, because the most promising ruby outcrop in that area, Aappaluttoq, is likely to be mined in the nearest future by the company True North Gems Greenland (TNGG). In the third year of the three-year long project on Greenlandic rubies the activities concentrated on the analytical investigation of the rubies collected in the Fiskenæsset complex in the summer of 2011 and on sapphire grains from other corundum outcrops in Greenland. Furthermore, it has been tried to apply other fingerprinting methods to the samples from the Aappaluttoq. This study focusses on ruby (red corundum), but also includes some data on sapphire (mainly bluish corundum). In the summer of 2011 circa 50 rock samples from 8 localities in the Fiskenæsset complex were collected with the aim to characterise the Fiskenæsset complex ruby occurrences using gemmological, geochemical and optical investigations (see Kalvig & Keulen, 2011).

From the collected material 32 rock samples were selected. These samples were crushed and the rubies larger than 2 mm were collected and described for their colour, transparency, size and other characteristics (see Kalvig & Keulen, 2011; Keulen & Kalvig 2013). Five of these samples and twelve samples from the GEUS archives have been investigated with laser ablation sector-field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) at GEUS in order to investigate the trace element distribution. Furthermore, four Greenlandic sapphire localities have been reinvestigated after preliminary investigations by Kalvig and Frei (2010) in order to have the same analytical quality on all Greenlandic samples. Both sets, rubies and sapphires, were compared to samples from international occurrences. These investigations have been reported in Chapter 2.

Trace element investigations with a LA-SF-ICP-MS instrument appear to be a promising tool in the fingerprinting of the Greenlandic ruby. Based on the trace elements Si, Ti, Cr, Fe, and Ga for the highest quality samples from Aappaluttoq, it seems possible to distinguish these from 25 internationally known ruby occurrences. For a most effective fingerprinting a combination of trace element analyses, oxygen isotope analyses (see Kalvig & Keulen, 2011) and other studies need to be performed with for example multivariate statistics. It should be stressed that the ICP-MS study ought to be confirmed on gemstone quality rocks in the future, as trace element patterns are not exactly identical for low and high quality stones.

A blind test has been performed in order to investigate the limitations of fingerprinting the Aappaluttoq occurrence. The sample from the Aappaluttoq occurrence was recognised correctly in this test, as was another sample from the Fiskenæsset complex. The samples from Aappalutoq, in contrast to most other localities in the Fiskenæsset complex, are not very rich in mineral inclusions. Anorthite inclusions, when they are observed, probably are a good characteristic of the Fiskenæsset rubies, as this type of inclusions does not occur often in rubies from other localities (Keulen & Kalvig 2013).

We performed Raman Spectroscopy analyses and fluorescence spectroscopy analyses at the Technical University of Denmark (DTU), which are reported in Chapter 3. Analyses were performed to characterise the Greenlandic rubies with a non-destructive method. Unfortunately, both spectrometry methods do not work very well on ruby samples that are relatively rich in the trace element chromium, as is the case for the Fiskenæsset complex samples and especially the Aappaluttoq sample.

The possibilities that the micro-X-ray fluorescence method provides to investigate corundum samples – element spot analyses and element mapping - have been tested on an instrument at the Roskilde University Center (RUC). The large advantage of the method is that it is non-destructive and works well on polished surfaces. Unfortunately, both techniques cannot be applied to the rubies from the Fiskenæsset complex. The data obtained in the spot analysis has a too low accuracy to be able to use as a fingerprinting tool. The samples from Aappaluttoq are not zoned and therefore did not yield a characteristic element map pattern that could be used to typify individual corundum crystals or the Aappaluttoq occurrence. A description of the micro-X-ray fluorescence method and the obtained results are given in Chapter 4.

A short summary of the earlier results for oxygen isotope analyses on rubies is given in Chapter 5. This is a useful method on raw corundum, but too destructive to be used extensively on polished stones.

Geochemical fingerprinting applying LA-SF-ICP-MS is so far the most promising method for fingerprinting samples. The method needs verification against true gemstone quality stones and a better comparison with samples from other, international occurrences. As geochemical fingerprinting applying LA-SF-ICP-MS is not entirely non-destructive, other possible methods need further investigation as well to establish the best, cheapest and least destructive method for determining the origin of polished stones.