PMGs in the Low-Sulfur Regions of the Giant Norilsk Ni-Cu-PGE Deposit, Sibera [presentation]
This presentation was delivered as a keynote presentation at the International Platinum Symposium Online in May of 2022.
The talk focuses on the relationship between Platinum Group Minerals (PGMs) and Bubbles, Sulfides and Chromite in the Norilsk deposit in Siberia. We use microXRF technologies and microCT technologies to study the relationships between these rare phases and the larger textures and mineralogy of the samples. More information on this research can be found in the publications; Schoneveld et al 2020 and Gritsenko et al (in prep).
Full abstract:
PGMS IN THE LOW SULFUR REGIONS OF THE GIANT NORILSK NI-CU-PGE DEPOSIT, SIBERIA
Louise Schoneveld1, Stephen J. Barnes1, Belinda Godel1, Margaux Le Vaillant1, Marina A. Yudovskaya2,3,
1Mineral Resources, CSIRO, Kensington, WA, Australia, 6151
2Institute of Geology of Ore Deposits, Mineralogy, Petrography, and Geochemistry, Russian Academy of Sciences , Moscow 119017, Russia
3 CIMERA, University of the Witwatersrand, Wits 2050, South Africa
The orebodies of Norilsk contain some of the world’s most valuable accumulations of Ni, Cu and platinum-group elements (PGEs). The PGEs occur throughout the deposit, but are particularly abundant within the upper “taxitic” zones i.e zones with variable grain size distributions, reworked country rock xenoliths, and mm to cm-scale irregular spinel-rich aggregates that occur as patches or disrupted seams and stringers. These taxitic gabbrodolerites contain ore-grade PGE concentrations alongside 2-3 wt.% magmatic sulfides, associations that are referred to as low-sulfide PGE-rich mineralisation. A key feature in these units are the spherical and sub-spherical features that we interpret as former gas bubbles, partially or entirely filled with low-temperature hydrothermal minerals, and locally also with magmatic phases including sulfide globules. In several instances these spherical features form clusters that are interpreted as former bubble foams and are commonly rimmed by spinel, preserving the relic spherical structure. Understanding the relationships between sulfide, gasses, spinel and PGM formation is crucial in discovering where to explore for these resources.
The wetting relationships between vapour bubbles, silicate melt, sulfide liquid, platinum-group minerals (PGMs), and oxides were visualised in detail using 3D µX-ray tomography and 2D X-ray fluorescence maps. These microanalytical methods reveal a short-range spatial variability in silicate and oxide minerals and visualisation in 3D allows for greater understanding of where the sparse PGMs occur within the full context of the surrounding minerals.
The PGMs do not appear to be concentrated in or on the edges relic bubbles or within the large sulfide accumulations in this zone, but are more commonly observed within the chromite stringers. It is likely these PGMs were transported into these zones with a PGE enriched sulfide which has since degassed in these low-pressure systems. This degassing released extremely large volumes of volatiles during the formation of the Siberian Traps Large Igneous Province and Norilsk, which is thought to have played a major role in the Permian–Triassic mass extinction event.
Figure 1: Sample of a “bubble foam” from the Kharaelakh upper taxitic layers. Sample KZ 1672-1910. A) an Australian Synchrotron XFM image with K; red, Fe; green and Sr; blue. Large feldspar grains form a framework with the bubble foam occupying the space between with PGM locations circled in yellow. B) HRXCT cross section through this sample, displaying the spinel (bright) forms almost completed spherical bubbles C) a cross section surface through a bubble showing a large sulfide encased within D) a block section of E,F. E) surfaces of the spinel grains from section D. F) Where large sulfide (yellow) are present, they are contained almost entirely within the bubbles. Red are platinum group minerals G) The relic bubbles are rimmed with spinel and preserve the almost completely spherical bubble shapes. Blue colouration to highlight the sphericity of the spinel bubble. H) image G removing the blue colouration in void space[1]
[1] SCHONEVELD, L., BARNES, S.J., GODEL, B., LE VAILLANT, M., YUDOVSKAYA, M.A., KAMENETSKY, V. & SLUZHENIKIN, S.F. (2020) Oxide-Sulfide-Melt-Bubble Interactions in Spinel-Rich Taxitic Rocks of the Norilsk-Talnakh Intrusions, Polar Siberia. Economic Geology, 10.5382/econgeo.4748.