Platinum group elements, layered mafic-ultramafic intrusions and halogens: A tale of two field campaigns…

The platinum group elements (PGEs: ruthenium, rhodium, palladium, osmium, iridium and platinum) are vital to many modern industrial processes, most notably in fuel production and vehicle emission control (i.e. catalytic converters), as well as key components in numerous forms of technology and fine jewellery.  Regrettably, the PGEs have an average concentration of 0.4 ppb in the Earth’s crust; with the majority of the Earth’s PGE budget locked up in the core (see Mungall & Naldrett 2008).  There is however a few unique places where the PGEs are found at ore-grade levels, i.e. 4 ppm and over, the most significant of which are hosted in layered mafic-ultramafic intrusions (essentially fossilised basaltic magma chambers). The Bushveld Complex in South Africa is the largest of these, with an aerial extent of ~65,000 km2 (over three times the size of Wales), and produces ~75% of the world’s platinum and palladium. Despite decades of research, there is very little consensus on how the PGEs are enriched by several orders of magnitude in these environments to produce an ore-grade deposit. With a rapid rise in demand, coupled with their scarcity within the Earth’s crust, there is an intensifying interest in the formation of mineable ore-grade PGE deposits.

Not so nice weather

Not-so-nice weather on the Isle of Rum. View looking towards the Central Layered Intrusion (and Highland cow!)

My PhD research is based on a long-standing hypothesis that places emphasis on the role of halogens, particularly chlorine, in concentrating PGEs in layered intrusions. The model envisages that halogen-rich fluids flush through large crystal piles whilst the intrusion is cooling, mobilising the PGEs and subsequently concentrating them in a smaller area (see Boudreau et al. 1986; Boudreau, 1999). However, due to the volatile nature and incompatibility of chlorine and the other heavy halogens (i.e. they prefer to exist in a fluid rather than a mineral); there is very little evidence of this process preserved once the fluids have moved through. To address this, I will be utilising the neutron irradiated noble gas mass spectrometry technique (Ruzié-Hamilton et al. 2016), which has unparalleled detection limits for halogens, on PGE-rich samples collected from two layered intrusions in the summer of 2016.

Rum Hallival

View of Hallival and Askival on the Isle of Rum.

The primary set of samples was obtained from the Rum Layered Suite (RLS), NW Scotland, a 60.53 ± 0.08 Myr (Hamilton et al., 1998) layered intrusion. The complex covers much of the central and southern areas of the island and comprises three primary intrusions; the Eastern, Western and Central Layered Intrusions. The Eastern Layered Intrusion consists of 16 individual cyclic units with Cr-spinel seams, usually 2-4 mm thick, often found at unit boundaries (the origin of which is also intensely debated). As well as looking rather spectacular, the seams can be host to PGE concentrations; the Unit 7-8 seam in particular has PGE concentrations of 2-3 ppm (O’Driscoll et al. 2009). The relatively young and unaltered nature of the RLS, plus the not-insignificant PGE contents, makes it an ideal primary location to test the role of halogen-rich fluids in the petrogenesis of PGE in layered mafic-ultramafic intrusions.

KODAK Digital Still Camera

The Unit 7-8 Cr-spinel seam with an anorthosite base and peridotite top. 

A second set of samples were collected from the 2701 ± 8 Ma (Sm-Nd, DePaolo & Wasserburg 1979; U-Pb age 2705 ± 4 Ma Premo et al. 1990) Stillwater Complex, SW Montana, USA. It is host to the J-M reef, the highest grade PGE-enriched horizon known in the world (reef averages 20-25 ppm platinum and palladium). The Stillwater is the layered intrusion on which the halogen-rich fluid model for PGE concentration was developed, and hence provides a fantastic comparison site to the younger and smaller RLS, but has however undergone more significant alteration.

Camping

Camping on the Stillwater Intrusion. 

My visit to the Stillwater Intrusion was part of the Geological Society of America Penrose Conference – Layered Mafic Intrusions and Associated Economic Deposits Meeting held in Red Lodge, Montana, at which I presented a short talk titled “Precious metal concentrations in layered mafic-ultramafic intrusions: insights from halogen and noble gas geochemistry”. The conference included several days of fieldwork (and one nights camping), directed by leading experts on the intrusion. During this time I saw classic localities such as the inch-scale doublet layering, “pillow” troctolite as referred to by Hess (1960), and surface outcrops of the PGE-rich Picket Pin and J-M reef.

Inch scale doublets2

The inch-scale doublets of the Stillwater intrusion.

Pillow up close

“Pillow” troctolite

The samples from Rum will be analysed later this year, and Stillwater samples in 2018. Watch this space!

References and further reading:

Boudreau,  A.E., (1999). Fluid Fluxing of Cumulates: the J-M Reef and Associated Rocks of the Stillwater Complex, Montana. Journal of Petrology, 40(5), 755–772.

Boudreau, A. E., Mathez, E. A., & McCallum, I. S., (1986). Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids. Journal of Petrology, 27(4), 967-986.

DePaolo, D.J ., & Wasserburg, G.J., (1979). Sm-Nd age of the Stillwater Complex and the mantle evolution curve for neodymium. Geochimica et Cosmochimica Acta, 43, 999-1008.

Hess, H.H., (1960). Stillwater Igneous Complex, Montana: a quantitative mineralogical study. Geological Society of America Memoirs, 80, 1-230.

Mungall, J.E. & Naldrett, A. J., (2008). Ore Deposits of the Platinum-Group Elements. Elements, 4, 253–258.

O’Driscoll, B., Day, James M.D., Daly, S.J., Walker, R.J., & McDonough, W.F., (2009). Rhenium-osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly. Earth and Planetary Science Letters, 286(1-2), 41–51.

Premo, W.R., Helz, R.T., Zientek, M.L. & Langston, R. B., (1990). U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma? Geology, 18, 1065–1068.

Ruzié-Hamilton, L., Clay, P. L., Burgess, R., Joachim, B., Ballentine, C. J., & Turner, G., (2016). Determination of halogen abundances in terrestrial and extraterrestrial samples by the analysis of noble gases produced by neutron irradiation. Chemical Geology, 437, 77-87.

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About Amy Kelly

I am currently a PhD student at the University of Manchester. My work is focussed on the potential role of halogens, such as chlorine, in concentrating precious metals in basaltic magma chambers. I am using a variety of techniques, in particular mass spectrometry, to ascertain where any halogen-rich fluids may have originated, as well as tracing their movement and extent to which they can create an ore-grade deposit.
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