Noront Resources

good stuff NN, here's some of the stuff from Cardiff Universitys studies on PGEs'

Iron meteorites that serve as analogues of the Earth’s core typically contain 10-20 times average CI chondrite values. Fertile mantle peridotites contain 0.005-0.01 times chondrite values and most crustal rocks typically contain <0.001 times chondrite values. Because the PGE are so depleted in the Earth’s crust they can serve as important tracers for addition of extraterrestrial material, as cosmic dust or as meteorites at large impact craters. The PGE have different melting points that range from 3050°C for Os to 1555°C for Pd and can undergo fractionation during melting of the mantle, which makes them important tracers of mantle melt processes. In the upper mantle, PGE appear to be hosted predominantly by intergranular sulphide minerals but alloys may be more important host for PGE in the lower mantle.

When released from the mantle during melting, the PGE are transferred into the crust through intrusion or eruption of mafic magma. The PGE are highly chalcophile and will become concentrated in an immiscible sulphide liquid if this forms in a magma chamber (e.g. Merensky Reef, Bushveld Complex, South Africa), a magma conduit (e.g. Voisey’s Bay, Canada) or in lava flows (e.g. komatiites at Kambalda, Australia). If the sulphide liquid interacts with a sufficient volume of silicate magma, the rock containing the collected sulphide liquid may be sufficiently enriched in PGE (typically >2 ppm Pt+Pd) to merit commercial extraction.

Cardiff hosts a dedicated fire assay laboratory for PGE analysis along with suitable ICP-OES and ICP-MS instruments (set up as part of a JIF award NER/H/S/2000/00862). This is the only facility in the UK to specialize in the analysis of PGE in impact craters and impact ejecta (supported by Leverhulme Trust award F/00 407/K) and it is widely used by collaborators in Europe1, North America2 and Africa3 who lack these facilities. The PGE group have also recently developed a set of synthetic laser ablation ICP-MS standards for the analysis of PGE in sulphide minerals. Cardiff is one of only a handful of laboratories to have applied this new technique to the study of PGE in magmatic sulphide ore deposits.

(1. Institute of Geochemistry, Vienna; and Humboldt Museum, Berlin. 2. Oberlin College; and University of California, Davis. 3Wits University; and University of Cape Town)

PGE geochemistry research is currently divided between the following themes:

Impact craters and impact ejecta

(B) photomicrograph of 2mm wide chondrite fragment surrounded by fine grained impact melt norite.

(A) Upper and lower contacts of boulder-sized (26cm long) chondrite fragment in impact melt.

Dr Iain McDonald is a leading expert on the geochemistry of the PGE in impact craters with over 10 years of experience in this field. Research work has focussed on using the PGE to identify and characterise the type of impacting object (asteroid/comet) at large craters such as Morokweng in South Africa (McDonald et al. 2001; Hart et al. 2002; Maier et al. 2003) and the Woodleigh crater in Australia (Reimold et al. 2003). Similar studies have also been carried out on small-medium sized craters such as Clearwater in Canada (McDonald 2002) and Bosumtwi in Ghana (Dai et al. 2005; McDonald et al. 2007). The most important finding to date has been the groundbreaking discovery of a boulder-sized asteroid fragment in the Morokweng impact crater (Figures 1 and 2) by Maier et al. (2006) that was published in Nature.

Figure 2: (A) Bulk chemical data from chondrite boulder (M3-766.54) and average L and LL chondrites normalised to Mg and CI chondrite (from Lodders 2003). (B) CI chondrite normalised PGE data from chondrite boulder and impact melt samples (M3-147 to M3-291). From Maier et al. 2006.

Dr McDonald has also been involved in international collaborative studies to investigate possible impact signatures coincident with major extinctions, such as those at the Permian-Triassic boundary (Coney et al. 2007) and the Jurassic-Cretaceous boundary (McDonald et al. 2006). Previous work has involved searching for impact signatures in diamictites and other anomalous layers proposed as impact deposits (Huber et al. 2001). Currently, these techniques are being applied to correlation studies of late Archaean – early Proterozoic impact ejecta layers in the Hamersley Basin of western Australia and the Transvaal Basin of South Africa (Figure 3).

Ni-Cu-PGE Ore Deposits

Figure 3: A polished rock slab showing a graded layer of impact spherules at Dales Gorge in the Hamersley Basin of Western Australia. Photograph courtesy of Bruce Simonson.

Figure 4: Sandsloot open pit platinum mine, operated by Anglo Platinum. Looking north.

Another major area for PGE geochemical research is the genesis of Ni-Cu-PGE mineral deposits in layered intrusions and other mafic-ultramafic bodies. The principal focus has been on the Bushveld Complex of South Africa, and particularly the Platreef orebody in the northern limb of the complex (Figure 4). The Platreef is a complex package of mineralised pyroxenites and gabbros, that contain ~8% of the world’s total resource of platinum and research has been aimed at understanding how bulk PGRE concentrations are controlled by different proportions of platinum-group minerals and PGE-bearing sulphides (Holwell et al. 2006; Holwell and McDonald 2006 and 2007; Hutchinson and McDonald in review). This work has been supported by the NERC through a CASE studentship to David Howell, supported Anglo American plc (NER/S/C/2003/11952) and through NERC Isotope Geoscience Facilities Steering Committee awards (NERC IP/839/1104 and IP/909/0506).

A key element of this work has been analyses for PGE in sulphide minerals. Cardiff hosts a facility for the in-situ analysis of PGE and semi-metals (As, Sb, Te and Bi) in sulphide minerals using laser ablation ICP-MS (Figure 5). This facility has been developed by Dr McDonald over the last 3 years and is one of a handful of laboratories worldwide with a track record in applying this technique to magmatic ore deposits (e.g. McDonald 2005; Prichard et al. 2005; Holwell and McDonald 2007).

Figure 5: Time resolved LA-ICP-MS analysis for Ni, Co, Pd and Ir in pyrrhotite (po) and pentlandite (pn) from the Platreef. Note that Pd mimics the profiles for Ni and Co in pentlandite, suggesting that Pd is present in solid solution in pentlandite. Pyrrhotite is a more important host for Ir than pentlandite.