Recent research by Kamenetsky et al. published in Geology provides insight into the nature of the parental magmas potentially responsible for the formation of nickel-copper-platinum group element (Ni-Cu-PGE) deposits in intra-plate environments. Previous researchers have illustrated the importance of the mantle-derived magmas and subcontinental lithospheric mantle in the formation of Ni-Cu-PGE deposits (e.g., Arndt et al., 2005); however, finding the nature of occurrence and concentrations of chalcophile elements in primary magmas from the subcontinental lithosphere has been problematic. New research by Kamentsky et al. provide significant insight into this problem.
The work of Kamenetsky studied unusual rocks from the Bouvet Triple Junction of the Mid-Atlantic Ridge. These rocks are unusual and unlike normal basaltic oceanic crust, having continental (i.e., evolved) radiogenic isotope signatures suggesting derivation from the subcontinental lithospheric mantle. Moreover, they are exceptionally well preserved, glassy, and contain sulfide globules within the glasses (Figure 1). It is these sulfide globules that provide critical insight into the nature and composition of chalcophile elements in the subcontinental lithospheric mantle.
Figure 1. Reflected-light photograph sulfide globule in basalt glass from the Bouvet Triple Junction and X-ray maps of elemental distribution within the globule. Hotter colours reflect higher concentrations. From Kamenetsky et al. (2013).
Most sulfide globules within the basalt glass are less than five microns; however, one globule was large enough to allow detailed mineralogical and geochemical study. The large globule contains Fe-Ni sulfides interpreted to be immiscible sulfides that quenched during basalt eruption (Figure 1). The globules also contain Fe-oxyhydroxides, minor Cr-rich magnetite, and micro-nuggets rich in Pt, Pd, Au, and Sn, including: pure Pt; Pt-Sn alloys; Au, Pt-Au, Pt-Au-Sn alloys; and rustenburgite (Pt,Pd)3Sn (Figure 2). In addition, laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analysis of the droplet has elevated chalcophile element contents (46 ppm Pt; 41 ppm Pd; 3 ppm Rh; 15 ppm Ru; 7.5 ppm Os; 14 ppm Au; and 15 ppm Ag); the samples also have chondrite-normalized noble metal signatures broadly similar to PGE-bearing layered intrusive complexes (e.g., Merensky Reef).
Figure 2. Scanning electron images of sulfide globules wtih sulfides (light grey), Fe-oxyhydroxide (dark grey), Cr-magnetite (grey) and nuggets of noble metals (white). From Kamenetsky et al. (2013).
While there are many features that are required to form an economic Ni-Cu-PGE-rich sulfide deposits (e.g., Naldrett, 1997), the results from Kamenetsky et al. illustrate that melts from the subcontinental lithospheric mantle contain Ni, Cu and PGE contents up to 2-times the values present in melts derived from the depleted mantle (e.g., N-MORB). Hence, subcontinental lithospheric mantle melts are charged in Ni-Cu-PGE and therefore are important potential targets for exploration in the ancient geological record (see also Arndt et al., 2005 and Begg et al., 2010).