GEMOC ARC National Key Centre

LASER ABLATION MICROPROBE ICP-MS OF NOBLE METALS IN IMMISCIBLE SULFIDES

Noreen J. Evans^1,2, Brent I. A. McInnes^1,2 and Simon Jackson²

1. CSIRO, Exploration and Mining

2. GEMOC Macquarie
 
 

During a 1994 cruise of the RV Sonne high-K calc-alkaline basalts (shoshonites) were dredged from 1400 m water depth on the eastern flank of the Tanga volcano, Tabar-Lihir-Tanga-Feni island arc, east of New Ireland, Papua New Guinea. The glassy margins of the vesicular flows contain spherical, 15-60µm diameter Cu-rich sulfide globules. The spheres have not crystallized sulfide or oxide minerals which indicates they are quenched immiscible Cu-Fe-O-S liquids, trapped as the basaltic melt became sulfide saturated during eruption. Previously, we reported proton and electron microprobe analyses on these sulfides and their glassy host which allowed calculation of sulfide-silicate partition coefficients (McInnes and Evans, 1996). Herein we present recently acquired laser ablation microprobe ICP-MS data for the platinum-group elements which gives us greater insight into details of the metal distribution.

The sulfide inclusions were analyzed in a 100 micrometer thick section using a UV laser ablation microprobe (LAM)-ICP-MS at Memorial University of Newfoundland, Canada. Standardization was performed using a rapidly quenched NiS fire assay button, prepared from a starting mixture doped with PGE to give a nominal concentration of 200 ppm. Differences in ablation yield between analyses were corrected by internal standardization using sulfur (S), which had previously been determined by electron microprobe microanalysis. Data were collected using a time-resolved data acquisition protocol, allowing signals to be examined and selectively integrated to minimize contributions to the signal from the minerals hosting the sulfide inclusions. In our case, the contribution of metals from the glass is negligible. A NiS fire assay button of reference material SARM-7 was analyzed several times to check precision and accuracy of the technique. Except for Ir and Os (near or below detection limit), and Au (not quantitatively collected by NiS fire assay), precisions (1 ) were better than 20% and mean values were within 10% of certified values.

Table 1. Results of (LAM)-ICP-MS for 10 different sulfide globules (in ppm).
 

We have previously reported elevated Cu and Pd contents in these sulfides, but the current data (Table 1) shows that the levels of all PGE, Au and Ag are extremely high. This result indicates the sulfide liquids are capable of removing vast quantities of metal from the magmatic system. There is significant variability in metal content from grain to grain. For example, Pt content ranges from 203ppm to less than 70ppb. The presence of Pd, Ag and Pt spikes within the signal peak for individual grains implies heterogeneous distribution of PGE. We suspect PGE tellurides may account for the presence of spikes as proton microprobe data revealed 93 ppm Te in the sulfides. The individual signal plots show good correlation between the Cu, Pd Ag and Pt responses, suggesting these elements may be somehow associated in the sulfides.

References: McInnes, B.I.A. and Evans, N.J. Cu- and Pd-rich Immiscible sulfide liquids in submarine shoshonitic lavas from the Tabar-Lihir-Tanga-Feni Island Arc, Papua New Guinea. 13th AGC, Canberra, 1996.