GEMOC ARC National Key Centre

POTENTIAL APPLICATIONS OF LAM-ICP-MS TECHNOLOGY IN ECONOMIC GEOLOGY: A PRELIMINARY STUDY OF MOLYBDENITE AND PYRITE.

Phillip Blevin¹ and Simon Jackson²

1. GEMOC ANU
2. GEMOC Macquarie

The laser ablation microprobe (LAM)-ICP-MS offers enormous potential in advancing trace element studies  of minerals and materials through significantly improved detection limits for in situ analysis, and the ability to generate profiles through minerals by progressive ablation.  Molybdenite (MoS2) and pyrite from several ore deposits were examined to ascertain trace element occurrences and abundances, evaluate problems relating to inclusions and zoning, characterise the suitability of MoS2 samples for Re-Os dating, as well as exploring more general applications in ore deposit studies.  The results of the MoS2 analytical campaign are reported on here.

The sulfides were analyzed in 100 micrometer thick sections using a UV laser ablation microprobe (LAM)-ICP-MS at Macquarie University.  In the absence of a suitable sulfide reference material, standardisation was performed using a silicate reference material, NIST SRM 610, with a nominal concentration of 500 ppm of most elements.  Differences in ablation yield between analyses were corrected by internal standardization using Mo (MoS2) and Fe (pyrite) concentrations, which were calculated from stoichiometry or had previously been determined by electron microprobe microanalysis. This protocol is expected to give semi-quantitative results.  Indeed, averaged Re contents determined by this method agreed well with those obtained by previous workers using MoS2 separates from the same deposits. Data were collected using a time-resolved data acquisition protocol, allowing signals to be examined for assessment of zoning and to be selectively integrated to minimize contributions to the signal from inclusions and host minerals.  The elements analysed were Si, Ti, Cr, Fe, Ni, Cu, Zn, As, Se, Zr, Nb, Mo, Pd, Ag, Sn, Sb, Te, W, Re, Au, Pb and Bi.  Samples were sourced from numerous East Australian occurrences as well as from overseas porphyry Mo and porphyry Cu systems.

Trace element behaviour
Re, W, Se, Te, Bi and possibly Sn were found to comprise important lattice substitutions in MoS2.  This is evidenced by the high degree of correlation of these elements with Mo.  Zonation, particularly of Bi, was present in some instances.  More commonly, Bi correlates with either Pb, or with Se and Te.  This is consistent with the occurrence of discrete inclusions of galenobismutite and cosalite (Bi-Pb), ikunolite (Bi-Se), joseite and tetradymite (Bi-Te), bismuthinite and native bismuth.  Transient correlations of Sb and Cu with Bi suggests the presence of bismuthinite-stibnite and bismuthinite-aikinite solid solutions.  Ablation of MoS2 crystals oriented both parallel and perpendicular to the c axis suggest that most inclusions occur along cleavages, but an exsolution origin for some of the inclusions cannot be ruled out.  Au is present as isolated Au° inclusions and in Bi-bearing inclusions.  Pd also reports in some samples.  High Sn registrations at the commencement of ablation runs represent surface contamination during polishing.

Deposit and Metallogenic Studies
MoS2 from chalcophile dominated (porphyry Cu-Au) deposits have higher Re, and lower Bi, W and Sn contents and Se/Te ratios than MoS2 from lithophile (Mo, W, Sn) dominated deposits.  Re and W contents are sensitive indicators of the degree of compositional evolution of the related igneous rocks, and thus provide a useful genetic fingerprint.  The systematic variation of Re contents in MoS2 from different deposit types has implications for understanding the occurrence of Mo and Au in otherwise lithophile dominated ore deposits (and related  granitoids) that are also highly depleted in other siderophile and chalcophile elements.  In particular, the results imply that Re becomes uncoupled from Mo during magmatic differentiation.  Significant Au deposits are known to be related to these igneous rocks.

Assessment of Molybdenites for Re-Os dating
A suggested test for assessing MoS2 samples for Re-Os dating involves comparing their Re contents with their structural polytype.  "3R" MoS2 has been assumed to grow via a screw dislocation mechanism triggered by incorporation of Re.  By inference, low Re "3R" MoS2 crystals are inferred to have lost Re subsequent to crystallisation, and therefore potentially may yield unreliable Re-Os dates.  Results of this study suggest that W and Bi may also occur as lattice components within MoS2.  The degree of structural mismatch of Bi, W and Sn within the MoS2 lattice are on par or higher than that for Re, thus the presence of these elements during crystallisation should trigger 3R polytype formation.  Under these circumstances there should be no (a priori) reason to doubt the veracity of dates obtained from low Re, "3R" MoS2 samples obtained from lithophile dominated deposits.