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Pd–Ag TELLURIDES FROM A Cl-RICH ENVIRONMENT IN THE LUKKULAISVAARA LAYERED INTRUSION, NORTHERN RUSSIAN KARELIA

Andrei Y. Barkov^1,, Robert F. Martin¹, Mahmud Tarkian², Glenn Poirier³ and Yves Thibault⁴

¹ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
² Institute of Mineralogy and Petrology, University of Hamburg, Grindelallee 48, D-20146 Hamburg, Germany
³ Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
⁴ Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

E-mail address: barkov{at}eps.mcgill.ca

Rare Pd–Ag tellurides, telargpalite and the phase Pd₆AgTe₄ occur in pods <0.5 m across and stringers of altered coarse-grained gabbronorite enriched in base-metal sulfides (up to ~25 vol.%) in the Nadezhda deposit, located in a sill-like body of microgabbronorite within the layered series of the Lukkulaisvaara intrusion, Karelia, Russia. The magmatic texture and relics of igneous minerals are preserved in the pods. The associated minerals are various platinum-group minerals (PGM), a rare Re-rich sulfide, and highly aluminous secondary minerals, e.g., almandine, Cl-rich ferropargasite, microcrystalline staurolite, and corundum, which formed at the expense of primary plagioclase and enstatite at a deuteric stage. Micro-inclusions of Cl-rich amphibole are common in various PGM and associated chalcopyrite at Nadezhda. The telargpalite displays an atomic ratio (Pd + Ag):(Te + Bi + Pb) of 3. The Ag–Pd correlation is negative and strong, and implies a limited Ag-for-Pd substitution in telargpalite, the empirical formula being Pd_2–xAg_1+x(Te,Bi,Pb), where 0 < x < 0.3. A limited Pd-for-Ag substitution appears to occur in the unnamed Pd₆AgTe₄. The maximum reflectance of the Pd–Ag tellurides increases with an increase in their Pd:Ag ratio. The telargpalite may cut almandine, and thus seems to have formed at a temperature lower than the temperature of equilibration of garnet–hornblende and garnet–staurolite (~560 to 670°C). The unnamed Pd₆AgTe₄ occurs in a close association with a Cl-dominant analogue of ferropargasite (up to 4.5 wt.% Cl; 1.2 Cl atoms per formula unit). The pods and stringers rich in Pd, Pt and Ag probably formed by crystallization of isolated volumes of H₂O-saturated melt, in situ. The Pd–Ag tellurides formed in a volatile-rich deuteric environment, at relatively low temperatures, and Cl was prominent in the fluid causing the alteration.

Keywords: platinum-group minerals, Pd–Ag tellurides, telargpalite, Pd₆AgTe₄, Cl-rich amphibole, layered intrusion, Lukkulaisvaara, Karelia, Russia, Fennoscandian Shield.

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