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UNGAVAITE, Pd₄Sb₃, A NEW INTERMETALLIC MINERAL SPECIES FROM THE MESAMAX NORTHWEST DEPOSIT, UNGAVA REGION, QUEBEC, CANADA: DESCRIPTION AND GENETIC IMPLICATIONS

Andrew M. McDonald^1,, Louis J. Cabri^2,¶, Christopher J. Stanley³, Nikolay S. Rudashevsky⁴, Glenn Poirier⁵, James E. Mungall⁶, Kirk C. Ross⁷, Bruce R. Durham⁸ and Vladimir N. Rudashevsky⁹

¹ Mineral Exploration Research Centre and Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada
² Cabri Consulting Inc., 99 Fifth Avenue, Suite 122, Ottawa, Ontario K1S 5P5, Canada
³ Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
⁴ Center for New Technologies, Roentgena Street 1, RU-197101, St. Petersburg, Russia
⁵ CANMET/MMSL 555 Booth Street, Ottawa, Ontario K1A 0G1, Canada
⁶ Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
⁷ Mineral Exploration Centre and Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario P3E 2C6, Canada
⁸ Canadian Royalties Inc., 1176 Delnite Road, Timmins, Ontario P4N 7J8, Canada
⁹ Center for New Technologies, Roentgena Street 1, RU-197101, St. Petersburg, Russia

E-mail address: amcdonald{at}laurentian.ca

Ungavaite, Pd₄Sb₃, is a new intermetallic mineral species discovered in the Mesamax Northwest deposit, Cape Smith fold belt, Ungava region, northern Quebec. It is associated with monoclinic pyrrhotite, pentlandite, chalcopyrite, galena, sphalerite, cobaltite, a chlorite-group mineral and magnetite. Associated precious-metal minerals include another new mineral species, naldrettite (Pd₂Sb), sperrylite, sudburyite, michenerite, Au–Ag alloy, altaite, petzite (Ag₃AuTe₄) and hessite (Ag₂Te). Ungavaite occurs as rare anhedral grains with inclusions of Au–Ag alloy or with attached chalcopyrite and a chlorite-group mineral. Grains of ungavaite vary in size (equivalent circle diameter) from ca. 36 to 116 µm, with an average of 73 µm (n = 4). Neither cleavage nor fracture was observed. It is distinctly anisotropic, non-pleochroic, has weak bireflectance and does not exhibit discernible internal reflections. Ungavaite appears bright creamy white in association with pentlandite, pyrrhotite, the chlorite-group mineral and chalcopyrite. Reflectance values in air (and in oil) for R₁ and R₂ are: 50.2, 50.5 (37.6, 38.0) at 470 nm, 55.6, 55.9 (43.2, 43.5) at 546 nm, 57.9, 58.3 (45.9, 46.3) at 589 nm and 60.2, 60.7 (48.1, 48.5) at 650 nm. The average result of 16 electron-microprobe analyses on one particle is: Pd 54.53, Fe 0.13, Te 0.09, Sb 44.59, Bi 0.42, Hg 0.19, and As 0.20, total 100.15 wt. %, corresponding to empirical formula (based on seven atoms) Pd_4.062(Sb_2.893Fe_0.017Bi_0.017Hg_0.006Te_0.005) _2.938, ideally Pd₄Sb₃. The mineral is the Pd-dominant analogue of genkinite. It is tetragonal, crystallizing in one of the possible space-groups P4₁2₁2, P4₁22, P4₃2₁2, P4₂2₁2, or P4₂22. Cell dimensions are: a 7.7388 (4), c 24.145(1) Å, with V = 1446.02(1) ų and Z = 8. The calculated density is 7.264(1) g/cm³. The strongest six lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 3.008(90)(008), 2.268(100)(134), 2.147(30)(230), 1.9404(60)(400), 1.2043(30)(2218, 452), 1.2002(30)(624). The mineral formed in a narrow (Pd + Sb)-rich zone separating massive and disseminated sulfides. It is likely a product of the hydrothermal remobilization of Pd (and possibly Sb) from the pre-existing massive sulfides. Phase relations suggest that ungavaite developed at a temperature below 400°C, possibly through a solid-state order–disorder transformation.

Keywords: platinum-group mineral, ungavaite, Pd–Sb, new intermetallic mineral species, platinum-group elements, hydroseparation, Mesamax Northwest deposit, Cape Smith fold belt, Ungava, Quebec.