Kalungaite, PdAsSe, a new platinum-group mineral from the Buraco do Ouro gold mine, Cavalcante, Goias State, Brazil

摘要

Kalungaite, PdAsSe, is a new mineral discovered in the Buraco do Ouro gold mine, Cavalcante town, Goias State, Brazil. It occurs in a quartz-muscovite mylonite, related to a peraluminous granite, in platy anhedral aggregates along foliation planes. Associated ore minerals are gold, chalcopyrite, bohdanowiczite, an unnamed Pb-Bi-Se-S mineral, clausthalite, guanajuatite, stibiopalladinite, sperrylite and padmaite. Gangue minerals are muscovite, quartz and rare tourmaline and magnetite. Kalungaite is. lead-grey, has a metallic lustre, a black streak and is brittle with uneven fracture. No cleavage was observed. The mineral has a micro-indentation hardness of VHN_(25) = 438 (range of 429-455 kg/mm~2 from five indentations). Under reflected light, kalungaite is cream, or creamy grey adjacent to gold grains, has no internal reflections and is isotropic. Reflectance values in air (and in oil) are: 47.5 (33.3) at 470 nm, 46.9 (32.6) at 546 nm, 46.8 (32.6) at 589 am and 48.0 (34.0) at 650 nrn. The average of eight electron-microprobe analyses gives: Pd 41.32, As 27.49, Bi 0.35, Sb 1.59, Se 27.67 and S 1.22, total 99.64 wt. %, corresponding to Pd_(1.006)(As_(0.950)Sb_(0.034)Bi_(0.004)_(SIGMA 0.988)(Se_(0.908)S_(0.099))_(SIGMA 1.007). Kalungaite is cubic, space group Pa 3-bar, a = 6.089(4) A, V= 225.78 A~3, Z= 4. D_(cal) is 7.59 g/cm~3. The strongest seven X-ray powder-diffraction lines [d in A(I)(hkl)] are: 3.027(75)(002). 1.838(100)(l 13), 1.172(95)(115, 333), 1.077(80)(044, 144, 334), 0.988(70)(116, 235, 253), 0.929(90)(335) and 0.918(70X226). Kalungaite is interpreted as having formed from hydrothermal fluids of granitic origin, during syn-emplacement shearing and alteration, producing an unusual gold-platinum-group element deposit.