Search

Author(s): Barkov, AY; Bindi, L; Tamura, N; Martin, RF; Ma, C; Winkler, B; Shvedov, GI; Morgenroth, W | Abstract: Fleetite, Cu2RhIrSb2, a new species of platinum-group mineral (PGM), was discovered intergrown with an Os–Ir–Ru alloy in the Miass Placer Zone (Au–PGE), southern Urals, Russia. A single grain 50 lm across was found. Osmium, ruthenium, and iridium are the main associated minerals; also present are Pt–Fe alloys, laurite, Sb-rich irarsite, Rh-rich tolovkite, kashinite, anduoite, ferronickelplatinum, heazlewoodite, PGE-bearing pentlandite and digenite, as well as micrometric inclusions of forsterite (Fo93.7), chromite–magnesiochromite, and Mg-rich edenite. In reflected light, fleetite is light gray; it is opaque, isotropic, non-pleochroic, and non-bireflectant. We report reflectance values measured in air. A mean of seven point-analyses (wavelength-dispersive spectrometry) gave Cu 13.93, Ni 8.60, Fe 0.10, Ir 28.07, Rh 7.91, Ru 1.96, Sb 39.28, total 99.85 wt.%, corresponding to (Cu1.41Ni0.58Fe0.01)R2.00(Rh0.49Ni0.36Ru0.12)R0.97Ir0.95Sb2.08 on the basis of six atoms per formula unit, taking into account the structural results. The calculated density is 10.83 g/cm3. Single-crystal X-ray studies show that fleetite is cubic, space group Fd3m (#227), a = 11.6682(8) A, V = 1588.59(19) A3, and Z = 16. A least-squares refinement of X-ray powder-diffraction data gave a = 11.6575(5) A and V = 1584.22(19) A3. The strongest five reflections in the powder pattern [d in A(I)(hkl)] are: 6.70(75)(111), 4.13(100)(220), 3.52(30)(311), 2.380(50)(422), 2.064(40)(440). Results of synchrotron micro-Laue diffraction experiments are consistent [a ¼ 11.66(2) A]. The crystal structure of fleetite was solved and refined to R ¼ 0.0340 based upon 153 reflections with Fo . 4r(Fo). It is isotypic with Pd11Bi2Se2 and best described as intermetallic, with all metal atoms in 12-fold coordination. Fleetite and other late exotic phases were formed by reaction of the associated alloy phases with a fluid phase enriched in Sb, As, and S in circulation in the cooling ophiolite source-rock. The mineral is named after Michael E. Fleet (1938–2017) in recognition of his significant contributions to the Earth Sciences.

The magmatic Cu-Ni ± PGE (platinum group elements) sulfide deposit Wetlegs within the troctolitic Partridge River intrusion (PRI) of the 1.1 Ga Duluth Complex (northeastern Minnesota) contains disseminated low-grade Cu-Ni sulfide minerals distributed along its heterogeneous base. The main association consists of magmatic intercumulus pyrrhotite, chalcopyrite, pentlandite, and cubanite. Secondary copper-bearing sulfide minerals (bornite, covellite, yarrowite, and digenite) are found within variously altered troctolites. Several late stage arsenic-rich phases occur in altered troctolitic rocks and underlying metasedimentary footwall hosts (Virginia Formation). They appear as nickeline, maucherite, and diarsenides in the system rammelsbergite-safflorite-loellingite, and sulfarsenides of the cobaltite-gersdorffite solid-solution series. Based on their textural relationships and chemistries a three stage depositional history is suggested, starting with the formation of nickeline, followed by diarsenides enriched in Co (below 600 °C) and the late Co-rich sulfarsenides (between 550 °C and 400 °C) in the troctolitic host rock. Due to the higher Ni-and Fe-contents, higher formation temperatures are proposed for arsenides (below 625 °C) and sulfarsenides (~600 °C) in the recrystallized microveins of the footwall. The restricted appearance in altered troctolitic rocks and underlying recrystallized footwall metasediments (Virginia Formation) suggest a deposition of arsenic-rich phases by late-stage hydrothermal fluids. In addition, the metasedimentary Virginia Formation may be a potential source of elements necessary to form arsenides and sulfarsenides, which is further supported by elevated δ34S values (>9‰) for samples from the arsenide assemblages. The platinum-group minerals (PGMs; sperrylite, stibiopalladinite) and precious element-bearing phases are either associated with primary magmatic sulfides, or hydrothermally altered silicate patches (amphibole, chlorite, sericite, prehnite, carbonates, and serpentine). These associations suggest two sources for PGMs: fractionation of a sulfide-saturated melt, and a remobilization and re-deposition of platinum-group elements (PGEs) in the form of aqueous complexes during the late-stage hydrothermal events.

A second occurrence of the Cu–Pb–Bi sulfosalt larosite, ideally (Cu,Ag) 21 PbBiS 13 , is described from Cu–Ag vein mineralization associated with an amphibolite dyke at Tinnsja, southern Norway. The host assemblage consists of bornite – chalcocite – magnetite – wittichenite (± chalcopyrite, digenite, stromeyerite) and closely resembles that from the type locality Larosite crystals present an acicular morphology ( 20.47 Ag 3.68 Fe 0.22 Pb 1.01 Bi 1.06 S 15.00 calculated on the basis of 15 S atoms.

Nielsenite, PdCu 3 , a new mineral species discovered in the Skaergaard intrusion, Kangerdlugssuaq area, East Greenland, occurs in a tholeiitic gabbro associated with plagioclase, clinopyroxene, orthopyroxene, ilmenite, titanian magnetite, fayalite and accessory chlorite-group minerals, ferrosaponite, a member of the annite–phlogopite series, hornblende, actinolite, epidote-group minerals, calcite, ankerite, apatite and baddeleyite. The mineral is found in composite microglobules principally composed of bornite–chalcocite, chalcocite, along with rare digenite, chalcopyrite, cobaltpentlandite, Co-rich pentlandite, and sphalerite. Associated platinum-group minerals include skaergaardite, keithconnite, vasilite, zvyagintsevite, (Cu,Pd,Au), (Pd,Cu,Sn) and (Pt,Fe,Cu,Pd) alloys and unnamed Au 3 Cu and PdAuCu. Nielsenite occurs as discrete grains or in sulfide-bearing, droplet-shaped to irregular grains that are 5–50 μm (ave: 16 μm) in size. The mineral is steel-grey in color with a metallic luster, a black streak and a sectile tenacity. No discernible forms or faces were observed. Neither cleavage nor fracture was observed, and no micro-indentation measurements were made. The mineral is non-pleochroic and exhibits neither discernible internal reflections nor evidence for twinning. It appears bright creamy white under reflected light. Reflectance values (in %) in air (in oil) are: 57.6 (47.5) at 470 nm, 60.85 (50.8) at 546 nm, 62.8 (53.0) at 589 nm and 66.7 (57.5) at 650 nm. The average result of 11 analyses is (in wt.%): Pd 29.86, Pt 3.08, Au 3.70, Cu 61.96, Fe 0.59, Pb 0.17, total 99.36%. The empirical formula (normalized to 4 apfu ) is: (Pd 0.862 Au 0.058 Pt 0.049 Fe 0.028 Pb 0.003 )(Cu 2.996 Fe 0.004 ) ∑3 or, ideally, PdCu 3 . The mineral is tetragonal, space group P 4 mm , with a 3.7125(8), c 25.62(1) A, V 353.2(1) A 3 for Z = 4. The strongest six lines on the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 2.137(100)(117), 1.8596(70)(200), 1.8337(40)(0014), 1.3126(60)(220), 1.1188(55)(317), and 1.0663 (30)(2214). Nielsenite is considered to be isostructural with synthetic tetragonal PdCu 3 ( P 4 mm ). The mineral is commonly associated with skaergaardite (PdCu), which is considered to form at ~600°C under conditions of fairly high f (S 2 ) (>7 log units). Synthetic PdCu 3 forms at or below 508°C and is characterized by a relatively high degree of Pd–Cu disorder, consistent with observations made for nielsenite. The name honors Troels F.D. Nielsen, geologist with the Geological Survey of Denmark and Greenland.

An investigation of ore minerals from the Thrust-Slice “2400’” of the Musonoi Extension mine, Katanga, Democratic Republic of Congo, has provided new insights into the origin of copper sulfides and selenides. The early sulfides are pyrite, covellite, spionkopite, and digenite, which crystallize, with authigenic quartz, during diagenesis. Metasomatic enrichment in U and Se, which is responsible for the unusual mineralogy of the Musonoi deposit, firstly gave rise to the crystallization of Cu–Pd-bearing trogtalite and various palladium selenides (oosterboschite, verbeekite and unidentified Pd–Cu–Pt–Se phases). Subsequently, Se-rich copper minerals, berzelianite and Se-bearing carrollite formed, followed by the crystallization of Se-bearing digenite. Temperature estimates based on these assemblages indicate at least 200°C. Meteoric alteration affected these primary minerals and mainly resulted in the leaching of copper. Replacement phases are spionkopite, yarrowite, and athabascaite, which formed at temperatures below 100°C. Electron-microprobe analyses show the existence of a complete solid-solution series between high digenite and berzelianite. The variation of the a unit-cell parameter along this solid solution follows the equation y = −0.3446 x 3 + 0.5485 x 2 − 0.0358 x + 5.5758, R2 = 0.96. A similar mechanism of S-for-Se substitution accounts for the precipitation of Se-bearing spionkopite and S-bearing athabascaite. These observations provide new data on the sequence of transformation affecting copper sulfides and selenides at low-temperature, meteoric conditions.

Phase relations in the dry condensed system Cu–Rh–S were determined at 900, 700, 540 and 500°C. At 900°C, the system contains digenite, four rhodium sulfides (Rh 17 S 15 , Rh 3 S 4 , Rh 2 S 3 and RhS ~3 ), three ternary sulfides (CuRh 2 S 4 , Cu x RhS 3+ x and a fibrous sulfide in the range Cu 11.9 Rh 24.6 S 63.6 – Cu 20.4 Rh 19.8 S 59.7 ), Cu–S sulfide melt, and Cu–Rh alloys. At 700°C, the ternary association changes to CuRh 2 S 4 , CuRhS 3 , and CuRhS 4 . All these phases persist with little change at 540°C. At 500°C, CuS appears, and the number of phases is reduced; only CuRhS 4 persists as a ternary phase. Ranges of solid solution for Cu–Rh alloys drop drastically with temperature. The solubility of Cu in Rh sulfides increases moderately as temperature decreases from 900° to 540°C. The solubility of Rh in digenite and Cu–S melt is very limited at all temperatures.

Indium minerals and In-rich phases in Brazil are restricted to Paleo- to Mesoproterozoic tin-bearing within-plate magmatic zones in central Brazil. The most important concentration of indium is in the Mangabeira granitic massif, which is also the type locality of yanomamite (InAsO4·2H2O). The Sn–In-mineralized area of this massif is comprised of Li-rich siderophyllite granite, topaz–albite granite, quartz – Li-mica greisens and a vein of In-rich quartz–topaz rock, with up to 1 wt% In, mainly composed of quartz, topaz, zinnwaldite, arsenopyrite and cassiterite. Accessory minerals are sphalerite, ferberite, lollingite, chalcopyrite, bismuthinite, galena, stannite-group minerals, tennantite, argentite and roquesite. Secondary minerals comprise bornite, digenite, covellite, scorodite, phenakite, native copper, yanomamite, dzhalindite, metazeunerite and rare arsenates, like pharmacosiderite, segnitite, chenevixite, and goudeyite, and unknown Bi, Ba and Sn arsenates. Indium-rich stannite, as well as red In-rich and brown In-poor varieties of sphalerite, also are present. Zinc is replaced by indium, copper and iron in the sphalerite structure, probably according to the scheme Cu + In + Fe = 3Zn. Indium-rich sphalerite intergrown with roquesite, forming a texture interpreted as the product of roquesite exsolution, is documented here for the first time. The data obtained are consistent with the existence of the pseudoternary system stannite – sphalerite – roquesite and of a discontinuous solid-solution between yanomamite and scorodite.

Various platinum-group minerals (PGM), phases rich in the platinum-group elements (PGE), and unusual species occur in the Kirakkajuppura area, Penikat layered complex, Finland. The PGM are intimately associated with actinolite and clinochlore. The base-metal sulfides [chalcopyrite, millerite, bornite, chalcocite, ferroan chalcocite(?), digenite or roxbyite: Cu9S5] are in minor or trace amounts. The observed PGM and rare species are: vysotskite–braggite series [PdS–(Pt,Pd)S], zvyagintsevite [Pd3Pb], a zvyagintsevite-like phase [(Pd,Pt)3(Pb,Cu)], unnamed Pd–Pb oxide [Pd7PbO8 or Pd3.5Pb0.5O4], various Cu–Fe–PGE thiospinels (large grains and unusual micro-aggregates), laflammeite [Pd3Pb2S2], laurite–erlichmanite series [(Ru,Os,Ir)(S,As)2 –(Os,Ru,Ir)(S,As)2], irarsite–hollingworthite series [(Ir,Rh,Ru)As1− x S1+ x – (Rh,Ir,Ru)As1− x S1+ x ; x ≤ 0.2], konderite-like Pb–(Pd)-rich sulfides and a Fe-dominant analogue of konderite [(Cu,Ni,Fe)3+ x (Fe,Pb)(Rh,Ir,Pd)8− x S16; x < 0.5], unnamed Rh1− x (Ni,Fe,Cu)2+ x S3 (0 ≤ x ≤ 0.1), a Pd–Pt–Cu alloy [Pt-rich, Cu-rich palladium: Pd89.42–91.22Pt5.94–6.16Cu2.62–4.64], skaergaardite–hongshiite [(Pd0.92 Pt0.10)∑ 1.02Cu0.98], keithconnite [Pd20(Te,Bi)7], plumbian keithconnite [Pd20(Te6Pb)], unnamed Pd5(As,Te,Sn,Pb)2, Pd-rich, Cu-rich gold [Au76.1–81.1Pd13.7–16.0Cu6.1–6.3Ag1.2–3.4], Zn-rich copper (natural?), unnamed Pb4O(VO4)2, and an undefined Pb-rich iron silicate (hydrous?). In addition, unnamed (Th,Ca)(V5+,Si,P)O4 and associated baddeleyite, barite, and a phosphatian huttonite or thorite occur in a pegmatitic gabbro from Kirakkajuppura. The Kirakkajuppura PGE mineralization probably formed by efficient hydrothermal remobilization and transport of PGE from their “primary” zones [Pd–Pt zones associated with the low-sulfide SJ reef and Ru–Os-mineralized chromitite zones], resulting in the local deposition of the extremely S-poor, PGE-rich assemblage, which is intimately associated with hydrous silicates and is located near the northern end of the Penikat complex.

Milotaite, chemically PdSbSe, is a new palladium mineral species discovered in a low-temperature selenium-rich assemblage at Předbořice, Czech Republic. It was observed in only one polished section, where it occurs as rare subhedral grains not exceeding 25 μm in size. It is embedded in eucairite and tiemannite, and randomly intergrown with bornite and digenite. The other associated phases are chrisstanleyite, fischesserite, native gold, silver-rich hakite, tyrrellite, clausthalite, chameanite and uraninite. The gangue is calcite. The synthetic equivalent of PdSbSe is megascopically silvery grey in color, has a metallic luster and a grey streak. It is brittle with an uneven fracture; no cleavage or parting can be observed. Values of VHN100 (12 indentations on three grains of synthetic PdSbSe) range between 420 and 514, mean 465 kg/mm2, which corresponds to a Mohs hardness of approximately 4½. In plane-polarized light, milotaite is white, isotropic and opaque, and lacks internal reflections. Reflectance data of both milotaite and its synthetic equivalent are provided. The average result of five electron-microprobe analyses of milotaite (two grains) is Pd 34.17, Cu 0.78, Ag 0.35, Sb 38.03, Se 26.38, total 99.71 wt.%, corresponding to Pd0.98Cu0.04Ag0.01Sb0.95Se1.02; synthetic PdSbSe (17 analyses on 4 grains) gave Pd 34.46, Sb 38.86, Se 26.60, total 99.91 wt.%, corresponding to Pd0.99Sb0.97Se1.04 (total atoms = 3). The synthetic PdSbSe has a cubic structure with a 6.3181(5) A, V 252.20 (5) A3, space group P 213, and Z = 4. The calculated density is 8.09 g/cm3. The crystal structure was solved using synthetic PdSbSe. The strongest eight lines in the calculated powder-diffraction pattern derived from the results of the crystal-structure refinement [ d in A(I)( hkl )] are: 2.825(100)(201), 1.905(98)(311), 2.579(81)(211), 3.159(53)(200), 2.233(32)(220), 1.752(27)(320), 1.688(25)(312) and 1.378(18)(412). The mineral is named after Milota Makovicky, University of Copenhagen, for her outstanding investigations of sulfide and sulfarsenide systems with platinum-group elements.

Bornite, chalcopyrite, idaite, covellite, yarrowite, spionkopite, anilite, digenite, djurleite, and chalcocite have been identified in quartz veins from the Vielsalm area, Stavelot Massif, Belgium. The identification of these copper sulfides is confirmed by electron-microprobe analyses. The association of primary sulfides indicates that the bornite-bearing quartz veins crystallized above 300‐350°C, whereas the chalcocite-bearing quartz veins crystallized below 200°C. Relations among sulfides indicate the occurrence of two sequences of transformation, responsible of the formation of secondary sulfides: chalcocite-H ⇒ chalcocite-M ⇒ djurleite ⇒ low digenite or (anilite + djurleite) ⇒ yarrowite + spionkopite ⇒ covellite + oxidation minerals, and bornite ⇒ idaite + chalcopyrite ⇒ covellite + oxidation minerals. The crystallization of chalcocite-M, djurleite, and low digenite or (anilite + djurleite) took place between 103.5 and 72°C. The associations idaite + chalcopyrite, yarrowite + spionkopite, and covellite + oxidation minerals, were produced under meteoric conditions. The source of Cu and of the other elements present in the quartz veins must be sought in the host chloritoid-bearing schists.