Palladian gold and palladium arsenide–antimonide minerals from Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

Self Cite

Hematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe2O3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Cauê (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as ‘jacutinga’), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to ∼1.3 wt.% TiO2) is lepidoblastic and defines a pervasive tectonic foliation (S1). In contrast, Ti-free, chromiferous hematite (to ∼6.4 wt.% Cr2O3) characteristically occurs as inclusions in palladiferous gold within S1-truncating ‘jacutinga’. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting ‘jacutinga’. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins (‘jacutinga’).

Section: Study Areas and Geological Settingmentioning

confidence: 99%

Evidence for metre-scale variations in hematite composition within the Palaeoproterozoic Itabira Iron Formation, Minas Gerais, Brazil

Cabral¹,

Lehmann²,

Galbiatti³

et al. 2006

Self Cite

“…obtained from sample K9 showed the mertieite-II structure, and the other three samples analysed, which have very similar chemical compositions, are also assumed to be mertieite-II. Mertieite-II has been found in numerous PGEbearing deposits as free grain concentrate, as inclusions or as complex intergrowths of 0.1À0.5 mm size (mostly <0.25 mm) (Cabri, 2002, and references therein;Cabral et al, 2002;Augé et al, 2002;Gervilla and Kojonen, 2002). The occurrence of an idiomorphic crystal of mertieite-II as a mm-sized crystal is first reported here.…”

Section: Mertieite-iimentioning

confidence: 67%

Euhedral crystals of ferroan platinum, cooperite and mertieite-II from alluvial sediments of the Darya river, Aldan Shield, Russia

Shcheka¹,

Solianik²,

Lehmann³

et al. 2004

Self Cite

Idiomorphic crystals (up to 3.5 mm) of ferroan platinum, cooperite and mertieite-II were found in a heavy-mineral concentrate from stream sediments of the Darya river in the Aldan Shield, Russia. Pt-Fe crystals display cubic and thin platy habits; occasionally they are twinned. The chemical composition ranges from Pt2.64Fe1.00 to Pt2.88Fe1.00 with Os, Ru, Ir, Rh and Pd below the analytical detection limit of the electron microprobe. X-ray diffractometry of Pt-Fe crystals suggests a F-centred cubic lattice, characteristic of ferroan platinum. Some of the ferroan platinum crystals have large (about 100 μm wide) cooperite overgrowth rims or are covered by a Au-Ag alloy. Cooperite also occurs as large euhedral crystals (up to 3 mm across, partly twinned). Crystals of mertieite-II are speckled with μm-sized (2–5 μm) inclusions of sperrylite and intergrown with minerals of cooperite-braggite solid solution, Pt-Pd-Hg alloy, keithconnite and Au-Ag alloy. Fractures along crystallographic planes of the mertieite-II crystals are filled with Pd-Pt-Fe-Sb-As-Hg-Te-Bi-bearing oxides. The coarse-grained PGM from the Darya have a geological setting similar to the Kondyor PGE placer 75 km to the northeast and are probably related to clinopyroxenite-hornblende-magnetite units of Alaskan-/Uralian-type intrusions.

“…However, one Fe?-Hg-Se-bearing Pd-Pt phase was observed as a 1 mm wide thread in a goldkaolinite-specularite in ll. Palladium arsenide-antimonide inclusions mertieite-II [Pd 8 (Sb,As) 3 ] and isomertieite [Pd 11 Sb 2 As 2 ] comprise the characteristic inclusion assemblage of the Gongo Soco gold (Cabral et al, 2002c). Isomertieite is found as coarsegrained (>100 mm), anhedral to subhedral crystals.…”

Section: Mineral Chemistry and Microstructurementioning

confidence: 99%

“…The nugget gold has a more or less constant composition, with 88.9士0.7 wt.% Au, 5.2±0.1 wt.%Pd, 2.7±0.1 wt.% Cu, 2.3±0.1 wt.% Ag and 1.1士0.4 wt.%〇 Hg (n = 12). (Cabral et al, 2002c). Isomertieite is found as coarse grained (>100 mm), anhedral to subhedral crys tals.…”

Section: Mineral Chem Istry and M Icrostructurementioning

confidence: 99%

A two-stage process of native palladium formation at low temperatures: evidence from a palladian gold nugget (Gongo Soco iron ore mine, Minas Gerais, Brazil)

Cabral¹,

Lehmann²

2003

Self Cite

Native palladium is rare and occurs in oxidizing environments (hematite stability field) and consistently has very low contents of other metals. On the other hand, gold may contain significant amounts of alloyed palladium. A nugget of palladian gold, obtained from a specularite-rich auriferous vein ('jacutinga') of the Gongo Soco iron ore mine, Quadrilátero Ferrṍfero, Minas Gerais, allows the definition of a paragenetic sequence of native palladium formation. The cm-sized nugget is brecciated and consists of fragments of palladian gold [Au 0.79 Pd 0.09 Cu 0.07 Ag 0.04 Hg 0.01 ] with inclusions of isomertieite [Pd 11 Sb 2 As 2 ], cemented by kaolinite-hematite-palladian gold infill. The isomertieite inclusions are preferentially fractured and display various degrees of alteration to a Pd-O phase. Native palladium occurs as a crystallized component within the Pd-O phase, with microstructural and chemical features suggesting a desiccation process. It is proposed that native palladium forms by (1) oxidation of a pre-existent palladium mineral to a Pd-O phase, and (2) subsequent crystallization of native palladium from the Pd-O phase, by oxygen loss and dehydration. This process operates in a low-temperature hydrothermal or weathering environment and leads to strong fractionation of palladium and gold.