On the Association of Palladium-Bearing Gold, Hematite and Gypsum in an Ouro Preto Nugget

Abstract

An ouro preto (black gold) nugget from Gongo Soco, Minas Gerais, Brazil, has a mineral assemblage of hematite and gypsum hosted by Pd-bearing gold. The hematite inclusion is microfractured and stretched. Scattered on the surface of the gold is a darkcolored material that consists partially of Pd-O with relics of palladium arsenide-antimonides, compositionally close to isomertieite and mertieite-II. The Pd-O coating has considerable amounts of Cu, Fe and Hg, and a variable metal:oxygen ratio, from O-deficient to oxide-like compounds. The existence of a hydrated Pd-O compound is suggested, and its dehydration or deoxygenation at low temperatures may account for the O-deficient Pd-rich species, interpreted as a transient phase toward native palladium. Although gypsum is a common mineral in the oxidized (supergene) zones of gold deposits, the hematite-gypsumbearing palladian gold nugget was tectonically deformed under brittle conditions and appears to be of low-temperature hydrothermal origin.Keywords: ouro preto, hematite-gypsum mineral assemblage, Pd-bearing gold, Pd-O coating, isomertieite, mertieite-II, Gongo Soco, Minas Gerais, Brazil. SOMMAIREUne pépite de ouro preto (or noir) provenant de Gongo Soco, Minas Gerais, au Brésil, contient l'assemblage hématite + gypse dans un hôte d'or palladifère. L'hématite en inclusion est microfissurée et étirée. La surface de la pépite est recouverte d'un matériau foncé qui correspond en partie à une gaine de Pd-O avec des reliques d'arséniures et d'antimoniures de palladium dont la composition ressemble à celles de isomertieïte et mertieïte-II. Cette couche à Pd-O contient des quantités importantes de Cu, Fe et Hg, et montre un rapport variable de métal à oxygène, allant de matériau déficitaire en oxygène à des composés ressemblant à des oxydes. A notre avis, il y a des indications d'un composé Pd-O hydraté, et sa déshydratation ou sa perte en oxygène à faibles températures rendrait compte des variétés riches en Pd et appauvries en O, que nous interprétons comme phases transitoires dans une progression vers le palladium natif. Quoique le gypse est un minéral répandu dans la zone oxydée (supergène) des gisements d'or, la pépite d'or palladifère à hématite-gypse a été déformée tectoniquement sous conditions cassantes, et semble avoir une origine hydrothermale à faible température.(Traduit par la Rédaction)

Platinum enrichment at low temperatures and related microstructures, with examples of hongshiite (PtCu) and empirical ‘Pt2HgSe3’ from Itabira, Minas Gerais, Brazil

“…Another omnipresent feature is the presence of microcracks, either natural or induced by the incidence of the electron beam (e.g. Oberthür, 2002; Cabral et al. , 2003; Oberthür et al.…”

“…McDonald et al. , 1999; Cabral et al. , 2003), in which native Pd is the stable phase of Pd under supergene conditions.…”

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

“…Gold grains found in 'jacutinga' are typically palladiferous. This gold commonly has dark-coloured areas, composed in part of Pd-O-bearing coatings, and represents the historically famous ouro preto (Johnson and Lampadius, 1837;Jedwab and Cassedanne, 1998;Cabral et al, 2003a, and references therein).…”

Palladian Gold: Chemical Composition, Minerals in Association, and Physicochemical Conditions of Formation at Different Types of Gold Deposits

“…The Hg content varies in the range of 0.07 to 13.9 wt.%, and the Ag content is lower at 2.2 wt.%. Native gold with Pd and Hg impurities and higher Ag contents is spread at many other deposits-Serra Pelada (Brazil) [97], Gongo Soco (Brazil) [78,98], Mayat river and Bol'shaya Kuonamka river (Russia) [47], and Dziwiszow (Poland) [65].…”

“…The palladian gold is characterized by a variable Pd content reaching up to 6 wt.% and sporadically observed Hg content up to 1 wt.% [97]. Pd,Hg-bearing gold at these deposits can also contain Cu and Ag impurities to 3 and more wt.% [78]. Palladian gold contains inclusions of platinum group minerals (PGMs): isomertieite, mertieite-II, chrisstanleyite, selenides Pd 5 (Hg,Sb,Ag) 2 Se 6 , and (Pd,Sb,Ag,Hg) 5 Se 4 .…”