HONGSHIITE, PtCu, FROM ITABIRITE-HOSTED Au Pd Pt MINERALIZATION (JACUTINGA), ITABIRA DISTRICT, MINAS GERAIS, BRAZIL

Abstract

Hongshiite occurs associated with palladian gold, and more rarely with native platinum, in the sulfide-free, hematite-rich jacutinga mineralization of the Quadrilatero Ferrifero, central Minas Gerais, Brazil. Hongshiite occurs in grains up to about 1.5 mm in cross-section. The crystals (>0.1 mm) are generally covered by a porous rim from which copper is preferentially leached, eventually forming a rind of nearly pure native platinum. The morphological and chemical features of hongshiite are similar to features caused by the weathering of primary gold in a lateritic environment. Submicrometer-sized crystals, acicular in shape, replace an octahedral precursor phase, interpreted to be magnetite. The magnetite could have acted as a local redox barrier to the passage of oxidizing hydrothermal fluids leading to hongshiite precipitation. Sperrylite, isomertieite, atheneite, sudovikovite, a “guanglinite”-like phase, an undefined copper selenide, and barite occur as euhedral to anhedral inclusions in the larger crystals of hongshiite. Patches of a Au–Cu–Pt alloy within hongshiite have a composition nearly intermediate between tetra-auricupride and hongshiite, and could be called platinian tetra-auricupride, suggesting an extension of a solid solution toward hongshiite. The presence of tiny rosette-like crystals of native palladium in the auriferous jacutinga mineralization is also documented.