Abstract

The study of the distribution of PGE, Au, Ni, Cu, Co, and S in the Proterozoic Niquelandia mafic-ultramafic layered intrusion (Brazil), 65 km long and 35 km wide, shows the existence of two distinct stratigraphic zones of contrasting PGE content. The Ultramafic Unit and the lowermost part of the Central Mafic Unit have relatively high PGE contents, whereas all units above this level have very low PGE contents. Data on sulfur contents, as well as the presence of interstitial base-metal sulfides starting from the intermediate level of the Ultramafic Unit, suggest that the parental magma was close to the saturation level, and that sulfur saturation occurred during fractional crystallization of the magma. The observed distribution of the precious metals is modeled in terms of the depletion of the magma in its PGE content due to the segregation of a sulfide liquid. The transition zone between PGE-undepleted and PGE-depleted rocks is considered to be the most favored stratigraphic level for stratiform PGE deposits in the Niquelandia Layered Intrusion. Similar to many PGE-mineralized units in layered intrusions, the transition zone occurs close to the level at which plagioclase first appears as a dominant cumulus phase; this emphasizes the potential for magmatic PGE deposits at this stratigraphic level in the Niquelandia Layered Intrusion.

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