Evidence for As Contamination and the Partitioning of Pd into Pentlandite and Co + Platinum Group Elements into Pyrite in the Fazenda Mirabela Intrusion, Brazil

Knight, Robert D., Prichard, Hazel M. and Ferreira Filho, Cesar F. (2017) Evidence for As Contamination and the Partitioning of Pd into Pentlandite and Co + Platinum Group Elements into Pyrite in the Fazenda Mirabela Intrusion, Brazil. Economic Geology, 112 (8), 1889-1912. (doi:10.5382/econgeo.2017.4533).

Abstract

The Fazenda Mirabela intrusion in Brazil hosts two zones of mineralization, the Santa Rita Ni-Cu sulfide ore zone occurring predominantly in an orthopyroxenite layer, and an underlying basal platinum group element (PGE) anomaly hosted in S-poor dunite. We show that in the northern and southern (marginal) zones of the intrusion, (Pt,Pd,Ni,Cu)(Fe,Bi,Te)2 minerals are accompanied by As-bearing platinum group minerals (PGM) sperrylite (PtAs2), and irarsite ([RhIrPt]AsS). These As-bearing PGM are extremely rare in the central zone of the intrusion, suggesting that As has been introduced into the margins of the intrusion from the country rocks via crustal assimilation or synmagmatic hydrothermal processes. Other PGM and precious metal minerals (PMM) including hessite (Ag2Te), Pd-Ag tellurides, electrum, and native Au are observed in the Santa Rita ore zone and do not show any lateral variation. In the semimetal- and S-poor dunite hosting the basal PGE anomaly, PGE are predominantly retained in the base metal sulfides and only minor PGE alloys are present. In this S-poor dunite Pt, Au and semimetals partitioned into a fractionated Cu-rich liquid, which formed during the crystallization of monosulfide solid solution (MSS) and was largely removed by high-temperature magmatic fluids. Palladium was not affected by this process and significant concentrations of Pd are identified in pentlandite, suggesting that Pd preferentially partitioned into MSS as it crystallized and subsequently pentlandite as it exsolved. Pyrite in the Santa Rita ore zone is Co rich and contains higher concentrations of IPGE (Os, Ir, Ru) and Rh than pyrrhotite and pentlandite, suggesting that pyrite has not replaced and inherited the PGE concentrations of these sulfides. The Pd-poor nature of pyrite also indicates that it has not replaced Pd-rich pentlandite. It is proposed that the IPGE and Co preferentially partitioned into pyrite as it exsolved from MSS.

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