Mineralogy and Geochemistry of Cu-Rich Ores from the McCreedy East Ni-Cu-PGE Deposit (Sudbury, Canada): Implications for the Behavior of Platinum Group and Chalcophile Elements at the End of Crystallization of a Sulfide Liquid

Abstract

Copper-rich massive sulfides are an important source of Pt and Pd in magmatic Ni-Cu-platinum group elements (PGE) ore deposits. At the McCreedy East deposit, Sudbury, they constitute a classic magmatic assemblage of chalcopyrite, cubanite ± pentlandite, located in sharp-walled footwall veins. These Cu-rich ores represent the subsolidus (<600°C) exsolution products of intermediate solid solution (ISS) that crystallized (950°–800°C) from a highly fractionated sulfide liquid rich in Pt, Pd, Ag, As, Bi, Cd, Pb, Se, Sn, Te, and Zn. Laser ablation-inductively coupled plasma-mass spectrometry and scanning electron microscope analyses have revealed the distribution of PGE and trace elements in these ores, which is important for a better understanding of the petrogenesis of Cu-rich sulfide deposits and to improve PGE extraction. Chalcopyrite and cubanite are the dominant hosts of Se and Sn, with Co in pentlandite. Lead is hosted by galena and Zn and Cd by sphalerite, with only a small proportion of these elements present at trace level in ISS (now equally distributed between chalcopyrite and cubanite). Platinum, Pd, Au, As, Bi, and Te, however, are not concentrated in the base metal sulfides and are accounted for almost entirely by the platinum group minerals (PGM) assemblage, which is dominated by Pt-Pd-Bi-Te phases, such as michenerite [(Pt,Pd)BiTe] and froodite [PdBi2], with minor sperrylite [PtAs2] and Sn-bearing PGM, such as niggliite [PtSn], paolovite [Pd2Sn], and an unnamed Pt-Sn-Te phase. The PGM form complex, composite grains hosted at the grain boundaries of the base metal sulfides. They typically comprise a core of Sn- or As-bearing PGM (stable at higher temperatures) hosted in Bi-Te-PGM (lower temperatures), which are commonly surrounded by accessory tellurides (Ag, Bi, Pb bearing) and sulfides (galena, sphalerite, and stannite [Cu2FeSnS4]). Primary chloride minerals such as cottunite [PbCl2] and ferropyrosmalite [(Fe,Mn)8Si6O15(OH,Cl)10] also form composite grains with hessite [Ag2Te] and galena. In contrast to much of the previous work at Sudbury, which has invoked the role of late-magmatic and/or hydrothermal fluids in the collection of precious metals, we show that, in this case, PGM in Cu-rich ore have a magmatic origin. Due to the incompatibility of Pt, Pd, Bi, and Te during the crystallization of ISS, these elements became concentrated in a small volume of late-stage S-bearing melt trapped between intermediate solid solutions. A sequence of PGM, followed by accessory tellurides and sulfides, crystallized from this late-stage melt and formed composite grains. Toward the end of crystallization, the small amount of Cl that was soluble in the sulfide liquid crystallized as primary chloride minerals at low temperatures (<500°C), either from the late-stage melt or from an exsolved Cl-rich late-magmatic fluid. Some of the primary PGM have been partially altered by Cl-rich fluids (late magmatic and/or hydrothermal) that leached Bi in preference to Pd to form an unnamed Pd-Bi-O-Cl phase. Many PGM also show scalloped edges and truncated grain boundaries, indicating partial corrosion and dissolution, most likely by late-magmatic/hydrothermal fluids that probably remobilized and deposited PGE in the footwall surrounding the veins.