Phase Relations in the Cu-Fe-S, Cu-Ni-S, and Fe-Ni-S Systems

Abstract

The phase relations in the Cu-Fe-S, Fe-Ni-S, and Cu-Ni-S systems were investigated by silica-tube quenching, differential thermal analysis, and high-temperature X-ray powder diffraction experiments. In addition, portions of the Cu-Fe-S and Fe-Ni-S systems were studied by gold-tube quenching and differential thermal analysis experiments under high confining pressures. At elevated temperatures extensive liquid immiscibility fields span the sulfur-rich region of each of the three systems, whereas homogeneous liquid fields dominate the phase relations in their central portions. The average composition of the Sudbury Cu-Fe-Ni sulfide ore, when projected onto the Cu-Fe-S plane, is accounted for above 860° C by a mixture of copper containing hexagonal pyrrhotite and copper-rich sulfide liquid. Thus at high temperatures a mechanism exists that may be responsible for certain copper-rich segregations observed in this type of ore. The minerals of the Cu-Ni-S system, with the rare exception of millerite, do not occur in Sudbury-type ores. Knowledge of the phase relations in this system is prerequisite, however, for systematic investigations of the complex Cu-Fe-Ni-S system. Applications of the phase relations in the Cu-Fe-S and Fe-Ni-S systems to typical ore assemblages show that extensive reequilibration took place among the sulfides after their initial deposition. The sulfides in Sudbury-type ores commonly have compositions and crystal structures that can be produced in the laboratory only at low temperatures.

Metallogenic fingerprint of a metasomatized lithospheric mantle feeding gold endowment in the western Mediterranean basin

“…11), are more consistent with their formation in a portion of the Fe-Ni-Cu-S system dominated by the early (850-900 °C) crystallization of the quaternary solid solution comprising the compositional range between hzss and iss solid solutions (Peregoedova and Ohnenstetter, 2002). Once cooling, this quaternary solid solution may increase its bulk Ni/Cu ratio breaking down to pentlandite from hzss at ∼760 °C and chalcopyrite (and/or bornite) from iss below 557 °C (Fleet and Pan, 1994;Kullerud et al, 1969;Peregoedova and Ohnenstetter, 2002), explaining the observed pn ± cp ± bn assemblages in the Tallante peridotites (Fig. 4).…”

“…4). At the temperature conditions (850-1050 °C) estimated for the equilibration of this set of mantle xenoliths (Hidas et al, 2016), phase relations in the Fe-Ni-Cu-S system predict that Fe-rich monosulfide solid solution (mss) should be the first solid to crystallize from sulfide melt droplets (Kullerud et al, 1969). If so, the fractionation of such sulfide melts must be governed by the sequential crystallization of monosulfide (mss) and/or intermediate (iss) [(Cu 1 ± x Fe 1 ± y )S 2 ] solid solutions (Ballhaus et al, 2001).…”

“…If so, the fractionation of such sulfide melts must be governed by the sequential crystallization of monosulfide (mss) and/or intermediate (iss) [(Cu 1 ± x Fe 1 ± y )S 2 ] solid solutions (Ballhaus et al, 2001). In this scenario, pentlandite may crystallize either at ∼865 °C as a result of peritectic reaction between Fe-rich mss (pyrrhotite) and residual Ni-rich liquid (Sugaki and Kitakaze, 1998), or at a lower temperature (<610 °C) as the subsolidus re-equilibration product of former mss reacting with heazlewoodite [(Ni, Fe) 3 ± x S 2 ] solid solution (hzss) (Kullerud et al, 1969). However, such a crystallization pathway does not explain the compositions of BMS from Tallante because: (1) their reconstructed bulk compositions have Cu contents (up to 14.8 wt%) exceeding the solubility limit of Cu in mss (maximum of 7.5 wt% Cu in mss structure at 935 °C; Cabri, 1973), and (2) pyrrhotite (the low temperature product of equilibration of the high-temperature mss) does not occur in our composite sulfide inclusions, ruling out the formation of pentlandite by re-equilibration of former mss, either by subsolidus or peritectic reaction (Mansur et al, 2019).…”

Tectono-magmatic controls on decratonic gold deposits

“…The elevated Sr/Y ratios of many intermediate rocks in the CTD (Figs. 3f and S3a), particularly of the Cu/Au ore-related stock/dikes, suggest that the parental mafic magmas of these rocks probably experienced major amphibole (± garnet) fractionation and only limited plagioclase (Kullerud et al 1969). The displacement of some data points outside the MSS field towards higher S contents is due to the relatively large uncertainty associated with S analyses by LA-ICP-MS. (Richards and Kerrich 2007).…”

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

“…Sulfides are included into diamond as hightemperature monosulfide solid solution (Mss), exsolving to Fe-, Ni-, and Cu-rich end members during cooling associated with volcanic exhumation (Kullerud et al, 1969). This exsolution causes fractionation of major and trace elements within the inclusions (Richardson et al, 2001), and unless the inclusions can be extracted and studied in their entirety, this process limits the ability to extract accurate isotopic information for geochronology.…”