PHYSICOCHEMICAL CONDITIONS OF THE PENTLANDITE FORMATION IN Cu-BEARING SULFIDE PARAGENESIS (according to experimental data)

Conditions of the pentlandite formation in Cu-bearing sulfide parageneses of Me9Ss section of the quaternary Cu–Fe–Ni–S system have been studied experimentally. Specimens were synthesized by ampoule technique in three regions: slow cooling from melt and long-term annealing at 550 °C and 760 °C with simultaneous determination of sulfur fugacity by the pyrrhotite method. Mineragraphic, X-ray, micro-X-ray and thermal methods of analysis were used. It is established that introduction of Cu into the system does not essentially change the pentlandite formation process consisting of several stages. The first stage is peritectic reaction between the melt and monosulfide solid solution (Mss) resulting in a quaternary non-annealing solid solution, first established in this work, between heazlewoodite (in the Fe–Ni–S) system and intermediate solid solutions (Iss) (in the Cu–Fe–S). The second stage is the solid-phase peritectoid reaction between Mss and quaternary solid solution leading to pentlandite; in the presence of copper the temperature of this reaction reduces by 20–30 °C against 613 °C in the Fe–Ni–S system. In the specimens annealed at 550 °C, in associations with Cu-bearing phases – lss and bornite – Cu-pentlandite is obtained (up to 11 at.% Cu), which begins to form instead of common pentlandite (up to 2 at.% Cu) under the conditions when lgfS2 reaches 7.8 (atm) up to its complete expelling. It is shown that the Ni contents in pentlandite as well as in Mss increase with sulfur fugacity, which in turn is higher in Cu-enriched systems.