Cu–Fe Bearing Zinc Sulfide from Laloki Stratabound Massive Sulfide Deposit, Papua New Guinea: Chemical Characterization

Abstract: A strange, unidentified, Cu-Fe bearing zinc sulfide occurs in the Laloki massive sulfide deposit, Papua New Guinea. The mineral is optically uniform in texture but is chemically variable and zoned even within a single grain. Copper contents vary from 0.1 up to 8.85 wt%. Iron reaches 18.31 wt% at maximum and decreases as Cu increases. It is remarkable, however, that the total Fe+Cu remains essentially unchanged between roughly 18 and 20 wt%. Zn and S are least variable, giving 45.85–47.84 wt% and 33.48–34.58 wt%, respectively. Other trace elements such as Cd and Mn are in general less than 0.2 wt%. It is strongly suggested that the mineral in question constitutes a unique Fe-Cu substitutional solid solution series belonging essentially to the Zn–Fe–Cu–S system.
The ideal chemical formula of the solid solution series can well be presented as Zn₁₀(Fe, Cu)₅S₁₅ or Zn₂(Fe, Cu)S₃, where Fe is always greater than Cu. It is intriguing that chalcopyrite blebs are recognizable restrictively only in nearby portions of the Cu-rich end member with the ideal composition close to Zn₁₀Fe₃Cu₂S_15. It has been confirmed by vacuum-sealed heating experiments that this mineral is decomposed to produce chalcopyrite and Fe-bearing normal sphalerite at temperatures below 200C. This would provide another evidence for the existence of such distinct phase as suggested here.