Djerfisherite (K(Na,Cu)₆(Fe,Cu,Ni)₂₄S₂₆Cl) is a potassium-bearing sulfide mineral that occurs in a variety of reducing, alkaline geological settings, including enstatite chondrites, kimberlites, and alkaline–ultramafic complexes. Its complex chemistry, marked by extensive isomorphic substitution among Fe, Cu, and Ni, as well as solid-solution relationships with related minerals like bartonite (KFe₃S₄), renders it a valuable petrogenetic indicator. However, the genetic mechanisms of djerfisherite—whether formed during high-temperature magmatic processes or via lower-temperature fluid-mediated reactions—remain uncertain. To address this, we conducted high-pressure experiments to simulate djerfisherite formation during mantle metasomatism. Using a gas bomb apparatus, we reacted a sulfide mixture (pyrite/chalcopyrite/pentlandite in 5:1:2 ratio) with K–Cl–rich aqueous fluids at 5 kbar and 600°C for 7 days. Two fluid compositions were tested, differing in KCl:K₂CO₃ ratios (1:1 and 1:2). In the 1:1 series, products included potassium carbonates, sulfate glass, iron oxide, residual KCl, and minor, non-stoichiometric sulfides with bartonite-like compositions (KFe₃S₄). Two generations of pyrite were observed, with later grains enriched in Cu and K. In contrast, the 1:2 series yielded potassium sulfides with compositions intermediate between bartonite and djerfisherite, forming a continuous solid-solution series and containing 1–3 wt.% Ni—evidence of substantial elemental substitution. This shift correlates with higher carbonate activity, which promotes djerfisherite stabilization. Our results demonstrate that djerfisherite can crystallize during late-stage, fluid-driven mantle metasomatism through interaction between primary mantle sulfides and K–Cl–CO₃–rich fluids. Critically, the KCl:K₂CO₃ ratio governs sulfide phase stability: carbonate-rich fluids favor djerfisherite, while chloride-dominated fluids stabilize bartonite-like phases. These findings support a fluid-mediated origin for djerfisherite in kimberlites and underscore the role of fluid composition in controlling sulfide mineral assemblages in the Earth’s mantle.

Funding: Supported by state contracts of the Institute of Experimental Mineralogy RAS (FMUF-2022-0001) and the Institute of Geochemistry and Analytical Chemistry RAS (FMMZ-2024-0056/0030).