Formation of Chromitite Seams and Associated Anorthosites in Layered Intrusion by Reactive Volatile-rich Fluid Infiltration

Abstract Drilling related to development of the platinum-group element deposit J-M Reef Stillwater Complex returned samples a rare chromitite seam between anorthosite and norite in discordant anorthositic body. Plagioclase core An concentrations are marginally higher modestly reversely zoned on side (average Ancore = 83·8; average – Anrim –1·1) as compared with 82·5; +1·0). The anorthosites also characterized by slightly smaller plagioclase grain size than (1·41 mm 1·54 mm, respectively). chromite can contain single polyphase inclusions orthopyroxene, plagioclase, amphibole, biotite Cl-rich apatite. These other compositional textural features, well inference from bodies Banded series, all consistent chromatographic model precipitation at reaction front protolith reacts aqueous fluid saturated alone. Chromitite formation is modeled using an infiltration metasomatic model, which becomes progressively undersaturated pyroxene it rises into hotter part crystal pile. As this pyroxene-undersaturated moves through noritic protolith, dissolves Cr-bearing orthopyroxene produce anorthosite. Chromite precipitates owing liberation Mg Cr pyroxene. Continuous redissolution reprecipitation occurs dissolution direction flow, collecting lost Owing dissolved mainly neutral divalent cation complex, CrCl(OH)0, solution, but incorporated trivalent chromite, required redox involve concurrent sulfide chromite. This mechanism differs some recent models that themselves replacement not original magma nor formed loss mafic material partial melting. results show need for experimental mineral solubility data T P conditions appropriate upper crustal mafic–ultramafic intrusions.