Metamorphism and element redistribution: investigations of Ag-bearing and associated minerals in the Arctic volcanogenic massive sulfide deposit, southwest Brooks Range, Northwest Alaska

Abstract

The Arctic Cu-Zn-Pb-Ag volcanogenic massive sulfide (VMS) deposit is the largest deposit in the Ambler Belt, SW Brooks Range. Electron microprobe and XRF examination of ~200 samples shows appreciable (>0.5 %) Ag present in galena, fahlore, and bornite; each contains variable Ag. Core logging and XRF analyses show that complex elemental zoning is present, consistent with folding but with only minor (cmscale?) spatial metal migration. Arctic galena averages 0.02 - 0.9 wt% Ag and displays a Bi-Ag correlation, indicating a coupled substitution of Bi³⁺ and Ag⁺ for Pb²⁺. Fahlore [(Cu,Ag)₁₀(Fe,Zn)₂(As,Sb)₄S₁₃] contains 0.1 -16 wt% Ag; variable As-Sb contents indicate a range from tennantite to tetrahedrite. Ag increases with Sb; high Ag is only present in tetrahedrite. FeS in sphalerite [(Zn,Fe)S] of ~0.5 to ~13 mol% shows spatial patterns, with higher FeS closer to graphitic rocks. FeS in sphalerite increases with decreasing fS₂; higher FeS sphalerite at Arctic is present with pyrrhotite, arsenopyrite and (Ag-rich) tetrahedrite. Redistribution of Ag took place during regional metamorphism: the original high FS₂-fO₂ VMS assemblage was altered with conversion of barite to Ba silicates (loss of O₂ and S₂). Variable fS₂-fO₂ conditions caused by graphite in host rocks versus barite in VMS rocks caused variable Ag mineralogy.