Metamorphism and element redistribution: Investigations of Ag-bearing and associated minerals in the Arctic volcanogenic massive sulfide deposit, southwest brooks range, Northwest Alaska.

摘要

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 (cm scale?) 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.