Rua Cove: Anatomy of Volcanogenic Fe-Cu Sulfide Deposit in Ophiolite on Knight Island, Alaska: ABSTRACT

Abstract

Tabular deposits of massive Fe-Cu sulfide and subjacent fracture-and void-controlled sulfide in volcanic breccia represents a sea-floor and subsea-floor hydrothermal system in the Tertiary ophiolitic terrane on Knight Island. The massive sulfide bodies at Rua Cove occur within a sequence of mafic volcanic rocks that include pillow lava, pillow-fragment breccia, mixed volcanic-chert breccia, and hyaloclastite. The principal massive sulfide horizon, composed of pyrrhotite and chalcopyrite with thin partings of talc + chlorite + quartz, is concordant with fine-grained volcaniclastic units altered to an assemblage of chlorite + quartz + sphene + ilmenite. Discordant, diffuse feeder-zone mineralization below the massive sulfide body includes fracture- and void-filling aggregates of pyrrhotite + chalcopyrite + talc + quartz and late-stage veins of quartz + epidote + chlorite + pyrrhotite + chalcopyrite. The formation of talc appears to postdate pervasive chloritization of mafic breccia and hyaloclastite. The thick section of mixed volcanic breccia and hyaloclastite suggests a depositional environment near the base of a steep volcanic edifice. Sulfide deposition was contemporaneous with formation of breccia and hyaloclastite. Rapid burial by fine-grained volcaniclastic material and succeeding lava flows inhibited sea-floor weathering of the Fe-rich massive sulfide. The occurrence of additional nested massive sulfide bodies, isolated fragments of massive sulfide in the volcanic breccia, and sulfide-rich matrix supporting fragments cut by quartz-sulfide veins suggests recurrent hydrothermal activity and sulfide deposition near an active volcanic center. The spatial association with sheeted dikes at depth and intercalated and overlying flysch-type strata of the Orca Group indicate that ineralization occurred in a rift setting near a continental margin. End_of_Article - Last_Page 667------------