AMPHIBOLE DOMINANT AU-CU SKARN AT THE PEAK DEPOSIT, EASTERN INTERIOR, ALASKA: DISTAL SKARN RELATED TO HIGH-CL HYDROTHERMAL SYSTEM

Recent exploration efforts in Eastern Interior Alaska have revealed significant ca. 70 Ma Au-Cu-Ag sulfide-amphibole skarn mineralization within metamorphic rocks of the Yukon Tanana Terrane, 20 km SE of Tok, AK. This is both younger and much more Cu-rich than the typical typical ca. 90 Ma Au intrusive related gold deposits in the Tintina Gold Belt (e.g., Ft Knox, Pogo). The largest skarn discovery to date is Contango ORE Inc’s 1.1 Moz Au-Eq Peak Zone deposit, hosted by an argillaceous marble layer in quartzite and micaceous quartz schist. Silicates at the Peak Zone are dominantly high Cl^-, randomly oriented, Fe-rich amphibole, lesser chlorite, and minor (<1%) residual clinopyroxene (Hd_80-90). Amphibole is ferropargasitic in composition and contain 2.5 weight % K₂O and up to 4.2 weight % Cl^-. Late Pb-Zn-Sb veins are most distal and cut schist wallrocks and amphibole skarn. The predominant sulfide mineral is pyrrhotite with lesser arsenopyrite; ore minerals are chalcopyrite, native gold, minor pyrargyrite (Ag₃SbS₃), trace Jonassonite (AuBi₅S₄) and maldonite (Au₂Bi). Native bismuth, galena, hedlyite (Bi₇Te₃) and Joseite-A (Bi₄TeS₂) are also present. The deposit displays a broad Au:Cu zoning that points toward an area lacking drilling which may contain the responsible pluton.

Unmineralized garnet (Ad_38-43)-pyroxene (Hd_80-85) and pyroxene-rich skarn occurs in different fault blocks from the Peak zone and may represent proximal skarn related to a different intrusive center. Amphibolites in the region are true metabasites, display metamorphic fabrics, and lack the high Fe-Cl compositions of the amphiboles in mineralized skarn. Residual clinopyroxene in the Peak zone is rare and most of the hornblende is coarse radial splays that shows no evidence for replacement of pyroxene. Consequently, I propose that the sulfides and hornblende were mostly direct replacements of argillaceous marble and not formed by retrograde alteration of pyroxene-rich skarn. The high Cl-amphiboles indicate the fluids were Cl-rich and presumably carried Cu via Cl complexing, unlike the Cl-poor fluids associated with ca. 90 Ma Cu-poor deposits of the region.