MINERAL OCCURRENCES IN THE TOK RIVER AREA, EASTERN ALASKA RANGE.

The Tok River area (Tanacross A-5 and A-6 quadrangles) of the eastern Alaska Range hosts a variety of ages and styles of mineralization, including stratabound basemetal sulfide, intrusion-related gold-copper-silver, and structurally controlled gold. Stratabound base-metal (Zn-Pb-Cu-Ag-Au) sulfide prospects occur in the Tanacross A-6 Quadrangle, where they are known in part as the Eagle trend. Mineralization is hosted by thin marble beds within a metamorphic package dominated by siliciclastic and rare volcanic protoliths. Sulfide minerals include sphalerite, galena, chalcopyrite, pyrrhotite, and pyrite; associated minerals include calcite, ankerite, siderite, albite, phengite, chlorite, ferro-actinolite, stilpnomelane, Fe-clinopyroxene, Ca-Mn garnet, ilvaite, and barite. Mineral assemblages, compositions, and textures indicate that base-metal sulfide mineralization experienced Permian through Jurassic regional metamorphism, and Pb-isotopic ratios are closely similar to those of Devonian to Mississippian syngenetic deposits of Interior Alaska. We interpret the Eagle trend to be a continuation of the PP-LZ trend of the Delta mineral belt, and that the Tok River area prospects are most similar to sedimentary-exhalative (SEDEX)-type systems. The Hona prospect comprises Au-Cu-Ag mineralization developed in association with a multiphase hypabyssal granodiorite stock of Late Cretaceous (ca. 75 Ma) age. Mineralization occurs with disseminated pyrrhotite, chalcopyrite, trace molybdenite, and secondary biotite within the intrusion, and with disseminated and veinlet-hosted pyrite and chalcopyrite in adjacent metamorphic rocks. Silicification, sodic, and argillic alteration occur locally, but well-developed sericitic alteration is lacking. The prospect is similar in age to the nearby Peak gold skarn and is associated with intrusions of similar composition. The geochemical signature of Hona mineralization (Au-Cu-Ag-As-Bi-Te) is similar to that of the Peak skarn and distinct from the coeval Taurus Cu-Mo-Au porphyry system. Structurally controlled gold prospects of the White Gold trend appear to have formed well after mid-Cretaceous metamorphic cooling. Mafic and felsic dikes of likely Paleogene age occur locally with structurally controlled gold mineralization and are both pre-/syn- and post-mineralization in timing. Advanced argillic alteration affects Au-As-mineralized dikes as well as mineralized country rock; such alteration reflects highly acidic hydrothermal fluids and is inconsistent with an orogenic gold deposit model. If the White Gold prospects were emplaced during the Paleocene, they are similar in age and style to the Hajdukovich property near Delta Junction and may be genetically linked to ca. 55 Ma extensional tectonics and magmatism. [ABSTRACT FROM AUTHOR]