Outline of Geology and Mineral Resources of Upper Kuskokwim Region, Alaska

Abstract

Abstract The Paleozoic rocks in the Farewell area consist of a relatively undeformed, shallow-water carbonate platform facies to the west in contrast with coeval polydeformed deep-water basin and flysch deposits to the south. Ordovician to Devonian shallow-water strata overlain by Carboniferous chert and argillite comprise the first of these two assemblages. The second assemblage consists of Cambro-Ordovician limestone and shale, Ordovician chert and shale, Silurian turbidites, and sandstone and limestone of Lower(?) Devonian age. Middle to Late Devonian shallow-water carbonate, chert, and clastic rocks and Triassic mafic igneous rocks overlie older lithologies. Tertiary continental clastic rocks and Late Cretaceous through Tertiary calcaklaline igneous rocks intrude the Paleozoic units. Three periods of folding have deformed layered rocks in the Farewell area, and crustal shortening of at least 50 kilometers has taken place. Right-lateral movement (about 60 kilometers) and vertical displacements occurred along the Farewell fault during Cenozoic time; Quaternary activity is entirely vertical. The McGrath area contains Upper Paleozoic to Triassic chert, argillite, and limestone unconformably overlain by Lower Cretaceous limey shales and siltstone. These rocks are, in turn, succeeded by a coarsening upward sequence of lithic sandstone, shale, siltstone, and conglomerate of late Early to Late Cretaceous age. The 2,400-meter-thick Cretaceous section formed in a northeast-trending, elongate, structurally controlled basin in which deep-water, high-energy slope or fan deposits were succeeded by marginal prograding deltas in a shallow marine environment. Volcanic agglomerate and volcaniclastic sandstone near the top of the Cretaceous section probably indicate orogeny, uplift, and transition to a nonmarine environment in latest Cretaceous time. Intruding and overlying the layered rocks of the McGrath area are zoned alkaline to subalkaline volcano-plutonic complexes localized along major northwest-trending fractures of the Iditarod-Nixon fault system. The tectonic history of the region includes Late Devonian-Triassic continental margin rifting with intrusion of mafic tholeiites and deposition of chert and relatively shallow-water sediments. Following Late Jusassic-Early Cretaceous amalgamation with the Yukon-Tanana block, the Nixon-Iditarod “aulocogen” was infilled with slope and prograding delta and fluvial deposits of the Kuskokwim Group during Early to Late Cretaceous time. By Late Cretaceous time, numerous horsts and grabens became sites of mafic dike swarms, tholeiitic subaerial volcanism, and volcano-plutonic complexes. Regional compression halted further development of the extensional basin. Mineral resources of the study area reflect these tectonic developments. Subduction-related Late Cretaceous-Early Tertiary calc-alkaline igneous activity in the Farewell area produced base-metal skarn, breccia pipe, and stockwork mineralization in plutonic rocks and carbonate strata. Cretaceous igneous complexes of the McGrath area host mineral deposits typical of those found in extensional rift environments worldwide and, in addition to gold, contain anomalous amounts of uranium, zirconium, hafnium, niobium, cobalt, nickel, and tungsten. Chromium-enriched dike swarms in the McGrath area are typical of rift environments. Some dikes later served as favorable structural sites for deposition of mercury and antimony, probably derived from Kuskokwim Group flysch.