NEW GEOLOGIC MAPPING IN WESTERN WRANGELLIA, TALKEETNA MOUNTAINS, ALASKA

As part of a multi-year effort to understand the geologic framework and Ni-Cu-Co-PGE potential of western Wrangellia, geologists from the Alaska Division of Geological & Geophysical Surveys (DGGS) Mineral Resources Section and the University of Alaska Fairbanks completed geologic mapping in the Talkeetna Mountains C-4, C-3, and B-4 quadrangles. This area was previously mapped at a reconnaissance scale and the new work sheds light on the stratigraphy, magmatic history, metallogeny, and deformation of the Wrangellia terrane in the Talkeetna Mountains.

The oldest rocks in the field area are thick accumulations of mafic to intermediate and locally felsic volcaniclastic breccias with volumetrically minor volcanic flows and sedimentary interbeds; these units have volcanic arc trace element geochemistry and we interpret them to be part of the Late Paleozoic Skolai Arc. These are overlain by a heterogeneous sedimentary unit consisting of interbedded sandstone, siltstone, mudstone, conglomerate, and chert. Limestone containing crinoid ossicles and Late Permian Spiriferella and Horridonia brachiopod genera occurs at several stratigraphic intervals within this sedimentary section.

Amygdaloidal metabasalt correlated to the Late Triassic Nikolai Greenstone comprise a sequence of flows at least one kilometer thick in the map area and overlie Upper Paleozoic sediments on a suspect unconformity. This unit records a pronounced shift in trace element geochemistry and magnetic susceptibility patterns, with the lowermost flows having island arc tholeiite composition, and the upper, high magnetic susceptibility flows characterized by within-plate basalt geochemistry and an enriched REE profile. Numerous sills of geochemically similar Late Triassic gabbro ranging from less than one meter up to several hundred meters thick intrude the Upper Paleozoic sedimentary sequence. At the top of the section, structural lows preserve isolated exposures of suspected Late Triassic carbonate, phyllite, and quartzite. Metasedimentary rocks assigned to the Kahiltna assemblage in the northwest corner of the map area are amphibolite grade and are structurally juxtaposed against less metamorphosed Wrangellia rocks.