METALLOGENY OF THE WRANGELLIA TERRANE IN THE TALKEETNA MOUNTAINS, SOUTHERN ALASKA

Late Triassic flood basalts of the Nikolai Greenstone are the defining geologic unit of Wrangellia terrane in southern Alaska. They form an igneous province extending 2500 km along the Cordillera, to as far south as Vancouver, B.C. Recent geologic mapping, age dating and lithogeochemistry extend the Nikolai Greenstone at least 80 km southwestward, from the southern Alaska Range through the Talkeetna Mountains. The unique metallogeny of large basaltic provinces is well represented by the Nikolai, indicating potential along this trend for basaltic Cu, sediment-hosted stratabound Cu, and magmatic Ni-Cu-PGE (platinum group elements) deposits. The type section of the Nikolai, in the Wrangell Mountains to the east, displays native Cu and Ag in amygdaloidal basalt. Bonanza-grade Cu-Ag mineralization permeates the overlying carbonate rocks to form Kennecott-type deposits.

A belt of copper and silver geochemical anomalies and copper sulfide mineral occurrences coincide with the Nikolai Greenstone in the Talkeetna Mountains. These Nikolai-hosted occurrences contain up to 3.3% Cu and 81 ppm Ag; chalcopyrite is associated locally with hematite, copper carbonates and epidote alteration. In the Alaska Range and Talkeetna Mountains, where argillaceous units overlie the Nikolai, stratiform copper sulfide deposits such as the Denali (Pass Creek) are similar to White Pine, Michigan, which formed where oxidized Cu- and Ag-bearing fluids derived from flood basalt, encountered a reduced lithology (shale) above the basalt.

In the Alaska Range large, layered mafic-ultramafic sills, which were magma chambers for the Nikolai Greenstone, contain elevated values of platinum group elements, nickel and copper. Abundant gabbro sills of the same composition as the Nikolai basalts crosscut the Mississippian to Lower Triassic stratigraphic section throughout the Talkeetna Mountains, suggesting that magmatic feeders were common in this part of Wrangellia. The "Kluane mafic-ultramafic belt", comprised of magmatic feeders for Nikolai Greenstone in the Yukon and B. C., is a newly recognized Ni-Cu-PGE province.

No ultramafic rocks have been mapped to date in the Talkeetna Mtns, but field geophysical data suggest the possibility of buried ultramafic bodies, and Ni-Cu-PGE stream sediment geochemical anomalies occur in close proximity to the basalts.