LOWER JURASSIC VOLCANIC AND SEDIMENTARY ROCKS DEPOSITIONALLY OVERLIE UPPER TRIASSIC HOST ROCKS AT THE PALMER VMS PROPERTY IN THE ALEXANDER TRIASSIC METALLOGENIC BELT, SOUTHEAST ALASKA

New U-Pb zircon ages for volcanic and sedimentary rocks at the Palmer volcanogenic massive sulfide property in the Alexander Triassic Metallogenic Belt (ATMB) in southeast Alaska constrain the tectonic setting and timing of mineralization.

At the Palmer property stratigraphic sections within a few km differ. At the Palmer deposit basalt and volcaniclastic rocks are overlain by U-Pb-dated Late Triassic rhyolite. Massive sulfide zones occur above, below and distal to the rhyolite body and are overlain by argillite, thick basalt flows, lenses of calcareous siltstone, Late Triassic conodont-bearing limestone, and a cap of mafic flows at Mt. Morlan. A polymictic volcaniclastic debris flow interlayered with tuff and basalt on Mt. Morlan has a single ~195 Ma zircon population. The Pump Valley section consists of conglomerate on an unconformity, limestone, pillow basalt, tuff, calcareous sediments and a cap of black argillite which has a minimum ~144 Ma zircon age population, a dominant ~186 Ma population, a ~221 Ma population and oldest zircons at ~320 Ma. At the occurrence named AG, basalt containing intercalated mineralized rhyolite and breccia is overlain by mafic volcaniclastic rocks and capped by argillite containing poorly preserved Triassic Heterastridium? sponges.

Palmer host rocks overlie regionally mapped Devonian mafic volcanic rocks, Upper Devonian fossiliferous limestone, Mississippian limestone, chert and argillite, and Permian brachiopod-bearing black argillite, chert, siltstone and carbonate of the Porcupine Slate. An argillite in Porcupine Creek (PC) 7 km east of the Palmer property yielded a minimum zircon age population at ~159 Ma and additional populations at ~169 and ~182 Ma. Zircons from the matrix of a conglomerate in the PC section have a dominant population at ~160 Ma. These data indicate the PC section includes Jurassic as well as Upper Paleozoic argillite.

The Permian, Triassic and Jurassic black argillites in the Palmer area indicate intervals of a low-oxygen depositional environment such as a restricted basin. Intra-arc rift basins are the inferred setting for the ATMB. The robust Jurassic zircon age populations in the Palmer area are an important new stratigraphic element in the ATMB and indicate persistence of volcanic activity in an inferred rift basin environment in the Early Jurassic.