U-PB ZIRCON AGES AND MINERALOGY OF THE DORA BAY INTRUSIVE COMPLEX AND HEAVY REE-BEARING PEGMATITE DIKES

The Dora Bay intrusive complex (DBIC) is a predominantly syenitic peralkaline intrusive body located in SE Alaska in the Alexander Terrane. Peralkaline intrusions are important sources of rare earth elements (REE), which are in great demand for their use in high-tech applications.

The DBIC and associated pegmatites are less-mineralized than, but a potential age and geochemical analogue to the Bokan Mountain complex (BMC) dated at ~177 Ma that is located ~30 km south of DBIC, and is the most significant heavy REE deposit in the United States..

The main phase of the DBIC is a medium-grained equigranular syenite. Associated REE-bearing (total REE >1% by weight) pegmatite dikes are coarse grained with euhedral phenocrysts of arfvedsonite, aegirine, and richterite up to 2 cm in length. Petrographic analyses coupled with XRD, SEM, and handheld XRF of mineral splits have identified REE phases including apatite, bastnaesite, monazite, titanite, and zircon. Additionally, DBIC pegmatites locally contain eudialyte, a rare alkali-Zr-silicate that hosts important critical elements (e.g. U, Pb, Nb, Ta, Zr, Hf, and REE).

Laser ablation ICP-MS data were obtained for the main syenite body and a radioactive, ~10 cm-wide pegmatite dike that intruded into the country rock. Zircons from the syenite, which included Th-rich grains (>2000 ppm Th), yield largely concordant U-Pb ages of 182.2 +/-5.5 Ma (30 spot analyses). Zircons from the pegmatite are highly enriched in uranium (~1000-4000 ppm) and have varying amounts of common Pb. However, a Concordia Age of 184.3 +/- 5.5 Ma derived from the 6 most concordant spot analyses is essentially the same as the discordia intercept age (184.4 +/- 5.5) of all 40 analyses. The source of the common Pb is unknown, but closed-system behavior of the U-Pb system in zircons is apparent. These new age results indicate that the syenite and the pegmatites were coeval and that the DBIC magmatism was temporally, if not cogenetic to, emplacement of the BMC.