GEOLOGY AND GEOCHEMISTRY OF THE DORA BAY PERALKALINE INTRUSIVE COMPLEX AND ASSOCIATED EUDIALYTE-BEARING HREE-ENRICHED DIKES, PRINCE OF WALES ISLAND, ALEXANDER TERRANE, SOUTHEAST ALASKA

The Dora Bay intrusive complex (DBIC) intrudes lower Paleozoic island arc volcanosedimentary rocks of the Alexander terrane in southeast Alaska. Like the better-mineralized Bokan Mtn. peralkaline intrusive complex (BMIC) 30 km to the south, the DBIC is Early Jurassic in age, exhibits late-stage pegmatites and dikes, and is associated with numerous U-Th-HREE-eudialyte-molybdenite-iron oxide mineral occurrences within and around its margins. Characterization of the DBIC and comparison to the BMIC thus provides an opportunity to examine the factors leading to HREE enrichment in peralkaline granites.

Compilation of existing geologic mapping, fieldwork, and airborne magnetic and resistivity surveys shows that the DBIC is a roughly circular body ~4.5 km in diameter. Geochemical data indicate alkaline granite, syenite, and nepheline syenite compositions for the DBIC and associated (± eudialyte-bearing) pegmatites and dikes. Discriminant diagrams show A-type, within-plate granites indicative of a Jurassic rifting event. Medium crystalline, equigranular, arfvedsonite- and aegirine-bearing syenite composes the majority of the complex. Molar Na₂O-K₂O-Al₂O₃ ratios (>1) indicate peralkaline to slightly metaluminous and sodic compositions, with SiO₂ = 60.7, Na₂O = 7.6, K₂O = 3.5, Fe_TO₃ = 7.6, TiO₂ = 0.4, Zr = 3162, TREO = 972 (avg. wt. % for oxides; ppm for Zr and TREO [Y+La+REE]; Fe_TO₃ = total Fe as Fe₂O₃; n=31). Previous mapping of a dioritic phase of the DBIC could not be confirmed by our work.

REE-enriched pegmatitic enclaves and discrete arfvedsonite-aegirine-albite bearing pegmatite dikes ± the rare Zr-silicate mineral eudialyte are a common late-stage magmatic feature within and near the margin of the DBIC. Pegmatite dikes extend outward for at least a kilometer and typically are more silicic, radioactive, and have higher TREO contents. Eudialyte-bearing pegmatites and dikes; SiO₂ = 61.0, Na₂O = 8.1, K₂O = 4.5, Fe_TO₃ = 5.8, TiO₂ = 0.2, Zr = 6630, TREO = 2724 (n=15). Pegmatites; SiO₂ = 68.2, Na₂O = 6.7, K₂O = 3.2, Fe_TO₃ = 10.6, TiO₂ = 0.3, Zr = 5687, TREO = 1245 (n=11). Pegmatite dikes; SiO₂ = 72.1, Na₂O = 5.1, K₂O = 2.6, Fe_TO₃ = 5.7, TiO₂ = 0.3, Zr = 5508, TREO = 4265 (n=11). Pegmatite dikes (± eudialyte, n=9) contain the highest abundances of TREO and range from 0.5 to 1.5 wt. % with flat to slightly HREE-enriched REE patterns.