MINERALOGIC AND GEOCHEMICAL IMPLICATIONS FOR ENRICHMENT OF U-TH-REE IN SYENITE DIKES ON THE EASTERN SEWARD PENINSULA, ALASKA

Metamorphic rocks and high-potassium intrusive rocks on the eastern Seward Peninsula are cut by a set of widely distributed northeast-trending syenite dikes that have meter to km strike lengths and are enriched in U, Th and rare earth elements(REE). These dikes cluster in an area of more than 300 km² within the late Early Cretaceous syenitic to monzonitic Kachauik pluton and sparsely intrude nearby stocks and metamorphic rocks as far as 60 km to the west. Sharp planar contacts and aphanitic textures indicate the dikes were emplaced into the Kachauik after it cooled. Some dikes have cm- to m-scale alteration halos in host rocks, indicating association with hydrothermal fluids. The dikes have distinctive mineralogy and texture, composed of an aphanitic groundmass of K-feldspar, albite and apatite and phenocrysts of K-feldspar, nepheline, amphibole, clinopyroxene, biotite, melanite garnet, allanite and pseudoleucite. Quartz, zircon, titanite and sulfide minerals are extremely rare. Evidence for hydrothermal alteration includes common fluorite, cancrinite, vesuvianite, sericite, chlorite and growth rims on allanite in the dikes and in adjacent host rocks. Allanite occurs as discrete crystals and as microinclusions in feldspar and fluorite, suggesting both igneous and hydrothermal origins. Melanite and allanite typically have black cores and halos indicating radiation damage and are the primary hosts of U, Th and REE. Microprobe data indicate vesuvianite in Kachauik syenite adjacent to dikes contains elevated U, Th and REE. Melanites in the dikes contain an average of 6% TiO₂. Energy dispersive spectroscopy reveals the presence of interstitial tausonite, a Sr-Ti perovskite, in melanite-bearing dikes. Whole-rock chemistry for these dikes shows 2000-4000 ppm Sr and Ba, 500-1000 ppm Zr, 25-40 ppm U, 75-125 ppm Th, as much as 7400 ppm F, and > 800 ppm total REE. Mineralogy and chemistry suggest a crystallization trend in which Ti partitioned into melanite and tausonite rather than titanite and Zr partitioned into major silicates rather than zircon or baddeleyite. Primary and alteration mineralogy, mineral chemistry and whole rock chemistry suggest these silica undersaturated dikes contain incompatible elements that were concentrated in late stage igneous melts and further enriched by high-F hydrothermal fluids.