GEOCHEMICAL CONSTRAINTS ON MELT SOURCE AND FRACTIONATION OF LITHIUM-MINERALIZED RARE METAL GRANITES, SEWARD PENINSULA, ALASKA

Rare metal granites (RMGs) are mined for tin (Sn) and tungsten (W) and may be exploited for other lithophile critical minerals (LCMs) such as lithium (Li) and tantalum (Ta), warranting an understanding of their distribution in the mineral system. On the western Seward Peninsula, Alaska, a belt of shallowly emplaced (≤ 1 kb) ca. 85 Ma S-type RMGs (SPRMGs) is structurally juxtaposed with exhumed pelitic restites (> 5 kb) of the Kigluaik Metamorphic Complex (KMC). We present new geochemical data from the SPRMGB, and from restites and anatectic granites of the KMC, to investigate lithophile element trends and test possible genetic links of a granite source region to Sn-W-Li-Ta mineralization at the Lost River and Kougarok RMG prospects.

Whole-rock major- and trace-element abundances were analyzed from several SPRMGs that vary in mapped exposure area. Almost all the granites are characterized by LCM abundances above average crust. Larger plutons (> 45 km²) show Li values from 80 – 240 ppm Li. Smaller plutons (< 25 km²) contain 200 – 420 ppm Li. At the Lost River and Kougarok cupolas, fresh granites contain up to 2,000 – 3,000 ppm Li. Rubidium (Rb) / strontium (Sr) shows a progressive trend from 1 – 40 in larger plutons, to > 100 at Kougarok and Lost River. These data suggest Li enrichment through magmatic fractionation of a plagioclase-rich assemblage. Tin, Ta, and W, in contrast, vary greatly across the SPRMGs, but are generally 5 – 10x average crust in most samples. Greisenization (fluorite, quartz, Li-mica, tourmaline, topaz) at Kougarok and Lost River further enriches these elements, with Sn, W, and Li locally exceeding 7,000 ppm.

Residual sillimanite migmatite from the KMC show Li abundances slightly above average crust, but are depleted in Sn, Ta, and W. In contrast, ca. 90 Ma anatectic granite and pegmatite associated with the migmatite contain up to 60 ppm Li, 9 ppm Sn, 2 ppm Ta, and 70 ppm W, Sr 0.9x average crust, and Rb/Sr < 13. This is compatible with extraction of LCMs during biotite dehydration melting. Collectively, our data support models of RMG genesis that invoke anatexis of pelitic rocks, followed by magmatic fractionation and ascent of the partial melts to the upper crust to form LCM deposits. This area may provide a trans-crustal perspective on genetically related ore systems formed at disparate crustal levels.