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The Mount Fairplay igneous complex is an approximately 24-square-kilometer (9-square-mile) intrusive body located in eastern Interior Alaska. The complex is latest Cretaceous (ca. 67 Ma) in age and consists of two recognized intrusive suites or groups: (1) an alkalic group comprising quartz-poor lithologies (monzonite, syenite, alkali-feldspar syenite) and (2) a slightly younger, sub-alkalic group comprising quartzrich lithologies (granite and quartz monzonite). Ilmenite and magnetite compositions of the alkalic rocks indicate magmatic oxidation states in the vicinity of the fayalite-quartz-magnetite (FQM) buffer; in contrast, data for the quartz-rich suite indicates conditions much more oxidized than the FQM buffer. Both groups typically contain diopsidic clinopyroxene, biotite, and amphibole, and the most mafic of the alkalic group contains olivine or iddingsite. Microprobe studies show that amphibole, biotite, and feldspar compositions vary according to both primary magmatic and secondary, post-magmatic processes or alteration. Some parts of the complex, particularly the alkali-feldspar syenite, are enriched in rare earth elements (REE) uranium (U), niobium (Nb), and thorium (Th). Magmatic apatite at Mount Fairplay has low-REE cores and REE-enriched rims; however, in hydrothermally altered samples, magmatic apatite is replaced by REE-depleted apatite, which is accompanied by secondary REE minerals including monazite, allanite, huttonite, and cheralite. Titanite, the bulk of which appears to be secondary, contains significant Nb (0.2-5 percent Nb2O5) and light REEs (0.2-3 percent total LREE oxide). The alkali-feldspar syenite has the highest REE and Nb concentrations, both in titanite and in whole-rock analyses. The alkalic rocks at Mount Fairplay also contain elevated fluorine (F) in concentrations well above those typical for igneous titanite. Other secondary minerals, including amphibole, biotite, and scapolite, contain elevated concentrations of chlorine (Cl) and probably also F, and these halogen enrichments are apparently restricted to the alkalic group of rocks and absent from the quartz-rich group. One sample of alkali-feldspar syenite also contains fluorite. Together, these features indicate that hydrothermal fluids containing relatively high Cl and fluorine (F) concentrations affected parts of the Mount Fairplay complex. [ABSTRACT FROM AUTHOR]