GEOCHRONOLOGY OF AU-VEINS & SOME ASSOCIATED ROCKS, SEWARD PENINSULA AND SOUTHERN BROOKS RANGE, NORTHERN ALASKA

Gold-quartz veins in Northern Alaska are broadly classified as ‘orogenic/metamorphic' as they occur in multiply deformed, greenschist-overprinted blueschist metamorphic rocks with the nearest magmatic bodies 10s to 100s of km away. Lack of reliable radiometric ages for these mica-poor veins makes them poorly constrained geologically and the currently popular model relates them to a ca 90-100 Ma magmatic welt and related metamorphic dewatering. Limited dating of veins from the Southern Brooks range has so far shown evidence only for a 112-115 Ma vein event. In contrast, our > 70 40Ar/39Ar age determinations on glaucophane, barroisite, winchite, phengite, and biotite from the Nome-Council area on veins and rocks indicates several vein forming episodes, all older than the bulk of magmatism on the Seward Peninsula.

Well-constrained glaucophane ages > 205 Ma indicate peak blueschist conditions in the late Triassic; an undisturbed biotite plateau age of 184 Ma from a schist sample indicates rapid cooling (thrusting?) by mid-Jurassic. Barroisite and Winchite and Na-phengite plateau ages of 138- 144 Ma indicate fluid influx associated with upper greenschist conditions by the late Jurassic. Na-poor phengites commonly yield ‘messy' spectra and integrated, plateau, or weighted ages of ~ 125-128 Ma, suggesting continued fluid flux and recrystallization in the early Cretaceous. Vein mica ages suggest three main vein events. (1) Early, deformed veins yield complex spectra with ages of ~ 112-115 Ma. (2) More common, sharp-walled, through-going veins with micas yielding excellent plateau ages of 106-109 Ma; these veins have 310/steep orientations. Very rare mafic alkalic dikes (Ti-garnet & analcime-bearing foid monzodiorite) are usually altered beyond dating; biotite from one gave an age of 107 Ma. We infer these veins are extension-related; limited oxygen isotope data suggests that convection of meteoric water along faults was a major factor. (3) A minor late vein event is suggested by 1 vein mica age of 102 Ma. Alkalic mafic dikes occur with 030 strikes; biotite from these uniformly yield 80-84 Ma ages, indicating late Cretaceous extension at a different orientation. Larger Au-vein deposits on the Seward Peninsula (e.g., the Rock Creek deposit) also yield multiple vein ages, indicating repeated mineralization events.