NEW GEOCHRONOLOGIC CONSTRAINTS ON THE FORMATION AND HYDROTHERMAL ENRICHMENT OF THE BOKAN MOUNTAIN PERALKALINE IGNEOUS COMPLEX AND ASSOCIATED HREE DEPOSITS, SOUTHEASTERN ALASKA, USA

The Early Jurassic Bokan Mtn. igneous complex (BMIC) intrudes lower Paleozoic island arc rocks of the Alexander terrane in SE Alaska. The BMIC comprises heavy rare earth element (HREE)-enriched pegmatites and felsic dikes, and U-Th-HREE-F mineral occurrences including the Ross-Adams U mine. Recent exploration at Bokan Mtn. has focused on the Dotson Zone, a 2-km-long, 50-m-wide zone of felsic dikes extending from the SE margin of the pluton. Current resources are 4.79 million tonnes (Mt) indicated and 1.05 Mt inferred at 0.6% TREO (39% HREO; Ucore, May, 2015). 

New SHRIMP and LA-ICPMS U-Pb ages on zircon (Z) and monazite (M) and ⁴⁰Ar/³⁹Ar ages on sodic amphibole from the major phases of the BMIC reveal a complex history of intrusion and emplacement of pegmatites and dikes, followed by hydrothermal enrichment of HREE and Z growth during at least two subsequent episodes. The oldest units (SHRIMP-Z ages) comprise a (20-100-m-wide) margin of: 1) altered aegirine (AG) granite (182.0 ± 2.7 and 179.9 ± 2.0 Ma), which transitions into 2) arfvedsonite (AF)-AG-quartz (Q) porphyritic granite, and 3) a mixed unit of these rock types (altered AG granite 176.2 ± 3.0 Ma). The bulk of the BMIC is a coarse-grained Q-AF porphyritic granite (181.9 ± 1.8 to 177.7 ± 1.5 Ma; four AF separates) that is contemporary with subordinate very fine-grained AF granite (181.8 ± 0.4 Ma; ⁴⁰Ar/³⁹Ar AF plateau age) in the core of the complex. The HREE-enriched I&L felsic dike system crosscuts the altered AG granite margin and is, in turn, truncated by the Q-AF porphyritic granite, thus establishing their relative ages.

An initial hydrothermal event is recorded in two AF separates from the BMIC main phase (173 ± 2 Ma) and in hydrothermal Z from the Dotson Zone (172.1 ± 3.4 and 170.6 ± 11.4 Ma; SHRIMP) and 169.9 ± 1.0 Ma (LA-ICP-MS). A second, much younger event is recorded in Z (121.4 ± 8.8; SHRIMP, and 123.1 ± 3.2 and 121 ± 11 Ma; LA-ICPMS) and M (115 ± 11 Ma; SHRIMP) from the Dotson Zone. The youngest rocks in the BMIC are widespread AG dikes (116.1 ± 2.7 Ma; SHRIMP-Z).

We suggest that emplacement of the BMIC and hydrothermal HREE-enrichment of felsic dikes took place over 10-12 m.y. A separate pulse of alkaline magmatism, or an as-yet unidentified metamorphic event, resulted in emplacement of AG dikes and further hydrothermal enrichment of the Dotson Zone approximately 55-50 m.y. later.