Aquamarine Mineralization in the Black River Batholith, Yukon Territory, Canada

The central Yukon Territory is renowned for its exceptionally abundant beryl occurrences. In 2003, widespread medium-blue aquamarine mineralization was discovered in the Black River Batholith, south-central Yukon. This aquamarine occurrence is notable because it is hosted in granodiorite, rather than being associated with pegmatites. Four distinct beryl occurrences have been documented in the batholith: (1) beryl with tourmaline, separated from the granodiorite by an aplite zone; (2) beryl with tourmaline bordering the granodiorite, with no aplite; (3) a set of beryl-quartz veins devoid of tourmaline, striking 70° and dipping 60° south; and (4) a second set of veins striking 315° and sub-vertical.

This study aims to: first, compare the different types of aquamarine occurrences and identify distinguishing features; second, compare this occurrence with the Tsa Da Glisa emerald deposit (15 km northeast) and the true blue aquamarine locality (150 km northwest); and third, determine an accurate age for the Black River Batholith and compare it with nearby intrusions.

The Black River Batholith, in the Finlayson Lake map area, is an Eocene-aged granitic intrusion emplaced within Cambrian to Lower Cambrian sedimentary rocks of the Kechika Group and Rosella Formation. It forms part of the geologically complex Canadian Cordillera, an orogenic belt formed by terrane accretion onto the Laurentian craton. Structurally, the batholith lies between the Tintina Fault to the north and the St. Cyr Thrust Fault to the southeast, with an elongated morphology aligned with these structures. These features suggest a complex tectonic evolution, potentially influenced by post-accretionary transcurrent faulting.

The batholith is predominantly peraluminous, consisting of K-feldspar-phyric monzogranite to granodiorite, with accessory minerals like biotite, muscovite, and tourmaline. Previous geochronological investigations yielded discordant ages, with K-Ar dating of biotite ranging from 46.9 ± 2.6 Ma to 68 Ma. Nearby lithologically similar intrusions yielded K-Ar ages of 70 to 100 Ma, indicating either a distinctive emplacement history for the Black River Batholith, deficiencies in earlier dating methods, or subsequent reheating events.

This study employs U-Pb zircon geochronology and whole-rock geochemical analysis to clarify the temporal and compositional relationships of the Black River Batholith with nearby intrusions and test its affiliation with the Cassiar Suite. Geochemical analysis of beryl-bearing and non-beryl-bearing granitoids will help delineate the chemical provenance of the beryl occurrences. Additionally, we will use SEM and EPMA to investigate compositional variations among beryl occurrences and zoning within individual beryl crystals, providing insights into their chemical affinities and source divergences. This integrated approach aids in reconstructing the Black River Batholith’s emplacement history and enhances our understanding of the region's magmatic evolution.

This study is significant because it addresses long-standing uncertainties regarding the batholith’s emplacement age, its relationship to nearby intrusions, and the origins of its unusual beryl occurrences. These occurrences have not been studied in detail before and may provide valuable insights into magmatic-hydrothermal processes in the region.