Are there Colombian-type emeralds in Canada's Northern Cordillera? Insights from regional silt geochemistry, and the genesis of emerald at Lened, NWT (2017)

The unusual black shale hosted emerald deposits of Colombia are an important source of the finest quality emeralds. The majority of world emerald deposits are related to granite pegmatites and metamorphic environments, however Colombian-type emerald is amagmatic and is associated with sedimentary-hydrothermal brines of largely evaporitic origin where the key emerald-forming elements are remobilized from black shale during regional compressional tectonism. The valuable gem quality mineralization of Colombian emerald deposits is so far unique, however the basic geological conditions that led to mineralization may not be as unusual. Similar black shale basins hosting evaporites are found worldwide, and could be prospective for Colombian-type emerald mineralization. Canada’s Selwyn Basin and periphery is one similar setting with the distinction of hosting two interesting beryl showings: the Mountain River Beryl (a variation of the Colombian beryl model) and the Lened emerald showing (in which black shale provided vanadium as an emerald chromophore). This thesis presents the results of applying Colombian-type emerald exploration criteria to Selwyn Basin stream sediment geochemistry data, in addition to clarifying the origin of emerald at the Lened showing located within the Selwyn Basin. The unusual black shale hosted emerald deposits of Colombia are an important source of the finest quality emeralds. The majority of world emerald deposits are related to granite pegmatites and metamorphic environments, however Colombian-type emerald is amagmatic and is associated with sedimentary-hydrothermal brines of largely evaporitic origin where the key emerald-forming elements are remobilized from black shale during regional compressional tectonism. The valuable gem quality mineralization of Colombian emerald deposits is so far unique, however the basic geological conditions that led to mineralization may not be as unusual. Similar black shale basins hosting evaporites are found worldwide, and could be prospective for Colombian-type emerald mineralization. Canada’s Selwyn Basin and periphery is one similar setting with the distinction of hosting two interesting beryl showings: the Mountain River Beryl (a variation of the Colombian beryl model) and the Lened emerald showing (in which black shale provided vanadium as an emerald chromophore). This thesis presents the results of applying Colombian-type emerald exploration criteria to Selwyn Basin stream sediment geochemistry data, in addition to clarifying the origin of emerald at the Lened showing located within the Selwyn Basin. We identified several prospective areas in Selwyn basin for further Colombian-type emerald exploration based on Na, K, Be, Cr/V, and rare earth element values, however there are important caveats regarding analytical techniques and the regional nature of silt sampling. We found that Lened is a unique skarn-hosted hydrothermal emerald occurrence (~100 Ma) in which Be was provided by a nearby granite pluton of similar age. The chromophore V was provided by local black shale/mudstone.