Dostal and Shellnutt reported in 2015 that the Jurassic Bokan Mountain complex (BMC), composed of arfvedsonite and/or aegirine-bearing peralkaline A-type granitic rocks, is a circular body about 3 km in diameter located in southeastern Alaska. Like many other highly fractionated granitic bodies, the BMC granites were affected by late
magmatic or post-magmatic processes, which, however, did not modify the contents of major elements. The granitic rocks are distinctly enriched in high-field-strength elements (HFSEs), rare earth elements (REEs), Y, Th, and U but depleted in Ba, Sr, and Eu and have high positive ɛNd(T) values.
Unlike the variations in the major elements, Sr and Ba, which can be accounted by fractional crystallization, the abundance of REE, Y, HFSE, U, and Th (the elements which are hosted in accessory phases) were modified by F-rich hydrothermal fluids. The BMC hosts significant rare metal mineralization related to the late-stage crystallization history of the complex involving late magmatic and/or post-magmatic fluids.
The mineralization includes two types:
1) a U–Th deposit which was exploited at the former Ross-Adams mine, and
2) REE and Y mineralization mostly hosted in felsic dikes.
Thermodynamic modelling of granites and spatially associated mafic rocks using the program Rhyolite-MELTS implies that the granites can be derived from the mafic rocks by fractional crystallization. It is suggested that such a process (i.e. derivation of peralkaline granitic magma from the alkali or transitional basaltic magmas derived by partial melting from a lithospheric source metasomatically enriched in rare metals) can be invoked for other peralkaline granitic rocks hosting rare metal deposits.
The authors further concluded that their calculations are consistent with a model suggesting that magma with a composition similar to the syn-plutonic lamprophyric dikes from BMC can produce a residual liquid which resembles the observed compositions of the granitic rocks of the complex under geologically reasonable conditions. The exact parental magma composition is unknown; however, the models place some constraints on the possible initial composition and magmatic conditions. This conclusion helps to elucidate one of the most debated stages of the origin of the rare metal deposits associated with peralkaline granitic rocks, i.e. the origin of the host rocks.
Rare metal mineralization (REE, Y, HFSE, U, and Th) of peralkaline felsic rocks is closely related to the crystallization of magma, especially to the late stages of the evolution. According to this model (e.g. Mungall and Martin 1996; Chakhmouradian and Zaitsev 2012; Dostal et al. 2014), the parent of the granitic rocks – the alkali or transitional basaltic magmas – was derived from a metasomatically enriched lithospheric source by a low degree of partial melting. The melts underwent fractional crystallization achieving a granitic composition (Figure 10). The fractional crystallization was protracted in part due to high contents of halogens and alkalis. The bulk of rare metals is hosted by ore minerals, which are frequently disseminated in the rocks (e.g. Richardson and Birkett 1996; Salvi and Williams-Jones 2005; Chakhmouradian and Zaitsev 2012; Dostal et al. 2014).
Rare metal-bearing minerals started to crystallize only during the late stages of crystallization when magmas become fluid-saturated and rich in rare metals. The mineralizing fluids were dominantly magmatic, released during the last stages of the magma evolution. The fluids did not significantly modify the major element composition of the granites. Such a process (i.e. derivation of the peralkaline granites from mafic magmas) can probably be applied to other rare metal deposits associated with peralkaline granitic rocks such as the Strange Lake, Labrador/Quebec (Canada), Khaldzan-Buregtey (Mongolia), and Ghurayyah (Saudi Arabia; Drysdall and Douch 1986; Küster 2009).
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