BMC lies within the Alexander Terrane of the North American Cordillera (Figure 1). The Cordillera consists of allochthonous oceanic and pericratonic terranes that were accreted to the northwestern margin of Laurentia (the North American craton) between late Paleozoic and early Cenozoic time (e.g., [21]). The Alexander terrane is a large outboard allochthonous terrane, which is considered to have an exotic nature with respect to the Laurentian margin (e.g., lack paleontological affinities with North America; [22,23]). The terrane (Figure 1) was accreted to the North American craton by the middle Cretaceous (~115–95 Ma), well after the emplacement of the Bokan complex.

The basement of southern Prince of Wales Island is composed of the Neoproterozoic to Lower Cambrian Wales Group made up of arc-related volcanic rocks, sedimentary rocks and granitoid intrusions, which were all metamorphosed during the Middle Cambrian to Early Ordovician Wales orogeny. These rocks were unconformably overlain by Ordovician–Early Silurian oceanic arc-related metavolcanic and metasedimentary rocks and intruded by plutons composed mainly of Late Ordovician to Early Silurian quartz monzonites, quartz diorites, diorites and gabbros. The Paleozoic units were metamorphosed during the Klakas orogeny (Middle Silurian to earliest Devonian; [26]). These rocks, which represent an oceanic arc complex [26], host the Bokan stock. Mesozoic rocks are rare in this part of the terrane. Early Jurassic igneous rocks in the Alexander terrane are limited to the BMC, the Dora Bay pluton composed of peralkaline granites and syenite and located about 30 km N of the BMC and bimodal Moffat volcanic rocks 100 km to SE of the BMC (Figure 1), all dated at ~175–185 Ma [25,27].

The Bokan Mountain pluton is a typical anorogenic granitic complex, emplaced at a shallow depth. It produced the contact metamorphic assemblages which included andalusite (chiastolite)-bearing hornfels. Around the intrusion, the Paleozoic country rocks, particularly granitoids, are albitized. A wide (>1 km; [28]) aureole of albitization was produced by infiltration of hydrothermal fluids related to the pluton.

The Bokan granites have variable grain size, ranging from fine to coarse-grained. They are composed typically of quartz and feldspars with 2–10 vol.% of mafic minerals (sodic amphibole-arfvedsonite and sodic clinopyroxene-aegirine in various proportions). Locally, the mafic minerals comprise about 30 vol.% of the rocks. The granites are mainly made up of quartz (~25–45 vol.%), K-feldspar (20–30 vol.%) and albite (30–50 vol.%). The intrusion (Figure 2) is composed of two main zones [29,30]: (1) a core made up of arfvedsonite-bearing granite (porphyry) that forms the main part of the body and (2) an outer ring comprised predominantly of aegirine-bearing granite. The aegirine granite forms a nearly complete circular zone around the core [29,31]. A transition unit between these two zones is composed of aegirine-arfvedsonite granite and is typically about 15 m wide. In addition, there is a discontinuous outermost border zone composed of pegmatitic pods with aegirine and arfvedsonite crystals enclosed in aplitic rocks. Fine-grained aegirine granite, which outcrops in the central part of the stock probably represents the preserved roof of the intrusion [5]. The intrusion is associated with the quartz-feldspatic (felsic) dikes, which occur throughout the intrusions as well as outside, mainly along shear or fault zones. The dikes, composed mainly of quartz, albite and K-feldspar, form elongated lenses, which are steeply dipping and commonly 0.1 to 3 m thick. They include both aplitic and pegmatitic types. Most of them are rich in REE, Nb, Ta, Zr, Pb, Th and U and are closely related to the Bokan intrusion [18,19,32].

The granitic rocks have diverse texture although two main textures are dominant—primary porphyritic and late protoclastic [5,18,29]. The porphyritic texture contains phenocrysts of subhedral to euhedral quartz that ranges from 5 to 20 mm in diameter and K-feldspar enclosed in a fine-to medium-grained groundmass made up of quartz, albitic feldspar and minor K-feldspar. The protoclastic textures are characterized by broken phenocrysts of quartz and feldspars, bending of feldspar twining and pronounced undulose extinction of quartz crystals. Protoclastic textures resulted from an overpressure of fluids generated during the exsolution of the vapor phase from the magma during late stages of crystallization [5].

K-feldspar occurs as microperthite and cross-hatched microcline while plagioclase (An < 10) forms both a separate phase or an intergrowth or replacement of K-feldspar. Albite pervasively replaces K-feldspar, both in phenocrysts and in the groundmass [18,19]. The groundmass is dominated by albite and quartz with subordinate K-feldspar where the primary assemblage is overprinted by the metasomatic assemblage. All the major minerals have formed during at least two stages, one magmatic and the other related to late-stages (late magmatic to post-magmatic). Albite ranges from magmatic crystals to late- to post-magmatic albitic growth from a fluid phase. The albite is generally euhedral without signs of alteration. Quartz has also crystallized during two stages, initially, as the early megacrysts and then in the matrix and as veinlets. Well-developed early quartz crystals were intruded by the quartz veinlets of the second generation. There are also at least two generations of K-feldspar. Early K-feldspar, now occurring as a cross-hatched microcline, is enclosed/embayed by microperthitic K-feldspar that is the main K-feldspar phase. Around the Ross-Adam deposit, Cuney and Kyser [5] recognized two stages of alteration of the granites. The first stage included albitization of K-feldspar followed by the second stage, where albitization was accompanied by quartz dissolution leading to the generation of secondary syenitic rocks. This rock type is composed primarily of albite and aegirine with accessory fluorite, monazite, zircon and Fe-Ti oxides. The U-Th mineralization is closely associated with the syenitic rock [5,33,34]. The ore zone appears to be a strongly altered syenitic rock.

The sodic amphibole (arfvedsonite) occurs as subhedral prismatic crystals 1–4 mm long, which are distinctly pleochroic (lilac to blue) and interstitial. It also displays at least two-stages of growth. The inclusion-rich amphibole cores (stage 1) are surrounded an inclusion-free overgrowths (stage 2). Amphibole is commonly replaced by a mixture of secondary minerals (frequently hematite and quartz) or by fibrous aegirine and hematite with interstitial quartz and minor fluorite. Aegirine forms a variety of crystal shapes ranging from equant to prismatic or skeletal. Like arfvedsonite, it also shows a two-stage growth history. Philpotts et al. [32] argued that some aegirine grains might be of hydrothermal origin. Pyroxene and amphibole may be spatially associated but in other samples only one or the other is present. Where both minerals are present, the anhedral to euhedral amphibole partly encloses and corrodes the pyroxene but in some cases pyroxene post-dates amphibole. This association suggests overlapping crystallization of these two minerals. Amphibole and pyroxene are poikilitic, enclosing crystals of quartz, microcline and albite. Individual major rock forming minerals display only limited chemical variations [18,32]. Granitic rocks of the BMC host numerous accessory mineral phases. Particularly common are zircon, monazite, xenotime, titanite, fluorite and Fe-Ti oxides. Some of the accessory minerals including monazite could be late/post-magmatic.

The analyzed rocks from a wider vicinity of the Ross-Adams mine were subdivided into four groups—unaltered granites (group 1), albitized granites (group 2), mineralized albitized granites (group 3) and syenites (group 4) although there are transitions among the groups. Group 2 differs from group 1 by distinct albitization. Albite pervasively replaces K-feldspar, both phenocrysts (especially along the fractures and cleavage planes) and grains in the groundmass. The groundmass is dominated by albite and quartz. The accessory mineralogy appears to be the same as in group 1. Group 3 contains notably higher amounts of accessory minerals. In addition to ubiquitous accessory minerals such as zircon, monazite, xenotime, titanite, fluorite and Fe-Ti oxides, more than twenty U-Th-, REE- and HFSE-bearing minerals have been identified at the Bokan complex (e.g., [19,32,35]). Some of these minerals are variably present in mineralized altered granites and may occur in two or more generations. Group 4 includes a syenite, a fine-grained rock, which is likely the result of albitization and desilicification of the rocks in the proximity of the mineralization [5]. Quartz occurs in this rock only in subordinate amounts.

3. History of Mining and Exploration