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A suite of samples, constituting a 3-km transect southeast from the margin of the Bokan Mountain peralkaline granite stock along a Y-REE-Zr-Nb-enriched micro-pegmatite and aplite vein-dike system, have been studied geochemically and petrographically in order to assess the nature and origin of the enrichment. Noteworthy minerals include arfvedsonite, taeniolite, gittinsite, and a host of rare-earth-, zirconium-, and niobiumbearing phases. Textures range from primary igneous assemblages, through autometasomatic replacements, to late-stage fracture fillings of hydrothermal aegirine, zircon, iron oxide, iron hydroxide, and clay minerals. Fluid inclusions are abundant. Mineral rare-earth abundance patterns show diversity that contrasts with the limited range of whole-rock patterns. Yttrium and heavy rare earths are unusually enriched in these samples. The rocks display a pronounced negative europium anomaly (Eu/Eu* = 0.27±0.03) that is consistent with their sampling the same evolutionary stage of an igneous system that has undergone extensive feldspathic fractionation. The enrichment of trace elements in the vein dikes likely resulted from separation of solid, fluid, and vapor phases. Evidence for immiscible fluid separation include silicate-, fluoride-, and sulfatephases. Neodymium isotope measurements indicate depleted source rocks for the magma and subsequent open geochemical systems. INTRODUCTION Bokan Mountain (lat 54°55'N., long 132°9'W.) is located between the South Arm of Moira Sound and the West Arm of Kendrick Bay on southern Prince of Wales Island in the Dixon Entrance Dl quadrangle, 60 km southwest of the town of Ketchikan (Figs. 1,2). In 1955, airborne and subsequent ground surveys identified areas of high radioactivity due to concentrations of uranium and thorium minerals. Follow-up work led to the development of the Ross Adams mine which produced 94,000 short tons of high-grade uranium ore, averaging one percent U3O8, between 1957 and 1971. Subsequently, significant resources of rare earth elements (REE), yttrium, zirconium, and niobium were discovered at Bokan Mountain. Details of the local geology and of the thoriumand uranium-rich mineral occurrences have been reported by MacKevett (1963), Staatz (1978), Thompson and others (1982), De Saint-Andre and others (1983), Warner & Mardock (1987), Thompson (1988), Barker & Mardock (1988), and Warner & Barker (1989). Prince of Wales Island is in the Craig subterrane of the allochthonous Alexander terrane (Berg and others, 1972; 1978; Churkin & Eberlein, 1977), which consists of up to 35,000 feet of predominantly marine sedimentary and volcanic strata, and plutonic rocks, of latest Precambrian(?) to Middle(?) Jurassic age (Gehrels & Saleeby, 1987; Gehrels & Berg, 1994). Samson and others (1989) suggest that the terrane has the characteristics of an oceanic island arc. Paleomagnetic, paleontologic, and detrital zircon studies (Hillhouse, 1977; Hillhouse & Gromme, 1984; Haeussler and others, 1992; Bazard and others, 1994; Gehrels and others, 1994; Savage, 1994; Gehrels & Berg, 1994) place the terrane within 10 to 20 degrees of the equator and apparently in close proximity to a continent prior to the Late Jurassic, followed by rifting and transport northwards to its present location. Continental docking of the Alexander terrane (and the adjoining Wrangellia terrane) began in the Jurassic (McClelland & Gehrels, 1990; Saleeby, 1994) and was complete by about Middle Cretaceous as indicated by the intrusion of subductionrelated magmatic-epidote-bearing plutons and the development of the Jurassic-Cretaceous Chugach accretionary prism outboard of the superterrane. In the southernmost portion of the Craig subterrane, the oldest rocks recognized are arc-type sediments and volcanics of the Wales Group that were metamorphosed and deformed during the Middle Cambrian to Early Ordovician Wales orogeny. These rocks form the basement for an arc-type volcanoplutonic-sedimentary complex of Early Ordovician to Early Silurian age which underlies most of the southern portion of the terrane and comprises the host rocks for the Bokan Mountain stock. The subsequent (Middle Silurian to earliest Devonian) Klakas orogeny resulted in metamorphism, intrusion, uplift, and erosion of the arc edifice (Gehrels & Berg, 1994). Upper Paleozoic shallow marine carbonates, clastic sediments, and lesser volcanic rocks, indicative of a tectonically stable environment, occur as erosional remnants in the Craig subterrane; they are largely absent in the Bokan Mountain area. Bokan Mountain is a circular stock, about 10 km2 in area, composed of peralkaline granite of Jurassic age (Armstrong, 1985). The geologic map of Gehrels (1992) divides the Bokan Mountain stock into a core of riebeckite granite porphyry (with subordinate aplitic felty-aegirine granite) surrounded by an outer annulus that is predominantly aegirine granite porphyry (Fig. 2). The Bokan Mountain stock intrudes country rock that consists predominantly of Late Ordovician and (or) Early Silurian quartz monzonite and granite and Late Ordovician quartz diorite and diorite (Fig. 2). The stock is surrounded by a 3 km-wide albitized contact-metamorphic aureole. Collot (in De Saint-Andre and others, 1983) interpreted the stock as the apex of a large composite intrusion made up largely of outer aegirine granite around a core of amphibole granite. Collot identified the amphibole in the granite as arfvedsonite. Collot distinguished three main facies from border to center: (1) albitic aegirine granite, (2) albitic arfvedsonite granite, and (3) finegrained albitic arfvedsonite-aegirine granite. Collot considered the Bokan Mountain granite to be hypoaluminous, an extreme example of peralkalinity, and highly differentiated. Thompson and others (1982) and Thompson (1988) mapped 12 intrusive phases in the complex; in their interpretation, a key feature is the subsidence or collapse of the central part of the complex that led to the emplacement of ring dikes, devolatilization of the magma, and a change from aegirine granite to riebeckite granite crystallization. The thorium-uranium deposits occur as pegmatites, vein dikes, and pipe-like bodies that are spatially and genetically associated with the stock, and are believed to have formed during devolatilization of the magma; the mineralization is thought to be of magmatohydrothermal origin (Thompson and others, 1982; Thompson, 1988). Alteration is characterized by albitization and silicification of the igneous host rock and destruction of potassium feldspars. Iron and manganese oxides locally mark the position of shear zones and faults along which the fluids travelled. Later movement on these faults has offset some of the mineralization (MacKevett, 1963). Temperature estimates obtained from water-rich and carbon dioxide-rich fluid inclusions in late-stage quartz, and independently from sulfur isotopes in coexisting sulfide phases, range from 150 to 450°C; «S 18O of calcite indicates magmatic rather than meteoric water (Thompson, 1988). Although most of the early work focused on the uranium occurrences, MacKevett (1963) recognized abnormal REEenrichrnents in some rocks at Bokan Mountain. The resources of REE, yttrium, zirconium, and niobium in Bokan Mountain granites and vein dikes have been emphasized in recent reports from the U.S. Bureau of Mines (Warner & Mardock, 1987; Warner & Barker, 1989). Bokan Mountain is of interest for a number of reasons: (1) Unusually high abundances of yttrium, zirconium, hafnium, and REE indicate an unusually high degree of fractionation; (2) Ratios of HREE (heavy REE, gadolinium to lutetium) to LREE (light REE, lanthanum to samarium) are much higher than HREE/LREE ratios of typical REE ore deposits and this could have economic significance should demand rise for one or more of the HREE; (3) the Bokan Mountain stock magma likely formed from relatively simple source material, either in the mantle or in juvenile rocks of the Alexander terrane (Samson and others, 1989); and (4) a possible petrologic scenario for the genesis of the Bokan Mountain stock has been outlined by Thompson (1988). The purpose of the present paper is to develop a detailed petrogenetic model for the origin of the late-stage granites and vein dikes at Bokan Mountain based on a comprehensive geochemical and petrographic study giving particular emphasis to the REE. Much of the earlier geochemical data were determined by semiquantitative emission spectroscopy. The more accurate analytical data we report here are particularly necessary for the petrogenetic interpretation of the REE data. We also obtained samariumneodymium isotopic systematics in the hope of reconciling discrepancies in previously reported ages. Sampling by Staatz (1978), Warner & Mardock (1987), Barker & Mardock (1988), and Warner & Barker (1989) included many composite and channel samples that are well suited for estimating potential grades and tonnages but not suited for paragenetic studies or for petrogenetic interpretation. The petrography of the late-stage granites and the REE-bearing minerals are here examined in detail. A comprehensive model for the evolution of Bokan Mountain is expected to be useful for elucidating the petrogenesis of other similar but less well studied alkalic igneous bodies in southeastern Alaska. PREVIOUS STUDIES OF THE BOKAN MOUNTAIN VEIN DIKES The term "vein dike" is used here for dikes and veins at Bokan Mountain that include pegmatitic, aplitic, and quartz-cored varieties. The vein dikes show bifurcating and anastomosing tendencies, particularly away from the stock, and they are, perhaps, more precisely thought of as systems rather than as individual bodies. The Dotson vein dike is a major mineralized (U, Th, REE, etc.) feature at Bokan Mountain. The northwestern terminus appears to be at a right-lateral offset of the "I & L" vein-dike system (Staatz, 1978), which continues to the periphery of the stock (Fig. 2)

Petrogenesis of late-stage granites and Y-REE-Zr-Nb-enriched vein dikes of the Bokan Mountain stock, Prince of Wales Island, southeastern Alaska