Primary, agpaitic and deuteric stages in the evolution of accessory Sr, REE, Ba and Nb-mineralization in nepheline-syenite pegmatites at Pegmatite Peak, Bearpaw Mts, Montana

The pegmatites at Pegmatite Peak (Bearpaw Mts., Montana) crystallized from an evolved fraction of nepheline-syenitic melt enriched in Sr, Ba, light RE E and Nb. These rocks are composed essentially of microcline (up to 1.1 wt. % Na2O and 1.0 wt.% BaO), altered nepheline (replaced by analcime, zeolites , muscovite and gibbsite), and prismatic aegirine set in an aggregate of fi brous and radial aegirine. The early accessory assemblage includes Mg-Fe mi ca, rutile, zircon, titaniferous magnetite and thorite. Precipitation of th ese phases was followed by crystallization of a plethora of rare minerals e nriched in Sr, Ba, light REE and Nb. Three major stages are distinguished i n the evolution of this mineralization: primary, agpaitic and deuteric. Pri mary repositories for Sr, REE and Nb included betafite, loparite-(Ce), cric htonite and ilmenite-group minerals. Betafite (Ta-poor, REE- and Th-rich) i s present in very minor amounts and did not contribute significantly to the sequestration of incompatible elements from the nepheline-syenite melt. Lo parite-(Ce) evolved predominantly by depletion in Sr and Ca and enrichment in Nb, Na and REE, i.e. from strontian niobian loparite (up to 22.0 wt.% Sr O) to niobian loparite (up to 17.6 wt.% Nb2O5) Crichtonite contains minor N a, Ca and K, lacks detectable Ba and REE, and is unusually enriched in Mn ( 7.0-13.6 wt.% MnO). The ilmenite-group minerals evolved from manganoan ilme nite to ferroan pyrophanite, and have relatively low Nb contents (less than or equal to 0.9 wt.% Nb2O5) During the agpaitic stage, the major repositor ies for incompatible elements were silicates, including lamprophyllite, tit anite and chevkinite-group minerals. Lamprophyllite is generally poor in Ba , and contains relatively minor Ca and K; only few small crystals exhibit r ims of barytolamprophyllite with up to 26.3 wt.% BaO. Titanite is devoid of Al and depleted in Fe, but significantly enriched in Nb, Sr, REE and Na: u p to 6.4, 4.5, 4.4. and 2.9 wt.% oxides, respectively. The chemical complex ity of titanite suggests involvement of several substitution mechanisms: Ca 2++Ti4+double left arrow Na1++Nb5+, Ca(2+)double left arrow Sr2+, 2Ca(2+)do uble left arrow Na1++REE3+, and Ca2++O(2-)double left arrow Na1+ + (OH)(1-) Chevkinite-group minerals evolved from Sr-rich (strontiochevkinite) to PEE -rich compositions [chevkinite-(Ce)]. Strontiochevkinite from Pegmatite Pea k is compositionally similar to the type material from Sarambi, and has hig h ZrO2 (up to 7.8 wt.%) and low FeOT (less than or equal to 2.5 wt.%) conte nts. During the final stages of formation of the pegmatites, a deuteric F-b earing fluid enriched in Sr and REE precipitated carbonates and minor phosp hates confined to fractures and cavities in the rock. In this youngest asse mblage of minerals, ancylite-(Ce) is the most common Sr-REE host. Some disc rete crystals of ancylite show significant enrichment in Th (up to 6.0wt.% ThO2). Ancylite-(Ce) and bastnaesite associated with "metaloparite" and TiO 2 (anatase?) comprise a replacement assemblage after primary loparite. The typical replacement pattern includes a logarite core with locally developed "metaloparite", surrounded by a basenaesite-anatase intermediate zone and an ancylite rim. Fluorapatite is rare, and has very high Sr, Na and REE con tents, up to 21.4, 2.6 and 12.9 wt.% oxides, respectively. Compositionally, this mineral corresponds to the solid solution series between fluorapatite and belovite-(Ce). At this stage, hollandite-group minerals became a minor host for Ba; they d emonstrate the evolutionary trend from priderite (5.2 wt.% K2O, 7.4 wt.% Ba O) to Ba-Fe hollandite (19.2-21.4 wt.% BaO). Thus, the evolution of Sr, REE , Ba and Nb mineralization was a complex, multi-stage process, and involved primary crystallization, re-equilibration phenomena and late-stage deuteri c alteration.