Discovery of a New Type of Carbohydrothermal Pegmatite at Moose Creek Valley, Ice River Alkaline Complex, British Columbia – Evidence for Extensive Ti Mobilization

Abstract A unique carbohydrothermal pegmatite was discovered on the western slope of Moose Creek Valley, 23 km south of the town of Field, British Columbia, Canada, on the eastern edge of the Mississippian-Devonian-aged Ice River Alkaline Complex. The pegmatite intrudes Unit 12 of the syenite subcomplex at 2258 m elevation in a steep couloir on the western side of the ridge between Sentry Peak and Zinc Mountain. It outcrops horizontally over a short N–S strike of 2.5 m and is a consistent 12 cm thick. The pegmatite has a heterogeneous texture, with massive pegmatitic and vuggy zones along with minor brecciation and fracture-filling. The primary assemblage at the margins consists of massive prismatic natrolite crystals up to 10 cm in length, blocky calcite, and euhedral titanite (up to 6 cm) along with fibrous aegirine. A total of 27 mineral species in five separate parageneses have been identified from the pegmatite to date. Many of these minerals are being described from the Ice River Alkaline Complex for the first time, including anatase, brookite, gibbsite, gonnardite, henrymeyerite, lucasite-(Ce), nordstrandite, srilankite, thorite, vinogradovite, wadeite, and wairakite. This is also the first known Canadian locality for the rare Ti oxides lucasite-(Ce) and srilankite. The Moose Creek Valley pegmatite formed from the mixing of exsolved, SiO2-undersaturated, alkali-Ti-Fe-rich carbohydrothermal fluids derived from the ultramafic layered subcomplex and the primary carbonatite. Titanium from primary minerals in the main complex rocks (titanite, Ti-bearing mafic minerals, perovskite) was scavenged and remobilized via Ti-OH− complexes in the exsolved fluids. Relative fluctuations in aOH− and aCO32− and the Na/Ca ratio in the cooling fluid resulted in alternating stages of Na- and Ca-autometasomatism within the pegmatite. The pegmatite appears to have acted as a closed system, with no evidence of crustal contamination, extensive interactions with the wall rock, or the input of additional fluids or melts. All assemblages support crystallization in a SiO2-undersaturated, alkali- and volatile-rich environment at high pH (7–9) and temperatures ranging from 400 to 150 °C.