MIXED LCT-NYF RAU I PEGMATITE, YUKON – EXAMPLE OF IN SITU CONTAMINATION

The Rau pegmatite field is located approx. 100 km NE of Mayo, Yukon Territory. Multiple fluorine-rich barren and mixed-family pegmatites are hosted in limestone and dolomite of the Bouvette Formation. Origin of the pegmatites can probably be related to the nearby Rackla pluton which is a biotite-muscovite granite with aplitic phases near the margins.

The Rau I pegmatite, geochemically the most evolved pegmatite dyke of the field, is hosted in dolomite-rich limestone. Its exocontact contains common F-rich tremolite, fluoborite, norbergite, Rb-bearing fluorphlogopite, F-rich talc, sulfides, and calcite ≫ dolomite. The pegmatite border zone contains common uvitic tourmaline, F-rich tremolite, F,Rb-rich phlogopite and fluorite. The pegmatite wall and intermediate zones are characterized by common F-rich schorl, muscovite, apatite, and rare beryl. The centre of the pegmatite contains common amazonite, albite, pink and blue tourmaline, muscovite, siderite and fluorite. Primary accessory minerals enclosed in albite include common U,Th-rich zircon (Zr/Hf ~ 15-18, locally up to 1.7), thorite, monazite-(Ce), and columbite-group minerals (CGM). Secondary Ta,Nb-oxide minerals, scheelite and uraninite are common, rare calcioancylite-(Ce) formed after monazite. Tourmaline varies from dravite and uvite to schorl-foitite in boder to intermediate zones to fluor-schorl, fluor-elbaite, and dravite in the pegmatite core.

CGM show primary trend from manganocolumbite to manganotantalite with Mn/(Fe+Mn) ~ 0.95, Ta/(Nb+Ta) ~ 0.05-0.5, and minor (FeTa) → (ScTi) substitution. Secondary CGM are characterized by dissolution-reprecipitation textures and significant Fe-enrichment resulting in Mn/(Fe+Mn) ~ 0.2. Secondary CGM are accompanied by F-dominant microlite, scheelite, and by rare Ta,Nb-rich cassiterite, wodginite and Sc,W,Ti-rich ixiolite/wodginite; the latter is close to the ideal compositions of “wolframowodginite1” [Fe²⁺₄(Sc₂Sn₂)(W₂Ta₆)O₃₂] and of Sc₂SnTa₂O₁₀ (theoretical Sc,Sn-analogue of qitianlingite).

The Rau I pegmatite is a textbook example of in-situ pegmatite contamination. Formation of a B,F-rich exocontact skarn zone and high contents of Ca, Mg, carbonates and sulphides result from the interchange of mass and fluids between the cooling pegmatite body and the dolomite host rock.