Paper Number: 4283
Local controls on mineral transformations: replacement of (Y-Nb-REE)-rich titanite by rutile
Michaut, P.1, Cuchet, S.2, Crumbach, M.2, Burgt, A.2, Etschmann, B.E.1, Pearce, M.A.3, Howard, D.4 and Brugger, J.1
1School of Earth, Atmosphere and the Environment, Monash University, Clayton 3800, Vic, Australia, barbara.etschmann@monash.edu
2BCC group, Lausanne, Switzerland
3CSIRO Mineral Resources Flagship, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
4Australian Synchrotron, 800 Blackburn Road, Clayton 3800, Vic, Australia
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Although Ti, Nb and REE are assumed to be immobile during most hydrothermal and metamorphic processes, many examples of (mostly) small-scale mobility of these elements have been reported [1,2]. Large (> 1 cm), idiomorphic titanite-(I) crystals growing in deformed quartz-feldspar veins within quartz-feldspar-muscovite±biotite gneisses from the Penninic nappes of the Binntal Valley, Switzerland, at amphibolite-facies peak metamorphic conditions are enriched in Y, Nb, REE ± As [3]. Some of these crystals have broken down into assemblages of titanite-(II)+gramaccioliite-(Y)+calcite±(Nb-rich-rutile). The host gneiss shows little evidence of alteration. It is common to observe fresh titanite-(I) in close proximity to titanite-(II). Titanite-(II) displays a patchy distribution of Y and Nb, and gramaccioliite-(Y) forms exsolution-like lamellae within these crystals. However, EBSD reveals that there is no crystallographic relationship between gramaccioliite-(Y) and the host titanite. The titanite-(II) assemblages often further decay to Nb-rich-rutile+gramaccioliite-(Y)±minor phases.
This new pathway of titanite composition [4] illustrates the importance of local conditions and small amounts of fluids for controlling rare metal mineralogy and geochemistry. The unusual reaction pathways of the Binntal titanite-(I) results from the crystallization of titanite probably under the influence of metamorphic fluids enriched in Y, Nb, REE ± As, resulting in an unusual chemistry. During retrograde alteration, the high levels of rare metals become unstable within the crystal structure of titanite. However, due to relatively low diffusion rates of Y + REE + Nb in titanite, titanite-(I) can survive metastably unless it interacts with small amounts of CO2-rich fluids. Importantly, despite textures reminiscent of a solid-state, diffusion-driven process (exsolution), the gramaccioliite-(Y) lamellae are clearly formed via a fluid-driven process. Once initiated, the process usually proceeds to a final assemblage of Nb-rich-rutile+gramaccioliite-(Y).
References: [1] Janots E, Berger A and Engi M (2011) Lithos 121: 1-11; [2] Lottermoser B (1992) Ore Geology Reviews 7: 25-41; [3] Cuchet S, Crumbach MA and Burgt A 2014. Schweizer Strahler 48: 2-55; [4] Middleton AW, Förster HJ, Uysal I T, Golding SD and Rhede D (2013) Chemical Geology 335: 105-117.