| dc.description.abstract | The skarns at Gryll’s Bunny lie within a succession of Devonian aged metabasites and metapelites (the Mylor Slate Formation) within the metamorphic aureole of the Permian Land’s End granite. Interactions between the granite and wall rocks caused the formation of exoskarns within the metapelitic and meta-basic wall rocks. Prograde exoskarn zones contain grossular-domain garnet, hornblende, alkali feldspar, titanite, apatite, tourmaline, axinite, biotite, fluorite and quartz. These have a retrograde overprint characterised by hydrous minerals such as epidote, biotite, sericite, muscovite and chlorite. Fe-Ti mineralisation occurs as disseminated, vein and/or massive forms of variable grain sizes.
From electron microprobe analysis, the composition of garnet in the garnet skarn is andradite-grossular, whereas that in the metabasites is grossular. Amphibole in the metabasites is magnesiosadanagaite-sadanagaite, whereas that in the garnet and amphibole skarn is nearer to the composition of pargasite-ferropargasite-hastingsite.
As a result of geochemical studies on the magnetite, metamorphic magnetite in the metabasites have relatively high V/Ti ratios and high V, Ga and Mn concentrations with complicated exsolution and replacement textures. However, they were modified as a result of skarnification to show lower V/Ti ratios and lower Ti, Al, Mg, Mn and Fe concentrations and gained a euhedral texture.
The temperature and salinity of the skarn-forming fluids, as estimated from fluid inclusion studies, were 291.2->600 ⁰C and 2.4-13.7 (wt. % NaCl equivalent) for the prograde stage, 222-428 ⁰C and 3.9- 14.8 (wt. % NaCl equivalent) for the retrograde stage.
The magnetite mineralisation originated from the meta-basic host rock involving a certain amount of Sn concentration, indicating that some Sn was present in the system during regional metamorphism before the emplacement of the Land`s granite. Granite-derived fluids caused metasomatism and recrystallisation of silicate and oxide minerals, Sn and F enrichment and Ca mobilisation. After granite emplacement, the skarns were affected by B-, P- bearing hydrothermal fluids, which caused precipitation of Cu-containing magnetite and subsequently tin mineralisation as Gryll`s Bunny tin floors. | en_GB |
