Characteristics of Magnetite and Calc-Silicate Minerals in the Gryll’s Bunny Skarn in the Land's End Aureole, SW England

The skarns at Gryll’s Bunny are dominated by garnet and magnetite with small amounts of hornblende, epidote, apatite and tourmaline. They formed discordantly within a succession of metabasalts and metapelites (the Mylor Slate formation) within the metamorphic aureole of the Land’s End granite. The skarns subdivide into discrete mineralogical types that include garnet-skarn with medium-coarse grained garnet, epidote, tourmaline, amphibole and biotite; hornblende-skarn with coarse-grained tabular hornblende, medium grained garnet, epidote, titanite, apatite and tourmaline; the foliated metapelite contains fine-grained hornblende and garnet with alkali feldspar, sericite, muscovite, titanite, quartz, epidote, apatite and tourmaline; cassiterite-rich “tin floors” are overlain by (variably metasomatized) metabasite that include horizontal bodies of tourmalinite with cassiterite, titanite, chlorite and apatite.

The lithologies contain variable amounts of magnetite that can be classified into 5 types. Magnetite in the metapelite (type 1) is very fine grained. Magnetite in hornblende-skarn associated with the metapelite (type 2) is fine grained with ilmenite lamellae and is associated with maghemite. Magnetite in the hornblende skarn adjacent to garnet skarn (type 3) contains abundant ulvöspinel lamellae. Magnetite in the garnet skarn is medium to coarse-grained with a granular recrystallized texture and spinel exsolutions (type 4). All of these types have been partially replaced by hematite along edges and cracks. Magnetite related with the tourmaline zone (type 5) is generally euhedral and free of exsolution lamellae. In addition, the tourmaline-cassiterite zone has abundant titanite with ilmenite laths.

Fluid inclusions in garnet, amphibole and epidote of the metasomatized rocks, garnet related with type-3 magnetite has higher homogenization temperature (291- >600 ^oC) and almost similar low-moderate salinity (2.4-13.7 wt. % NaCl equiv.) than that of type-2 magnetite (222-428 ^oC and 3.9-14.8 wt. % NaCl equiv).

EPMA and LA-ICP-MS analysis demonstrate that garnets are of grossular (60-76)-andradite (13-32) composition and rich in TiO₂; amphiboles are sadanagaite-pargasite, tourmalines are shorl-feruvite and apatites are fluor- and hydroxyl-apatite composition. V/Ti and Ga/Ti in magnetite decrease progressively from type 1 to 5, indicating that type 1 and 2 retain characteristics of their mafic host rock as well as metamorphic process, development towards type 5 is interpreted by the increasing significance of granitic fluids. All of the magnetite types have elevated Sn and Zn whilst Zr, Mg and Al are low. The homogeneity of type 5 magnetite supports a purely metasomatic origin at the final stage of skarn development.

Key words: SW England, Gryll’s Bunny skarns, Botallack, Magnetite, Mineralogy, Geochemistry, Fluid inclusions