Indium mineralisation in SW England: Host parageneses and mineralogical relations

• This is the first time that indium mineralisation has been systematically documented in SW England.
• Indium in SW England is primarily associated with Early-Permian peraluminous granite magmatism.
• Cadmium and Ga were abundantly available prior to the emplacement of the granites.
• In deposited primarily in the sulphide portions of granite-related skarn and vein systems.
• The highest In concentrations are associated with the Carnmenellis and St Agnes pluton, and in this respect follows Sn.
• Indium is hosted by chalcopyrite and sphalerite with less in cassiterite, stannite group minerals and tennantite.
• The majority of the indium budget is in chalcopyrite.
• Roquesite and other discrete In minerals formed where In-bearing chalcopyrite and sphalerite have been replaced.
• No In was detected in löllingite, arsenopyrite, galena, bornite, chalcocite and pyrrhotite.
• No In was detected in rutile, haematite, magnetite, tourmaline, biotite, and chlorite.
• Scattered In in pyrite relate to impurities rather than solid solution.
• The In deportment varies between parageneses with different sphalerite morphologies.
• The mechanism of In incorporation into sphalerite varies with different morphologies.
• Remobilisation of In from granite-related parageneses led to localised concentration into Triassic crosscourse veins.

The SW England ore region contains significant amounts of indium (In) in Early Permian granite-related skarn and lode parageneses and, to a lesser extent, Triassic epithermal “crosscourse” veins. Ore parageneses that predate granite emplacement (Devonian and Lower Carboniferous sedimentary exhalative and vein parageneses) are largely devoid of In. Cadmium (Cd) and gallium (Ga) occur widely in all sulphide-bearing parageneses across the region with sphalerite concentrations locally reaching 1.74 wt% Cd and 1750 ppm Ga.Indium displays a strong affinity to sulphide-bearing magmatic-hydrothermal parageneses. It occurs in silicate-sulphide skarns, polymetallic sulphide lodes and sulphide-bearing portions of greisen-bordered sheeted vein systems and quartz-tourmaline lodes across the region. Magnetite-silicate skarns and quartz-tourmaline lodes that are devoid of sulphide, in contrast, were comparatively unfavourable for In precipitation. The highest In concentrations are found in mineral lodes associated with the Carnmenellis and St Agnes granites, which are the districts that had the highest historical production of Sn. Total In concentrations in these areas locally exceed 430 ppm, while concentrations elsewhere are systematically below 200 ppm.The principal In hosts are chalcopyrite, sphalerite and stannite group minerals with local concentrations within cassiterite and tennantite. Surprisingly, chalcopyrite accounts for the majority of the In budget throughout the region, although concentrations are highest in sphalerite and stannite group minerals. Sphalerite locally contains up to 1.42 wt% In, chalcopyrite has up to 2200 ppm and stannite group minerals up to 6800 ppm. No In was detected in löllingite, arsenopyrite, rutile, haematite, magnetite, tourmaline, biotite, chlorite, galena, bornite, chalcocite or pyrrhotite. Scattered concentrations in pyrite relate to impurities rather than incorporation by solid solution. Roquesite and possibly dzhalindite or native In formed locally where In-bearing chalcopyrite or sphalerite has been replaced by bornite and quartz. The In partitioning between sphalerite and chalcopyrite varies broadly between 1:1 and 10:1.Sporadic In was included in Triassic crosscourse veins as a result of interactions between migrating CaCl2-rich basinal brines and earlier formed granite-related parageneses. The interactions involved at least two distinct components: 1) Incorporation of clasts of magmatic-hydrothermal veins in crosscourse veins during faulting, and 2) Dissolution and re-precipitation of magmatic-hydrothermal vein minerals in crosscourse fluids. Local concentrations reach 140 ppm In.The magmatic-hydrothermal parageneses in SW England are comparable to the South China Tin Belt, Mount Pleasant, as well as Erzgebirge/Krušné Hory. Magmatic-hydrothermal fluids associated with peraluminous granites have developed a variety of skarn, greisen, lode and veins parageneses by interactions with their host rocks and contained fluids. Crosscourse epithermal mineralisation occurred as In was transported by CaCl2-rich basinal brines in a similar manner as In-bearing veins in the West Shropshire ore field.

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