Indium mineralisation in SW England: Host parageneses and mineralogical relations

Abstract

The SW England ore region contains significant 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), in contrast, 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, veins and breccias across the region. Magnetite-silicate skarns and quartz-tourmaline lodes, veins and breccias 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. 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. Sphalerite locally contains up to 1.42 wt% In, chalcopyrite has up to 2200 ppm and stannite group minerals up to 6800 ppm. Although In concentrations are highest in sphalerite and stannite group minerals, chalcopyrite accounts for the majority of the In budget throughout most of the region. Roquesite and possibly dzhalindite or native In formed locally where In-bearing chalcopyrite or sphalerite have been replaced by bornite and quartz. The In partitioning between sphalerite and chalcopyrite varies broadly between 1:1 and 10:1.

In the granite-related parageneses, the In deportment varies across the different parageneses, and this variation is systematically related to different sphalerite morphologies. Chalcopyrite contains most of the In budget within parageneses that have large, unzoned sphalerite grains with chalcopyrite disease or only sparse, minute grains of sphalerite. In contrast, zoned sphalerite without chalcopyrite disease carries the majority of the In budget in the evolved parts of the polymetallic sulphide lodes. Although chalcopyrite is also abundant here, it is much less significant as a host.

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 region-wide dominance of chalcopyrite as an In host is unexpected and appears to be at variance with magmatic-hydrothermal parageneses elsewhere. This suggests that processing strategies that focus exclusively on sphalerite are likely to have little success. Detailed mineral deportment assessments must be a pre-requisite for the design of successful processing strategies in the region. Interestingly, we have no reason to believe that SW England is different to other regions with granite-related magmatic-hydrothermal parageneses, and it is possible that the global potential for In recovery from chalcopyrite has been underestimated.

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.