Mineralogical deportment of indium in the Neves-Corvo deposit - Implications for recovery and extraction

Neves-Corvo is a world-class volcanic-hosted massive sulphide (VHMS) deposit located in the Iberian Pyrite Belt (IPB). It is both one of the largest and richest deposits in the IPB. Besides its size, other notable features include its high tin content and the unusual ore types within which most of this tin was originally hosted. High indium concentrations (30 - 50 ppm in whole ore) also make it an attractive source of this rare metal. However, the mine does not currently profit from this potential.

It was the aim of this study to generate the data required for a detailed quantitative evaluation of different valorisation options for indium at Neves-Corvo. In addition to data on the spatial distribution of the element, this also requires a detailed understanding of its mineralogical deportment. While previous studies had reported elevated indium concentrations in a number of different minerals (e.g. stannite, sphalerite, cassiterite, tetrahedrite-tennantite, chalcopyrite) as well as the occurrence of two discrete indium minerals (roquesite, CuInS2, and sakuraiite, (Cu, Zn, Fe)3(In,Sn)S4), modal mineralogy and consequently the deportment of indium in the studied samples were never quantified.

In the present work, a combination of whole ore geochemistry, automated scanning electron microscope (SEM)-based image analysis and electron probe microanalysis (EPMA) was used to study the mineralogical deportment of indium in a representative set of more than 70 ore and process samples. It was found that, depending on ore type, sphalerite and/or chalcopyrite are the most important host minerals while stannite, roquesite and sakuraiite are not important due to their generally low abundance. Indium concentrations in sphalerite are highly variable, but are usually 2 - 3 times higher than in coexisting chalcopyrite. The exact concentrations depend on the total concentration of indium in the ore in relation to the abundance of these two minerals. This apparent equilibrium partitioning behaviour is thought to be mostly a consequence of the extensive syntectonic recrystallisation of the ore minerals.

The clear dominance of sphalerite and chalcopyrite as indium carriers has obvious consequences for valorisation options at the mine. First, the indium contained in the ores should be recoverable from the ores with the zinc and copper concentrates. Second, the production of separate concentrates enriched to the minimum concentrations required by smelters will only be possible by the separate processing of indium-rich ores.

To the best knowledge of the authors, this is the first detailed deportment study for indium ever conducted. The results have obvious implications not only for indium valorisation at Neves-Corvo, but also in other massive sulphide deposits affected by extensive tectonically induced recrystallisation. They should therefore be of interest to the wider geometallurgical and economic geology communities.

This is a contribution to the ZHINC project (PTDC/CTE-GIX/114208/2009). Thanks are due to the authors' host institutions for the provision of the funding and infrastructure necessary to conduct this work.