Advanced Identification and Quantification of In-Bearing Minerals by Scanning Electron Microscope-Based Image Analysis
Abstract The identification and accurate characterization of discrete grains of rare minerals in sulfide base-metal ores is usually a cumbersome procedure due to the small grain sizes (typically <10 μm) and complex mineral assemblages in the material. In this article, a new strategy for finding and identifying indium minerals, and quantifying their composition and abundance is presented, making use of mineral liberation analysis (MLA) and electron probe microanalysis (EPMA). The method was successfully applied to polymetallic massive sulfide ores from the Neves-Corvo deposit in Portugal. The presence of roquesite and sakuraiite could be systematically detected, their concentration quantified by MLA measurements, and their identity later confirmed by EPMA analyses. Based on these results, an almost complete indium deportment could be obtained for the studied samples. This validates the approach taken, combining automated mineralogy data with electron microprobe analysis. A similar approach could be used to find minerals of other common minor and trace elements in complex base-metal sulfide ores, for example Se, Ge, Sb, or Ag, thus permitting the targeted development of resource technologies suitable for by-product recovery.
Indium and selenium distribution in the Neves-Corvo deposit, Iberian Pyrite Belt, Portugal
ABSTRACT High concentrations of indium (In) and selenium (Se) have been reported in the Neves-Corvo volcanic-hosted massive sulfide deposit, Portugal. The distribution of these ore metals in the deposit is complex as a result of the combined effects of early ore-forming processes and late tectonometamorphic remobilization. The In and Se contents are higher in Cu-rich ore types, and lower in Zn-rich ore types. At the deposit scale, both In and Se correlate positively with Cu, whereas their correlations with Zn are close to zero. This argues for a genetic connection between Cu, In and Se in terms of metal sourcing and precipitation. However, re-distribution and re-concentration of In and Se associated with tectonometamorphic deformation are also processes of major importance for the actual distribution of these metals throughout the whole deposit. Although minor roquesite and other In-bearing phases were recognized, it is clear that most In within the deposit is found incorporated within sphalerite and chalcopyrite. When chalcopyrite and sphalerite coexist, the In content in sphalerite (avg. 1400 ppm) is, on average, 2–3 times higher than in chalcopyrite (avg. 660 ppm). The In content in stannite (avg. 1.3 wt.%) is even higher than in sphalerite, but the overall abundance of stannite is subordinate to either sphalerite or chalcopyrite. Selenium is dispersed widely between many different ore minerals, but galena is the main Se-carrier. On average, the Se content in galena is ~50 times greater than in either chalcopyrite (avg. 610 ppm) or sphalerite (avg. 590 ppm). The copper concentrate produced at Neves-Corvo contains very significant In (+Se) content, well above economic values if the copper smelters recovered it. Moreover, the high In content of sphalerite from some Cu-Zn ores, or associated with shear structures, could possibly justify, in the future, a selective exploitation strategy for the production of an In-rich zinc concentrate.
La cassitérite zonée du gisement de Sokhret Allal (Granite des Zaër; Maroc Central): Composition chimique et phases fluides associées
The chemical composition of a zoned cassiterite from the tungsten-tin deposit of Sokhret Allal (Zaër granite, Central Morocco) has been analyzed with an electron microprobe. The mineral zoning is related to variable Fe, Ti, and Nb contents: the darker zones are enriched in titanium and iron whereas lighter zones are purer. Fluid inclusions contemporaneous with cassiterite deposition are primary, and the fluids are aqueous with a composition of 97–99 mol% H2O and 3–1 mol% eq. NaCl (temperature=270–330 °C) and were trapped in the liquid-stability field. The contribution of these data to the central Morocco W-Sn metallogenesis is discussed.RésuméLa composition chimique d'une cassitérite zonée provenant du gisement de tungstène-étain de Sokhret Allal (Granite des Zaër; Maroc Central) a été analysée par microsonde électronique. La zonation optique du minéral correspond à des variations en éléments mineurs (Fe, Ti, Nb): les bandes sombres apparaissent corrélées à une augmentation en titane et fer tandis que les bandes claires sont plus pures. Les inclusions fluides contemporaines du dépôt de la cassitérite sont primaires et constituées de fluides typiquement aqueux de composition (97 a 99 moles % H2O, 3 à 1 moles % équivalent NaCl, température=270–330 °C) et sont piégées en phase liquide. La contribution de ces nouvelles données à la métallogenèse W-Sn du Maroc Central est discutée.