Mineralogy and geochemistry of indium-bearing polymetallic vein-type deposits: Implications for host minerals from the Freiberg district, Eastern Erzgebirge, Germany

Abstract

Located at the northwestern border of the Bohemian Massif in the European Variscides, the Erzgebirge is one of the most important Sn–W–Mo, Ag, Cu–Zn–Pb–In, U, and Bi–Co–Ni metallogenetic provinces in Europe. The ca. 1100 silver-base metal veins in the Freiberg metamorphic core complex are characterized by two principal types of late-Variscan polymetallic vein-type mineralization: (1) Quartz-bearing As(–Au)–Zn–Cu(–In–Cd)–Sn–Pb–Ag–Bi–Sb polymetallic sulphide association (‘kb’ ore-type), and (2) Carbonate- or quartz-bearing Ag–Sb polymetallic sulphide association (‘eb’ and ‘eq’ ore-type). High indium concentrations in the Freiberg silver-base metal vein district and other base metal and tin-polymetallic deposits suggest that the Erzgebirge is among the largest In-enriched ore provinces known worldwide. The first modern geochemical bulk ore and microprobe analyses addressing In distribution in the Freiberg district are presented in this paper and are compared with published data. Polymetallic veins in the Freiberg district show a wide range of In concentrations up to 0.15 wt.% with an average of 176 ppm ( n = 82). The ‘kb’ ore-type veins in the ‘Freiberg’ (up to 1560 ppm In, mean 253 ppm, n = 36), ‘Muldenhütten’ (up to 785 ppm In, mean 284 ppm, n = 10), and ‘Brand’ ore fields (up to 638 ppm In, mean 156 ppm, n = 15) occur the highest In resources in the Freiberg district. Two types of In concentration can be distinguished, based on microanalytical study. The first type was found in sphalerites of the Zn–Sn–Cu sequence (as presented in one of the figures in this article) which show In contents up to 0.38 wt.%, significant Cd up to 1.11 wt.%, and Ga contents up to 0.17 wt.%. Iron-rich sphalerites (mean 12.9 wt.% Fe, n = 202) from a representative Ag-base metal vein are characterized by In contents between 0.03 and 0.38 wt.% (mean 0.16 wt.% In, n = 202). A negative correlation exists between (Zn + Fe) and Cd, and (Zn + Fe) with In, reflecting structural substitution of In and Cd in sphalerite. The second type of In enrichment was identified as microscopic Zn–Cu–Sn–In–S grains in pyrite of a Cu-rich ‘kb’ vein. Quantitative electron images of these grains (up to 6 μm) shows high levels of Zn (5.6 to 52.8 wt.%), Cu (4.1 to 19.6 wt.%), Sn (0.3 to 17.2 wt.%), and In (1.3 to 2.9 wt.%). In the ternary (Cu + Ag)–(Sn + In)–(Zn + Fe) diagram, compositions of the Zn–Cu–Sn–In–S phase in a representative Cu-rich ‘kb’ ore-type sample fall along a linear compositional trend between Fe–Cu–In-rich sphalerite (Zn 0.76Fe 0.11Cu 0.06In 0.01S) and the ideal fields of petrukite and sakuraiite (Cu 0.29Zn 0.08Fe 0.32In 0.02Sn 0.13S). Both types of In enrichment support that the In-mineralization is associated with the Zn–Sn–Cu sequence (‘indium stage’) of the ‘kb’ ore-type association. Based on mineralogical, geochemical, isotopic, fluid inclusion, age relationships and structural data, the high In concentrations in base metal veins in the Erzgebirge may indicate an influence of fluids expelled from magmas during emplacement of post-collisional lamprophyric and rhyolitic dikes. The high In concentration of Cd- and Fe-rich sphalerites from Ag-base metal and Sn-polymetallic deposits in the Erzgebirge is an additional argument for a genetic link between these mineralization stages. Such evidence is significant for exploration of magma-affiliated In deposits in post-collisional settings.