Lithological units and Sn deportment of a compositionally complex skarn ore (Hämmerlein, Erzgebirge, Germany)

The Hämmerlein seam is part of the world class Tellerhäuser deposit in the Erzgebirge, Germany, and represents a compositionally complex polymetallic Sn-In-Zn skarn. Current resources amount to 100000 t Sn at a cut-off grade of 0.2 wt. %. In addition, 2100 t of In and 270000 t of Zn have been estimated [1]. In the late 1970s, 50000 t of ore from the Hämmerlein seam were mined and processed experimentally in a pilot plant, but grade and recovery remained below expectations. Cited reasons for poor recovery the complex mineralogy and variability in grain sizes of valuable minerals [2]. The predicted rise in global Sn consumption and limited availability of high grade deposits [3] render the Tellerhäuser deposit an interesting exploration target [1]

A consortium of German research institutions currently conducts new beneficiation experiments on the Hämmerlein orebody. Determination of the Sn deportment and the characterization of the different lithological (skarn) units are the first steps in this process. For this purpose, three transects in the central part of the Hämmerlein orebody were mapped and a suite of hand specimen collected to represent all relevant lithotypes within the studied part of the orebody. Thin sections were prepared and analyzed using the Mineral Liberation Analyzer (MLA) to obtain quantitative data about mineralogy, mineral grain sizes, intergrowths, and associations. The remaining material of the hand specimen was crushed to 99 % < 250 µm. This granular material was split to produce grain mounts for further mineralogical studies and in order to prepare sample powders for geochemical analysis.

The Hämmerlein skarn orebody can be subdivided into the following three macroscopically distinct lithotypes: 1. magnetite-dominated (40 – 80 wt. % magnetite), 2. sulphide-dominated (> 20 wt. % sphalerite) and 3. silicate-dominated (> 60 wt.% silicates). In the silicate-dominated unit a gradual transition of different silicate minerals enables further discrimination of a chlorite-rich, an amphibole-chlorite-rich, an epidote-pyroxene-rich and a garnet-rich subunit. The hanging and footwall are best described as mica schist and gneiss, respectively.

The primary host mineral for Sn is cassiterite (SnO2) with grain sizes between 1 µm and 1 mm. Some of the cassiterite has fibrous crystal habit. Significant amounts (ca. 1.4 wt. %) of coarse-grained (50 µm to 1 mm) cassiterite is present in the chlorite subunit. The amphibole-chlorite subunit contains an average of 0.3 wt. % cassiterite. Samples from other parts of the Hämmerlein orebody indicate significant amounts of cassiterite in the magnetite- and the sulphide-dominated lithotypes as well.

Stokesite (CaSnSi3O9 ∙ 2H2O) is the second most abundant Sn mineral. It appears in fine-grained aggregates in the amphibole-chlorite subunit and in the magnetite-dominated ore type reaching concentrations of ca. 0.1 wt. %. Notable Sn concentrations were detected by WDX in some examples of titanite (≤ 21 wt. %), epidote (≤ 4.4 wt. %), amphibole (≤ 1.9 wt. %), garnet (≤ 2.3 wt. %), and iron oxides (≤ 2.3 wt. %).

Our preliminary results illustrate that the Sn mineralisation of the Hämmerlein skarn is indeed very complex. Cassiterite dominates, but other minerals (most notably stokesite) do contribute significantly to the deportment. Further studies will aim to quantify the variability of deportment and other resource characteristics, in order to guide mineral processing test work.