Distribution of trace elements in willemite from the Belgium non-sulphide deposits

Samples of willemite (Zn₂SiO₄) mineralization from the historical non-sulphide Zn–Pb deposits of La Calamine (eastern Belgium) have been recovered from collections of the Geological Survey of Belgium. Textural and chemical analyses are used to evaluate the critical element distribution (Ge, In, Ga) and deportment. willemite occurs as a variety of types that continuously formed between the protore stage (sulphides) and the late supergene stage (carbonates and hydrated phases). Different types of willemite may be distinguished on the basis of their shape and zoning characteristics, supporting a polyphase non-sulphide mineralization after the protore stage. This is also marked by a significant change of major-element compositions in the late generation of willemite. Laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) measurements of minor and trace elements also reveal a strong variability between the different willemite types, although no straightforward relation with the shape of willemite crystals or their zoning is demonstrated. Among the trace elements, we note anomalous high contents of P, Cd, As, Pb, Ag and Sb, the three latter ones being related to tiny galena inclusions. While Ga and In contents are very low (less than 4 ppm) or below detection limits, respectively, significant Ge contents up to 250 ppm were measured. Such contents are consistent with values reported from willemite mineralization throughout the world. The concentrations measured in willemite are similar to those in sphalerite (averaging 250 ppm), supporting a role as precursor for sphalerite. However, the supergene origin of willemite in Belgian deposits is controversial and the influence of low-temperature hydrothermal fluids for willemite precipitation cannot be ruled out. This also questions the origin of Ge further incorporated in zinc silicates.