Willemite (zinc silicate) forms in a variety of geological environments and by processes ranging from low-temperature alteration of zinc-sulphide ores in arid environments to magmatic-hydrothermal transport and deposition related to highly evolved alkaline magmas. Economic willemite is found solely in carbonate-hosted deposits (e.g., Franklin, New Jersey; Vazante, Brazil; Beltana, South Australia; Kabwe, Zambia), where zinc grades can be greater than 40 wt%.
It is controversial whether carbonate-hosted willemite deposits form by supergene or hypogene alteration of pre-existing sulphide deposits or by primary formation from hydrothermal fluids. Recent data on Vazante, Beltana, and Kabwe indicate formation at temperatures in excess of 150°C and under oxidizing (hematite stable) conditions.
In order to understand the conditions that result in primary willemite formation, especially instead of sphalerite, we have calculated the solubilities of sphalerite and willemite over wide ranges of temperature (25 – 300°C), chloride concentration (0 – 5 molal), dissolved sulphur concentration (0 – 0.1 m), pH (3 - 10) and oxidation potential. The figure below shows the solubilities of sphalerite (dotted surface) and willemite (meshed surface) as a function of temperature and log aO2(g) (referenced to aCH4(g)/aCO2(g)=10,000) at constant pH, Stot, Cltot and quartz saturation. The magnetite-hematite buffer is shown for reference by thick lines on the solubility surfaces. In the presence of sulphur, willemite is predicted to form instead of sphalerite under more oxidising (e.g., magnetite-hematite, sulfate predominant) or alkaline (high pH) conditions, especially at temperatures greater than approximately 150°C. Based on static and preliminary dynamic models, it is likely that willemite will precipitate in response to water-rock interaction and/or fluid mixing processes, e.g., Beltana deposit in South Australia.