Zinc is a natural component of the earth’s crust and an inherent part of our environment. Zinc is present not only in rock and soil, but also in air, water and the biosphere. Plants, animals and humans contain zinc.
Minerals and metals are mostly obtained from the earth’s crust. The average natural level of zinc in the earth’s crust is 70 mg/kg (dry weight), ranging between 10 and 300 mg/kg (Malle 1992).
In some areas, zinc has been concentrated to much higher levels by natural geological and geochemical processes (5-15% or 50,000-150,000 mg/kg). Such concentrations, found at the earth’s surface and underground, are being exploited as ore bodies.
Zinc ore deposits are widely spread throughout the world. Zinc ores are extracted in more than 50 countries. China, Australia, Peru, Europe and Canada are the biggest zinc mining countries. Zinc is normally associated with lead and other metals including copper, gold and silver. There are four major types of zinc deposits:
Volcanic hosted massive sulphides (VMS)
VHMS deposits are polymetallic and are an important economic source of copper and zinc often associated with significant concentrations of silver, gold, cadmium, bismuth or tin.
Carbonate hosted (Mississippi Valley & Irish types)
Limestone and dolomite are the most common host rocks. The zinc lead content usually ranges from 5%-10% with zinc usually predominating over lead. Concentrations of copper, silver and barite of fluorite may also be present.
Sediment hosted (sedex deposits)
The host rocks are mainly shale, siltstone, and sandstone. Sedex deposits represent some of the world’s largest accumulations of zinc, lead and silver. The mineral has a high silver content. The lead/zinc content ranges from 10-20%.
Intrusion related (high sulphidation, skarn, manto, vein)
These deposits are typically found in carbonate rocks in conjunction with magmatic-hydrothermal systems and are characterized by mineral association of calcium and magnesium. Typically the ore body contains more lead than zinc and is associated with silver.
The most commonly found zinc mineral is sphalerite (ZnS) also known as zinc blende, which is found in almost all currently mined zinc deposits. The mineral crystallizes from the hydrothermal solution as purezinc sulphide.
The mineral marmatite is a complex zinc-iron sulphide, which is commonly found but rarely exploited as it is not easy to smelt.
Zinc deposits close to the earth’s surface are often converted to oxides and carbonates. Small quantities of zinc carbonate - the mineral calamine (smithsonite) in North America - often refer to the hydrated silicate mineral also known as hemimorphite.
Iron and lead sulphides, in the form of the minerals pyrite and galena are always associated in significant quantities while smaller quantities of other metals are commonly found.
Metamorphically formed oxide zinc ores such as franklinite or zincite are limited to only a few deposits.