ZINC OXIDE AND ZINC SILICATE DEPOSITS–A NEW LOOK

Non-sulfide zinc deposits can be broken into three major categories: supergene zinc oxide deposits, hypogene zinc silicate (sulfide) deposits, and metamorphic zinc silicate deposits. Supergene deposits form from the oxidation of generally carbonate-hosted, sulfide-bearing deposits. This relationship is due to the high reactivity of carbonate minerals with the oxidized acidic, zinc-rich fluids derived from the breakdown of sphalerite-rich bodies. Resulting zinc oxide bodies are commonly twice to an order of magnitude higher in grade than the original sulfide bodies. Supergene zinc deposits occur both in arid climates (similar to classic supergene copper oxide deposits) and in actively oxidizing tropical environments. Supergene deposits may be subdivided into three categories: direct replacement of sulfides, wallrock replacements, and saprolitic accumulations. Though most are extremely small, large examples (such as LanPing, Yunnan, China) are known. Hypogene zinc silicate (sulfide) deposits consist of veins and irregular pipes of willemite-sphalerite-(hematite) in carbonate rocks. They appear to form by the interaction of oxidized, silica-saturated, sulfur-poor hydrothermal fluids with carbonate host rocks. The largest known deposit of this type is Vazante, Brazil; other important deposits are Beltana, Australia and Kabwe, Zambia. The deposits commonly display silicification of the host carbonates in the vicinity of mineralization. Vazante and Beltana have undergone intense ferroan dolomitization in the area of mineralization prior to willemite formation. Textural evidence indicates that early willemite precipitates with sulfides and iron oxides (Vazante and Kabwe) or alone (Beltana). The deposits occur within sedimentary sequences which contain, or for which there is evidence once contained, evaporites. Metamorphic zinc silicate deposits are restricted to two examples (Franklin Furnace and Sterling Hill, New Jersey). They consist of stratabound lenses of franklinite-willemite -zincite in metacarbonate rocks within high-grade metamorphic terrains. Their genesis is controversial though they may be related to the BHT family of deposits.