The sulphide ores, chiefly sphalerite or blende, are the most important, but in the oxidized zone they are often altered to carbonates—smithsonite, calamine and hydrozincite. The oxides—franklinite, willemite and zincite—are important in only one district in New Jersey. The carbonates, although they carry a low percentage of zinc, often occur in concentrated ore bodies, and yield readily to metallurgical treatment. Therefore, calamine-smithsonite ores form a large proportion of the production of many important districts, but blende will hereafter be of increasing importance in the world production.
Zinc ores occur in deposits of several distinct genetic types. In the order of their importance they are:
(a) Deposits formed in sedimentary rocks, without apparent connection with igneous rocks, as bedded replacements usually of limestones and dolomites. The ores of this type usually contain lead (galena) and iron (pyrite or marcasite) minerals, often those of manganese and cadmium, sometimes those of arsenic, cobalt and nickel, but seldom gold, silver, copper, or antimony. Barite and fluorite are sometimes present.
The deposits of this type are of world-wide distribution. Their greater purity and the simplicity of the treatment necessary, particularly of the ores in their oxidized zones, have caused them to be exploited first and most extensively and to be until recently the dominant factor in the world production of zinc. To this type belong the deposits of the Mississippi Valley and Silesia, which together produced 34 per cent. of the world’s output in 1913.
(b) Veins associated with igneous rocks and disseminated sulphide replacements of igneous rock. In this group come the deposits of Butte, Leadville, and the Coeur d’Alene, and the disseminated deposits of the Bawdwin (Burma Mines Co.), Ridder (Siberia) and Salmon River (B.C.). These ores are usually complex, comprising minerals of zinc, lead, copper, iron, gold and silver, and often arsenic, antimony and other metals. In one group, the silver-lead deposits, zinc seems a minor factor, but with depth replaces lead as the predominant metal. Deposits of these complex ores have in recent years become important sources of zinc through recognition of the zinc ores in their oxidized zones, through zinc becoming the dominant metal with increasing depth at many mines, and especially through improvements in methods of concentrating complex ores and extracting metal from the concentrates. Ores of this type will be of increasing importance in the future because of their world-wide distribution.
(c) Igneous metamorphic deposits containing franklinite, willemite, zincite, a little blende, and a gangue of calcite, rhodonite, garnet, pyroxene, hornblende, magnetite and tremolite. This type is characteristic of the Franklin and Adirondack deposits of the northeastern metallographic province of the United States and is also found at Magdalena and Hanover, New Mexico and the Horn Silver mine, in Utah.
(d) Metamorphosed deposits. Originally these may have been of any of the preceding types but are now disguised by regional metamorphism. The best example is the important deposit of Ammeberg, Sweden. Blende there occurs disseminated in bands in gneissoid granulite, which also contains bands of disseminated pyrrhotite and arsenopyrite.
GEOGRAPHICAL DISTRIBUTION
The chief zinc ore deposits of the world are in the countries listed in the [table] below; the order of the countries is that of their importance in the industry in 1913, as nearly as can be estimated from incomplete data.
The three major metallographic provinces of the world as indicated by present exploitation are those of Broken Hill, N.S.W., Silesia, and the Mississippi Valley.