GEOLOGICAL OCCURRENCE
Zinc and lead are commonly associated in mineral deposits, sometimes intimately mixed, sometimes segregated enough so that one metal predominates, but seldom free entirely one from the other. The geological and geographical distribution of the two metals is, therefore, nearly identical. Galena is the most common and important of the lead minerals. Cerussite, anglesite and pyromorphite usually result from the oxidation of galena, the sulphate being usually an intermediate state in the oxidation to the carbonate. Pyromorphite and wulfenite are of minor importance. Jamesonite is more an ore of antimony than of lead. Sphalerite (zinc blende) weathers more readily than galena, and therefore zinc in many places is carried below water level more rapidly and completely than lead. For this reason some mines change from predominantly lead mines to zinc mines with greater depth. Apart from the effect of such secondary enrichment, this change is often encountered in primary ores with increase in depth.
Lead ores occur in deposits of several distinct genetic types. Many deposits lie at shallow depth in sedimentary rocks, without apparent connection with igneous rocks, occurring as tabular replacements of receptive beds. In regions of slightly disturbed strata the ore shoots tend to follow pitching troughs. Ores of this type usually contain lead (galena), zinc (sphalerite), and iron (pyrite) minerals; many contain manganese and cadmium; some contain cobalt and nickel; but few carry gold, silver, copper, or antimony. Deposits of this type are of world-wide distribution, and many are extensive and commercially important. The greater purity of the ore and the simplicity of treatment (particularly for the ores in the oxidized zones), caused them to be exploited first and most extensively, and to dominate formerly the world production of lead. To this type belong, with many others, the deposits of the Mississippi Valley and Silesia, that together yielded 15 per cent. of the world’s production in 1913.
Other important deposits are closely associated with igneous rocks, and are characterized by complex ores. They comprise vein deposits, disseminated replacements of igneous rocks, and silver-lead replacements in limestone.