The data of lead isotope geology related to problems of Ore Genesis

Searching questions about the origins of ore deposits have been asked for many years. Where do ore fluids originate-within the crust, or deeper? Can specific sources of ore metals be identified? When were ores deposited? How long did mineralization continue? What was the rate and paragenesis of deposition? Which deposits are cogenetic, and what metallogenetic provinces do they define? Isotope analyses of lead from rocks and ores provide a new tool for attacking such problems of ore genesis. About two thousand such analyses now available from more than 20 laboratories have been compiled and studied for this purpose. The data now available suggest partial answers to some questions and serve to define other, new problems, not previously recognized. Their greatest import, however, is to illustrate the potential future significance of lead-isotope studies. Ore-lead, as shown two decades ago, ranges widely in isotopic composition. Recent studies show that traces of lead in ordinary rocks of the earth's crust exhibit apparently similar variations. All these variations are attributable primarily to mixing of old lead with accumulations of new radiogenic lead from decay of radioactive uranium and thorium. Each sample of lead preserves in its isotopic composition a record of its own mixing history: a composite record of the time this lead was in different environments, its past associations with uranium and thorium, and the geochemical processes that moved it from place to place. Integrated study of all these variables will throw light on many difficult problems of ore genesis when adequate techniques are developed. Progress will be slow, however, until more accurate lead-isotope analyses can be made at lower cost and more objective criteria developed for interpreting measured isotopic compositions in terms of geologic history.