LEAD (PB) BEHAVIOUR IN CRUSTAL EVOLUTION

The propensity of certain metals to concentrate themselves into ore deposits at specific periods in the earth's evolution has been well documented. Most notable are high concentrations of gold in Archean greenstones, enormous volumes of iron in Proterozoic banded iron formations, and the restriction of most copper +/- molybdenite base metal porphyry deposits to Mesozoic and Cenozoic age. Though it has received little attention, lead (Pb) also expresses temporal/spatial behaviour through earth history. Sulphide deposits with significant Pb content are virtually absent in the Archean, whereas giant volcanic-hosted and clastic-hosted massive sulphide Pb-Zn deposits appear in abundance in the Mesoproterozoic. In the context of crustal evolution, Pb follows a pattern coincident with the development of a negative Eu associated with post-Archean igneous rocks. Most Archean igneous rocks exhibit an overall HREE depletion implying derivation from melting of mantle peridotite. In comparison, younger igneous rocks record a negative europium anomaly suggesting derivation from melting of a plagioclase-bearing source such as Archean Tonalite-Trondjhemite-Granite crust (TTG). As with europium in plagioclase, Pb favours potassium feldspar as its silicate host. It is postulated that processes associated with the melting of TTG to form Eu-depleted melts, also removed the Pb held in potassium feldspar. The cratonized Archean lithosphere, fluxed by Cl-S fluids derived from the subduction of highly altered oceanic lithosphere, set the stage for prolific Pb-sulphide mineralization in the Mesoproterozoic. Since that time, sulphide deposits with a high Pb component are formed only near cratonic basement or near sedimentary material derived from it. This includes juvenile terranes in which pre-accretionary base metal deposits are largely devoid of Pb, whereas high Pb-sulphide deposits appear in deposits formed after accretion onto or near Precambrian nuclei.