Abstract

Assembling of scattered data regarding the lead of pegmatites, granitic rocks and associated lead ores yields some significant facts and deductions. Pegmatitic feldspars contain two to twenty times as much lead as granites or their feldspars. The lead in major pegmatites commonly duplicates ore lead isotopically, whereas the lead of granites associated with lead ores is much more variable and often distinctly anomalous. These data do not support the common assumption that ores are derived from igneous rocks by a process of magmatic differentiation. Indeed, they justify the suspicion that pegmatites may not be an end product of differentiation, but instead may result from the external addition of a mineralized fluid to the margins of a cooling intrusive along fracture zones, the reaction producing a low melting eutectic of granitic base with exotic accessories. The few data available for minor pegmatites suggest that their lead is more variable, possibly indicative of diverse origins.The meager data on lead in sedimentary rocks and in igneous rocks outside the granite family are inconclusive. Some appear to contain normal lead and others clearly are anomalous.

First Page Preview

First page PDF preview
You do not currently have access to this article.