Abstract

The recent arresting conclusion by Arthur Holmes that lead and accompanying metals and gangue-elements of ores cannot have been derived from granitic or basaltic magmas, because of isotopic differences between "ore lead" and "rock lead," but must have come from a still deeper source is so fundamental and of such great practical importance that it should be carefully tested before being given the acceptance that its alluring qualities might encourage. The present paper examines the validity of (a) Holmes' initial and underlying assumption that all originally existing lead was of uniform atomic weight, the same as found for "ore lead"; (b) the steps leading to calculated values for granitic and for basaltic "rock lead" that differ slightly from "ore lead"; (c) the assumption that such differences imply complete independence of these magmas from the ore-source through all time; (d) consequences in the magma depth-range and in the still deeper region if Holmes' reasoning and results be applied consistently. The finding reached is that the various evidences of magmatic origin of ores carefully assembled through the decades survive Holmes' stimulating challenge without casualty. Some aspects of radioactivity are considered with respect to geological application.

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