Non-uniformly scaled tendencies in evolution of ore composition of volcanic-associated massive sulfide deposits
Changes in average grades and ratios of major and minor elements of volcanogenic massive sulfide ores reveal two tendencies differing in scale of their manifestation in geological time and space. One of them is expressed by evolution of geochemical features of deposits formed in relatively short periods of ore formation in some separately taken ore provinces. It results from evolution of tectonic regime and corresponding changes in composition of magmas. Tendency of another kind is manifested globally and expressed by secular changes in ore chemistry of deposits formed in the last 3.5 Ga during various epochs of massive sulfide ore formation. Its plausible reason is density differentiation of the matter of the Earth, gradual decrease of temperature in the upper mantle, and concerned evolution of tectonic regimes and processes of magma generation.