GEOCHEMICALLY ANOMALOUS, ULTRAMAFIC AND MAFIC ROCKS OF THE PRECAMBRIAN BASEMENT IN SOUTHEASTERN LITHUANIA

Abstract

The europium minimum often observed in Archean ultramafic and mafic rocks reflects both the effect of protomagmatic processes (fractional crystallization, accompanied by removal of feldspars and clinopyroxene from the system, and contamination by crustal material in intermediate magma chambers), and secondary redistribution of rare earth elements (REE) as a result of metamorphism and metasomatism [7]. Recorded REE anomalies, especially in the rocks of southeastern Lithuania, are also emphasized by the anomalous distribution of both rock-forming and trace elements. Geochemical data indicate that rocks in some cases, e.g., the Kambalda komatiites in Australia, despite the virtually total reworking of their original mineral composition by low-temperature metamorphism, have mainly retained their primary bulk composition, produced by the magmatic process and characteristic of ultramafic melts. In other ultramafic and mafic, rocks such as those in southeastern Lithuania and in part, on the Aldan shield, the concentrations of SiO2, Cr, Ni and REE and the distribution patterns of the latter elements are geochemically anomalous and cannot be explained by any one model. Statistical treatment of data on 124 gold deposits in such greenstone belts, which have produced 5054 tons of gold, shows that 4451 tons of metal from 99 deposits ultramafic and mafic volcanics. Gold deposits are selectively confined to parts of belts metamorphosed in the greenschist facies, although a number of large gold-ore bodies are localized in rocks metamorphosed under epidote-amphibolite and even amphibolite facies conditions. Additional aspects of the subject are discussed.