Factors of Concentration of Chalcophile Elements in the Evolution of Plateau-Basaltic Magmas

The geochemical evolution of ore-bearing trappean intrusions particularly from the Siberian platform is discussed. Analysis of empirical geochemical and experimental data shows, on the one hand, that it is reasonable to suggest that chalcophile elements were concentrated together with the sulfide melt in the lower horizons of the bedded bodies due to gravitational accumulation of the sulfide melt and, on the other hand, that the mechanism of sulfurization of basic magmas is the most possible one to explain the formation of sulfide liquid in sufficient quantities to concentrate ore components in economic amounts.

Within the framework of the hypothesis of magmatic sulphurization, the source of ore elements is the parental magma. The source of sulfur seems to be of crustal nature. Since the sulfide melt formed after the sulfurization mechanism is in equilibrium with the silicate part of the system, it does not differ from the sulfide melt formed due to liquation of the sulfur dissolved in the magma. There are as yet no data enough for a quantitative estimation of the contribution from the sulfide-silicate liquation which is, undoubtedly, manifested steadily. However, it may be stated that the role of sulfide-liquid liquation in the ore genesis is negligible.

In terms of the above mechanism, the conditions necessary for chalcophile elements to be concentrated at the magmatic stage in ore amounts are estimated: 1. Primary enrichment of ore elements in magma at the pre-chamber stage effected by the formation of magnesian melts. 2. The contact of the intruding melt with sulfur-containing sequences. 3. Deep differentiation of the intruding melt to facilitate gravitational accumulation of sulfide liquid.