ORE-FORMING MAGMATIC SYSTEMS OF MIDOCEANI C RIDGES

On the basis of analogy of magmatic mineralization in basalts and othe r eruptive rocks of the Earth's crust of the MOR crest with that in la yered basite and basite-hyperbasite plutons, a qualitative model has b een constructed for the orthomagmatic ore-forming system of a magmatic chamber and related fluid magmatogenic system of the MOR. It distingu ishes three successive stages of evolution for quickly-spreading ridge s: (1) A cumulus zone forms in a shallow magmatic chamber under differ entiation, and a sulfide liquid, released in the process of subliquidu s decomposition of a basite melt, developing an immiscible residual li quid owing to crystallization, and saturation of the melt with volatil es, is segregated in the adcumulus liquid. (2) Ore and petrogenic comp onents are actively transported by a volatile phase from the zone of o re melt segregation to the upper part of the chamber and out of it. Th is stage develops upon reaching the volume saturation of the adcumulus melt by volatiles. A mixed magmatogene fluid system forms over the zo ne where magmatic gases and interstitial solutions mix. Hydrothermal s olutions from this zone may discharge on the sea bottom. The inerstiti al solutions create hydrothermal recycling cells in the side parts of these magmatogene fluid systems. (3) Upon complete crytallization of a shallow magmatic chamber, a recycling fluid system forms and function s. It dies out after the Earth's crust temperature anomaly has disappe ared. Similar phenomena must also occur in the mantle magmatic chamber s. With the quasistationary feeding of shallow magmatic chambers from deeper ones, the third stage does not take place in some segments of M OR. In slowly-spreading ridges, with no large shallow-depth chambers i n the recent magmatic systems, the above-mentioned processes run in in termediate mantle magmatic chambers. The third stage of the evolution of magmatogene ore-forming systems is not expressed in them because of no conditions appropriate for its development in the mantle.