MINERALOGICAL AND GEOCHEMICAL PECULIARITIES OF Fe-Ti-P MAFIC ROCKS OF THE STREMYHOROD INTRUSION (KOROSTEN PLUTON)

Abstract

Small-scale layered intrusions of mafic composition enriched in ilmenite, magnetite and apatite are known in association with the rocks of the anorthositic series of the Korosten pluton. Two Fe-Ti-P deposits have been explored in the most well studied Fedorivka and Stremyhorod intrusions. The geological setting, mineral and chemical composition of mafic intrusions is similar. They are considered to be typical layered intrusions, with gradual accumulation of ore minerals in the upper part of the layered series. This review examines some mineralogical and geochemical features of ores of the Stremyhorod intrusion. Ores are represented by olivine (melano-) gabbro with peridotitic layers. Olivine (Fa46—55), augite, plagioclase (An45—56) and biotite are the main rock-forming minerals. Among the ore minerals, the most common are homogeneous ilmenite (Hem4—6), while magnetite (Uspl2—39) with lamellae's of ilmenite, and apatite occur in a subordinate amount. Apatite is enriched in LREE ((La/Yb)n = 11.9-14.8), has moderate negative Eu-anomalies (0.37-0.45) and increased concentrations of Y and Sr. According to the order of appearance of rock-forming and ore minerals, crystallization proceeded under reducing conditions with early melt saturation in ТіО2, which led to the crystallization of ilmenite, probably even before the appearance of clinopyroxene. The estimation of the melt temperature by plagioclase composition (1060-1100 ºC) and Fo-minal content in olivine (1090-1100 ºC) yielded similar results. The equilibrium conditions were reached at fO2 –8.8 to –9.3, so crystallization of ores of the Stremyhorod intrusion took place at reducing conditions (ΔFMQ = –0.74 - –0.54). Considering the small size of such intrusions, in comparison to the gabbro-anorthosite massifs in the Korosten pluton, as well as ore mineral enrichment and late emplacement relative to the host gabbro-anorthosites, we suppose that these intrusions were formed after separation of residual melts from partly crystallized rocks of the anorthositic series. The composition of such melts (ferrodioritic/jotunitic, basaltic) is still obscure. The Mg# in the equilibrium liquid calculated for ore rocks of the Stremyhorod intrusion is 22-26%, which is slightly higher than that of the Fedorivka intrusion (Mg# 14-22%), and overlaps with jotunites of the Korosten pluton. The process of separation (filter-pressing) of Fe-Ti-P enriched melts and its subsequent migration to the upper level were accompanied by the partial capture of anorthositic material, which affects the geochemical characteristics of the ore gabbroids.