Geochemistry and ore formation

3-14

Authors: Kryvdik S.G., Sharygin V.V., Gatsenko V.O., Lunev E.S.

The article represents new microprobe data for micas from malignites, their xenoliths and dike subalkaline gabbroids of the Pokrovo-Kyriyivo massif. Micas from gabbroids are phlogopites with moderate Al2O3 (14-16 wt. %) and TiO2 (4,3-6,3 wt.%). Malignite xenoliths contain tetraferriphlogopite, which is high in magnesian number (0,8 Mg #), low in alumina (7,9-11,2 wt.% Al2O3) and TiO2 (0,7-2,5 wt.%). Two generations of micas are documented in malignites. The first early generation includes varieties, which form single large rhomboid, rectangular or irregular grains with numerous inclusions of nepheline, clinopyroxene, titanite, magnetite and Sr-rich fluorapatite. These micas in malignites are low in Al2O3 (6.4-8.8 wt.%), high in TiO2 (up to 4.9 wt. %) and F (to 3.3 wt. %). The amount of the K cations is more than Al (K > Al), and the sum (Si + Al) is < 4. In chemical composition these micas are intermediate between tetraferriannite and tetraferriphlogopite. However, the reserve absorption scheme is rarely observed in these micas (optically tetraferriphlogopites are present only in mica-rich xenoliths from malignite). The second late generation is represented by small crystals from miarole-like minute inclusions (commonly in gцtzenite), which also contain zeolites, fluorite, strontianite and later (II generation) nepheline. Micas of second generation are characterized by low TiO2 (0.8-2.5 wt.%), MgO (1.4-10 wt.%), F (0-0.2 wt.%), and high content of Al2O3 (12.6-15.9 wt.%) which is 1.3-1.5 Al cations in the formula. They are represented by annite to siderophуllite. Annite is formed in a medium and low temperature conditions. It is assumed that complement deficiency of cations in tetrahedra (Si + Al <4) preferably Ti4+, but not Fe3+, is characteristic for micas of alkaline rocks and some carbonatites. Low Al2O3 content in early micas of malignites (K > Al) is due to agpaitic conditions of malignite formation and high K content. Alumina-understurated micas are crystallized together with such characteristic agpaitic minerals as alkaline pyroxene and amphibole, nepheline and sodalite, gцtzenite, catapleiite, Sr-rich fluorapatite. Fluorine is typical for high-magnesian micas. It decreases up to a total absence with increasing of Fe2O3. This is probably due to the decreasing temperature because fluorine is not incorporated in Fe2O3-rich annite, which crystallize in microinclusions even if fluorite is present (i.e., when the system was saturated with fluorine). It is also possible that during the formation of malignite the Fe increase in system is a division between the fluorine-rich mica and fluid – melt as determined by experiment for the system of iron-rich biotite – granite melt.

22-42

Authors: Kravchenko G.L., Rusakov N.F.

For the first time Kamyshevatsky metaultrabasite massif on the NW extension of the Sorokinska tectonic zone (STS) is revealed and mapped. Rocks of this massif are presented by apodunite and apoperidotite serpentinites, peridotites, olivine-bearing and olivine-free pyroxenites and their metamorphosed varieties, i.e. hornblendites, aktinolitites, tremolitites and phlogopite (chlorite) -tremolite-anthophyllite rocks. The primary composition of the massif corresponds to dunites, harzburgites, lherzolites and olivine pyroxenites.The massif is slightly differentiated and in the peripheral parts consist of olivine pyroxenites, and in the center of serpentinites and peridotites. Absence of distinctly displayed contact alternations in ultrabasites can evidence on injection of porridge-like magma as a protrusion or on intensive display of regional metamorphism and granitization which fully shaded these alterations. Petrographic, petrochemical and geochemical features of rocks, their rock-forming and ore minerals are described in detail. The presence of TiO2, Al2O3, CaO and alkalis, increased total iron content and low ratio of M/F and Cr/Al indicate on belonging of the studied rocks to ultraferrobasites (gabbro-peridotite association). It is suggested that these rocks are predominantly metamorphosed at the expanse of serpentine dehydration during the granitoid intrusions. The data on internal structure of the massif, its relationship with STS and some similarities and differences concerning metaultrabasite with metakomatiites that have been recently identified in the southern part of STS are presented.The similarity between these rocks consists in availability porphyraceous texture with large crystals of olivine in them, as well as practically the same chemical composition of serpentinites and komatiites with the spinifextextures in metaultrabasiteKamyshevatsky massifwere not found. As the rocks of this region have been transformed by multiple dynamic and thermal influences, the primary structure in these rocks could hardly been preserved. It is probable that the metaultrabasiteand metakomatiiteshave a secondary (metamorphogeneuos) origin at which they could have crystallized in the processes of serpentine dehydration. The latter may have a place at high contact thermal influence of granitoids or at the regional metamorphism of the moderate and low degrees. It is established that metaultrabasites of Kamyshevatsky massif are genetically closely related to those ones in STS this massif represent direct NW-continuation of layering intrusions which are spread in edge parts of STS.