ULTRABASIC ROCKS OF THE ZAKHIDNO-LASHCHIVSKYI MASSIF (POBUZKYI ORE DISTRICT)

Abstract

The Zakhidno-Lashchivskyi massif, located in the southwestern part of the Ukrainian shield, is confined to the Holovanivska suture zone. It has a lenticular shape, north-western strike and zonal structure. From the point of view of metallogeny, the massif is located in the Pobuzkyi ore district, specialized in Ni, Co, Fe, Cr and precious metals. The Pobuzka group of deposits is confined to the crust of weathering of ultrabasic rocks of the Kapitanivsko-Dereniukhinskyi complex which is Archaean in age (AR3kp–dr). Ultrabasic rock bodies up to several square kilometres in size are spread over an area of about 500 km2. Petrographic studies have shown that apoperidotite (apolherzolite, apoharzburgite) serpentinites, serpentinized ultrabasic rocks, orthopyroxenites and olivine pyroxenites, harzburgites, and lherzolites are common among the rocks of the Zakhidno-Lashchivskyi massif, and clinopyroxenites occur occasionally. Apoperidotite serpentinites are almost entirely composed of serpentine (about 99 %), contain also ore minerals and iron oxides. Ortho- and clinopyroxene, olivine, secondary amphibole, serpentine, spinel, and ore minerals are determined in orthopyroxenites. The olivine content increases in olivine pyroxenites and harzburgites, compared to orthopyroxenites. In some cases, due to the increase in the amount of monoclinic pyroxene in pyroxenites, compared to rhombic pyroxene, rocks can be identified as lherzolites. There are also olivine websterites – pyroxenites, in which the olivine content reaches 50 %. According to the results of microanalytical studies, typical serpentinites of the Zakhidno-Lashchivskyi massif contain serpentine, pyroxenes, and calcite; ore minerals are represented by magnetite and chromite. Two generations of serpentine have been identified: antigorite and lizardite in the form of foliated aggregates and fibrous chrysotile. Magnetite is characterized by a change in its chemical composition from the centre to the periphery of the grain: the NiO content increases from 0.91 to 13.43 wt. %. The nickel-bearing crust of weathering, which has a zonal structure, is associated with the ultrabasic rocks. The main productive horizon of the crust is composed of nontronites and ferruginized nontronites, in which nickel-bearing nontronite, montmorillonite, and other minerals have been identified.