JultikaUniversity of Oulu repository

This thesis focuses on the magnetite composition of the Mustavaara V-Fe-Ti deposit located in the Porttivaava block of the Koillismaa layered intrusion in the Taivalkoski area, north-eastern Finland. The main purpose was to describe the lithology of a drill core crossing the ore-bearing magnetite gabbro unit and its hanging wall and foot wall rocks and to document the distribution of vanadium in magnetite. The study material consisted of twenty polished thin sections and whole-rock ICP-OES analyses and DDT test results provided by Mustavaaran Kaivos Oy. The thin sections were used in microprobe studies of the magnetite composition. The Mustavaara V-Fe-Ti ore is hosted by the 2440 Ma Koillismaa layered intrusion complex, which intruded in the contact zone of the overlying volcanic rocks and the Archean gneiss complex, forming a sheet-like body. The magmatic body was tectonically dismembered into several blocks after its formation. The ore-hosting magnetite gabbro is located in the upper portions of the intrusion’s stratigraphical column, reaching 100–200 m in thickness and 19 km in lateral extent in the Porttivaara block of the intrusion. The ore unit is divided into four main units on the basis of the magnetite abundance: Disseminated rock, Ore Upper, Middle and Lower layers. The layers differ from each others on the basis of the modal abundance and V₂O₃ content of magnetite. The Upper and Lower layers are richest in V₂O₃. The middle layer is the thickest layer but lowest in vanadium. Indexes of differentiation, such as An% in plagioclase and variation of Ti and V contents as a function of depth, indicate that the magnetite gabbro did not crystallize from a single magma pulse, as the magma evolution shows multiple reversals. Magnetite occurs as euhedral grains in the dissemination and becomes subhedral with increasing abundance. Magnetite has been silicified near rock fractures, which bears only small significance on a larger scale. The only significant silicification is present in the Ore Lower layer where the ore unit is divided by a thin, intensely silicified gabbro. The microprobe analyses of this study show that the magnetite V₂O₃ content ranges from 1.0 to 1.8 wt%, is broadly homogeneous and as such renders magnetite ideal for beneficiation.