Summary:
The Mustavaara deposit is characteristic of a vanadiferous titanomagnetite deposit (VTM), which are typical mafic layered intrusions.
Mustavaara magnetite gabbro lies in the upper part of the Porttivaara intrusion (part of Koillismaa igneous complex) where it forms a coherent layer following the general layering of the intrusion. The magnetite gabbro is plagioclaseaugite-ilmenomagnetite adcumulate with sharp lower contact and gradual upper contact which is characteristic for VTM deposits.
The Mustavaara deposit is a vanadium bearing magnetite gabbro horizon which is part of a large mafic layered intrusive complex, known as the Koillismaa Layered Igneous Complex. The age of the intrusion is approximately 2,400 to 2,500 Ma. This sheet like body intruded into the Archean basement as a single body with a total length of 100 km and a thickness of up to 3 km. This body has separated into at least 5 distinct large blocks due to later folding and faulting. The block containing the Mustavaara deposit is the Porttivaara Block, with a total approximate length of 19 km.
The layered intrusion is divided into a marginal series (50 to 250 m thick) at the base and a thick layered series (up to 2,500 m thick) making up the bulk of the intrusion. The layered series is mainly composed of norites, gabbronorites, leucogabbros and anorthosites. The ore- bearing Mustavaara magnetite-gabbro occurs in the upper part of the layered series, surrounded by anorthosite rocks. The ore-horizon is a sheet-like body that trends east-west over a total strike length of approximately 15km within the Porttivaara Block. The ore-horizon generally dips 35°- 45° to the north. The thickness progressively increases from east to west up to a maximum of 200 m, while the average magnetite concentrate decreases to the west from 20% in the east to less than 10% in the western part.
Genetically, the vanadium rich magnetite gabbro is considered to be of magmatic origin formed as a segregation from an iron-rich liquid.
According to Karinen et al. (2015), the Mustavaara vanadiferous titanomagnetite (VTM) deposit was formed by gravity concentration of iron oxide crystals, or by sorting of a magnetite slurry. In the Panzhihua Complex, VTM deposit formation via gravitational accumulation has been suggested as well as fractional crystallisation of Fe-Ti-V oxides (Gao et al. 2019). Repetition of these mechanisms has led to the formation of several stratified layers of magnetite ore. In Mustavaara the magnetite crystals contain thin lamellae of ilmenite and are generally referred to as ilmenomagnetite. Iron content of the ilmenomagnetite concentrate is around 62 - 63% Fe and the average vanadium contents in the magnetite concentrate from the four units is approximately 0.9% V. At Mustavaara the amount of ilmenomagnetite in magnetite gabbro changes in such a regular way that the ilmenomagnetite layer can be divided into four separate layers. The three lowermost layers comprise the main constituent of the Mustavaara ore deposit and they are called: ore lower layer (OLL), ore middle layer (OML) and ore upper layer (OUL). The highest ilmenomagnetite content of 20 – 35 wt.% is in OLL (footwall contact), which forms a narrow and continuous layer (0.2 – 4 m) just above and following the footwall contact. OUL (hanging wall contact) forms a thicker (20 – 40 m) and continuous layer along the hanging wall contact in the main ore layer and has ilmenomagnetite content of 18 – 25 wt.%. In the thickest (10 – 50 m) OML the ilmenomagnetite content is 10 – 15 wt.%.
The fourth layer above the hanging wall contact consists of weakly disseminated ilmenomagnetite and is labelled as the ore disseminated layer (ODL). The ODL is the most inhomogeneous layer, which contains scattered anorthosite, and anorthosite gabbro fragments and compact magnetite veins. The anorthosite gabbro waste rock blocks do not necessarily follow the general layering and are randomly oriented. Upwards, the ilmenomagnetite dissemination gradually decrease and the amount of anorthosite gabbro blocks and layers in the magnetite gabbro increase. This happens until the rock is a heterogeneous anorthosite gabbro containing specks of magnetite gabbro. In this disseminated layer the ilmenomagnetite content usually varies from 2 - 10 wt.%.
The magnetite gabbro horizon dips 30 to 40° to the north. In the westernmost part (800 m) the thickness of the coherent ilmenomagnetite ore layer (OLL+OML+OUL) is 60 – 95 metres. Eastward (700 m) the thickness of the ore layer is quite consistent at 40 – 50 metres. In the easternmost part, over a distance of 200 m, the dip starts to steepen from 40 to 70°. Over the same distance the thickness of the ore layer starts to become thinner, from 40 meters to 10 metres thick, until it finally dies out completely.