Geological characteristics and genetic mechanism of the Mustavaara magnetite-ilmenite deposit, Finland

The Mustavaara gabbro in Finland is a small intrusion emplaced at 1881Ma into tonalitic bedrock near the boundary of the Mid Finland granitoid complex. The proposed model for ore genesis is based on stepwise or continuous flow of magma through the sill-like middle zone and the extraction of suspended materials. The abundance of ilmenite, the high ilmenite-to-magnetite ratio (3∶1) and the limited volume of apatite in the ilmenite ore hosted by the Mustavaara gabbro are different from characteristics of many other ilmenite deposits. Mustavaara represents a new type of Fe-Ti oxide deposit. To produce titanium ore, in most cases, the parent magma is likely to be at least slightly enriched in titanium. However, for titanium deposits in small mafic intrusions, such as Koivusaarenneva, an open (magma flow-through) system is an essential genetic component to produce economic deposits. The Mustavaara gabbro is an excellent example of a small mafic intrusion that may be an economically important source for titanium (ilmenite) because the intrusion is a part of a magma channelway. The difference between the two deposit types in China and Finland is probably attributed to different tectonic settings and different physicochemical conditions in the evolution process of the ore magma.