JultikaUniversity of Oulu repository

Abstract

Finland has been a historically significant producer of vanadium. The Mustavaara Fe-Ti-V deposit in Lapland had a decade-long history of mining from 1976 to 1985. The deposit is hosted in magnetite gabbro within the upper zone of the Koillismaa layered intrusion. The deposit is divided into four ore layers (Disseminated, Upper, Middle, and Lower ore layers). Each ore layer differs in modal abundance of oxide grains, as well as in V-content of oxides. The highest oxide abundances are found in the Lower ore layer, at 20–35 wt. % ilmenomagnetite, while the highest V contents of ilmenomagnetite are found in the Upper and Lower ore layers, at 1.7 wt. % V2O3.

In this study, the systematic analysis of plagioclase grains was conducted from the footwall, to the magnetite gabbro, to the hanging wall units using an in-situ approach via LA-MC-ICP-MS. The samples from the studied rock units display constant initial 87Sr/86Sr (2.44 Ga) of about 0.7030. The initial 87Sr/86Sr ratio is similar to the coeval siliceous high-magnesium basalt (SHMB) series mafic dykes in the Fennoscandian shield, as well as to some other co-genetic mineralized intrusions in the Tornio-Näränkävaara belt. This indicates that the parental magma of Mustavaara belongs to the SHMB series magma, and that this type of magma is capable of producing an Fe-Ti-V deposit, in addition to known Cr and PGE mineralization.

In-situ trace element concentrations of plagioclase have been analyzed using LA-HR-ICP-MS, to constrain the magma evolution history and ore-formation mechanisms of the iron oxide deposit. A previous study found that the anorthite content decreases from the footwall of the magnetite gabbro unit upwards, and from the hanging wall downward, and suggested that the magnetite gabbro unit may represent the most involved magma. From this study, the highly incompatible elements (e.g., LREE) in plagioclase show a mirror trend with the anorthite content, confirming the double fractionation trend revealed by major elements. In addition, this study confirms that the Upper ore layer (UOL) may represent the most evolved portion of the magma. V and Ti show a similar distribution pattern to anorthite, reaching their lowest concentrations at the UOL, indicating that their bulk partition coefficients are greater than 1. Additionally, traverse analyses of plagioclase from rim to core to rim have been conducted. The results show that highly incompatible element concentrations increase from core to rim, whereas Ti and V concentrations decrease from core to rim. Based on these observations, together with the euhedral texture of magnetite in oxide-poor samples, it is suggested that magnetite and plagioclase and clinopyroxene may have co-precipitated from magma. Some enrichment of oxide due to gravity settling may have formed the magnetite gabbro unit in Mustavaara.