Conflicting zircon vs. titanite U-Pb age systematics and the deposition of the host volcanic sequence to Kiruna-type and IOCG deposits in northern Sweden, Fennoscandian shield

Abstract

The Northern Norrbotten region, and in particular the Kiruna area, hosts a number of large apatite iron oxide deposits (e.g. the huge Kiirunavaara ore) of significant economic importance. Age data from rock lithologies hosting these ores, represented by metamorphosed rocks of the Porphyrite and Kiirunavaara Groups, are complex to interpret. This is illustrated by (LA-ICP-MS) data for titanite, and to some extent for rutile, which scatter considerably yielding ages within a span from ca. 2.1 Ga to 1.7 Ga. These analysed hydrothermal minerals, characterized by complex BSE images revealing darker and brighter zones, are located in ore zones and associated with e.g. strong scapolitisation, albitisation and actinolitisation. Previous (TIMS) zircon ages of host rocks, on the other hand, define a more narrower age interval between ca. 1900 and 1870 Ma, and this is supported by new U-Pb zircon results presented here. Furthermore, one coherent set of SIMS data for titanite from the Luossavaara ore favour that crystallization took place at ca 1.88 Ga, although laser ICP data from the same locality are much more complex. An implication arising from published pre-1.9 Ga laser ablation ages for titanites is that the emplacement of host rocks started already at around 2.1 Ga. As the depositional time of these rocks is crucial for the understanding of the overall crustal formation in northern Norrbotten, additional rocks were selected for age dating. New zircon age data (LA-ICP-MS and SIMS) give support to a scenario where host rocks to ores started to develop at around 1900 Ma and this calls for a re-evaluation of published LA-ICP-MS data of hydrothermal mineral phases.Here, we present four models that aim to explain how pre-1.9 Ga titanite ages, believed to have a questionable geological significance, may develop. The principal idea is that ≤2.1 Ga alteration events were not responsible for the crystallization of the hydrothermal minerals, instead it is believed that apparent old age domains carry excess radiogenic lead due to the effect of ≤1.9 Ga hydrothermal processes. Currently, the interpretation of U-Pb isotope data in the study area remains enigmatic, and further radiometric analyses are required.