Abstract
The origin of Kiruna-type iron oxide–apatite ores is controversial, and debate presently centres on a ‘magmatic’ versus a ‘hydrothermal’ mode of formation. To complement recent investigations on the Grängesberg iron oxide–apatite ore deposit in the northwestern part of the Palaeoproterozoic Bergslagen ore province in central Sweden, we investigated the oxygen isotope composition of the host rocks of this large iron oxide–apatite ore body. As the metavolcanic and metagranitoid country rocks around the Grängesberg ore body either pre-date or are coeval with ore formation, they would be expected to record an extensive isotopic imprint if the ore body had formed by large-scale hydrothermal processes involving an externally sourced fluid. A direct magmatic formation process, in turn, would have produced localized alteration only, concentrated on the immediate vicinity of the ore body. Here, we test these two hypotheses by assessing the oxygen isotope variations in the host rocks around the main Grängesberg iron oxide–apatite ore body. We analysed oxygen isotopes in quartz from metavolcanic (n = 17) and metagranitoid host rocks (n = 14) from the vicinity of the ore body, and up to 2 km distance along and across the strike of the ore body. Remarkably, we find no significant variation in δ18O values with distance from the ore body, or any deviations in country rock δ18O from common magmatic and/or regional values. Only two samples show shifts to values more negative than the common magmatic range, indicating highly localized hydrothermal overprint only. As a large-scale, low-temperature hydrothermal origin of the ore body through voluminous fluid percolation would be expected to have left a distinct imprint on the oxygen isotope values of the country rocks, our results are more consistent with an ortho-magmatic origin for the Grängesberg iron oxide–apatite ore.
Highlights
Introduction and approachThe genesis of ‘Kiruna-type’ iron oxide–apatite ores is the focus of a long-standing debate (e.g. Geijer 1910, Parak 1975, Weidner 1982, Naslund 1983, Nyström and Henriquez 1994, Barton and Johnson 1996, Sillitoe and Burrows 2002)
Using the example of the Grängesberg Mining District in Central Sweden, we investigate if the surrounding country rocks to the main ore body, which comprisevolcanic andplutonic rocks that are possibly coeval with ore formation, record any evidence for large-scale hydrothermal activity
With respect to oxygen isotope systematics, the Grängesberg metavolcanic and metaplutonic host rocks record δ18O values exclusively between + 5.8 and + 8.6‰, which essentially overlaps with the range of δ18O values for ‘normal’ intermediate arc magmas
Summary
Introduction and approachThe genesis of ‘Kiruna-type’ iron oxide–apatite ores is the focus of a long-standing debate (e.g. Geijer 1910, Parak 1975, Weidner 1982, Naslund 1983, Nyström and Henriquez 1994, Barton and Johnson 1996, Sillitoe and Burrows 2002). Whereas a lively debate between the two endmember schools (magmatic vs low-temperature hydrothermal) is ongoing, one aspect that has not yet been fully exploited is the relationship of the Kiruna-type ores with their surrounding host rocks. Hydrothermal iron ore formation has been suggested to encompass large-scale hydrothermal circulation such as seafloor exhalation, sub-seafloor hydrothermal replacement of host rocks, or hydrothermal fluid interaction with an evaporitic source If a large-scale hydrothermal origin is considered, fluids from external sources would likely be involved, which would affect the O isotope composition of the immediate host rocks to the ore body and produce an aureole that is distinct from the regional isotope composition (cf Taylor 1971, 1974; Beaty et al 1988; Dilles et al 1992; King et al 1997). In a large-scale hydrothermal aureole, fluids would progressively cool away from the heat source as well as experience progressive fluid–rock interaction, leading to variable isotope signals with distance from the heat source (e.g. Nabelek 1987; Beaty et al 1988; Ayers et al 2006; Donoghue et al 2010; Berg et al 2018)
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