Petrological and structural character of c. 1.88 Ga meta-volcanosedimentary rocks hosting iron oxide-copper-gold and related mineralisation in the Nautanen–Aitik area, northern Sweden

• The petrological and deformation characteristics of a Palaeoproterozoic meta-volcanosedimentary sequence in the Nautanen–Aitik area (near Gällivare, northern Sweden) are presented. The investigated sequence (part of the Muorjevaara group) predominantly comprises metavolcaniclastic rocks displaying variable grain size, textural and syn-depositional features (e.g. grading, cross-laminae, compositional banding). Locally, poorly sorted agglomerate-like horizons with coarser clasts (lapilli-to block-size) occur. Interbedded sections of mainly fine-grained, volcanogenic (epiclastic) metasedimentary rocks contain syn-depositional features that suggest a sub-aqueous, relatively shallow depositional environment. Locally preserved cross-laminae in this unit provide evidence of way-up and paleocurrent directions. Intercalations of pelite, mica schist and amphibolitic schist also occur throughout the sequence. In general, least altered samples indicate a predominantly intermediate (basaltic andesitic to andesitic), calc-alkaline composition for the sequence.U-Pb SIMS zircon dating of a meta-andesite horizon, intercalated within the sequence, has yielded a precise U-Pb concordia age of 1 878 ± 7 Ma (2σ, n = 12). This date constrains the timing of intermediate volcanism and the deposition of syn-volcanic epiclastic material. By inference, it also provides an estimate for the age of Muorjevaara group rocks hosting the Nautanen Cu-Au and Aitik Cu-Au-Ag deposits. When combined with lithogeochemical signatures, the new age also confirms a genetic link between the metavolcaniclastic package and gabbroic to dioritic intrusions in the area (e.g. the c. 1.88 Ga Aitik stock). Additionally, several zircon cores record 207Pb/206Pb apparent ages between c. 1.90 and 1.89 Ga, suggesting inheritance of marginally older volcanic ± plutonic material not exposed at the present erosion level.The Nautanen–Aitik area contains the roughly north-northwest-trending Nautanen deformation zone (NDZ), a major composite brittle-ductile structure hosting hydrothermal iron oxide-Cu-Au (IOCG)-style mineralisation (e.g. the Nautanen deposit). Here, the bedrock is relatively intensely altered and sheared, and has been transposed into a sub-parallel high-strain zone predominantly consisting of a composite planar penetrative fabric. Outside the high-strain zone, relatively large-scale approximately north-northwest-aligned folds and discordant brittle structures occur. Magnetic susceptibility and VLF-resistivity modelling of the NDZ confirm that composite planar structures mainly dip steeply to the west-southwest and continue at depth.

Subject headings

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

Keyword

The changing Earth

Den föränderliga jorden

Publication and Content Type

ref (subject category)

art (subject category)