Architecture and mineral potential of the Paleoproterozoic Karrat Group, West Greenland - Results of the 2017 Season

The main goal of the 2017 field season was to revise the geological maps of the southern area of Karrat Group exposures. This revision will encompass the 1:100 000 sheets of Maarmorilik 71V.2 Syd, Nuugaatsiaq 71V.2 Nord, Pannertooq 72V.2 Syd, and Svartenhuk 71V.1 Nord, originally compiled between 1980 and 1991. This third field season followed up on fieldwork carried out in 2015 and 2016 and, as the other two field seasons, was jointly financed by the Geological Survey of Denmark and Greenland (GEUS) and the Ministry of Mineral Resources of Greenland (MMR).
Within this framework, and since the focus of the first two seasons had been on the Paleo- proterozoic Karrat Group, the 2017 fieldwork targeted Archean rocks, namely in Panner- tooq (head of Ukkusissat Fjord), Upernivik Ø, Kigarsima/Tornit and the area south of Maarmorilik. This work led to the identification, in what was originally mapped as Archean orthogneiss, of significant paragneiss and quartzite sequences, of uncertain age. These supracrustal sequences often appear infolded with Archean orthogneiss, so some of them could possibly be unrelated to the stratigraphy of the Karrat Group. Similar infolding ob- served in the Qeqertarssuaq Formation, mapped in Kangilleq Fjord, suggests that this for- mation could possibly also be unrelated to the Karrat Group, as traditionally defined. Cor- roborating this interpretation is the fact that higher-P metamorphic assemblages, evi- denced by garnet amphibolite and kyanite micaschist, were documented in the Qeqertars- suaq Formation, but not in overlying formations. This contrasting metamorphic evolution can be interpreted as evidence for an early thermal event, prior to Qaarsukassak Formation deposition. Alternatively, the disparate metamorphic conditions could be accounted for by juxtaposition of different tectonic units during the Rinkian orogeny. T o further constrain the depositional ages of the paragneiss and quartzite (including those of the Qeqertarssuaq Formation), follow up detrital zircon geochronology is warranted. However, regardless of what the subsequent analytical work reveals, these findings appear to already imply signifi- cant revisions to the existing maps.
Within the Paleoproterozoic volcano-sedimentary succession, fieldwork allowed for the identification of the presence of the Qaarsukassak Formation (informal) in Kussinersuaq (Umiammakku Isbræ), Rinks Isbræ, Qingaarssuaq (Kangerlussuaq Fjord), Kigarsima/T ornit (Kangerluarsuk Fjord) and Kangerluarsuup Sermia. This formation hosts the stratabound mineralisation in the Rio Tinto Zinc (RTZ) Discovery area (Kangerluarsuk Fjord), where it was first defined. While no primary Zn mineralisation was observed at the defined localities, with only faint zinc zap responses obtained at Kussinersuaq, and other localities not tested for mineralisation, these findings significantly stretch the areal extent of the stratabound Zn- hosting Qaarsukassak Formation, and are therefore of economic significance. Detailed follow up photogeological mapping and interpretation of hyperspectral scenes of this min- eralisation host should consequently be carried out. Further work within the Paleoprotero- zoic Karrat Group, included the study of the mafic volcanic rocks of the Kangilleq Fm (in- formal), was aimed at recognising horizons with distinct geochemical signatures (alkaline vs. subalkaline), as identified in samples collected in previous seasons, in order to elucidate petrogenesis of the volcanic rocks.
The structural setting and metamorphism of the Prøven Igneous Complex (PIC) is key to understanding the geological evolution of the region and its lower contacts and some internal structures were studied. This work demonstrated that the PIC comprises one or more tabular intrusions. In the west, near Upernavik, the complex was emplaced close to the basement-cover contact. Farther to the east and south, it seems to have been emplaced at a higher stratigraphic position within the Paleoproterozoic sedimentary sequence. Subse- quently, the complex was displaced to the NW (north side of the PIC) and SE (south side of the PIC). Wherever it was studied, both North and South, this contact is a shear zone. The PIC contains abundant enclaves of meta-sedimentary rocks, particularly near the lower contact. The enclaves are most likely to be from a Paleoproterozoic sequence - the Karrat Group - although this assumption is unproven. In the instances where possible cross- cutting intrusive relations are found with meta-sedimentary rocks at the base of the PIC, the cross-cutting igneous rocks are late syn-tectonic biotite granite and leucogranites, rather than elements of the PIC proper. Earlier workers assumed that these late syn-tectonic granites and the PIC were part of the same magmatic event and were both late syn- tectonic. Our new field evidence from the northern contact of the PIC, consistent with re- cently published geochronology (Sanborn-Barrie et al. 2017), shows that the PIC was af- fected by intense fabric formation and folding and that its contacts with its host rocks, where we have seen them, are always concordant as a consequence of intense defor- mation. We conclude that the published interpretation that the PIC was emplaced relatively late during Rinkian orogenic evolution should be rejected (Grocott & Pulvertaft, 1990 and references therein).