Architecture and mineral potential of the Paleoproterozoic Karrat Group, West Greenland: Results of the 2016 Season

This report presents the preliminary results of the second field season of a joint project be- tween the Geological Survey of Denmark and Greenland (GEUS) and the Ministry of Min- eral Resources of Greenland (MMR) focusing on the Paleoproterozoic Karrat Group. The motivation for the expedition stems from an evaluation of the mineral potential along with understanding the stratigraphic architecture of the Karrat Group and aims to address stra- tigraphy, structure, and economic geology. Field work extended to northern Karrat Group outcrops, between Upernavik and Kullosuaq, and allowed further studies of the southern Karrat Group outcrops between Svartenhuk and Maarmorilik. The relationship between the Karrat Group and the Prøven Igneous Complex (PIC), emplaced between these two areas, was also assessed. The 2016 field season was aided by oblique aerial photography (“pho- toflying”), and hyperspectral techniques. Hyperspectral scenes were collected along cliff faces in the areas with the most prospective mineral potential, including Kangerluarsuk Fjord and Maarmorilik. These studies have built on the observations made in 2015, and have led either to the consolidation or to the revision of interpretations suggested after that season.
The exposed Karrat Group extends from ~71° to ~75°N and is composed of five formations: the Qeqertarssuaq (Lower Karrat Group), Qaarsukassak, Mârmorilik, Kangilleq, and Nûkavsak formations (Upper Karrat Group), of which the Qaarsukassak amd Kangilleq formations are informal. The lower and upper Karrat Group are separated by a regional erosional unconformity, which represents a revision to the Karrat stratigraphy established during the 2015 field season. The Karrat Group has not been differentiated north of Uper- navik due to the high metamorphic grade and polyphase deformation. Major goals of the field work included delineating contact relationships between units, documenting sedimen- tary structures that can define depositional environment(s), mapping the lateral extent and nature of extrusive mafic volcanism and volcaniclastic rocks in the upper Karrat Group (Kangilleq Formation), and determining the relative age relationships between units, which is elusive since several units are not in contact with one another. Integrated stratigraphy, sedimentology, volcanology, and ongoing geochronology will lead to a better understanding of the basin tectonic setting(s) in which the Karrat Group was deposited.
The deformation history of the area put forward after the 2015 season has been reviewed, and this succession is suggested: a NW-vergent D1 north of the Prøven Igneous Complex and channel flow in an orogen-scale NW-SE compression; D2 is characterised by E-W extension near Archean orthogneiss at Kangiusap Kuua and distal compression further to the E (previously assigned to D4); D3 is represented by structuring of the Kigarsima nappe overprinting all the other structures on Karrat Island and the peninsula to the SE (formerly described as D2); and, finally, a NW-SE compression, documented in the Maarmorilik area, is now relabeled as D4 (previously assigned to D3).
The time-stratigraphic horizon above and laterally to the Kangilleq Formation volcanism represents a period of VMS-formation on (or just below) the seafloor. Sampling of massive sulfides has yielded anomalous base metal concentrations (typically <0.3% Cu+Pb+Zn). However, because it is known that massive sulfide horizons can display zonations from low-grade to gold- and/or base metal-rich zones, these horizons potentially constitute tar- gets for future base metal and/or gold exploration in the Karrat region.
A Mississippi-Valley Type (MVT) origin is suggested for the Zn-Pb mineralisation in the Qaarsukassak Formation (RTZ Discovery), which might therefore have formed at the same time as the mineralisation in the Mârmorilik Formation. This is based on proximity of the Qaarsukassak Formation mineralisation to Archean rocks and the association of its distri- bution with marble and chert horizons. The possible role of evaporites and their diapirism in the mineralising process has been highlighted in the Mârmorilik Formation, by establishing the proximity of the known Lower Mârmorilik Formation showings to what can be interpret- ed to have been an evaporite horizon (now a “mottled pelite”). From this horizon, evaporites possibly ascended and were emplaced, as diapirs, within the Upper Mârmorilik Formation, underneath the pelites that would have acted as cap rock (where the Black Angel deposit is). Diapirism was likely triggered by deformation, during which evaporites flowed along thrust planes, as has been tentatively documented through the hyperspectral survey.
Possible silicate-silicate liquid immiscibility accompanied by silicate-sulfide immisciblity, occurred in a gabbro complex identified on the eastern side of Nutaarmiut island, to the southeast of Upernavik. The degree of PGE-Au enrichment in the sulfide has not been tested yet, but could potentially be of economic significance.