Architecture and mineral potential of the Paleoproterozoic Karrat Group, West Greenland. Results of the 2015 season

Stratigraphic investigations revealed important new information on the nature and relationships between units of the meta-sedimentary and meta-volcanic Karrat Group. The oldest unit is the Qeqertarssuaq Formation, which is composed dominantly of quartzite and mica schist in contact with Archean crystalline basement rocks (“Archean basement,” herein). The Mârmorilik Formation, which consists dominantly of dolomite and calcite marbles, is also in depositional contact with Archean basement rocks, but not with the Qeqertarssuaq Formation. Therefore, the relative age of the Mârmorilik and Qeqertarssuaq formations is unclear. A new stratigraphic unit, the Qaarsukassak Formation, is formally established as a result of the field season’s mapping in Kangerluarsuk Fjord. The Qaarsukassak Formation occurs locally in depositional contact with Archean crystalline basement and stratigraphically below the youngest unit of the Karrat Group, the Nûkavsak Formation. Calcite marble horizons within the Qaarsukassak Formation appear to control the distribution of Zn-Pb mineralisation at the Discovery and Kangerluarsuup Sermia showings. The understanding of the distribution of this formation is therefore of significance for exploration work.

In Kangigdleq Fjord, a thick package of mainly mafic volcanic rocks occurs stratigraphically below the Nûkavsak Formation. Preliminary interpretations indicate that these metavolcanic rocks are mainly subaqueous and transitional to alkaline in chemistry intra-plate basalts. Their thickness is variable through the region, with thicknesses of up to 850 m, at Qangattarsuaq, interpreted to be part of a larger submarine volcanic structure. The metavolcanic rocks either have a LREE-enriched REE profile or a flat REE chondrite-normalized profile, comparable to samples from the Bravo Lake Formation (Baffin Island, Nunavut, Canada). Contact relationships between the Nûkavsak Formation and meta-volcanic rocks
were observed to be transitional over several tens of metres, suggesting that there was no time break between the deposition of these two units. From these new observations, stratigraphic revisions are suggested, which would either elevate the meta-volcanic rocks to a standalone formation (the Kangigdleq Formation) separate from the Qeqertarssuaq Formation or would include the meta-volcanic rocks into the Nûkavsak Formation, since these units show conformable relationships in the field.

The Nûkavsak Formation contains primary sedimentary structures (including flute casts, scour structures, flame structures, and cross-bedding) and also contains abundant clasts of reworked, older Karrat Group rocks, including abundant meta-volcanic pebbles and cobbles. This was not previously reported, but provides important stratigraphic and basin tectonic constraints. The association between semi-massive sulfide horizons within the Nûkavsak Formation and the Kangigdleq Formation, suggests that the former are related to submarine volcanic activity and could correspond to volcanogenic massive sulphide systems (VMS). These horizons are anomalous, yet low in base metals and gold. However, these horizons and other related rusty-weathering horizons, including sulfidic cherts or siliceous meta-mudstones and gossanous meta-sandstone horizons, appear to account for several of the stream sediment and float base metal anomalies that have been previously reported.

An unconformity is inferred from field relationships between the Lower (Qeqertarssuaq Formation) and Upper (Kangigdleq-Nûkavsak formations) Karrat Group. The Qeqertarssuaq Formation records at least one more phase of deformation and is transposed, i.e. it is infolded with Archean basement. A post-Qeqertarssuaq Formation unconformity is also supported by two more observations: that meta-volcanic rocks of the Kangigdleq Formation rest on different units of the Qeqertarssuaq Formation, and that clasts of recognized Qeqertarssuaq Formation were found to occur in the Nûkavsak Formation. Ongoing geochronological studies will better constrain the depositional age of the Karrat Group overall, test hypotheses regarding possible unconformities within the Karrat Group, and elucidate sedimentary provenance.

Some revisions to the existing map units are recommended. For example, rock units currently mapped as amphibolite with map code ‘A’ are not equated across the 71 V.2 South and 71 V.2 North map sheets (Henderson and Pulvertaft, 1987). Sometimes this map unit represents meta-volcanic rocks of the Kangigdleq Formation, which are stratigraphically above the Qeqertarssuaq Formation and are mostly in the greenschist facies, but sometimes this unit is amphibolite of uncertain protolith within the Qeqertarssuaq Formation.

The overall structural grain of the study area is characterized by complex interference of the structures related to four stages of deformation, D1 through D4. Early structures observed only in the Qeqertarsuaq Formation and the Umanak Gneiss pre-date the development of an unconformity and are attributed to the Qeqertarssuaq Stage (D1). The deformation post-dating the unconformity can be separated into three distinct stages named the Kigarsima Stage (D2), related to a WSW-ENE to SW-NE compression, the Maarmorilik Stage (D3), related to NW-SE compression and the Svartenhuk Stage (D4), with extension around intrusions and distal E-W compression. The area south of Kangilleq Fjord is characterized by fold structures with NW-trending axial traces that formed during the Kigarsima Stage (D2). The main regional overprint is by D4 (Svartenhuk Stage) folds with N-trending axials traces. The area between Umiammakku and Inngia Fjord is complex with also mainly D2 and D4 interference patterns. D1 (Qeqertarssuaq Stage), restricted to the Qeqertarsuaq Formation and the Umanak Gneiss, and D3 (Maarmorilik Stage) appear not to have a major influence on the regional-scale structural grain. The area between Inngia Fjord and Kangiusap Kuua is defined by N-trending structures related to extension proximal to intrusions in the west and E-vergent distal deformation.

Anomalous, but low, Au values were confirmed in quartz veins in Qeqertarssuaq Formation quartzites at Kussinersuup Aaffaa and at Inngia Fjord, and in quartz veins in Nûkavsak Formation metasandstoness and phyllites at Southern Kangiusap Kuua (Eastern Svartenhuk). These are interpreted to be related to a mesozonal orogenic gold mineral system, superimposed, at the former localities, on a Zr, Nb, Y, U, Th, lanthanide and, possibly, Au-enriched paleoplacer.