Flinks Dal Formation

Rocks south of Motzfeldt Sø are of noticeably different character to those of the MSF and are much more typical of Gardar rocks in general. Following the division of Bradshaw and Tukiainen (1983) these units belong to the Flinks Dal and Motzfeldt Sø Formations, though they bear little resemblance to the Motzfeldt Sø units east of the lake. The FDF has been divided into a number of intrusive members (Bradshaw, 1988 and Bradshaw and Tukiainen, 1983) sequentially emplaced inwards in a series of nested plutons. The three intrusive members of the FDF are the FDF – Nepheline syenite (SM5), FDF – Porphyritic nepheline syenite (SM2 and SM4) and FDF – Foyaite (SM4). Each of these intrusive phases are white nepheline syenites with differing textural and modal mineral abundances. These units have been previously described by Emeleus and Harry (1970), Jones (1980), Bradshaw (1988) and Schönenberger & Markl (2008) therefore the reader is referred to these sources for detailed descriptions. This section will overview field observations from each of the FDF units and will discuss features which can aid in the interpretation of the genesis of mineralisation in the MSF.

The largest and oldest of these units is the FDF - porphyritic nepheline syenite (SM2 & SM4). This unit is largely homogeneous over much of its outcrop area, comprising typically medium-grain porphyritic nepheline syenites. These typically contain phenocrysts of platy, tabular alkali-feldspars and squat euhedral nepheline in varying states of alteration (white, green or pink). The groundmass contains the same felsic mineralogy as the phenocryst assemblage. The mafic mineralogy is dominated by fresh blue-black interstitial amphibole and green pyroxene. One characteristic feature of this unit is the varying presence of porphyritic melanocratic enclaves. These vary in size from ca. 5 cm to 40 cm and have a phenocryst assemblage of elongate tabular alkali-feldspars up to 10 mm in length. The smaller inclusions are often well rounded (Fig. 2.5b), however larger examples often have more angular or sub- rounded shapes (Fig. 2.5a). The smaller inclusions may therefore represent the partially assimilated remnants of larger xenoliths or autoliths of earlier partially-crystallised syenite formed as part of the same magmatic episode. However, some samples show convolute margins and feldspar phenocrysts from the host syenite are frequently found penetrating the syenite-inclusion contact, suggesting that the porphyritic melanocratic units may have been partially molten during their incorporation. On the tops of plateaus there are localised areas which show weak red colouration from localised oxidation of alkali-feldspars, similar to that in the altered MSF units at low elevations.

Figure 2.5. (a) Subangular melanocratic enclaves within FDF-porphyritic nepheline syenite (N61° 11’ 40.5” W044° 06’ 56.5”). (b) Samll rounded melanocratic enclaves in FDF-porphyritic nepheline syenite (N61° 11’ 40.5” W044° 06’ 56.5”). (hammer handle is 80 cm long).

FDF, wholly surrounded by the FDF – porphyritic nepheline syenite and covering an area of about 10 km2. Typically the unit contains squat rectangular alkali-feldspars up to 5 cm in length. Anhedral nepheline crystals up to 3 cm in size show varying states of alteration, similar to the FDF - porphyritic nepheline syenite. The mafic mineralogy is dominated by dark green pyroxenes and subordinate amphiboles. In some localities biotite is a secondary phase after pyroxene. On the high plateaus this rock takes on a yellowed and slightly oxidised colouration locally from alteration of feldspar and nepheline crystals, in a similar localised nature as observed in other FDF units. Within the FDF – Nepheline syenite are a number of porphyritic microsyenite units containing phenocrysts of elongate platy micas up to 5 mm in size.

Figure 2.6. (a) Looking south from north of Motzfeldt Sø at contact between the FDF (left) and the MSF (right) showing dark brick red alteration. Viewer is standing at N61° 11’ 44.9” W045° 01’ 12.3” looking WNW. (b) Contact between FDF (bottom) and MSF (top). Both formations show pervasive brick red alteration. (hammer handle 80cm long) shows orientation of contact (N61° 12’ 34.4” W045° 06’ 19.5”). (c) Contact between light- coloured FDF (right) and redder MSF (left). Alteration is largely constrained to the MSF. (d) close-up of contact

On the peninsula of land bordered by the Sermia Avanardleq and Qorqup Sermia glaciers there is an area of characteristic brick red altered syenite attributed to the MSF (SM1). The units in this region are very similar in character to the MSF northeast of Motzfeldt Sø. Although this area does not show as much textural heterogeneity as the units northeast of Motzfeldt Sø, it still has the characteristic brick-red colour and high degree of alteration as elsewhere in the MSF. The contact of the MSF in central Motzfeldt with the FDF – Porphyritic nepheline syenite can be traced laterally and vertically from the top of the current exposure on the high plateaus of central Motzfeldt (ca. 1300m) down to ca. 900m in the river valleys flowing north into Qôrqup Sermia. The contact can also be observed from north of Motzfeldt Sø, where it can clearly be seen in the cliffs above the lake (Fig. 2.6a). In the high elevation exposures the contact is sharp and follows an E-W trend dipping steeply towards the north (Fig. 2.6b&c).

The contact in this region shows a fine-medium grain chilled margin in the FDF, though chilling is not so significant as to suggest emplacement of the FDF into a cold host. The contact within approximately 30 cm is also characterised by enrichment of green pyroxenes (Fig. 2.6d). Despite the FDF being largely unaltered there is a marginal zone of alteration extending into the FDF for approximately 10 cm. Traversing to deeper levels in the centre the contact becomes less obvious as the contrasting alteration between the MSF and FDF is less pronounced (e.g. N61° ll’ 44.9” W045° 01’ 12.3”) (Fig. 2.7a). At these lower elevations the boundary between the MSF and FDF is still a sharp intrusive contact with the same orientation, however there is little chilling and abundant xenoliths of largely unaltered and slightly coarser MSF syenites are hosted in the FDF (Fig. 2.7b), in addition to the porphyritic melanocratic enclaves discussed above. Close to and along the contact at lower elevations, rare pegmatitic patches up to several meters in thickness (Fig 2.7c) occur, containing very large euhedral feldspars up to 20cm in length, intercumulus nepheline, euhedral amphiboles and rare spherulitic growth of pyroxene needles (Fig. 2.7d). The occurrence of pegmatites is particularly rare in the FDF and during this study were only observed in few localities (e.g. N61° 12’ 36.5” W045° 07’ 57.5” and N61° 12’ 34.9” W045° 07’ 50.1”). The scarceness of pegmatites may be attributed to the relatively anhydrous nature of FDF melts.

the region in the Inúngurassuaq area the intrusive contact appears to mirror the boundary between the altered MSF and relatively unaltered FDF. Although in many places the extent of alteration appears to follow the location of the contact, it is not controlled by it and in many locations the alteration is found penetrating well into the “unaltered” units of the FDF, likewise the “altered” units of the MSF appear relatively unaltered in areas at much lower elevations. Similar trends in alteration have been observed in the MSF north-west of Motzfeldt Sø.

Figure 2.7: (a) Intrusive contact between the MSF (top) and FDF (bottom). Both units appear relatively unaltered. (b) Xenoliths/enclaves of relatively unaltered MSF syenite host within unltered FDF. (c) pegmatite patches close to intrusive contact in FDF. (d) spherulitic pyroxene within FDF pegmatites. Hammer handle is 4 cm wide. All locations witin 20 m of N61˚ 12’ 36.5” W045˚ 07’ 54.6”.