The units to the northeast of Motzfeldt Sø comprise syenite units belonging to the Motzfeldt Sø Formation (MSF) (Bradshaw & Tukiainen, 1983). These units have been described in detail by Bradshaw (1988) and are recognised to comprise three members; MFS- Marginal Arfvedsonite Syenite, MFS-Altered syenite and MFS-Nepheline syenite. Associated with the emplacement of the MSF is late-stage sheeting of evolved paralkaline microsyenite sheets. Detailed field relations are given by Bradshaw (1988). Subdivision of the region into these units is often subjective given the extreme textural and mineralogical heterogeneity within each unit and highly variable hydrothermal overprinting. As a result rocks of the present study will be described in detail and any assignment to the units of Bradshaw (1988) will only be made loosely.
The MSF shows an extremely high degree of heterogeneity. The syenites show striking textural variation from coarse-grained pegmatitic units to fine microsyenitic and aplitic units over distances of a few metres. The rock is typically a coarsely crystalline feldspathic syenite comprising euhedral brick-red oxidised alkali-feldspars, intercumulus amphiboles and subordinate green pyroxene (spherulitic occurrences are common at high levels). It includes facies which are porphyritic, containing large (ca. 15mm) alkali feldspars to phaneritic syenites with poikiolitic intergrowths of alkali-feldspar and arfvedsonite amphibole (Fig. 2.2a). Finer grained facies are also common, often having strongly porphyritic textures containing elongate tabular alkali-feldspars (ca. 10mm) in a microsyenite groundmass. This facies often exhibits a strong mineral alignment. Cutting much of the brick-red units of the MSF are sheets, isolated pods and lenses of brick-red aplitic syenites bordered by coarse pegmatites, this is best observed in cliff sections on the high plateaus of the MSF where the outcrop has a striped appearance where the concentration of these units is intense (e.g. N61˚ 11’ 13.4” W044˚ 58’ 16.2”) (Fig. 2.2b).
Throughout the MSF localised occurrences of high concentrations of pyrochlore group minerals are found and have been the site of continued economic interest for Angus and Ross Plc. In addition to pyrochlore, zircon and other Nb, Ta and REE bearing minerals are found in this distinct facies syenite within the MSF. The characteristic pyrochlore host lithology is a leucocratic microsyenite, which occurs throughout the MSF as inclined sheets
traversing to high levels in the formation. On the high plateaus, in rocks inferred to be close to the roof of the formation, occurrences of this facies are relatively common, particularly in the Angus and Ross field locality 4 where the highest concentrations of economically interesting mineralisation are found.
Figure 2.2. (a) Poikiolitic intergrowth of euhedral alkali-feldspars in black amphibole (hand lens for scale). (b) dark aplitic sheet bordered by coarse pegmatites. (c) Attenuated contact between texturally varying facies of altered MSF syenites (hammer handle is 80cm). (d) mafic cumulate mineral layering (hammer handle is 80cm).
The mineralogy of this distinct syenite facies is dominated by highly oxidised alkali- feldspar. Nepheline is also present but heavily altered. The mafic mode is generally very low and fine grained, occurring as sporadic clusters of amphibole, often replaced by secondary magnetite. Fluorite is abundant either as intercumulus rock forming minerals or as late veins. Pyrochlore group minerals have a dark brown to honey-yellow colouration in hand specimen and often have a halo of intense red oxidation in the feldspars surrounding them. Pyrochlore occurs disseminated throughout the rock, although several outcrops contain layers or horizons
disseminated throughout the rock. Enriched horizons often parallel the mineral fabric of the rock suggesting that these may be pyrochlore rich cumulate horizons.
Figure 2.3. Hand specimen of pyrochlore rich sample GJM05-44. Pyrochlore is found disseminated through the samples though occurs in discrete enriched horizons where pyrochlore accounts for up to 90% of the rock’s mineralogy.
Units located on the high plateaus show the most complex mineralogies and highest degree of textural variation. In addition to the many facies of syenite previously discussed there are occurrences of strongly pink coloured fine-crystalline syenite facies hosting large (ca. 10-15mm) pseudohexagonal green phenocrysts thought to represent alteration products of nepheline. Units close to the roof also show notable increases in the occurrence of intercumulus and vug-filling quartz. Intercumulus and vein filling purple fluorite is also common, making up 10 modal % of the rock in some areas. Contacts between texturally and mineralogically contrasting facies of syenite are rarely sharp and laterally traceable. In most areas several facies of syenite and nepheline syenite are found intimately intermingled over the scale of several meters (Fig. 2.2c). Areas where these textural relations are observed are often on the high plateaus or close to the highest surface exposure of the formation. From the field relations of these units it is suggested that the present land surface on the high plateaus is
the interplay of multiple melt batches interacting, mixing and mingling. This is supported by a lack of chilling on the contacts between units, suggesting that many of these melts were emplaced contemporaneously under very similar thermal and chemical conditions.
In addition to showing a high degree of textural variation rocks from high levels in the intrusion are also extremely friable through alteration of alkali-feldspars associated with the high degree of inferred subsolidus alteration. In addition to the pervasive oxidation of alkali- feldspars and secondary replacement of primary mafic phases discussed above, much of the outcrop east of Motzfeldt Sø contains macroscopic hematite, covering fracture surfaces and grain boundaries, giving intensely altered areas a distinctive blue-black sub-metallic lustre. In intensely altered samples the mafic mineralogy is often wholly replaced by hematite and other Fe-oxides and alkali-feldspars are coated by specular hematite.
The excellent 3-dimensional exposure provided by recent glaciation allows insights into the texture and alteration variability of units in the vertical dimension. A section from the high plateaus to the valley floor towards Motzfeldt Sø was carried out as part of the present study. On descending, the alteration becomes less intense and the rock loses the characteristic brick-red colouration. In low lying units close to Motzfeldt Sø shore at an elevation of <300 m, the rock becomes texturally more homogeneous and has a relatively fresh appearance and lacks the brick red alteration found at higher levels. At that locality, alteration is restricted to narrow zones of intense alteration and the rocks appearance is more similar to those units observed in the FDF. It is inferred that textural heterogeneity and alteration increase towards the top of the intrusion and is most intense on the high plateaus where the roof of the formation was.
Within the Angus and Ross licence area at Locality 5 (Fig. 1.2) an ~150 m vertical sequence of inter-bedded quartz conglomerates, quartz arenites and basaltic lavas, belonging to the Eriksfjord formation, are preserved in situ. Located at the base of the sequence, where the highly altered MSF syenite is in direct contact with quartz arenites, autobrecciation occurs for ~10 m into the quartzite (Fig. 2.4). Angular brecciated fragments of hematite coated quartzite, ranging in size from 5mm-30cm, are rimmed by cryptocrystalline quartz and supported by coarse crystalline euhedral fluorite. This contact represents the roof-zone contact of the MSF syenites with the base of the Eriksfjord formation. Localised fluorite
fluorite throughout the MSF and the presence of extensive fluorite mineralisation in the preserved roof zone of the formation suggests that the fluorine content of the MSF melts was relatively high and exceptionally high in the roof-zone of the formation.
Fig. 2.4. (a and b) autobrecciated Eriksfjord arenites supported by crystalline fluorite. (a) shows fracture surfaces on the Eriksfjord are coated in specular hematite. (b) shows small angular quartzite fragments supported by coarse crystalline fluorite (pen for scale).