Abstract
The first systematic investigations of the central part of the Early Proterozoic Ketilidian orogen in the vicinity of Søndre Sermilik in the early 1960s suggested that this part of the orogen comprised a mixture of the Julianehåb granite, altered supracrustal rocks and older orthogneisses. Recent field work has shown that the area consists only of a variably deformed suite of granitic to dioritic plutonic rocks and a range of hornblende-bearing dykes of the appinite suite which all belong to the Julianehåb batholith. Steep to vertical shear zones with widths from a few centimetres to more than one kilometre are a significant element of the structure. The principal shear zones trend north-east and they are parallel to the schistosity and subhorizontal linear structures in the granitoid rocks. Kinematic indicators in many of the shear zones indicate sinistral transcurrent displacements. The relationships between granite fabrics, shear zones and mafic dykes suggest that the Julianehåb batholith was emplaced during subduction from the south towards the Archaean craton in the north-west in a sinistral transpressional system. Effects of hydrothermal alteration, mainly in the form of quartz veining, silicification, chloritisation, epidotisation and pyritisation, are common within and adjacent to the largest shear zones. These effects are believed to be related to late stages of the evolution of the batholith. Gold anomalies appear to be closely tied to the hydrothermal phenomena.
Highlights
The first systematic investigations of the central part of the Early Proterozoic Ketilidian orogen in the vicinity of Søndre Sermilik in the early 1960s suggested that this part of the orogen comprised a mixture of the Julianehåb granite, altered supracrustal rocks and older orthogneisses
This report describes the results of field work in the area adjacent to Søndre Sermilik, a major fjord north of Nanortalik in South Greenland (Figs I and 2)
The field work, undertaken in 1993, was the second fjeld season of the project SUPRASYD (Dawes & Schønwandt, 1992; Nielsen et al, 1993; Garde & Schønwandt, 1994) which is aimed at a reassessment of the geology of the Early Proterozoic Ketilidian orogen of South Greenland, especially the economic mineral potential of the supracrustal rocks
Summary
Surveys in the early part of this century led to the opening of a graphite mine in pelitic schists on Amitsoq island (Fig. l) which lies south-west of Søndre Sermilik. Gold was first detected in the Ketilidian belt in 1986 by panning stream sediments (Nielsen, 1987) and a regional distribution of gold was subsequently proved by stream sediment analysis (Steenfelt, 1987, 1990; Steenfelt & Tukainen, 1991) These findings led to detailed exploration by mining companies (Appel et al, 1993), and chip sampling by Nuna Oil A/S disclosed local gold mineralisation in the parts per million range in the Søndre Sermilik area A significant auriferous quartz vein with visible gold hosted by metabasalts was located in the Ketilidian supracrustal rocks south of Søndre Sermilik. The vein has yielded an average of 51 ppm Au over a width of 0.46 m (Gowen et al, 1993)
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