Alkaline Rocks and Carbonatites of the World

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Werner Bjerge

stripes

Occurrence number: 
063-00-009
Country: 
Greenland
Location: 
Longitude: -24, Latitude: 71.92

This complex, having a diameter of about 17 km, is emplaced in lithologically variable sediments of Upper Palaeozoic age which are slightly hornfelsed near the contacts, and inclusions of which are abundant in parts of the complex. There is a partially preserved roof of feldspar porphyries, breccias and various pyroclastic rocks. The complex comprises three sub-complexes. In the southeast is the oldest centre of dominantly basic rocks including pyroxenites, kaersutite-bearing alkali gabbros grading into syenogabbros, and dykes and veins of syenite and granite, together with volcanic agglomerates and breccias. Olivine, amphibole and biotite are present in the pyroxenite and gabbros in which some layering is developed. The northern sub-complex is dominantly composed of syenites and granites which are generally banded and with the development of pegmatitic patches. The granites are not peralkaline, but the syenites, which grade from quartz-bearing to analcime- and sodalite-bearing varieties, contain aegirine-augite, riebeckite and arfvedsonite. The south and west of the complex is occupied principally by analcime-bearing syenites which grade into nepheline-sodalite foyaites. These rocks contain perthite, albite, up to 30% nepheline + sodalite, cancrinite and analcime, aegirine/aegirine-augite, barkevikite, arfvedsonite, biotite and accessories including lavenite, lamprophyllite, mosandrite and narsarsukite. Igneous and sedimentary xenoliths are widespread and there is evidence of fenitization of basic rocks. Leucocratic dykes, principally bostonites, cut the sediments and are probably related to the northwest centre. Basic dykes and sills are similarly widely distributed but tinguaites are restricted to the area of the nepheline syenite sub-complex. Rock analyses are available in Bearth (1959) and Brooks et al. (1982) have made a detailed mineralogical study.

Economic: 
A small isolated intrusion of alkaline granite and syenite cutting Carboniferous sediments to the west of the main complex and known as Malmbjerg is, together with the surrounding sediments, extensively mineralized, being enriched in Fe, Mn, Ti, Zr, Nb, W, Bi and Mo. The deposits have been extensively investigated and reserves close to 200 million tons of ore grading 0.25% MoS2 estimated (Sorensen et al., 1978, p. 258).
Age: 
Two fission-track ages on sphene gave a weighted mean of 30.3±1.3 Ma, but a discordant age on apatite of 20.5±1.3 Ma was considered to represent cooling due to erosion of overburden (Gleadow and Brooks, 1979, p. 54). A whole rock Rb-Sr isochron yielded an age of 31±2 Ma, and K-Ar on biotite 28±1 Ma (Rex et al., 1979).
References: 
BEARTH, P. 1959. On the alkali massif of the Werner Bjerge in East Greenland. Meddelelser om Gronland, 153(4): 1-63. BROOKS, C.K. and GILL, R.C.O. 1982. Compositional variation in the pyroxenes and amphiboles of the Kangerdlugssuaq intrusion, East Greenland: further evidence for the crustal contamination of syenite magma. Mineralogical Magazine, 45: 1-9. GLEADOW, A.J.W. and BROOKS, C.K. 1979. Fission track dating, thermal histories and tectonics of igneous intrusions in East Greenland. Contributions to Mineralogy and Petrology, 71: 45-60. REX, D.C., GLEDHILL, A.R., BROOKS, C.K. and STEENFELT, A. 1979. Radiometric ages of Tertiary salic intrusions near Kong Oscars Fjord, East Greenland. Rapport, Gronlands Geologiske Undersogelse, 95: 106-9. SORENSEN, H., NIELSEN, B.L. and JACOBSEN, F.L. 1978. Denmark and Greenland. In S.H.U. Bowie, A. Kvalheim and H.W. Haslam (Eds). Mineral deposits of Europe, 1: 251-61. The Institution of Mining and Metallurgy and The Mineralogical Society, London
Fig. 88 Werner Bjerge (after Bearth, 1959, Plate 22).
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