Precambrian geology of Nuussuaq and the area north-east of Disko Bugt, West Greenland

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Garde, A. A., & Steenfelt, A. (1999). Precambrian geology of Nuussuaq and the area north-east of Disko Bugt, West Greenland. Geology of Greenland Survey Bulletin, 181, 6-40. https://doi.org/10.34194/ggub.v181.5108 _______________ The Precambrian terrain of eastern Nuussuaq and north-east Disko Bugt largely consists of late Archaean (c. 2800 Ma) orthogneisses, intercalated with units of strongly deformed Archaean supracrustal rocks. The latter are up to several kilometres wide and comprise both metavolcanic and metasedimentary rocks within which local occurrences of gold have been found. In central Nuussuaq a layered complex of anorthosite, leucogabbro, gabbro and ultramafic rocks is tectonically intercalated with Archaean orthogneisses, and an intrusive complex of Archaean tonalites and trondhjemites, largely unaffected by Archaean and Proterozoic deformation, occurs in the area north-east of Disko Bugt. Here an up to c. 3.5 km thick sequence of early Proterozoic shallow marine clastic sediments and minor marble unconformably overlies Archaean rocks. Several suites of basic dykes are present, and dykes and small plugs of ultramafic lamprophyre and lamproite (age c. 1750 Ma) are common in the central part of the region. Most of the region was overprinted by early Proterozoic deformation and metamorphism. Prominent Proterozoic flat-lying ductile shear zones with north- or north-westward movement of the hanging wall are overprinted by open folds.

A reassessment of the timing of early Archaean crustal evolution in West Greenland

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Moorbath, S., & Kamber, B. S. (1998). A reassessment of the timing of early Archaean crustal evolution in West Greenland. Geology of Greenland Survey Bulletin, 180, 88-93. https://doi.org/10.34194/ggub.v180.5091 _______________ In last year’s Review of Greenland activities, Kalsbeek (1997) divided the recent history of geochronology into three successive periods: 1. single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2. Rb-Sr and Pb/Pb whole-rock isochrons and multigrain zircon U-Pb isotope data; 3. the present, where ‘single’ zircon U-Pb data are predominantly used. To these three, we would propose adding a fourth, namely a combination of all three, in order to achieve the maximum age information within complex terrains. For an early Precambrian terrain like that of West Greenland, we consider that the combined use of at least the last two approaches is essential (to which should be added the Sm-Nd method). In recent years, study of the geochronological evolution of the Godthåbsfjord and Isua regions has been dominated by rapid and precise ion-probe U-Pb dating of complex-structured zircons, and it has become fashionable to regard the wide range of zircon dates, and particularly the oldest, as giving the age of rock formation. Dates obtained from whole-rock Rb-Sr, Sm-Nd and Pb/Pb regressions have been regarded as too imprecise for adequate age resolution, whilst constraints on crustal evolution imposed by initial Sr, Nd and Pb isotope ratios have been summarily dismissed or totally ignored. We consider that this sole dependence on ion-probe dating of zircon can lead (as, indeed, in the early Archaean of West Greenland) to a potential misinterpretation of the timing of crustal evolution, especially in those cases where little or no information regarding the relationship between measured date and internal grain structure is available. Figure 1 shows the localities mentioned in the text.

Mineralogy of the near-surface expression of Au-As-Cu mineralization in an arid environment

In the arid, Late Precambrian terrain of southern Israel, a complex suite of minerals and amorphous species were deposited in host gneiss from fluids under near-neutral conditions within 1 m of the surface. The morphology of secondary gold appears to relate to its host mineral (skeletal-dendritic with quartz; multi-faceted crystals with arsenates; spherical droplets with iron oxide). The gold is very fine-grained, and was most likely complexed as a thiosulphate.Three amorphous phases are present (iron oxide, chrysocolla, Cu-Mn-(Fe-As) silicate). At least in part, gold and baryte appear to have crystallized out of a metal-Fe-oxide gel. Other minerals, including apatite, anglesite, and conichalcite, may have grown from appropriate crystallites present in the gel.The conichalcite occurs mainly as bladed to acicular radial spherulites. In the presence of lead, a solid solution phase between duftite and conichalcite (‘Pb-conichalcite’) was formed.