GEUS Bulletin (Geological Survey of Denmark and Greenland)
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Titanium minerals in Cameroon
No full textThe mineral rutile (TiO2) is a major ore of titanium, which is used in products such as white pigment and titanium metal. The global consumption of titanium minerals in 2011 was c. 6.7 million tonnes of which 0.7 million tonnes were rutile and 6 million tonnes ilmenite (TiFeO3). Rutile is almost pure TiO2 and therefore more valuable than ilmenite (c. 1500 /t, respectively). Compared with ilmenite, rutile can be processed with lower consumption of chemicals and yields less waste products. Rutile was mined in Cameroon between 1935 and 1955 when a total of 15 000 tonnes of rutile were extracted from alluvial deposits. The French Bureau de Recherches Géologiques et Minières conducted a drilling programme in Cameroon in the 1980s which identified c. 2.6 million tonnes of rutile in discontinuous occurrences with concentrations of c. 1%. Most of the occurrences are located in small- to medium-sized riverbeds with a thickness of 1.5–4.5 m. The main alluvial rutile area is located around the town of Akonolinga, 80 km east of Yaoundé, the capital of Cameroon (Fig. 1). The rutile in the alluvial deposits was derived from the bedrock by weathering, and at some sites major, residual rutile deposits are reported from Quaternary lateritic deposits. The Geological Survey of Denmark and Greenland conducted a project together with the Institut de Recherches Géologiques et Minières in Cameroon to gain a better understanding of how rutile formed in the bedrock before it was weathered out and try to tie the rutile in the alluvial deposits to its source rocks. This is done by studying the compositional variation of the rutile in the alluvial deposits and comparing it with possible bedrock sources. The compositional variation of ilmenite and monazite ((La,Ce)PO4)) and the age distribution of zircon (ZrSiO4) in alluvial sand and bedrock were also investigated. The chemical compositions of minerals in the sediments are used to infer the bedrock source of the minerals. This has particular application in many areas of Cameroon, such as the southern part of the country which is characterised by low relief and dense rain forest with bedrock outcrops that are sparse and difficult to find. Cameroon is a country in west central Africa (Fig. 1) and is called ‘Africa in miniature’ because of its cultural, geological and landscape diversity. The landscape includes beaches, deserts, mountains, rain forests and savanna. The highest point is the active volcano Mount Cameroon (4095 m), and the country is home for over 200 different linguistic groups with French and English as the official languages. Compared with other African countries, Cameroon is politically and socially stable. The country covers an area of 475 442 km2 with a population of c. 20 million of which 70% are Christians and 20% Muslims
Drowning of the Miocene Billund delta, Jylland: land–sea fluctuations during a global warming event
No full textLower Miocene strata from boreholes and, in particular, at outcrops in the Lillebælt and Limfjorden areas of Jylland provide a natural laboratory for studying the drowning of a major delta system during a period of global warming. Detailed studies of sedimentary structures, fossil algae, spores and pollen give information about depositional environments, local temperatures and precipitation. By comparing with the global climatic record from the same period, a detailed reconstruction of the flooding of a low-relief delta system can be made, with emphasis on the global warming after the glacial event Mi1a. The local temperature increase following the Mi1a event is estimated to be c. 5°C
A Baltic Ice Lake lowstand of latest Allerød age in the Arkona Basin, southern Baltic Sea
No full textAfter the last deglaciation, the Baltic Sea underwent a complex salinity history and dynamic shore-level development with several lacustrine and marine stages: the Baltic Ice Lake, the Yoldia Sea, the Ancylus Lake and the Littorina Sea (Björck 1995). In connection with shallow seismic profiling in the south-western Baltic Sea, two marked and widespread erosional unconformities have been identified (Jensen et al. 1997, 1999; Lemke et al. 1998; Larsen 2004). The older unconformity occurs within sediments deposited in the Baltic Ice Lake, whereas the younger one separates Baltic Ice Lake sediments from Holocene lake and mire deposits. The latter unconformity is dated to the transition between the Younger Dryas and the Holocene, corresponding to c. 11.7 cal. ka BP and formed due to a sudden drop in the level of the icedammed Baltic Ice Lake of around 25 m, caused by ice recession from Mt. Billingen in south central Sweden
Seismic activity in Denmark: detection level and recent felt earthquakes
No full textThe Geological Survey of Denmark and Greenland (GEUS) records seismological data at six locations in Denmark (Fig. 1) and all data from these stations are manually reviewed for events like earthquakes and explosions. The identified events are analysed and located, in many cases using supporting data from stations outside Denmark. Seismic events have been recorded instrumentally in Denmark since 1929, but earthquakes felt in Denmark have been reported as far back as 1515 (Lehmann 1956; Gregersen et al. 1998; GEUS 2012). This article reports on the developments in detection level of both man-made events and natural earthquakes within the Danish Exclusive Economic Zone (EEZ) from 2000 to 2012. Changes in detection level are mainly due to the availability of data from new seismic stations in Sweden and Norway as well as from a GEUS test station at Gøttrup in NW Jylland. As a case study, the list of events on and around Bornholm is reviewed. Also described here are the reported intensities at two recent felt events in Denmark (North Sea magnitude 4.3 on 19 February 2010 and Kattegat magnitude 4.1 on 6 August 2012)
Fingerprinting of corundum (ruby) from Fiskenæsset, West Greenland
No full textSince the late 1960s, it has been known that pink and red corundum occur in the area near Fiskenæsset (Qeqertarsuatsiaat) in southern West Greenland. Corundum is hosted in the Fiskenæsset complex, which is part of the Archaean basement of the North Atlantic Craton. To date, c. 40 corundum localities with a wide range of quality are known in the area – a few localities yield stones of gem quality. The most promising locality, Aappaluttoq, is likely to be mined in the foreseeable future by the Canadian company True North Gems (Figs 1, 2A). Red corundum of gem quality is called ruby; gem quality corundum of other colours (e.g. pink, yellow or blue) is called pink sapphire, yellow sapphire etc., while the blue gem corundum is sapphire. Red, pink and blue corundum are also known in smaller quantities from other areas in Greenland
Assessing urban groundwater table response to climate change and increased stormwater infiltration
No full textThe global climate is expected to show continued warming throughout the coming century. As a direct consequence of higher temperatures, the hydrological cycle will undergo significant changes in the spatial and temporal distribution of precipitation and evapotranspiration. In addition to more frequent and severe droughts and floods, climate change can affect groundwater recharge rates and groundwater table elevation (Bates et al. 2008). Some previous studies of climate change impact on groundwater have suggested alarming reductions in ground-water recharge and lowering of water tables. Other studies, especially those focusing on regions of higher latitudes, have indicated a potential rise in water tables due to increased precipitation and recharge (Scibek & Allen 2006; Woldeamlak et al. 2007
The norite belt in the Mesoarchaean Maniitsoq structure, southern West Greenland: conduit-type Ni-Cu mineralisation in impact-triggered, mantle-derived intrusions?
No full textWith the recent discovery of the giant, deeply eroded, 3 Ga Maniitsoq impact structure in southern West Greenland (Garde 2010), an enigmatic, c. 75 by 15 km large, curvilinear belt of undeformed norite intrusions with Ni-Cu mineralisation was re-interpreted as representing crustally contaminated melts derived from the mantle in the wake of the impact (Fig. 1; Garde et al. 2012). The norite belt (Nielsen 1976; Secher 1983) was discovered in the early 1960s by the mining and exploration company Kryolitselskabet Øresund A/S, and more than one hundred shallow exploration holes were drilled by the company in the period 1965–1971. The mineralisation has subsequently been investigated by Cominco Ltd., Falconbridge Ltd. and NunaMinerals A/S. In 2011, the re-interpretation of the norite belt, and recent availability of improved airborne geophysical exploration tools, prompted the Canadian company North American Nickel Inc. (NAN) to resume exploration
Lineament mapping and geological history of the Kangerlussuaq region, southern West Greenland
No full textHow could future ice ages affect deep nuclear waste repositories in crystalline basement rocks? Deep repositories may be affected by a number of glacially induced processes including, but not limited to, (1) fault activation or re-activation and associated seismicity, (2) changing hydraulic and chemical groundwater dynamics and (3) enhanced erosion. Such processes are likely to affect not only man-made barriers in spent fuel repositories such as copper canisters and bentonite clay buffers, but also the rock masses that contain and isolate the repositories. In order to increase our understanding of this problem, an international study (the Greenland Analogue Project) was set up in 2008. The aim of the study was to use crystalline bedrock at the margin of the Inland Ice near Kangerlussuaq airport in West Greenland as an analogue for future nuclear fuel waste repositories affected by glaciation in Fennoscandia and Canada. Accordingly, a wide range of field surveys were conducted for the analogue project (Fig. 1). This paper describes a detailed structural investigation of lineament zones and the establishment of an event succession for fault and fracture zone evolution in central parts of the study area (Figs 1B, 2), as well as an interpretation of the distribution of fracture and fault zones with potentially increased permeability. Three deep holes were drilled in the study area, and instruments were installed in two of them for subsequent down-hole sampling and monitoring of groundwater to a depth of c. 600 m. The cores were used to compare the subsurface fracture patterns with those established on the basis of surface mapping
Calibration of spectral gamma-ray logs to deltaic sedimentary facies from the Cretaceous Atane Formation, Nuussuaq Basin, West Greenland
No full textGamma-ray logs are widely used as a lithology indicator in wells as part of standard petrophysical interpretations. In cored wells, gamma-ray logs should always be calibrated to the lithology in order to correct the petrophysical model. Gamma radiation is emitted from three elements, K, Th and U (potassium, thorium and uranium) which occur in minerals such as feldspar, mica, glauconite, clay minerals, zircon, titanite and apatite as well as in organic complexes. Organic-rich mudstones usually have high gamma-radiation values and quartz-rich sandstones low values. In many places, upward-coarsening successions are recognisable from the gamma log. The gamma log records the sum of radiation from K, Th and U, and their relative contributions are measured in a spectral gamma-ray log. The present case study focuses on spectral gamma-ray characterisation of the deltaic Atane Formation which shows well-developed, upward-coarsening delta-front deposits in outcrops (Fig. 1C)
Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland
No full textThe 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c. 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation.
A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c. 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations.
Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting.
There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift