Gamma-ray spectrometry in the field was used in connection with an intensive study of the abundance and distribution of U and Th in peralkaline rocks of the Illmaussaq intrusion, south Greenland. U and Th were determined from counts accumulated in the 1.76 and 2.62 MeV intervals of a portable T spectrometer.In order to permit a direct comparison with conventional surface sampling we define an effective sample in field T spectrometry as the volume of rock in which the variance of a particular radioelement (U, Th, or K) is equal to the estimation variance of a Tspectrometric determination of this radioelement. This volume contributes about 60% of the measured T rays. It is roughly one-tenth the volume which contributes 90%, the sample size which until now has been widely accepted. The mass of the effective sample for a NaI (T1) detector whose sides are surrounded by 5 cm of lead, placed on a rock surface, is •20 kg and •15 kg for 2.62 and 1.76 MeV T rays, respectively. Calibrations for U and Th were achieved by use of large concrete standards of known radioelement content, and having a y-ray mass absorption nearly equal to that of the rock to be measured. The calibration for U based on the concrete standards was more accurate than a calibration based on rock samples collected from measurement localities. The accuracy of the field T spectrometer, depending on standard errors associated with calibration equations, was about 2.5% for Th, and ranged from 2.5 to 7.5% for U, as Th/U varied from ! to 5. Analytical precision for single measurements, combining the effects of counting statistics and instrument instability, ranged between 1 and 8% for Th, and between 1 and 12% for U. As many as 100 measurements per day with a leadcollimated detector were made along grids with 1-meter spacings in rock with a rough surface and an uneven distribution of radioelements. Results confirmed that specific rock types have characteristic U and Th contents. The measurement system was accurate enough so that where adequate radioelement contrasts existed, contours of U and Th corresponded closely to rock type and followed structural trends in the rock.
The U–Th–REE deposit located at the Kvanefjeld plateau in the north-west corner of the Ilímaussaq alkaline complex, South Greenland, consists of lujavrites which are melanocratic agpaitic nepheline syenites. The fine-grained lujavrites of the Kvanefjeld plateau can be divided into a northern and a southern part with an intermediate zone between them. The northern part is situated along the north contact of the Ilímaussaq complex and continues east of the Kvanefjeld plateau as a lujavrite belt along the contact. This part has relatively ‘low’ contents of U, Th, and REE, and hyperagpaitic mineralogy is restricted to its highest-lying parts. The fine-grained lujavrites of the intermediate and southern part of the Kvanefjeld plateau occur between and below huge masses of country rocks which we show are practically in situ remnants of the roof of the lujavrite magma chamber. These lujavrites have high contents of U, Th, and REE, and hyperagpaitic varieties with naujakasite, steenstrupine and villiaumite are widespread. We present a model for the formation of the fine-grained lujavrites of the Kvanefjeld plateau. In this model, an off-shoot from the large lujavrite magma body in the central part of the complex intruded into a fracture zone along the north contact of the Ilímaussaq complex and was forcefully emplaced from north-west to south-east. The intruding lujavrite magma was bounded to the west, north, and at its roof by strong volcanic country rocks, and to the south by the weaker, earlier rocks of the complex. The magma stored in the fracture crystallized, squeezing volatile and residual ele-ments upwards. A subsequent violent explosion opened up fractures in the weaker southern rocks, and the residual volatile-enriched magma was squeezed into fractures in augite syenite, naujaite, and also in the overlying volcanic roof rocks. The removal of the volatile-rich lujavrite magma in the upper part of the fracture-bounded magma chamber made room for the rise of volatile-poor magma from the lower part of the magma chamber, and these lujavrites crystallized to form the northern continuous lujavrite belt. Transfer and accumulation of volatile and residual elements in a lujavrite magma crystallizing below an impervious cover played a key role in the formation of the Kvanefjeld U–Th–REE deposit, as it also did in the crystallization of the lujavrite magma body in the central part of the Ilímaussaq complex.
The Kvanefjeld area is situated in the northernmost part of the nimaussaq intrusion. The area represents a section through the roof zone of this intrusion. The roof is composed of sandstone, basaltic lavas, sheets of gabbro and dykes of dolerite and trachyte. Large masses of anorthosite are also found in the roof zone, The oldest members of the intrusion are augite syenite and alkali syenite which together with large masses of naujaite (poikilitic sodalite syenite) are enclosed in various types of fine-grained lujavrite. The lujavrite also intrudes the lavas of the roof. The bodies and veins of lujavrite are mainly located in zones of deformation in the rocks of the roof zone. The lavas of the roof are strongly altered in contact with the lujavrite and are locally enriched in epistolite-murmanite minerals. The latest member of the intrusion is a medium- to coarsegrained lujavrite which forms sheets and veins in most of the abovementioned rocks. The earlier fine-grained lujavrites and the contactmetasomatized lavas have concentrations of steenstrupine, monazite and thorite (?) in contact with the medium- to coarse-grained lujavrite and may contain up to 0.3 % U and three to four times this amount of thorium. This mineralization has been studied by mineralogical, geochemical and radiometric methods and in a number of drill holes. Analcime-rich veins rich in niobium and beryllium minerals are of widespread occurrence. The present paper gives a preliminary account of the geology of this region with special reference to the structural geology. A detailed examination of the economic geology of the Kvanefjeld area is currently being undertaken.
In the peralkaline Ilimaussaq intrusion in South Greenland minor quantities of beryllium minerals are widespread in hydrothermal veins. Concentrations of veins rich in beryllium minerals are known from the Taseq slope and the Kvanefjeld area in the northern part of the intrusion. Up to now 10 beryllium minerals have been found, the most important being chkalovite. The hydrothermal veins range in width from about 1 mm to 2 m but are mostly a few cm thick. The most important minerals in the veins are analcime, sodalite, ussingite, natrolite, aegirine, arfvedsonite, epistolite and chkalovite. A field beryllometer based on the photoneutron method of determining beryllium is described. With a 100 mCi Sb124 activation source the instrument has a limit of detection ofless than 10 ppm BeO. The effective measuring area is ca. 40 cm2. Beryllium has a log-normal distribution in the rocks of the area.
The agpaitic part of the Ilímaussaq alkaline complex, South Greenland, is made up of a roof zone, an intermediate zone and a floor zone. Dykes and sills of peralkaline microsyenite intersect the rocks of the roof and floor zones, but do not appear to intersect the lujavritic nepheline syenites which make up the intermediate zone. The microsyenites consist of Na-poor microcline, K-poor albite, aegirine and arfvedsonite which are practically identical to those of the agpaitic nepheline syenites of the complex. Neptunite and pectolite are the commonest minor minerals. The microsyenites are silica-saturated, –oversaturated, or, more rarely, undersaturated. The agpaitic part of the Ilímaussaq complex is considered to have been formed in a closed magma chamber; the lujavrites of the intermediate zone representing residual melts left after the consolidation of the roof and floor zones. That the microsyenite intrusions intersect the roof and floor zones but not the youngest lujavrites lying between these zones presents a geometrical problem which is discussed at some length. It is difficult to explain the microsyenites as products of fractionation or contamination of melts within the agpaitic magma chamber. Furthermore, the microsyenites differ mineralogically and chemically from the abundant microsyenitic dykes of the regional Tugtutôq-Ilímaussaq dyke swarm. It is therefore proposed that they originated in the source region which fed the agpaitic melts of the Ilímaussaq complex and that their emplacement in fractures was accompanied by a loss of volatiles and incompatible elements.
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