The Motzfeldt 87 project. Final report

The Motzfeldt 87 Project comprises a detailed investigation of the niobium tantalum-uranium-bearing pyrochlore mineralisation at Motzfeldt Se, South Greenland. The aim of the project is to gain a general impression of the metal distribution in the mineralised zones and to delineate areas with high metal concentrations. The project had a duration of one year covering a field season and subsequent laboratory work, compilation and evaluation of results.

The mineralisation is hosted in the Hotzfeldt Centre which belongs to the Proterozoic Gardar alkaline province. The centre is build up of several intrusive phases of syenite and nepheline syenite. Pyrochlore is concentrated in a 200-300 m wide zone along the outher margin of the intrusion. The mineral is hosted in both peralkaline microsyenite and altered syenite, with tantalum content (Nb/Ta = 11) highest in the latter. A vertical zonation of the Nb/Ta ratio exists with the highest tantalum contents at lower levels.

The field work carried out in July and August 1987 involved twenty persons and comprised an airborne gamma-spectrometric survey, a major chip sampling programme and minor reconnaissance.

Five areas were flown in detail with a helicopter borne, multi-channel gamma spectrometer. A total of 454 line ~ were flown and 63 443 measurements were recorded over a total area of 18 km . The aeroradiometric results were used to guide the chip sampling programme. The data have been compiled into 12 contoured anomaly maps for uranium, thorium and U/Th ratio (appendices 8-19).

Systematic chip sampling of four main localities situated on steep mountain sides were carried out by a team of six mountaineers. The 928 samples collected have been assayed for Nb, Ta, U and Th, and analysed for Be, Ce, La, Li, Ho, Sn, Y and Zr. The geochemical data have been treated statistically; and based on kriging estimates 20 contoured anomaly maps for niobium, tantalum, Nb/Ta ratio, uranium and thorium have been prepared (appendices 21-30). Areas and average metal contents have been calculated for various cut-off values of tantalum and niobium (table 9). The most favourable ar~a contains c. 3770 ppm Nb, 430 ppm Ta, 180 ppm U and 120 ppm Th over 1260 m².

Aeroradiometric and geochemical data have been correlated, whereby it has been possible to prepare 12 extrapolated geochemical maps of niobium, tantalum and uranium (appendices 31-42).

The tantalum potential of the investigated localities have been evaluated on the base of the processed data and geological modelling. As a result two localities are given a high priority for tantalum exploration, one locality a medium potential, and three localities a low potential.

It is concluded, that the miner~lised zone of the Motzfeldt Centre has a potential of more than 500 x 10⁶ tons rock with average contents of 1400 ppm Nb, 120 ppm Ta, 60 ppm U and 90 ppm Th. The potential fo5 mineralisation with more than 250 ppm Ta (305 ppm Ta₂O₅) is at least 30 x 10⁶ tons.