We have just published on Ore Geology Reviews the culmination of many years worth of work in the Motzfeldt region of South Greenland, considered as one of the world’s largest Ta-Nb deposits. The work was commenced in 1995 as part of a regional petrographic study on the earliest rocks from the Gardar rift and has developed into a long-standing project lasting nearly 25 years. It has followed the fortunes of the region through a succession of licence holders.
Finch AA, McCreath JA, Reekie CDJ, Hutchison W, Ismaila A, Armour-Brown A, Andersen T, Simonsen SL (2019) From Mantle to Motzfeldt: A Genetic Model for Syenite-hosted Ta,Nb-mineralisation. Ore Geology Reviews, 107, 402-416. Finch et al 2019 Motzfeldt with Commons Licence
The wider significance of this work includes the fact that alkaline igneous rocks host some of the world’s most valuable deposits of critical raw metals such as Ta and Nb. Although there is a volume of literature on carbonatite and peraluminous granitoid-hosted Ta and Nb, there is very little on Nb and Ta in syenite. This paper presents a coherent model which explains the petrogenesis from mantle to surface exposure. We track high field strength elements from their source in the mantle, through ascent to emplacement, crystallisation and through the effects of aggressive late-stage hydrothermal alteration. The paper provides an explanation of the complex field geology and the unusual mineralogical and isotopic geochemistry. It also provides pointers for future exploration.
Roof zones generally are important magmatic systems in which exceptional geochemistry is observed. As is explored further in the HiTech AlkCarb consortium, roof zones concentrate many critical elements, such as Nb and Ta as explored here. The research at Motzfeldt was partly published from support from Work Package 2 of the HiTech AlkCarb consortium.
A substantial archive of data relating to the area has also been created.
A genetic model for the Motzfeldt Tantalum-Niobium-rich syenite in south-west Greenland, considered to be one of the world’s largest Ta prospects, is presented. The Motzfeldt primary magma formed early in regional Gardar (1273±6 Ma) rifting. Isotope signatures indicate that the Hf had multiple sources involving juvenile Gardar Hf mixed with older (Palaeoproterozoic or Archaean) Hf. We infer that other High Field Strength Elements (HFSE) similarly had multiple sources. The magma differentiated in the crust and ascended before emplacement at the regional unconformity between Ketilidian basement and Eriksfjord supracrustals. The HFSE-rich magmas crystallised Ta-rich pyrochlore which formed pyrochlore-rich crystal mushes, and it is these pyrochlore-rich horizons, rich in Ta and Nb, that are the focus of exploration. The roof zone chilled and repeated sheeting at the roof provided a complex suite of cross-cutting syenite variants, including pyrochlore microsyenite, in a ‘Hot Sheeted Roof’ model. The area was subject to hydrothermal alteration which recrystallized alkali feldspar to coarse perthite and modified the mafic minerals to hematite, creating the friable and striking pink-nature of the Motzfeldt Sø Centre. Carbon and oxygen isotope investigation of carbonate constrains fluid evolution and shows that carbonate is primarily mantle-derived but late-stage hydrothermal alteration moved the oxygen isotopes towards more positive values (up to 21 ‰). The hydrothermal fluid was exceptionally fluorine-rich and mobilised many elements including U and Pb but did not transport HFSE such as Ta, Hf and Nb. Although the U and Pb content of the pyrochlore was enhanced by the fluid, the HFSE contents remained unchanged and therefore Hf isotopes were unaffected by fluid interaction. While the effect on hydrothermal alteration on the visual appearance of the rock is striking, magmatic processes concentrated HFSE including Ta and the hydrothermal phase has not altered the grade. Exploration for HFSE mineralisation commonly relies on airborne radiometric surveying which is particularly sensitive to the presence of U, Th. A crucial lesson from Motzfeldt is that the best target is unaltered pyrochlore which was identified less easily by radiometric survey. Careful petrological/mineral studies are necessary before airborne survey data can be fully interpreted.