From Mantle to Motzfeldt: A genetic model for syenite-hosted Ta,Nb-mineralisation

Abstract

A genetic model for the Motzfeldt Tantalum-Niobium-rich syenite in southwest 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.

Sulphur isotopes of alkaline magmas unlock long-term records of crustal recycling on Earth

“…In brief, Ilímaussaq hosts agpaitic nepheline syenites (peralkaline rocks with complex Na-Ca-HFSE minerals) and is one of the most reduced and chemically evolved magmatic series known 43 . Motzfeldt predominantly hosts miaskitic nepheline syenites (peralkaline rocks with zircon and Fe-Ti oxides), which formed at reduced magmatic conditions (less extreme than Ilímaussaq, but still below the QFM buffer 40 ) and underwent intense late-stage oxidation 44 . Ivigtût is an alkaline granite stock that is associated with the world’s largest deposit of cryolite (Na 3 AlF 6 ).…”

Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

“…If natural pyrochlores contain a lot more U than Th (e.g. [ 15 ]) then either the melt must have had exceptionally high U/Th [ 52 ], or, perhaps more common, the pyrochlores were precipitated from or altered by a hydrothermal fluid, which can transport U much more efficiently than Th [ 15 ]. A recent paper shows nicely how the U and Th composition of pyrochlore varies with increasing degree of alteration: Whereas primitive pyrochlores contain wt.% of Th and only little U, the U content of pyrochlore increases and the Th content decreases with increasing degree of hydrothermal alteration [ 51 ].…”

“…In this study we focus on the Nb-and Ta-rich accessory phases pyrochlore, microlite, and fersmite, which are accessory phases in alkaline silicate rocks [14][15][16][17][18][19][20] and, perhaps more commonly, carbonatites [18,[21][22][23]. Note that these mineral phases are the most important hosts for Nb and Ta in ore deposits (e.g., [18,[24][25][26][27].…”

Mineralogical constraints on Nb–REE mineralization of the Zhujiayuan Nb (−REE) deposit in the North Daba Mountain, South Qinling, China

“…There is a current controversy over the role that magmatism and post‐magmatic alteration play during HFSE and REE mineralization in alkaline igneous rocks. On the one hand, certain studies suggest that magmatism caused the formation of these deposits (Finch et al, ; Huang, Wang, Zhang, Li, & Qin, ; Huang, Zhang, Santosh, & Zhang, ; Kogarko, ; Kovalenko et al, ; Linnen & Keppler, ; Mccreath, Finch, Herd, & Armour‐Brown, ). On the other hand, studies have suggested that a combination of magmatism and post‐magmatic alteration resulted in the formation of these deposits (Kempe, Götze, Dandar, & Habermann, ; Salvi & Williams‐Jones, , , ; Schmitt, Trumbull, Dulski, & Emmermann, ; Sheard, Williams‐Jones, Heiligmann, Pederson, & Trueman, ; Wang, Zhao, & Chen, ; Yang et al, ).…”

“…For example, the Khaldzan‐Buregtey Zr–Nb–REE deposit contains mineralization distributed in albitized, silicified, and epidotized igneous rock (Kovalenko et al, ). The majority of the pyrochlore in the Motzfeldt syenite‐hosted Nb–Ta deposit has a metamorphic structure (Finch et al, ; Mccreath et al, ). Post‐magmatic alteration, especially albitization and greisenization, in the Boziguo'er rare metal granite is ubiquitous (Huang et al, , ).…”