A 50 m.y. melting model for the rare metal-rich Koktokay pegmatite in the Chinese Altai: Implications from a newly identified Jurassic granite

The petrogenesis of the world famous Koktokay No. 3 pegmatite in the Chinese Altai Mountains is still enigmatic due to its superenrichment of rare metals and the apparent absence of a parental granite. We present results from a granite apophysis that was recently discovered in the No. 3 pegmatite open pit. Results show that it has low K/ Rb (32.35-38.76), Zr/Hf (13.78-23.30), Nb/ Ta (1.00-6.02), and extremely low K2O/Na2O (0.12-0.20) ratios, which, together with its mica composition and the occurrences of garnet, indicate that it is a highly evolved muscovite albite granite. Columbite and apatite from the granite apophysis yielded U-Pb ages of 182.3 +/- 1.0 Ma and 184.9 +/- 4.3 Ma, respectively, which are younger than the Triassic ages (ca. 210 Ma) of the main magmatic stage but fall into the age range of the No. 3 pegmatite series (220-175 Ma). Both the granite and the apatite grains within it share nearly identical rare earth element patterns with the magmatic stage of the No. 3 pegmatite. The whole-rock eHf(t) values (t = 183 Ma) range from -0.11 to +1.32, consistent with those of the No. 3 pegmatite and indicating a similar source. We propose that the Jurassic granite represents a late pulse of magma injected as apophyses from a deep-seated magma chamber (perhaps from the lower crust), which overlapped with the early pegmatite and promoted the rare metal mineralization. The No. 3 pegmatite and the Jurassic granite may represent a continuous magmatic system with a 50 m.y. melting process, generating this unusual giant pegmatite intrusion with abundant rare metals.

Automated summary

The petrogenesis of the Koktokay No. 3 pegmatite in the Chinese Altai Mountains is still unknown, as it is highly enriched in rare metals and lacks a parental granite. However, the recent discovery of a granite apophysis in the No. 3 pegmatite open pit sheds new light on its origin. The granite apophysis is a highly evolved muscovite albite granite, with similar rare earth element patterns as the No. 3 pegmatite. It is suggested that the Jurassic granite represents a late pulse of magma injected from a deep-seated magma chamber, which overlapped with the early pegmatite and promoted the rare metal mineralization.