Metallogenic electromagnetism study reveals the pegmatite rare metal mineral system in Koktokay, Altai, Western China

• BMT data reveal the mineral system of the world’s wellknown rare metal deposit in Koktokay. • The mineral system in Koktokay is controlled by magmatical system with a deep-seated chamber at some 15 km. • Metallogenic electromagnetism study provide crucial evidence for undertanding the genesis of Koktokay rare metal deposit. The Koktokay (KOK) pegmatite field in the Altai Metallogenic Belt in Xinjiang is one of the largest beryllium mines in the world, as well as an important lithium, niobium, and tantalum deposit for China. The KOK No. 3 pegmatite ring is a very typical pegmatite ring among the world’s pegmatite systems. Great progress has been achieved in studies of the KOK rare metal ore deposit, but the metallogenic model and controlling structure of the deposit remain controversial. In addition, the prospects of mineral resources in the KOK district and vicinity are unclear. In this study, we propose a metallogenic model for understanding the rare metal mineral system of the KOK based on an metallogenic electromagnetism study using multiscale electromagnetic exploration and petro-electromagnetism analyses. The geo-electrical structure of the shallow part (to 1,000 m) and deep part (to 30 km) of the KOK rare metal mining district are explored through audio magnetotelluric sounding (AMT) and broadband frequency magnetotelluric sounding (BMT). The results of petro-electromagnetism analyses (complex impedance measurements) of rock samples show that the KOK pegmatite has low-resistivity properties. Resistivity contour maps at iso -depths of 200 m and 400 m from AMT exploration show that a high-resistivity granite mass surrounds the KOK pegmatite field. The pegmatite, along with the wall rocks, are characterized as medium to relatively low resistivity. The BMT results reveal low-resistivity bodies within a depth range of 15–30 km in the southern part of the KOK pegmatite field, which are speculated to reflect residual magma chambers based on the comprehensive interpretation from geophysical, geochemical and geological analysis. The BMT results also indicate resistivity differences between the pathway from the speculated ancient magma chambers to the terminal mineral deposits and surrounding rocks. The source of this system is speculated as a magma chamber, which features low resistivity and lies in the lower crust of the south of the No. 3 pegmatite deposit. Geochemical analysis results indicate that the rare metal pegmatite has experienced highly differentiated evolution. Based on a comprehensive understanding of the geophysical observations and geochemical constraints, we conclude that the mineral system of the rare metal pegmatite at the KOK is controlled by a magmatic system. The KOK No. 3 pegmatite deposit lies in the north of the pegmatite field. We suggest that the south of the KOK pegmatite field also has favorable resource potential.