Deep tectonic characteristics and ore-prospecting potential of the Zetang Cr-bearing ultramafic rock mass in Tibet

Abstract：Numerous Cr-bearing ultramafic rock masses with high ore-prospecting potential are distributed around Luobusha, the largest chromite deposit in China. The Zetang rock mass was selected to conduct profile probing using gravimetric, magnetic and magnetotelluric methods. The results show that the Zetang rock mass was cut into several parts by approximately NS-trending faults, with the west parts moving farther northward than the east ones and distributed in NWW directions on the whole. The magnetic anomalies in the mass were caused by the magnetite resulting from the serpentinization and the residual magnetization when the rock mass was formed, and such magnetic anomalies indicate the limits of the superficial part of the rock mass. The gravity anomalies reveal that there are high-density bodies existing locally in the mass, which accord with the magnetic anomalies, and the outcropped ore spots at the earth's surface are mostly distributed within the limits. The surface layer of the mass is a thin high-resistance layer, with a thickness of at least one hundred meters in general, and the maximum thickness of the high-resistance body reaches 1.5 km in the middle section of the mass profile but its area is not large. Under the thin high-resistance bodies, there is a thin low-resistance layer. Both sides of the profile are dominated by high-resistance bodies, but the middle section contains quite a few low-resistance bodies, which are connected with the low-resistance bodies in the deep part at the depth of 5–25 km and thus belong to the low-resistance bodies in the middle crust. It is inferred that these low-resistance bodies were caused by partial melting, suggesting that there probably existed passages for the upward migration of mantle-derived materials. Through summarizing the characteristics of various geophysical fields, it is thought that the sectors with more local gravity anomalies and low-resistance body anomalies in a magnetic anomaly area are most favorable for further detailed exporation and ore-prospecting work.