The magnetic anomaly characteristics of Dongpo, Xigaze and some other ophiolite rock masses along the Yarlung-Zangbo suture zone and their oreprospecting significance

Aeromagnetic survey △T maps clearly show that there are two SN-trending aeromagnetic anomaly zones along the Yarlung-Zangbo suture zone, and geological researches have confirmed that the ophiolite rock masses along the Yarlung-Zangbo suture zone extend nearly 2000 km in banded distribution and in ES direction; however, it has been known that the main chromiumcontaining ophiolite rock masses are distributed on the south bank of the Yarlung Zangbo River. There are discontinuously distributed chromium-containing ophiolite rock masses observed on the ground of Luobusha and Zedang, which have small sizes and whose geological study degree is relatively high. There is Xigaze rock mass outcropped westward on the ground, and there are huge rock masses, including Dangqiong, Purang, and Dongpo, further to the west. Aeromagnetic survey maps show that the rock masses, including Xigaze, Dongpo, Purang, and Dangqiong, are obviously consistent with aeromagnetic anomalies. Ground magnetic survey for the above rock masses accurately has determined the relationship between the known rock masses and the aeromagnetic anomalies, and the Luobusha and Zetang rock masses in the eastern section of the Yarlung Zangbo River belong to small rock masses within the negative anomaly range on the northern margin of the strong aeromagnetic anomaly in the south. The chromium-containing ophiolite rock masses along the river in the west are distributed discontinuously, exhibiting short banded or large three-dimensional masses; ground magnetic survey anomalies prominently reflect the unevenness of distribution of the ground magnetite rock masses, with strong serpentinization and strong magnetic anomaly on the margin of the rock masses (including surrounding rocks near rock masses), thus to accurately delimit the range of the rock masses. The aeromagnetic anomalies reflect that there are relatively enriched magnetic minerals in the rock masses; the magnetic minerals might come from the minerals containing such components as FeO, and Fe₂O₃ in the deep part, which upwelled to the upper crust with magma and formed Fe₃O₄ due to serpentinization, thus generating strong magnetism when the temperature was lower than 480℃. Inversion calculation shows that the magnetic bodies are within the depth range of 3-5 km; although the magnetite existing in the ophiolite rock masses and their surrounding rocks was not directly from mantle materials, the magma materials in the chromium- containing ophiolite rock masses were main material contributors to the magnetite. The distribution range of the magnetite roughly corresponds to the orecontaining ophiolite rock masses, thus the aeromagnetic and ground magnetic survey anomalies, together with such methods as magnetotelluric method and reflection seismic method, can yield the geophysical characteristics of the deep structure of the rock masses, in favor of delimiting prospective areas for ore prospecting.