Mapping chromite deposits with audio magnetotellurics in the Luobusa ophiolite of southern Tibet

Abstract

The exploration of podiform chromites in the Indus Yarlong Zangbo suture zone of southern Tibet has proved difficult because most known deposits pinch out and then reappear in the same direction. Several ground-based geophysical approaches such as gravity, magnetic, and controlled-source audio-frequency magnetotelluric (CSAMT) methods have been applied to explore for these chromite deposits but have mostly failed to delineate prospective areas. We have evaluated a successful podiform chromite exploration case history that is based on AMT. More than 8000 AMT stations were used in this study within a [Formula: see text] area of the ophiolite belt. Line separations were 80 or 40 m, and the station separation was 20 m. We implemented Bostick conversion and nonlinear conjugate gradient inversions for data interpretation, whereas 2D resistivity sections and 3D resistivity imaging were used to elucidate the inner structure and distribution of rock faces within the Luobusa ophiolite. Results from rock physics and drilling further indicate that resistivity-anomaly domains from these AMT results are correlated with rock faces in terms of fresh harzburgite, altered harzburgite and dunite, and they can thus be connected to concealed deposits. Therefore, we have developed three resistivity-anomaly models for chromite exploration, and we delineated several prospective regions containing exploitable deposits within the Luobusa ophiolite. Seven of the nine verified boreholes discussed in this paper intersected with chromite deposits; one comprises the largest and highest grade chromite deposit in China to date. Our AMT results provide the impetus for future chromite exploration in Tibet and enable a refined understanding of the structure and distribution of rock faces within the Luobusa ophiolite.

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

“…1d) ophiolites are located in the western and eastern parts of the Yarlung Zangbo Suture Zone, respectively. Most of the ophiolites in the Yarlung Zangbo Suture, including Purang, lack significant podiform chromitites, whereas the Luobusa ophiolite hosts the largest chromite deposit in China [48].…”

“…It is a tectonically bounded slice, at least 3 km thick, and has been thrust northward onto the Tertiary molasse deposits of the Luobusa Formation and the Gangdese batholiths. To the south, it is separated from Triassic flysch deposits by a steep reverse fault [16,48] (Fig. 1d).…”

“…Individual chromitites are typically lens shaped, although many occur as large, massive blocks and tabular or pencil-like bodies [66]. Typical chromitite bodies range from 20 to 250 m long, 10 to 100 m wide and 0 to 5 m thick and have an average grade of 48 wt% Cr 2 O 3 [48,67], which places them in the metallurgic category. Most of the bodies are hosted in harzburgite, although minor deposits also occur in the clinopyroxene-bearing harzburgite along the southern margin of the ophiolite [67].…”

Magnetotelluric Signal-Noise Identification and Separation Based on ApEn-MSE and StOMP

“…It has been widely used in many fields, such as the survey and exploration of petroleum and natural gas, the investigation of geothermal fields, and the prediction of natural earthquakes, etc. [ 3 , 4 , 5 , 6 ]. In addition, it is used to study the electrical structure of the crust and upper mantle, in which the discovery of the highly conductive layer provides an important basis for tectonic mechanics.…”

Study on Climate Impacts on Asphalt Pavement in Tibet, China

“…The climatic conditions in Tibet are complex, and the differences in latitude and altitude lead to multiple climatic zones in Tibet (Zhu et al, 2019;He et al, 2018;Wang & Ma, 2018). Subtropical, warm, temperate, sub-frigid and cold zones are distributed from southeast to northwest, and the annual average temperature in Tibet is low and the daily temperature difference is large.…”