Gravity method for investigating the geological structures associated with W-Sn polymetallic deposits in the Nanling Range, China

摘要

The crustal structures of the Nanling Range (South China), which is endowed with abundant W-Sn polymetallic mineral resources related to granites, was investigated with gravity surveys. Since the rock densities of granitic intrusions differ significantly from the host sedimentary rocks, an integrated gravity data analysis (e.g., field decomposition, edge detection and inverse modeling) could help explore the concealed geological structures in the region. Subsequently, the analysis would assist in understanding the spatial distribution of W-Sn polymetallic deposits. Multiscale decomposition of Bouguer gravity data based on wavelet transform showed the multiscale gravity components of causative sources at different depths, and highlighted lots of concealed bodies (e.g., granites and sedimentary basin) especially in the 2nd and 3rd order wavelet details. Based on these decomposed gravity anomalies, the edge detection method played a significant role in delineating the multiscale concealed faults or lithological contacts. These lineaments, especially for their bends and intersections, are closely associated with the spatial distribution of ore deposits due to the fact that most of the deposits occur along these lineaments. The potential distribution of concealed granitic intrusions was mapped by using singularity analysis, which notably identified the weak gravity anomalies caused by concealed granites. It was observed that most of the ore deposits are located inside or near these potential areas. Three-dimensional (3D) density modeling of the local ore cluster area, Xianghualing-Qitianling-Qianlishan (XQQ) indicated that the granites extend much underneath the outcrops. This result derived from gravity inversion suggested that XQQ granitic plutons are connected together in the middle-lower crust and this system might provide sufficient materials and heat to form a large number of polymetallic ore deposits. Also, from a calculation of the Moho depths in the area, our findings showed that ore deposits often occur alongside the Moho variations which are often associated with various tectonic activities (e.g., crustal fault and magma intrusion). In summary, our research suggested that the bends and intersections of lineaments; concealed granites and their buffer zones; and the Moho variations serve as significant indicators for ore mineralization and they can assist in the prediction of potential W-Sn polymetallic deposits in the Nanling Range. (C) 2015 Elsevier B.V. All rights reserved.