Deep-to-Shallow Structural Architecture and Regional Exploration Target Screening in the Duolong Porphyry Cu-Au District, Tibet: Spectrum-Constrained 2D Multiscale Wavelet Analysis

The Duolong district in the central Bangong-Nujiang Suture Zone (BNSZ) is a major porphyry–epithermal Cu-Au cluster in Tibet. We combine radially averaged log-power spectra with a five-level db4 two-dimensional discrete wavelet transform (2D-DWT) to compare depth-sensitive frequency bands with their plan-view expressions in complete Bouguer gravity and reduced-to-the-pole (RTP) aeromagnetic grids. The five-level transform yields signed detail reconstructions d1–d5, ordered from fine to coarse scales, and a final cumulative low-pass approximation a5. The gravity/magnetic spectral estimates are 1.02/1.09 km for the shallow ensemble, 5.12/4.85 km for the intermediate ensemble, and 18.60/21.58 km for the deepest resolved ensemble. These values describe statistical source-depth ranges rather than discrete interfaces. The 2.98 km difference between the deep-source estimates (14.8% of the paired mean) is treated as methodological uncertainty and/or a difference in the depths of the density and magnetization transitions rather than as quantitative identity. The coarsest detail d5 and low-pass approximation a5 express a broad upper-to-middle-crustal background transition and do not image the Moho or a mantle source. Co-located mid-scale gravity and magnetic anomalies are consistent with a composite intrusive–basement source region, but the absence of site-specific density and susceptibility measurements prevents a unique lithological assignment. A southeastern anomaly zone centered approximately at 84.0° E, 31.7° N is therefore presented as a first-order regional screening hypothesis within a broad 3–8 km source-depth band, not as a quantitatively closed drill target. The interpretation distinguishes geophysical observations from geological hypotheses and explicitly incorporates the limitations of spectral and wavelet separation.