Application of 3-D inversion of AMT data in exploration of concealed nonmetallic deposits: A Case Study on the Huashitou Mountain fluorite deposit in Beishan metallogenic belt, China

Abstract

This study utilized audio-frequency magnetotelluric (AMT) data collected from a concealed fluorite deposit on Huashitou Mountain in the Beishan metallogenic belt to obtain a 3-D resistivity model beneath the deposit. The fluorite ore-forming mechanism was determined and is discussed in combination with previous high-resolution magnetic and induced-polarization imaging results. Our study revealed a large high-resistivity body, R2 (> 1000 Ω·m), corresponding to the low induced-polarization and low-magnetic region that extends from 40 to > 80 m in depth. We suggest that this corresponds to the Indosinian intrusive granite, and potentially serves as a fluorite mineralization rock mass. A highly conductive zone, C3 (< 3 Ω·m), extending from approximately 20 to 300 m, or even deeper, was interpreted as a deep fault/fracture zone functioning as a fluid pathway for fluorite mineralization. Two smaller conductive zones, C1 and C2 (< 20 Ω·m), appear above the high-resistivity granite intrusion. These zones correspond well with high induced-polarization anomalies (> 2.5 % threshold), relatively high-△T anomalies (−50 nT), and hematite alteration areas shown on the geological cross-sections. These zones were interpreted as two near-surface secondary fault structures. The aforementioned fault structures are believed to have formed before the main mineralization period of the fluorite deposit and were subsequently reactivated during intense tectonic-magmatic activity in the Late Hercynian. Hot fluids enriched with volatile components (e.g., F and CO₂) were transported along the deep fault, represented by C3, from the lower crust to the near-surface secondary fault zones (represented by C1 and C2). These fluids interacted with the calcic plagioclase in alkaline granite magma to form fluorite (mainly composing CaF₂) through calcium extraction. Further, the process led to hydrothermal alteration, resulting in associated iron-oxide mineralization or silicification alteration zones, as well as alterations related to a sedimentary layer such as kaolinization and chloritization. These zones exhibit high conductivity because of their significant porosity and the presence of fluids or metal sulfides.