The world-class carbonate-hosted Fankou Zn-Pb deposit in China. Part II. Alterations: C-O isotopic halos footprint the nature and transfer pathway of ore-fluids, and quantitative water-rock interaction process

Abstract

The giant Fankou Zn-Pb deposit (more than 10 Mt Zn + Pb metals, grading at 15 %), situated in northern margin of the Quren Basin, is one of the largest Zn-Pb deposits in China. The stratiform orebodies are jointly bounded by both the strata (in the specific layers of Middle to Upper Devonian Donggangling and Tianziling carbonate sequences) and the fault system (a three-order thrusting fault system). Ore textures of carbonate dissolution, replacement-infilling and cementation are widespread in Fankou, analogous to a MVT (Mississippi Valley-type) deposit. Owing to the poorly developed fluid inclusions, the nature of the Fankou ore fluids remains indeterminate. To address this problem, we herein utilize micro-petrography, carbon-oxygen (C-O) isotopes and quantitative isotopic exchange modelling to trace the ore-fluids and water-rock interaction process. A total of ninety-three (93) carbonate samples, covering the majorities of stratigraphic horizons, planar locations, and various alteration intensities, were systematically collected. Micro-petrography reveals the primitively sedimentary bioclastic limestone suffer from heterogeneous dolomitization and carbonatization followed by massive Zn-Pb precipitation. These carbonate samples display a widely-ranged, δ18O- and δ13C-depleted isotope signature (δ18O = 12.57 to 23.82 ‰, δ13C = −7.79 to −0.46 ‰), relative to normal marine carbonates (δ18O = 25 ‰, δ13C = 0 ‰). Further modelling of isotopic exchange indicates a low-temperature (100 to 250 °C), δ18O-depleted (−3 to 7 ‰) and δ13C-depleted (−10 to −7‰) ore fluid equilibrated with the host rock by a relatively low water-rock ratio (W/R = 1 to 10). In terms of space, we identified a relatively decreasing trend of δ18O and δ13C isotopic compositions of carbonates, from southeast to northwest, and from bottom to up. Based upon the previous structural analysis, we interpret that the δ18O and δ13C isotopic distribution patterns are caused by a northwestwards transfer pathway of ore-fluids along the thrusting fault system, and the metalliferous, δ18O- and δ13C-depleted fluids are sourced from the interior of the Quren Basin. Collectively, we clarify the deep southeastern part of the Fankou Zn-Pb deposit and the interiors of the Quren Basin is the favorable exploration target in future.