The role of deep-sourced metamorphic fluids in Au enrichment: A case study of the Bangbu Au deposit, southern Tibet (Western China)

Accurately identifying the evolutionary process and source of ore-forming fluids is essential for elucidating the mineralization mechanisms of large-scale gold deposits and is pivotal in discerning the genetic types of lode gold deposits. The Bangbu gold deposit (>40 t @ 7.0 g/t), situated in southern Tibet within the eastern segment of the Yarlung Tsangpo metallogenic belt, represents a significant gold repository that originated from the main collision stage of the Tibetan Plateau. This deposit provides an excellent opportunity for examining the genesis of large lode gold deposits within a compressional tectonic environment. The orebodies are predominantly associated with faults systems, while the hosts rock comprise the sedimentary-metamorphic rock of the Langjiexue Group. Petrographic analysis and crosscutting relationships identify three ore-forming stages: (I) quartz + coarse-grained pyrite, (II) quartz + fine-grained pyrite + native gold + polymetallic sulfide, (III) ore-barren calcite. The He and Ar isotopic compositions indicates that the mineralization fluids are of a hybrid crust-mantle origin (³He/⁴He = 0.12–0.63 Ra; ⁴⁰Ar*/⁴He = 0.27–2.67), with mantle contributions ranging from 1.65 % to 10.93 % (He_mantle). The hydrogen–oxygen isotope data corroborates the deep-derived metamorphic origin of the ore-forming fluids, aligning with the proposed genetic model. In-situ sulfur isotopic compositions of pyrite (δ³⁴S_V-CDT = +1.82 ‰ to + 5.27 ‰) systematically document mantle reservoir contributions.

Fluid inclusion (FI) studies identified four FI types in quartz from these stages: liquid (H₂O)-rich two-phase (L-type), vapor (H₂O)-rich two-phase (V-type), and CO₂ (liquid and gas)-bearing inclusions (C1- and C2-types). Further identification of four types of fluid inclusion assemblages (FIA), designated as A (L + V-types), B (L + C1-types), C (L + V + C1-types), and D (only L-type). The development of stage I is characterized by four distinct types of FIAs, exhibiting homogenization temperatures from 234 °C to 334 °C and salinities ranging between 2.40 and 6.29 wt% NaCl equivalent. This corresponds to a medium-temperature, low-salinity NaCl–H₂O–CO₂ hydrothermal system (mol% CO₂ = 7.48–9.21). In contrast, stage II encompasses two types of FIAs (A and D), with homogenization temperatures between 198 °C and 282 °C and salinities from 0.53 to 5.85 wt% NaCl equivalent, indicative of a NaCl–H₂O-dominated hydrothermal system. Stages I and II pyrite related to native gold, and can be divided into four generations (Py1a, Py1b, Py2a, and Py2b). The porous/sieve-like textures of Py1a and Py2a indicate Au remobilization, predominantly governed by coupled dissolution-reprecipitation (CDR). Elevated As concentrations (up to 1.81 wt%) in pyrite, positively correlated with Au, further demonstrate that CDR-driven evolution of deep metamorphic fluids caused pyrite lattice distortion. This deformation appears to facilitate the incorporation of gold, thereby enhancing its enrichment within the mineral. Consequently, this study posits that the Bangbu gold deposit exemplifies a quintessential orogenic gold deposit.