Gold Occurrence and Evolution of OreForming Fluids of the Qinglonggou Gold Deposit, Northern Margin of Qaidam Basin, Qinghai Province, China

The Qinglonggou gold deposit is a largescale deposit located within the Tanjianshan gold orefield on the northern margin of the Qaidam Basin. Recent exploration has achieved significant breakthroughs at depth and along its periphery; however, the occurrence of gold in deep ores and the evolution of the oreforming fluids remain debated. This research methodology integrated detailed field investigations and drill core logging with systematic mineralogical and geochemical analyses. These included electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for in-situ trace elements, and laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) for in-situ sulfur isotope analysis of pyrite, arsenopyrite, and native gold from the main mineralization stages (III–V).

The results indicate that gold primarily occurs as nanoscale inclusions within main-stage pyrite (Py3, Py4, Py5) and arsenopyrite, with minor amounts found as native gold filling fractures in pyrite or calcite. Geochemical data from pyrite reveal an evolution in fluid composition from Stage III to Stage V, marked by systematic variations in Co/Ni ratios, As content, and trace elements (e.g., Au, Ag, Cu, Sb). This reflects a transition from moderate-temperature to higher-temperature conditions. Stage IV witnessed intense fluid boiling, which was a critical mechanism for the large-scale precipitation of gold. In-situ sulfur isotope analyses demonstrate a multi-sourced sulfur system: Stage III sulfur is predominantly magmatic-hydrothermal (δ³⁴S: +5‰ to +20‰), Stage IV shows significant seawater influence (δ³⁴S up to +25‰), and Stage V indicates a mixed source. Furthermore, platinum-group element (PGE) signatures and high Bi contents suggest a potential contribution of mantle-derived or deep magmatic components to the ore-forming materials.

In conclusion, the Qinglonggou deposit formed through multiple overprinting hydrothermal events. Early mineralization (Stage III) produced As-rich, Au-poor pyrite in an island-arc setting. The main gold mineralization (Stage IV) was triggered by fluid boiling accompanied by seawater mixing, leading to gold enrichment in arsenopyrite and native gold. A later fluid pulse (Stage V), possibly involving new As-rich fluid and mantle-derived components, further complicated the system. This study provides key geochemical constraints on the metallogenic processes in the Tanjianshan area.