Effects of arsenic on the distribution and mode of occurrence of gold during fluid-pyrite interaction; a case study of pyrite from the Qiucun gold deposit, China

• Published in: The American mineralogist Vol. 107; no. 5; pp. 914 - 929
• Main Authors: Zhang He, Zhang He, Cai Yuanfeng, Cai Yuanfeng, Sha Gang, Sha Gang, Brugger, Joel, Pring, Allan, Ni Pei, Ni Pei, Qian, Gujie, Luo Zhenjiao, Luo Zhenjiao, Zhang Yang, Zhang Yang, Tan Wei, Tan Wei
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.05.2022; Walter de Gruyter GmbH
• Subjects: Ablation; arsenian pyrite; Arsenic; arsenides; arsenopyrite; Asia; boundary interactions; case studies; China; computed tomography data; Crystal defects; Crystallization; Defects; Distribution; Economic geology; epithermal gold deposit; epithermal processes; Far East; fluid phase; fluid-rock interaction; Fluorescence; Fractures; Fugacity; Fujian China; Gold; gold ores; Grain boundaries; ICP mass spectra; Inductively coupled plasma mass spectrometry; Laser ablation; Lasers; Lattice vibration; mass spectra; Mass spectrometry; Mass spectroscopy; metal ores; metals; mineral deposits, genesis; mineral-water interface; Mineralization; Mineralogy; nonsilicates; Nucleation; Physicochemical processes; Pyrite; Qiucun Deposit; Solid solutions; spectra; Stacking faults; Structural analysis; sulfides; Sulfur; Sulphur; Synchrotrons; Tomography; Trace elements; Unit cell; water-rock interaction; X-ray analysis; X-ray fluorescence; China
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2021-7675

This paper presents the results of an investigation of the incorporation of Au within pyritic ore from the Qiucun epithermal Au deposit, China. The new data provide insights into the mode of occurrence of Au during fluid-rock interactions within epithermal systems. The distribution and mode of occurrence of Au within arsenian pyrite were investigated using a chemical and structural characterization-based approach combining laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis, megapixel synchrotron X-ray fluorescence analysis (MSXRF), and atom probe tomography (APT). The resulting data indicate that invisible Au is present at elevated concentrations in the form of a homogeneous solid solution within As-rich pyrite domains, which yields Au concentrations that positively correlate with As. Arsenic-induced lattice defects, such as stacking faults and the expansion of the pyrite unit cell, provide evidence of the effect of As on the incorporation of Au into pyrite. The nucleation and crystallization of electrum preferentially occurred at the fluid-pyrite reaction interface or along fractures and grain boundaries within the pre-existing pyrite. This study indicates that changes in physicochemical conditions (e.g., temperature, pH, and sulfur fugacity) during fluid-pyrite interactions are key controls on the development of nanometer- or micrometer-scale clusters of gold. The systematic compositional and textural observations documented in this study provide new insights into the mechanisms responsible for the different modes of occurrence of Au (ionic vs. particulate) and enable us to further understand the processes involved in the formation of Au mineralization.