New insights into pyrite in the Western Mecsek uranium ore deposit: its morphology, invisible gold and trace element composition

This study presents the occurrence of invisible gold and associated trace elements in pyrite from the Late Permian Kővágószőlős Sandstone Formation within the Western Mecsek uranium ore deposit, Hungary. Eight pyrite-rich drill-core samples were analyzed using optical microscopy, EMPA, LA-ICP-MS, and sequential extraction coupled with ICP-OES. Optical microscopy and EMPA analyses identified three pyrite morphologies: framboidal (1–20 μm), cement (50–200 μm), and euhedral (1–20 μm). Framboidal pyrite, of bacterial origin, occurs within organic matter, whereas cement and euhedral pyrites are associated with quartz and feldspar. Cement pyrite occasionally hosts minor sulfosalt and galena inclusions. LA-ICP-MS detected gold in pyrite grains (average 0.33 ppm), while sequential extraction with ICP-OES confirmed gold at 0.42 ppm. Elevated concentrations of As, Zn, and Cu range from 0.2 to 2.3%, along with trace elements such as W (0.05-0.13 ppm), Ba (7- 40 ppm), and Ni (95-244 ppm), were also identified. Gold is interpreted to occur primarily as a solid solution within the pyrite lattice, whereas other trace elements are likely associated with metal nanoparticles on or within the pyrite grains. These findings emphasize the complex geochemistry of pyrite and highlight the necessity for further studies, including sulfur isotope analysis, to elucidate ore-forming processes and gold mineralization mechanisms.