Trace element, sulfur and lead isotopes in pyrite of Xinjiazui gold deposit in the northwestern margin of Yangtze Block, China: Implications for ore–forming processes

• Multistage gold-bearing veins arise from a single hydrothermal fluid system. • Ore-forming materials (S and metals) primarily derived from metamorphic basement. • Ore-forming is coupled with the Back-Longmenshan tectonic belt in the Mesozoic. The ore-forming processes and sources of material in hydrothermal gold deposits remain pivotal subjects in economic geology, with recent studies underscoring their critical importance. The Xinjiazui gold deposit, located on the northwestern margin of the Yangtze Block, holds a gold reserve of 3.7 tons. Structurally, the ore bodies are constrained by brittle-ductile shear zones, hosted within the black rock series, and spatially associated with quartz diorite dikes, that the origin of its ore-forming materials and the specific metallogenic processes remain contentious, which substantially restricts the understanding of regional gold mineralization regularities. Utilizing advanced analytical techniques such as EPMA and LA–ICP–MS, in-situ trace element and isotopic analyses on pyrite from the Xinjiazui deposit were conducted to elucidate these debates. Pyrite can be divided into six types, including syn -sedimentary pyrite appearing in framboidal (Py 0 a) and euhedral granular texture (Py 0 and Py 0 b) within surrounding rocks; and, the pyrite is mainly concentrated in Stage I (Py 1 a-Py 1 b) and Stage II (Py 2 ) of the hydrothermal mineralization epoch. Both Py1a and Py1b also exhibit low levels of Au (≤0.25 ppm, Avg) and As(< 500 ppm, Avg), as well as similar Pb isotopic compositions ( 206 Pb/ 204 Pb: 19.57 – 19.98, 207 Pb/ 204 Pb: 15.74 – 15.86, 208 Pb/ 204 Pb: 38.40 – 38.77) that are considerably overlapped with the hosting Cambrian carbonaceous siliceous slate, yet they have distinct S isotopic compositions (Py 1 a: 19.18 to 19.43 ‰, Py 1 b: 11.15 to 12.19 ‰) reflect derivation from different sources, consistent to their respective hosting rocks (Cambrian slate vs. the veins from stage II). In contrast, Py 2 shows significantly increased contents of Au and As (3.54 ppm, 10423 ppm, Avg), and its Pb isotopic composition ( 206 Pb/ 204 Pb: 18.12 – 18.82, 207 Pb/ 204 Pb: 15.59 – 15.83, 208 Pb/ 204 Pb: 38.38 – 39.14) is consistent with the Silurian carbonaceous phyllite and quartz diorite dikes. The δ 34 S isotope of Py 2 (8.79 to 12.47 ‰) overlaps with the Neoproterozoic deep metamorphic basement, Silurian carbonaceous phyllite and quartz diorite dikes. Despite this isotopic similarity, the low contents of W, Sn, Mo, and Bi in Py 2 related to magmatic rocks, along with the absence of ore minerals in quartz–diorite dike breccia, suggests a weak genetic link between the Xinjiazui gold deposit and quartz diorite dikes. Moreover, the low background gold content in the Cambrian slate (5.46 ppb) and Silurian phyllite (5.13 ppb), both near the Clark value (4.0 ppb), indicates the black rock series (the above-mentioned slate and phyllite) are unlikely to be the primary sources for gold. Instead, the Neoproterozoic deep metamorphic basement likely contributed the majority of S and Au to the gold mineralization, and variations in trace elements, S and Pb isotopes across the stages I and II of mineralization may result from progressive evolution of basement-derived fluids. Increasing gold content with decreasing fluid temperature from Stage I to Stage II further suggests temperature played a key role in gold precipitation. Overall, ore formation at the Xinjiazui gold deposit resulted from a single-source fluid system evolving through multiple stages linked to regional tectonic transitions in the Mesozoic.