Ore genesis of the Gala gold deposit in the Garzê–Litang suture zone, western Sichuan Province: constraints from textures, trace elements and in-situ sulfur isotopes of pyrite

As a key gold mineralization domain in southwestern China, the Garzê–Litang suture zone (GLSZ) is tectonically located between the Yangtze block and Yidun island arc, hosting numerous gold deposits. The Gala gold deposit, in the northern GLSZ, is a representative orogenic gold deposit in western Sichuan, with 12 t Au reserves and an average grade of 3.8 g/t. Gold is primarily hosted in quartz-sulfide veins, characterized by auriferous pyrite, arsenopyrite, and trace native gold. Here we identified the metallogenic stage into three episodes and five stages: pre-ore episode (stage I), main hydrothermal-ore episode (stage II, stage III, and stage IV), and post-ore episode (stage V). This study distinguishes five pyrite generations (Py1–Py5), Py1 belongs to the pre-ore stage and Py5 primarily belongs to the post-ore stage, while the mian hydrothermal-ore stage includes Py2, Py3, and Py4. Py1 is coarse-grained, euhedral, and texturally homogeneous; Py2 in quartz-dolomite-pyrite vein exhibits medium-grained subhedral-euhedral morphology and overgrows on Py1; Py3 is subdivided into Py3a (porous core with oscillatory zoning) and Py3b (rim overgrowths surrounding Py3a); Py4 is texturally homogeneous and closely associated with arsenopyrite; Py5 is fine-grained, anhedral, and occurs as veinlet-style aggregates. LA-ICP-MS in-situ trace element data show that Py1 and Py5 have much lower Au and As contents than Py2, Py3 (Py3a, Py3b), and Py4. Gold is predominantly invisible, existing as lattice-bound Au + (solid solution in pyrite) or microscale gold-bearing inclusions, with minor native gold along quartz vein fractures. Sulfur isotopic analyses show that Au-As-enriched Py2, Py3, and Py4 have similar low δ 34 S values (Py2: −9.09 to − 8.07 ‰; Py3a: −10.32 to − 9.25 ‰; Py3b: −11.20 to − 10.12 ‰; Py4: −16.91 to − 12.93 ‰), indicating that the sulfur may have been derived from the metamorphism of sedimentary strata (i.e., Qugasi Formation). In contrast, Au-As-depleted Py1 has relatively higher δ 34 S values (−5.37 to 1.83 ‰), similar to typical magmatic sulfur. Py5 has the lowest δ 34 S values (–32.44 to −17.87 ‰), suggesting a bacterially mediated metamorphic sulfur source. Fluid boiling and water–rock interaction are the main mechanisms controlling gold precipitation in the Gala deposit. Combining this study’s results with regional geology, we suppose that the Gala gold mineralization is primarily related to a metamorphic-hydrothermal system that formed during Early Jurassic post-collisional intracontinental extensional tectonic setting.