Geochronology, ore-forming processes and metal source of the Walega gold deposit, Eastern Kunlun Orogenic Belt, China: Constraints from monazite in-situ U-Pb dating and pyrite geochemistry

• Hydrothermal monazite dated at ∼ 229 Ma records the age of Walega gold deposit. • Three generations of pyrite have been identified in the Walega gold deposit. • The ore-forming materials originated from the late hydrothermal phase of the granitic magmatism. The Gouli goldfield, hosting intense magmatism related gold mineralization, is a highly distinctive gold producer in the East Kunlun Orogenic Belt. Genesis of gold deposits in this region is enigmatic, primarily due to uncertainties of the ore-forming timing, the critical mineralization processes and the material sources. Here, we conducted comprehensive in-situ textural and geochemistry (trace elements, mapping and sulfur isotopes) analyses on pyrite and in-situ U-Pb dating on monazite from the hydrothermal stages in Walega gold deposit from the north-central Gouli goldfield, with the aim to exactly constrain these uncertainties. Three types of pyrite are classified in the Walega gold deposit, based on their structural and geochemical characteristics. Py1 is mostly found in pyrite-quartz veins (metallogenic stage I), and often occurs as the nucleus of Py2 and Py3 in stages II or III, or is replaced by transitional pyrite (T-Py) or Py2. T-Py, a nano-micron pyrite particle aggregate, is the transition pyrite between Py1 and Py2, and usually shows colloidal, pseudocrystalline, and banded textures. Py2 is mainly found in pyrite-arsenopyrite-quartz veins (metallogenic stage II), which has a close syngenetic relationship with the arsenopyrite, and is commonly found as core (Py1)-rim (Py2) structure. Py3 is mainly found in quartz-polymetallic sulphide veins (metallogenic stage III), as a common core (Py1)-mantle (Py2)-rim (Py3) structure. In metallogenic stage III, the darkest pyrite (Py1) is encapsulated by brighter pyrite (Py2) co-precipitated with gold and arsenopyrite, and the brighter pyrite (Py2) is encapsulated by the brightest pyrite (Py3) co-precipitated with polymetallic minerals. Py1 typically contains lower concentrations of As (median 11.97 ppm) and Au (median 0.02 ppm), with δ 34 S values ranging from + 4.0 ‰ to + 6.1 ‰. Py2 exhibits the highest concentrations of As (median 7559 ppm) and the second-highest concentrations of Au (median 1.86 ppm), with δ 34 S values (+4.0 ‰ to + 5.9 ‰) similar to those of Py1 and Py3. T-Py shows the second-highest As content (median 1871 ppm) but the highest Au content (median 2.86 ppm). The As and Au contents of Py3 fall between those of Py1 and Py2, and its δ 34 S values ranging from + 4.4 ‰ to + 6.2 ‰. U-Pb dating of hydrothermal monazite in metallogenic stage I and III constrains the mineralization age to ∼ 229 Ma, consistent with the age of magmatic-hydrothermal gold mineralization of Gouli goldfield. The chemical composition and structural characteristics of pyrite, along with monazite geochronology, suggest that the ore-forming fluids in the Walega gold deposit are derived from multistage magmatic-hydrothermal processes and that both the fluids and ore-forming materials originated from the late hydrothermal phase of the granitic magmatism.