Multistage sulfide and hydrothermal quartz as a record of precision ore-forming process in the epithermal Cu-Zn-Au system in the Kalatag arc, East Tianshan, NW China

The Meiling is a newly discovered Late Paleozoic epithermal Cu-Zn-Au deposit in the Kalatag arc, East Tianshan, NW China. Some key issues such as metal substitution mechanisms, physicochemical characteristics of oreforming fluids and sulfide precipitation process are still unclear. In this contribution, we carried out a detailed trace element geochemistry of sulfides and quartz and in-situ sulfur isotopic compositions to characterize the ore-forming elements enrichment process. The ore-forming process can be divided into four stages, including quartz-pyrite, quartz-chalcopyrite-pyrite, quartz-chalcopyrite-sphalerite-pyrite and quartz-calcite-pyrite stage, respectively. Three generations of pyrite are identified in the Meiling deposit, including subhedral to anhedral pyrite (Py1), fine-grained pyrite (Py2) and irregular pyrite (Py3). Three generations of hydrothermal quartz (Qtz1, Qtz2, and Qtz3) are identified. Titanium, Al and Li compositions of Qtz1 are mostly higher than those of Qtz2 and Qtz3, indicating a higher formation pressure and temperature. Average Li compositions show a decreasing trend from Qtz1 to Qtz3, namely 42.8 ppm, 11.3 ppm, 5.3 ppm, respectively. Similar trends also appear in the Al compositions, indicating that the Li and Al compositions may control the CL intensity of quartz. The increasing pH from Qtz1 to Qtz3, combined with the alteration characteristic and the sequence of sulfide precipitation, indicating a clear relationship between pH and mineralization process in the Meiling deposit. LA-ICP-MS data shows that Py1 displays the highest compositions of Te (avg. 44.66 ppm) and Pb (avg. 361.95 ppm). Py2 contains the highest compositions of Cu (avg. 5643.85 ppm), Zn (avg. 436.72 ppm), As (avg. 15300.55 ppm), and Ag (avg. 146.70 ppm), whereas the Py3 contains the most Se (avg. 405.40 ppm), Au (avg. 18.65 ppm), Ni (avg. 19.45 ppm) and Co (avg. 12.64 ppm) compositions. In Py2, the high compositions of Cu and Pb may be due to the existence of inclusions in chalcopyrite and galena. Chalcopyrite is enriched in Ag, Te, Sb, Zn, Cd, In and Se and poor in Ge, Au and Bi. Sphalerite is characterized by high compositions of Ag, Cd, Fe, Mo, Cu, Ga and Sb. Cobalt/Ni ratios of Py1(avg. 1.7), Py2 (avg. 4.9), and Py3 (avg. 1.8), indicating a hydrothermal origin. Sulfur isotopic compositions of sulfides suggest that quartz porphyry is an important material source at Meiling deposit. Tellurium composition in Py1 (avg. 44.7 ppm) was significantly higher than that in Py2 (avg. 1.10 ppm) and Py3 (avg. 0.90 ppm), indicating that the fluid fO2 in stage II and III was higher than that in stage I. Moreover, stage II and III have nearly the same fO2 values. Arsenic enters pyrite in the form of As- instead of S- in the Meiling deposit, causing its lattice defects to promote Au+ to enter into pyrite. Au-As-rich Py3 and visible gold are most likely to be formed by direct precipitation from the new Au-As rich ore-forming fluid. Abundant porous pyrite and trace element ratios (e.g., Ag/As, Ag/Co) indicates that boiling plays an important role in the precipitation process of Py2 and Py3. In summary, trace element compositions of multistage sulfide and quartz can effectively reconstruct hydrothermal fluid evolution and metal enrichment process of Meiling epithermal Cu-Zn-Au mineralization system.