Textural and chemical variations of sphalerite in the Giant Shuangjianzishan Ag-Pb-Zn deposit, South Great Xing’an Range, China: Implications for ore-forming processes

Abstract

The Shuangjianzishan vein-type Ag-Pb-Zn deposit (145 Mt of ore grading 128.5 g/t Ag, 1.8 wt% Pb and 0.5 wt% Zn) in the South Great Xing’an Range (NE China) is hosted in NW- and NE-trending veins crosscutting the Lower Permian Dashizhai Formation, which is intruded by a granite porphyry. Although Ag-Pb-Zn sulfide mineralization has been extensively studied, the detailed ore-forming processes remain unclear. We have identified four stages of sulfide assemblages containing three generations of sphalerite: (1) Quartz arsenopyrite sphalerite (Sp1) stage; (2) Galena Ag-minerals sphalerite (Sp2) stage; (3) Ag-minerals pyrrhotite sphalerite (Sp3) stage; and (4) Pyrite calcite stage. Stages 2 and 3 are economically important. The textures of Sp1, Sp2, and Sp3 directly record the ore-forming processes, and the mineralization of silver shows an evolution trend from simple sulfides to complex sulfosalts. LA-ICP-MS trace element analyses showed that the Sp1 contains low content of silver and other trace elements (Fe, Mn, Sn, Cu, and Pb), indicating an anoxic to an euxinic environment. Stage 2 contributed largely to the silver endowment, with relatively high silver content in sphalerite or Ag-bearing mineral inclusions in Sp2a, Sp2b, and galena. Most of the silver minerals were deposited during stage 3, with sulfide and sulfosalt assemblages (e.g., pyrargyrite + canfieldite + tetrahedrite with minor pyrrhotite) replacing Sp2b. Selenium is significantly enriched in Sp3 and selenium-rich canfieldite from Stage 3. Sulfur isotopes determined by in situ LA-multicollector (MC)-ICP-MS analyses of sphalerite vary significantly from −9.6 to 2.2 ‰ (δ34SSp1 = -4.1 ∼ 1.1 ‰; δ34SSp2 = -4.2 ∼ 2.2 ‰; δ34SSp3 = -9.6 ∼ -2 ‰). Sulfur was possibly derived from the magmas forming the granite porphyry (δ34S = 1.4 ‰) and the magmatic sulfur reservoir was oxidized by mixing with the Dashizhai stratum sulfur (δ34S: −4.9 to −6.5 ‰). This resulted in lowering of the fluid δ34S value during the deposition of Sp3. The textural, chemical and isotopic data indicate distinct ore-forming episodes at Shuangjianzishan. We suggest that the deposition of abundant Ag-bearing minerals was induced by a new volume of Ag-rich ore-forming fluids and triggered by extensive decompression and cooling during Stage 3. The sulfur isotope and chemical geothermometers suggest the involvement of relatively high-temperature (>330 °C) hydrothermal fluid during Stage 2. Unlike a single cooling mineralization process, the superposition of relatively high-temperature mineralizing fluids may be an important factor in the large-scale Ag-Pb-Zn mineralization at Shuangjianzishan.