In situ trace element compositions of sulfides constraining the genesis of the worldclass Shuangjianzishan Ag-Pb-Zn deposit, NE China

Abstract

The worldclass Shuangjianzishan Ag-Pb-Zn deposit, containing proven reserves of >21,665 t of silver @128.5 g/t, >3.3 Mt of lead @1.6 wt%, and >1.9 Mt of zinc @0.6 wt%, is the largest silver deposit in Asia. This giant Ag mineralization system has attracted extensive attention since its discovery. However, the incorporation mechanism of Ag in ore sulfides and the deposit genetic type are still greatly debated. We have conducted in situ LA-ICP-MS trace element analyses for sulfides in this deposit to address the above issues. Three paragenetic stages of mineralization were recognized at Shuangjianzishan, including pyrite + quartz + K-feldspar stage (I), galena + sphalerite + Ag-bearing sulfosalt + quartz + calcite + chlorite + sericite stage (II), and post-ore quartz stage (III). Most sulfides and silver minerals were deposited in stage II. In situ time-resolved depth profiles, principal component analysis (PCA), and correlation plots of trace element for sulfides indicate that trace elements are dominantly present in sulfides as solid solutions and nano-inclusions, and less as micro-inclusions. For silver, the most critical element in the studied deposit, its substitution mechanisms vary largely among sphalerite, pyrite, and galena. In sphalerite, Ag enters the crystal lattice associated with coupled substitution of Zn by Fe, Sn, Sb, and Ga. In pyrite, Ag enters the lattice through the coupled substitution of Fe by Cu, Zn, and Sn. In galena, Ag enters lattice by the coupled substitution of Pb by Sb and Zn. Based on the GGIMFis geothermometer calculations, the temperatures for ore stage II range from 290° to 333 ℃ (mean of 319 ℃), which are almost identical to the results of the sulfur isotope geothermometry and fluid inclusion thermometry from the previous studies. Furthermore, the trace element distributions in the Shuangjianzishan sulfides, e.g., the enrichment of Fe, Mn, Sn, Cu, Ag, Sb and Pb and the depletion of Ga, Ge, Co and In in sphalerite, as well as the geological and geochemical features, infer that the studied deposit is similar to the typical epithermal type deposit. We therefore propose that the worldclass Shuangjianzishan Ag-Pb-Zn deposit is an epithermal deposit, and this contribution indicates that in situ LA-ICP-MS trace element analysis of sulfides could help shed light on identifying the genetic type for the hydrothermal ore deposits.