Constraints on the genesis of the Laochang Pb–Zn ore, Gejiu district, Yunnan: Evidence from sulfide trace element and isotope geochemistry

Abstract

The Laochang deposit is an important part of the Gejiu Sn–Cu polymetallic district, containing abundant reserves of tin, copper, lead, zinc and tungsten. The recently discovered sulfide Pb–Zn ore is located 1000 m underground in the Laochang deposit, and its genesis is not clear. In this study, the trace element concentrations and sulfur and lead isotope compositions of sulfides from the newly discovered Laochang Pb–Zn ore were analyzed to constrain mineralization conditions and genesis. Our results show that sphalerite from Laochang Pb-Zn ore contains high Fe, Cu, Ga, Ag, In and Sn, low Co, Ni, Ge and As and moderate Mn and Cd. Pyrite is enriched in Ag, Sb, Pb and As and poor in Co, Ni, Se, Ti and Mn. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) ablation profiles of sphalerite show that Fe, Cd, Mn, Ga, and In occur as solid solutions and/or lattice-bound ions, whereas Ag, Cu, Pb, Sb, and Sn are present in both mineral inclusions and solid solutions. Moreover, the profiles of pyrite indicate that Co, Ni and As enter the pyrite lattice by ion replacement and that Sn, Ag, Pb and Sb occur in both solid solutions and mineral inclusions.The Fe/Zn calculation temperature of sphalerites ranges from 299 °C to 344 °C (mean of 320 °C), while the sphalerite geothermometer (GGIMFis) temperatures range from 268 °C to 312 °C, with an average of 290 °C. These results indicate that the Laochang Pb–Zn ore formed under moderate- to high-temperature conditions. The sphalerites in the Laochang Pb–Zn ore is enriched in Ga and In, is poor in Ge, and has moderate contents of Cd which implies that it may be related to magmatic-hydrothermal fluid. Pyrites in this study have relatively low Co and Ni concentrations and low Co/Ni ratios, which also indicate magmatic-hydrothermal origin.The overall sulfur isotope compositions of sulfides from the Laochang Pb–Zn ore, ranging from −1.2 ‰ to +3.7 ‰, indicate that the S may be derived from the Laoka granite with some contribution from the wallrock. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of galena in the five samples range from 18.400 to 18.447, 15.656 to 15.697, and 38.478 to 38.968, respectively. These results imply that the ore-forming metals in the Pb–Zn ore were derived from the upper crust. Therefore, we believe that the Pb–Zn ore may be of magmatic-hydrothermal origin and represents the distal mineralization of the Laoka metallogenic system.