TRACE ELEMENTS AND ISOTOPES OF SULFIDES IN PB-ZN ORES FROM LAOCHANG DEPOSIT, GEJIU DISTRICT, YUNNAN, CHINA: EVIDENCE FOR THE EXISTENCE OF EXHALATIVE SEDIMENTARY MINERALIZATION

Laochang deposit is one of the important parts of the world-class Gejiu Sn-polymetallic deposit. Although there are many papers have been published to explain the ore genesis, controversy still exists. In this paper, the major and trace elements and S isotope of sphalerite and S, Pb isotopes of galena have been analyzed to explain the genesis of stratiform Pb-Zn ore. The Zn, S and Fe concentrations of sphalerites range from 44.51%~59.71%, 31.08%~33.06% and 6.33% ~18.90%, respectively, indicating that the sphalerite is marmatite. Trace elements exhibit that Fe, Mn, Cu, Ga, Cd and Sn are enrichment while Co, Ni, Ge, As, Ag, In, Sb and Pb are poor in the Laochang sphalerite. Time resolved laser ablation profiles reveal that Fe, Mn, Ga, Cd, In, As, Co and Ni are mainly present in solid solution or as nanoscale inclusions and Cu, Ag, Sb, Pb and Sn are present in both solid solution and mineral inclusions. The Cd concentrations, Zn/Cd and Ga/In ratios indicate that ore-formation temperature belongs to medium-high temperature. The sphalerite is rich in Ga and poor in Ge, which is similar to the SEDEX deposit Dabaoshan and Bainiuchang.

The measured δ³⁴S values for sphalerite range from 0.8‰ to 3.7‰, while the galena δ³⁴S values are between -1.2‰ and 0.3‰. The sulfur isotope compositions indicate that S has the characteristics of the sulfur of deep-seated magma source. The Pb isotopic compositions of galena yield ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values from 18.400 to 18.445, 15.656 to 15.694, and 38.478 to 38.968, respectively, indicating an upper crustal lead source with a little addition of mantle lead source. So, we conclude that the Laochang Pb-Zn deposit is of submarine exhalation sedimentary origin.