Extreme Cd enrichment induced by organic matter involvement in the MVT Pb-Zn deposit: Constraints from S-Zn-Cd isotopes in the Niujiaotang deposit, southwestern China

• Cadmium mainly occurs in the lattice of sphalerite through directly substituting Zn • High δ 34 S values suggest TSR occurring and providing abundant S for the mineralization. • Low δ 66 Zn and δ 114/110 Cd values support the Cd transportation by organic complexes. • Organic matters played critical roles in both the Cd transportation and precipitation. Dispersed elements such as Ge, Ga and Cd attract increasing attention due to their wide use in modern high-tech or emerging industries. These elements are typically enriched in Mississippi valley-type (MVT) Pb-Zn deposits, which have not been well understood for the enrichment mechanisms due to their low-concentration and dispersed features. The Niujiaotang Zn-Cd deposit located at the southwestern China is one of the MVT Pb-Zn deposits extremely enriched in Cd, hosting more than 5000 tonnes of Cd with an average Cd grade of 1.4 wt% in sphalerite. To understand the critical factors controlling the abnormal enrichment of Cd, the elemental compositions and S-Zn-Cd isotopes of Cd-bearing sphalerite were investigated. The results show that the sphalerite in the Niujiaotang deposit can be divided into two generations (Sp1 and Sp2) from the early to the late hydrothermal stage, of which the Sp1 (3208–20521 ppm, mean of 10,812 ppm) has higher Cd concentrations than those of the Sp2 (Cd = 364–13500 ppm, mean of 4222 ppm). The Cd-rich Sp1 is coexistent with bitumen and abundant in organic gas-bearing fluid inclusions (e.g., CH 4 and C 2 H 6 ), suggesting that Cd enrichment might be associated with organic matter involvement. The high δ 34 S values (14.0‰ to 34.6‰) of sulfides are similar to those of sulfates in the strata, corroborating that TSR (thermochemical sulfate reduction) occurred between sulfates and organic matter. The low δ 66 Zn sphalerite (–0.27‰ to 0.09‰) and δ 114/110 Cd sphalerite (–0.72‰ to –0.10‰) values, in combination with the widespread paleo-oil reservoirs in the study region, suggesting that organic ligands derived from oil-field brines contributed to the transportation of Zn and Cd. The above results support that organic matter played critical roles in both the metal transportation and precipitation, which thus promoted the abnormal enrichment of Cd.